Non-aqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a negative electrode and a nonaqueous electrolyte; the positive electrode contains a positive electrode active material; the positive electrode active material contains a lithium transition metal composite oxide that contains Ni, Mn and Al; and the respective proportions of Ni, Mn and Al in the metal elements other than Li contained in the lithium transition metal composite oxide are 50% by atom or more (Ni), 10% by atom or less (Mn) and 10% by atom or less (Al). In cases where the lithium transition metal composite oxide contains Co, the proportion of Co in the metal elements other than Li is 1.5% by atom or less. The nonaqueous electrolyte contains an organic phosphorus compound represented by general formula (1). In general formula (1), each of R1 and R2 independently represents an alkyl group having 1 to 4 carbon atoms; and R3 represents a fluorinated alkyl group having 1 to 4 carbon atoms.

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, comprise a positive electrode, a negative electrode, and a non-aqueous electrolyte. To ensure the superior characteristics of non-aqueous electrolyte secondary batteries, efforts are being made to improve the battery's components.

[0003] Patent Document 1 proposes a non-aqueous electrolyte containing a compound (A) having an organic group having 1 to 20 carbon atoms, which may have substituents on the nitrogen atom of isocyanuric acid, and a nitrile compound, isocyanate compound, difluorophosphate compound, or fluorosulfonate salt.

[0004] Patent Document 2 describes formula 1:Li x Ni 1-y-z-v-w Co y Al z M1 v M2 w We propose a positive electrode active material for a non-aqueous electrolyte secondary battery, which consists of a lithium-containing composite oxide represented by O2, where element M1 in formula 1 is at least one selected from the group consisting of Mn, Ti, Y, Nb, Mo, and W, and element M2 is at least two selected from the group consisting of Mg, Ca, Sr, and Ba, and element M2 contains at least Mg and Ca, and formula 1 satisfies 0.97≦x≦1.1, 0.05≦y≦0.35, 0.005≦z≦0.1, 0.0001≦v≦0.05, and 0.0001≦w≦0.05, and the composite oxide is composed of primary particles aggregated to form secondary particles, the average particle size of the primary particles of the composite oxide is 0.1 μm or more and 3 μm or less, and the average particle size of the secondary particles of the composite oxide is 8 μm or more and 20 μm or less. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-194930 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-310181 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] In recent years, the price of Co in lithium transition metal composite oxides has been soaring. Reducing the Co content in lithium transition metal composite oxides is cost-effective, but the cycle characteristics of non-aqueous electrolyte secondary batteries deteriorate. This is presumably because the lattice structure of the lithium transition metal composite oxide becomes unstable and the deterioration is accelerated by side reactions. [Means for Solving the Problems]

[0007] One aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode contains a positive electrode active material, and the positive electrode active material contains a lithium transition metal composite oxide containing Ni, Mn, and Al. The ratios of Ni, Mn, and Al in the metal elements other than Li contained in the lithium transition metal composite oxide are Ni: 50 atomic% or more, Mn: 10 atomic% or less, and Al: 10 atomic% or less, respectively. When the lithium transition metal composite oxide contains Co, the ratio of Co in the metal elements other than Li is 1.5 atomic% or less. The non-aqueous electrolyte has the general formula (1):

[0008] [Chemical Formula]

[0009] and contains an organic phosphorus compound represented by the formula. In the general formula (1), R 1 and R 2 are each independently an alkyl group having 1 to 4 carbon atoms, and R 3 is a fluorinated alkyl group having 1 to 4 carbon atoms. It relates to a non-aqueous electrolyte secondary battery. [Advantages of the Invention]

[0010] According to the present disclosure, even when a lithium transition metal composite oxide containing no Co or a lithium transition metal composite oxide having a small Co content is used, a non-aqueous electrolyte secondary battery excellent in cycle characteristics can be provided.

[0011] The novel features of the present invention are described in the appended claims. However, the present invention will be better understood with reference to the following detailed description taken in conjunction with the drawings, in terms of both its configuration and content, as well as other objects and features of the present invention.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic perspective view of a part of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a cutout.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the non-aqueous electrolyte secondary battery according to the present disclosure will be described with examples. However, the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not more than the upper limit. When a plurality of materials are exemplified, one of them may be selected and used alone, or two or more of them may be combined and used.

[0014] In addition, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0015] Non-aqueous electrolyte secondary batteries include at least lithium-ion batteries and lithium metal secondary batteries.

[0016] The non-aqueous electrolyte secondary battery according to this disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material. The positive electrode active material includes a lithium transition metal composite oxide containing Ni, Mn, and Al.

[0017] Reducing the Co content and increasing the Ni content of the lithium transition metal composite oxide is cost-effective and allows for the securing of high capacity. Therefore, in the non-aqueous electrolyte secondary battery according to this disclosure, the Ni content of the lithium transition metal composite oxide is increased. On the other hand, in the non-aqueous electrolyte secondary battery according to this disclosure, the lithium transition metal composite oxide either does not contain Co or the proportion of Co among metal elements other than Li is limited to 1.5 atomic percent or less. Hereinafter, the lithium transition metal composite oxide in the non-aqueous electrolyte secondary battery according to this disclosure will also be referred to as "composite oxide NMA".

[0018] The proportions of Ni, Mn, and Al among the metal elements other than Li in the composite oxide NMA are Ni: 50 atomic% or more, Mn: 10 atomic% or less, and Al: 10 atomic% or less, respectively, and the composite oxide NMA does not contain Co, or the proportion of Co among the metal elements other than Li is 1.5 atomic% or less.

[0019] Mn and Al contribute to stabilizing the crystal structure of composite oxide NMA with reduced Co content. However, because the Co content of composite oxide NMA is limited to 1.5 atomic percent or less and the Ni content is high, the crystal structure tends to be unstable, and metals such as Al and Ni may leach out of the composite oxide NMA. When metals leach out, the positive electrode capacity decreases and the cycle characteristics (or capacity retention rate) deteriorates. In particular, in composite oxide NMA with a high Ni content, the leached Ni may form an oxide film on the particle surface of the composite oxide NMA with a structure that hinders the intercalation and release of Li ions, leading to an increase in internal resistance. In addition, in positive electrodes containing composite oxide NMA, the non-aqueous electrolyte is easily oxidized and decomposed, which reduces the cycle characteristics (or capacity retention rate) and increases resistance.

[0020] In view of the above, the non-aqueous electrolyte secondary battery relating to this disclosure uses a composite oxide NMA and a non-aqueous electrolyte containing an organophosphorus compound represented by the following general formula (1) (hereinafter also referred to as compound A).

[0021] [ka]

[0022] In general formula (1), R 1 and R 2 Each of these is an alkyl group having 1 to 4 carbon atoms. In general formula (1), R 3 These are fluorinated alkyl groups having 1 to 4 carbon atoms.

[0023] By incorporating compound A into the non-aqueous electrolyte, the oxidation resistance of the non-aqueous electrolyte is improved, and a high-quality coating derived from compound A is formed on the surface of the positive electrode active material particles. This coating exhibits excellent ionic conductivity and stability, suppressing the degradation of the positive electrode active material (dissolution of metal from the positive electrode active material) due to contact between the positive electrode active material and the non-aqueous electrolyte. As a result, it is believed that the decrease in cycle characteristics associated with the degradation of the positive electrode active material is suppressed.

[0024] Incidentally, even when compound A is combined with lithium transition metal composite oxides that have a higher Co content than composite oxide NMA, it is not possible to significantly improve cycle characteristics or suppress the increase in internal resistance. Compound A exhibits these effects significantly when combined with composite oxide NMA. The reason why the effect is significant with composite oxide NMA is thought to be that composite oxide NMA has higher resistance than lithium transition metal composite oxides with a higher Co content, and its particles are relatively brittle. The particles of composite oxide NMA are prone to cracking, metal leaching is also more pronounced, and the increase in resistance associated with charging and discharging is more readily apparent. Therefore, in composite oxide NMA, the improvement in characteristics due to the coating derived from compound A is greater. On the other hand, lithium transition metal composite oxides with a higher Co content are superior in this respect, so there is less need to use compound A.

[0025] (Organophosphorus compounds) In general formula (1), R 1 and R 2 Each of these is an alkyl group having 1 to 4 carbon atoms. The alkyl group having 1 to 4 carbon atoms may be linear or branched. Examples of alkyl groups having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, and tert-butyl groups. Among these, methyl and ethyl groups are preferred.

[0026] Let's assume R 1 and R 2 When at least one of the components is a fluorinated alkyl group having 1 to 4 carbon atoms, the solubility of reaction products derived from organophosphorus compounds tends to increase, and the ability to form a coating decreases.

[0027] In general formula (1), R 3This refers to a fluorinated alkyl group having 1 to 4 carbon atoms. A fluorinated alkyl group is defined as an alkyl group in which at least one hydrogen atom is replaced by a fluorine atom. In a fluorinated alkyl group having 1 to 4 carbon atoms, the remaining hydrogen atoms that are not replaced by a fluorine atom may be further replaced by halogen atoms other than fluorine (e.g., chlorine atoms, bromine atoms). The fluorinated alkyl group may be linear or branched.

[0028] At least one hydrogen atom of the alkyl group is substituted with a fluorine atom, and R 3 It is presumed that the direct bonding of phosphorus atoms to the phosphorus atoms affects the ionic conductivity and stability of the coating.

[0029] Let's assume R 3 If R is an alkyl group with 1 to 4 carbon atoms that does not contain a fluorine atom, it is thought that the coating derived from the organophosphorus compound will not be able to exist stably. Also, R 3 Even if the phosphorus atom is a fluorinated alkyl group with 1 to 4 carbon atoms, if it is bonded to the phosphorus atom via an oxygen atom, the resistance of the coating tends to increase.

[0030] Examples of fluorinated alkyl groups having 1 to 4 carbon atoms include fluoromethyl group, difluoromethyl group, trifluoromethyl group, 2-fluoroethyl group, 2,2-difluoroethyl group, 2,2,2-trifluoroethyl group, 2-fluoropropyl group, 3-fluoropropyl group, 2,2-difluoropropyl group, 3,3-difluoropropyl group, 3,3,3-trifluoropropyl group, 2-fluorobutyl group, 3-fluorobutyl group, 4-fluorobutyl group, 2,2-difluorobutyl group, 3,3-difluorobutyl group, 4,4-difluorobutyl group, and 4,4,4-trifluorobutyl group. Among these, fluoromethyl group, difluoromethyl group, and trifluoromethyl group are preferred.

[0031] A specific example of compound A is preferably at least one selected from the group consisting of diethyl(fluoromethyl)phosphonate, diethyl(difluoromethyl)phosphonate, and diethyl(trifluoromethyl)phosphonate. Among these, diethyl(difluoromethyl)phosphonate is more preferred. Compound A may be used alone or in combination of two or more types.

[0032] (Fluorinated cyclic carbonate) The non-aqueous electrolyte may further contain a fluorinated cyclic carbonate. A fluorinated cyclic carbonate refers to a compound in which at least one hydrogen atom of a cyclic carbonate is replaced by a fluorine atom. When compound A and a fluorinated cyclic carbonate are used together, a film is formed from the combination of compound A and the fluorinated cyclic carbonate, further enhancing the ionic conductivity of the film. This ensures superior cycle characteristics and further suppresses the increase in internal resistance. In particular, when the NMA composite oxide described later is used as the positive electrode active material, the effect of using compound A and a fluorinated cyclic carbonate together is significantly observed. However, fluorinated cyclic carbonate alone is disadvantageous in terms of the ionic conductivity of the film.

[0033] When a fluorinated cyclic carbonate (e.g., FEC) is added to a non-aqueous electrolyte, the amount of gas generated tends to increase when the battery is stored at high temperatures in a charged state. However, when a fluorinated cyclic carbonate is added to the non-aqueous electrolyte together with compound A, gas generation is significantly suppressed.

[0034] Fluorinated cyclic carbonates include, for example, compounds represented by the following general formula (2).

[0035] [ka]

[0036] In general formula (2), R 4 ~R 7Each of these is independently a hydrogen atom or a methyl group, and at least one of the hydrogen atoms of the hydrogen atom and the hydrogen atom of the methyl group is substituted with a fluorine atom. The remaining hydrogen atoms that are not substituted with a fluorine atom may be substituted with a halogen atom other than a fluorine atom (e.g., a chlorine atom, a bromine atom).

[0037] Specific examples of fluorinated cyclic carbonates include fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1-fluoropropylene carbonate, 3,3,3-trifluoropropylene carbonate, and 2,3-difluoro-2,3-butylene carbonate. Among these, FEC is preferred from the viewpoint of reducing film resistance. Fluorinated cyclic carbonates may be used individually or in combination of two or more types.

[0038] The non-aqueous electrolyte secondary battery related to this disclosure will be described in more detail below, component by component.

[0039] [Positive electrode] The positive electrode contains a positive electrode active material. The positive electrode typically comprises a positive electrode current collector and a layered positive electrode mixture (hereinafter referred to as the positive electrode mixture layer) held by the positive electrode current collector. The positive electrode mixture layer can be formed by coating the surface of the positive electrode current collector with a positive electrode slurry, which is obtained by dispersing the components of the positive electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary.

[0040] The positive electrode mixture may contain a positive electrode active material as an essential component, and may also contain binders, thickeners, conductive agents, etc., as optional components.

[0041] (Cathode active material) The positive electrode active material contains a composite oxide NMA. The composite oxide NMA contains Ni, Mn, and Al, and may contain trace amounts of Co, or may not contain Co. From the viewpoint of reducing manufacturing costs, a lower Co content is desirable, and the proportion of Co to the metal elements other than Li is 1.5 atomic% or less, preferably 1.0 atomic% or less, more preferably 0.5 atomic% or less, and most preferably no Co at all. On the other hand, from the viewpoint of increasing capacity, in the composite oxide NMA, the proportions of Ni, Mn, and Al to the metal elements other than Li are Ni: 50 atomic% or more, Mn: 10 atomic% or less, and Al: 10 atomic% or less, respectively. The Ni content to the metal elements other than Li is preferably 80 atomic% or more, more preferably 90 atomic% or more, and may be 92 atomic% or more. The Mn content may be 7 atomic% or less, 5 atomic% or less, or 3 atomic% or less. The Al content may be 9 atomic% or less, 7 atomic% or less, or 5 atomic% or less. The composite oxide NMA has, for example, a layered crystalline structure (e.g., a rock salt-type crystalline structure).

[0042] A composite oxide NMA is, for example, formulated with the formula: Li α Ni (1-x1-x2-y-z) Co x1 Mn x2 Al y M z O 2+β It is represented as follows: Element M is an element other than Li, Ni, Mn, Al, Co, and oxygen.

[0043] In the above formula, α, which represents the atomic ratio of lithium, is, for example, 0.95 ≤ α ≤ 1.05. α increases or decreases with charging and discharging. In (2 + β), which represents the atomic ratio of oxygen, β satisfies -0.05 ≤ β ≤ 0.05.

[0044] The atomic ratio of Ni, 1 - x1 - x2 - y - z(=v), is, for example, 0.685 or more, may be 0.8 or more, may be 0.90 or more, and may be 0.92 or more. Also, v representing the atomic ratio of Ni may be 0.95 or less. v may be 0.685 or more and 0.95 or less (0.685 ≤ v ≤ 0.95), may be 0.80 or more and 0.95 or less, may be 0.90 or more and 0.95 or less, and may be 0.92 or more and 0.95 or less.

[0045] The higher the atomic ratio v of Ni, the more lithium ions can be extracted from the composite oxide NMA during charging, and the higher the capacity can be increased. However, Ni in the composite oxide NMA with such increased capacity tends to have a higher valence. Also, when the atomic ratio of Ni increases, the atomic ratios of other elements relatively decrease. In this case, especially in the fully charged state, the crystal structure tends to become unstable, metals (such as Ni) are likely to dissolve, and it easily becomes inactivated by changing to a crystal structure where reversible intercalation and deintercalation of lithium ions are difficult due to repeated charge and discharge. As a result, the cycle characteristics are likely to deteriorate. Therefore, when using a composite oxide NMA with a high Ni content, the effect of improving the cycle characteristics by adding compound A to the non-aqueous electrolyte can be significantly obtained.

[0046] The atomic ratio x1 representing Co is, for example, 0.015 or less (0 ≤ x1 ≤ 0.015), may be 0.01 or less, and may be 0.005 or less. When x1 is 0, it includes the case where Co is below the detection limit.

[0047] The atomic ratio x2 representing Mn is, for example, 0.1 or less (0 < x2 ≤ 0.1), may be 0.07 or less, may be 0.05 or less, and may be 0.03 or less. x2 may be 0.01 or more and may be 0.02 or more. Mn contributes to the stabilization of the crystal structure of the composite oxide NMA, and the inclusion of inexpensive Mn in the composite oxide NMA is advantageous for cost reduction.

[0048] The y representing the atomic ratio of Al is, for example, 0.1 or less (0 < y ≤ 0.1), and may be 0.09 or less, may be 0.07 or less, or may be 0.05 or less. y may be 0.01 or more, or may be 0.02 or more. Al contributes to the stabilization of the crystal structure of the composite oxide NMA. Also, it is preferable to satisfy 0.05 ≤ x2 + y ≤ 0.1. In this case, the effects of Compound A and the effect of suppressing the increase in internal resistance after repeated charge and discharge become more prominent.

[0049] The z representing the atomic ratio of element M is, for example, 0 ≤ z ≤ 0.10, may be 0 < z ≤ 0.05, or may be 0.001 ≤ z ≤ 0.005.

[0050] Element M may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. Among them, when at least one selected from the group consisting of Nb, Sr, and Ca is contained in the composite oxide MNA, it is considered that the surface structure of the composite oxide NMA is stabilized, the resistance is reduced, and the elution of the metal is further suppressed. It is more effective if element M is unevenly distributed in the vicinity of the particle surface of the composite oxide NMA.

[0051] The content of the elements constituting the composite oxide NMA can be measured by an inductively coupled plasma atomic emission spectrometer (ICP - AES), an electron probe microanalyzer (EPMA), an energy dispersive X - ray spectrometer (EDX), or the like.

[0052] The composite oxide NMA is, for example, secondary particles in which a plurality of primary particles are aggregated. The particle size of the primary particles is generally 0.05 μm or more and 1 μm or less. The average particle size of the secondary particles of the composite oxide is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less.

[0053] In this specification, the average particle size of secondary particles refers to the particle size at which the integrated volume value in the particle size distribution measured by laser diffraction scattering method becomes 50% (volume-average particle size). Such a particle size is sometimes referred to as D50. For the measuring device, for example, the "LA-750" manufactured by HORIBA, Ltd. can be used.

[0054] The composite oxide NMA can be obtained, for example, by the following procedure. First, a solution containing an alkali such as sodium hydroxide is added dropwise to a solution of a salt containing the metal elements constituting the composite oxide NMA, while stirring, to adjust the pH to the alkaline side (for example, 8.5 to 12.5), thereby precipitating a composite hydroxide containing the metal elements (Ni, Mn, Al, Co if necessary, and element M if necessary). Subsequently, the composite hydroxide is calcined to obtain a composite oxide containing the metal elements (hereinafter also referred to as "raw material composite oxide"). The calcination temperature at this time is not particularly limited, but for example, it is 300°C to 600°C.

[0055] Next, the raw material composite oxide, a lithium compound, and a compound containing element M as needed are mixed, and the mixture is calcined under an oxygen stream to obtain the composite oxide NMA. The calcination temperature at this time is not particularly limited, but for example, it is between 450°C and 800°C. Each calcination may be carried out in one stage, in multiple stages, or while increasing the temperature.

[0056] Lithium compounds such as lithium oxide, lithium hydroxide, and lithium carbonate may be used. When mixing the raw material composite oxide and the lithium compound, a compound containing element M can be mixed in to make element M concentrated near the particle surface of the composite oxide NMA.

[0057] The positive electrode active material may contain lithium transition metal composite oxides other than composite oxide NMA, but it is preferable that the proportion of composite oxide NMA is high. The proportion of composite oxide NMA in the positive electrode active material is, for example, 90% by mass or more, and may be 95% by mass or more. The proportion of composite oxide in the positive electrode active material is 100% by mass or less.

[0058] (others) For example, resin materials can be used as binders. Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, and rubber-like materials (e.g., styrene-butadiene copolymer (SBR)). A single binder may be used, or two or more may be used in combination.

[0059] Examples of thickening agents include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, and methylcellulose. A single thickening agent may be used alone, or two or more may be used in combination.

[0060] Examples of conductive agents include conductive fibers and conductive particles. Examples of conductive fibers include carbon fibers, carbon nanotubes, and metal fibers. Examples of conductive particles include conductive carbon (carbon black, graphite, etc.) and metal powders. Conductive agents may be used individually or in combination of two or more types.

[0061] The dispersion medium used in the positive electrode slurry is not particularly limited, but examples include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0062] For example, a metal foil may be used as the positive electrode current collector. The positive electrode current collector may be porous. Examples of porous current collectors include nets, perforated sheets, and expanded metal. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium. The thickness of the positive electrode current collector is not particularly limited, but for example, it may be 1 to 50 μm, or 5 to 30 μm.

[0063] [Negative electrode] The negative electrode includes at least a negative electrode current collector and may also include a negative electrode active material. Typically, the negative electrode comprises a negative electrode current collector and a layered negative electrode mixture (hereinafter referred to as the negative electrode mixture layer) held by the negative electrode current collector. The negative electrode mixture layer can be formed by coating the surface of the negative electrode current collector with a negative electrode slurry, which is obtained by dispersing the components of the negative electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary.

[0064] The negative electrode mixture may contain a negative electrode active material as an essential component, and may also contain binders, thickeners, conductive agents, etc., as optional components.

[0065] (Negative electrode active material) As the negative electrode active material, metallic lithium, lithium alloys, etc., may be used, but materials capable of electrochemically intercalating and releasing lithium ions are preferably used. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode may contain one type of negative electrode active material, or a combination of two or more types.

[0066] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Carbonaceous materials may be used individually or in combination of two or more types.

[0067] Graphite is preferred as a carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0068] Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium ion conducting phase (matrix). Examples of silicon oxides include SiOx particles. x is, for example, 0.5 ≤ x < 2, and may also be 0.8 ≤ x ≤ 1.6. The lithium ion conducting phase can be at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase.

[0069] (others) For example, the materials exemplified for the positive electrode can be used as the binder, thickener, conductive agent, and dispersion medium for the negative electrode slurry.

[0070] For example, a metal foil may be used as the negative electrode current collector. The negative electrode current collector may be porous. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, etc. The thickness of the negative electrode current collector is not particularly limited, but for example, it may be 1 to 50 μm, or 5 to 30 μm.

[0071] [Non-aqueous electrolytes] Non-aqueous electrolytes typically consist of a non-aqueous solvent, a lithium salt, and additives.

[0072] The non-aqueous electrolyte contains compound A as an additive. The content of compound A in the non-aqueous electrolyte may be 2% by mass or less, 1.5% by mass or less, 1% by mass or less, or 0.5% by mass or less. When the content of compound A is within this range, excessive film formation on the positive electrode surface is suppressed, and the effect of suppressing the increase in internal resistance when charging and discharging is repeated can be enhanced. In non-aqueous electrolyte secondary batteries, the content of compound A in the non-aqueous electrolyte changes during storage or charging and discharging. Therefore, it is sufficient that compound A remains in the non-aqueous electrolyte sampled from the non-aqueous electrolyte secondary battery at a concentration above the detection limit. The content of compound A in the non-aqueous electrolyte may be 0.01% by mass or more.

[0073] The content of compound A in the non-aqueous electrolyte used in the manufacture of non-aqueous electrolyte secondary batteries may be 0.01% by mass or more, or 0.1% by mass or more, or 0.3% by mass or more. For example, the content of compound A in the non-aqueous electrolyte used in the manufacture of non-aqueous electrolyte secondary batteries may be 1.5% by mass or less, or 1% by mass or less, or 0.5% by mass or less. These lower and upper limits can be combined arbitrarily.

[0074] The non-aqueous electrolyte may further contain the above-mentioned fluorinated cyclic carbonate as an additive. The content of fluorinated cyclic carbonate in the non-aqueous electrolyte is preferably 1.5% by mass or less, and may be 1% by mass or less or 0.5% by mass or less. When the content of fluorinated cyclic carbonate is within this range, excessive film formation on the positive electrode surface is suppressed, and the effect of suppressing the increase in internal resistance when charging and discharging is repeated can be enhanced. In non-aqueous electrolyte secondary batteries, the content of fluorinated cyclic carbonate in the non-aqueous electrolyte changes during storage or charging and discharging. Therefore, it is sufficient that fluorinated cyclic carbonate remains in the non-aqueous electrolyte taken from the non-aqueous electrolyte secondary battery at a concentration above the detection limit. The content of fluorinated cyclic carbonate in the non-aqueous electrolyte may be 0.01% by mass or more.

[0075] The content of fluorinated cyclic carbonate in the non-aqueous electrolyte used in the manufacture of non-aqueous electrolyte secondary batteries may be 0.01% by mass or more, or 0.1% by mass or more, or 0.3% by mass or more. For example, the content of fluorinated cyclic carbonate in the non-aqueous electrolyte used in the manufacture of non-aqueous electrolyte secondary batteries may be 1.5% by mass or less, or 1% by mass or less, or 0.5% by mass or less. These lower and upper limits can be combined arbitrarily.

[0076] The content of compound A and fluorinated cyclic carbonate in the non-aqueous electrolyte can be determined, for example, by gas chromatography under the following conditions. Equipment used: Manufactured by Shimadzu Corporation, GC-2010 Plus Column: J&W HP-1 (film thickness 1 μm, inner diameter 0.32 mm, length 60 m) Column temperature: Increase the temperature from 50°C to 90°C at a rate of 5°C / min, maintain at 90°C for 15 minutes, then increase the temperature from 90°C to 250°C at a rate of 10°C / min, maintain at 250°C for 15 minutes. Split ratio: 1 / 50 Linear speed: 30.0cm / sec Inlet temperature: 270℃ Injection volume: 1μL Detector: FID 290℃ (sens.101)

[0077] In non-aqueous electrolytes, the mass ratio of fluorinated cyclic carbonate to compound A (=fluorinated cyclic carbonate / compound A) may be, for example, 0.5 to 1.5 or 0.8 to 1.2. When the mass ratio of both components is within this range, the balance of the film composition formed on the particle surface of the composite oxide NMA is improved. That is, a film is formed that has excellent ionic conductivity and a strong effect in suppressing metal elution and suppressing the increase in internal resistance when charging and discharging is repeated.

[0078] (Non-aqueous solvent) Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous electrolyte may contain one non-aqueous solvent or a combination of two or more non-aqueous solvents.

[0079] (Lithium salt) Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bisoxalate borate, lithium difluorooxalate borate, lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonate imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonate imide (LiN(C2F5SO2)2). The nonaqueous electrolyte may contain one lithium salt or a combination of two or more lithium salts.

[0080] The concentration of lithium salt in the non-aqueous electrolyte is, for example, between 0.5 mol / L and 2 mol / L.

[0081] The non-aqueous electrolyte may further contain, as an additive, at least one selected from the group consisting of vinylene carbonate and vinylethylene carbonate.

[0082] [Separator] It is desirable to interpose a separator between the positive and negative electrodes. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0083] One example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, is housed together with a non-aqueous electrolyte in an outer casing. However, it is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0084] The following describes the structure of a rectangular non-aqueous electrolyte secondary battery as an example of a non-aqueous electrolyte secondary battery related to this disclosure, with reference to Figure 1.

[0085] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser-welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a seal 8 after injection.

[0086] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0087] [Examples]

[0088] Examples 1-4 and Comparative Examples 1-8 A non-aqueous electrolyte secondary battery was fabricated and evaluated using the following procedure. (1) Preparation of the positive electrode 95 parts by mass of positive electrode active material particles were mixed with 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of aluminum foil, the coating was dried, and then the foil was rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm³) on both sides of the aluminum foil. 3 A positive electrode was obtained by forming a positive electrode.

[0089] The positive electrode active material particles were prepared using the following procedure. Aqueous solutions were prepared by dissolving nickel sulfate, aluminum sulfate, and, if necessary, cobalt sulfate or manganese sulfate. The concentration of nickel sulfate in the aqueous solution was set to 1 mol / L, and the concentrations of the other sulfates were adjusted so that the ratio of Ni to each metal element was as shown in Table 1.

[0090] At 50°C, while stirring the aqueous solution, an aqueous solution containing 30% by mass of sodium hydroxide was added dropwise until the pH of the mixture reached 12, thereby precipitating the hydroxide. The hydroxide was recovered by filtration, washed with water, and dried. The dried material was calcined at 500°C for 8 hours under a nitrogen atmosphere to obtain a composite oxide.

[0091] The obtained composite oxide and lithium hydroxide were mixed so that the total atomic ratio of Li, Ni, Co, Mn, and Al was 1:1. The mixture was calcined in an electric furnace in an oxygen atmosphere by heating from room temperature to 650°C at a heating rate of 2.0°C / min. Then, it was calcined again by heating from 650°C to 750°C at a heating rate of 0.5°C / min. The resulting calcined product was washed with water and dried to obtain the composite oxide NMA (positive electrode active material particles).

[0092] (2) Fabrication of the negative electrode A silicon composite material and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, sodium salt of CMC (CMC-Na), SBR, and water were mixed in a predetermined mass ratio to prepare the negative electrode slurry. Next, the negative electrode slurry was applied to the surface of a copper foil, which was to be used as the negative electrode current collector. After the coating was dried, it was rolled to form negative electrode mixture layers on both sides of the copper foil.

[0093] (3) Preparation of non-aqueous electrolytes A non-aqueous electrolyte (electrolyte) was prepared by dissolving LiPF6 and, if necessary, the organophosphorus compounds (component 1) and FEC (component 2) shown in Table 1 in a mixed solvent of EC and EMC (EC:EMC = 3:7 (volume ratio)). The concentration of LiPF6 in the electrolyte was 1.0 mol / L. The concentrations (initial concentrations) of component 1 and component 2 in the prepared non-aqueous electrolyte were the values ​​(mass%) shown in Table 1.

[0094] (4) Fabrication of non-aqueous electrolyte secondary batteries An aluminum positive electrode lead was attached to the positive electrode obtained above, and a nickel negative electrode lead was attached to the negative electrode obtained above. In an inert gas atmosphere, the positive and negative electrodes were wound in a spiral shape via a polyethylene thin film (separator) to create a wound electrode group. The electrode group was housed in a bag-shaped outer casing made of a laminate sheet with an aluminum layer, the non-aqueous electrolyte was injected, and the outer casing was sealed to create a non-aqueous electrolyte secondary battery. When housing the electrode group in the outer casing, parts of the positive and negative electrode leads were exposed to the outside of the outer casing. In Table 1, E1 to E4 are Examples 1 to 4, and C1 to C8 are Comparative Examples 1 to 8.

[0095] (5) Evaluation The non-aqueous electrolyte secondary batteries obtained in the examples and comparative examples were evaluated as follows. (a) Initial DC resistance (DCIR) Under 25°C conditions, the battery was charged with a constant current of 0.3 It until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.05 It. Subsequently, it was discharged at a constant current of 0.3 It for 100 minutes to bring the State of Charge (SOC) to 50%.

[0096] For a battery with a state of charge (SOC) of 50%, the voltage was measured after discharging it for 10 seconds at currents of 0A, 0.1A, 0.5A, and 1.0A. The DCIR (initial DCIR) was calculated from the absolute value of the slope of a straight line approximating the relationship between the discharge current and the voltage after 10 seconds using the least squares method.

[0097] (b) Charge-discharge cycle test Under conditions of 25°C, the battery was charged with a constant current of 0.5 It until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.02 It. Subsequently, it was discharged with a constant current of 0.5 It until the voltage reached 3.0V. This charging and discharging cycle was repeated 200 times, each time constituting one cycle.

[0098] (c) DCIR increase rate (ΔDCIR) Except for using a battery that had undergone 200 charge-discharge cycles in the charge-discharge cycle test described in (b) above, the DCIR (DCIR after 200 cycles) was calculated in the same manner as in (a) above. The ratio of the DCIR after 200 cycles to the initial DCIR was defined as the DCIR increase rate and calculated using the following formula. DCIR increase rate (%) = {(DCIR at 200 cycles - initial DCIR) / initial DCIR} × 100

[0099] (d) Capacity retention rate (MR) In the charge-discharge cycle test described in (b) above, the discharge capacity at the first cycle and the discharge capacity at the 200th cycle were measured, and the capacity retention rate was calculated using the following formula, which was used as an indicator of the cycle characteristics. Capacity retention rate (%) = (Discharge capacity at 200 cycles / Discharge capacity at 1 cycle) × 100

[0100] (e) Amount of gas generated during high-temperature storage Furthermore, the following high-temperature storage characteristics were evaluated for batteries E1, E2, C3, and C4. Under conditions of 25°C, the battery was charged with a constant current of 0.3It until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.02It. The charged battery was then stored at 80°C for 72 hours, and the amount of gas generated inside the battery during storage was examined.

[0101] The evaluation results are shown in Table 1. For the first component in Table 1, A1 is diethyl(difluoromethyl)phosphonate, A2 is diethyl(trifluoromethyl)phosphonate, B1 is diethylmethylphosphonate, B2 is dimethylvinylphosphonate, and B3 is bis(2,2,2-trifluoroethyl)methylphosphonate. A1 to A2 are organophosphorus compounds represented by general formula (1), while B1 to B3 are not organophosphorus compounds represented by general formula (1).

[0102] [Table 1]

[0103] In C3, which uses the composite oxide NMA that does not contain Co, the MR decreased by 4.9% (89.8% → 84.9%) and the ΔDCIR increased by 7.2% (17.0% → 24.2%) compared to C1, which uses a composite oxide that contains a relatively large amount of Co.

[0104] In C3, which uses the composite oxide NMA that does not contain Co, the MR decreased by 2.3% (87.2% → 84.9%) and the ΔDCIR increased by 2.7% (21.5% → 24.2%) compared to C2, which uses the composite oxide that contains a relatively large amount of Co.

[0105] In E1 and C3, a composite oxide NMA without Co was used as the positive electrode active material, while A1 was added to the non-aqueous electrolyte in E1, and not in C3. In E1, the MR increased significantly by 4.5% compared to C3 (84.9% → 89.4%), and the ΔDCIR decreased significantly by 5.3% (24.2% → 18.9%).

[0106] On the other hand, in C8 and C2, composite oxides containing a relatively large amount of Co were used as the positive electrode active material, with A1 added to the non-aqueous electrolyte in C8 and not added to the non-aqueous electrolyte in C2. In C8, the capacity retention rate (MR) increased by only 1.4% compared to C2 (87.2% → 88.6%), and the DCIR increase rate (ΔDCIR) decreased by only 3.3% (21.5% → 18.2%).

[0107] In E1, which uses a composite oxide NMA that does not contain Co, the effect of adding A1 to the non-aqueous electrolyte was significantly more pronounced compared to C8, which uses a composite oxide that contains a relatively large amount of Co.

[0108] In E2, which contained A1 and FEC in a non-aqueous electrolyte, the MR increased significantly by 5.4% (84.9% → 90.3%) and the ΔDCIR decreased significantly by 6.2% (24.2% → 18.0%) compared to C3.

[0109] In C4, which contained FEC instead of A1 as a non-aqueous electrolyte, MR increased by 2.1% (84.9% → 87.0%) and ΔDCIR decreased by 3.3% (24.2% → 20.9%) compared to C3.

[0110] In E2, compared to E1, MR increased by a further 0.9% (89.4% → 90.3%), and ΔDCIR decreased by a further 0.9% (18.9% → 18.0%). By using a non-aqueous electrolyte containing FEC together with A1, the improvement in cycle characteristics and the reduction in internal resistance were more pronounced.

[0111] In C4 and C3, a composite oxide NMA without Co was used as the positive electrode active material, while FEC was added to the non-aqueous electrolyte in C4, and FEC was not added to the non-aqueous electrolyte in C3. In C4, the amount of gas generated during high-temperature storage increased significantly to 6.3 mL compared to C3 (24.2 mL → 30.5 mL).

[0112] On the other hand, in E2 and E1, a composite oxide NMA without Co was used as the positive electrode active material, while in E2, A1 and FEC were added to the non-aqueous electrolyte, and in E1, A1 was added to the non-aqueous electrolyte, but FEC was not added. In E2, the amount of gas generated during high-temperature storage increased by only 1.9 mL compared to E1 (24.5 mL → 26.4 mL), and gas generation due to FEC was significantly suppressed when FEC was added together with A1.

[0113] Similarly, in E3-E4, where A2 was added to the non-aqueous electrolyte, MR increased significantly and ΔDCIR decreased remarkably.

[0114] In C5-C7, B1-B3 were included as the first component in the non-aqueous electrolyte, resulting in a decrease in MR and an increase in ΔDCIR. Note that in B1, the R of general formula (1) 3 This is a methyl group. In B2, the R of general formula (1) 3 This is a vinyl group. In B3, the R of general formula (1) 1 and R 2 However, it is a 2,2,2-trifluoroethyl group, R 3 This is a methyl group. [Industrial applicability]

[0115] The non-aqueous electrolyte secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like. Furthermore, because it offers high capacity while maintaining excellent cycle characteristics, it is also suitable for automotive applications. However, the applications of the non-aqueous electrolyte secondary battery are not limited to these.

[0116] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of symbols]

[0117] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug

Claims

1. It comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material. The positive electrode active material includes a lithium transition metal composite oxide containing Ni, Mn, and Al. The proportions of Ni, Mn, and Al among the metal elements other than Li contained in the lithium transition metal composite oxide are, respectively Ni: 50 atomic% or more, Mn: 10 atomic percent or less, Al: 10 atomic% or less And, When the lithium transition metal composite oxide contains Co, the proportion of Co among the metal elements other than Li is 1.5 atomic percent or less. The aforementioned non-aqueous electrolyte is given by general formula (1): 【Chemistry 1】 It contains an organophosphorus compound represented by the general formula (1), R １ and R ２ Each of these is independently an alkyl group having 1 to 4 carbon atoms. R ３ This is a non-aqueous electrolyte secondary battery in which the fluorinated alkyl group has 1 to 4 carbon atoms.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the organophosphorus compound comprises at least one selected from the group consisting of diethyl (fluoromethyl)phosphonate, diethyl (difluoromethyl)phosphonate, and diethyl (trifluoromethyl)phosphonate.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the organophosphorus compound in the non-aqueous electrolyte is 2% by mass or less.

4. Furthermore, a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, comprising a fluorinated cyclic carbonate.

5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the fluorinated cyclic carbonate comprises fluoroethylene carbonate.

6. The lithium transition metal composite oxide is given by the following formula: Li α Ni （１－ｘ１－ｘ２－ｙ－ｚ） Co ｘ１ Mn ｘ２ Al ｙ M ｚ O ２＋β It is expressed as, in the formula, ０．９５≦α≦１．０５、 0.685≦1-x1-x2-y-z≦0.95, 0 ≤ x 1 ≤ 0.015, 0 < x² ≤ 0.1, 0 < y ≤ 0.1, 0 ≤ z ≤ 0.1, and -0.05 ≤ β ≤ 0.05 satisfies, A non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein M is an element other than Li, Ni, Mn, Al, Co, and oxygen.

7. The non-aqueous electrolyte secondary battery according to claim 6, wherein the element M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y.

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