JP7451995B2 - Energy storage element - Google Patents

Description

The present invention relates to a power storage element.

Nonaqueous electrolyte secondary batteries, typified by lithium ion nonaqueous electrolyte secondary batteries, have high energy density and are widely used in electronic devices such as personal computers and communication terminals, automobiles, and the like. The non-aqueous electrolyte secondary battery generally includes an electrode body having a pair of electrodes electrically isolated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and transfers ions between the two electrodes. It is configured to charge and discharge by doing so. In addition, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as power storage elements other than non-aqueous electrolyte secondary batteries.

Carbon materials such as graphite, non-graphitic carbon, and amorphous carbon are widely used as the active material of the negative electrode of such power storage elements. For example, a lithium ion secondary battery using a non-graphitizable carbonaceous material with a deep charge/discharge capacity per unit mass as a negative electrode active material has been proposed for the purpose of increasing capacity (see Patent Document 1).

Japanese Patent Application Publication No. 7-335262

However, when the charge depth of the negative electrode is deepened by using non-graphitizable carbon as the negative electrode active material for the purpose of increasing the capacity of the battery, the negative electrode potential becomes base, and metal lithium precipitates during charging, resulting in the failure of the charge/discharge cycle. There is a risk that the capacity retention rate will decrease. Therefore, even when non-graphitizable carbon is used as the negative electrode active material of a battery with a deep charge depth for the purpose of increasing capacity, there is a need for a power storage element that can suppress the decrease in capacity retention after charge/discharge cycles. ing.

The present invention has been made based on the above-mentioned circumstances, and is aimed at suppressing the decrease in capacity retention rate after charge/discharge cycles when non-graphitizable carbon is used as the active material of a negative electrode with a deep charging depth. The purpose is to provide a power storage element that can

One aspect of the present invention, which has been made to solve the above problems, includes: a negative electrode including a negative electrode mixture layer containing a negative electrode active material; a positive electrode including a positive electrode mixture layer containing a positive electrode active material; and a separator interposed therebetween, the separator has a porosity of 50% or more, the negative electrode active material contains non-graphitizable carbon as a main component, and the true density of the non-graphitizable carbon is A. When [g/cm ³ ], the charged electricity amount B [mAh/g] of the negative electrode in a fully charged state satisfies the following formula 1.
-660×A+1433≦B≦-830×A+1800 ・・・1

According to the present invention, when non-graphitizable carbon is used as the active material of a negative electrode with a deep charging depth, it is possible to provide a power storage element that can suppress a decrease in capacity retention rate after charge/discharge cycles.

FIG. 1 is a schematic perspective view showing the configuration of a power storage element according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing a power storage device configured by collecting a plurality of power storage elements according to an embodiment of the present invention. 2 is a graph showing the relationship between the true density of non-graphitizable carbon in a negative electrode active material and the amount of electricity charged in a fully charged negative electrode in a test example.

As a result of various experiments, the present inventor determined that the true density A of the non-graphitizable carbon contained in the negative electrode mixture layer and the charge carrier (in the case of a lithium ion secondary battery, lithium ion ), we realized that there is a certain correlation between the amount of charge carriers that can be stored while suppressing the precipitation (charged electricity amount B), and furthermore, we set the porosity range of the separator to an appropriate range. The present invention has been completed based on the discovery that by doing so, it is possible to more effectively suppress the decrease in capacity retention rate after charge/discharge cycles.
That is, in the electricity storage element according to one embodiment of the present invention, a negative electrode includes a negative electrode mixture layer containing a negative electrode active material, a positive electrode includes a positive electrode mixture layer containing a positive electrode active material, and a space between the negative electrode and the positive electrode. and an intervening separator, the separator has a porosity of 50% or more, the negative electrode active material contains non-graphitizable carbon as a main component, and the true density of the non-graphitizable carbon is A [g /cm ³ ], the charging electricity amount B [mAh/g] of the negative electrode in a fully charged state satisfies the following formula 1.
-660×A+1433≦B≦-830×A+1800 ・・・1

In the case where non-graphitizable carbon is used as the active material of the negative electrode with a deep charge depth for the purpose of increasing the capacity, the power storage element can suppress a decrease in the capacity retention rate after charge/discharge cycles. Although the reason for this is not certain, the following reasons are presumed. When non-graphitizable carbon is used as the active material of the negative electrode, if the depth of charge of the negative electrode is increased, the potential of the negative electrode shifts to a more base state, which may cause charge carriers to easily precipitate. In the electricity storage element, when the true density of non-graphitizable carbon is A [g/cm ³ ], the charged electricity amount B [mAh/g] of the negative electrode in a fully charged state is −830×A+1800 or less. Therefore, the amount of charged electricity B becomes appropriate, and excessive deposition of charge carriers can be suppressed. In addition, in the range where the charged electricity amount B of the negative electrode is -660 x A + 1433 or more, the porosity of the separator is 50% or more, which makes it possible to reduce the movement resistance of charge carriers in the separator. The potential of can be kept relatively noble. Therefore, as a result of being able to suppress the precipitation of charge carriers, it is possible to suppress a decrease in capacity retention rate after charge/discharge cycles.
Here, the term "fully charged state" refers to a state in which the battery is charged to the rated upper limit voltage for ensuring the rated capacity determined by the battery design. In addition, if there is no description regarding the rated capacity, when charging is performed using the charging control device adopted by the energy storage element, it will not be charged until the charging end voltage is reached when the charging operation is controlled to stop. A state in which For example, after the electricity storage element is charged at a constant current of (1/3) CA until it reaches the rated upper limit voltage or the charge end voltage, the voltage becomes 0.01CA at the rated upper limit voltage or the charge end voltage. A state in which constant current and constant voltage (CCCV) charging is performed up to the point where the battery is charged is a typical example of the "fully charged state" referred to herein.

It is preferable that the true density A of the non-graphitizable carbon is 1.5 [g/cm ³ ] or less. When the true density A of the non-graphitizable carbon is within the above range, the amount of lithium ions that can be stored between the crystal structures of the non-graphitizable carbon can be within a favorable range.

The positive electrode active material has a lithium transition metal oxide containing nickel, cobalt, and manganese as a main component, and the molar ratio of nickel to the sum of nickel, cobalt, and manganese in the lithium transition metal oxide is 0.5 or more. preferable. By setting the molar ratio of nickel to the sum of nickel, cobalt, and manganese in the lithium transition metal oxide within the above range, the capacity retention rate of the electricity storage element after charge/discharge cycles can be increased.

Hereinafter, a power storage element according to an embodiment of the present invention will be explained in detail.

<Electricity storage element>
A power storage element according to an embodiment of the present invention includes a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. Hereinafter, a non-aqueous electrolyte secondary battery will be described as a preferable example of a power storage element. The above-mentioned positive electrode and negative electrode usually form an electrode body in which they are alternately stacked by stacking or winding with a separator in between. This electrode body is housed in a battery container, and the battery container is filled with a nonaqueous electrolyte.

[Negative electrode]
The negative electrode includes a negative electrode base material and a negative electrode mixture layer that is directly or indirectly laminated on at least one surface of the negative electrode base material. The negative electrode mixture layer contains a negative electrode active material. The negative electrode may include an intermediate layer disposed between the negative electrode base material and the negative electrode mixture layer.

(Negative electrode base material)
The negative electrode base material is a conductive base material. As the material of the negative electrode base material, metals such as copper, nickel, stainless steel, nickel-plated steel, or alloys thereof are used, and copper or copper alloys are preferable. In addition, examples of the form of the negative electrode base material include foil, vapor deposited film, etc., and foil is preferable from the viewpoint of cost. In other words, copper foil is preferable as the negative electrode base material. Examples of the copper foil include rolled copper foil, electrolytic copper foil, and the like. Note that "conductive" means that the volume resistivity measured in accordance with JIS-H-0505 (1975) is 1 x 10 ⁷ Ω・cm or less, and "non-conductive" ” means that the volume resistivity is more than 1×10 ⁷ Ω·cm.

The average thickness of the negative electrode base material is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 25 μm or less, even more preferably 4 μm or more and 20 μm or less, and particularly preferably 5 μm or more and 15 μm or less. By setting the average thickness of the negative electrode base material within the above range, it is possible to increase the energy density per volume of the electricity storage element while increasing the strength of the negative electrode base material. The "average thickness of the base material" refers to the value obtained by dividing the punched mass when punching a base material of a predetermined area by the true density and punched area of the base material.

(Negative electrode mixture layer)
The negative electrode mixture layer is formed from a so-called negative electrode mixture containing a negative electrode active material.

The negative electrode active material contains non-graphitizable carbon as a main component. When the negative electrode active material contains non-graphitizable carbon as a main component, the capacity retention rate of the electricity storage element after charge/discharge cycles can be increased. Further, the negative electrode mixture may contain other negative electrode active materials other than the non-graphitizable carbon. In addition, the above-mentioned "main component in the negative electrode active material" means the component with the highest content, and refers to a component contained in 90% by mass or more with respect to the total mass of the negative electrode active material.

(Non-graphitizable carbon)
Non-graphitizable carbon is a carbon substance in which the average lattice spacing d(002) of the (002) plane is larger than 0.36 nm and smaller than 0.42 nm, as measured by X-ray diffraction in a discharge state. Non-graphitizable carbon usually refers to a material in which minute graphite crystals are arranged in random directions and nano-order voids are present between the crystal layers. Examples of the non-graphitizable carbon include phenol resin fired products, furan resin fired products, furfuryl alcohol resin fired products, and the like.

Here, the "discharge state" refers to a state in which the open circuit voltage is 0.7 V or more in a monopolar battery using a negative electrode containing a carbon material as a negative electrode active material as a working electrode and metal Li as a counter electrode. Since the potential of the metal Li counter electrode in an open circuit state is approximately equal to the redox potential of Li, the open circuit voltage in the monopolar battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. . In other words, if the open circuit voltage of the monopolar battery is 0.7V or higher, it means that a sufficient amount of lithium ions, which can be intercalated and released during charging and discharging, is released from the carbon material that is the negative electrode active material. .

The true density A of the non-graphitizable carbon is not particularly limited as long as the relationship between the true density A and the amount of charged electricity B satisfies the above formula, but its upper limit is preferably 1.8 g/ ^cm3 , and 1. .7 g/cm ³ is more preferred. In some embodiments, the true density A may be 1.65 [g/cm ³ ] or less, 1.58 [g/cm 3 ] or less, or 1.52 [g/cm ³ ] or less. ³ ] or less, or 1.5 or less. By setting the upper limit of the true density A within the above range, the amount of lithium ions that can be stored between the crystal structures of the non-graphitizable carbon can be set within a favorable range. The lower limit of the true density A is preferably 1.4 g/cm ³ , more preferably 1.45 g/cm ³ . When the lower limit of the true density is within the above range, it is possible to suppress an increase in irreversible capacity due to an increase in impurities derived from raw materials or an increase in reaction active surfaces. That is, within the above range, the amount of lithium ions that can be stored can be increased while suppressing the irreversible capacity. True density is measured by the pycnometer method using butanol.

The lower limit of the content of the non-graphitizable carbon relative to the total mass of the negative electrode active material is preferably 90% by mass. By setting the content of non-graphitizable carbon to the above lower limit or more, the capacity retention rate of the electricity storage element after charge/discharge cycles can be further increased. On the other hand, the upper limit of the content of the non-graphitizable carbon relative to the total mass of the negative electrode active material may be, for example, 100% by mass.

(Other negative electrode active materials)
Other negative electrode active materials that may be included in addition to non-graphitizable carbon include easily graphitizable carbon, graphite, metals such as Si and Sn, oxides of these metals, or combinations of these metals and carbon materials. Examples include complexes of.

The content of the negative electrode active material in the negative electrode mixture layer is not particularly limited, but its lower limit is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, more preferably 98% by mass.

(Other optional ingredients)
The negative electrode mixture contains optional components such as a conductive agent, a thickener, and a filler as necessary.

The non-graphitizable carbon also has conductivity, but the conductive agent is not particularly limited as long as it is a conductive material. Examples of such conductive agents include graphite, carbonaceous materials, metals, conductive ceramics, and the like. Examples of the carbonaceous material include non-graphitized carbon, graphene-based carbon, and the like. Examples of non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, carbon black, and the like. Examples of carbon black include furnace black, acetylene black, Ketjen black, and the like. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerene. Examples of the shape of the conductive agent include powder, fiber, and the like. As the conductive agent, one type of these materials may be used alone, or two or more types may be used in combination. Further, these materials may be used in combination. For example, a composite material of carbon black and CNT may be used. Among these, carbon black is preferred from the viewpoint of electronic conductivity and coatability, and acetylene black is particularly preferred.

Examples of the binder include fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), thermoplastic resins such as polyethylene, polypropylene, polyacrylic acid, polyimide, etc.; ethylene-propylene-diene rubber (EPDM); Examples include elastomers such as sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber; polysaccharide polymers, and the like.

The binder content in the negative electrode mixture layer is preferably 1% by mass or more and 10% by mass or less, more preferably 3% by mass or more and 9% by mass or less. By setting the content of the binder within the above range, the negative electrode active material particles can be stably held.

Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. When the thickener has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like.

The filler is not particularly limited. The main components of fillers include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicate, magnesium hydroxide, and hydroxide. Calcium, hydroxides such as aluminum hydroxide, carbonates such as calcium carbonate, poorly soluble ionic crystals such as calcium fluoride, barium fluoride, barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc, montmorillonite, Examples include substances derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. When using a filler in the negative electrode mixture layer, the proportion of the filler in the entire negative electrode mixture layer can be approximately 8.0% by mass or less, and usually approximately 5.0% by mass or less (for example, 1.0% by mass or less). % by mass or less).

(middle class)
The intermediate layer is a coating layer on the surface of the negative electrode base material, and contains conductive particles such as carbon particles to reduce contact resistance between the negative electrode base material and the negative electrode mixture layer. The structure of the intermediate layer is not particularly limited, and can be formed, for example, from a composition containing a resin binder and conductive particles.

In the electricity storage element, when the true density of the non-graphitizable carbon is A [g/cm ³ ], the charged electricity amount B [mAh/g] of the negative electrode in a fully charged state satisfies the following formula 1.
-660×A+1433≦B≦-830×A+1800 ・・・1
In the electricity storage element, when the true density of non-graphitizable carbon is A [g/cm ³ ], the charged electricity amount B [mAh/g] of the negative electrode in a fully charged state is −830×A+1800 or less. Therefore, the amount of charged electricity B becomes an appropriate amount, and excessive precipitation of metallic lithium can be suppressed. In addition, in the range where the charged electricity amount B of the negative electrode is -660 x A + 1433 or more, the porosity of the separator is 50% or more, so that the movement resistance of lithium ions in the separator can be reduced. The potential of the negative electrode can be made relatively noble. Therefore, as a result of being able to suppress the precipitation of metallic lithium, it is possible to suppress a decrease in the capacity retention rate after charge/discharge cycles.

The amount of electricity B charged to the negative electrode is measured using the following procedure.
(1) Discharge the target battery in the glove box to the end of discharge (low SOC region) and disassemble it.
(2) In the above-mentioned glove box controlled to have an atmosphere with an oxygen concentration of 5 ppm or less, the positive electrode plate and the negative electrode plate are taken out and a small laminate cell is assembled.
(3) After charging the small laminate cell to the fully charged state, constant current constant voltage (CCCV) discharge is performed to 0.01 CA at the lower limit voltage when the rated capacity is obtained in the electricity storage element.
(4) In a glove box controlled to have an oxygen concentration of 5 ppm or less, the small laminate cell is dismantled, the negative electrode is taken out, and the small laminate cell is reassembled into a small laminate cell with lithium metal placed as a counter electrode.
(5) Additional discharge is performed at a current density of 0.01 CA until the negative electrode potential reaches 2.0 V (vs. Li/Li ⁺ ), and the negative electrode is adjusted to a fully discharged state.
(6) The total amount of electricity in (3) and (5) above is divided by the mass of the negative electrode of the positive and negative electrode opposing parts in the small laminate cell to determine the amount of charged electricity.

[Positive electrode]
The positive electrode includes a positive electrode base material and a positive electrode mixture layer that is directly or indirectly laminated on at least one surface of the positive electrode base material. The positive electrode mixture layer contains a positive electrode active material. The positive electrode may include an intermediate layer disposed between the positive electrode base material and the positive electrode mixture layer.

(Positive electrode base material)
The positive electrode base material 21 is a conductive base material. As the material of the positive electrode base material, metals such as aluminum, titanium, tantalum, stainless steel, or alloys thereof are used. Among these, aluminum and aluminum alloys are preferred from the viewpoint of potential resistance, high conductivity, and cost balance. Moreover, the form of the positive electrode base material 21 includes foil, a vapor deposited film, etc., and foil is preferable from the viewpoint of cost. That is, as the positive electrode base material 21, aluminum foil is preferable. Note that examples of aluminum or aluminum alloy include A1085 and A3003 defined in JIS-H4000 (2014).

(Positive electrode mixture layer)
The positive electrode mixture layer is formed from a so-called positive electrode mixture containing a positive electrode active material. The positive electrode active material can be appropriately selected from known positive electrode active materials, for example. As a positive electrode active material for a lithium ion nonaqueous electrolyte secondary battery, a material that can insert and release lithium ions is usually used. Examples of the positive electrode active material include a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure, a lithium transition metal oxide having a spinel type crystal structure, a polyanion compound, a chalcogen compound, and sulfur. Examples of lithium transition metal composite oxides having α-NaFeO ₂ type crystal structure include Li[Li _x Ni _1-x ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Co _{(1- x-γ)} ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Co _(1-x) ]O ₂ (0≦x<0.5), Li[Li _x Ni _γ Mn _(1-x-γ) ]O ₂ (0≦x<0.5, 0<γ<1), Li[Li _x Ni _γ Mn _β Co _(1-x-γ-β) ]O ₂ (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li _x Ni _γ Co _β Al _(1-x-γ-β) ]O ₂ (0≦ Examples include x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Examples of lithium transition metal oxides having a spinel crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ . Examples of the polyanion compound include LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F, and the like. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, and the like. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials.

The positive electrode active material is preferably a nickel-containing lithium transition metal oxide containing nickel. It is preferable that the molar ratio of nickel to the total of metal elements other than lithium in the nickel-containing lithium transition metal oxide is 0.5 or more (for example, 0.5 or more and 1 or less), and 0.55 or more (for example, 0.6 0.9 or less) is more preferable. As an example of a particularly preferable positive electrode active material, the main component is a lithium transition metal oxide containing nickel, cobalt, and manganese, and the molar ratio of nickel to the sum of nickel, cobalt, and manganese in the lithium transition metal oxide is 0.5 or more. (For example, 0.5 or more 0.9
Hereinafter, examples include those having a ratio of 0.6 to 0.8 (typically 0.6 to 0.8). By setting the molar ratio of nickel to the sum of nickel, cobalt, and manganese in the lithium transition metal oxide within the above range, the capacity retention rate of the electricity storage element after charge/discharge cycles can be increased.

In the positive electrode mixture layer, one type of these materials may be used alone, or two or more types may be used in combination. In the positive electrode mixture layer, one type of these compounds may be used alone, or two or more types may be used in combination.

The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited, but its lower limit is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, more preferably 98% by mass.

The amount of electricity B charged to the negative electrode in a fully charged state is, for example, the mass N of the negative electrode active material per unit area in the negative electrode mixture layer relative to the mass P of the positive electrode active material per unit area in the positive electrode mixture layer. It can be adjusted by changing the ratio N/P. In one embodiment, when the true density of the non-graphitizable carbon is A [g/cm ³ ], the mass P of the positive electrode active material per unit area in the positive electrode mixture layer is It is preferable that the ratio N/P of the mass N of the negative electrode active material per unit area satisfies the following formula 2.
0.57×A-0.53≦N/P≦0.70×A-0.65...2
When an N/P satisfying the above formula 2 is applied to a conventional battery using non-graphitizable carbon, the depth of charge becomes deeper than the normal depth, and metallic lithium tends to precipitate. However, in the electricity storage element, since the porosity of the separator is 50% or more, the movement resistance of lithium ions in the separator can be reduced, and therefore the potential of the negative electrode can be made relatively noble. Therefore, as a result of being able to suppress the precipitation of metallic lithium, it is possible to suppress a decrease in the capacity retention rate after charge/discharge cycles.

(Other optional ingredients)
The positive electrode mixture contains optional components such as a conductive agent, a binder, a thickener, and a filler, as necessary. Optional components such as a conductive agent, a binder, a thickener, and a filler can be selected from the materials exemplified for the negative electrode.

The conductive agent is not particularly limited as long as it is a conductive material. Such a conductive agent can be selected from the materials exemplified for the negative electrode above. When using a conductive agent, the proportion of the conductive agent in the entire positive electrode mixture layer can be approximately 1.0% by mass to 20% by mass, and usually approximately 2.0% by mass to 15% by mass (e.g. 3.0% by mass to 6.0% by mass).

The binder can be selected from the materials exemplified for the negative electrode. When using a binder, the proportion of the binder in the entire positive electrode mixture layer can be approximately 0.50% by mass to 15% by mass, and usually approximately 1.0% by mass to 10% by mass (for example, 1.0% by mass). 5% by mass to 3.0% by mass).

Examples of the thickener include polysaccharide polymers such as carboxymethylcellulose (CMC) and methylcellulose. Further, when the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group by methylation or the like in advance. When using a thickener, the proportion of the thickener in the entire positive electrode mixture layer can be approximately 8% by mass or less, and usually approximately 5.0% by mass or less (for example, 1.0% by mass or less). ) is preferable.

The filler can be selected from the materials exemplified for the negative electrode. When using a filler, the proportion of the filler in the entire positive electrode mixture layer can be approximately 8.0% by mass or less, and usually approximately 5.0% by mass or less (for example, 1.0% by mass or less). It is preferable to do so.

(middle class)
The intermediate layer is a coating layer on the surface of the positive electrode base material 21, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode base material 21 and the positive electrode mixture layer. Similar to the negative electrode, the structure of the intermediate layer is not particularly limited, and can be formed, for example, from a composition containing a resin binder and conductive particles.

[Nonaqueous electrolyte]
As the non-aqueous electrolyte, a known non-aqueous electrolyte that is commonly used in general non-aqueous electrolyte secondary batteries (power storage elements) can be used. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Note that the non-aqueous electrolyte may be a solid electrolyte or the like.

As the non-aqueous solvent, a known non-aqueous solvent that is commonly used as a non-aqueous solvent for a general non-aqueous electrolyte for a power storage device can be used. Examples of the nonaqueous solvent include cyclic carbonates, chain carbonates, esters, ethers, amides, sulfones, lactones, and nitriles. Among these, it is preferable to use at least a cyclic carbonate or a chain carbonate, and it is more preferable to use a cyclic carbonate and a chain carbonate in combination. When using a cyclic carbonate and a chain carbonate together, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is not particularly limited, but may be, for example, from 5:95 to 50:50. preferable.

Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), and difluoroethylene. carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diphenyl carbonate, and among these, EMC is preferred.

As the electrolyte salt, a known electrolyte salt that is commonly used as an electrolyte salt of a general non-aqueous electrolyte for a power storage device can be used. Examples of the electrolyte salts include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, and the like, with lithium salts being preferred.

Examples of the lithium salt include inorganic lithium salts such as LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiClO ₄ , LiN(SO ₂ F) ₂ , LiSO ₃ CF ₃ , LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ Hydrogen in C ₂ F ₅ ) ₂ , LiN(SO ₂ CF ₃ )(SO ₂ C ₄ F ₉ ), LiC(SO ₂ CF ₃ ) ₃ , LiC(SO ₂ C ₂ F ₅ ) ₃ is replaced with fluorine Examples include lithium salts having a hydrocarbon group. Among these, inorganic lithium salts are preferred, and LiPF ₆ is more preferred.

The lower limit of the concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol/dm ³ , more preferably 0.3 mol/dm ³ , even more preferably 0.5 mol/dm ³ , and 0.7 mol/dm ³ is particularly preferred. On the other hand, this upper limit is not particularly limited, but is preferably 2.5 mol/dm ³ , more preferably 2.0 mol/dm ³ , and even more preferably 1.5 mol/dm ³ .

Other additives may be added to the non-aqueous electrolyte. Moreover, room temperature molten salts, ionic liquids, etc. can also be used as the non-aqueous electrolyte.

[Separator]
A separator is interposed between the negative electrode and the positive electrode. As the separator, for example, woven fabric, nonwoven fabric, porous resin film, etc. are used. Among these, porous resin films are preferred from the viewpoint of strength, and nonwoven fabrics are preferred from the viewpoint of liquid retention of the nonaqueous electrolyte. The main components of the separator include, from the viewpoint of strength, polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacrylonitrile, polyphenylene sulfide, polyimide, and fluororesin. Among these, polyolefins such as polyethylene and polypropylene are preferred. Further, these resins may be combined.

The lower limit of the porosity of the separator is 50%. In some embodiments, the porosity of the separator may be 52% or more, 55% or more, or 58% or more (eg, 60% or more). The upper limit of the porosity of the separator is preferably 70%, more preferably 65%. By setting the porosity within the above range, the effect of suppressing a decrease in capacity retention during charge/discharge cycles can be further enhanced. The porosity is the ratio of the pore volume to the total volume of the porous resin layer, and is measured according to the "pore volume ratio" defined in JIS-L1096 (2010).

The average thickness of the separator is not particularly limited, but its lower limit is preferably 3 μm, more preferably 5 μm, and even more preferably 7 μm. In some embodiments, the average thickness of the separator may be, for example, 8 μm or more, and typically 10 m or more. On the other hand, the upper limit of the average thickness of the separator is preferably 30 μm, more preferably 25 μm. In some embodiments, the average thickness of the separator may be, for example, 20 μm or less, and typically 15 μm or less. The technology disclosed herein can be preferably implemented, for example, in an embodiment in which the average thickness of the separator is 3 μm or more and 30 μm or less (or 8 μm or more and 15 μm or less).

Note that an inorganic layer may be laminated between the separator and the electrode (for example, the positive electrode). This inorganic layer is a porous layer also called a heat-resistant layer. Furthermore, a separator in which an inorganic layer is formed on one or both sides of a porous resin film can also be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components. The technology disclosed herein can be implemented in an embodiment in which an inorganic layer is not laminated between the separator and the negative electrode.

The inorganic particles contained in the inorganic layer preferably have a weight loss of 5% or less at 500° C. in the atmosphere, and more preferably have a weight loss of 5% or less at 800° C. in the atmosphere. Inorganic compounds are examples of materials whose weight loss is less than a predetermined value. Examples of inorganic compounds include oxides such as iron oxide, silicon oxide, aluminum oxide, titanium oxide, barium titanate, zirconium oxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, aluminosilicate; magnesium hydroxide, water Hydroxides such as calcium oxide and aluminum hydroxide; Nitrides such as aluminum nitride and silicon nitride; Carbonates such as calcium carbonate; Sulfates such as barium sulfate; Hardly soluble ionic crystals such as calcium fluoride and barium fluoride Covalent crystals such as silicon and diamond; Substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, or artificial products thereof. . As the inorganic compound, these substances may be used alone or in combination, or two or more kinds may be used in combination. Among these inorganic compounds, silicon oxide, aluminum oxide, or aluminosilicates are preferred.

[Specific configuration of power storage element]
The shape of the power storage element of this embodiment is not particularly limited, and examples include a cylindrical battery, a pouch film battery, a square battery, a flat battery, a coin battery, a button battery, and the like.

FIG. 1 shows a square non-aqueous electrolyte secondary battery 1 as an example of a power storage element. Note that this figure is a transparent view of the inside of the battery container. An electrode body 2 having a positive electrode and a negative electrode wound with a separator in between is housed in a rectangular battery container 3. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode current collector 41 . The negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode current collector 51 .

[Method for manufacturing power storage element]
The method for manufacturing the electricity storage element includes producing a negative electrode, producing a positive electrode, preparing a non-aqueous electrolyte, and stacking or winding a positive electrode and a negative electrode through a separator to form electrode bodies that are alternately stacked. , accommodating the electrode body in a container, and injecting the non-aqueous electrolyte into the container. The above-mentioned positive electrode can be obtained by laminating the above-mentioned positive electrode mixture layer on the positive electrode base material directly or via an intermediate layer. The above-mentioned positive electrode mixture layer is laminated by applying a positive electrode mixture paste to the positive electrode base material. Further, the negative electrode can be obtained by laminating the negative electrode mixture layer on the negative electrode base material directly or via an intermediate layer, similarly to the positive electrode. The negative electrode mixture layer is laminated by applying a negative electrode mixture paste containing non-graphitizable carbon to the negative electrode base material. The positive electrode mixture paste and the negative electrode mixture paste may contain a dispersion medium. As the dispersion medium, for example, aqueous solvents such as water and a mixed solvent mainly composed of water; organic solvents such as N-methylpyrrolidone and toluene can be used.

The above-mentioned negative electrode, positive electrode, non-aqueous electrolyte, etc. can be housed in a case by a known method. After housing, the storage port can be sealed to obtain a power storage element. Details of each element constituting the electricity storage element obtained by the above manufacturing method are as described above.

According to the electricity storage element, when non-graphitizable carbon is used as the active material of the negative electrode with a deep charging depth, it is possible to suppress a decrease in the capacity retention rate after charge/discharge cycles.

[Other embodiments]
Note that the power storage element of the present invention is not limited to the above embodiments, and various changes may be made without departing from the gist of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment or a known technique. Additionally, some of the configurations of certain embodiments may be deleted. Furthermore, well-known techniques can be added to the configuration of a certain embodiment.

In the above embodiments, the description has been given mainly of the form in which the electricity storage element is a non-aqueous electrolyte secondary battery, but other electricity storage elements may be used. Other power storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), and the like. Examples of nonaqueous electrolyte secondary batteries include lithium ion nonaqueous electrolyte secondary batteries.

The present invention can also be realized as a power storage device including a plurality of the above power storage elements. In this case, the technology of the present invention may be applied to at least one power storage element included in the power storage device. Furthermore, a battery pack can be constructed by using one or more power storage elements (cells) of the present invention, and furthermore, a power storage device can be constructed using this battery pack. The power storage device described above can be used as a power source for automobiles such as electric vehicles (EVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs). Further, the power storage device can be used in various power supplies such as an engine starting power supply, an auxiliary machine power supply, and an uninterruptible power supply (UPS).

FIG. 2 shows an example of a power storage device 30 in which a power storage unit 20 in which two or more electrically connected power storage elements 1 are assembled is further assembled. Power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1 and a bus bar (not shown) that electrically connects two or more power storage units 20. The power storage unit 20 or the power storage device 30 may include a state monitoring device (not shown) that monitors the state of one or more power storage elements.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[Example 1 to Example 26]
(Preparation of negative electrode)
A negative electrode mixture paste was prepared by mixing non-graphitizable carbon as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. . The mass ratio of non-graphitizable carbon, styrene-butadiene rubber, and carboxymethyl cellulose (CMC) was 97.4:2.0:0.6 in terms of solid content.

The negative electrode mixture paste was prepared by adjusting the viscosity by adjusting the amount of water and kneading using a multi-blender mill. This negative electrode mixture paste was applied to both sides of the copper foil so that a non-laminated portion was formed at one edge of the copper foil. Next, a negative electrode mixture layer was produced by drying. After the drying, the negative electrode mixture layer was roll pressed to a predetermined packing density to obtain a negative electrode.

(Preparation of positive electrode)
A positive electrode is made using lithium nickel cobalt manganese composite oxide as a positive electrode active material, acetylene black (AB) as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a nonaqueous dispersion medium. A mixture paste (material for forming a positive electrode mixture layer) was prepared. Note that the mass ratio of the positive electrode active material, binder, and conductive agent was 94.5:4.0:1.5 in terms of solid content. This positive electrode mixture paste was applied to both sides of the aluminum foil so that a non-laminated portion was formed at one edge of the aluminum foil. Next, a positive electrode mixture layer was produced by drying. After the drying, the positive electrode mixture layer was roll-pressed to a predetermined packing density to obtain a positive electrode. Further, the N/P of Examples 1 to 28 is shown in Table 1. Here, the molar ratio of nickel, cobalt, and manganese (Ni:Co:Mn ratio) in the lithium nickel cobalt manganese composite oxide, which is the positive electrode active material, was set to 6.0:2.0:2.0.

(Nonaqueous electrolyte)
The non-aqueous electrolyte was prepared by dissolving LiPF ₆ in a solvent containing propylene carbonate (PC) and diethyl carbonate (DEC) at a volume ratio of 30:70 so that the salt concentration was 1.2 mol/ ^dm3 . It was prepared using

(Separator)
A polyethylene microporous membrane with a thickness of 14 μm was used as the separator.

(electricity storage element)
The above positive electrode, negative electrode, and separator were laminated to produce an electrode body. Thereafter, the non-laminated portion of the positive electrode base material and the non-laminated portion of the negative electrode base material were welded to a positive electrode current collector and a negative electrode current collector, respectively, and the mixture was sealed in a container. Next, after welding the container and the lid plate, the non-aqueous electrolyte was injected and the container was sealed. In this way, batteries (power storage elements) of Examples 1 to 28 were obtained. The design rated capacity of this battery is 40.9 Ah.

[evaluation]
(True density of non-graphitizable carbon)
The true density of non-graphitizable carbon was measured using the following procedure.
The non-graphitizable carbon in a discharged state was immersed in water to remove the binder and thickener, and then vacuum-dried at 25° C. for 12 hours, and then the non-graphitizable carbon was taken out. Next, this non-graphitizable carbon was dried at 120° C. for 2 hours and cooled to room temperature in a desiccator. The mass (m1) of a pycnometer was accurately weighed, approximately 3 g of non-graphitizable carbon was added, and the mass was accurately weighed (m2). Next, 1-butanol was gently added to the pycnometer to a depth of about 20 mm from the bottom, placed in a vacuum desiccator, and gradually evacuated to maintain the pressure at 2.0 kPa to 2.6 kPa. After this pressure was maintained for 20 minutes and the generation of bubbles had ceased, the pycnometer was removed from the vacuum dessicator and further 1-butanol was added. The pycnometer was immersed in a constant temperature water bath at 30±0.5°C for 30 minutes, and the 1-butanol liquid level was adjusted to the marked line. The pycnometer was taken out, the outside was thoroughly wiped, and the mass was accurately weighed. The pycnometer was immersed again in the constant temperature water bath for 15 minutes, the 1-butanol liquid level was adjusted to the marked line, the pycnometer was taken out, the outside was thoroughly wiped, and the mass was measured. This process is repeated three times, and the average value of each mass when repeated three times is defined as (m4). Next, fill the same pycnometer with 1-butanol, immerse it in a constant temperature water bath in the same way as above, measure the mass after aligning it with the marked line, repeat the process 4 times, and calculate the average value of each mass after 4 repetitions. (m3). In addition, the process of putting degassed water into a pycnometer just before use, immersing it in a thermostatic water bath in the same manner as above, aligning it with the marked line, and then weighing it is repeated four times, and the average value is taken as (m5). The true density A was calculated using the following formula 2. In addition, in the following formula 2, d is the specific gravity at 30° C., and d=0.9946.
A=(m2-m1)/(m2-m1-(m4-m3))×((m3-m1)/(m5-m1))×d...2

(Charging electricity amount of negative electrode)
The amount of electricity charged to the negative electrode was measured by the method described above.

(Porosity of separator)
The porosity of the separator was measured according to the "pore volume ratio" specified in JIS-L1096 (2010).

(Capacity retention rate after charge/discharge cycle)
(1) Initial discharge capacity confirmation test Constant current constant voltage (CCCV) charging was performed in a constant temperature oven at 25°C under the conditions of a charging current of 13.6A and a charge end voltage of 4.32V until the charging current became 0.4A or less. This was followed by a 20-minute rest period. Thereafter, constant current (CC) discharge was performed at a discharge current of 40.9 A and a discharge end voltage of 2.4 V. The discharge capacity at this time was defined as the "initial discharge capacity".
(2) Capacity retention rate For each storage element after measuring the "initial discharge capacity," the charging current is 0.4 A or less under the conditions of a charging current of 13.6 A and a charging end voltage of 4.32 V in a thermostatic chamber at 45°C. Constant current/constant voltage (CCCV) charging was performed until the voltage was reached, and then a rest period of 10 minutes was provided. Thereafter, constant current (CC) discharge was performed at a discharge current of 40.9 A and a discharge end voltage of 2.4 V, followed by a rest period of 10 minutes. This charge/discharge cycle was performed for 1000 cycles. After 1000 cycles, the discharge capacity was measured under the same conditions as the "initial discharge capacity" measurement test, and the discharge capacity at this time was defined as the "capacity after 1000 cycles." The "capacity after 1000 cycles" relative to the "initial discharge capacity" was defined as the capacity retention rate after charge/discharge cycles.

Table 1 below shows the amount of electricity charged in the negative electrodes of Examples 1 to 26, the N/P ratio, the capacity retention rate after charge/discharge cycles, the evaluation of the capacity retention rate, and the evaluation results of the porosity of the separator. Further, FIG. 3 shows the relationship between the true density of non-graphitizable carbon in the negative electrode active material and the amount of electricity charged in the negative electrode in a fully charged state in Examples 1 to 26.

As shown in Table 1 and FIG. 3, when the true density of the non-graphitizable carbon is A [g/cm ³ ], the charged electricity amount B [mAh/g] of the negative electrode in the fully charged state is: -660×A+1433≦B≦−830×A+1800, and Examples 1 to 9, Example 15, Example 19, and Example 23, in which the porosity of the separator is 50% or more, have a capacity retention rate after charge/discharge cycles. was good.

On the other hand, the charged electricity amount B [mAh/g] of the negative electrode is in the range of -660×A+1433≦B≦−830×A+1800, but the porosity of the separator is less than 50% Example 11, Example 16, and Example In Examples 20 and 24, the capacity retention rate decreased.
In addition, in Examples 12, 17, 18, 21, 22, 25, and 26 in which the charged electricity amount B [mAh/g] of the negative electrode is less than -660×A+1433, regardless of the porosity of the separator. The capacity retention rate was good.
Furthermore, in Examples 10, 13, and 14 in which the charged electricity amount B [mAh/g] of the negative electrode exceeds -830 × A + 1800, the capacity retention rate is decreased.
The electricity storage element uses a separator with a large porosity when the charged electricity amount B [mAh/g] of the negative electrode in a fully charged state is within a specific range satisfying -660×A+1433≦B≦-830×A+1800. It can be seen that by using them in combination, it is possible to suppress a decrease in the capacity retention rate even when the depth of charge is relatively deep.

The above results showed that in the electricity storage element, when non-graphitizable carbon is used as the active material of the negative electrode with a deep charging depth, a decrease in capacity retention rate after charge/discharge cycles can be suppressed.

INDUSTRIAL APPLICATION This invention is suitably used as an electrical storage element including the non-aqueous electrolyte secondary battery used as a power supply of electronic devices, such as a personal computer and a communication terminal, and an automobile.

1 Energy storage element 2 Electrode body 3 Battery container 4 Positive electrode terminal 41 Positive electrode current collector 5 Negative electrode terminal 51 Negative electrode current collector 20 Energy storage unit 30 Energy storage device

Claims (3)

a negative electrode comprising a negative electrode mixture layer containing a negative electrode active material;
a positive electrode comprising a positive electrode mixture layer containing a positive electrode active material;
and a separator interposed between the negative electrode and the positive electrode,
The porosity of the separator is 50% or more,
The negative electrode active material contains non-graphitizable carbon as a main component,
When the true density of the non-graphitizable carbon is A [g/cm ³ ], an electricity storage amount B [mAh/g] of the negative electrode in a fully charged state satisfies the following formula 1.
-660×A+1433≦B≦-830×A+1800 ・・・1 The power storage element according to claim 1, wherein the non-graphitizable carbon has a true density A of 1.5 [g/cm ³ ] or less. The positive electrode active material mainly contains a lithium transition metal oxide containing nickel, cobalt and manganese,
The energy storage device according to claim 1 or 2, wherein the molar ratio of nickel to the total of nickel, cobalt, and manganese in the lithium transition metal oxide is 0.5 or more.

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