Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery (1) includes: a positive electrode (10) that includes a lithium manganese oxide (LMO) as a positive electrode active material; a negative electrode (20) that includes lithium (Li) and SiOX (0 ≤ X < 2) as a negative electrode active material; and an electrolyte (50) that includes an organic solvent and a supporting salt. The capacity balance {negative electrode capacity (mAh) / positive electrode capacity (mAh)}, expressed on the basis of the capacity of the negative electrode and the capacity of the positive electrode, is in the range 1.56–2.51. The molar ratio (Li / SiOX) of the lithium (Li) and the SiOX (0 ≤ X < 2) of the negative electrode active material is in the range 3.8–4.9. The molar ratio (Li / LMO) of the lithium (Li) and the lithium manganese oxide (LMO) is 8.0 or less.

Description

Nonaqueous electrolyte secondary battery

[0001] This application claims priority to Japanese Patent Application No. 2024-18835, filed on February 9, 2024, the contents of which are incorporated herein by reference.

[0002] A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte impregnated in the positive electrode, the negative electrode, and the separator, all contained within a sealed container. Non-aqueous electrolyte secondary batteries have a high energy density and are lightweight, so they are used in power sources for electronic devices, power storage units for power generation devices, and the like.

[0003] Patent Document 1 describes the composition of the organic solvent and supporting salt used in the electrolyte. Patent Document 1 also describes providing a nonaqueous electrolyte secondary battery that maintains sufficient discharge capacity even in low-temperature environments and is capable of operating over a wide temperature range by optimizing the composition of the negative electrode active material. In the nonaqueous electrolyte secondary battery described in Patent Document 2, the overdischarge characteristics are improved by increasing the concentration of the supporting salt in the electrolyte in the electrolyte system described in Patent Document 1.

[0004] International Publication No. WO 2016 / 143543 International Publication No. WO 2022 / 181207

[0005] However, in the nonaqueous electrolyte secondary battery described in Patent Document 2, although the effect of improving overdischarge characteristics as the concentration of the supporting salt in the electrolyte solution increases, the discharge capacity itself tends to decrease as the concentration of the supporting salt increases.

[0006] An object of one aspect of the present invention is to provide a nonaqueous electrolyte secondary battery that has a sufficient discharge capacity and excellent overdischarge characteristics.

[0007] As a result of investigations into solving the above problems, the inventors of the present invention have found that the capacity balance between the negative electrode and the positive electrode and the ratio of lithium (Li) and SiO in the negative electrode active material are important factors. Ｘ (0≦X<2) and the molar ratio (Li / SiO ＸThe inventors have found that by optimizing the molar ratio (Li / LMO) of lithium (Li) and lithium manganese oxide (LMO), it is possible to increase the discharge capacity and improve the overdischarge characteristics.

[0008] The nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode containing lithium manganese oxide (LMO) as a positive electrode active material, and a negative electrode containing lithium (Li) and SiO Ｘ (0≦X<2) and an electrolyte solution containing an organic solvent and a supporting salt, wherein a capacity balance represented by a capacity of the negative electrode and a capacity of the positive electrode {negative electrode capacity (mAh) / positive electrode capacity (mAh)} is in the range of 1.56 to 2.51, and Ｘ (0≦X<2) and the molar ratio (Li / SiO Ｘ ) is in the range of 3.8 to 4.9, and the molar ratio (Li / LMO) of the lithium (Li) to the lithium manganese oxide (LMO) is 8.0 or less.

[0009] By maintaining the capacity balance between the negative electrode and the positive electrode (negative electrode capacity (mAh) / positive electrode capacity (mAh)) within the above range, a certain margin of capacity can be secured on the negative electrode side. Therefore, even if the decomposition of the negative electrode active material due to the battery reaction proceeds rapidly, a certain level of negative electrode capacity can be secured. This prevents a decrease in discharge capacity even if the nonaqueous electrolyte secondary battery is stored or used for a long period of time under harsh temperature and humidity conditions.

[0010] Negative electrode active material lithium (Li) and SiO Ｘ By setting the molar ratio of lithium (Li) and SiO in the negative electrode active material to the above range, it is possible to make charging abnormalities less likely to occur. Ｘ By setting the molar ratio to be within the above range, it is possible to prevent the discharge capacity from decreasing even when the nonaqueous electrolyte secondary battery 1 is used or stored in a high-temperature environment for a long period of time.

[0011] By setting the molar ratio (Li / LMO) of lithium (Li) to lithium manganese oxide (LMO) within the above range, the electrical characteristics after storage of the nonaqueous electrolyte secondary battery under high-temperature, high-humidity, and overdischarge conditions, i.e., the overdischarge characteristics, become good, and thus degradation of the nonaqueous electrolyte secondary battery can be suppressed even when overdischarge occurs.

[0012] In the nonaqueous electrolyte secondary battery having the above configuration, the positive electrode contains at least Li as the lithium manganese oxide used as the positive electrode active material. １＋ｘ Co ｙ Mn ２－ｘ－ｙ O ４ (0≦x≦0.33, 0<y≦0.2) is preferred.

[0013] By using a positive electrode containing a compound having the above composition as the lithium manganese oxide used as the positive electrode active material, discharge characteristics are improved, particularly in low-temperature environments, and sufficient discharge capacity is obtained over a wide temperature range, thereby improving battery characteristics.

[0014] In the above configuration, SiO Ｘ (0≦X<2) preferably has at least a portion of its surface covered with carbon.

[0015] SiO Ｘ When at least a portion of the surface is coated with carbon (0≦X<2), the conductivity of the negative electrode 20 is improved and an increase in internal resistance in a low-temperature environment is suppressed, thereby suppressing a voltage drop at the initial stage of discharge and making it possible to further stabilize the discharge characteristics.

[0016] The electrolytic solution preferably contains, as the organic solvent, a mixed solution containing propylene carbonate (PC), ethylene carbonate (EC), and dimethoxyethane (DME) in a volume ratio range of {PC:EC:DME}={0.5 to 1.5:0.5 to 1.5:1 to 3}, and, as the supporting salt, lithium bis(fluorosulfonyl)imide (LiFSI) at 0.6 to 1.5 (mol / L).

[0017] The use of ethylene carbonate (EC) and propylene carbonate (PC) as the cyclic carbonate solvents allows for good capacity retention, particularly at high temperatures. The use of dimethoxyethane (DME) as the chain ether solvent allows for improved low-temperature characteristics while maintaining capacity at room temperature. Furthermore, adjusting the mixing ratio of the EC, PC, and DME significantly improves the ability to maintain discharge capacity, particularly at low temperatures.

[0018] By using a solvent having the above composition for the electrolyte and adjusting and optimizing the composition and content of the supporting salt, the effect of maintaining the discharge capacity in a low-temperature environment can be significantly achieved.

[0019] The nonaqueous electrolyte secondary battery preferably includes a separator disposed between the positive electrode and the negative electrode, and a storage container having an internal storage space in which the positive electrode, the negative electrode, the separator, and the electrolytic solution are disposed.

[0020] The nonaqueous electrolyte secondary battery has battery elements disposed in the storage space of a storage container that houses the positive electrode and the negative electrode, and thus has a structure with excellent electrical insulation and sealing properties, which can suppress volatilization of the electrolyte solution and intrusion of moisture contained in the atmosphere into the battery.

[0021] The storage container is preferably a coin-shaped container including a cylindrical positive electrode can with a bottom, and a negative electrode can fixed to the opening of the positive electrode can with a gasket interposed therebetween, forming a storage space between the negative electrode can and the positive electrode can.

[0022] The nonaqueous electrolyte secondary battery has battery elements arranged in a storage space in which the positive electrode can and the negative electrode can are sealed in an optimal structure, resulting in a structure with excellent electrical insulation and sealing properties, which can suppress volatilization of the electrolyte and the intrusion of moisture contained in the atmosphere into the battery interior. As a result, even a small coin-type nonaqueous electrolyte secondary battery can achieve high output and capacity characteristics.

[0023] According to one aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery that has a sufficient discharge capacity and excellent overdischarge characteristics.

[0024] 1 is a cross-sectional view schematically showing a coin-type (button-type) non-aqueous electrolyte secondary battery according to an embodiment of the present invention; 2 is a graph showing test results of measuring the discharge capacity of the non-aqueous electrolyte secondary battery after storage under overdischarge conditions; and 3 is a graph showing test results of measuring the discharge capacity of the non-aqueous electrolyte secondary battery according to an embodiment of the present invention in a low-temperature environment.

[0025] Hereinafter, embodiments of the nonaqueous electrolyte secondary battery of the present invention will be described in detail, with reference to Fig. 1. Specifically, the nonaqueous electrolyte secondary battery described below is configured by housing an active material used as a positive electrode or a negative electrode and an electrolyte solution in a container, but the configuration according to the present invention can also be applied to electrochemical cells such as lithium ion capacitors.

[0026] 1, the nonaqueous electrolyte secondary battery 1 of this embodiment is a so-called coin (button) type battery. The nonaqueous electrolyte secondary battery 1 includes, in a storage container 2, a positive electrode 10 capable of absorbing and desorbing lithium ions, a negative electrode 20 capable of absorbing and desorbing lithium ions, a separator 30 disposed between the positive electrode 10 and the negative electrode 20, and an electrolyte solution 50 containing at least a supporting salt and an organic solvent.

[0027] More specifically, the nonaqueous electrolyte secondary battery 1 includes a storage container 2 having a positive electrode can 12 and a negative electrode can 22. The positive electrode can 12 is cylindrical with a bottom. The negative electrode can 22 is cylindrical with a lid (hat-shaped). The negative electrode can 22 is fixed to the opening 12a of the positive electrode can 12 with a gasket 40 interposed therebetween. The negative electrode can 22 forms a storage space between itself and the positive electrode can 12. The periphery of the opening 12a of the positive electrode can 12 is crimped inward, i.e., toward the negative electrode can 22. This seals the storage space. The storage container 2 is a coin-shaped container.

[0028] In the storage space sealed by the storage container 2, a positive electrode 10 provided on the positive electrode can 12 side and a negative electrode 20 provided on the negative electrode can 22 side are disposed opposite each other with a separator 30 interposed therebetween. The storage space is filled with an electrolyte 50. In the example shown in FIG. 1 , a lithium foil 60 is interposed between the negative electrode 20 and the separator 30.

[0029] The gasket 40 is inserted along the inner peripheral surface of the positive electrode can 12. The gasket 40 is connected to the outer periphery of the separator 30 and holds the separator 30. The positive electrode 10, the negative electrode 20, and the separator 30 are impregnated with the electrolyte solution 50 filled in the storage container 2.

[0030] In the nonaqueous electrolyte secondary battery 1 of the example shown in FIG. 1 , the positive electrode 10 is electrically connected to the inner surface of the positive electrode can 12 via the positive electrode current collector 14. The negative electrode 20 is electrically connected to the inner surface of the negative electrode can 22 via the negative electrode current collector 24. In this embodiment, the nonaqueous electrolyte secondary battery 1 including the positive electrode current collector 14 and the negative electrode current collector 24 is described as an example, but is not limited to this. The nonaqueous electrolyte secondary battery 1 may be configured such that, for example, the positive electrode can 12 also serves as the positive electrode current collector and the negative electrode can 22 also serves as the negative electrode current collector.

[0031] The nonaqueous electrolyte secondary battery 1 of this embodiment is configured as described above, and is capable of storing (charging) and releasing (discharging) electric charge by the movement of lithium ions from one of the positive electrode 10 and the negative electrode 20 to the other.

[0032] The positive electrode 10 contains lithium manganese oxide (LMO) as a positive electrode active material. The negative electrode 20 contains lithium (Li) and SiO as a negative electrode active material. Ｘ (0≦X<2).

[0033] [Positive Electrode Can and Negative Electrode Can] The positive electrode can 12 is cylindrical with a bottom and has a circular opening 12a in a plan view. Any conventionally known material can be used without any restrictions for the positive electrode can 12. Examples of the material for the positive electrode can 12 include stainless steel such as SUS329J4L and NAS64.

[0034] The negative electrode can 22 is configured in a covered cylindrical (hat-shaped) shape, and its tip 22a is configured to fit into the positive electrode can 12 through the opening 12a. As with the material of the positive electrode can 12, the material of the negative electrode can 22 may be a conventionally known stainless steel, such as SUS304-BA. The negative electrode can 22 may also be made of a clad material in which copper, nickel, or the like is pressure-welded to stainless steel.

[0035] The positive electrode can 12 and the negative electrode can 22 are fixed together by crimping the periphery of the opening 12a of the positive electrode can 12 to the negative electrode can 22 with a gasket 40 interposed therebetween. The positive electrode can 12 and the negative electrode can 22 seal the storage space. The thickness of the metal plate material used for the positive electrode can 12 and the negative electrode can 22 is generally about 0.1 to 0.3 mm. The average thickness of the entire positive electrode can 12 and the negative electrode can 22 is about 0.20 mm.

[0036] 1, the negative electrode can 22 has a shape in which the leading end 22a is folded back along the outer surface of the negative electrode can 22. The negative electrode can 22 may have a shape in which the leading end does not have a fold.

[0037] Examples of non-aqueous electrolyte secondary batteries to which the configuration of this embodiment can be applied include batteries of 920 size (outer diameter φ9.5 mm×height 2.0 mm), which is a common size for coin-type non-aqueous electrolyte secondary batteries.

[0038] [Gasket] The gasket 40 is formed in an annular shape along the inner circumferential surface of the positive electrode can 12. An annular groove 41 is formed in the gasket 40. The tip portion 22a of the negative electrode can 22 is disposed inside the annular groove 41. The material of the gasket 40 is preferably a resin having a heat distortion temperature of 230°C or higher. If the resin used for the gasket 40 has a heat distortion temperature of 230°C or higher, deformation of the gasket and leakage of the electrolyte 50 can be suppressed even when the nonaqueous electrolyte secondary battery 1 is used or stored in a high-temperature environment or when heat is generated during use of the nonaqueous electrolyte secondary battery 1.

[0039] Examples of materials for the gasket 40 include plastics such as polypropylene resin (PP), polyphenylene sulfide (PPS), polyethylene terephthalate (PET), polyamide, liquid crystal polymer (LCP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA), polyether ether ketone resin (PEEK), polyether nitrile resin (PEN), polyether ketone resin (PEK), polyarylate resin, polybutylene terephthalate resin (PBT), polycyclohexane dimethylene terephthalate resin, polyether sulfone resin (PES), polyamino bismaleimide resin, polyetherimide resin, and fluororesin. Among these, polypropylene resin is preferred from the viewpoint of being able to prevent the gasket from deforming significantly during use or storage in a high-temperature environment and further improving the sealing performance of the nonaqueous electrolyte secondary battery.

[0040] The above-mentioned materials (such as polypropylene resin) to which glass fiber, mica whiskers, ceramic fine powder, etc. are added in an amount of 30 mass % or less can also be suitably used for the gasket 40. By using such materials, it is possible to prevent the gasket from being significantly deformed by high temperatures and to prevent the electrolyte 50 from leaking.

[0041] A sealant may be applied to the inner surface of the annular groove 41 of the gasket 40. Examples of the sealant that can be used include asphalt, epoxy resin, polyamide resin, and butyl rubber adhesive. After the sealant is applied to the inside of the annular groove 41, it is dried before use.

[0042] The gasket 40 is sandwiched between the positive electrode can 12 and the negative electrode can 22, and at least a portion of the gasket 40 is compressed. The compression ratio at this time is not particularly limited, and may be set within a range that can reliably seal the interior of the nonaqueous electrolyte secondary battery 1 and that does not cause rupture in the gasket 40.

[0043] [Electrolyte Solution] The electrolyte solution 50 includes at least an organic solvent and a supporting salt. The electrolyte solution 50 is, for example, a mixed solvent (mixed solution) containing propylene carbonate (PC), ethylene carbonate (EC), and dimethoxyethane (DME) as the organic solvent in a volume ratio of {PC:EC:DME}={0.5-1.5:0.5-1.5:1-3}. The electrolyte solution 50 also contains lithium bis(fluorosulfonyl)imide (LiFSI) as the supporting salt at, for example, 0.6-1.5 (mol / L). The LiFSI content may be 1.0 (mol / L) or more and 1.5 (mol / L) or less. The electrolyte solution 50 is typically obtained by dissolving the supporting salt in a nonaqueous solvent such as an organic solvent. The properties of the electrolyte solution 50 are determined taking into account the desired heat resistance, viscosity, and the like.

[0044] When the organic solvent is a mixed solvent containing PC, EC, and DME in an appropriate mixing ratio, a nonaqueous electrolyte secondary battery 1 can be realized that can maintain a sufficient discharge capacity over a wide temperature range, including low-temperature environments. Specifically, by using PC and EC, which have a high dielectric constant and high solubility of supporting salts, as the cyclic carbonate solvent, the discharge capacity of the nonaqueous electrolyte secondary battery 1 can be increased. Because PC and EC have high boiling points, they form an electrolyte solution that is less likely to volatilize even when used or stored in a high-temperature environment.

[0045] By mixing PC, which has a lower melting point than EC, with EC as the cyclic carbonate solvent, it is possible to improve low-temperature characteristics. By using DME, which has a low melting point, as the chain ether solvent, low-temperature characteristics are improved. In addition, DME has low viscosity, which improves the electrical conductivity of the electrolyte. Furthermore, DME solvates with Li ions, thereby increasing the discharge capacity of non-aqueous electrolyte secondary batteries.

[0046] The cyclic carbonate solvent has a structure represented by the following chemical formula 1. Examples of the cyclic carbonate solvent include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), trifluoropropylene carbonate (TFPC), chloroethylene carbonate (ClEC), trifluoroethylene carbonate (TFEC), difluoroethylene carbonate (DFEC), and vinylene carbonate (VEC).

[0047] In the nonaqueous electrolyte secondary battery 1, from the viewpoints of ease of forming a film on the negative electrode 20, improvement of low-temperature characteristics, and further improvement of capacity retention rate at high temperatures, two types of cyclic carbonate solvents, PC and EC, can be used as the cyclic carbonate solvent having a structure represented by the following (Chemical Formula 1):

[0048]

[0049] In the above (Chemical Formula 1), R1, R2, R3, and R4 represent any one of hydrogen, fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms, and a fluorinated alkyl group. In addition, R1, R2, R3, and R4 in the above (Chemical Formula 1) may be the same or different.

[0050] In this embodiment, as described above, by using PC and EC as the cyclic carbonate solvent, which have high dielectric constants and high solubility of supporting electrolytes, it is possible to obtain a large discharge capacity. Because PC and EC have high boiling points, they form an electrolyte solution that is less likely to volatilize even when used or stored in a high-temperature environment. Furthermore, by using PC, which has a lower melting point than EC, as the cyclic carbonate solvent in combination with EC, excellent low-temperature characteristics can be obtained.

[0051] The chain ether solvent has a structure represented by the following (chemical formula 2). Examples of the chain ether solvent include 1,2-dimethoxyethane (DME) and 1,2-diethoxyethane (DEE). In this embodiment, in particular, from the viewpoint of improving the conductivity and further improving the low-temperature characteristics while maintaining the capacity at room temperature, DME, which easily solvates with lithium ions, is used as the chain ether solvent having the structure represented by the following (chemical formula 2).

[0052]

[0053] In the above (Chemical Formula 2), R5 and R6 represent any one of hydrogen, fluorine, chlorine, an alkyl group having 1 to 3 carbon atoms, and a fluorinated alkyl group. R5 and R6 may be the same or different.

[0054] In this embodiment, as described above, the use of DME, which has a low melting point, as the chain ether solvent improves low-temperature characteristics. Furthermore, the low viscosity of DME improves the electrical conductivity of the electrolyte. Furthermore, DME solvates Li ions, allowing the nonaqueous electrolyte secondary battery to achieve a large discharge capacity.

[0055] In this embodiment, the blending ratio of the organic solvents in the solvent of the electrolytic solution 50 is set to a volume ratio in the range of {PC:EC:DME}=0.5-1.5:0.5-1.5:1-3. The blending ratio in the solvent is more preferably a volume ratio in the range of 0.8-1.2:0.8-1.2:1.5-2.5, and most preferably approximately {PC:EC:DME}={1:1:2}.

[0056] When the blending ratio of the organic solvent is within the above range, the effect of improving low-temperature characteristics without impairing the capacity retention rate at high temperatures or room temperature, as described above, is more pronounced. More specifically, when the blending ratio of propylene carbonate (PC), a cyclic carbonate solvent, is equal to or greater than the lower limit of the above range, the effect of improving low-temperature characteristics is significantly achieved by mixing PC, which has a lower melting point than EC, with EC. On the other hand, since PC has a lower dielectric constant than EC, the concentration of the supporting electrolyte cannot be increased. Therefore, if the content is too high, it may be difficult to obtain a large discharge capacity. Therefore, it is preferable to limit the blending ratio to equal to or less than the upper limit of the above range.

[0057] When the blending ratio of ethylene carbonate (EC), a cyclic carbonate solvent, in the organic solvent is equal to or greater than the lower limit of the above range, the dielectric constant of the electrolyte solution 50 and the solubility of the supporting salt are increased, resulting in a larger discharge capacity of the nonaqueous electrolyte secondary battery. EC has a high viscosity and therefore poor electrical conductivity. Because EC has a high melting point, an excessively high content may result in a decrease in low-temperature characteristics. Therefore, it is preferable to limit the blending ratio to the upper limit of the above range. By setting the blending ratio of EC in the organic solvent within the above range, it is possible to suppress an increase in internal resistance in low-temperature environments.

[0058] When the blending ratio of dimethoxyethane (DME), a chain ether solvent, in the organic solvent is equal to or greater than the lower limit of the above range, the low-melting-point DME is contained in the organic solvent in a predetermined amount, resulting in a significant effect of improving low-temperature characteristics. DME has low viscosity, which improves electrical conductivity and enables a large discharge capacity to be obtained by solvating with Li ions. Since DME has a low dielectric constant, the concentration of the supporting electrolyte cannot be increased, and if the content is too high, it may be difficult to obtain a large discharge capacity. Therefore, it is preferable to limit the blending ratio to equal to or less than the upper limit of the above range. By setting the blending ratio of DME in the organic solvent within the above range, it is possible to suppress the voltage drop at the beginning of discharge.

[0059] The use of ethylene carbonate (EC) and propylene carbonate (PC) as the cyclic carbonate solvents allows for good capacity retention, particularly at high temperatures. The use of dimethoxyethane (DME) as the chain ether solvent allows for improved low-temperature characteristics while maintaining capacity at room temperature. Furthermore, adjusting the mixing ratio of the EC, PC, and DME significantly improves the ability to maintain discharge capacity, particularly at low temperatures.

[0060] A Li compound can be used as the supporting salt used in the electrolytic solution 50. Examples of Li compounds include lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), with LiFSI being preferred.

[0061] The content of the supporting salt in the electrolyte solution 50 can be determined taking into consideration the type of supporting salt, etc., as well as the type of positive electrode active material described below. In this embodiment, the content of the supporting salt in the electrolyte solution 50 can be, for example, 0.6 to 1.5 (mol / L). By including the above-mentioned lithium compound as the supporting salt in the electrolyte solution 50 in a molar ratio within the above range, a sufficient discharge capacity can be remarkably obtained over a wide temperature range, including low-temperature environments, and the battery characteristics can be improved.

[0062] If the concentration of the supporting salt in the electrolytic solution 50 exceeds the upper limit of the above range, the discharge capacity may decrease. If the concentration of the supporting salt in the electrolytic solution 50 falls below the lower limit, the internal resistance may increase.

[0063] When LiFSI is used alone as the supporting electrolyte and the electrolyte solution 50 contains LiFSI at a concentration of 0.6 to 1.5 (mol / L), the voltage drop at the beginning of discharge can be suppressed. Furthermore, the discharge characteristics in low-temperature environments can be improved, and sufficient discharge capacity can be obtained over a wide temperature range. Because LiFSI has excellent conductivity, the above effects become more pronounced.

[0064] In this embodiment, by using an organic solvent having the above composition for the electrolyte solution 50, it is possible to suppress an increase in the viscosity of the electrolyte solution, particularly in a low-temperature environment of −30° C. to −40° C., and to suppress the impediment of charge transfer. This improves discharge characteristics in a low-temperature environment, and makes it possible to maintain a sufficient discharge capacity over a wide temperature range. In this embodiment, by employing a configuration in which the supporting electrolyte contained in the electrolyte solution 50 contains the above-mentioned lithium compound in a molar ratio within the above range, the above effect is more pronounced, and the battery characteristics are further improved.

[0065] In this embodiment, in addition to optimizing the composition of the electrolyte solution 50, the negative electrode active material of the negative electrode 20 is lithium (Li) and SiO Ｘ (0≦X<2) This improves the discharge characteristics in a low-temperature environment, and the effect of maintaining a sufficient discharge capacity over a wide temperature range becomes more pronounced.

[0066] [Positive Electrode] The positive electrode 10 contains a positive electrode active material including lithium manganese oxide (LMO). For example, the positive electrode 10 can be a mixture of the positive electrode active material, polyacrylic acid as a binder, and graphite or the like as a conductive additive.

[0067] The positive electrode active material contained in the positive electrode 10 is, for example, LiMn ２ O ４ Ya, Li ４ Mn ５ O １２ Among the lithium manganese oxides, Li １＋ｘ Co ｙ Mn ２－ｘ－ｙ O ４ (0≦x≦0.33, 0<y≦0.2), in which a portion of Mn is substituted with Co. In this way, by adding a transition metal element such as Co or Ni to a lithium manganese oxide and using a positive electrode active material in which a portion of the lithium manganese oxide is substituted with the transition metal element, the discharge characteristics are further improved.

[0068] In this embodiment, by using a positive electrode active material made of a lithium manganese oxide of the above composition for the positive electrode 10, discharge characteristics are improved, particularly in low-temperature environments, and the effect of obtaining sufficient discharge capacity over a wide temperature range is more pronounced, further improving battery characteristics. In this embodiment, the positive electrode active material may contain not only one type of the lithium manganese oxides described above, but also a plurality of types.

[0069] When a granular positive electrode active material made from the above materials is used, its particle diameter (D50) is not particularly limited, and is preferably 0.1 to 100 μm, and more preferably 1 to 10 μm. If the particle diameter (D50) of the positive electrode active material is below the lower limit of the above preferred range, the nonaqueous electrolyte secondary battery becomes difficult to handle due to increased reactivity when exposed to high temperatures. If the particle diameter (D50) of the positive electrode active material exceeds the upper limit of the above preferred range, the discharge rate may decrease. Note that the "particle diameter (D50) of the positive electrode active material" refers to the particle diameter measured using a conventionally known laser diffraction method, and refers to the median diameter.

[0070] The content of the positive electrode active material in the positive electrode 10 is determined taking into consideration the discharge capacity required for the nonaqueous electrolyte secondary battery 1, etc. The content of the positive electrode active material in the positive electrode 10 is preferably 50 to 95 mass %. When the content of the positive electrode active material is equal to or greater than the lower limit of the above-mentioned preferred range, a sufficient discharge capacity is likely to be obtained. When the content of the positive electrode active material is equal to or less than the upper limit of the above-mentioned preferred range, the positive electrode 10 is easily formed.

[0071] The positive electrode 10 may contain a conductive additive (hereinafter, the conductive additive used in the positive electrode 10 may be referred to as a "positive electrode conductive additive"). Examples of the positive electrode conductive additive include carbonaceous materials such as furnace black, ketjen black, acetylene black, and graphite. As the positive electrode conductive additive, one of the above may be used alone, or two or more may be used in combination.

[0072] The content of the positive electrode conductive additive in the positive electrode 10 is preferably 4 to 40 mass %, more preferably 10 to 25 mass %. When the content of the positive electrode conductive additive is equal to or greater than the lower limit of the above-mentioned preferred range, sufficient conductivity is easily obtained. When the content of the positive electrode conductive additive is equal to or greater than the lower limit of the above-mentioned preferred range, the electrode is easily molded into a pellet. On the other hand, when the content of the positive electrode conductive additive in the positive electrode 10 is equal to or less than the upper limit of the above-mentioned preferred range, sufficient discharge capacity is easily obtained by the positive electrode 10.

[0073] The positive electrode 10 may contain a binder (hereinafter, the binder used in the positive electrode 10 may be referred to as the "positive electrode binder"). A conventionally known substance can be used as the positive electrode binder, and examples thereof include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyacrylic acid (PA), carboxymethyl cellulose (CMC), and polyvinyl alcohol (PVA). Among these, polyacrylic acid is preferred. Cross-linked polyacrylic acid is particularly preferred. The positive electrode binder may be one of the above materials used alone, or two or more of them may be used in combination.

[0074] When polyacrylic acid is used as the positive electrode binder, it is preferable to adjust the pH of the polyacrylic acid in advance to 3 to 10. In this case, for example, an alkali metal hydroxide such as lithium hydroxide or an alkaline earth metal hydroxide such as magnesium hydroxide can be used to adjust the pH. The content of the positive electrode binder in the positive electrode 10 can be, for example, 1 to 20 mass %.

[0075] The size of the positive electrode 10 is determined according to the size of the nonaqueous electrolyte secondary battery 1. The thickness of the positive electrode 10 is determined according to the size of the nonaqueous electrolyte secondary battery 1. When the nonaqueous electrolyte secondary battery 1 is a coin-type battery for backup use in various electronic devices, the thickness of the positive electrode 10 is, for example, about 300 to 1000 μm.

[0076] The following method can be used to manufacture the positive electrode 10. A positive electrode mixture is obtained using a positive electrode active material. At least one of a positive electrode conductive additive and a positive electrode binder is added to the positive electrode mixture as needed. This positive electrode mixture is then pressure-molded into a desired shape. The pressure during pressure molding is determined taking into consideration the type of positive electrode conductive additive, etc., and is, for example, 0.2 to 5 ton / cm. ２ It can be said that:

[0077] A conventionally known material can be used as the positive electrode current collector 14. The positive electrode current collector 14 is made of, for example, a conductive resin adhesive containing carbon as a conductive filler.

[0078] [Negative Electrode] The negative electrode 20 contains lithium (Li) and SiO as negative electrode active materials. Ｘ (0≦X<2) As the negative electrode 20, for example, a mixture of a negative electrode active material, an appropriate binder, polyacrylic acid as a binding agent, and graphite or the like as a conductive additive can be used.

[0079] Negative electrode active material lithium (Li) and SiO Ｘ (0≦X<2) and the molar ratio (Li / SiO Ｘ ) is in the range of 3.8 to 4.9. The negative electrode active material lithium (Li) and SiO Ｘ (0≦X<2) and the molar ratio (Li / SiO Ｘ The molar ratio (Li / SiO Ｘ The molar ratio (Li / SiO Ｘ ) may be, for example, 3.9 to 4.9. Ｘ The molar ratio (Li / SiO Ｘ ) may be, for example, 4.1 to 4.6.

[0080] Negative electrode active material lithium (Li) and SiO Ｘ By setting the molar ratio of lithium (Li) and SiO in the negative electrode active material to the above range, it is possible to make charging abnormalities less likely to occur. ＸBy setting the molar ratio to be within the above range, it is possible to prevent the discharge capacity from decreasing even when the nonaqueous electrolyte secondary battery 1 is used or stored in a high-temperature environment for a long period of time.

[0081] The molar ratio (Li / SiO Ｘ If the molar ratio (Li / SiO) is less than 3.8, the lithium content will be insufficient when the battery is used or stored for a long period of time in a high-temperature environment, and the discharge capacity will decrease. Ｘ If the molar ratio (Li / SiO) exceeds 4.9, the amount of Li is too large, and charging abnormalities tend to occur. Ｘ ) exceeds 4.9, metallic Li is converted into SiO Ｘ Since the carbon dioxide remains without being absorbed into the electrolyte, the internal resistance may increase and the discharge capacity may decrease.

[0082] SiO ｘ (0≦X<2) is, for example, SiO. ｘ By using (0≦X<2), the nonaqueous electrolyte secondary battery 1 can be used at a high voltage and the cycle characteristics are improved. The negative electrode 20 contains lithium (Li) and SiO ｘ (0≦X<2) and further, carbon, Si, WO ２ and W.O. ３ By using the above-mentioned material as the negative electrode active material in the negative electrode 20, the reaction between the electrolyte solution 50 and the negative electrode 20 during charge / discharge cycles is suppressed, the decrease in capacity can be suppressed, and the cycle characteristics can be improved.

[0083] SiO ｘ (0≦X<2) may have at least a portion of the surface coated with carbon (C). This improves the conductivity of the negative electrode 20 and suppresses an increase in internal resistance in a low-temperature environment. Therefore, the voltage drop at the initial stage of discharge is suppressed, and the discharge characteristics can be further stabilized. SiO ｘ (0≦X<2) may be in particulate form. ｘ In the case of (0≦X<2), it is sufficient that at least a part of the surface is covered with carbon, but if the entire surface is covered, the above effect can be enhanced.

[0084] SiO ｘ The method for coating the particle surface (0≦X<2) with carbon is not particularly limited, but examples thereof include physical vapor deposition (PVD) using a gas containing an organic substance such as methane or acetylene, and chemical vapor deposition (CVD).

[0085] SiO as the negative electrode active material ｘ When a negative electrode active material having a particle diameter (D50) of 0≦X<2 is used, its particle diameter (D50) is not particularly limited, and is preferably 0.1 to 30 μm, and more preferably 1 to 10 μm, for example. If the particle diameter (D50) of the negative electrode active material is within the above range, conductivity is maintained even if expansion or contraction of the negative electrode occurs during charging and discharging of the non-aqueous electrolyte secondary battery, and therefore deterioration of charge-discharge characteristics such as cycle characteristics is suppressed. If the particle diameter (D50) of the negative electrode active material is less than the lower limit of the above preferred range, for example, the reactivity of the non-aqueous electrolyte secondary battery increases when exposed to high temperatures, making it difficult to handle. If the particle diameter (D50) of the negative electrode active material exceeds the upper limit of the above preferred range, the discharge rate may decrease. Note that, when the negative electrode active material (SiO ｘ (0≦X<2)) particle diameter (D50) is SiO ｘ (0≦X<2) is the particle size in a state where at least a part of the surface is coated with carbon.

[0086] The content of the negative electrode active material in the negative electrode 20 is determined taking into consideration the discharge capacity required for the nonaqueous electrolyte secondary battery 1, etc. The content of the negative electrode active material in the negative electrode 20 is, for example, preferably 50% by mass or more, more preferably 60 to 80% by mass. When the content of the negative electrode active material made of the above material in the negative electrode 20 is equal to or greater than the lower limit of the above preferred range, a sufficient discharge capacity is likely to be obtained. When the content of the negative electrode active material is equal to or less than the upper limit of the above preferred range, the negative electrode 20 is easily formed.

[0087] The negative electrode 20 may contain a conductive additive (hereinafter, the conductive additive used in the negative electrode 20 may be referred to as the "negative electrode conductive additive"). The negative electrode conductive additive is the same as the positive electrode conductive additive. The negative electrode 20 may contain a binder (hereinafter, the binder used in the negative electrode 20 may be referred to as the "negative electrode binder"). Examples of negative electrode binders include polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polyacrylic acid (PA), carboxymethyl cellulose (CMC), polyimide (PI), and polyamideimide (PAI). Among these, polyacrylic acid is preferred, and cross-linked polyacrylic acid is more preferred.

[0088] The negative electrode binder may be one of the above-mentioned binders alone, or two or more of them may be used in combination. When polyacrylic acid is used as the negative electrode binder, it is preferable to adjust the pH of the polyacrylic acid to 3 to 10 in advance. In this case, the pH can be adjusted by adding, for example, an alkali metal hydroxide such as lithium hydroxide or an alkaline earth metal hydroxide such as magnesium hydroxide. The content of the negative electrode binder in the negative electrode 20 is, for example, 1 to 20 mass%.

[0089] The size and thickness of the negative electrode 20 are similar to those of the positive electrode 10. In the nonaqueous electrolyte secondary battery 1 shown in Fig. 1, a lithium foil 60 is provided on the surface of the negative electrode 20, i.e., between the negative electrode 20 and a separator 30 described below.

[0090] The negative electrode 20 can be manufactured by the following method. First, SiO ｘ A negative electrode mixture is obtained using the above method. At least one of a negative electrode conductive additive such as graphite and a negative electrode binder is added to the negative electrode mixture as needed. This negative electrode mixture is then pressure-molded into a desired shape. The pressure during pressure molding is determined taking into consideration the type of negative electrode conductive additive, and is, for example, 0.2 to 5 ton / cm. ２Then, lithium foil is placed on the surface of the pressure-molded negative electrode mixture, and the mixture is sealed in a container together with a positive electrode, a separator, and an electrolyte to prepare a battery. After that, the battery is left standing at room temperature or after heating, whereby negative electrode 20 in which lithium is incorporated into the negative electrode mixture can be obtained.

[0091] The negative electrode current collector 24 can be made of the same material as the positive electrode current collector 14 .

[0092] [Separator] The separator 30 is interposed between the positive electrode 10 and the negative electrode 20. As the separator 30, an insulating film having high ion permeability, excellent heat resistance, and a predetermined mechanical strength is used.

[0093] The separator 30 can be made of any material conventionally used for separators in nonaqueous electrolyte secondary batteries and that satisfies the above-described characteristics. Examples of the separator 30 include glass, such as alkali glass, borosilicate glass, quartz glass, and lead glass, and nonwoven fabrics and fibers made of resins, such as polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyethylene terephthalate (PET), polyamide-imide (PAI), polyamide, polyimide (PI), aramid, cellulose, fluororesin, and ceramics. Among the above materials, nonwoven fabrics made of glass fibers are more preferable for the separator 30. Glass fibers have excellent mechanical strength and high ion permeability, which allows for reduced internal resistance and improved discharge capacity.

[0094] The thickness of the separator 30 is determined taking into consideration the size of the nonaqueous electrolyte secondary battery 1 and the material of the separator 30. The thickness of the separator 30 can be, for example, about 5 to 300 μm.

[0095] [Capacity Balance Between Negative Electrode and Positive Electrode] The capacity balance (negative electrode capacity (mAh) / positive electrode capacity (mAh)) represented by the capacity of the negative electrode 20 and the capacity of the positive electrode 10 is in the range of 1.56 to 2.51. The capacity balance is preferably 1.7 or more. The capacity balance is preferably 1.8 or more. The capacity balance is more preferably 2.0 or more. The capacity balance is preferably 2.4 or less. The capacity balance may be 2.3 or less.

[0096] By maintaining the capacity balance between the negative electrode 20 and the positive electrode 10 within the above range, a certain margin of capacity can be secured on the negative electrode side. For example, even if decomposition of the negative electrode active material due to the battery reaction proceeds quickly, a certain level of negative electrode capacity can be secured. Therefore, even if the nonaqueous electrolyte secondary battery 1 is stored or used for a long period of time in a harsh high-temperature and high-humidity environment, the decrease in discharge capacity is suppressed, and the storage characteristics are improved.

[0097] If the capacity balance between the negative electrode 20 and the positive electrode 10 is less than 1.56, deterioration will increase during long-term use in a high-temperature environment, making it difficult to maintain capacity. On the other hand, if the capacity balance between the negative electrode 20 and the positive electrode 10 exceeds 2.51, sufficient discharge capacity will not be obtained.

[0098] [Molar Ratio of Lithium to Lithium Manganese Oxide] The molar ratio (Li / LMO) of lithium (Li) used in the negative electrode 20 to lithium manganese oxide (LMO) used in the positive electrode 10 is 8.0 or less. Li / LMO may be 7.8 or less.

[0099] By setting the molar ratio (Li / LMO) of lithium (Li) to lithium manganese oxide (LMO) within the above range, the electrical characteristics after storage of the nonaqueous electrolyte secondary battery under high-temperature, high-humidity, and overdischarge conditions, i.e., the overdischarge characteristics, become good, and thus degradation of the nonaqueous electrolyte secondary battery can be suppressed even when overdischarge occurs.

[0100] The molar ratio (Li / LMO) of lithium (Li) to lithium manganese oxide (LMO) may be, for example, 6.7 or more. When Li / LMO is in this range, the capacity balance falls within the above range (1.56 to 2.51), and the Li / SiOＸ By keeping the capacity balance within the above range, it is possible to prevent a decrease in discharge capacity even when the nonaqueous electrolyte secondary battery is used under severe temperature and humidity conditions. Ｘ By keeping the value within the above range, charging abnormalities and the like are less likely to occur, and further, even when the battery is used or stored in a high-temperature environment for a long period of time, a decrease in discharge capacity can be suppressed.

[0101] <Effects of the Non-Aqueous Electrolyte Secondary Battery of the Present Embodiment> In the non-aqueous electrolyte secondary battery 1 of the present embodiment, the capacity balance represented by the capacity of the negative electrode and the capacity of the positive electrode is optimized as described above, and the lithium and SiO Ｘ (0≦X<2) and the molar ratio (Li / SiO Ｘ ) is set to the above range. Furthermore, the molar ratio of lithium to lithium manganese oxide (Li / LMO) is set to the above range. This makes it possible to obtain a sufficient discharge capacity and improve overdischarge characteristics.

[0102] The non-aqueous electrolyte secondary battery 1 includes a separator 30 disposed between a positive electrode 10 and a negative electrode 20, and a storage container 2 in which the positive electrode 10, the negative electrode 20, the separator 30, and the electrolyte solution 50 are disposed in an internal storage space.

[0103] The nonaqueous electrolyte secondary battery 1 has each battery element disposed in the storage space of the storage container 2 that houses the positive electrode 10 and the negative electrode 20, resulting in a structure with excellent electrical insulation and sealing properties, which can suppress volatilization of the electrolyte solution and intrusion of moisture contained in the atmosphere into the battery.

[0104] The storage container 2 is a coin-shaped container that includes a cylindrical positive electrode can 12 with a bottom, and a negative electrode can 22 that is fixed to the opening 12a of the positive electrode can 12 with a gasket 40 interposed therebetween and forms a storage space between the negative electrode can 22 and the positive electrode can 12.

[0105] The nonaqueous electrolyte secondary battery 1 has battery elements arranged in the storage space of the storage container 2, which has an optimally sealed structure of the positive electrode can 12 and the negative electrode can 22. This provides a structure with excellent electrical insulation and sealing properties, and prevents the volatilization of the electrolyte and the intrusion of moisture from the atmosphere into the battery. This allows the battery to achieve high output and capacity characteristics even in a small, coin-shaped nonaqueous electrolyte secondary battery.

[0106] <Other Forms of Nonaqueous Electrolyte Secondary Battery> In the present embodiment, a coin-shaped nonaqueous electrolyte secondary battery has been described as an embodiment of the nonaqueous electrolyte secondary battery, which includes a housing container formed by crimping a positive electrode can and a negative electrode can. However, the present invention is not limited to this. The nonaqueous electrolyte secondary battery may be, for example, a nonaqueous electrolyte secondary battery having a structure in which the opening of a ceramic container body is sealed with a ceramic lid by heat treatment such as seam welding using a metal sealing member.

[0107] The configuration according to this embodiment can also be applied to electrochemical cells such as lithium ion capacitors.

[0108] <Uses of Non-Aqueous Electrolyte Secondary Battery> As described above, the non-aqueous electrolyte secondary battery 1 of this embodiment can obtain sufficient discharge capacity and exhibit good overdischarge characteristics, and therefore can be suitably used as a backup power source with a voltage value of 2 to 3 V, for example.

[0109] The silicon oxide contained in the negative electrode active material is SiO Ｘ (0<X≦2) is also acceptable.

[0110] Next, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to these examples. The nonaqueous electrolyte secondary battery according to the present invention can be appropriately modified and implemented within the scope that does not deviate from the gist of the present invention.

[0111] <Preparation of Electrolyte Solution and Fabrication of Non-Aqueous Electrolyte Secondary Battery> A coin-type non-aqueous electrolyte secondary battery shown in FIG. 1 was fabricated. An electrolyte solution having the composition shown in Table 1 was prepared. Li was used as the positive electrode active material. １．１４ Co ０．０６ Mn １．８０ O４ The negative electrode active material contained lithium (Li) and SiO whose entire surface was coated with carbon. This nonaqueous electrolyte secondary battery was a coin-shaped (920 size) nonaqueous electrolyte secondary battery (lithium secondary battery) having an outer diameter of 9.5 mm and a thickness of 2.0 mm.

[0112] (Preparation of Electrolyte Solution) An organic solvent was prepared according to the blending ratio (volume %) shown in Table 1, and a supporting salt was dissolved in this organic solvent to prepare an electrolyte solution 50. In this case, propylene carbonate (PC), ethylene carbonate (EC), and dimethoxyethane (DME) were mixed as the organic solvent in a volume ratio of {PC:EC:DME}={1:1:2} to prepare a mixed solvent. Lithium bis(fluorosulfonyl)imide (LiFSI) was dissolved as a supporting salt in the obtained mixed solvent at the concentration shown in Table 1 to obtain an electrolyte solution 50.

[0113] (Fabrication of Battery) A positive electrode 10 was fabricated as follows. １．１４ Co ０．０６ Mn １．８０ O ４ ) was used as the positive electrode active material. This positive electrode active material was mixed with graphite as a conductive additive and polyacrylic acid as a binder to prepare a positive electrode mixture. The ratio of lithium manganese oxide: graphite: polyacrylic acid was 95:4:1 (mass ratio). 52 to 56 mg of the obtained positive electrode mixture was pressed at a pressure of 14 to 15 MPa and pressure-molded into a disk-shaped pellet with a diameter of 5.8 mm.

[0114] Next, the resulting pellet (cathode 10) was bonded to the inner surface of a stainless steel (NAS64: t = 0.20 mm) cathode can 12 using a carbon-containing conductive resin adhesive, and these were integrated to obtain a cathode unit. This cathode unit was heated and dried in air at 120°C for 11 hours. A sealant was applied to the inner surface of the opening 12a of the cathode can 12 in the cathode unit.

[0115] The negative electrode 20 was fabricated as follows. A negative electrode active material containing SiO powder with carbon (C) formed on the entire surface was prepared. Here, the ratio of SiO to C in the SiO powder was previously adjusted to 94:6 to 96:4 by weight. This negative electrode active material was mixed with graphite as a conductive agent, polyacrylic acid as a binder, and ethylene-tetrafluoroethylene copolymer (ETFE) as a release agent to form a negative electrode mixture. The ratio of negative electrode active material: graphite: polyacrylic acid: ETFE was 75:20:4:1 (mass ratio). 10.0 to 14.1 mg of the obtained negative electrode mixture was pressure-molded at a pressure of 20 MPa and pressure-molded into a disk-shaped pellet with a diameter of 6.3 mm.

[0116] Next, the resulting pellet (negative electrode 20) was bonded to the inner surface of an anode can 22 made of a clad material (t=0.20 mm) consisting of a copper layer, a stainless steel layer, and a nickel layer using a conductive resin adhesive containing carbon as a conductive filler, and these were integrated to obtain a negative electrode unit. This negative electrode unit was then dried by heating under reduced pressure in air at 160°C for 11 hours. Lithium foil 60 punched to a diameter of 5.4 to 6.0 mm and a thickness of 0.38 to 0.46 mm was then pressure-bonded onto the pellet-shaped negative electrode 20 to form a lithium-negative electrode laminate.

[0117] In this example, the positive electrode current collector 14 and the negative electrode current collector 24 shown in FIG. 1 were not provided, and the positive electrode can 12 was made to function as a positive electrode current collector, and the negative electrode can 22 was made to function as a negative electrode current collector, to fabricate a nonaqueous electrolyte secondary battery.

[0118] Next, the nonwoven fabric made of glass fibers was dried and then punched into a disk shape with a diameter of 7.4 mm to form the separator 30. This separator 30 was placed on the lithium foil 60 that had been pressure-bonded onto the negative electrode 20, and a polypropylene gasket 40 was placed at the opening of the negative electrode can 22.

[0119] Next, the positive electrode can 12 and the negative electrode can 22 were filled with the electrolyte 50 prepared in the above-described procedure in a total amount of 25.6 μL per battery.

[0120] Next, the negative electrode unit was crimped to the positive electrode unit so that the separator 30 was in contact with the positive electrode 10. Then, the opening of the positive electrode can 12 was fitted to seal the positive electrode can 12 and the negative electrode can 22, and the battery was then left to stand at room temperature for 4 days and then at 60°C for 2 days to obtain a nonaqueous electrolyte secondary battery.

[0121]

[0122] [Test Examples 1 to 9] In producing each of the electrodes, the capacity balance (negative electrode capacity (mAh) / positive electrode capacity (mAh)) represented by the capacity of the negative electrode 20 and the capacity of the positive electrode 10 was adjusted to the value shown in Table 2. Table 2 shows the molar ratio (Li / SiO) of lithium (Li) to SiO in the negative electrode active material. Table 2 also shows the molar ratio (Li / LMO) of lithium (Li) to lithium manganese oxide (LMO).

[0123] <Evaluation Method> The nonaqueous electrolyte secondary batteries of Test Examples 1 to 9 obtained by the above procedure were subjected to the evaluation tests described below.

[0124] The nonaqueous electrolyte secondary batteries were placed in an overdischarged state by short-circuiting through a 10 kΩ resistor, and were then placed in this state for 20 days at a temperature of 60°C, after which the discharge capacity was measured. Three samples were used for each test example, and the Li / LMO and discharge capacity described below for each sample are shown in Table 2. The measurement results are shown in Table 2 and FIG. 2.

[0125] The charge / discharge conditions for measuring the discharge capacity were as follows: The overdischarge resistor was removed, and the battery was left at room temperature for 1 hour, after which charging was performed under the following charge conditions. Next, the discharge capacity was measured under the following discharge conditions. Charge conditions: constant current constant voltage (CCCV) charge, charge current 200 μA, charge voltage 3.1 V, charge time 72 hours, room temperature. Discharge conditions: constant current (CC) discharge, discharge current 25 μA, end voltage 2.0 V.

[0126]

[0127] 2, in the test examples in which the molar ratio (Li / LMO) of lithium (Li) to lithium manganese oxide (LMO) was 8.0 or less, the electrical characteristics (overdischarge characteristics) after storage of the nonaqueous electrolyte secondary battery under high temperature and overdischarge conditions were good. This shows that degradation of the nonaqueous electrolyte secondary battery can be suppressed even when overdischarge occurs.

[0128] [Test Examples 10 to 14] Next, nonaqueous electrolyte secondary batteries of Test Examples 10, 11, 12, 13, and 14 were fabricated. Test Examples 10 to 14 had the same capacity balance and Li / SiO as Test Example 1, and the same Li / LMO (7.95) as Test Example 1, except for the concentration of LiFSI, which was the supporting electrolyte of the electrolyte solution. That is, the capacity balance of Test Examples 10 to 14 was the same as that of Test Example 1. The Li / SiO of Test Examples 10 to 14 was the same as that of Test Example 1. The Li / LMO of Test Examples 10 to 14 was the same as that of Test Example 1. The concentrations of LiFSI in Test Examples 10 to 14 were 1.0 mol / L (1.0 M), 1.2 mol / L (1.2 M), 1.5 mol / L (1.5 M), 1.8 mol / L (1.8 M), and 2.0 mol / L (2.0 M), respectively. The discharge capacities of the nonaqueous electrolyte secondary batteries of Test Examples 10 to 14 at low temperatures were measured as described below.

[0129] First, as a pretreatment, the nonaqueous electrolyte secondary battery was sequentially discharged and charged at room temperature under the following conditions: Discharge conditions: constant current (CC) discharge, discharge current 25 μA, end voltage 2.0 V. Charging conditions: constant current constant voltage (CCCV) charge, charge current 200 μA, charge voltage 3.1 V, charge time 72 h.

[0130] Next, the pretreated nonaqueous electrolyte secondary batteries were charged and discharged under the following conditions. Charging conditions: constant current constant voltage (CCCV) charging, charging current 200 μA, charging voltage 3.1 V, charging time 2 hours. Cooling was initiated after the start of charging, and the nonaqueous electrolyte secondary batteries were cooled from room temperature to −40°C. Discharging conditions: constant current (CC) discharge, discharge current 25 μA, end voltage 2.0 V, −40°C. Three samples were used for each test example. The measurement results of the supporting electrolyte concentration and discharge capacity for each test example are shown in the graph of FIG. 3.

[0131] As shown in Figure 3, when the supporting electrolyte concentration was in the range of 1.0 to 1.5 mol / L (Test Examples 10 to 12), the discharge capacity was maintained and the variation was small. In contrast, when the supporting electrolyte concentration was 1.8 mol / L and 2.0 mol / L (Test Examples 13 and 14), the discharge capacity decreased significantly and the variation also increased. From the above, it can be seen that when using an electrode in which the capacity balance, Li / SiO, and Li / LMO were adjusted to be within the above ranges and an electrolyte with the above supporting electrolyte concentration was used, the discharge capacity could be maintained in a low-temperature environment.

[0132] According to the present invention, a nonaqueous electrolyte secondary battery that has sufficient discharge capacity and excellent overdischarge characteristics can be provided. By applying the present invention to nonaqueous electrolyte secondary batteries used in various electronic devices, for example, the present invention can also contribute to improving the performance of various electronic devices.

[0133] REFERENCE SIGNS LIST 1 nonaqueous electrolyte secondary battery 2 storage container 10 positive electrode 12 positive electrode can 12a opening 20 negative electrode 22 negative electrode can 30 separator 40 gasket 50 electrolyte

Claims

1. A positive electrode containing lithium manganese oxide (LMO) as a positive electrode active material, and a negative electrode containing lithium (Li) and SiO Ｘ (0≦X<2) and an electrolyte solution containing an organic solvent and a supporting salt, wherein a capacity balance {negative electrode capacity (mAh) / positive electrode capacity (mAh)} expressed by a capacity of the negative electrode and a capacity of the positive electrode is in a range of 1.56 to 2.51, and Ｘ (0≦X<2) and the molar ratio (Li / SiO Ｘ ) is in the range of 3.8 to 4.9, and a molar ratio (Li / LMO) of the lithium (Li) to the lithium manganese oxide (LMO) is 8.0 or less.

2. The positive electrode contains at least Li as the lithium manganese oxide used as the positive electrode active material. １＋ｘ Co ｙ Mn ２－ｘ－ｙ O ４ The nonaqueous electrolyte secondary battery according to claim 1 , wherein x is a saturating agent, and y is a saturating agent.

3. The SiO Ｘ The nonaqueous electrolyte secondary battery according to claim 1 , wherein (0≦X<2) at least a portion of the surface is coated with carbon.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolytic solution contains as the organic solvent a mixed solution containing propylene carbonate (PC), ethylene carbonate (EC), and dimethoxyethane (DME) in a volume ratio range of {PC:EC:DME}={0.5-1.5:0.5-1.5:1-3}, and as the supporting salt, lithium bis(fluorosulfonyl)imide (LiFSI) at 0.6 to 1.5 (mol / L).

5. The nonaqueous electrolyte secondary battery according to claim 1, comprising: a separator disposed between the positive electrode and the negative electrode; and a container having an internal storage space in which the positive electrode, the negative electrode, the separator, and the electrolytic solution are disposed.

6. The nonaqueous electrolyte secondary battery according to claim 5, wherein the storage container is a coin-shaped container comprising: a cylindrical positive electrode can with a bottom; and an anode can fixed to an opening of the positive electrode can with a gasket interposed therebetween, forming a storage space between the anode can and the positive electrode can.

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