JP2017134986A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery using a fluorinated solvent capable of suppressing a resistance increase rate after high rate cycle.SOLUTION: The nonaqueous electrolyte secondary battery includes: a positive electrode; a negative electrode; a nonaqueous electrolytic solution; and a case containing the positive electrode, the negative electrode, and the nonaqueous electrolytic solution. The non-aqueous electrolyte contains a non-aqueous solvent including a fluorinated cyclic carbonate and chain fluorinated carbonate, and a lithium salt. The lithium carrier concentration is 0.70 mol/L or more and 1.01 mol/L or less.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Examples of the solvent used by mixing with the non-aqueous electrolyte include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, chloroethylene carbonate, and vinylene carbonate. And a linear or cyclic carbonate compound such as dimethylvinylene carbonate.

  In a non-aqueous electrolyte secondary battery in which the maximum potential of the positive electrode exceeds 4.3 V on the basis of metallic lithium, the simple chain or cyclic carbonate compound as described above has a positive electrode surface with a high potential when fully charged. May be oxidatively decomposed. For this reason, it has been proposed to use a solvent containing a fluorinated cyclic carbonate or a fluorinated chain carbonate as a solvent material having oxidation resistance.

  Here, the “cyclic carbonate” refers to a carbonate compound having a carbonate skeleton (O—CO—O) closed in a cyclic structure with a C—C bond in the chemical structure. “Chain carbonate” refers to an acyclic (chain) carbonate compound having a carbonate skeleton (O—CO—O) in its chemical structure. “Fluorinated” means that part of the compound is replaced by fluorine. For example, “fluorinated chain carbonate” refers to a compound that is a chain carbonate and is partially substituted with fluorine. “Fluorinated cyclic carbonate” refers to a compound that is a cyclic carbonate and is partially substituted with fluorine. Cyclic carbonates and chain carbonates are less likely to be oxidized by fluorination (obtains oxidation resistance). Here, a compound that has not been “fluorinated” is referred to as “non-fluorinated”. For example, a non-fluorinated chain carbonate is referred to as “non-fluorinated chain carbonate”. Non-fluorinated cyclic carbonates are referred to as “non-fluorinated chain carbonates”.

  For example, Japanese Unexamined Patent Application Publication No. 2014-211962 proposes a nonaqueous electrolyte solution for a nonaqueous electrolyte secondary battery in which the maximum potential of the positive electrode is 4.5 V or more based on metallic lithium. The publication includes (A) non-fluorinated cyclic carbonate, (B) fluorinated cyclic carbonate, and (C) fluorinated chain carbonate, and accounts for the total of (A), (B), and (C). A configuration is proposed in which the proportion of non-fluorinated cyclic carbonate exceeds 10% by volume. Then, by adjusting to the above range, the non-aqueous electrolytic solution made of a fluorine-based organic solvent is used, and the maximum ultimate potential is 4.5 V (vs. Li / Li +) or more in the charging condition. It is said that the oxidative decomposition of the water electrolyte can be suitably suppressed.

JP 2014-211962 A

  This inventor is considering using the fluorinated solvent which combined the fluorinated cyclic carbonate and the fluorinated chain carbonate as an electrolyte solution which has oxidation resistance. According to the knowledge of the present inventor, when the fluorinated solvent is used, the resistance increase rate after the high-rate cycle tends to be higher than when the non-fluorinated solvent is used. In applications such as hybrid vehicles and electric vehicles that are repeatedly charged and discharged at a high current rate, it is desirable to keep the resistance increase rate after a high rate cycle low. In applications such as hybrid vehicles and electric vehicles, use in a low temperature environment of about −10 ° C. is sufficiently assumed. For this reason, it is desired to suppress the increase in resistance in a low temperature environment of −10 ° C. or lower.

  The proposed non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, a non-aqueous electrolyte, and a case containing a positive electrode, a negative electrode, and a non-aqueous electrolyte. The nonaqueous electrolytic solution includes a nonaqueous solvent composed of a fluorinated cyclic carbonate and a fluorinated chain carbonate and a lithium salt, and a lithium carrier concentration is 0.70 mol / L or more and 1.01 mol / L or less. is there. According to such a configuration, it is possible to achieve both a reduction in the resistance increase rate after the high rate cycle and a reduction in the battery resistance under a low temperature environment.

FIG. 1 is a graph showing the relationship between the lithium carrier concentration and the rate of increase in resistance after a high-rate cycle for samples 1 to 3 using a non-fluorinated solvent. FIG. 2 is a graph showing the relationship between the lithium carrier concentration and the resistance increase rate after the high-rate cycle for samples 4 to 11 in which a fluorinated solvent was used.

  Hereinafter, an embodiment of the nonaqueous electrolyte secondary battery proposed here will be described. The embodiments described herein are, of course, not intended to limit the present invention in particular. The invention is not limited to the embodiments described herein unless specifically stated.

  The present inventor has a maximum potential of the positive electrode of 4.3 V (vs. Li / Li +) or more (more specifically, 4.5 V (vs. Li / Li +) or more, further 5.0 V or more). We are investigating the use of fluorinated solvents for such high-potential non-aqueous electrolyte secondary batteries. As described above, the fluorinated solvent tends to increase the resistance increase rate after the high rate cycle. The resistance increase rate after the high rate cycle becomes a problem particularly in applications in which charging and discharging are repeated at a high current rate.

  The reason why the resistance increase rate after the high-rate cycle becomes high is that charging and discharging at a high current rate are repeated, so that the salt concentration unevenness tends to increase in the battery, the lithium ions are biased, and the output characteristics deteriorate (resistance Is considered to increase). A fluorinated solvent tends to have a higher viscosity and a lower degree of dissociation of the electrolyte than a non-fluorinated solvent. For this reason, in the nonaqueous electrolyte secondary battery using a fluorinated solvent, the present inventor believes that an increase in resistance due to salt concentration unevenness after a high rate cycle is likely to appear significantly.

  The present inventor focused on the lithium carrier concentration of the electrolytic solution and studied the influence on the problem of keeping the resistance increase rate after the high rate cycle low. Here, the lithium carrier concentration is the concentration (molar concentration) of lithium ions in the electrolytic solution. The lithium carrier concentration is obtained, for example, by the product of the concentration (molar concentration) of the lithium salt in the electrolytic solution and the degree of dissociation of the lithium salt with respect to the solvent of the electrolytic solution. The lithium carrier concentration indicates the concentration of lithium ions in the electrolytic solution, and is not necessarily the same as the lithium salt concentration indicating the concentration of the lithium salt in the electrolytic solution. The inventor paid attention to the lithium carrier concentration of the electrolytic solution. The nonaqueous electrolytic solution secondary battery using the fluorinated solvent is different from the nonaqueous electrolytic solution secondary battery using the nonfluorinated solvent. I got a good knowledge.

  With non-fluorinated solvents, increasing the lithium carrier concentration tended to increase the rate of resistance increase after high rate cycling. On the other hand, in the fluorinated solvent, it has been found that when the lithium carrier concentration is increased to some extent, the rate of increase in resistance after the high rate cycle can be kept low. When a non-aqueous solvent comprising a fluorinated cyclic carbonate and a fluorinated chain carbonate is used, the lithium carrier concentration of the non-aqueous electrolyte containing a lithium salt is approximately 0 from the viewpoint of keeping the rate of increase in resistance after a high rate cycle low. It was found that it should be 70 mol / L or more. In addition, in a non-aqueous electrolyte containing a fluorinated solvent, in order to increase the lithium carrier concentration, it is better to increase the proportion of the fluorinated cyclic carbonate having a high degree of dissociation of the electrolyte or increase the lithium salt concentration. As a result, the viscosity of the non-aqueous electrolyte increases. When the viscosity of the non-aqueous electrolyte increases, battery resistance in a low temperature (for example, −10 ° C.) environment increases. Based on this viewpoint, it was found that the lithium carrier concentration is preferably 0.70 mol / L or more and 1.01 mol / L or less.

  The relationship between the lithium carrier concentration of the non-aqueous electrolyte and the rate of increase in resistance after high rate cycling of the battery can be verified, for example, by preparing and testing the following evaluation battery. Below, the structural example of the battery for evaluation is shown. The configuration of the evaluation battery described here is merely an example of the evaluation battery, and is not limited to the configuration of the present invention.

In the production of the positive electrode, LiNi _0.5 Mn _1.5 O ₄ (active material) as the positive electrode active material, acetylene black (AB) as the conductive additive, and PVdF (polyvinylidene fluoride) as the binder. Prepared. The active material, AB, and PVdF are mixed with N-methyl-2-pyrrolidone (NMP) as a dispersion solvent in a mass ratio of 87: 10: 3. Thereby, the positive electrode slurry containing the positive electrode active material is produced. The positive electrode slurry is applied on an aluminum foil prepared as a positive electrode current collector. By drying the applied positive electrode slurry, a positive electrode active material layer is formed on the aluminum foil. Then, the positive electrode active material layer is pressed by a roll press so that the density of the positive electrode active material layer formed on the aluminum foil is 2.3 g / cm ³ . Thereby, a sheet-like positive electrode (positive electrode sheet) is obtained.

  In the production of the negative electrode, a carbon material as a negative electrode active material, a styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were prepared. The carbon material, SBR, and CMC are mixed with water as a dispersion solvent in a mass ratio of 98: 1: 1. Thereby, a negative electrode slurry containing the negative electrode active material is produced. Specifically, as the carbon material, a natural graphite material having an average particle diameter of 20 μm, a lattice constant C0 of 0.67 nm, and a crystal thickness Lc in the C-axis direction of 27 nm is used. The negative electrode slurry is applied on a copper foil prepared as a negative electrode current collector. The coating amount of the negative electrode slurry is adjusted so that the weight ratio between the positive electrode active material layer and the negative electrode active material layer after drying is 2: 1 per unit area. By drying the negative electrode slurry, a negative electrode active material layer is formed on the copper foil. And the negative electrode active material layer formed on copper foil is pressed by roll press. Thereby, a sheet-like negative electrode (negative electrode sheet) is obtained.

A microporous polyethylene film having a thickness of 20 μm is used for the separator of the evaluation battery. The positive electrode sheet and the negative electrode sheet are overlapped and wound through a separator. The wound wound body is vacuum-dried at 80 ° C. for 12 hours. The vacuum-dried wound body is accommodated in an aluminum battery case. Thereafter, an electrolytic solution in which a predetermined amount of LiPF ₆ was dissolved in a predetermined solvent was poured into the battery case, and the battery case was sealed to obtain an evaluation battery. Here, electrolytic solutions in which the concentration of LiPF ₆ and the solvent were changed were prepared, and various evaluation batteries having different electrolytic solutions were obtained. Each evaluation battery was evaluated for lithium carrier concentration (mol / L), viscosity (mPa · s), initial resistance (25 ° C. 10 C), initial resistance (−10 ° C. 5 C), and resistance increase rate.

Here, the viscosity of the electrolytic solution was measured with a rheometer.
The lithium carrier concentration of the electrolytic solution is determined by the product of the salt concentration and the dissociation degree of LiPF ₆ (salt concentration × dissociation degree). Here, the dissociation degree is calculated by (ion conductivity (σ _NMR ) obtained from diffusion coefficient measured by nuclear magnetic resonance ( _NMR )) / (ion conductivity (conductivity)). Here, the ionic conductivity (σ _NMR ) obtained from the diffusion coefficient measured by nuclear magnetic resonance (NMR) is obtained by the following equation.
σ _NMR = (ZF ² / RT) × (D _Li × C _Li + D _PF6 × C _PF6 )
here,
D _Li is the Li self-diffusion coefficient (m ² / s) measured by NMR;
D _PF6 is the self-diffusion coefficient (m ² / s) of PF ₆ measured by NMR;
C _Li is the Li concentration (mol / L);
C _{PF6 @,} the concentration of PF ₆ (mol / L);
Z is the valence of each ion;
F is the Faraday constant (9.649 × 10 ⁴ (C / mol));
R is a gas constant (8.314 (J / (K · mol)));
It is.
Further, the viscosity, the lithium carrier concentration and the degree of dissociation are all measured at 25 ° C.

  In the measurement of the initial capacity, constant current charging is performed until the current rate becomes 4.9 V at a current rate of 1/3 C, and then constant current discharge is performed until it reaches 3.5 V at a current rate of 1/3 C. The charge and discharge processes with this as one cycle were performed three times. Thereafter, constant current charging was performed until the current value reached 4.9 V at a current value of 1/3 C, and full voltage charging was performed by performing constant voltage charging at a constant voltage of 4.9 V until the current value reached 1/50 C. Thereafter, the capacity discharged when discharging to 3.5 V at a current value of 1/5 C by the constant current method was defined as the initial capacity. Such an operation was performed in a temperature environment of 25 ° C. The initial capacity of all the evaluation batteries was about 5 Ah. Here, the voltage of the battery is evaluated by an open circuit voltage (OCV). Moreover, when the voltage of a battery is measured by closed circuit voltage (CCV), it is good to convert into open circuit voltage (OCV) suitably.

  In the resistance measurement, the SOC of the battery is adjusted to 50% by constant current charging of the electric capacity corresponding to 50% of the initial capacity at a current value of 1/5 C in the temperature environment of 25 ° C. did. The overvoltage was measured when constant current discharge was performed for 10 seconds at a current value of 10C at an SOC of 50%. The resistance (initial resistance (25 ° C. 10 C)) was evaluated by dividing this value by the current value. Similarly, after adjusting the SOC of the battery to 50%, a low-temperature resistance (initial resistance (−10) is measured by measuring an overvoltage when a constant current is discharged at a current value of 5 C for 10 seconds in a temperature environment of −10 ° C. 5C)) was measured.

  In the measurement of the rate of increase in resistance after the high-rate cycle, in a temperature environment of 25 ° C., the evaluation battery adjusted to 50% SOC is charged for 10 seconds at a current value of 30 C, and after the charging, the current value of 4 C is 75 seconds. The discharge to be discharged was regarded as one cycle, and this was repeated 2000 cycles. Then, after 2000 cycles, the resistance is measured by the same method as the initial resistance (25 ° C., 10 C) to obtain the resistance after 2000 cycles. Then, the resistance after 2000 cycles was divided by the initial resistance (25 ° C. 10 C) to calculate the rate of increase in resistance (resistance after 2000 cycles / initial resistance (25 ° C. 10 C)). This resistance increase rate was evaluated as the resistance increase rate after the high rate cycle.

Table 1 is a table showing the relationship among each evaluation battery, initial resistance, and resistance increase rate. Further, the test battery, the concentration of the components and composition ratios and electrolyte solvents of each electrolyte (LiPF ₆₎ are shown, respectively. Each evaluation battery is different in the solvent component of the electrolytic solution and the concentration of the electrolyte (LiPF ₆ ). The remaining configuration of each evaluation battery is basically the same. As shown in Table 1, as long as the kind of solvent and the composition ratio are the same, even if the salt concentration is changed, the degree of dissociation does not change greatly.

In Table 1, LiPF ₆ concentration is the molar concentration of LiPF ₆ per liter of solvent. The solvent is a mixed solvent in which a cyclic carbonate and a chain carbonate are mixed. Table 1 shows the components of the cyclic carbonate and the chain carbonate of each battery for evaluation, and the composition ratio (here, the volume ratio) of the cyclic carbonate and the chain carbonate.

  In Table 1, “EC” is Ethylene Carbonate (Chemical Formula 1). “EMC” is Ethyl Methyl Carbonate (Chemical Formula 2). “FEC” is Fluoro Ethylene Carbonate (Chemical Formula 3). “TFMEC” is Tri Fluoro Methyl Ethylene Carbonate (Chemical Formula 4). “TFEMC” is Methyl 2,2,2-Trifluoroethyl Carbonate (Chemical Formula 5). Here, “EC” is a non-fluorinated cyclic carbonate. “EMC” is a non-fluorinated chain carbonate. “FEC” and “TFMEC” are fluorinated cyclic carbonates. “TFEMC” is a fluorinated chain carbonate. Each chemical formula is as follows.

  According to the knowledge of the present inventors, the cyclic carbonate tends to dissociate the supporting electrolyte more than the chain carbonate, but tends to have a high viscosity. For example, as shown in samples 4, 5, and 7, as the proportion of cyclic carbonate increases, the lithium carrier concentration increases (sample 5 <sample 4 <sample 7). On the other hand, the cyclic carbonate has a higher electrolyte solution viscosity than the chain carbonate. For example, as the proportion of cyclic carbonate increases, the viscosity of the electrolytic solution increases (sample 5 <sample 4 <sample 7). Moreover, the fluorinated solvent has a higher viscosity than the non-fluorinated solvent, and the dissociation degree of the supporting electrolyte is low, and the lithium carrier concentration tends to be low (sample 1> sample 4). Further, when “TFMEC” is used, the lithium carrier concentration tends to be smaller than when “FEC” is used (sample 4> sample 6).

  In addition, when the present inventor examined the relationship between the lithium carrier concentration and the rate of increase in resistance after the high rate cycle, the sample using a fluorinated solvent and the sample using a non-fluorinated solvent have completely different properties. It was found.

FIG. 1 is a graph showing the relationship between the lithium carrier concentration and the rate of increase in resistance after a high-rate cycle for samples 1 to 3 using a non-fluorinated solvent. As shown in FIG. 1, in a non-aqueous electrolyte secondary battery using a non-fluorinated solvent, for example, as shown in Samples 1 to 3, the composition ratio of cyclic carbonate and chain carbonate is the same. Even in this case, the lithium carrier concentration increases as the concentration of LiPF ₆ increases. As the lithium carrier concentration increases, the initial resistance and the resistance increase rate tend to increase.

  FIG. 2 is a graph showing the relationship between the lithium carrier concentration and the resistance increase rate after the high-rate cycle for samples 4 to 11 in which a fluorinated solvent was used. In a nonaqueous electrolyte secondary battery using a fluorinated solvent, for example, as shown in Samples 4 to 11, when the lithium carrier concentration is low, the resistance increase rate after the high rate cycle tends to be high.

  Further, for example, the sample 5 has a lower lithium carrier concentration and viscosity than the sample 4, but the resistance increase rate after the high rate cycle is higher than that of the sample 4. Sample 6 has the same viscosity as sample 4 but only 0.4 mPa · s higher, but the lithium carrier concentration is about 20% lower and the resistance increase rate after the high-rate cycle is about 30% higher. Further, as seen in Samples 7 to 11, when the lithium carrier concentration is 0.44 mol / L or more, when the lithium carrier concentration increases, the resistance increase rate after the high rate cycle tends to improve. It was. That is, when the lithium carrier concentration was higher than a certain level, the resistance increase rate after the high rate cycle tended not to become so large (sample 8 to sample 11).

Thus, in the fluorinated solvent, as the concentration of LiPF ₆ increases, the lithium carrier concentration increases and the viscosity also increases. However, unexpectedly, even if the lithium carrier concentration increases, the rate of increase in resistance after the high rate cycle does not increase (for example, samples 7 to 11). Considering such a tendency, as in Samples 8 to 11, nonaqueous electrolysis using a nonaqueous electrolytic solution containing a nonaqueous solvent composed of a fluorinated cyclic carbonate and a fluorinated chain carbonate and a lithium salt is used. In the liquid secondary battery, the lithium carrier concentration of the nonaqueous electrolytic solution is preferably 0.70 mol / L or more.

However, as shown in Table 1, as the concentration of LiPF ₆ increases, the lithium carrier concentration increases and the viscosity also increases. When the lithium carrier concentration becomes too high, for example, the initial resistance at a low temperature (for example, −10 ° C.) tends to increase. Considering such a tendency, as in Samples 9 to 11, non-aqueous electrolysis using a non-aqueous electrolytic solution containing a non-aqueous solvent composed of a fluorinated cyclic carbonate and a fluorinated chain carbonate and a lithium salt is used. For the liquid secondary battery, the lithium carrier concentration of the non-aqueous electrolyte is preferably 0.70 mol / L or more and 1.01 mol / L or less. With such a configuration, even when an electrolytic solution using a fluorinated solvent having oxidation resistance is used, a non-aqueous electrolytic solution secondary in which the rate of increase in resistance after a high rate cycle is kept low and the initial resistance at low temperature is low. Battery can be provided.

Such a configuration can be suitably applied to a non-aqueous electrolyte secondary battery using a positive electrode active material that exhibits a high potential when fully charged, such as LiNi _0.5 Mn _1.5 O ₄ . Further, according to the inventor's knowledge, in the above configuration, the fluorinated cyclic carbonate and the fluorinated chain carbonate can be appropriately changed. However, the viscosity of the non-aqueous electrolyte is, for example, 13.7 mPa · s at 25 ° C. It is good to adjust so that it may become the following. The viscosity of the nonaqueous electrolytic solution may be low. For example, the viscosity of the nonaqueous electrolytic solution may be adjusted to be 8.9 mPa · s to 13.7 mPa · s at 25 ° C.

  The non-aqueous electrolyte secondary battery proposed here has been variously described, but the embodiments and examples given here do not limit the present invention unless otherwise specified.

  For example, the configuration of the nonaqueous electrolyte secondary battery, such as the positive electrode, the negative electrode, and the separator, is not limited to the configuration of the evaluation battery described above unless otherwise specified. For example, the positive electrode active material, the negative electrode active material, the separator, the solvent component of the electrolytic solution, and the like can be variously modified as long as there is no contradiction with the configuration of the present invention as disclosed in Patent Document 1. is there.

  For example, the fluorinated cyclic carbonate is not limited to the above-described FEC and TFMEC. Specific examples of the fluorinated cyclic carbonate include monofluoroethylene carbonate (FEC), 4,4-difluoroethylene carbonate, difluoroethylene carbonate such as 4,5-difluoroethylene carbonate, trifluoroethylene carbonate, tetrafluoroethylene carbonate, fluoro Methylethylene carbonate, difluoromethylethylene carbonate, trifluoromethylethylene carbonate (TFMEC), bis (fluoromethyl) ethylene carbonate, bis (difluoromethyl) ethylene carbonate, bis (trifluoromethyl) ethylene carbonate, fluoroethylethylene carbonate, difluoroethyl Ethylene carbonate, trifluoroethyl ethylene carbonate, tetrafluoroethylene Such as Le ethylene carbonate. Also in this invention, the fluorinated cyclic carbonate illustrated here can be employ | adopted suitably. A plurality of types of fluorinated cyclic carbonates may be used in combination. Of these, the above-described FEC and TFMEC are preferably used.

  Further, the fluorinated chain carbonate is not limited to the above-described TFEMC. Specific examples of the fluorinated chain carbonate include, for example, fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, trifluoromethyl methyl carbonate, fluoromethyl difluoromethyl carbonate, bis (fluoromethyl) carbonate, bis (difluoromethyl) carbonate, bis (Trifluoromethyl) carbonate, (2-fluoroethyl) methyl carbonate, ethylfluoromethyl carbonate, (2,2-difluoroethyl) methyl carbonate, (2-fluoroethyl) fluoromethyl carbonate, ethyl difluoromethyl carbonate, (2, 2,2-trifluoroethyl) methyl carbonate (TFEMC), (2,2-difluoroethyl) fluoromethyl carbonate, (2-fluoroethyl) D) Difluoromethyl carbonate, ethyl trifluoromethyl carbonate, ethyl- (2-fluoroethyl) carbonate, ethyl- (2,2-difluoroethyl) carbonate, bis (2-fluoroethyl) carbonate, ethyl- (2,2, 2-trifluoroethyl) carbonate, ethyl- (2,2,2-trifluoroethyl) carbonate, 2,2-difluoroethyl-2′-fluoroethyl carbonate, bis (2,2-difluoroethyl) carbonate, 2, 2,2-trifluoroethyl-2′-fluoroethyl carbonate, 2,2,2-trifluoroethyl-2 ′, 2′-difluoroethyl carbonate, bis (2,2,2-trifluoroethyl) carbonate, penta Fluoroethyl methyl carbonate, pentaf Oro ethyl fluoromethyl carbonate, pentafluoroethyl ethyl carbonate, bis etc. (pentafluoroethyl) carbonate. In the present invention, the fluorinated chain carbonate exemplified here can be appropriately employed. A plurality of types of fluorinated chain carbonates may be used in combination. Among these, the above-described TFEMC is preferably used.

Further, the lithium salt is not limited to LiPF ₆ described above. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF _3). One type or two or more types of lithium compounds (lithium salts) such as SO ₂ ) ₃ , LiI, and LiN (FSO ₂ ) ₂ can be used. For example, in addition to LiPF ₆ described above, LiBF ₄ , LiN (FSO ₂ ) _{2 and the} like are preferably used.

The non-aqueous electrolyte may contain an optional additive as necessary as long as the object of the present invention is not significantly impaired. The additive may be used for the purpose of, for example, improving the output performance of the battery, improving the storage stability (suppressing the decrease in capacity during storage, etc.), improving the cycle characteristics, and improving the initial charge / discharge efficiency. Examples of preferred additives include fluorophosphate (preferably difluorophosphate, for example, lithium difluorophosphate represented by LiPO ₂ F ₂ ), lithium bisoxalate borate (LiBOB), and lithium difluorooxalate. Examples thereof include borate (LiDFOB) and vinylene carbonate (VC). Further, for example, additives such as cyclohexylbenzene and biphenyl that can be used as a countermeasure against overcharge may be used.

  The ratio of the fluorinated cyclic carbonate and the fluorinated chain carbonate and the concentration of the lithium salt in the non-fluorinated solvent may be set to an appropriate ratio so that the above-described lithium carrier concentration can be realized. Preferably, as described above, the viscosity of the non-aqueous electrolyte is adjusted to be 8.9 mPa · s or more and 13.7 mPa · s or less.

Claims (1)

A positive electrode, a negative electrode, a non-aqueous electrolyte, and a case containing the positive electrode, the negative electrode, and the non-aqueous electrolyte;
The non-aqueous electrolyte contains a non-aqueous solvent composed of a fluorinated cyclic carbonate and a fluorinated chain carbonate, and a lithium salt, and has a lithium carrier concentration of 0.70 mol / L or more and 1.01 mol / L. The following is a nonaqueous electrolyte secondary battery.

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