JP7489948B2 - Electrode structure, secondary battery, battery pack, vehicle, and stationary power source - Google Patents

Description

Embodiments of the present invention relate to electrode structures and secondary batteries, etc.

Non-aqueous electrolyte batteries such as lithium-ion batteries are used as power sources in a wide range of fields. Non-aqueous electrolyte batteries come in a wide variety of forms, from small ones for use in various electronic devices to large ones for use in electric vehicles. Non-aqueous electrolyte batteries use non-aqueous electrolytes that contain flammable substances such as ethylene carbonate, so safety measures are necessary.

Instead of a non-aqueous electrolyte, development is underway to develop aqueous electrolyte batteries that use an aqueous electrolyte containing a non-flammable aqueous solvent.

JP 2010-56027 A JP 2018-160443 A JP 2018-163893 A JP 2019-57373 A

Appl. Mater. Interfaces 2014, 6, 526-531 Appl. Mater. Interfaces 2016, 8, 23688-23695

The objective is to provide an electrode structure, a secondary battery, and a battery pack that can realize a secondary battery that exhibits high charge/discharge efficiency and suppresses self-discharge, as well as a vehicle and a stationary power source equipped with the battery pack.

According to an embodiment, the battery includes a negative electrode including a negative electrode active material-containing layer, and a separator including at least three layers, a first composite layer, a porous layer, and a second composite layer, in this order. The negative electrode active material-containing layer and the first composite layer are in contact with each other, and the first composite layer and the second composite layer include solid particles and a polymer material. The solid particles include inorganic solid particles having ionic conductivity of alkali metal ions. The inorganic solid particles include one selected from the group consisting of oxide-based ceramics such as alumina, silica, zirconia, yttria, magnesium oxide, calcium oxide, barium oxide, strontium oxide, and vanadium oxide, and nitride-based ceramics such as sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, barium sulfate, hydroxyapatite, lithium phosphate, zirconium phosphate, titanium phosphate, silicon nitride, titanium nitride, and boron nitride. When the separator is cut, the relationship between the force FH applied in the horizontal direction and the force FV applied in the vertical direction associated with the peeling of the first composite layer from the negative electrode active material-containing layer is 1.35≦FH/FV≦2.2.

FIG. 1 is a cross-sectional view illustrating an example of an electrode structure according to an embodiment. 5 is a schematic view of a modified example of a separator included in the electrode structure according to the embodiment. FIG. FIG. 2 is a cross-sectional view illustrating an example of an electrode group according to the embodiment. FIG. 1 is a cross-sectional view illustrating an example of a secondary battery according to an embodiment. 4. FIG. 4 is a cross-sectional view taken along line IV-IV of the secondary battery shown in FIG. FIG. 4 is a partially cutaway perspective view illustrating a secondary battery according to another embodiment of the present invention; FIG. 7 is an enlarged cross-sectional view of a portion B of the secondary battery shown in FIG. 6 . FIG. 1 is a perspective view illustrating an example of a battery pack according to an embodiment. FIG. 1 is a perspective view illustrating an example of a battery pack according to an embodiment. FIG. 13 is an exploded perspective view illustrating a battery pack according to another embodiment of the present invention. FIG. 11 is a block diagram showing an example of an electrical circuit of the battery pack shown in FIG. 10 . FIG. 1 is a partially see-through view illustrating an example of a vehicle according to an embodiment. FIG. 1 is a block diagram showing an example of a system including a stationary power supply according to an embodiment.

In general, the potential window of an aqueous electrolyte is narrower than that of a non-aqueous electrolyte. Therefore, in an aqueous electrolyte battery, depending on the combination of the positive and negative electrodes, water in the aqueous electrolyte may be electrolyzed during the initial charge. Therefore, the separator used in an aqueous electrolyte battery is required to prevent water from coming into contact with the electrodes, that is, to be dense enough to provide high water blocking properties.

In addition, in secondary batteries that use lithium metal or zinc metal for the electrodes, and in secondary batteries that use electrolytes containing lithium ions or zinc ions, there is a risk that deposits such as lithium dendrites or zinc dendrites may form on the electrodes during charging and discharging. If these dendrites break through the separator, an internal short circuit may occur. Separators are required to be dense in order to make them less likely to be broken through by these dendrites.

An example of a separator with particularly high density is a solid electrolyte membrane. A solid electrolyte membrane is a membrane made only of inorganic solid particles with ion conductivity. A solid electrolyte membrane is completely waterproof because it does not allow the passage of solvents and allows only specific ions to pass through selectively. However, a solid electrolyte membrane has low flexibility and is therefore not sufficiently durable. In addition, in order to use a solid electrolyte membrane as a separator, a certain thickness is required, making it difficult to increase the energy density of the battery.

To solve this problem, a polymer composite membrane has been proposed in which inorganic solid particles are bonded together with a polymer material. Although polymer composite membranes do not exhibit the same complete water impermeability as solid electrolyte membranes, they are highly dense and can be impregnated with minute amounts of aqueous electrolyte. Furthermore, polymer composite membranes are more flexible than solid electrolyte membranes and can be made thinner.

However, when such a composite electrolyte membrane is used as a separator, there is a concern that the surface may crack due to swelling caused by the aqueous electrolyte, making it easier for the aqueous electrolyte to come into contact with the electrodes.

The following describes the embodiments with reference to the drawings as appropriate. Note that common components throughout the embodiments are given the same reference numerals, and duplicate explanations may be omitted. Each figure is a schematic diagram to facilitate the explanation and understanding of the embodiments, and the shapes, dimensions, ratios, etc. may differ from those of the actual device, but these can be appropriately modified in design by taking into account the following explanation and known techniques. Also, unless otherwise specified, values obtained by pH and other measurements are values measured at atmospheric pressure and 25°C.

[First embodiment]
According to a first embodiment, an electrode structure comprises a negative electrode having a negative electrode active material-containing layer, and a separator having at least three layers, namely, a first composite layer, a porous layer, and a second composite layer, in this order, the negative electrode active material-containing layer and the first composite layer are in contact with each other, the first composite layer and the second composite layer contain solid particles and a polymer material, and when the interface between the negative electrode active material-containing layer and the first composite layer is measured by interface cutting evaluation, the relationship between a force FH applied in the horizontal direction and a force FV applied in the vertical direction satisfies 1.35≦FH/FV≦2.2.

Figure 1 is a cross-sectional view that shows a schematic example of an electrode structure according to an embodiment. The electrode structure 1 shown in Figure 1 includes a separator 4 and a negative electrode 3. The separator 4 includes a porous layer 4a, a first composite layer 4b1, and a second composite layer 4b2. The porous layer 4a is sandwiched between the first composite layer 4b1 and the second composite layer 4b2. Therefore, at least three layers, the first composite layer, the porous layer, and the second composite layer, are included in this order. The first composite layer and the second composite layer may be collectively referred to as a composite layer.

In FIG. 1, the porous layer 4a is present over the entire separator 4 from one end to the other end, but it is sufficient that the porous layer 4a is present up to 90% of the way from the center of the separator 4 toward the end. The composite layer 4b contains solid particles and a polymeric material. The composite layer is preferably composed of inorganic solid particles and a polymeric material. By using a separator in which a porous layer is present between composite layers, as in the electrode structure according to this embodiment, cracks that occur in the separator consisting of only the composite layer can be reduced when the electrode structure including the separator consisting of only the composite layer is impregnated with an aqueous electrolyte. This is because the porous layer can act as a buffer material. Since the cracks can be reduced in this way, the thickness of the separator can be reduced while maintaining the strength of the composite layer, and the energy density of the battery can also be improved.

In the case of an aqueous electrolyte battery, it is preferable to use different aqueous electrolytes for the positive and negative electrodes in order to prevent a decrease in the efficiency of the battery while suppressing self-discharge. If a crack occurs in the separator, the aqueous electrolytes of the positive and negative electrodes pass through the separator and come into direct contact with each other, resulting in mixing. Therefore, in the separator that can suppress the occurrence of cracks provided in the electrode structure according to this embodiment, the porous layer can act as a buffer material, so that the aqueous electrolyte on the positive electrode side and the aqueous electrolyte on the negative electrode side can be prevented from mixing. Therefore, self-discharge can be improved while preventing a decrease in the efficiency of the battery.

In FIG. 1, the separator has a three-layer structure with one porous layer and two composite layers, but the structure is not limited to this. For example, FIG. 2 is a schematic diagram of a modified separator provided in the electrode structure. As shown in FIG. 2, it can also have a five-layer structure with two porous layers and three composite layers. In other words, when there are n porous layers (n≧1, n is an integer), the composite layer has n+1 layers, and the porous layers and composite layers are alternately stacked, and the composite layer is disposed in the layer in contact with the active material-containing layer. The more porous layers and composite layers there are, the higher the cell resistance becomes, so a three-layer structure with one porous layer and two composite layers is preferable.

The negative electrode 3 includes a negative electrode current collector 3a and a negative electrode active material-containing layer 3b. In FIG. 1, the negative electrode current collector is laminated on the negative electrode active material layer, but the negative electrode current collector may be provided inside the negative electrode active material layer. In that case, it is preferable to use a mesh-shaped negative electrode current collector as the negative electrode current collector.

The negative electrode active material-containing layer and the first composite layer of the electrode structure according to this embodiment are in contact with each other. When the interface between the negative electrode active material-containing layer and the first composite layer is measured by an interface cutting evaluation, the relationship between the horizontal force FH and the vertical force FV is FH/FV≧1.35. This relationship results in a strong bond between the negative electrode active material-containing layer and the composite layer. The bond means that the negative electrode active material-containing layer and the first composite layer are bonded to each other by their respective binders. The negative electrode active material-containing layer hardly swells even when exposed to an aqueous electrolyte. Therefore, even if the composite layer swells due to the aqueous electrolyte due to the bond between the negative electrode active material-containing layer and the composite layer, the negative electrode active material-containing layer can suppress the swelling of the composite layer, and therefore the occurrence of cracks due to the swelling of the composite layer can be suppressed.

In addition, if liquid pools form at the interface between the negative electrode active material-containing layer and the composite layer, water decomposition occurs in the area where they come into contact with the negative electrode surface, which causes a decrease in self-discharge performance. By strongly bonding the negative electrode active material-containing layer and the composite layer as in the present application, liquid pools can be prevented from forming at the interface, and self-discharge performance can be improved.

The relationship between the horizontal force FH and the vertical force FV when the interface between the negative electrode and the composite layer of the electrode structure is measured by interface cutting evaluation is preferably 1.35≦FH/FV≦2.2. This range allows the negative electrode active material-containing layer and the composite layer to be more firmly bonded, so that the occurrence of cracks and breakage due to swelling in the composite layer can be further suppressed. More preferably, 1.4≦FH/FV≦2.2, and even more preferably, 1.5≦FH/FV≦2.2.

When the interface between the negative electrode and the composite layer of the electrode structure is measured by interface cutting evaluation, the relationship between the horizontal force FH and the vertical force FV is preferably FH/FV≦2.2. If 2.2<FH/FV, the negative electrode and the composite layer are too strongly bonded, which inhibits ion conduction at the interface and becomes a resistive component, which is undesirable.

In this embodiment, the electrode structure includes a negative electrode and a separator. In the electrode structure according to this embodiment, the thickness of the separator can be reduced due to the strong bond between the negative electrode and the separator, and therefore the energy density can be improved. In addition, when the types of aqueous electrolytes on the positive electrode side and the negative electrode side are changed, excessive mixing of the aqueous electrolyte on the positive electrode side and the aqueous electrolyte on the negative electrode side can be suppressed. In addition, since the negative electrode and the separator are strongly bonded, the presence of liquid pools between the separator and the negative electrode can be suppressed, and therefore a secondary battery with a reduced self-discharge rate can be realized.

Here we explain the specific configuration of the electrode structure according to this embodiment.

(Negative electrode)
The negative electrode included in the electrode structure includes a negative electrode active material-containing layer and a negative electrode current collector. The negative electrode active material-containing layer includes a negative electrode active material. The current collector is sometimes called a negative electrode current collector. One surface of the negative electrode active material-containing layer is in contact with the composite layer.

The negative electrode active material-containing layer contains a negative electrode active material (electrode active material) and, optionally, a conductive agent and a binder. The negative electrode active material-containing layer desirably contains, as an active material, a negative electrode active material containing a compound having a lithium ion insertion-extraction potential of 1 V to 3 V (vs. Li/Li ⁺ ) relative to the oxidation-reduction potential of lithium.

In an aqueous electrolyte battery having a negative electrode containing a compound having a lithium ion insertion-extraction potential in the above range in the negative electrode active material, water contained in the solvent of the aqueous electrolyte may be electrolyzed inside and near the negative electrode during the initial charge. This is because the potential of the negative electrode decreases due to the insertion of lithium ions into the negative electrode active material during the initial charge. When the negative electrode potential decreases below the hydrogen generation potential, a portion of the water is decomposed into hydrogen (H ₂ ) and hydroxide ions (OH ⁻ ) inside and near the negative electrode. This increases the pH of the aqueous electrolyte present inside and near the negative electrode.

The hydrogen generation potential at the negative electrode depends on the pH of the aqueous electrolyte. That is, when the pH of the aqueous electrolyte in contact with the negative electrode increases, the hydrogen generation potential at the negative electrode decreases. In a battery using a negative electrode active material with a lower limit of the lithium ion insertion-extraction potential of 1 V or more (vs. Li/Li ⁺ ), the potential of the negative electrode is lower than the hydrogen generation potential during the initial charge, but after the initial charge, the potential of the negative electrode tends to become higher than the hydrogen generation potential, making it difficult for water to decompose at the negative electrode.

Therefore, in a battery using the electrode structure as a composite member of the negative electrode and the separator, the pH of the aqueous electrolyte present in and near the negative electrode can be maintained at a high level after the initial charge. Therefore, by using a compound having a lower limit of the lithium ion insertion-extraction potential of 1 V or more (vs. Li/Li ⁺ ) as the negative electrode active material contained in the active material-containing layer of the electrode structure, a secondary battery with high capacity and excellent stability can be realized.

Examples of compounds having a lithium ion insertion-extraction potential of 1 V or more and 3 V or less (vs. Li/Li ⁺ ) based on the oxidation-reduction potential of lithium include titanium oxides and titanium-containing oxides. Examples of titanium-containing oxides include lithium titanium composite oxides, niobium titanium composite oxides, and sodium niobium titanium composite oxides. The negative electrode active material may contain one or more of titanium oxides and titanium-containing oxides.

Titanium oxide includes, for example, titanium oxide with a monoclinic structure, titanium oxide with a rutile structure, and titanium oxide with an anatase structure. The composition of titanium oxide with each crystal structure before charging can be expressed _{as TiO2} , and the composition after charging can be expressed as _LiyTiO2 (subscript y is 0≦y≦1). The structure of titanium oxide with a monoclinic structure before charging can be expressed as _TiO2 (B).

Examples of the lithium titanium oxide include lithium titanium oxides having a spinel structure (e.g., a compound represented by the general formula Li4 _{+ jTi5O12 ,} where -1≦j≦3), lithium titanium oxides having a ramsdellite structure (e.g., a compound represented by Li2 _{+ jTi3O7 ,} where -1≦j≦3), a compound represented by _{Li1+ yTi2O4 ,} where 0≦y≦1, a compound represented by _{Li1.1+ yTi1.8O4} , where 0≦y≦1, a _compound represented by _{Li1.07+ yTi1.86O4} , where 0≦y≦1, a compound represented by _LikTiO2 , where 0<k≦1, etc. The _{lithium titanium} oxide may also be a lithium titanium composite oxide having a different element introduced therein.

The niobium titanium composite oxide includes, for example, a compound represented by Li _χ TiMe _α Nb _2±β O _7±σ, where 0≦χ≦5, 0≦α≦0.3, 0≦β≦0.3, and 0≦σ≦0.3, and Me is one or more selected from the group consisting of Fe, V, Mo, and Ta.

The sodium niobium titanium composite oxide is, for example, represented by the general formula Li _2+d Na _2-e Me1 _f Ti _6-g-h Nb _g Me2 _h O _14+δ , where 0≦d≦4, 0≦e<2, 0≦f<2, 0<g<6, 0≦h<3, -0.5≦δ≦0.5, Me1 includes one or more selected from Cs, K, Sr, Ba, and Ca, and Me2 includes one or more selected from Zr, Sn, V, Ta, Mo, W, Fe, Co, Mn, and Al. It includes an orthorhombic type Na-containing niobium titanium composite oxide.

The negative electrode of the electrode structure preferably uses titanium oxide with anatase structure, titanium oxide with monoclinic structure, lithium titanium oxide with spinel structure, niobium titanium composite oxide, or a mixture of these as the negative electrode active material. On the one hand, when titanium oxide with anatase structure, titanium oxide with monoclinic structure, or lithium titanium oxide with spinel structure is used as the negative electrode active material, the electrode structure can obtain a high electromotive force by combining it with a positive electrode using lithium manganese composite oxide as the positive electrode active material, for example. On the other hand, the use of niobium titanium composite oxide can exhibit high capacity.

The negative electrode active material may be contained in the active material-containing layer in the form of particles, for example. The negative electrode active material particles may be primary particles, secondary particles which are aggregates of primary particles, or a mixture of a single primary particle and a secondary particle. The shape of the particles is not particularly limited, and may be, for example, spherical, elliptical, flat, fibrous, etc.

The secondary particles of the negative electrode active material can be obtained, for example, by the following method. First, the raw materials of the negative electrode active material are reacted and synthesized to produce a negative electrode active material precursor with an average particle size of 1 μm or less. The negative electrode active material precursor is then subjected to a firing process and a pulverization process using a pulverizer such as a ball mill or a jet mill. Next, in the firing process, the negative electrode active material precursor is aggregated and grown into secondary particles with a large particle size.

The average particle size (diameter) of the secondary particles of the negative electrode active material is preferably 3 μm or more, and more preferably 5 μm or more and 20 μm or less. In this range, the surface area of the negative electrode active material is small, so water decomposition can be further suppressed.

It is desirable that the average particle diameter of the primary particles of the negative electrode active material is 1 μm or less. This shortens the diffusion distance of Li ions inside the negative electrode active material and increases the specific surface area. Therefore, excellent high input performance (fast charging) can be obtained. On the other hand, if the average particle diameter of the primary particles of the negative electrode active material is small, the particles are more likely to aggregate. If the particles of the negative electrode active material aggregate, the aqueous electrolyte in the secondary battery is more likely to be unevenly distributed at the electrode, which may lead to a depletion of ion species at the positive electrode. Therefore, it is preferable that the average particle diameter of the primary particles of the negative electrode active material is 0.001 μm or more. It is more preferable that the average particle diameter of the primary particles of the negative electrode active material is 0.1 μm or more and 0.8 μm or less.

The primary particle size and secondary particle size refer to the particle size at which the volume cumulative value is 50% in the particle size distribution determined by a laser diffraction particle size distribution measuring device. For example, a Shimadzu SALD-300 is used as a laser diffraction particle size distribution measuring device. The light intensity distribution is measured 64 times at 2 second intervals. The sample used for this particle size distribution measurement is a dispersion diluted with N-methyl-2-pyrrolidone so that the concentration of active material particles is 0.1% to 1% by mass. Alternatively, the measurement sample is one in which 0.1 g of active material is dispersed in 1 ml to 2 ml of distilled water containing a surfactant.

The negative electrode active material has a specific surface area, as measured by the BET method using nitrogen (N ₂ ) adsorption, in the range of, for example, 3 m ² /g to 200 m ² /g. When the specific surface area of the negative electrode active material is within this range, the affinity between the negative electrode and the aqueous electrolyte can be further increased. This specific surface area can be determined, for example, by a method similar to the method for measuring the specific surface area of the negative electrode active material-containing layer described later.

It is desirable for the porosity of the negative electrode active material-containing layer to be 20% or more and 50% or less. This makes it possible to obtain a high-density negative electrode that has excellent affinity with the aqueous electrolyte. It is more preferable for the porosity of the negative electrode active material-containing layer to be 25% or more and 40% or less.

The porosity of the negative electrode active material-containing layer can be obtained, for example, by mercury intrusion porosimetry. Specifically, first, the pore distribution of the negative electrode active material-containing layer is obtained by mercury intrusion porosimetry. The total pore volume is calculated from this pore distribution. The porosity can be calculated from the ratio of the total pore volume to the volume of the negative electrode active material-containing layer.

The specific surface area of the negative electrode active material-containing layer measured by the BET method using nitrogen (N ₂ ) adsorption is more preferably 3 m ² /g or more and 50 m ² /g or less. If the specific surface area of the negative electrode active material-containing layer is less than 3 m ² /g, the affinity between the negative electrode active material and the aqueous electrolyte is reduced. As a result, the interface resistance of the negative electrode increases, and the output performance and charge/discharge cycle performance of the secondary battery may be reduced. On the other hand, if the specific surface area of the negative electrode active material-containing layer exceeds 50 m ² /g, the distribution of ion species ionized from the electrolyte salt contained in the aqueous electrolyte is biased toward the negative electrode, which leads to a shortage of the ion species at the positive electrode, and the output performance and charge/discharge cycle performance may be reduced.

This specific surface area can be determined, for example, by the following method. When the electrode structure including the negative electrode active material-containing layer to be measured is incorporated in a secondary battery, the secondary battery is disassembled and a part of the negative electrode active material-containing layer is taken. In nitrogen gas at 77K (the boiling point of nitrogen), the nitrogen gas pressure P (mmHg) is gradually increased while measuring the nitrogen gas adsorption amount (mL/g) of the sample for each pressure P. The pressure P (mmHg) is divided by the saturated vapor pressure P0 (mmHg) of nitrogen gas to obtain a relative pressure P/P0, and the nitrogen gas adsorption amount for each relative pressure P/P0 is plotted to obtain an adsorption isotherm. A BET plot is calculated from this nitrogen adsorption isotherm and the BET formula, and the specific surface area is obtained using this BET plot. Note that the BET multipoint method is used to calculate the BET plot.

The conductive agent is mixed as necessary to improve current collection performance and reduce contact resistance between the negative electrode active material and the negative electrode current collector. Examples of conductive agents include carbonaceous materials such as acetylene black, ketjen black, graphite, and coke. The conductive agent may be one type, or two or more types may be mixed and used.

The binder has the function of binding the negative electrode active material and the conductive agent. The binder may be one type, or two or more types may be mixed and used. Examples of other binders include, but are not limited to, at least one selected from the group consisting of cellulose-based polymers such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), fluorine-based rubber, styrene butadiene rubber, acrylic resin or its copolymer, polyacrylic acid, and polyacrylonitrile. The polymer materials described as the polymer materials used in the composite layer can also be used as the binder for the negative electrode active material-containing layer.

The compounding ratios of the negative electrode active material, conductive agent, and binder in the negative electrode active material-containing layer are preferably in the ranges of 70% by mass or more and 95% by mass or less, 3% by mass or more and 20% by mass or less, and 2% by mass or more and 10% by mass or less, respectively. When the compounding ratio of the conductive agent is 3% by mass or more, the conductivity of the negative electrode active material-containing layer can be improved, and when it is 20% by mass or less, the decomposition of the aqueous electrolyte on the conductive agent surface can be reduced. When the compounding ratio of the binder is 2% by mass or more, sufficient electrode strength can be obtained, and when it is 10% by mass or less, the insulating part of the electrode can be reduced.

The material of the negative electrode current collector is a substance that is electrochemically stable in the electrode potential range when the alkali metal ions are inserted or removed. The negative electrode current collector is, for example, zinc foil, aluminum foil, or an aluminum alloy foil containing one or more selected from magnesium (Mg), titanium (Ti), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), and silicon (Si), and is preferably surface-coated with a metal with a high hydrogen overvoltage such as Zn or Sn. The thickness of the negative electrode current collector is preferably 5 μm or more and 20 μm or less. A negative electrode current collector having such a thickness can balance the strength and weight of the electrode.

The negative electrode collector may be in the form of a plate or a mesh structure. A mesh structure refers to, for example, a structure in which metal wires are woven, or a metal plate is punched to form multiple through holes. Therefore, the negative electrode collector has spaces between conductive parts. The negative electrode active material can enter the spaces between the conductive parts. The conductive parts refer to the metal wires in the case of a negative electrode collector made of metal wires, and the part excluding the through holes in the case of a negative electrode collector made of metal plate. The negative electrode collector may have a two-dimensional structure or a three-dimensional structure, i.e., a nonwoven fabric structure.

The negative electrode current collector may have at least a portion that is not in contact with the negative electrode active material-containing layer. This portion may function as a current collecting tab. Alternatively, a current collecting tab separate from the negative electrode current collector may be electrically connected to the electrode.

(Separator)
The separator comprises at least three layers, a first composite layer, a porous layer, and a second composite layer, in this order. The porous layer is sandwiched between the first composite layer and the second composite layer. The end of the porous layer may be covered by the composite layer. Also, when the separator is viewed from the stacking direction of the composite layer and the porous layer, the porous layer may have a larger area than the composite layer so as to protrude from the end of the composite layer. The first composite layer and the second composite layer may be the same size or different sizes.

The porosity of the first composite layer and the second composite layer is smaller than the porosity of the negative electrode active material-containing layer. The porosity can be measured from an image obtained by observing a cross section with a scanning electron microscope (SEM).

The softening point temperature T1 of the polymer material Pc contained in the first composite layer is lower than the softening point temperature T2 of the polymer material Pn used as a binder contained in the negative electrode active material-containing layer (T2>T1). By making T1 and T2 T2>T1, it is possible to prevent the binder from covering the negative electrode active material. If the negative electrode active material is covered with the binder, the electronic conductivity decreases, which is undesirable because it reduces the charge/discharge efficiency. Furthermore, by making T2>T1, the polymer material Pc contained in the first composite layer can be bound at a temperature equal to or lower than the softening point temperature of the polymer material Pn. Therefore, it is possible to prevent the polymer material Pn from softening, and by preventing the polymer material Pn from covering the negative electrode active material, ion conduction is not hindered, so that the resistance can be reduced. In addition, by selecting a temperature of T1 or more and T2 or less in the heat press process when preparing the composite layer, the first composite layer can be formed to be dense and flat, and the negative electrode active material layer can be formed to form a structure in which the polymer material coverage on the active material surface is suppressed and the resistance increase is suppressed. The softening point can be changed even when using the same polymer material if the molecular weight is different.

The greater the difference in softening point temperature, the more the polymer material can be prevented from covering the surface of the negative electrode active material and the more dense the first composite layer can be. Therefore, in order to achieve a greater difference in softening point temperature, it is preferable that the polymer material Pc contained in the composite layer is different from the polymer material Pn used as a binder contained in the negative electrode active material-containing layer.

The composite layer includes solid particles and a polymeric material. The solid particles are organic solid particles or inorganic solid particles. The organic solid particles and the inorganic solid particles may be used in combination. The composite layer is preferably made of inorganic solid particles and a polymeric material.
Examples of inorganic solid particles contained in the composite layer include oxide-based ceramics such as alumina, silica, zirconia, yttria, magnesium oxide, calcium oxide, barium oxide, strontium oxide, and vanadium oxide, carbonates and sulfates such as sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, and barium sulfate, phosphates such as hydroxyapatite, lithium phosphate, zirconium phosphate, and titanium phosphate, and nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride. The inorganic particles listed above may be in the form of a hydrate.

The inorganic solid particles preferably include inorganic solid particles having ionic conductivity for alkali metal ions. Specifically, inorganic solid particles having ionic conductivity for lithium ions and sodium ions are more preferable.

Examples of inorganic solid particles having lithium ion conductivity include oxide-based solid electrolytes and sulfide-based solid electrolytes. As the oxide-based solid electrolyte, it is preferable to use a lithium phosphate solid electrolyte having a NASICON (Sodium (Na) Super Ionic Conductor) type structure and represented by the general formula Li _1+x M ₂ (PO ₄ ) _3. In the general formula, M is, for example, one or more selected from the group consisting of titanium (Ti), germanium (Ge), strontium (Sr), zirconium (Zr), tin (Sn), aluminum (Al), and calcium (Ca). The subscript x is within the range of 0≦x≦2. The ionic conductivity of the lithium phosphate solid electrolyte represented by the general formula LiM ₂ (PO ₄ ) ₃ is, for example, 1×10 ⁻⁵ S/cm or more and 1×10 ⁻³ S/cm or less.

Specific examples of lithium phosphate solid electrolytes having a NASICON structure include an LATP compound represented by Li1 _{+ wAlwTi2 -w} ( _PO4 ) ₃ , where 0.1≦w≦0.5; a compound represented by _{Li1+ yAlzM12 -z} ( _PO4 ) ₃ , where M1 is one or more selected from the group consisting of Ti, Ge, Sr, Zr, Sn, and Ca, and where 0≦y≦1 and 0≦z≦1; a compound represented by Li1 _{+ xAlxGe2 -x} ( _PO4 ) ₃ , where 0≦x≦2; and a compound represented by Li1 _{+ xAlxZr2 -x} ( _PO4 ) ₃ , where 0≦x≦2; Li1 _{+u+ vAluMα2 - uSivP3 -vO 12} , where Mα is 1 or more selected from the group consisting of Ti and Ge, and 0<u≦2, 0≦v<3; and a compound represented by Li1 _{+2tZr1 -tCat ( PO4} ) ₃ , where 0≦t<1. _{Li1+2tZr1 -tCat ( PO4} ) ₃ is preferably used as the inorganic solid particles because it has high water resistance, low reducing properties, and low cost.

Examples of oxide-based solid electrolytes include, in addition to the lithium phosphate solid electrolyte, amorphous LIPON compounds represented by Li _p PO _q N _r , where 2.6≦p≦3.5, 1.9≦q≦3.8, and 0.1≦r≦1.3 (e.g., Li _2.9 PO _3.3 N _0.46 ); compounds having a garnet structure represented by La _{5+s As} La _3-s Mβ ₂ O ₁₂ , where A is 1 or more selected from the group consisting of Ca, Sr, and Ba, Mβ is 1 or more selected from the group consisting of Nb and Ta, and 0≦s≦0.5; compounds represented by Li ₃ Mγ _2-s L _{2 O 12} , where Mγ is 1 or more selected from the group consisting of Ta and Nb, and L may contain Zr, _where 0≦s≦ _0.5 ; _{3O12 ,} where 0≦s≦0.5; and LLZ compounds ₍ e.g. _{, Li7La3Zr2O12} ), where M2 is 1 or more selected from the group consisting of Nb and Ta, and ₀ ≦ _x ≦ ₂ . The solid electrolyte may be one type, or two or more types may be mixed and used. The ionic conductivity of LIPON is, for example, ₁ × ^10-6 S/cm or more _{and 5} × ^10-6 S/ _cm or less. The ionic conductivity of LLZ is, for example, 1× ^10-4 S/cm or more and 5× ^10-4 S/cm or less.

As the inorganic solid particles having ionic conductivity of sodium ions, a sodium-containing solid electrolyte may be used. The sodium-containing solid electrolyte has excellent ionic conductivity of sodium ions. Examples of the sodium-containing solid electrolyte include β-alumina, sodium phosphorus sulfide, and sodium phosphorus oxide. The sodium ion-containing solid electrolyte is preferably in the form of glass ceramics.

The inorganic solid particles are preferably a solid electrolyte having a lithium ion conductivity of 1×10 ⁻⁵ S/cm or more at 25° C. The lithium ion conductivity can be measured, for example, by an AC impedance method, as will be described in detail later.

The shape of the inorganic solid particles is not particularly limited, but may be, for example, spherical, elliptical, flat, or fibrous.

The average particle size of the inorganic solid particles is preferably 15 μm or less, and more preferably 12 μm or less. If the average particle size of the inorganic solid particles is small, the density of the composite layer can be increased.

The average particle size of the inorganic solid particles is preferably 0.01 μm or more, and more preferably 0.1 μm or more. If the average particle size of the inorganic solid particles is large, aggregation of the particles tends to be suppressed.

The average particle size of inorganic solid particles means the particle size at which the volumetric cumulative value is 50% in the particle size distribution determined by a laser diffraction particle size distribution measuring device. The sample used for this particle size distribution measurement is a dispersion diluted with ethanol so that the concentration of inorganic solid particles is 0.01% by mass to 5% by mass.

The inorganic solid particles contained in the first and second composite layers may be the same or different. The inorganic solid particles contained in the composite layers may be a single type or a mixture of multiple types.

In the composite layer, inorganic solid particles are preferably the main component. From the viewpoint of increasing the ionic conductivity of the composite layer, the proportion of inorganic solid particles in the composite layer is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more. From the viewpoint of increasing the film strength of the composite layer, the proportion of inorganic solid particles in the composite layer is preferably 98 mass% or less, more preferably 95 mass% or less, and even more preferably 90 mass% or less. The proportion of inorganic solid particles in the composite layer can be calculated by thermogravimetric analysis (TG) analysis.

It is preferable to use organic solid particles that do not swell in an aqueous electrolyte, and it is more preferable to use organic solid particles that do not swell in an aqueous electrolyte and that have ionic conductivity for lithium ions and sodium ions.
The average particle size and particle shape of the organic solid particles, and the proportion of the organic solid particles in the composite layer are preferably the same as those of the inorganic solid particles described above.

The polymeric material contained in the first composite layer and the second composite layer enhances the adhesion between the solid particles. The weight average molecular weight of the polymeric material is, for example, 3000 or more. When the weight average molecular weight of the polymeric material is 3000 or more, the adhesion between the solid particles can be further enhanced. The weight average molecular weight of the polymeric material is preferably 3000 or more and 5,000,000 or less, more preferably 5000 or more and 2,000,000 or less, and even more preferably 10,000 or more and 1,000,000 or less. The weight average molecular weight of the polymeric material can be determined by gel permeation chromatography (GPC).

The polymeric material may be a polymer made of a single monomer unit, a copolymer made of multiple monomer units, or a mixture thereof. The polymeric material preferably contains a monomer unit made of a hydrocarbon having a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F). In the polymeric material, the proportion of the portion made of the monomer unit is preferably 70 mol % or more. Hereinafter, this monomer unit is referred to as a first monomer unit. In addition, in the copolymer, the unit other than the first monomer unit is referred to as a second monomer unit. The copolymer of the first monomer unit and the second monomer unit may be an alternating copolymer, a random copolymer, or a block copolymer.

If the proportion of the portion composed of the first monomer unit in the polymer material is lower than 70 mol %, the water-proofing properties of the first and second composite layers may be reduced. In the polymer material, the proportion of the portion composed of the first monomer unit is preferably 90 mol % or more. It is most preferable that the polymer material is a polymer in which the proportion of the portion composed of the first monomer unit is 100 mol %, that is, composed only of the first monomer unit.

The first monomer unit may be a compound having a functional group in its side chain containing one or more elements selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), and a main chain formed by carbon-carbon bonds. The hydrocarbon may have one or more functional groups containing one or more elements selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F). The functional group in the first monomer unit increases the conductivity of alkali metal ions passing through the composite layer.

The hydrocarbon constituting the first monomer unit preferably has a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), and nitrogen (N). When the first monomer unit has such a functional group, the conductivity of the alkali metal ions in the composite layer tends to be higher and the internal resistance tends to be lower.

The functional group contained in the first monomer unit preferably contains one or more selected from the group consisting of a formal group, a butyral group, a carboxymethyl ester group, an acetyl group, a carbonyl group, a hydroxyl group, and a fluoro group. Moreover, the first monomer unit more preferably contains at least one of a carbonyl group and a hydroxyl group in the functional group, and even more preferably contains both.

The first monomer unit can be represented by the following formula:

In the above formula, R1 is preferably selected from the group consisting of hydrogen (H), an alkyl group, and an amino group, and R2 is preferably selected from the group consisting of a hydroxyl group (-OH), -OR1, -COOR1, -OCOR1, -OCH(R1)O-, -CN, -N(R1) ₃ , and -SO ₂ R1.

The first monomer unit may be, for example, one or more selected from the group consisting of vinyl formal, vinyl alcohol, vinyl acetate, vinyl acetal, vinyl butyral, acrylic acid and its derivatives, methacrylic acid and its derivatives, acrylonitrile, acrylamide and its derivatives, styrene sulfonic acid, polyvinylidene fluoride, and tetrafluoroethylene.

The polymer material preferably includes one or more selected from the group consisting of polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, polyvinylidene fluoride, and polytetrafluoroethylene.

Below is an example of the structural formula of a compound that can be used as a polymer material.

The structural formula of polyvinyl formal is as follows: In the following formula, it is preferable that a is 50 or more and 80 or less, b is 0 or more and 5 or less, and c is 15 or more and 50 or less.

The structural formula of polyvinyl butyral is as follows: In the following formula, it is preferable that l is 50 or more and 80 or less, m is 0 or more and 10 or less, and n is 10 or more and 50 or less.

The structural formula of polyvinyl alcohol is as follows: In the following formula, n is preferably 70 or more and 20,000 or less.

The structural formula of polymethyl methacrylate is as follows: In the following formula, n is preferably 30 or more and 10,000 or less.

The second monomer unit is a compound other than the first monomer unit, i.e., a compound that does not have a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N), and fluorine (F), or a compound that has such a functional group but is not a hydrocarbon. Examples of the second monomer unit include ethylene oxide and styrene. Examples of the polymer composed of the second monomer unit include polyethylene oxide (PEO) and polystyrene (PS).

The types of functional groups contained in the first monomer unit and the second monomer unit can be identified by infrared spectroscopy (Fourier Transform Infrared Spectroscopy; FT-IR). Furthermore, it can be determined by nuclear magnetic resonance (NMR) that the first monomer unit is composed of a hydrocarbon. Furthermore, the proportion of the portion composed of the first monomer unit in a copolymer of the first monomer unit and the second monomer unit can be calculated by NMR.

The polymer material may contain an electrolyte. The proportion of electrolytes that may be contained in a polymer material can be determined from its water absorption rate. Here, the water absorption rate of a polymer material is the value obtained by subtracting the mass Mp of the polymer material before immersion in water at a temperature of 23°C for 24 hours from the mass Mp' of the polymer material after immersion in water at a temperature of 23°C for 24 hours, and dividing the result by the mass Mp of the polymer material before immersion ([Mp'-Mp]/Mp x 100). The water absorption rate of a polymer material is thought to be related to the polarity of the polymer material.

When a polymeric material with high water absorption is used, the alkali metal ion conductivity of the composite layer tends to increase. In addition, when a polymeric material with high water absorption is used, the binding strength between the solid particles and the polymeric material is increased, so the flexibility of the composite layer can be increased. The water absorption of the polymeric material is preferably 0.01% or more, more preferably 0.5% or more, and even more preferably 2% or more.

The strength of the composite layer can be increased by using a polymer material with low water absorption. That is, if the water absorption rate of the polymer material is too high, the composite layer may swell due to the electrolyte. Also, if the water absorption rate of the polymer material is too high, the polymer material in the composite layer may flow into the electrolyte. The water absorption rate of the polymer material is preferably 15% or less, more preferably 10% or less, even more preferably 7% or less, and particularly preferably 3% or less.

From the viewpoint of increasing the flexibility of the composite layer, the proportion of the polymer material in the composite layer is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 10% by mass or more. Furthermore, the higher the proportion of the polymer material, the more likely it is that the denser the composite layer will be.

From the viewpoint of increasing the lithium ion conductivity of the composite layer, the proportion of the polymer material in the composite layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. The proportion of the polymer material in the composite layer can be calculated by thermogravimetry (TG) analysis.

The polymeric materials contained in the first composite layer and the second composite layer may be the same or different. The polymeric materials may be a single type or a mixture of multiple types.

The composite layer may contain a plasticizer and an electrolyte salt in addition to the solid particles and polymer material. For example, if the composite layer contains an electrolyte salt, the alkali metal ion conductivity of the separator can be further increased.

From the viewpoint of preventing internal short circuits, the thickness of the composite layer is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 7 μm or more. From the viewpoint of increasing ion conductivity and energy density, the thickness of the composite layer is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less.

The thickness of the first composite layer and the second composite layer may be the same or different. When the thicknesses are different, it is preferable that the first composite layer in contact with the first surface of the negative electrode active material-containing layer is thicker than the other second composite layer. By making the first composite layer thicker, the water blocking property can be improved. In addition, the first composite layer and the second composite layer may have the same configuration or different configurations.
The above thicknesses are preferred for each of the first and second composite layers, but not necessarily for both composite layers.

The porous layer carries on either side a first composite layer and a second composite layer.
The porous layer is, for example, a nonwoven fabric or a self-supporting porous membrane. For example, polyethylene (PE), polypropylene (PP), cellulose, polyethylene terephthalate (PET) or polyvinylidene fluoride (PVdF) is used as the material of the nonwoven fabric or the self-supporting porous membrane. The porous layer is preferably a cellulose nonwoven fabric.

The porous layer can contain many pores and can be impregnated with a large amount of aqueous electrolyte. The porous layer typically does not contain solid particles. For example, the proportion of the area of the cross section of the porous layer that is occupied by solid particles can be 5% or less.

The thickness of the porous layer is, for example, 1 μm or more, and preferably 3 μm or more. If the porous layer is thick, the mechanical strength of the separator increases, and the secondary battery is less likely to have an internal short circuit. The thickness of the porous layer is, for example, 30 μm or less, and preferably 10 μm or less. If the porous layer is thin, the internal resistance of the secondary battery decreases, and the volumetric energy density of the secondary battery tends to increase. The thickness of the porous layer can be measured, for example, by a scanning electron microscope.

As described above, the separator may contain an aqueous electrolyte in the composite layer or the porous layer. The electrode active material-containing layer may further contain an aqueous electrolyte. That is, the electrode structure may contain a first aqueous electrolyte in the negative electrode active material-containing layer and a second aqueous electrolyte in the separator. When used as a secondary battery, the electrode structure may further contain a third aqueous electrolyte in the positive electrode active material-containing layer. The first aqueous electrolyte, the second aqueous electrolyte, and the third aqueous electrolyte may have the same composition, or may have different compositions. Hereinafter, unless otherwise specified, the first aqueous electrolyte, the second aqueous electrolyte, and the third aqueous electrolyte are collectively referred to simply as "aqueous electrolytes". The aqueous electrolyte will be described later.

The separator provided in the electrode structure according to this embodiment has a porous layer between the first composite layer and the second composite layer. Therefore, when the aqueous electrolytes on the positive electrode side and the negative electrode side are different, the porous layer acts as a buffer to prevent the aqueous electrolyte on the positive electrode side from mixing excessively with the aqueous electrolyte on the negative electrode side. Preventing this excessive mixing can suppress self-discharge.

<Production Method>
Next, a method for manufacturing an electrode structure according to an embodiment will be described. In summary, the method for manufacturing an electrode structure includes preparing an anode active material layer having an anode active material-containing layer on a surface of an anode current collector, or an anode active material-containing layer having at least a part of an anode current collector inside the anode active material-containing layer, applying a composite layer-forming slurry to both sides of a porous film to form a composite layer, and laminating the anode active material-containing layer and a separator to obtain a joint of the anode and the separator.

The method for forming the negative electrode active material-containing layer on the surface of the negative electrode current collector is not particularly limited. For example, it can be obtained by the following method. First, the negative electrode active material, the conductive agent, and the binder are suspended in an appropriate solvent to prepare a slurry. Next, this slurry is applied to one or both sides of the negative electrode current collector. The coating film on the negative electrode current collector is dried to form the negative electrode active material-containing layer. Thereafter, the negative electrode current collector and the negative electrode active material-containing layer formed thereon are pressed. The negative electrode active material-containing layer may be a mixture of the negative electrode active material, the conductive agent, and the binder formed into a pellet shape.

There is no particular restriction on the method for forming the negative electrode active material-containing layer in which at least a portion of the negative electrode current collector is provided inside the negative electrode active material-containing layer. For example, the process of immersing the negative electrode current collector in a negative electrode active material mixture slurry prepared by suspending an active material, a conductive agent, and a binder in an appropriate solvent, dip-coating, and drying is repeated multiple times. Furthermore, a method of applying the same slurry again to the above coating until the desired basis weight is achieved, and pressing the resulting coating film to obtain the negative electrode active material-containing layer can be used. In this way, the negative electrode current collector can be embedded in the negative electrode active material-containing layer.

The separator including the composite layer is prepared, for example, as follows.
First, a slurry for forming a first composite layer on a first surface of a porous membrane that is to be a base layer is prepared by mixing solid particles, a polymer material, and a solvent, and stirring the mixture.

It is preferable to use a solvent capable of dissolving polymeric materials. Examples of the solvent include alcohols such as ethanol, methanol, isopropyl alcohol, normal propyl alcohol, and benzyl alcohol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and diacetone alcohol; esters such as ethyl acetate, methyl acetate, butyl acetate, ethyl lactate, methyl lactate, and butyl lactate; ethers such as methyl cellosolve, ethyl cellosolve, butyl cellosolve, 1,4-dioxane, and tetrahydrofuran; glycols such as ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, butyl carbitol acetate, and ethyl carbitol acetate; Glycol ethers such as ethyl carbitol, ethyl carbitol, and butyl carbitol; aprotic polar solvents such as dimethylformamide, dimethylacetamide, acetonitrile, valeronitrile, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, and γ-butyrolactam; cyclic carboxylic acid esters such as gamma butyrolactone, gamma valerolactone, gamma caprolactone, and epsilon caprolactone; and chain carbonate compounds such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methyl carbonate, and methyl-n-propyl carbonate are used.

Next, the composite layer forming slurry is applied to one side of a porous substrate layer such as a cellulose-based nonwoven fabric by, for example, a doctor blade method to obtain a coating film. This coating film is dried at a temperature of 50°C or higher and 150°C or lower.

Next, a slurry for forming a second composite layer on the other side of the porous substrate layer is prepared. The slurry for forming the second composite layer is obtained in the same manner as the slurry for forming the first composite layer. As the slurry for forming the second composite layer, the slurry for forming the first composite layer may be used, or a slurry using other solid particles or polymeric materials may be used. The slurry for forming the second composite layer is applied to the other side of the porous substrate layer, for example, by a doctor blade method, to obtain a coating film. This coating film is dried at a temperature of 50°C or higher and 150°C or lower. In this way, a laminate is obtained in which the dried coating film is provided on both sides of the porous layer. This laminate is a separator precursor.

This laminate may be subjected to roll pressing. In the roll pressing, a press device equipped with two rollers, one above the other, is used so that the coating film on both sides of the laminate is simultaneously roll pressed. A heated roller may also be used. In this case, the heating temperature of the roller that comes into contact with the coating film on one side is set to a temperature within ±20°C of the softening point of the polymeric material in the coating film. By performing roll pressing on the coating film at a temperature close to the softening point of the polymeric material contained in the first composite layer, the polymeric material in the coating film is heated and softened. In addition, by applying pressure to the coating film, the softened polymeric material may be arranged to fill voids in the coating film.

The heating temperature of the roller in contact with the coating film on the other side can be changed as appropriate to suit the desired structure. For example, when the coating film on the other side is roll-pressed at room temperature of 25°C, the polymer material does not soften, and a second composite layer with uniform density along the thickness direction of the coating film is obtained. Furthermore, when roll-pressed at a temperature near the softening point of the polymer material, a second composite layer with high coating density, similar to the first composite layer, is obtained.

The heating temperature of the roller is preferably lower than the melting point of the polymeric material contained in the first composite layer and the second composite layer. If the heating temperature is increased above the melting point of the polymeric material, the polymeric material on the surface side of the coating film may melt and the voids may be completely lost. This is because if the voids are completely lost, the ionic conductivity of the composite layer decreases. If the ionic conductivity of the composite layer decreases, the ionic conductivity of the separator also decreases.

The softening point and melting point of polymeric materials may vary depending on the molecular weight and the monomer unit ratio. For example, the softening point of PVdF is 135°C to 145°C, and the melting point is 170°C to 180°C. The softening point of polyvinyl formal is 120°C to 130°C, and the melting point is 190°C to 200°C. The softening point of polyvinyl butyral is 120°C to 130°C, and the melting point is 190°C to 200°C.

In this manner, a separator having a porous layer, a first composite layer, and a second composite layer is obtained. Here, a method of simultaneously performing roll press processing on the coating films provided on both sides of the porous layer has been described, but the roll press processing may be performed on each side at a time.

The composite layer may also be formed by applying two or more types of slurry to form a multi-layer structure.

In this way, a separator is obtained in which a composite layer is provided on both sides of a porous layer. The laminate is then pressed, for example, at room temperature (25°C) using a press device equipped with two rollers, one above the other, to obtain a first composite layer having different densities on the surface side and the inside. The heating temperature of the roller on the first composite layer side may be set to near the softening point of the polymer material.

The negative electrode active material-containing layer and the separator obtained as described above are laminated to obtain a negative electrode and separator bonded body. The lamination is performed, for example, by a heated roll press process. For the roll press process, for example, a press device equipped with two rollers, one above the other, is used. The heating temperature at this time is preferably set between the softening point temperature T1 of the polymer material Pc contained in the composite layer and the softening point temperature T2 of the polymer material Pn used as a binder contained in the negative electrode active material-containing layer. In the above, an example in which a press process was performed in the preparation of both layers was described, but the negative electrode active material-containing layer and the separator before the press process may be laminated and then pressed at the same time.

In this manner, a bonded assembly of a separator and a negative electrode bonded to the separator, that is, the electrode structure according to this embodiment, can be obtained.

After impregnating this electrode structure with an aqueous electrolyte, a positive electrode active material mixture slurry is applied to the positive electrode current collector on the composite layer side of the electrode structure that is not joined to the negative electrode active material-containing layer, and a press process is performed to stack the prepared positive electrode and press it to produce an electrode group. The electrode group formed by stacking the electrode structure and the positive electrode may be a stack type electrode group produced by pressing the stacked electrode group, or a wound type electrode group produced by winding and pressing the electrode group.

<Measurement method>
Various measurement methods will be described below. When the electrode structure to be measured is incorporated in a secondary battery, the secondary battery is discharged, and then the battery is disassembled to remove the electrode structure.

(Method of measuring horizontal force FH and vertical force FV when measuring the interface between the negative electrode active material containing layer and the first composite layer by interface cutting evaluation)
The relationship between the horizontal force FH and the vertical force FV when the interface between the negative electrode active material-containing layer and the first composite layer in the electrode structure is measured by the interface cutting evaluation can be measured by the surface/interface cutting method. Before the measurement, both sides of the electrode structure are washed with pure water, and then immersed in pure water and left for 48 hours or more. After that, both sides are further washed with pure water and dried in a vacuum drying oven at 100 ° C. for 48 hours or more to prepare a measurement sample of the electrode structure.

The peel strength can be measured by the surface and interface cutting method using a cutting strength measuring device such as the SAICAS (Surface and Interface Cutting Analysis System; SAICAS) (registered trademark). The surface and interface cutting method is also sometimes called the SAICAS method. For example, a DN-GS manufactured by Daipla Wintes Co., Ltd. can be used as the measuring device. For example, a ceramic blade made of borazon material with a blade width of 1.0 mm is used as the cutting blade. For example, the measurement conditions are a blade angle of 20 degrees and a relief angle of 10 degrees.

First, the separator is cut in the vertical direction with a pressing load of 1N (constant load mode). Here, the blade is moved to a predetermined depth in the separator by cutting at a constant horizontal speed of 2 μm/sec and vertical speed of 0.2 μm/sec at a shear angle of 45°. When the horizontal load (horizontal force) on the blade decreases due to the peeling of the first composite layer from the negative electrode active material-containing layer, the vertical load is controlled to 0.5 N to keep the blade position constant in the vertical direction. Then, the horizontal force (horizontal load) is measured at a horizontal speed of 2 μm/sec. After the horizontal force associated with the peeling becomes constant, the measurement is continued over a 1 mm long area, and the horizontal force FH and vertical force FV in the peel strength measured in this length area are measured. Note that both the measurement temperature and the sample temperature are room temperature (25°C). The porous layer and the second composite layer are not peeled off, and the measurement is performed by cutting from the surface of the second composite layer to the interface between the negative electrode active material-containing layer and the first composite layer.

The FH and FV values measured from the electrode structure using the surface/interface cutting method are the same as the FH and FV values measured from the electrode structure after the positive electrode is peeled off from the electrode group, which is a laminate of the positive electrode and electrode structure.

(Method of measuring porosity of electrode structure)
In measuring the porosity of the electrode structure, the electrode group is separated from other components of the battery. The electrode group is rinsed on both sides with pure water, and then immersed in pure water and left for 48 hours or more. Then, ultrasonic vibration is further applied to peel off the positive electrode and remove the electrode structure. The electrode structure is rinsed on both sides with pure water, dried in a vacuum drying oven at 100°C for 48 hours or more, and then the porosity is evaluated. In addition, the porosity is measured at any multiple points (for example, 5 points) in the electrode structure, and the average is taken. This operation is performed on the separator composite layer and the negative electrode active material-containing layer, respectively.

The porosity of the composite layer is measured by considering the first composite layer and the second composite layer together as one composite layer. For example, when measuring at five locations, the results of measuring three locations on the first composite layer and two locations on the second composite layer are used. Measurements may be made at five locations on the first composite layer, or at five locations on the second composite layer. This is because the porosity of the first composite layer and the second composite layer of the separator of the electrode structure according to this embodiment is the same, so the measurement results of only one of them may be used, or both may be used.

The porosity is determined by observing a cross-section polished by ion milling under a SEM at 5,000x magnification, binarizing the obtained image into pore regions and material regions, determining the area _SA of pore region A, and dividing the area SA of pore region _A by 100 times the area S of the observation region ( _SA /(area S of observation region × 100)).
This binarization process can be carried out using image processing software, and the void regions and filled region can be separated.

The electrode structure according to the first embodiment comprises a negative electrode having a negative electrode active material-containing layer, and a separator having at least three layers, a first composite layer, a porous layer, and a second composite layer, in this order, in which the negative electrode active material-containing layer and the first composite layer are in contact with each other, the first composite layer and the second composite layer contain solid particles and a polymeric material, and the relationship between the horizontal force FH and the vertical force FV when the interface between the negative electrode active material-containing layer and the first composite layer is measured by interface cutting evaluation is 1.35≦FH/FV≦2.2. With this configuration, a secondary battery that exhibits high charge/discharge efficiency and suppresses self-discharge can be realized.

Second Embodiment
In the second embodiment, an electrode group 50 including the electrode structure according to the first embodiment and a positive electrode is provided.

Figure 3 is a cross-sectional view that shows a schematic example of an electrode group according to the second embodiment. As shown in Figure 3, the electrode group 50 includes an electrode structure 1 and a positive electrode 5. The electrode structure 1 is the electrode structure according to the first embodiment. The positive electrode includes a positive electrode active material-containing layer 5a and a positive electrode current collector 5b. The first composite layer is in contact with the negative electrode active material-containing layer, and the second composite layer is in contact with the positive electrode active material-containing layer.

(Positive electrode)
The positive electrode may include a current collector and a positive electrode active material-containing layer provided on at least one surface of the current collector. The positive electrode active material-containing layer includes a positive electrode active material, and optionally a conductive agent and a binder.

The positive electrode current collector includes metals such as stainless steel, aluminum (Al) and titanium (Ti). The positive electrode current collector has, for example, a foil, a porous body or a mesh shape. In order to prevent corrosion due to a reaction between the positive electrode current collector and the aqueous electrolyte, the surface of the positive electrode current collector may be coated with a different element. The positive electrode current collector is preferably one having excellent corrosion resistance and oxidation resistance, such as Ti foil. In addition, when Li ₂ SO ₄ is used as the aqueous electrolyte, Al may be used as the positive electrode current collector because corrosion does not progress.

The positive electrode current collector may also include a portion on its surface that does not have a positive electrode active material-containing layer. This portion can function as a positive electrode current collecting tab.

The positive electrode active material-containing layer contains a positive electrode active material. The positive electrode active material-containing layer may be supported on both the front and back surfaces of the positive electrode current collector.

As the positive electrode active material, a compound having a lithium ion insertion-extraction potential of 2.5 V to 5.5 V (vs. Li/Li ⁺ ) relative to the oxidation-reduction potential of lithium can be used. The positive electrode may contain one type of compound alone as the positive electrode active material, or may contain two or more compounds as the positive electrode active material.

Examples of compounds that can be used as the positive electrode active material include lithium manganese composite oxide, lithium nickel composite oxide, lithium cobalt aluminum composite oxide, lithium nickel cobalt manganese composite oxide, spinel-type lithium manganese nickel composite oxide, lithium manganese cobalt composite oxide, lithium iron oxide, lithium fluorinated iron sulfate, and phosphate compounds having an olivine crystal structure (for example, a compound represented by Li _k FePO ₄ where 0<k≦1, a compound represented by Li _k MnPO ₄ where 0<k≦1), etc. Phosphate compounds having an olivine crystal structure have excellent thermal stability.

Examples of compounds that can obtain a high positive electrode potential include lithium manganese composite oxides such as a compound represented by Li _k Mn ₂ O ₄ having a spinel structure and having 0<k≦1, and a compound represented by Li _k MnO ₂ and having 0<k≦1; lithium nickel aluminum composite oxides such as a compound represented by Li _k Ni _1-i Al _i O ₂ and having 0<k≦1, 0<i<1; lithium cobalt composite oxides such as a compound represented by Li _k CoO ₂ and having 0<k≦1; lithium nickel cobalt composite oxides such as a compound represented by Li _k Ni _1-i-t Co _i Mn _t O ₂ and having 0<k≦1, 0<i<1, 0≦t<1; lithium manganese cobalt composite oxides such as a compound represented by Li _k Mn _i Co _1-i O ₂ and having 0<k≦ ₁ , 0<i<1; Examples of such _lithium phosphate oxides include spinel-type lithium manganese nickel composite oxides such as a compound represented by Mn _1-i Ni _i O ₄ where 0<k≦1, 0<i<1; lithium phosphate oxides having an olivine structure such as a compound represented by Li k FePO ₄ where 0<k≦1, a compound represented by Li _k Fe _1-y Mn _y PO ₄ where 0<k≦1, 0<y≦1, and a compound represented by Li _k CoPO ₄ where 0<k≦1; and fluorinated iron sulfates (for example, a compound represented by Li _k FeSO ₄ F where 0<k≦1).

The positive electrode active material preferably contains one or more selected from the group consisting of lithium cobalt composite oxide, lithium manganese composite oxide, and lithium phosphate having an olivine structure. The working potential of these compounds is 3.5 V (vs. Li/Li ⁺ ) to 4.2 V (vs. Li/Li ⁺ ). That is, the working potential of these compounds as active materials is relatively high. By using these compounds in combination with the above-mentioned negative electrode active materials such as spinel-type lithium titanate and anatase-type titanium oxide, a high battery voltage can be obtained.

The positive electrode active material is contained in the positive electrode in the form of particles, for example. The positive electrode active material particles can be single primary particles, secondary particles which are aggregates of primary particles, or a mixture of primary particles and secondary particles. The shape of the particles is not particularly limited, and can be, for example, spherical, elliptical, flat, or fibrous.

The average particle size (diameter) of the primary particles of the positive electrode active material is preferably 10 μm or less, more preferably 0.1 μm or more and 5 μm or less. The average particle size (diameter) of the secondary particles of the positive electrode active material is preferably 100 μm or less, more preferably 10 μm or more and 50 μm or less. The primary particle size and secondary particle size of the positive electrode active material can be measured in the same manner as for the negative electrode active material particles.

The positive electrode active material-containing layer may contain a conductive agent and a binder in addition to the positive electrode active material. The conductive agent is mixed as necessary to improve current collection performance and reduce contact resistance between the positive electrode active material and the current collector. Examples of conductive agents include carbonaceous materials such as acetylene black, ketjen black, graphite, and coke. The conductive agent may be one type, or two or more types may be mixed together.

Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber, polypropylene (PP), polyethylene (PE), carboxymethyl cellulose (CMC), polyimide (PI), polyacrylimide (PAI), and the like. One type of binder may be used, or two or more types may be mixed and used. The polymer materials described as the polymer materials used in the composite layer may also be used as binders in the positive electrode active material-containing layer.

The compounding ratios of the positive electrode active material, conductive agent, and binder in the positive electrode active material-containing layer are preferably 70% by mass or more and 95% by mass or less, 3% by mass or more and 20% by mass or less, and 2% by mass or more and 10% by mass or less, respectively. When the compounding ratio of the conductive agent is 3% by mass or more, the conductivity of the positive electrode can be improved, and when it is 20% by mass or less, the decomposition of the aqueous electrolyte on the conductive agent surface can be reduced. When the compounding ratio of the binder is 2% by mass or more, sufficient electrode strength can be obtained, and when it is 10% by mass or less, the insulating part of the electrode can be reduced.

The positive electrode can be obtained, for example, by the following method. First, a positive electrode active material, a conductive agent, and a binder are suspended in an appropriate solvent to prepare a slurry. This slurry is applied to one or both sides of a positive electrode current collector. The coating on the positive electrode current collector is dried to form a positive electrode active material-containing layer. Thereafter, the positive electrode current collector and the positive electrode active material-containing layer formed thereon are pressed. As the positive electrode active material-containing layer, a mixture of the positive electrode active material, the conductive agent, and the binder formed into a pellet shape may be used.

The interfacial strength σn between the composite layer and the negative electrode is greater than the interfacial strength σp between the composite layer and the positive electrode (σn>σp). The interfacial strengths σn and σp can be measured using the surface/interface cutting method described above.

The electrode group according to the second embodiment is equipped with the electrode structure according to the first embodiment, so that a secondary battery equipped with this electrode group also exhibits high charge/discharge efficiency and can suppress self-discharge.

[Third embodiment]
According to a third embodiment, there is provided a secondary battery including the electrode group according to the second embodiment and an aqueous electrolyte. The aqueous electrolyte includes water.

The secondary battery according to the third embodiment may further include a container that contains the electrode group and the aqueous electrolyte.

Furthermore, the secondary battery according to the third embodiment may further include a negative electrode terminal electrically connected to the negative electrode and a positive electrode terminal electrically connected to the positive electrode.

The secondary battery according to the third embodiment may be, for example, a lithium ion secondary battery. The secondary battery also includes an aqueous electrolyte secondary battery that contains an aqueous electrolyte.

The aqueous electrolyte may be held in the electrode group. The aqueous electrolyte may include a first aqueous electrolyte held in the electrode (negative electrode) in the electrode group and a second aqueous electrolyte held in the separator, and may further include a third aqueous electrolyte held in the positive electrode.

The first aqueous electrolyte, the second aqueous electrolyte, and the third aqueous electrolyte may have the same composition, or may have different compositions. Hereinafter, unless otherwise specified, the first aqueous electrolyte, the second aqueous electrolyte, and the third aqueous electrolyte will be collectively referred to simply as "aqueous electrolytes."

(Aqueous electrolyte)
The aqueous electrolyte contains an aqueous solvent and an electrolyte salt. The aqueous electrolyte is, for example, liquid. The liquid aqueous electrolyte is prepared by dissolving an electrolyte salt as a solute in an aqueous solvent.

As the electrolyte salt, for example, a lithium salt, a sodium salt, or a mixture thereof is used. One type or two or more types of electrolyte salts can be used.

Examples of lithium salts that can be used include lithium chloride (LiCl), lithium bromide (LiBr), lithium hydroxide ( _{LiOH), lithium sulfate (Li2SO4 ) ,} lithium nitrate ( _LiNO3 ), lithium acetate ( _CH3COOLi ), lithium oxalate ( _Li2C2O4 ) _{, lithium carbonate (Li2CO3 )} , lithium bis( _{trifluoromethanesulfonyl} )imide (LiTPSI; LiN( _SO2CF3 ) ₂ ), lithium bis( _{fluorosulfonyl} )imide (LiPSI; LiN( _SO2F ) ₂ ), and lithium bisoxalate borate (LiBOB; LiB[(OCO) ₂ ] ₂ ).

As the sodium salt, sodium chloride (NaCl), sodium sulfate (Na ₂ SO ₄ ), sodium hydroxide (NaOH), sodium nitrate (NaNO ₃ ), sodium trifluoromethanesulfonylamide (NaTPSA), or the like is used.

The lithium salt preferably contains LiCl. The use of LiCl can increase the lithium ion concentration of the aqueous electrolyte. The lithium salt preferably contains at least one of LiSO ₄ and LiOH in addition to LiCl.

In addition to the lithium salt, a zinc salt such as zinc chloride or zinc sulfate may be added to the electrolyte. By adding such a compound to the electrolyte, a zinc-containing coating layer and/or an oxidized zinc-containing region can be formed on the electrode in a battery using the electrode contained in the electrode structure as the negative electrode. These zinc-containing members are effective in suppressing hydrogen generation in the electrodes on which they are formed.

The molar concentration of lithium ions in the aqueous electrolyte is preferably 3 mol/L or more, more preferably 6 mol/L or more, and even more preferably 12 mol/L or more. If the concentration of lithium ions in the aqueous electrolyte is high, electrolysis of the aqueous solvent in the electrode is easily suppressed, and hydrogen generation from the electrode tends to be low.

In the case of an aqueous electrolyte, it is preferable that the amount of aqueous solvent is 1 mol or more per mol of the solute salt. More preferably, the amount of aqueous solvent is 3.5 mol or more per mol of the solute salt. The aqueous solvent contains, for example, 50% or more by volume of water.

The aqueous electrolyte preferably contains, as anion species, one or more selected from the group consisting of chloride ions (Cl ⁻ ), hydroxide ions (OH ⁻ ), sulfate ions (SO ₄ ²⁻ ), and nitrate ions (NO ³⁻ ).

The pH of the aqueous electrolyte is preferably 3 or more and 14 or less, and more preferably 4 or more and 13 or less. In all embodiments, the pH is measured at 25±2°C.

After the initial charge, the pH of the aqueous electrolyte is preferably different between the negative electrode side and the positive electrode side. In the secondary battery after the initial charge, the pH of the first aqueous electrolyte on the negative electrode side is preferably 3 or more, more preferably 5 or more, and even more preferably 7 or more. In addition, in the secondary battery after the initial charge, the pH of the third aqueous electrolyte on the positive electrode side is preferably in the range of 0 to 7, and more preferably 0 to 6.

The pH of the aqueous electrolyte on the negative and positive electrode sides can be obtained, for example, by dismantling the secondary battery and measuring the pH of the aqueous electrolyte present between the separator and the negative and positive electrodes, respectively.

As the aqueous solvent, a solution containing water can be used. The aqueous solution may be pure water or a mixed solvent of water and an organic solvent.

The aqueous electrolyte may be a gel electrolyte. The gel electrolyte is prepared by mixing the above-mentioned liquid aqueous electrolyte with a polymer compound to form a composite. Examples of the polymer compound include polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), and polyethylene oxide (PEO).

The presence of water in an aqueous electrolyte can be confirmed by GC-MS (Gas Chromatography-Mass Spectrometry) measurements. The salt concentration and water content in an aqueous electrolyte can be measured, for example, by ICP (Inductively Coupled Plasma) emission spectrometry. The molar concentration (mol/L) can be calculated by weighing a specified amount of aqueous electrolyte and calculating the salt concentration contained therein. The number of moles of solute and solvent can also be calculated by measuring the specific gravity of the aqueous electrolyte.

(Method for measuring pH of aqueous electrolyte)
The method for measuring the pH of the aqueous electrolyte is as follows.
The secondary battery is disassembled, the electrolyte contained in the electrodes and the separator of the electrode group is extracted, the amount of the liquid is measured, and the pH value is measured with a pH meter. The pH measurement is performed, for example, as follows. For this measurement, for example, F-74 manufactured by Horiba, Ltd. is used, and the measurement is performed in an environment of 25±2°C. First, standard solutions of pH 4.0, 7.0, and 9.0 are prepared. Next, the F-74 is calibrated using these standard solutions. An appropriate amount of the electrolyte (electrolytic solution) to be measured is prepared and placed in a container, and the pH is measured. After the pH measurement, the sensor part of the F-74 is washed. When measuring another measurement target, the above-mentioned procedure, i.e., calibration, measurement, and washing, is performed each time.

<Outer layer material>
For the outer layer member in which the electrode structure and the aqueous electrolyte are housed, a metal container, a laminate film container, or a resin container can be used.

Metal containers that can be used are rectangular or cylindrical metal cans made of nickel, iron, stainless steel, etc. Resin containers that can be used are made of polyethylene or polypropylene, etc.

The thickness of each of the resin container and the metal container is preferably in the range of 0.05 mm to 1 mm. The thickness is more preferably 0.5 mm or less, and even more preferably 0.3 mm or less.

Examples of laminate films include multilayer films in which a metal layer is coated with a resin layer. Examples of metal layers include stainless steel foil, aluminum foil, and aluminum alloy foil. Polymers such as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET) can be used for the resin layer. The thickness of the laminate film is preferably in the range of 0.01 mm to 0.5 mm. The thickness of the laminate film is more preferably 0.2 mm or less.

Secondary batteries can be used in various shapes, such as rectangular, cylindrical, flat, thin, and coin-shaped. The secondary battery may also be a secondary battery with a bipolar structure. Secondary batteries with a bipolar structure have the advantage that multiple cells connected in series can be made from a single cell.

<Terminals>
The negative electrode terminal can be formed from a material that is electrochemically stable and conductive in a potential range of 1 V to 3 V with respect to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). Specifically, the material of the negative electrode terminal can be zinc, copper, nickel, stainless steel, or aluminum, or an aluminum alloy containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. As the material of the negative electrode terminal, zinc or a zinc alloy is preferably used. The negative electrode terminal is preferably made of the same material as the negative electrode current collector (for example, the current collector included in the negative electrode structure) in order to reduce the contact resistance with the negative electrode current collector.

The positive electrode terminal can be formed from a material that is electrically stable and conductive in a potential range of 2.5 V to 4.5 V with respect to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). Examples of the material for the positive electrode terminal include titanium, aluminum, and aluminum alloys containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. The positive electrode terminal is preferably formed from the same material as the positive electrode collector in order to reduce the contact resistance with the positive electrode collector.

The secondary battery according to the embodiment will be described in detail below with reference to Figs. 4 and 5. Fig. 4 is a cross-sectional view showing an example of a secondary battery according to the embodiment. Fig. 5 is a cross-sectional view taken along line IV-IV of the secondary battery shown in Fig. 4.

The electrode group is housed in an exterior member 2 made of a rectangular cylindrical metal container. The electrode group includes an electrode structure 1 and a positive electrode 5, and the electrode structure includes a negative electrode 3 and a separator 4. The electrode group 1 has a structure in which the separator 4 is arranged between the positive electrode 5 and the negative electrode 3 and wound in a spiral shape to form a flat shape. An aqueous electrolyte (not shown) is held in the electrode structure 1. As shown in FIG. 4, a strip-shaped negative electrode current collector tab 16 is electrically connected to each of multiple points on the end of the negative electrode 3 located on the end face of the electrode structure 1. In addition, a strip-shaped positive electrode current collector tab 17 is electrically connected to each of multiple points on the end of the positive electrode 5 located on this end face. The multiple negative electrode current collector tabs 16 are connected to the negative electrode terminal 6 in a bundled state as shown in FIG. 5. In addition, although not shown, the positive electrode current collector tabs 17 are also electrically connected to the positive electrode terminal 7 in a bundled state.

The metal sealing plate 10 is fixed to the opening of the metal exterior member 2 by welding or the like. The negative electrode terminal 6 and the positive electrode terminal 7 are each pulled out to the outside through an extraction hole provided in the sealing plate 10. A negative electrode gasket 8 and a positive electrode gasket 9 are arranged on the inner circumferential surface of each extraction hole of the sealing plate 10 to prevent short circuits due to contact with the negative electrode terminal 6 and the positive electrode terminal 7, respectively. By arranging the negative electrode gasket 8 and the positive electrode gasket 9, the airtightness of the secondary battery 100 can be maintained.

A control valve 11 (safety valve) is arranged on the sealing plate 10. When the internal pressure in the battery cell increases due to gas generated by electrolysis of the aqueous solvent, the generated gas can be released to the outside from the control valve 11. The control valve 11 can be, for example, a reset type that operates when the internal pressure becomes higher than a set value and functions as a sealing plug when the internal pressure decreases. Alternatively, a non-reset type control valve that does not recover its function as a sealing plug once it is activated may be used. In FIG. 4, the control valve 11 is arranged in the center of the sealing plate 10, but the control valve 11 may be located at the end of the sealing plate 10. The control valve 11 may be omitted.

The sealing plate 10 is also provided with a liquid inlet 12. The aqueous electrolyte can be injected through this liquid inlet 12. After the aqueous electrolyte is injected, the liquid inlet 12 can be closed with a sealing plug 13. The liquid inlet 12 and the sealing plug 13 may be omitted.

Figure 6 is a partially cutaway perspective view that shows a schematic diagram of another example of a secondary battery according to an embodiment. Figure 7 is an enlarged cross-sectional view of part B of the secondary battery shown in Figure 6. Figures 6 and 7 show an example of a secondary battery 100 that uses an exterior member made of a laminate film as a container.

The secondary battery 100 shown in Figs. 6 and 7 includes an electrode structure 1 shown in Figs. 6 and 7, an exterior member 2 shown in Fig. 6, and an aqueous electrolyte (not shown). The electrode structure 1 and the aqueous electrolyte are housed in the exterior member 2. The aqueous electrolyte is held in the electrode structure 1.

The exterior member 2 is made of a laminate film that includes two resin layers and a metal layer interposed between them.

As shown in FIG. 7, the electrode group is a laminated electrode group. The laminated electrode group includes a plurality of electrode structures and positive electrodes 5, and the electrode structures include negative electrodes 3 and separators 4. The electrode group has a structure in which the negative electrodes 3, separators 4, and positive electrodes 5 are laminated in this order.

Each negative electrode 3 includes a negative electrode current collector 3a and a negative electrode active material-containing layer 3b disposed on both sides of the negative electrode current collector 3a. Each separator 4 is supported on the negative electrode active material-containing layer 3b of the negative electrode 3. The electrode group also includes a plurality of positive electrodes 5. Each of the plurality of positive electrodes 5 includes a positive electrode current collector 5a and a positive electrode active material-containing layer 5b supported on both sides of the positive electrode current collector 5a. To avoid complicating the drawing, the positive electrode 5 and the composite layer 4 are drawn separated from each other, but as described above, the electrode group includes the negative electrode 3 and the positive electrode 5, each of which is in contact with the separator 4.

The negative electrode current collector 3a of each negative electrode 3 includes a portion 3c on one side where the negative electrode active material-containing layer 3b is not provided on any surface. This portion 3c serves as a negative electrode current collector tab. As shown in FIG. 7, the portion 3c serving as the negative electrode current collector tab does not overlap with the positive electrode 5. In addition, the multiple negative electrode current collector tabs (portions 3c) are electrically connected to a band-shaped negative electrode terminal 6. The tip of the band-shaped negative electrode terminal 6 is pulled out to the outside of the exterior member 2.

Although not shown, the positive electrode collector 5a of each positive electrode 5 includes a portion on one side where the positive electrode active material-containing layer 5b is not supported on any surface. This portion serves as a positive electrode current collector tab. The positive electrode current collector tab does not overlap with the negative electrode 3, as does the negative electrode current collector tab (portion 3c). The positive electrode current collector tab is located on the opposite side of the electrode structure 1 to the negative electrode current collector tab (portion 3c). The positive electrode current collector tab is electrically connected to a strip-shaped positive electrode terminal 7. The tip of the strip-shaped positive electrode terminal 7 is located on the opposite side of the negative electrode terminal 6 and is drawn out to the outside of the exterior member 2.

The secondary battery according to the third embodiment includes the electrode structure according to the first embodiment. Therefore, the secondary battery exhibits high charge/discharge efficiency, improved rate characteristics, and suppressed self-discharge.

[Fourth embodiment]
According to a fourth embodiment, there is provided a battery pack including a plurality of secondary batteries according to the third embodiment.

In the battery pack according to the embodiment, the individual cells may be electrically connected in series or parallel, or may be arranged in a combination of series and parallel connections.

Next, an example of a battery pack will be described with reference to the drawings.
Fig. 8 is a perspective view showing an example of a battery pack according to the embodiment. The battery pack 200 shown in Fig. 8 includes five cells 100a to 100e, four bus bars 21, a positive electrode lead 22, and a negative electrode lead 23. Each of the five cells 100a to 100e is a secondary battery according to the third embodiment.

The bus bar 21 connects, for example, the negative terminal 6 of one cell 100a to the positive terminal 7 of the adjacent cell 100b. In this way, the five cells 100 are connected in series by the four bus bars 21. That is, the battery pack 200 in FIG. 8 is a five-cell battery pack. Although no example is shown, in a battery pack including a plurality of cells electrically connected in parallel, for example, the plurality of negative terminals are connected to each other by a bus bar and the plurality of positive terminals are connected to each other by a bus bar, so that the plurality of cells are electrically connected.

The positive terminal 7 of at least one of the five cells 100a to 100e is electrically connected to a positive lead 22 for external connection. The negative terminal 6 of at least one of the five cells 100a to 100e is electrically connected to a negative lead 23 for external connection.

The battery pack according to the embodiment includes the secondary battery according to the embodiment. Therefore, the battery pack can exhibit high charge/discharge efficiency and improved rate characteristics, and self-discharge is suppressed.

[Fifth embodiment]
According to a fifth embodiment, there is provided a battery pack including the secondary battery according to the third embodiment. This battery pack can include the battery assembly according to the fourth embodiment. This battery pack may include a single secondary battery according to the third embodiment instead of the battery assembly according to the fourth embodiment.

Such a battery pack may further include a protection circuit. The protection circuit has a function of controlling charging and discharging of the secondary battery. Alternatively, a circuit included in a device that uses the battery pack as a power source (e.g., electronic equipment, automobiles, etc.) may be used as the protection circuit for the battery pack.

The battery pack may further include an external terminal for current flow. The external terminal for current flow is for outputting current from the secondary battery to the outside and/or for inputting current from the outside to the secondary battery. In other words, when the battery pack is used as a power source, current is supplied to the outside through the external terminal for current flow. When the battery pack is charged, charging current (including regenerative energy from the power of an automobile or the like) is supplied to the battery pack through the external terminal for current flow.

Next, an example of a battery pack according to an embodiment will be described with reference to the drawings.
FIG. 9 is a perspective view that illustrates an example of a battery pack according to the embodiment.

The battery pack 300 includes a battery assembly consisting of the secondary batteries shown in FIG. 6 and FIG. 7. The battery pack 300 includes a housing 310 and a battery assembly 200 housed in the housing 310. The battery assembly 200 is a battery assembly in which a plurality of (e.g., five) secondary batteries 100 are electrically connected in series. The secondary batteries 100 are stacked in the thickness direction. The housing 310 has openings 320 on the top and on each of the four side surfaces. The side surface from which the negative terminal 6 and the positive terminal 7 of the secondary battery 100 protrude is exposed to the opening 320 of the housing 310. The output positive terminal 332 of the battery assembly 200 is strip-shaped, with one end electrically connected to one of the positive terminals 7 of the secondary batteries 100 and the other end protruding from the opening 320 of the housing 310 and protruding from the top of the housing 310. On the other hand, the output negative electrode terminal 333 of the battery pack 200 is strip-shaped, with one end electrically connected to one of the negative electrode terminals 6 of the secondary batteries 100, and the other end protruding from the opening 320 of the housing 310 and protruding from the top of the housing 310.

Another example of such a battery pack will be described in detail with reference to Figs. 10 and 11. Fig. 10 is an exploded perspective view showing a schematic diagram of another example of a battery pack according to an embodiment. Fig. 11 is a block diagram showing an example of an electrical circuit of the battery pack shown in Fig. 10.

The battery pack 300 shown in Figures 10 and 11 includes a container 31, a lid 32, a protective sheet 33, a battery pack 200, a printed wiring board 34, wiring 35, and an insulating plate (not shown).

The storage container 31 shown in FIG. 10 is a bottomed, square container having a rectangular bottom. The storage container 31 is configured to be able to accommodate a protective sheet 33, a battery pack 200, a printed wiring board 34, and wiring 35. The lid 32 has a rectangular shape. The lid 32 covers the storage container 31 to accommodate the battery pack 200 and the like. Although not shown, the storage container 31 and the lid 32 are provided with openings or connection terminals for connection to external devices and the like.

The battery pack 200 includes a plurality of cells 100, a positive electrode lead 22, a negative electrode lead 23, and an adhesive tape 24.

At least one of the multiple single cells 100 is a secondary battery according to the embodiment. Each of the multiple single cells 100 is electrically connected in series as shown in FIG. 11. The multiple single cells 100 may be electrically connected in parallel, or may be connected in a combination of series and parallel connections. When the multiple single cells 100 are connected in parallel, the battery capacity is increased compared to when they are connected in series.

The adhesive tape 24 fastens the multiple cells 100 together. Instead of the adhesive tape 24, heat shrink tape may be used to secure the multiple cells 100 together. In this case, protective sheets 33 are placed on both sides of the battery pack 200, the heat shrink tape is wrapped around the battery pack 200, and the heat shrink tape is then thermally shrunk to bind the multiple cells 100 together.

One end of the positive electrode lead 22 is connected to the battery pack 200. One end of the positive electrode lead 22 is electrically connected to the positive electrode of one or more single cells 100. One end of the negative electrode lead 23 is connected to the battery pack 200. One end of the negative electrode lead 23 is electrically connected to the negative electrode of one or more single cells 100.

The printed wiring board 34 is installed along one of the short sides of the inner surface of the container 31. The printed wiring board 34 includes a positive connector 342, a negative connector 343, a thermistor 345, a protection circuit 346, wiring 342a and 343a, an external terminal 350 for supplying electricity, a positive wiring (positive wiring) 348a, and a negative wiring (negative wiring) 348b. One surface of the printed wiring board 34 faces one side of the battery pack 200. An insulating plate (not shown) is interposed between the printed wiring board 34 and the battery pack 200.

The other end 22a of the positive electrode lead 22 is electrically connected to the positive electrode connector 342. The other end 23a of the negative electrode lead 23 is electrically connected to the negative electrode connector 343.

The thermistor 345 is fixed to one side of the printed wiring board 34. The thermistor 345 detects the temperature of each of the cells 100 and transmits the detection signal to the protection circuit 346.

The external terminals 350 for electrical current flow are fixed to the other surface of the printed wiring board 34. The external terminals 350 for electrical current flow are electrically connected to a device that is external to the battery pack 300. The external terminals 350 for electrical current flow include a positive terminal 352 and a negative terminal 353.

The protection circuit 346 is fixed to the other surface of the printed wiring board 34. The protection circuit 346 is connected to the positive terminal 352 via the positive wiring 348a. The protection circuit 346 is connected to the negative terminal 353 via the negative wiring 348b. The protection circuit 346 is also electrically connected to the positive connector 342 via the wiring 342a. The protection circuit 346 is electrically connected to the negative connector 343 via the wiring 343a. Furthermore, the protection circuit 346 is electrically connected to each of the multiple single cells 100 via the wiring 35.

The protective sheet 33 is disposed on both inner surfaces of the long sides of the container 31 and on the inner surface of the short side that faces the printed wiring board 34 via the battery pack 200. The protective sheet 33 is made of, for example, resin or rubber.

The protection circuit 346 controls the charging and discharging of the multiple single cells 100. In addition, the protection circuit 346 cuts off the electrical connection between the protection circuit 346 and the external terminals 350 (positive terminal 352, negative terminal 353) for supplying electricity to an external device based on a detection signal transmitted from the thermistor 345 or a detection signal transmitted from each single cell 100 or the battery pack 200.

The detection signal transmitted from the thermistor 345 may be, for example, a signal that detects that the temperature of the cell 100 is equal to or higher than a predetermined temperature. The detection signal transmitted from each cell 100 or the battery pack 200 may be, for example, a signal that detects overcharging, overdischarging, or overcurrent of the cell 100. When detecting overcharging or the like for each cell 100, the battery voltage may be detected, or the positive electrode potential or negative electrode potential may be detected. In the latter case, a lithium electrode used as a reference electrode is inserted into each cell 100.

The protection circuit 346 may be a circuit included in a device (e.g., electronic device, automobile, etc.) that uses the battery pack 300 as a power source.

As described above, the battery pack 300 is provided with an external terminal 350 for current flow. Therefore, the battery pack 300 can output current from the assembled battery 200 to an external device and input current from the external device to the assembled battery 200 via the external terminal 350 for current flow. In other words, when the battery pack 300 is used as a power source, the current from the assembled battery 200 is supplied to the external device through the external terminal 350 for current flow. When the battery pack 300 is charged, a charging current from the external device is supplied to the battery pack 300 through the external terminal 350 for current flow. When the battery pack 300 is used as an in-vehicle battery, regenerative energy of the vehicle's power can be used as the charging current from the external device.

The battery pack 300 may include a plurality of assembled batteries 200. In this case, the assembled batteries 200 may be connected in series, in parallel, or in a combination of series and parallel connections. The printed wiring board 34 and the wiring 35 may be omitted. In this case, the positive electrode lead 22 and the negative electrode lead 23 may be used as the positive and negative terminals of the external terminals for current supply, respectively.

Such battery packs are used in applications that require excellent cycle performance, for example, when drawing a large current. Specifically, these battery packs are used, for example, as power sources for electronic devices, stationary batteries, and on-board batteries for various vehicles. Examples of electronic devices include digital cameras. These battery packs are particularly suitable for use as on-board batteries.

The battery pack according to the fifth embodiment includes the secondary battery according to the third embodiment or the battery pack according to the fourth embodiment. Therefore, the battery pack exhibits high charge/discharge efficiency, improves rate characteristics, and suppresses self-discharge.

Sixth embodiment
According to a sixth embodiment, a vehicle including the battery pack according to the fifth embodiment is provided.

In such a vehicle, the battery pack, for example, recovers regenerative energy from the vehicle's motive power. The vehicle may also include a mechanism (regenerator) that converts the kinetic energy of the vehicle into regenerative energy.

Examples of vehicles according to the embodiment include two- to four-wheel hybrid electric vehicles, two- to four-wheel electric vehicles, power-assisted bicycles, and rail vehicles.

The mounting position of the battery pack in the vehicle according to the embodiment is not particularly limited. For example, when the battery pack is mounted in an automobile, the battery pack can be mounted in the engine compartment, the rear of the vehicle body, or under the seat of the vehicle.

The vehicle according to the embodiment may be equipped with a plurality of battery packs. In this case, the batteries included in each battery pack may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections. For example, when each battery pack includes a battery pack, the battery packs may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections. Alternatively, when each battery pack includes a single battery, the batteries may be electrically connected in series, electrically connected in parallel, or electrically connected by a combination of series and parallel connections.

Next, an example of a vehicle according to an embodiment will be described with reference to the drawings.
FIG. 12 is a partially see-through view that illustrates an example of a vehicle according to an embodiment.
A vehicle 400 shown in Fig. 12 includes a vehicle body 40 and a battery pack 300 according to the fifth embodiment. In the example shown in Fig. 12, the vehicle 400 is a four-wheeled automobile.

The vehicle 400 may be equipped with multiple battery packs 300. In this case, the batteries (e.g., single cells or assembled batteries) included in the battery packs 300 may be connected in series, in parallel, or in a combination of series and parallel connections.

Figure 12 illustrates an example in which the battery pack 300 is mounted in an engine compartment located in front of the vehicle body 40. As described above, the battery pack 300 may be mounted, for example, at the rear of the vehicle body 40 or under a seat. This battery pack 300 can be used as a power source for the vehicle 400. In addition, this battery pack 300 can recover regenerative energy for the power of the vehicle 400.

The vehicle according to the sixth embodiment is equipped with the battery pack according to the fifth embodiment. Therefore, the vehicle has excellent driving performance and reliability.

[Seventh embodiment]
According to a seventh embodiment, there is provided a stationary power source including the battery pack according to the fifth embodiment.
The stationary power source may be equipped with the assembled battery according to the fourth embodiment or the secondary battery according to the third embodiment, instead of the battery pack according to the fifth embodiment. The stationary power source according to the embodiments can achieve a long life.

FIG. 13 is a block diagram showing an example of a system including a stationary power source according to the embodiment. FIG. 13 is a diagram showing an example of application to stationary power sources 112 and 123 as an example of use of battery packs 300A and 300B according to the embodiment. In the example shown in FIG. 13, a system 110 in which stationary power sources 112 and 123 are used is shown. The system 110 includes a power plant 111, a stationary power source 112, a consumer-side power system 113, and an energy management system (EMS) 115. In addition, a power grid 116 and a communication network 117 are formed in the system 110, and the power plant 111, the stationary power source 112, the consumer-side power system 113, and the EMS 115 are connected via the power grid 116 and the communication network 117. The EMS 115 uses the power grid 116 and the communication network 117 to perform control to stabilize the entire system 110.

The power plant 111 generates a large amount of electricity using fuel sources such as thermal power and nuclear power. Electricity is supplied from the power plant 111 through the power grid 116, etc. The stationary power source 112 is equipped with a battery pack 300A. The battery pack 300A can store the electricity supplied from the power plant 111. The stationary power source 112 can supply the electricity stored in the battery pack 300A through the power grid 116, etc. The system 110 is provided with a power conversion device 118. The power conversion device 118 includes a converter, an inverter, a transformer, etc. Therefore, the power conversion device 118 can convert between direct current and alternating current, convert between alternating currents with different frequencies, and perform voltage transformation (boosting and bucking), etc. Therefore, the power conversion device 118 can convert the electricity from the power plant 111 into electricity that can be stored in the battery pack 300A.

The consumer-side power system 113 includes a power system for factories, a power system for buildings, and a power system for homes. The consumer-side power system 113 includes a consumer-side EMS 121, a power conversion device 122, and a stationary power source 123. The stationary power source 123 is equipped with a battery pack 300B. The consumer-side EMS 121 performs control to stabilize the consumer-side power system 113.

The consumer-side power system 113 is supplied with power from the power plant 111 and from the battery pack 300A through the power grid 116. The battery pack 300B can store the power supplied to the consumer-side power system 113. Similarly to the power conversion device 118, the power conversion device 122 includes a converter, an inverter, a transformer, and the like. Therefore, the power conversion device 122 can convert between DC and AC, convert between ACs with different frequencies, and perform voltage transformation (boost and step-down), etc. Therefore, the power conversion device 122 can convert the power supplied to the consumer-side power system 113 into power that can be stored in the battery pack 300B.

The power stored in the battery pack 300B can be used, for example, to charge a vehicle such as an electric car. The system 110 may also be provided with a natural energy source. In this case, the natural energy source generates power using natural energy such as wind power and solar power. Power is supplied from the natural energy source in addition to the power plant 111 through the power grid 116.

[Example]
Example 1
<Manufacture of electrode structure>
First, the negative electrode was prepared. The negative electrode active material, the conductive agent, and the binder were dispersed in an N-methyl-2-pyrrolidone (NMP) solvent to prepare a negative electrode active material-containing slurry. The proportions of the conductive agent and the binder in the negative electrode active material-containing layer were 5 parts by weight and 1 part by weight, respectively, relative to 100 parts by weight of the negative electrode active material. Lithium titanium oxide (Li ₄ Ti ₅ O ₁₂ ) powder was used as the negative electrode active material. Graphite powder was used as the conductive agent. Polyvinylidene fluoride (PVdF) resin was used as the binder.

The prepared slurry was then applied to both sides of the negative electrode current collector, and the coating was dried to form a negative electrode active material-containing layer. A 50 μm thick Zn foil was used as the negative electrode current collector. When applying the slurry to the Zn foil, the negative electrode active material-containing slurry was applied only to one side of the Zn foil for the portion of the negative electrode to be prepared that was located in the outermost layer of the electrode group, and the negative electrode active material-containing slurry was applied to both sides of the Zn foil for the other portions. At this time, the drying temperature of the negative electrode active material-containing slurry was 120° C., and the slurry application speed was 1 m/min. Next, the negative electrode current collector and the negative electrode active material-containing layer were pressed at a press temperature of 25° C. and a press pressure of 5 kN to prepare a negative electrode. In Table 1, the pressing temperature and pressing pressure of the negative electrode current collector and the negative electrode active material-containing layer are described as the composite coating film pressing temperature and the composite coating film pressing pressure.

Next, the solid particles and the polymeric material were mixed with NMP to obtain _a slurry for forming the composite layer. The _inorganic solid particles LATP ( _{Li1.5Al0.5Ti1.5} ( _PO4 ) ₃ ) were used as the solid particles, and polyvinyl butyral was used as the polymeric material. The softening point of the polyvinyl butyral was 120°C. In the composite layer slurry, the mass ratio of the inorganic solid particles to the polymeric material was 88:12. This composite layer slurry was applied to one side of the cellulose nonwoven fabric by the doctor blade method at a coating speed of 0.5 m/min, and the resulting coating was dried at a temperature of 120°C. The other side of the cellulose nonwoven fabric was also coated under the same conditions to obtain a separator precursor in which a composite layer was provided on both sides of the cellulose nonwoven fabric, that is, a first composite layer and a second composite layer were provided.

Next, this separator precursor was subjected to roll pressing. For the roll pressing, a pressing device equipped with two rollers, one above the other, was used. The separator was obtained by pressing with the heating temperature of the rollers set to 120°C and the pressing pressure of the rollers set to 10 kN. The prepared negative electrode and separator were then pressed together again with rollers heated to 120°C and a pressing pressure of 10 kN to obtain an electrode structure with a separator provided on the surface of the negative electrode.

<Preparation of secondary battery>
A secondary battery was fabricated using the obtained electrode structure, a positive electrode prepared by the following method, and an aqueous electrolyte.

[Production of Positive Electrode]
A positive electrode active material, a conductive agent, a binder, and a solvent were mixed to prepare a positive electrode slurry. LiMn ₂ O ₄ was used as the positive electrode active material. Graphite powder was used as the conductive agent. Polyvinylidene fluoride (PVdF) was used as the binder. N-methyl-2-pyrrolidone (NMP) was used as the solvent. The mass ratio of the positive electrode active material, conductive agent, and binder in the slurry was 100:5:5. This positive electrode slurry was applied to both sides of a positive electrode current collector, dried, and then pressed to obtain a positive electrode. A Ti foil with a thickness of 12 μm was used as the positive electrode current collector.

[Preparation of electrode group]
The positive electrode and the electrode structure were laminated to obtain a laminate. At this time, the electrode structure was impregnated with a 12 M LiCl aqueous solution as an aqueous electrolyte, and then arranged so that the second composite layer surface was located on the positive electrode side. The laminate was spirally wound so that the negative electrode side was located on the outermost periphery, and then pressed with 5 kN to produce a flat electrode group.

[Battery Assembly]
The obtained electrode group was placed in a metal can, and an aqueous electrolyte prepared in the same manner as that impregnated in the electrode structure was poured thereinto to prepare a secondary battery.

<Performance evaluation of secondary batteries>
<Internal resistance measurement>
First, the state of charge (SOC) was adjusted to 50%. The adjusted battery was pulse-discharged at a rate of 10 C, and the discharge voltage after 10 seconds was measured. The internal resistance of the secondary battery was calculated from the voltage before discharge and the discharge voltage at 10 C.

<Charge/discharge test>
The charge and discharge efficiency was measured. Specifically, the secondary battery was first charged at a constant current of 5A in an environment of 25°C until the battery voltage reached 2.7V. This state was maintained for 30 minutes. Then, the secondary battery was discharged at a constant current of 5A until the battery voltage reached 2.1V. This state was maintained for 30 minutes. This series of operations constituted one charge and discharge cycle, which was repeated 50 times. The discharge capacity and charge capacity at the 50th cycle were measured for each secondary battery, and the charge and discharge efficiency (discharge capacity/charge capacity) was calculated using the measured values. Subsequently, the secondary battery was charged until the battery voltage reached 2.7V, and then a rest period of 24 hours was provided, after which the charge capacity was measured again, and the storage performance (charge capacity after 24 hours rest/charge capacity immediately before rest) was evaluated. The results are shown in Table 4.

In Table 4, the column labeled "Charge/Discharge Efficiency (%)" lists the discharge capacity after 50 cycles of testing, calculated as above, divided by the charge capacity. The column labeled "Discharge Capacity (mAh/g)" lists the discharge capacity at the 50th cycle of the 50-cycle test. The column labeled "Storage Performance (%)" lists the ratio of the charge capacity after 24 hours of rest to the capacity when fully charged at the 51st cycle.

The methods for measuring peel strength and porosity are described below, but these evaluations were performed after measuring the above resistance, charge/discharge efficiency, discharge capacity, and storage performance.

<Evaluation of peel strength>
The obtained electrode structure was evaluated by the surface/interface cutting method described above for the horizontal force FH and the vertical force FV when measuring the peel strength of the interface between the negative electrode active material-containing layer and the separator. Specifically, both sides of the electrode structure were washed with pure water, then immersed in pure water and left for 48 hours or more, and then further washed with pure water and ultrasonic vibration was applied to peel off the positive electrode and take out the electrode structure. Then, the electrode structure was dried in a vacuum drying oven at 100 ° C for 48 hours or more to prepare a measurement sample of the electrode structure.

Measurements were made using the surface and interface cutting method on the prepared electrode structure. The measuring device used was a DN-GS manufactured by Daipla Wintes Co., Ltd. For example, a ceramic blade made of borazon material with a blade width of 1.0 mm was used as the cutting blade, and the measurement conditions were a rake angle of 20 degrees and a relief angle of 10 degrees.

First, the composite layer was cut in the vertical direction with a pressing load of 1N (constant load mode). At this time, cutting was performed from the surface of the second composite layer. Here, cutting was performed at a constant horizontal speed of 2 μm/sec and vertical speed of 0.2 μm/sec, and a shear angle of 45°C. When the horizontal load (horizontal force) applied to the blade decreased due to the first composite layer being peeled off from the negative electrode active material-containing layer, the vertical load was controlled to be 0.5 N to keep the position of the blade constant in the vertical direction. Then, the horizontal force (horizontal load) was measured at a horizontal speed of 2 μm/sec. After the horizontal force associated with the peeling became constant, the measurement was continued over a 1 mm long area, and the average strength of the horizontal force measured in this length area was measured as the horizontal force FH and the vertical force FV in the peel strength. Both the measurement temperature and the sample temperature were room temperature (25°C). The measurement results of the electrode structure manufactured in Example 1 were that the force FH applied in the horizontal direction was 6.0 N/cm, the force FV applied in the vertical direction was 4.1 N/cm, and the relationship between the force FH applied in the horizontal direction and the force FV applied in the vertical direction was FH/FV = 1.46.

(Method of measuring porosity of electrode structure)
In measuring the porosity of the electrode structure, the electrode structure is separated from other components of the battery. The electrode group is washed on both sides with pure water, and then immersed in pure water and left for 48 hours or more. Then, ultrasonic vibration is further applied to peel off the positive electrode and remove the electrode structure. The electrode structure is washed on both sides with pure water, dried in a vacuum drying furnace at 100 ° C. for 48 hours or more, and then the surface polished by ion milling was observed at 5000 times the magnification with an SEM. The porosity was obtained by dividing the area S _A of the pore region A of the obtained SEM image by 100 times the area S of the observation region (S _A / (area S of the observation region × 100)).

This operation was performed five times at any area on the cross section of the composite layer and the cross section of the negative electrode active material layer, and the average was taken to calculate the porosity of each. The porosity of the composite layer was measured without distinguishing between the first composite layer and the second composite layer. The measurement results of the electrode structure produced in Example 1 showed that the porosity of the composite layer was 14%, and the porosity of the negative electrode mixture layer was 25%.

The composition and manufacturing conditions of the electrode structures in Example 1 described above and Examples 2-10 described below are summarized in Tables 1-3. Table 4 also summarizes the results of the surface/interface cutting method evaluation and the porosity evaluation of the electrode structures manufactured in Examples 1-10. The separator composition shows the solid particles and polymeric material used in the composite layer material, and their mass ratios. The separator manufacturing conditions show the drying temperature and coating speed of the composite layer forming slurry, as well as the roller heating temperature and pressing pressure during roll pressing.

Similarly, the separator composition, electrode structure manufacturing conditions, negative electrode composition, negative electrode fabrication conditions, peel strength, FH, FV ratio (FH/FV), porosity of each layer of the electrode structure, resistance, charge/discharge efficiency, discharge capacity, and storage performance used in Comparative Examples 1-3 described below are summarized in Tables 5-8. Items that could not be evaluated are marked with "-" under the corresponding item in the table.

Example 2
<Manufacture of electrode structure>
An electrode structure was produced in the same manner as in Example 1, except that the mass ratio of the solid particles to the polymer material in the slurry used to prepare the composite layer was changed to 97:3.

<Preparation of secondary battery>
A secondary battery was produced in the same manner as in Example 1, except that the obtained electrode structure was used instead of the electrode structure obtained in Example 1, and was evaluated.

When the same evaluation as in Example 1 was performed, the force FH applied in the horizontal direction was 5.1 N/cm, and the force FV applied in the vertical direction was 3.6 N/cm. In addition, the porosity of the composite layer was 21%, and the porosity of the negative electrode composite layer was 25%.

Example 3
<Manufacture of electrode structure>
An electrode structure was produced in the same manner as in Example 1, except that the mass ratio of the solid particles to the polymer material in the slurry used to prepare the composite layer was changed to 80:20.

<Preparation of secondary battery>
A secondary battery was produced in the same manner as in Example 1, except that the obtained electrode structure was used instead of the electrode structure obtained in Example 1, and was evaluated.

When the same evaluation as in Example 1 was performed, the force FH applied in the horizontal direction was 9.0 N/cm, and the force FV applied in the vertical direction was 4.3 N/cm. In addition, the porosity of the composite layer was 10%, and the porosity of the negative electrode composite layer was 25%.

Example 4
An electrode structure was produced in the same manner as in Example 1, except that alumina (Al ₂ O ₃ ) was used instead of LATP.

<Preparation of secondary battery>
A secondary battery was produced in the same manner as in Example 1, except that the obtained electrode structure was used instead of the electrode structure obtained in Example 1, and was evaluated.

In Example 4, the horizontal force FH was 6.2 N/cm, and the vertical force FV was 4.1 N/cm. The porosity of the composite layer was 19%, and the porosity of the negative electrode composite layer was 25%.

Examples 5-7 below show examples of using various materials for each component.

(Examples 5 to 7)
<Manufacture of electrode structure>
In Examples 5-7, electrode structures were produced in the same manner as in Example 1, except that the materials used for the separator and the negative electrode active material-containing layer were changed as shown in Tables 1-3.

<Preparation of secondary battery and performance evaluation>
A secondary battery was produced in the same manner as in Example 1, except that the obtained electrode structure was used instead of the electrode structure obtained in Example 1, and was evaluated.
The FH, FV and porosity of Examples 5-7 are summarized in Table 4.

Examples 8-10 below show examples of electrode structures fabricated under various process conditions.

<Manufacture of electrode structure>
In Example 8, the electrode structure was produced in the same manner as in Example 1, except that the separator and the negative electrode active material-containing layer were both stacked in a pre-press state and subjected to a heated roll press treatment at the same time. Therefore, the roller heating temperature, press temperature, and press pressure in the separator structure production conditions and the negative electrode active material-containing layer production conditions in Example 8 are indicated as "-" to indicate blanks. In Examples 8-10, the electrode structure was produced in the same manner as in Example 1, except that the production conditions of the separator and the negative electrode active material-containing layer were changed as shown in Tables 5-7.

<Preparation of secondary battery and performance evaluation>
A secondary battery was produced in the same manner as in Example 1, except that the obtained electrode structure was used instead of the electrode structure obtained in Example 1, and was evaluated.

The FH, FV, and porosity for Examples 8-10 are summarized in Table 8.

(Comparative Example 1)
A negative electrode and a positive electrode were prepared in the same manner as in Example 1. A separator was also prepared as follows. A 15 μm thick cellulose-based nonwoven fabric was prepared as a porous layer. Next, the solid particles and the polymeric material were mixed with N-methyl-2-pyrrolidone (NMP) to obtain a slurry for forming a composite layer. Inorganic solid particles LATP (Li1.5Al0.5Ti1.5(PO4)3) were used as the solid particles, and polyvinyl butyral was used as the polymeric material. The softening point of the polyvinyl butyral was 120° C. In the slurry, the mass ratio of the inorganic solid particles to the polymeric material was 88:12. This slurry was applied only on one side of the porous layer by a doctor blade method at a slurry application speed of 0.5 m/min, and the resulting coating was dried at a temperature of 120° C. In this manner, a separator precursor having a coating film provided on the porous layer was obtained.

Next, this separator precursor was subjected to roll press treatment. For the roll press treatment, a press device equipped with two rollers, one above and one below, was used. The heating temperature of the rollers was 120°C, and the pressing pressure of the rollers was 10 kN. In this way, a separator was obtained.

<Preparation of secondary battery>
In addition to the same negative electrode as in Example 1, two separators were prepared as described above as a first separator and a second separator, and a positive electrode and an aqueous electrolyte were prepared as in Example 1.

The positive electrode, the first separator, the negative electrode, and the second separator were stacked in this order to obtain a laminate. At this time, the first and second separators were arranged so that the composite layer was located on the negative electrode side and the porous layer was located on the positive electrode side. The laminate was wound in a spiral shape so that the negative electrode was located on the outermost periphery, and then pressed to produce a flat electrode group.

A secondary battery was produced and evaluated in the same manner as in Example 1, except that the obtained electrode group was used instead of the electrode group obtained in Example 1.

Regarding peel strength, in the electrode structure of Comparative Example 1, the horizontal force FH was 2.1 N/cm, and the vertical force FV was 2.3 N/cm. Although the surface of the composite layer and the surface of the negative electrode active material-containing layer are in contact with each other, these members are not joined, so FH and FV had equivalent values.

(Comparative Example 2)
<Production of negative electrode>
A slurry for producing an anode was prepared similar to that prepared in Example 1. This slurry was applied to both sides of a 50 μm-thick Zn foil and dried at 120° C. to provide a coating film of an anode active material-containing layer. A laminate of the Zn foil (current collector foil) and the coating film of the anode active material-containing layer was subjected to a roll press treatment to produce an anode consisting of the current collector foil and the anode active material-containing layer. The temperature of the roller was room temperature (25° C.), and the pressing pressure was 10 kN.

<Manufacture of electrode structure>
Next, a composite layer was formed on the surface of the negative electrode active material-containing layer of the negative electrode. A slurry for forming the composite layer was prepared similar to that prepared in Example 1. This slurry was used to perform coating, drying, and roll press treatment under the same conditions as those for forming the separator in Example 1, except that the slurry was directly coated on the surface of the negative electrode active material-containing layer instead of the porous layer. In other words, the separator in Comparative Example 2 did not include a porous layer, and the negative electrode active material-containing layer and the composite layer were in direct contact with each other. As a result, an electrode structure in which the current collector foil, the negative electrode active material-containing layer, and the separator were integrated was obtained.

<Preparation of secondary battery>
The obtained electrode structure was immersed in the same aqueous electrolyte as in Example 1. A secondary battery was produced and evaluated in the same manner as in Example 1, except that this electrode structure was used instead of the electrode structure obtained in Example 1.

The peel strength of the obtained electrode structure was evaluated in the same manner as in Example 1. In Comparative Example 2, the FH was 10.8 N/cm and the FV was 4.5 N/cm.

(Comparative Example 3)
<Preparation of secondary battery>
A secondary battery was produced in the same manner as in Example 1, except that the negative electrode obtained in Example 1 and a 15 μm-thick cellulose nonwoven fabric were used as the separator, and evaluation was carried out.

As shown in Tables 4 and 8, in Example 1-10, by adopting the manufacturing procedures shown in the tables, an electrode structure with 1.35≦FH/FV≦2.2 was obtained. From these results, it is judged that the adhesion on the first surface of the electrode structure was good and no twisting or the like occurred. Furthermore, in Example 1-10, a structure was obtained in which the porosity of the composite layer was smaller than that of the negative electrode composite layer.

The composition and manufacturing conditions of the electrode structures in Comparative Examples 1-3, as well as the results of evaluation using the surface and interface cutting method, are summarized in the table below.

As described above, in Comparative Example 1, the negative electrode active material-containing layer and the separator were not joined, so FH and FV were almost equal. On the other hand, in Comparative Example 2, conversely, the composite layer slurry penetrated excessively into the negative electrode composite layer during composite layer formation, resulting in a large FH value and FH/FV≧2.2.

The results of the battery performance evaluation of the secondary batteries according to Examples 1-10 and Comparative Examples 1-3 are shown in Tables 4 and 8. In Tables 4 and 8, the column labeled "Resistance (mΩ)" lists the internal resistance calculated as described above, and the column labeled "Charge/Discharge Efficiency (%)" lists the value obtained by dividing the discharge capacity after a 50-cycle test by the charge capacity. The column labeled "Discharge Capacity (mAh/g)" lists the discharge capacity at the 50th cycle in the 50-cycle test. The column labeled "Storage Performance (%)" lists the ratio of the charge capacity after a 24-hour rest to the capacity when the battery was fully charged at the 51st cycle.

As is clear from Tables 4 and 8, the storage performance of the secondary battery using the electrode structure of Example 1-10 was higher than that of the secondary battery of Comparative Example 1-3. Regarding the performance difference from Comparative Example 1, since the FH and FV at the interface between the negative electrode and the separator (the interface between the negative electrode active material-containing layer and the first composite layer) in the secondary battery of Example 1-10 are 1.35≦FH/FV≦2.2, the negative electrode and the composite layer are firmly bonded, so that it is possible to suppress the occurrence of cracks and breaks due to swelling in the composite layer, and the strength of the separator can be improved. In addition, it was possible to suppress the presence of a liquid pool between the separator and the negative electrode. Therefore, it was possible to suppress self-discharge while suppressing resistance. In contrast, in Comparative Example 1, since the separator and the negative electrode were simply laminated, it is presumed that a liquid pool was generated at the interface between them. In Comparative Example 1, it is determined that the electrolysis of water occurred due to the contact of the member constituting the negative electrode with the water in the liquid pool, and self-discharge progressed.

From the results of the peel strength measurement shown in Table 8, it is judged that there is no liquid pool in Comparative Example 2, but each performance in Comparative Example 2 was significantly low. This is presumably due to the fact that the manufacturing method adopted in the electrode structure according to Comparative Example 2 did not allow pre-impregnation with the electrolyte solution. First, since the drying temperature of each slurry forming the negative electrode active material-containing layer and the composite layer is high, the water solvent of the aqueous electrolyte evaporates, so the aqueous electrolyte cannot be impregnated until both the negative electrode active material-containing layer and the composite layer are formed. In Comparative Example 2, at the stage where both the negative electrode active material-containing layer and the composite layer are prepared, a current collector foil is provided on one side of the negative electrode active material-containing layer, and a composite layer is formed on the other side on the back side. Since the liquid cannot be permeated through the current collector foil, the aqueous electrolyte cannot be impregnated into the negative electrode active material-containing layer from the side on the current collector foil side. In Comparative Example 2, since the raw materials and process for forming the composite layer are the same as those in Example 1, it can be judged that a dense composite layer is obtained at least to some extent. It is difficult to allow liquid to penetrate through such a dense composite layer, and it is believed that the aqueous electrolyte is hardly impregnated into the negative electrode active material-containing layer from the composite layer side. As a result, the amount of aqueous electrolyte impregnated into the electrode structure in Comparative Example 2 is small, and it is believed that the battery performance is significantly deteriorated.

In Comparative Example 3, the electrolysis of water could not be suppressed in the first place, and in addition to the storage performance, the charge/discharge efficiency was also low.

According to one or more of the embodiments and examples described above, the electrode structure comprises a negative electrode having a negative electrode active material-containing layer, and a separator having at least three layers, a first composite layer, a porous layer, and a second composite layer, in this order, the negative electrode active material-containing layer and the first composite layer are in contact with each other, the first composite layer and the second composite layer contain solid particles and a polymeric material, and the relationship between the force FH applied in the horizontal direction and the force FV applied in the vertical direction when the interface between the negative electrode active material-containing layer and the first composite layer is measured by interface cutting evaluation is 1.35≦FH/FV≦2.2.

The above configuration makes it possible to provide an electrode structure, a secondary battery, and a battery pack that can realize a secondary battery that exhibits high charge/discharge efficiency, improves rate characteristics, and suppresses self-discharge, as well as a vehicle and a stationary power source equipped with the battery pack.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims.

1...electrode structure, 2...exterior member, 3...negative electrode, 3a...negative electrode current collector, 3b...negative electrode active material-containing layer, 3c...negative electrode active material, 4...separator, 4a...porous layer, 4b...composite layer, 5...positive electrode, 5a...positive electrode current collector, 5b...positive electrode active material-containing layer, 6...negative electrode terminal, 7...positive electrode terminal, 8...negative electrode gasket, 9...positive electrode gasket, 10...sealing plate, 11...control valve, 12...filling port, 13...sealing plug, 16...negative electrode current collector tab, 17...positive electrode current collector tab, 21...bus bar, 22...positive electrode side lead, 23...negative electrode side lead, 24...adhesive tape, 31...container, 32...lid, 33...protective sheet, 34...printed wiring board, 35...wiring, 40...vehicle body, 100...secondary battery, 110...system, 111...power plant, 112... Stationary power source, 113... consumer side power system, 115... energy management system, 116... power network, 117... communication network, 118... power conversion device, 121... consumer side EMS, 122... power conversion device, 123... stationary power source, 200... battery pack, 300... battery pack, 300A... battery pack, 300B... battery pack, 310... housing, 320... opening, 332... output positive terminal, 333... output negative terminal, 342... positive side connector, 343... negative side connector, 345... thermistor, 346... protection circuit, 342a... wiring, 343a... wiring, 350... external terminal for current supply, 352... positive side terminal, 353... negative side terminal, 348a... positive side wiring, 348b... negative side wiring, 400... vehicle.

Claims (16)

a negative electrode including a negative electrode active material-containing layer;
a separator including at least three layers, in this order, a first composite layer, a porous layer, and a second composite layer;
Equipped with
the negative electrode active material-containing layer and the first composite layer are in contact with each other, and the first composite layer and the second composite layer contain solid particles and a polymer material,
The solid particles include inorganic solid particles having an ionic conductivity of an alkali metal ion, the inorganic solid particles including one selected from the group consisting of alumina, silica, zirconia, yttria, magnesium oxide, calcium oxide, barium oxide, strontium oxide , vanadium oxide, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, barium sulfate, hydroxyapatite, lithium phosphate, zirconium phosphate, titanium phosphate, silicon nitride, titanium nitride , and boron nitride;
The polymer material includes a monomer unit made of a hydrocarbon having a functional group containing one or more selected from the group consisting of oxygen (O), sulfur (S), nitrogen (N) and fluorine (F),
An electrode structure in which, when the separator is cut, the relationship between a force FH applied in a horizontal direction and a force FV applied in a vertical direction accompanying peeling of the first composite layer from the negative electrode active material containing layer is 1.35≦FH/FV≦2.2. The electrode structure according to claim 1, wherein the porosity of the first composite layer and the second composite layer is smaller than the porosity of the negative electrode active material-containing layer. The electrode structure according to claim 1 or 2, wherein the softening point temperature T2 of the polymer material Pn contained in the negative electrode active material-containing layer is higher than the softening point temperature T1 of the polymer material Pc contained in the first composite layer (T2>T1). The electrode structure according to any one of claims 1 to 3, wherein the polymer material Pc contained in the first composite layer and the polymer material Pn contained in the negative electrode active material-containing layer are different. 5. The electrode structure according to claim 1, wherein the inorganic solid particles include _at least one selected from the group consisting of a compound represented by Li1 _{+ wAlwTi2 -w} ( _PO4 ) ₃ (wherein 0.1≦w≦0.5), a compound represented by Li1 ₊ yAlzM12 _-z ( _PO4 ) ₃ (wherein M1 is one or more selected from the group consisting of Ti, Ge, Sr, Zr, Sn, and Ca _, and 0≦y≦1 and 0≦z≦1), and a compound represented by Li5 _{+ xLa3M22 - xZrxO12} (wherein M2 is one or more selected from the group consisting of Nb and Ta, and 0≦x≦2). The electrode structure according to any one of claims 1 to 5, wherein the proportion of the monomer units in the polymer material is 70 mol % or more. The electrode structure according to claim 6, wherein the functional group of the monomer unit includes one or more selected from the group consisting of a formal group, a butyral group, a carboxymethyl ester group, an acetyl group, a carbonyl group, a hydroxyl group, and a fluoro group. 8. The electrode structure according to claim 1, wherein the negative electrode active material-containing layer contains a compound having a lithium ion insertion-extraction potential of 1 V or more and 3 V or less relative to an oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). The electrode structure according to any one of claims 1 to 8,
A positive electrode and
A secondary battery comprising an aqueous electrolyte containing water. The positive electrode comprises a positive electrode active material-containing layer,
10. The secondary battery according to claim 9, wherein the positive electrode active material-containing layer contains a positive electrode active material containing a compound having a lithium ion insertion-extraction potential of 2.5 V or more and 5.5 V or less relative to the oxidation-reduction potential of lithium (vs. Li/Li ⁺ ). A battery pack comprising the secondary battery according to claim 9 or 10. The battery pack according to claim 11, further comprising an external terminal for energizing and a protection circuit. The battery pack according to claim 11 or 12, comprising a plurality of the secondary batteries, the plurality of secondary batteries being electrically connected in series, in parallel, or in a combination of series and parallel. A vehicle equipped with a battery pack according to any one of claims 11 to 13. The vehicle according to claim 14, further comprising a mechanism for converting the kinetic energy of the vehicle into regenerative energy. A stationary power source comprising the battery pack according to any one of claims 11 to 13.

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