Lithium-ion battery

Abstract

To provide a lithium ion battery having a superior high-rate discharge characteristic.SOLUTION: A lithium ion battery according to the present invention comprises a cathode, an anode, a separator disposed between the cathode and anode, and an ionic liquid electrolyte containing a lithium salt. The cathode has a cathode collector sheet, and a porous cathode active material layer provided on the cathode collector sheet. The cathode active material layer has a thickness of 1 μm or more and 100 μm or less. In the ionic liquid electrolyte, concentration of the lithium salt is 1.6 mol / L or more and 4.0 mol / L or less. A quantity of the lithium salt in the ionic liquid electrolyte in pores of the cathode active material layer per specific surface area of the cathode active material layer, measured by a method of mercury penetration is 4.9×10-5 mol / m2 or more and 13.0×10-5 mol / m2 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to lithium-ion batteries. [Background technology]

[0002] Due to their high energy density, lithium-ion batteries are widely used in electronic and electrical devices such as smartphones and laptops. Lithium-ion batteries generally use a flammable non-aqueous electrolyte, in which lithium salts are dispersed in a non-aqueous solvent. Lithium-ion batteries can generate heat due to overcharging or short circuits between the positive and negative electrodes. Furthermore, the positive electrode active material can release oxygen from its crystals due to thermal decomposition or overcharging. This puts lithium-ion batteries at risk of abnormal heat generation and fire. In order to prevent accidents caused by abnormal heat generation and fire, it has been proposed to use ionic liquids as electrolyte solvents in lithium-ion batteries (see, for example, Patent Document 1). Ionic liquids are liquids composed of anions and cations, and generally have low vapor pressure and are non-flammable. Therefore, the use of ionic liquids as electrolyte solvents can improve the safety of lithium-ion batteries. On the other hand, because electric vehicle batteries and engine starter batteries need to output a large discharge current, lithium-ion batteries with excellent high-rate discharge characteristics are required. Generally, a battery is considered to have a high-rate C-rate (the speed of charging and discharging; in the case of constant current charge / discharge measurements, 1 C is defined as the current that fully charges (or discharges) the battery's theoretical capacity in one hour) of 5 C or higher. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-195129 Summary of the Invention [Problem to be solved by the invention]

[0004] The conductivity of lithium ions in ionic liquid electrolytes is relatively low. Furthermore, the viscosity of ionic liquid electrolytes is relatively high. This can cause a bottleneck in the rate of lithium ion migration between the positive and negative electrodes during discharge, resulting in a deterioration of the high-rate discharge performance of lithium-ion batteries. The present invention has been made in view of the above circumstances, and provides a lithium ion battery having excellent high-rate discharge characteristics. [Means for solving the problem]

[0005] The present invention provides a battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an ionic liquid electrolyte containing a lithium salt, wherein the positive electrode has a positive electrode current collector sheet and a porous positive electrode active material layer provided on the positive electrode current collector sheet, the positive electrode active material layer having a thickness of 1 μm or more and 100 μm or less, the concentration of the lithium salt in the ionic liquid electrolyte being 1.6 mol / L or more and 4.0 mol / L or less, and the amount of substance of the lithium salt in the ionic liquid electrolyte in pores of the positive electrode active material layer per specific surface area of ​​the positive electrode active material layer measured by mercury intrusion porosimetry being 4.9 × 10 -5 mol / m 2 Over 13.0 x 10 -5 mol / m 2 The present invention provides a lithium-ion battery characterized by the following: [Effects of the Invention]

[0006] The lithium ion battery of the present invention has the above-mentioned characteristics and therefore has excellent high-rate discharge characteristics. This was made clear by experiments conducted by the present inventors. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic cross-sectional view of a lithium-ion battery according to one embodiment of the present invention. [Figure 2] 1 is a partial cross-sectional view of a positive electrode included in a lithium ion battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0008] The lithium ion battery of the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an ionic liquid electrolyte containing a lithium salt. The positive electrode has a positive electrode current collector sheet and a porous positive electrode active material layer provided on the positive electrode current collector sheet. The thickness of the positive electrode active material layer is 1 μm or more and 100 μm or less. The concentration of the lithium salt in the ionic liquid electrolyte is 1.6 mol / L or more and 4.0 mol / L or less. The amount of substance of the lithium salt in the ionic liquid electrolyte in the pores of the positive electrode active material layer per specific surface area of ​​the positive electrode active material layer measured by mercury intrusion porosimetry is 4.9 × 10 -5 mol / m 2 Over 13.0 x 10 -5 mol / m 2 The present invention is characterized by the following:

[0009] The ionic liquid contained in the ionic liquid electrolyte is preferably MPP-FSI or EMI-FSI. The lithium salt is preferably LiFSI. The positive electrode active material layer preferably contains lithium iron phosphate as the positive electrode active material. The negative electrode preferably contains carbon as a negative electrode active material.

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0011] FIG. 1 is a schematic cross-sectional view of the lithium ion battery of this embodiment, and FIG. 2 is a partial cross-sectional view of the positive electrode.

[0012] The lithium ion battery 20 of this embodiment includes a positive electrode 2, a negative electrode 3, a separator 4 disposed between the positive electrode 2 and the negative electrode 3, and an ionic liquid electrolyte 5 containing a lithium salt. The positive electrode 2 has a positive electrode current collector sheet 6 and a porous positive electrode active material layer 7 provided on the positive electrode current collector sheet 6. The thickness of the positive electrode active material layer 7 is 1 μm or more and 100 μm or less. The concentration of the lithium salt in the ionic liquid electrolyte 5 is 1.6 mol / L or more and 4.0 mol / L or less. The amount of substance of the lithium salt in the ionic liquid electrolyte 5 in the pores of the positive electrode active material layer 7 per specific surface area of ​​the positive electrode active material layer 7 measured by mercury intrusion porosimetry is 4.9 × 10 -5 mol / m 2 Over 13.0 x 10 -5 mol / m 2 The present invention is characterized by the following: The lithium ion battery 20 may be a primary battery or a secondary battery.

[0013] The positive electrode 2 has a positive electrode current collector sheet 6 and a porous positive electrode active material layer 7 provided on the positive electrode current collector sheet 6 . The positive electrode current collector sheet 6 is a sheet that serves as a base material for providing the positive electrode active material layer 7, and is a conductor that electrically connects the positive electrode battery terminal (for example, the positive electrode can 16) and the positive electrode active material layer 7. The positive electrode current collector sheet 6 is, for example, aluminum foil.

[0014] The positive electrode active material layer 7 is a porous layer containing a positive electrode active material. The positive electrode active material layer 7 may be provided on one side of the positive electrode current collector sheet 6, or on both sides of the positive electrode current collector sheet 6. The thickness of the positive electrode active material layer 7 (the length from the contact surface between the positive electrode current collector sheet 6 and the positive electrode active material layer 7 to the surface of the positive electrode active material layer 7) is 1 μm or more and 100 μm or less. By making the thickness of the positive electrode active material layer 7 1 μm or more, the amount of the positive electrode active material contained in the positive electrode 2 can be increased, thereby increasing the capacity of the lithium ion battery. In addition, the positive electrode active material layer 7 can be easily formed by coating. By setting the thickness of the positive electrode active material layer 7 to 100 μm or less, it is possible to shorten the migration distance (diffusion distance) of lithium ions between the vicinity of the interface between the positive electrode active material layer 7 and the positive electrode current collector sheet 6 and the vicinity of the surface of the positive electrode active material layer 7, thereby making it possible to prevent a shortage of lithium ions in the ionic liquid electrolyte in the pores near the interface during discharge and an excess of lithium ions in the ionic liquid electrolyte in the pores near the interface during charge.

[0015] For example, as shown in FIG. 2 , when an electrode reaction progresses on the surface of the positive electrode active material 9 of the positive electrode active material layer 7 due to discharge, lithium ions in the ionic liquid electrolyte 5 filling the pores 13 are inserted as lithium atoms into the positive electrode active material 9. This reduces the lithium ion concentration in the ionic liquid electrolyte 5 filling the pores 13, creating a difference in lithium ion concentration between the ionic liquid electrolyte 5 outside the positive electrode active material layer 7 and the ionic liquid electrolyte 5 filling the pores 13. Due to this concentration difference, the lithium ions contained in the ionic liquid electrolyte 5 outside the positive electrode active material layer 7 diffuse into the ionic liquid electrolyte 5 in the pores 13, and lithium ions are supplied to the ionic liquid electrolyte 5 in the pores 13 where the lithium ion concentration has decreased. However, if the thickness of the positive electrode active material layer 7 is thicker than 100 μm, the migration distance (diffusion distance) of the lithium ions becomes longer, and sufficient lithium ions are not supplied to the ionic liquid electrolyte 5 in the pores 13 near the interface between the positive electrode active material layer 7 and the positive electrode current collector sheet 6, which may slow down the rate of the electrode reaction. Therefore, by setting the thickness of the positive electrode active material layer 7 to 100 μm or less, it is possible to suppress such a decrease in the rate of the electrode reaction, and the high-rate discharge characteristics of the lithium ion battery 20 can be improved.

[0016] The specific surface area of ​​the positive electrode active material layer 7 (measured by mercury porosimetry) is 1 m 2 / g or more 10m 2 / g, which allows the surface of the positive electrode active material 9 where the electrode reaction proceeds to be widened, and the charge / discharge characteristics of the lithium ion battery 20 to be improved. The pore volume of the positive electrode active material layer 7 (pore volume measured by mercury intrusion porosimetry) can be set to 0.05 ml / g or more and 0.8 ml / g or less. This allows a larger amount of ionic liquid electrolyte 5 to fill the pores, improving the charge / discharge characteristics of the lithium ion battery 20. Furthermore, the positive electrode active material layer 7 can have sufficient physical strength.

[0017] The positive electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer in the positive electrode. The positive electrode active material contained in the positive electrode active material layer 7 is, for example, an olivine-type LiFePO4, Li x Fe 1-y M y PO4 (where 0.05≦x≦1.2, 0≦y≦0.8, and M is at least one of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb), LiCoO2, LiNiO2, LiNi x Co 1-x O2(x=0.01~0.99), LiMnO2, LiMn2O4, LiCo x Mn y Ni z O2(x+y+z=1), etc. The positive electrode active material layer 7 can contain one of these positive electrode active materials alone or a mixture of two or more of them.

[0018] The positive electrode active material layer 7 can have a porous structure in which powder of the positive electrode active material 9 is bonded with a binder. This allows the positive electrode active material layer 7 to have pores 13 between the positive electrode active material particles. These pores 13 are filled with the ionic liquid electrolyte 5, and an electrode reaction proceeds on the surface of the positive electrode active material particles. For example, when charging the lithium ion battery 20, the lithium atoms contained in the positive electrode active material particles become lithium ions (Li + ) into the ionic liquid electrolyte 5, and when the lithium ion battery 20 is discharged, the lithium ions of the ionic liquid electrolyte 5 are inserted into the positive electrode active material particles as lithium atoms.

[0019] The positive electrode active material particles contained in the positive electrode active material layer 7 may have a conductive coating 10 on their surfaces. This can improve the conductivity of the particle surfaces where the electrode reaction proceeds, thereby reducing the internal resistance of the positive electrode. The conductive coating is, for example, a carbon coating.

[0020] The positive electrode active material layer 7 may contain a conductive additive. This can improve the conductivity of the positive electrode active material layer 7 and reduce the internal resistance of the positive electrode 2. The conductive additive is, for example, acetylene black. Alternatively, the conductive additive may be fine particles of coke-based soft carbon, which is easily graphitizable carbon. The positive electrode active material layer 7 may contain a binder, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), sodium carboxymethyl cellulose (CMC), acrylonitrile rubber, or an acrylonitrile rubber-PTFE mixture.

[0021] For example, a paste is prepared by mixing a powder of a positive electrode active material, a conductive additive, and a binder, and this paste is applied to the positive electrode current collector sheet 6 (for example, by roll-to-roll coating). The applied layer is then dried and pressed to form the positive electrode active material layer 7. Examples of solvents used to prepare the paste include water, dimethylformamide, N-methylpyrrolidone, isopropanol, and toluene. Alternatively, a paste may be prepared by mixing a powder of positive electrode active material, a conductive additive, a binder, an ionic liquid electrolyte, and a solvent, and the paste may be applied to the positive electrode current collector sheet 6. The applied layer may then be dried and pressed to form the positive electrode active material layer 7. In this case, a gelled ionic liquid electrolyte may be used. This allows the positive electrode active material layer 7 to retain the ionic liquid electrolyte in the vicinity of the positive electrode active material.

[0022] The positive electrode active material layer 7 may not have been subjected to a pressing process (an unpressed positive electrode active material layer 7). This increases the pore volume in the positive electrode active material layer 7, and prevents a shortage or excess of lithium ions from occurring in the ionic liquid electrolyte 5 in the pores. The positive electrode active material layer 7 may be pressed at a compression ratio of 2% to 24%, which increases the density of the positive electrode active material in the positive electrode active material layer 7 and increases the battery capacity of the lithium ion battery.

[0023] The negative electrode 3 is an electrode containing a negative electrode active material. The negative electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer at the negative electrode. Examples of the negative electrode active material include carbon materials (soft carbon, hard carbon, graphite, etc.), metallic lithium, lithium titanate (LTO), and Sn alloys.

[0024] The negative electrode 3 can include a negative electrode current collector sheet 11 and a negative electrode active material layer 12 provided on the negative electrode current collector sheet 11. The negative electrode current collector sheet 11 is a sheet that serves as a base for providing the negative electrode active material layer 12, and is a conductor that electrically connects the negative electrode battery terminal (e.g., a negative electrode can 17) and the negative electrode active material layer 12. The negative electrode current collector sheet 11 is, for example, copper foil. The negative electrode active material layer 12 is a layer containing a negative electrode active material. The negative electrode active material layer 12 may be provided on one side of the negative electrode current collector sheet 11, or may be provided on each of both sides of the negative electrode current collector sheet 11. The negative electrode active material layer 12 may contain, for example, fine particles of the negative electrode active material. The negative electrode active material layer 12 may contain a binder, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), acrylonitrile rubber, or an acrylonitrile rubber-PTFE mixture. The negative electrode active material layer 12 may contain a thickener, such as sodium carboxymethyl cellulose (CMC).

[0025] For example, a paste is prepared by mixing a powder of the negative electrode active material, a binder, and a thickener, and this paste is applied to the negative electrode current collector sheet 11. The applied layer is then dried and pressed to form the negative electrode active material layer 12. Examples of solvents used to prepare the paste include dimethylformamide, N-methylpyrrolidone, isopropanol, and toluene.

[0026] The separator 4 is in the form of a sheet and is disposed between the positive electrode 2 and the negative electrode 3. The separator 4, together with the positive electrode 2 and the negative electrode 3, can form an electrode laminate as shown in FIG. 1. By providing the separator 4, it is possible to prevent a short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. The separator 4 is not particularly limited as long as it can prevent the flow of short-circuit current and is permeable to ions that conduct between the positive and negative electrodes. For example, it can be a polyolefin microporous film, a cellulose sheet, a glass filter, or a nonwoven or woven fabric made of fibers such as polyolefin or cellulose.

[0027] The ionic liquid electrolyte 5 is an ion-conducting medium between the positive electrode and the negative electrode, and contains an ionic liquid composed of anions and cations, and a lithium salt dissolved in the ionic liquid. Ionic liquids are liquids composed of anions and cations. Ionic liquids generally have low vapor pressure and are difficult to burn, so the use of ionic liquid electrolyte 5 can improve the safety of lithium-ion battery 20.

[0028] The ionic liquid contained in the ionic liquid electrolyte 5 is composed of, for example, a bis(fluorosulfonyl)imide ion (hereinafter referred to as FSI ion) as an anion and a pyrrolidinium ion as a cation. Specifically, the ionic liquid is composed of an FSI ion and a methylpropylpyrrolidinium ion (hereinafter referred to as MPP ion) (MPP-FSI or MPP-FSA). The ionic liquid contained in the ionic liquid electrolyte 5 is composed of, for example, FSI ions as anions and imidazolium ions as cations. Specifically, the ionic liquid is composed of FSI ions and ethylmethylimidazolium ions (hereinafter referred to as EMI ions) (EMI-FSI or EMI-FSA).

[0029] The lithium salt contained in the ionic liquid electrolyte 5 (lithium salt dissolved in the ionic liquid) is lithium bis(fluorosulfonyl)imide (hereinafter referred to as LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (hereinafter referred to as LiTFSI). By using such a lithium salt, the lithium salt can be dissolved in the ionic liquid at a relatively high concentration.

[0030] The concentration of the lithium salt in the ionic liquid electrolyte 5 can be 1.6 mol / L or more and 4.0 mol / L or less. By setting the lithium salt concentration to 1.6 mol / L or more, it is possible to prevent a shortage of lithium ions in the ionic liquid electrolyte 5 in the pores 13 of the positive electrode active material layer 7 during high-rate discharge (increase in concentration overvoltage), thereby improving the high-rate discharge characteristics of the lithium-ion battery 20. Furthermore, by setting the lithium salt concentration to 4.0 mol / L or less, it is possible to prevent precipitation of the lithium salt. Furthermore, if the lithium salt concentration is higher than 4.0 mol / L, the lithium salt becomes less soluble in the ionic liquid, making it difficult to prepare the ionic liquid electrolyte 5.

[0031] The amount of lithium salt in the ionic liquid electrolyte 5 in the pores 13 of the positive electrode active material layer 7 per specific surface area of ​​the positive electrode active material layer 7 measured by mercury porosimetry is 4.9 × 10 -5 mol / m 2 Over 13.0 x 10 -5 mol / m 2 The following is the result. The electrode reaction proceeds on the surface of the positive electrode active material 9. In other words, the larger the specific surface area of ​​the positive electrode active material layer 7, the faster the consumption rate of lithium ions in the ionic liquid electrolyte 5 in the pores 13. In addition, the ionic liquid electrolyte 5 has a relatively high viscosity, and the supply rate of lithium ions from the ionic liquid electrolyte 5 outside the positive electrode active material layer 7 to the ionic liquid electrolyte 5 in the pores 13 is relatively slow. For this reason, the amount of lithium salt in the ionic liquid electrolyte 5 in the pores 13 per specific surface area of ​​the positive electrode active material layer 7 (i.e., the contact area between the positive electrode active material 9 and the ionic liquid electrolyte 5) is 4.9 × 10 -5 mol / m 2 By setting the amount of lithium salt to 13.0×10 or more, it is possible to prevent a shortage of lithium ions (increase in concentration overvoltage) in the ionic liquid electrolyte 5 in the pores 13 of the positive electrode active material layer 7 during high-rate discharge, and to rapidly promote the electrode reaction accompanying high-rate discharge. -5 mol / m 2 By setting the following, it is possible to suppress interference of lithium ions near the surface of the positive electrode active material 9, and to suppress an increase in the energy required for the electrode reaction (an increase in concentration overpotential). As a result, it is possible to improve the high-rate discharge characteristics of the lithium-ion battery 20. This was made clear by experiments conducted by the present inventors.

[0032] Constant current charge / discharge test Lithium ion batteries of Samples 1 to 27 shown in Tables 1 to 6 were fabricated using CR2032 coin cells (diameter: 20 mm, height: 3.2 mm), and a constant current charge / discharge test was carried out. The discharge rate was 10C. A cathode active material paste was applied to one side of an aluminum foil (cathode current collector sheet) and dried to form a cathode active material layer, thereby preparing a cathode. All batteries in Samples 1 to 27 used lithium iron phosphate (LiFePO4, LFP) (with a conductive coating (carbon coating) on ​​the particle surface) as the cathode active material. The applied mass of the cathode active material and the thickness of the cathode active material layer for each sample are shown in Tables 1, 3, and 5. Additionally, the cathode active material layers in the batteries in Samples 15 to 17, 19, 20, 22 to 24, 26, and 27 were subjected to a press treatment. The compression ratio for each sample is shown in Table 5.

[0033] [Table 1]

[0034] [Table 2]

[0035] [Table 3]

[0036] [Table 4]

[0037] [Table 5]

[0038] [Table 6]

[0039] The specific surface area A (pore area) and pore volume V within the positive electrode active material layer of a positive electrode active material layer formed in the same manner (without pressing) were measured using a mercury intrusion porosimeter. The positive electrode active material layers of Samples 1 to 27 were formed using positive electrode active material pastes prepared in the same manner, and therefore had the same specific surface area per unit mass, as shown in Tables 2, 4, and 6. The positive electrode active material layers of Samples 1 to 14, 18, 21, and 25 had a compression ratio of 0%, and therefore had the same pore volume V, as shown in Tables 2, 4, and 6. For Samples 15 to 17, 19, 20, 22 to 24, 26, and 27, the pore volume V was calculated based on the compression ratio, assuming that pressing did not change the specific surface area but only the pore volume.

[0040] Coin cells were fabricated using the fabricated positive electrode, separator (nonwoven fabric), lithium foil (negative electrode), and ionic liquid electrolyte. The ionic liquid electrolyte used was an ionic liquid containing dissolved lithium salt. The ionic liquids used were MPP-FSI (methylpropylpyrrolidinium-bis(fluorosulfonyl)imide) or EMI-FSI (ethylmethylimidazolium-bis(fluorosulfonyl)imide). The lithium salt used was Li-FSI (lithium-bis(fluorosulfonyl)imide). The type of ionic liquid used and the lithium salt concentration (C) are listed in Tables 1, 3, and 5. Tables 2, 4, and 6 also list the a value calculated using the formula: (lithium salt concentration (C)) × (pore volume (V) in the positive electrode active material layer) / (specific surface area (A) of the positive electrode active material layer). This a value indicates the amount of lithium salt in the ionic liquid electrolyte in the pores of the positive electrode active material layer per specific surface area of ​​the positive electrode active material layer.

[0041] A constant current charge / discharge test was carried out using the lithium ion batteries of Samples 1 to 27 (charge: CCCV, discharge: CC). The discharge rate was set to 10 C. The 10 C discharge capacities calculated from the measurement results are shown in Tables 2, 4, and 6.

[0042] The lithium ion batteries of Samples 1 to 5 differ in the lithium salt concentration C of the ionic liquid electrolyte, as shown in Table 1. Sample 1, which had the lowest lithium salt concentration, had a 10C discharge capacity of 99.9 mAh / g, as shown in Table 2. For Samples 2 to 4, the 10C discharge capacity increased as the lithium salt concentration increased. However, although Sample 5 had a higher lithium salt concentration than Sample 4, its 10C discharge capacity was smaller than that of Sample 4. In particular, the 10C discharge capacity of Sample 1 was lower than that of Samples 2 to 5. It is believed that the concentration overvoltage of Sample 1 was large due to a lack of lithium ions, resulting in a small 10 C discharge capacity. It is also believed that the concentration overvoltage of Sample 5 was larger than that of Sample 4 due to an excess of lithium ions. Furthermore, it was found that when the lithium salt concentration is between 1.6 mol / L and 4.0 mol / L, excess or deficiency of lithium ions during charge / discharge reactions is unlikely, enabling high-rate charge / discharge. Furthermore, it was found that when the lithium salt concentration is between 2.4 mol / L and 3.2 mol / L, excess or deficiency of lithium ions can be suppressed, resulting in a lithium-ion battery with excellent high-rate charge / discharge characteristics.

[0043] The lithium-ion batteries of Samples 6 to 8 used EMI-FSI as the ionic liquid electrolyte, as shown in Table 1. Sample 6 had a lithium salt concentration of 0.8 mol / L, similar to Sample 1, and as shown in Table 2, the 10 C discharge capacity was also relatively small, similar to Sample 1. Sample 7 had a lithium salt concentration of 2.4 mol / L, similar to Sample 3, and the 10 C discharge capacity was also relatively large, similar to Sample 3. Sample 8 had a higher lithium salt concentration than Sample 7, but its 10 C discharge capacity was lower than Sample 7. Therefore, Samples 6 to 8, which used EMI-FSI as the ionic liquid, were found to exhibit the same lithium salt concentration dependence as Samples 1 to 5, which used MPP-FSI as the ionic liquid.

[0044] The lithium-ion batteries of Samples 9 to 13 differed in the thickness of the positive electrode active material layer (amount of positive electrode active material applied), as shown in Table 3. Sample 9, which had the thickest positive electrode active material layer, had a 10C discharge capacity of 86.7 mAh / g, as shown in Table 4. For Samples 10 to 13, the 10C discharge capacity increased as the thickness of the positive electrode active material layer became thinner (as the amount of positive electrode active material applied decreased). In particular, the 10C discharge capacity of Sample 9 was lower than that of the other samples. In sample 9, the thickness of the positive electrode active material layer was large, which made it difficult for lithium ions from outside the positive electrode active material layer to reach the deep part of the positive electrode active material layer, and it is thought that this resulted in an increase in concentration overpotential due to a lack of lithium ions in this deep part, resulting in a low 10C discharge capacity. Furthermore, by making the sample thickness 100 μm or less, it was found that excess or deficiency of lithium ions due to the distance traveled by lithium ions during charge / discharge reactions is unlikely to occur, making high-rate charge / discharge of lithium-ion batteries possible.

[0045] As shown in Table 5, the lithium-ion batteries of samples 14 to 17 had a positive electrode active material coating amount of 1.78 g / □, the lithium-ion batteries of samples 18 to 20 had a positive electrode active material coating amount of 0.93 g / □, the lithium-ion batteries of samples 21 to 24 had a positive electrode active material coating amount of 0.70 g / □, and the lithium-ion batteries of samples 25 to 27 had a positive electrode active material coating amount of 0.45 g / □. In addition, the compressibility of the positive electrode active material layer was changed for these samples as shown in Table 5.

[0046] In these samples, the 10 C discharge capacity generally increased as the amount of applied positive electrode active material decreased, which is the same tendency as in samples 9 to 13. Furthermore, among samples with the same amount of applied positive electrode active material, the greater the compression ratio, the smaller the 10 C discharge capacity. This is thought to be because the pressing process reduces the pore volume V in the positive electrode active material layer, reducing the amount of ionic liquid electrolyte in the positive electrode active material layer and increasing the concentration overpotential due to a lack of lithium ions. The amount of lithium salt in the ionic liquid electrolyte in the pores of the positive electrode active material layer per specific surface area A of the positive electrode active material layer shown in Tables 2, 4, and 6 (value a = C × V / A) was 4.9 × 10 -5 mol / m 2 Over 13.0 x 10 -5 mol / m 2 It has been found that when the following conditions are met, excess or deficiency of lithium ions is unlikely to occur during charge / discharge reactions, and high-rate charge / discharge is possible. Furthermore, it was found that for the same a value, a thinner positive electrode active material layer is better. This is thought to be because the distance that lithium ions travel during charge / discharge reactions is shorter, making it less likely for the lithium ions to become unevenly distributed. [Explanation of symbols]

[0047] 2: Positive electrode 3: Negative electrode 4: Separator 5: Ionic liquid electrolyte 6: Positive electrode current collector sheet 7: Positive electrode active material layer 8: Lithium ion 9: Positive electrode active material 10: Conductive film 11: Negative electrode current collector sheet 12: Negative electrode active material layer 13: Pore 16: Positive electrode can 17: Negative electrode can 18: Gasket 20: Lithium ion battery

Claims

1. a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an ionic liquid electrolyte containing a lithium salt; the positive electrode has a positive electrode current collector sheet and a porous positive electrode active material layer provided on the positive electrode current collector sheet, the thickness of the positive electrode active material layer is 1 μm or more and 100 μm or less, the concentration of the lithium salt in the ionic liquid electrolyte is 1.6 mol / L or more and 4.0 mol / L or less; The amount of the lithium salt in the ionic liquid electrolyte in the pores of the positive electrode active material layer per specific surface area of ​​the positive electrode active material layer measured by mercury intrusion porosimetry is 4.9 × 10 -5 mol / m 2 Above 13.0 x 10 -5 mol / m 2 A lithium-ion battery characterized by:

2. 2. The lithium ion battery according to claim 1, wherein the ionic liquid contained in the ionic liquid electrolyte is MPP-FSI or EMI-FSI.

3. 3. The lithium ion battery according to claim 1, wherein the lithium salt is LiFSI.

4. 4. The lithium ion battery according to claim 1, wherein the positive electrode active material layer contains lithium iron phosphate as a positive electrode active material.

5. 5. The lithium ion battery according to claim 1, wherein the negative electrode contains carbon as a negative electrode active material.

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