JP6885252B2 - Lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a lithium ion secondary battery.

Japanese Unexamined Patent Publication No. 2017-004879 (Patent Document 1) discloses that the DMC ratio is 39 to 43% by volume in an electrolytic solution containing ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

JP-A-2017-004879

The electrolytic solution of a lithium ion secondary battery (hereinafter, may be abbreviated as "battery") contains a solvent and a lithium (Li) salt. The Li salt is dissolved in the solvent. Conventionally, a mixed solvent containing EMC and DMC has been used from the viewpoint of output in a normal temperature range (for example, 0 to 40 ° C.) and a low temperature range (for example, -30 to 0 ° C.).

When the viscosity of the electrolytic solution decreases, the ion mobility in the electrolytic solution increases. DMC has a low viscosity in the normal temperature range. Due to the high DMC ratio, it is expected that the output will be improved in the normal temperature range. However, DMC has a freezing point of about 3 ° C. Therefore, if the DMC ratio is high, the electrolytic solution may solidify in an extremely low temperature region of about −40 ° C. When the electrolytic solution solidifies, it is considered that the output is significantly reduced.

On the other hand, EMC has a freezing point of about −55 ° C. Therefore, it is expected that the output will be improved in the extremely low temperature region due to the high EMC ratio. However, when the EMC ratio becomes high, the reaction becomes active at the time of overcharging, and it is considered that the calorific value increases.

An object of the present disclosure is to achieve both output in a cryogenic region and suppression of heat generation during overcharging.

Hereinafter, the technical configuration and the action and effect of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of claims should not be limited by the correctness of the mechanism of action.

[1] The lithium ion secondary battery includes at least a positive electrode mixture layer, a negative electrode mixture layer, an inorganic filler layer, a separator and an electrolytic solution.
The positive electrode mixture layer contains at least a positive electrode active material and at least 1.4% by mass or more and 6% by mass or less of trilithium phosphate.
The separator is arranged between the positive electrode mixture layer and the negative electrode mixture layer.
The inorganic filler layer is arranged between the positive electrode mixture layer and the separator.
The electrolytic solution contains at least ethyl methyl carbonate and dimethyl carbonate.
The ratio of the volume of ethyl methyl carbonate to the volume of dimethyl carbonate is 0.94 or more and 2.11 or less.

In the configuration of the above [1], the ratio of the volume of EMC to the volume of DMC (hereinafter, may be abbreviated as “EMC / DMC ratio”) is 0.94 or more and 2.11 or less. If the EMC / DMC ratio deviates from the range of 0.94 or more and 2.11 or less, the output may decrease in the extremely low temperature region. Further, if the EMC / DMC ratio deviates from the range of 0.94 or more and 2.11 or less, the suppression of heat generation at the time of overcharging may be insufficient.

In the configuration of the above [1], the trilithium phosphate (Li ₃ PO ₄ ) content of the positive electrode mixture layer is 1.4% by mass or more and 6.0% by mass or less. This is expected to suppress heat generation during overcharging.

In the configuration of the above [1], the inorganic filler layer is arranged between the positive electrode mixture layer and the separator. The electrolytic solution can be retained in the inorganic filler layer. Therefore, since the inorganic filler layer is arranged between the positive electrode mixture layer and the separator, the electrolytic solution is abundantly present in the vicinity of the positive electrode active material. As a result, it is possible to suppress a decrease in the concentration of Li ions in the vicinity of the positive electrode active material at the time of output in the extremely low temperature region. That is, it is expected that the output will be improved in the extremely low temperature range.

Due to the synergistic effect of the above actions, in the configuration of the above [1], it is expected that both the output in the cryogenic region and the suppression of heat generation during overcharging are compatible.

[2] The electrolytic solution may further contain ethylene carbonate. The dimethyl carbonate may have a ratio of 28% by volume or more and 36% by volume or less with respect to the total of ethylene carbonate, ethylmethyl carbonate and dimethyl carbonate.

In the three-component mixed system of EC, EMC and DMC, the difference between the output at −30 ° C. and the output at −45 ° C. is remarkable. That is, the output drops significantly from −30 ° C. to −45 ° C. According to the configuration of the above [2], it is expected that the output decrease from -30 ° C to −45 ° C is suppressed.

FIG. 1 is a schematic view showing an example of the configuration of the battery of the present embodiment. FIG. 2 is a schematic view showing an example of the configuration of the electrode group. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. FIG. 4 is a graph showing the relationship between the EMC / DMC ratio and the IV resistance (−45 ° C.). FIG. 5 is a graph showing the relationship between the EMC / DMC ratio and ΔT. FIG. 6 is a graph showing the relationship between the DMC ratio and the IV resistance ratio (−45 ° C./-30 ° C.).

Hereinafter, embodiments of the present disclosure (which may be abbreviated as "the present embodiment" in the present specification) will be described. However, the following description does not limit the scope of claims.

<Lithium-ion secondary battery>
FIG. 1 is a schematic view showing an example of the configuration of the battery of the present embodiment. The battery 100 includes a case 90. The case 90 is square (flat rectangular parallelepiped). However, the case (exterior body) should not be limited to a square shape. The case may be cylindrical, for example. The case 90 includes a container 91 and a lid 92. The lid 92 is joined to the container 91 by, for example, laser welding.

The case 90 houses the electrode group 50 and the electrolytic solution. The alternate long and short dash line in FIG. 1 indicates the liquid level of the electrolytic solution. The electrolytic solution is also present in the electrode group 50. The case 90 is hermetically sealed. The case 90 may be made of, for example, an aluminum (Al) alloy or the like. However, the case may be, for example, a pouch made of an aluminum laminated film or the like as long as a predetermined airtightness can be ensured.

The lid 92 includes a positive electrode terminal 81 and a negative electrode terminal 82. The lid 92 may include, for example, a liquid injection hole, a gas discharge valve, a current shutoff mechanism (CID), and the like.

The container 91 may be pressed from both sides in the Y-axis direction of FIG. That is, the battery 100 may be constrained. The pressing force may be, for example, 30 to 1000 kgf.

FIG. 2 is a schematic view showing an example of the configuration of the electrode group. The electrode group 50 is a winding type. That is, the electrode group 50 is configured by laminating the positive electrode 10, the separator 30, the negative electrode 20, and the separator 30 in this order and winding them in a spiral shape. The positive electrode 10 is electrically connected to the positive electrode terminal 81 (FIG. 1). The negative electrode 20 is electrically connected to the negative electrode terminal 82 (FIG. 1). The electrode group may be a laminated type (stack type). The laminated electrode group can be configured by alternately laminating the positive electrode and the negative electrode while sandwiching the separator between the positive electrode and the negative electrode.

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22. The separator 30 is arranged between the positive electrode mixture layer 12 and the negative electrode mixture layer 22. Further, the inorganic filler layer 40 is arranged between the positive electrode mixture layer 12 and the separator 30. The positive electrode mixture layer 12, the negative electrode mixture layer 22, the inorganic filler layer 40, and the separator 30 are each impregnated with an electrolytic solution. That is, the battery 100 includes at least a positive electrode mixture layer, a negative electrode mixture layer, an inorganic filler layer, a separator, and an electrolytic solution.

《Positive electrode》
In the present embodiment, the positive electrode 10 is a strip-shaped sheet. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12. The positive electrode current collector 11 may have an arbitrary shape depending on the specifications of the battery 100. The positive electrode current collector 11 may be, for example, an Al foil, a titanium (Ti) foil, a stainless steel foil, or the like. The positive electrode current collector 11 may have a thickness of, for example, 10 to 100 μm.

The positive electrode mixture layer 12 is formed on the surface of the positive electrode current collector 11. The positive electrode mixture layer 12 may be formed on both the front and back surfaces of the positive electrode current collector 11. The positive electrode mixture layer 12 may have a thickness of, for example, 10 to 200 μm. In the present specification, the "thickness" of each configuration can be measured by, for example, a micrometer or the like. The thickness of each configuration may be measured in a cross-sectional microscope image or the like of each configuration. The thickness can be measured at least 3 times. At least three arithmetic means can be adopted as the measurement result.

The positive electrode mixture layer 12 may have a density of, for example, 2.30 to 2.76 g / cm ^3. The positive electrode mixture layer 12 may have a density of, for example, 2.34 g / cm ³ or more, or may have a density of 2.42 g / cm ³ or more. The positive-electrode mixture layer 12 may be, for example, have a density of 2.62 g / cm ³ or less may have a density of 2.55 g / cm ³ or less. The "density" of the positive electrode mixture layer 12 in the present specification indicates an apparent density. That is, the density can be calculated by dividing the mass of the positive electrode mixture layer 12 by the apparent volume of the positive electrode mixture layer 12. The apparent volume is calculated by multiplying the thickness of the positive electrode mixture layer 12 by the area of the positive electrode mixture layer 12. The same applies to the density of the negative electrode mixture layer 22 described later.

The positive electrode mixture layer 12 is porous. The positive electrode mixture layer 12 contains at least the _{positive electrode active material and Li 3} PO _4. The positive electrode mixture layer 12 may contain, for example, a positive electrode active material, a conductive material, Li ₃ PO _4, and a binder.

The positive electrode mixture layer 12 may contain, for example, 80 to 95% by mass of the positive electrode active material. From the viewpoint of the energy density of the battery 100, the positive electrode mixture layer 12 may contain 85% by mass or more of the positive electrode active material. The positive electrode active material can be particles. The positive electrode active material may have an average particle size of, for example, 1 to 30 μm. The "average particle size" in the present specification indicates a particle size in which the cumulative particle volume from the fine particle side is 50% of the total particle volume in the volume-based particle size distribution measured by the laser diffraction / scattering method. Such an average particle size is also referred to as "D50".

The positive electrode active material electrochemically occludes and releases Li ions. The positive electrode active material should not be particularly limited. The positive electrode active material can have various crystal structures. The positive electrode active material may have a crystal structure such as a layered rock salt type, a spinel type, or an olivine type. The positive electrode active material may be, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiFePO _4, or the like. One kind of positive electrode active material may be used alone, or two or more kinds of positive electrode active materials may be used in combination. From the viewpoint of the output in the extremely low temperature region and the energy density of the battery 100, it is desirable that the positive electrode active material has a layered rock salt type crystal structure.

The positive electrode active material has a layered rock salt type crystal structure and has the following formula (I):
_{Li a Ni x Co y Mn z} O 2 ... (I)
(However, in the formula, a, x, y, z are 1 <a <1.2, 0 <x <1, 0 <y <1, 0 <z <1, x> z, y> z, x + y + z = 1 is satisfied.)
May be represented by.

Examples of the positive electrode active material represented by the above formula (I) include Li _1.1 Ni _0.7 Co _0.2 Mn _0.1 O ₂ , Li _1.1 Ni _0.6 Co _0.3 Mn _0.1 O ₂ , Li _1.1 Ni _0.5 Co _0.3 Mn _0.2 O ₂ . Examples thereof include _{Li 1.1} Ni _0.4 Co _0.4 Mn _0.2 O ₂ and Li _1.1 Ni _0.4 Co _0.3 Mn _0.1 O _2.

The positive electrode active material may be doped with an element other than, for example, Li, Ni (nickel), Co (cobalt), Mn (manganese) and O (oxygen). The doping elements include, for example, Mg (magnesium), Al, Si (silicon), Ca (calcium), Ti, Zn (zinc), Ga (gallium), Y (yttrium), Zr (zirconium), Nb (niobium), etc. It may be at least one selected from the group consisting of Mo (molybdenum), Hf (hafnium), W (tungsten), Ce (cerium), Sm (samarium), Eu (europium) and Yb (yttrium). The positive electrode active material may be doped with, for example, 0.8 to 1.2% by mass of W. As a result, for example, it is expected that the output will be improved in the extremely low temperature region.

The positive electrode mixture layer 12 contains _{Li 3} PO ₄ of 1.4% by mass or more and 6.0% by mass or less. This is expected to suppress heat generation during overcharging. Li ₃ PO ₄ may be uniformly dispersed throughout the positive electrode mixture layer 12. Li ₃ PO ₄ may be arranged, for example, on the surface of the positive electrode active material. The positive electrode mixture layer 12 may contain, for example, 1.4% by mass or more and 2.8% by mass or less of Li ₃ PO ₄ , or 2.8% by mass or more and 6.0% by mass or less of Li ₃ PO ₄ . It may be included. Li ₃ PO ₄ may have an average particle size of, for example, 0.05 to 50 μm.

The positive electrode mixture layer 12 may contain, for example, 3 to 15% by mass of a conductive material. The conductive material assists the electron conduction in the positive electrode mixture layer 12. The conductive material should not be particularly limited. The conductive material may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, carbon black (for example, acetylene black or the like), activated carbon, carbon fiber, carbon nanotubes or the like. One kind of conductive material may be used alone, or two or more kinds of conductive materials may be used in combination. The positive electrode mixture layer 12 may contain, for example, 5% by mass or more of a conductive material. This is expected to improve the output, for example, in the extremely low temperature range. This is thought to be because the ohm loss at the time of output becomes small.

The positive electrode mixture layer 12 may contain, for example, 1 to 10% by mass of a binder. The binder binds the components in the positive electrode mixture layer 12 to each other, and binds the positive electrode mixture layer 12 and the positive electrode current collector 11. The binder should not be particularly limited. The binder is, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer [poly (VDF-co-HFP)], polyacrylic acid (PAA), carboxymethyl cellulose. It may be (CMC) or the like. One type of binder may be used alone, or two or more types of binders may be used in combination.

《Negative electrode》
In the present embodiment, the negative electrode 20 is a strip-shaped sheet. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22. The negative electrode current collector 21 may have an arbitrary shape depending on the specifications of the battery 100. The negative electrode current collector 21 may be, for example, a copper (Cu) foil, a Ti foil, a stainless steel foil, or the like. The negative electrode current collector 21 may have a thickness of, for example, 10 to 100 μm. The negative electrode mixture layer 22 may have a density of, for example, 0.8 to 2.0 g / cm ^3.

The negative electrode mixture layer 22 is formed on the surface of the negative electrode current collector 21. The negative electrode mixture layer 22 may be formed on both the front and back surfaces of the negative electrode current collector 21. The negative electrode mixture layer 22 may have a thickness of, for example, 10 to 200 μm.

The negative electrode mixture layer 22 is porous. The negative electrode mixture layer 22 contains at least the negative electrode active material. The negative electrode mixture layer 22 may contain, for example, a negative electrode active material and a binder.

The negative electrode mixture layer 22 may contain, for example, 80 to 99.5% by mass of the negative electrode active material. The negative electrode active material can be particles. The negative electrode active material may have an average particle size of, for example, 1 to 30 μm. The negative electrode active material electrochemically occludes and releases Li ions. The negative electrode active material should not be particularly limited. The negative electrode active material may be, for example, a carbon material (typically graphite) containing a crystal structure in which carbon hexagonal mesh surfaces are laminated, silicon, silicon oxide, a silicon-based alloy, tin, a tin-based alloy, or the like. One kind of negative electrode active material may be used alone, or two or more kinds of negative electrode active materials may be used in combination.

The negative electrode mixture layer 22 may contain, for example, 0.5 to 20% by mass of a binder. Binders should not be particularly limited. The binder may be, for example, CMC, styrene butadiene rubber (SBR), PAA or the like. One type of binder may be used alone, or two or more types of binders may be used in combination. The negative electrode mixture layer 22 may further contain a conductive material. The conductive material of the negative electrode mixture layer 22 may be, for example, carbon black, carbon fiber, or the like.

《Separator》
In the present embodiment, the separator 30 is a strip-shaped sheet. The separator 30 is arranged between the positive electrode mixture layer 12 and the negative electrode mixture layer 22. The separator 30 is electrically insulating. The separator 30 electrically separates the positive electrode mixture layer 12 and the negative electrode mixture layer 22. The separator 30 is porous. The separator 30 allows the electrolytic solution to permeate. The separator 30 may have a thickness of, for example, 10 to 50 μm.

The separator 30 can be, for example, a porous membrane made of polyethylene (PE), polypropylene (PP), or the like. The separator 30 may have a single-layer structure or a multi-layer structure (for example, a three-layer structure). The separator 30 may be composed of, for example, only a porous membrane made of PE. The separator 30 may be configured by, for example, laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order.

《Inorganic filler layer》
The inorganic filler layer 40 is arranged between the positive electrode mixture layer 12 and the separator 30. The inorganic filler layer 40 may be formed on the surface of the separator 30, may be formed on the surface of the positive electrode mixture layer 12, or may be formed on both the surface of the separator 30 and the surface of the positive electrode mixture layer 12. It may be formed. The inorganic filler layer 40 can be formed, for example, by applying a paste containing an inorganic filler to the surface of the separator 30 and drying it. The inorganic filler layer 40 may have a thickness of, for example, 0.5 to 10 μm.

As long as the inorganic filler layer 40 is arranged between the positive electrode mixture layer 12 and the separator 30, the inorganic filler layer 40 may also be arranged between the negative electrode mixture layer 22 and the separator 30. However, when the inorganic filler layer 40 is arranged between the negative electrode mixture layer 22 and the separator 30, the coating formed on the surface of the negative electrode mixture layer 22 and the inorganic filler layer 40 are in close contact with each other, resulting in an effective reaction. The area may be reduced. As a result, the output in the cryogenic region may decrease. Therefore, in the present embodiment, it is desirable that the inorganic filler layer 40 is arranged only between the positive electrode mixture layer 12 and the separator 30.

The inorganic filler layer 40 is porous. The inorganic filler layer 40 can retain the electrolytic solution abundantly. By arranging the inorganic filler layer 40 between the positive electrode mixture layer 12 and the separator 30, it is possible to suppress a decrease in the concentration of Li ions in the vicinity of the positive electrode active material at the time of output in a cryogenic region. .. The inorganic filler layer 40 may have a surface density (base weight) of, for example, 1 to 20 g / cm ^2. In such a range, the inorganic filler layer 40 is expected to have an electrolytic solution holding ability suitable for the present embodiment.

The inorganic filler layer 40 contains an inorganic filler. As used herein, "inorganic filler" refers to particles of an inorganic compound that are not involved in the charge / discharge reaction. The inorganic filler may have a melting point higher than the melting point of the constituent material of the separator 30. In that case, for example, heat resistance can be imparted to the separator 30 by forming the inorganic filler layer 40 on the surface of the separator 30.

The inorganic filler may have an average particle size of, for example, 0.1 to 5 μm. The inorganic filler may be, for example, alumina (for example, α-alumina), titania, boehmite, magnesia, zirconia or the like. One kind of inorganic filler may be used alone, or two or more kinds of inorganic fillers may be used in combination. The shape of the inorganic filler should not be particularly limited. The inorganic filler can be, for example, spherical, lumpy, needle-like, fibrous, flake-like or the like.

The inorganic filler layer 40 may further contain a binder. The inorganic filler layer 40 may contain, for example, 70 to 98% by mass of the inorganic filler and 2 to 30% by mass of the binder. The binder of the inorganic filler layer 40 may be exemplified as a binder of the positive electrode mixture layer 12 and the negative electrode mixture layer 22. The binder may be, for example, a polyimide, an ethylene-acrylic acid ester copolymer, or the like.

《Electrolytic solution》
The electrolytic solution contains a solvent and a Li salt. The solvent is aprotic. Solvents include EMC and DMC. That is, the electrolytic solution contains at least EMC and DMC. The sum of EMC and DMC in the solvent may have a ratio of, for example, 60-95% by volume.

The EMC / DMC ratio is 0.94 or more and 2.11 or less. If the EMC / DMC ratio is out of the range of 0.94 or more and 2.11 or less, the output may decrease in the extremely low temperature region. Further, if the EMC / DMC ratio is out of the range of 0.94 or more and 2.11 or less, the suppression of heat generation at the time of overcharging may be insufficient. The EMC / DMC ratio may be, for example, 1.05 or more, 1.06 or more, 1.12 or more, or 1.37 or more. .. The EMC / DMC ratio may be, for example, 1.79 or less, or 1.50 or less.

The solvent may further contain components other than EMC and DMC. The solvent may contain, for example, a high dielectric constant component. Examples of the high dielectric constant component include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL) and the like. The high dielectric constant component in the solvent may have a ratio of, for example, 5-40% by volume.

That is, the electrolytic solution may further contain EC. In that case, the DMC may have, for example, a ratio of 28% by volume or more and 36% by volume or less with respect to the total of EC, EMC and DMC. This is expected to suppress the decrease in output from −30 ° C. to −45 ° C. The DMC may have, for example, a ratio of 28% by volume or more and 34% by volume or less with respect to the total of EC, EMC and DMC.

The Li salt is dissolved in the solvent. The electrolytic solution may contain, for example, 0.5 to 2 mL / l Li salt. The electrolytic solution may contain, for example, 0.8 mL / l or more of Li salt, or 1.0 mL / l or more of Li salt. The electrolytic solution may contain, for example, a Li salt of 1.5 mL / l or less, or may contain a Li salt of 1.2 mL / l or less. It is expected that the more Li salts are dissolved in the electrolytic solution, the greater the freezing point depression of the electrolytic solution. Due to the freezing point depression, the output is expected to improve in the extremely low temperature range.

Li salts include, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li [N (SO ₂ F) ₂ ] (LiFSI), Li [N (CF ₃ SO _2). ) ₂ ] etc. may be used. Of these, LiPF ₆ tends to have high solubility and ion mobility. When the Li salt is LiPF ₆ , for example, improvement of charge / discharge characteristics is expected. One kind of Li salt may be used alone, or two or more kinds of Li salt may be used in combination.

The electrolytic solution may further contain various functional additives. The electrolytic solution may contain, for example, 0.1 to 5% by mass of a functional additive. The functional additive may be, for example, a film-forming agent, a gas generating agent (overcharge additive), or the like. Examples of the functional additive include vinylene carbonate (VC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), lithium bisoxalatoborate (LiBOB), lithium difluorophosphate, lithium fluorosulfonate, and cyclohexylbenzene. (CHB), biphenyl (BP), propansalton (PS), ethylene sulphite (ES) and the like can be mentioned.

<< Applications >>
The battery 100 of the present embodiment is expected to exhibit high output in a cryogenic region. The battery 100 of the present embodiment is expected to generate a small amount of heat even when it is misused such as when it is overcharged. Examples of applications in which such performance is utilized include driving power sources for hybrid vehicles (HVs), plug-in hybrid vehicles (PHVs), electric vehicles (EVs), and the like. However, the use of the battery 100 of the present embodiment should not be limited to a power source for driving an automobile. The battery 100 of this embodiment can be applied to all uses.

Hereinafter, examples of the present disclosure will be described. However, the following description does not limit the scope of claims.

<Experiment 1>
<< Example 1-1 >>
By mixing the positive electrode active material (represented by the above formula (I)), the conductive material (acetylene black), Li ₃ PO ₄ , the binder (PVdF) and the solvent (N-methyl-2-pyrrolidone), A positive electrode paste was prepared. The positive electrode paste was applied to the surface (both front and back surfaces) of the positive electrode current collector (Al foil) and dried to form a positive electrode mixture layer. The positive electrode mixture layer was compressed so that the positive electrode mixture layer had a predetermined density. As a result, a positive electrode was manufactured. The positive electrode was cut into strips.

A negative electrode paste was prepared by mixing a negative electrode active material (graphite), a binder (CMC, SBR) and a solvent (water). The negative electrode paste was applied to the surface (both front and back surfaces) of the negative electrode current collector (Cu foil) and dried to form a negative electrode mixture layer. The negative electrode mixture layer was compressed so that the negative electrode mixture layer had a predetermined density. As a result, a negative electrode was manufactured. The negative electrode was cut into strips.

A strip-shaped separator was prepared. The separator has a three-layer structure. That is, the separator is configured by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order. The paste was prepared by mixing the inorganic filler, binder and solvent. The paste was applied to the surface (one side) of the separator and dried to form an inorganic filler layer.

The positive electrode, the separator, the negative electrode and the separator were laminated in this order and spirally wound. As a result, a group of electrodes was manufactured. The inorganic filler layer formed on the separator was arranged so as to face the positive electrode. That is, the inorganic filler layer was arranged between the positive electrode mixture layer and the separator. The electrode group was formed into a flat shape.

A square case was prepared. The electrode group was housed in a case. The electrolyte was injected into the case. In Experiment 1, the electrolyte contains 1.2 mol / l LiPF ₆ . The case was sealed. From the above, a battery (square lithium ion secondary battery) was manufactured. Batteries are designed to be used in the voltage range of 3.0 to 4.1 V. The batteries have undergone the prescribed aging. The initial charge and discharge of the battery was carried out. In this experiment, the initial discharge capacity was the rated capacity.

<< Example 1-2 to Example 1-7, Comparative Example 1-1 to Comparative Example 1-10 >>
Batteries having the configurations shown in Table 1 below were manufactured. In the column of "inorganic filler layer" in Table 1 below, for example, "between the positive electrode and the separator" indicates that the inorganic filler layer is arranged between the positive electrode mixture layer and the separator.

《Low temperature output test》
In this specification, "C" is used as a unit of electric current. The current of "1C" discharges the rated capacity in 1 hour. The battery voltage was adjusted to 3.7V. The batteries were placed in a constant temperature bath set at −45 ° C. The battery was left in the same environment for 6 hours. After 6 hours, the battery was discharged by a current of 10C. The amount of voltage drop 0.1 seconds after the start of discharge was measured. The DC resistance (also referred to herein as "IV resistance") was calculated by dividing the voltage drop by the current. The results are shown in Table 1 above. The values shown in the IV resistance column of Table 1 above are relative values when the IV resistance of Example 1-7 is 100%. The smaller the value, the higher the output in the cryogenic region.

《Overcharge test》
A thermocouple (temperature sensor) was attached to the surface of the battery. The battery temperature was adjusted to -10 ° C. The battery was charged to a predetermined voltage exceeding 4.1 V by a current of 10 C. After reaching the predetermined voltage, the battery was left for 5 minutes. The difference (ΔT) between the temperature of the battery after 5 minutes and the temperature of the battery when the predetermined voltage was reached was calculated. The results are shown in Table 1 above. The smaller ΔT is, the more the heat generation during overcharging is suppressed.

《Evaluation》
Based on the results of the low temperature output test and the overcharge test, it was determined which of the following ranks A to D each battery belonged to. It is considered that the battery having the rank "A" has both the output in the extremely low temperature region and the suppression of heat generation at the time of overcharging.

A: The IV resistance (−45 ° C.) is less than 111% and ΔT is less than 26 ° C.
B: IV resistance (−45 ° C.) is less than 111% and ΔT is 26 ° C. or higher.
C: IV resistance (−45 ° C.) is 111% or more, and ΔT is less than 26 ° C.
D: IV resistance (−45 ° C.) is 111% or more, and ΔT is 26 ° C. or more.

"result"
As shown in Table 1 above, in the examples satisfying all of the following conditions (1) to (3), both the output in the cryogenic region and the suppression of heat generation during overcharging are compatible (that is, the rank is "". A "). On the other hand, in the comparative example in which any one or more of the following conditions (1) to (3) is not satisfied, the output in the cryogenic region and the suppression of heat generation at the time of overcharging are not compatible (that is, the rank is "B". ~ D ").

(1) The positive electrode mixture layer contains _{Li 3} PO ₄ of 1.4% by mass or more and 6.0% by mass or less.
(2) The inorganic filler layer is arranged between the positive electrode mixture layer and the separator.
(3) The EMC / DMC ratio is 0.94 or more and 2.11 or less.

FIG. 4 is a graph showing the relationship between the EMC / DMC ratio and the IV resistance (−45 ° C.). FIG. 5 is a graph showing the relationship between the EMC / DMC ratio and ΔT. In the range where the EMC / DMC ratio is 0.94 or more and 2.11 or less, both the output in the extremely low temperature range and the suppression of heat generation at the time of overcharging are compatible.

<Experiment 2>
<< Example 2-1 to Example 2-5, Comparative Example 2-1 to Comparative Example 2-4 >>
Batteries having the configurations shown in Table 1 above were manufactured. In Experiment 2, the electrolyte contains 0.8 mL / l of LiPF ₆ .

《Low temperature output test》
The IV resistance at −45 ° C. was measured in the same manner as in “Experiment 1” above. The IV resistance at -30 ° C was measured as well as the IV resistance at −45 ° C, except that the measurement temperature was changed to −30 ° C. The IV resistance ratio was calculated by dividing the IV resistance at −45 ° C. by the IV resistance at −30 ° C. The results are shown in Table 1 above. The smaller the IV resistivity ratio, the more the decrease in output from −30 ° C. to −45 ° C. is suppressed.

FIG. 6 is a graph showing the relationship between the DMC ratio and the IV resistance ratio (−45 ° C./-30 ° C.). In the range where the DMC ratio is 28% by volume or more and 36% by volume or less, the IV resistance ratio is remarkably lowered.

The embodiments and examples disclosed this time are exemplary in all respects and are not restrictive. The technical scope defined by the description of the claims includes all changes within the meaning and scope equivalent to the description of the claims.

10 positive electrode, 11 positive electrode current collector, 12 positive electrode mixture layer, 20 negative electrode, 21 negative electrode current collector, 22 negative electrode mixture layer, 30 separator, 40 inorganic filler layer, 50 electrode group, 81 positive electrode terminal, 82 negative electrode terminal, 90 cases, 91 containers, 92 lids, 100 batteries (lithium ion secondary batteries).

Claims (2)

Containing at least a positive electrode mixture layer, a negative electrode mixture layer, an inorganic filler layer, a separator and an electrolytic solution,
The positive electrode mixture layer contains at least a positive electrode active material and at least 1.4% by mass or more and 6.0% by mass or less of trilithium phosphate.
The separator is arranged between the positive electrode mixture layer and the negative electrode mixture layer.
The inorganic filler layer is arranged between the positive electrode mixture layer and the separator.
The inorganic filler layer is porous and
The inorganic filler layer has a surface density of ^{1 g / cm 2} or more and 20 g / cm ^{2 or less.}
The electrolytic solution contains at least ethyl methyl carbonate and dimethyl carbonate and contains at least.
The ratio of the volume of ethyl methyl carbonate to the volume of dimethyl carbonate is 0.94 or more and 2.11 or less.
Lithium-ion secondary battery. The electrolytic solution further contains ethylene carbonate and contains
Dimethyl carbonate has a ratio of 28% by volume or more and 36% by volume or less with respect to the total of ethylene carbonate, ethylmethyl carbonate and dimethyl carbonate.
The lithium ion secondary battery according to claim 1.

Images