Nonaqueous 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; and the positive electrode active material contains a lithium transition metal composite oxide that contains Ni, Mn and Al. 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 oxalate compound; and the oxalate compound contains a lithium cation and an anion of an oxalate complex.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] A non-aqueous electrolyte secondary battery, such as a lithium ion secondary battery, comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. Attempts have been made to improve the components of the battery in order to ensure excellent characteristics of the non-aqueous electrolyte secondary battery.

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

[0004] Patent Document 2 discloses a compound of Formula 1: Li ｘ Ni １－ｙ－ｚ－ｖ－ｗ Co ｙ Al ｚ M1 ｖ M2 ｗ O ２ wherein the element M1 in formula 1 is at least one element selected from the group consisting of Mn, Ti, Y, Nb, Mo, and W, the element M2 is at least two elements selected from the group consisting of Mg, Ca, Sr, and Ba, and the element M2 contains at least Mg and Ca, 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, primary particles of the composite oxide aggregate 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.

[0005] JP 2014-194930 A JP 2006-310181 A

[0006] The price of Co in lithium transition metal composite oxides has been rising in recent years. Reducing the Co content in lithium transition metal composite oxides is cost-effective, but it also reduces the cycle characteristics of non-aqueous electrolyte secondary batteries. This is thought to be because the lattice structure of the lithium transition metal composite oxide becomes unstable, accelerating degradation due to side reactions.

[0007] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material, and the positive electrode active material includes a lithium transition metal composite oxide including Ni, Mn, and Al, and the proportions of Ni, Mn, and Al among 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, 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 % or less, and the non-aqueous electrolyte includes an oxalate compound, and the oxalate compound includes a lithium cation and an anion of an oxalate complex.

[0008] According to the present disclosure, even when a lithium transition metal composite oxide that does not contain Co or a lithium transition metal composite oxide with a low Co content is used, a nonaqueous electrolyte secondary battery that has excellent cycle characteristics can be provided.

[0009] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] 1 is a partially cutaway schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0011] Below, embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure will be described using examples, but 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 are obtained. In this specification, the expression "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 lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, 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 equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0012] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0013] Non-aqueous electrolyte secondary batteries include at least lithium ion batteries, lithium metal secondary batteries, and the like.

[0014] A non-aqueous electrolyte secondary battery according to the present disclosure includes 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.

[0015] Reducing the Co content of the lithium transition metal composite oxide and increasing the Ni content would be cost-effective and would ensure high capacity. Therefore, in the nonaqueous electrolyte secondary battery according to the present disclosure, the Ni content of the lithium transition metal composite oxide is increased. Meanwhile, in the nonaqueous electrolyte secondary battery according to the present disclosure, the lithium transition metal composite oxide does not contain Co, or the proportion of Co among the metal elements other than Li is limited to 1.5 atomic % or less. Hereinafter, the lithium transition metal composite oxide in the nonaqueous electrolyte secondary battery according to the present disclosure will also be referred to as the "complex oxide NMA."

[0016] The proportions of Ni, Mn, and Al among the metal elements other than Li contained 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.

[0017] Mn and Al contribute to stabilizing the crystal structure of the composite oxide NMA with a reduced Co content. However, because the Co content of the composite oxide NMA is limited to 1.5 atomic % or less and the Ni content is high, the crystal structure is prone to instability, and metals such as Al and Ni may be eluted from the composite oxide NMA. The elution of metals reduces the positive electrode capacity and the cycle characteristics (or capacity retention rate). In particular, in composite oxide NMA with a high Ni content, the eluted Ni forms an oxide coating on the particle surface of the composite oxide NMA that prevents the absorption and release of Li ions, which may lead to an increase in internal resistance. Furthermore, the eluted metal may precipitate at the negative electrode, affecting the durability of the secondary battery.

[0018] In view of the above, the nonaqueous electrolyte secondary battery according to the present disclosure uses a composite oxide NMA and a nonaqueous electrolyte containing an oxalate compound. The oxalate compound contains a lithium cation and an anion of an oxalate complex. The anion of the oxalate complex produced by the oxalate compound has a high degree of dissociation, which is thought to increase the ionic conductivity of the entire nonaqueous electrolyte and form a coating on the surface of the composite oxide NMA particles, thereby suppressing metal elution. Therefore, excellent cycle characteristics can be ensured, and the coating derived from the oxalate compound has excellent ionic conductivity, so it is presumed that the effect of inhibiting electrode reactions is minimal.

[0019] The nonaqueous electrolyte may further contain an organosilicon compound represented by the general formula (2) described below. The organosilicon compound forms a strong coating on the particle surface of the composite oxide NMA that suppresses side reactions. This ensures better cycle characteristics and further suppresses an increase in internal resistance. If an organosilicon compound is used alone, the coating tends to be excessively thick, which may increase resistance to ion conduction. In contrast, a coating derived from a combination of an oxalate compound and an organosilicon compound has excellent ionic conductivity. In other words, the oxalate compound also has the effect of improving the ionic conductivity of the coating derived from the organosilicon compound.

[0020] However, even when an oxalate compound is combined with a lithium transition metal composite oxide having a higher Co content than the composite oxide NMA, the effect of improving cycle characteristics and the effect of suppressing an increase in internal resistance cannot be significantly achieved. The above effects are significantly achieved by combining an oxalate compound with the composite oxide NMA. The reason why the effect is significant in the composite oxide NMA is thought to be that the composite oxide NMA has a higher resistance and relatively fragile particles compared to lithium transition metal composite oxides having a higher Co content. Particles of the composite oxide NMA are prone to cracking and significant metal elution, which makes an increase in resistance during charge and discharge more apparent. Therefore, the composite oxide NMA exhibits a greater improvement in properties due to the coating derived from the oxalate compound. On the other hand, lithium transition metal composite oxides with a higher Co content are superior from this perspective, so there is less need to use an oxalate compound.

[0021] Hereinafter, each component of the nonaqueous electrolyte secondary battery of this embodiment will be described in more detail.

[0022] [Positive Electrode] The positive electrode contains a positive electrode active material. The positive electrode usually includes a positive electrode current collector and a layer of positive electrode mixture (hereinafter referred to as a positive electrode mixture layer) held on the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the components of the positive electrode mixture are dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the slurry. The dried coating may be rolled if necessary.

[0023] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0024] (Positive Electrode Active Material) The positive electrode active material includes a composite oxide NMA. The composite oxide NMA contains Ni, Mn, and Al, and may contain a trace amount of Co, or may not contain Co. From the viewpoint of reducing manufacturing costs, the lower the Co content, the more desirable it is. The proportion of Co in 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 is contained. On the other hand, from the viewpoint of increasing capacity, the proportions of Ni, Mn, and Al in 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. The Ni content in 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 crystal structure (for example, a rock salt crystal structure).

[0025] The composite oxide NMA is, for example, a compound represented by the formula: Li α Ni(1-x1-x2-yz)Co ｘ１ Mn ｘ２ Al ｙ Me ｚ O ２＋β The element Me is an element other than Li, Ni, Mn, Al, Co, and oxygen.

[0026] 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 charge and discharge. In (2+β), which represents the atomic ratio of oxygen, β satisfies −0.05≦β≦0.05.

[0027] 1-x1-x2-y-z (=v), which indicates the atomic ratio of Ni, is, for example, 0.685 or more, and may be 0.8 or more, 0.90 or more, or 0.92 or more. Furthermore, v, which indicates 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), 0.80 or more and 0.95 or less, 0.90 or more and 0.95 or less, or 0.92 or more and 0.95 or less.

[0028] The higher the atomic ratio v of Ni, the more lithium ions can be extracted from the composite oxide NMA during charging, thereby increasing the capacity. However, the valence of Ni in the composite oxide NMA with increased capacity tends to be high. Furthermore, as the atomic ratio of Ni increases, the atomic ratios of other elements become relatively small. In this case, the crystal structure tends to become unstable, especially in the fully charged state, and repeated charge and discharge changes the crystal structure to one that makes it difficult to reversibly absorb and release lithium ions, making it more likely to become inactive. As a result, the cycle characteristics are likely to deteriorate. In the nonaqueous electrolyte secondary battery according to this embodiment, despite using the composite oxide NMA with such a high Ni content, excellent cycle characteristics can be ensured by using a nonaqueous electrolyte containing an oxalate compound.

[0029] The x1, which indicates the atomic ratio of Co, is, for example, 0.015 or less (0≦x1≦0.015), and may be 0.01 or less, or may be 0.005 or less. When x1 is 0, this includes cases where Co is below the detection limit.

[0030] x2, which indicates the atomic ratio of Mn, is, for example, 0.1 or less (0 < x2 ≦ 0.1), and may be 0.07 or less, 0.05 or less, or 0.03 or less. x2 may be 0.01 or more, or 0.02 or more. Mn contributes to stabilizing the crystal structure of the composite oxide NMA, and containing inexpensive Mn in the composite oxide NMA is advantageous for cost reduction.

[0031] The atomic ratio y of Al is, for example, 0.1 or less (0<y≦0.1), and may be 0.09 or less, 0.07 or less, or 0.05 or less. y may be 0.01 or more, or 0.02 or more. Al contributes to stabilizing the crystal structure of the composite oxide NMA. It is also preferable that 0.05≦x2+y≦0.1 be satisfied. In this case, the effects of the oxalate compound and the effect of suppressing an increase in internal resistance after repeated charge and discharge are further enhanced.

[0032] The atomic ratio z of the element Me is, for example, 0≦z≦0.10, may be 0<z≦0.05, or may be 0.001≦z≦0.005.

[0033] The element Me 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. In particular, when at least one selected from the group consisting of Nb, Sr, and Ca is contained in the composite oxide MNA, it is thought that the surface structure of the composite oxide NMA is stabilized, resistance is reduced, and metal elution is further suppressed. It is more effective if the element Me is unevenly distributed near the particle surface of the composite oxide NMA.

[0034] The contents of the elements constituting the composite oxide NMA can be measured using an inductively coupled plasma atomic emission spectroscopy (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray spectroscopy (EDX).

[0035] The composite oxide NMA is, for example, a secondary particle formed by agglomeration of a plurality of primary particles. 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.

[0036] In this specification, the average particle size of secondary particles refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. Such a particle size is sometimes referred to as D50. For example, an "LA-750" manufactured by Horiba Ltd. can be used as a measuring device.

[0037] 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 stirred solution of a salt containing the metal elements that constitute the composite oxide NMA, and the pH is adjusted to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating a composite hydroxide containing the metal elements (Ni, Mn, Al, and optionally Co, and optionally element M). Next, the composite hydroxide is calcined to obtain a composite oxide containing the metal elements (hereinafter also referred to as the "raw composite oxide"). The calcination temperature is not particularly limited, but is, for example, 300°C to 600°C.

[0038] Next, the raw composite oxide, the lithium compound, and optionally a compound containing element M are mixed, and the mixture is calcined under an oxygen stream to obtain the composite oxide NMA. The calcination temperature is not particularly limited, but is, for example, 450° C. or higher and 800° C. or lower. Each calcination may be performed in one step, multiple steps, or while increasing the temperature.

[0039] When mixing the raw composite oxide with the lithium compound, by mixing a compound containing element M, it is possible to make element M unevenly distributed in the vicinity of the particle surface of the composite oxide NMA.

[0040] As the lithium compound, lithium oxide, lithium hydroxide, lithium carbonate, lithium halide, etc. may be used.

[0041] The positive electrode active material may contain a lithium transition metal composite oxide other than the composite oxide NMA, but preferably contains a large proportion of the composite oxide NMA. The proportion of the composite oxide NMA in the positive electrode active material is, for example, 90% by mass or more, or even 95% by mass or more. The proportion of the composite oxide in the positive electrode active material is 100% by mass or less.

[0042] (Others) As the binder, for example, a resin material is used. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, and rubber-like material (for example, styrene butadiene copolymer (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0043] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0044] Examples of the conductive agent include conductive fibers and conductive particles. Examples of the conductive fibers include carbon fibers, carbon nanotubes, and metal fibers. Examples of the conductive particles include conductive carbon (carbon black, graphite, etc.), metal powder, etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0045] The dispersion medium used in the positive electrode slurry is not particularly limited, but examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0046] The positive electrode current collector may be, for example, a metal foil. The positive electrode current collector may be porous. Examples of porous current collectors include nets, punched sheets, and expanded metals. 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 may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0047] [Negative Electrode] The negative electrode includes at least a negative electrode current collector and may include a negative electrode active material. The negative electrode typically includes a negative electrode current collector and a layer of a negative electrode mixture (hereinafter referred to as a negative electrode mixture layer) held on the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating may be rolled, if necessary.

[0048] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0049] (Negative electrode active material) As the negative electrode active material, metallic lithium, lithium alloys, etc. may be used, but materials capable of electrochemically absorbing 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.

[0050] Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination.

[0051] Among these, graphite is preferred as the carbonaceous material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0052] Examples of Si-containing materials include simple Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed in a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO ｘ The x may be, for example, 0.5≦x<2, or 0.8≦x≦1.6. The lithium ion conductive phase may be SiO ２ At least one selected from the group consisting of a silicate phase and a carbon phase may be used.

[0053] (Others) As the binder, thickener, conductive agent, and dispersion medium used in the negative electrode slurry, for example, the materials exemplified for the positive electrode can be used.

[0054] The negative electrode current collector may be, for example, a metal foil. The negative electrode current collector may be porous. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0055] [Non-aqueous Electrolyte] The non-aqueous electrolyte usually contains a non-aqueous solvent, a lithium salt (a lithium salt other than an oxalate compound), and an additive.

[0056] (Oxalate Compound) The non-aqueous electrolyte contains an oxalate compound as an additive. The oxalate compound contains a lithium cation and an anion of an oxalate complex. The oxalate compound can generate an anion of an oxalate complex in the non-aqueous electrolyte. Therefore, the anion of the oxalate complex is counted as an oxalate compound.

[0057] The oxalate compound preferably contains a compound represented by the following general formula (1):

[0058]

[0059] In general formula (1), M is P or B. When M is P, n is an integer of 1 to 3, and m = 6 - 2n is satisfied. When M is B, n is 1 or 2, and m = 4 - 2n is satisfied.

[0060] The oxalate compound is lithium difluorobisoxalate phosphate: LiPF ２ (C ２ O ４ ) ２ , lithium tetrafluorooxalate phosphate: LiPF ４ (C ２ O ４ ), lithium trisoxalate phosphate: LiP(C ２ O ４ ) ３ , lithium bis(oxalatoborate): LiB(C２ O ４ ) ２ , and lithium difluorooxalatoborate: LiBF ２ (C ２ O ４ Among them, lithium difluorobisoxalate phosphate is more preferred.

[0061] The content of the oxalate compound in the non-aqueous electrolyte may be 3% 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 the oxalate compound is within this range, excessive film formation on the positive electrode surface is suppressed, thereby enhancing the effect of suppressing an increase in internal resistance during repeated charge and discharge. In a non-aqueous electrolyte secondary battery, the content of the oxalate compound in the non-aqueous electrolyte changes during storage or charge and discharge. Therefore, it is sufficient that the oxalate compound remains in the non-aqueous electrolyte sampled from the non-aqueous electrolyte secondary battery at a concentration above the detection limit. The content of the oxalate compound in the non-aqueous electrolyte may be 0.01% by mass or more.

[0062] The content of the oxalate compound in the non-aqueous electrolyte used in the production of a non-aqueous electrolyte secondary battery may be 0.01% by mass or more, 0.1% by mass or more, or 0.3% by mass or more. The content of the oxalate compound in the non-aqueous electrolyte used in the production of a non-aqueous electrolyte secondary battery may be, for example, 1.5% by mass or less, 1% by mass or less, or 0.5% by mass or less. These lower and upper limits can be combined in any manner.

[0063] (Organosilicon Compound) The non-aqueous electrolyte may further contain, as an additive, an organosilicon compound represented by the following general formula (2).

[0064]

[0065] In general formula (2), R １ ~R ４ are each independently an alkyl group, an alkenyl group, an alkynyl group, or an alkoxy group, and R １ ~R ４At least one of is an alkenyl or alkynyl group.

[0066] The alkyl group of the alkyl group, alkenyl group, alkynyl group, or alkoxy group may be linear or branched. The number of carbon atoms in the alkyl group, alkenyl group, alkynyl group, or alkoxy group may be, for example, 1 to 10, 1 to 6, or 1 to 4.

[0067] Examples of alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, hexyl, 2-ethylhexyl, decyl, tetradecyl, and stearyl groups. Of these, methyl and ethyl groups are preferred. Examples of alkenyl groups include vinyl, allyl, prop-2-en-1-yl, 4-hexenyl, and 5-hexenyl groups. Of these, vinyl and allyl groups are preferred. Examples of alkynyl groups include ethynyl and propargyl groups. Examples of alkoxy groups include methoxy, ethoxy, n-propoxy, and isopropoxy groups. At least one hydrogen atom of the alkyl, alkenyl, alkynyl, or alkoxy group may be substituted with a halogen atom or the like.

[0068] The organosilicon compound preferably contains at least one selected from the group consisting of tetravinylsilane, tetraallylsilane, and dimethyldivinylsilane, with tetravinylsilane being more preferred.

[0069] R １ ~R ４ When at least one of the groups is an alkenyl group or an alkynyl group, the organosilicon compound may have a polymerizable unsaturated bond, and a strong coating containing an oligomer or polymer component derived from the compound may be formed on the surface of the positive electrode active material. The coating has an excellent protective function, further enhancing the effect of suppressing metal elution from the positive electrode active material.

[0070] The content of the organosilicon compound 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 the organosilicon compound is within this range, excessive film formation on the positive electrode surface is suppressed, and the effect of suppressing an increase in internal resistance during repeated charge and discharge can be enhanced. In a non-aqueous electrolyte secondary battery, the content of the organosilicon compound in the non-aqueous electrolyte changes during storage or charge and discharge. Therefore, it is sufficient that the organosilicon compound remains in the non-aqueous electrolyte sampled from the non-aqueous electrolyte secondary battery at a concentration above the detection limit. The content of the organosilicon compound in the non-aqueous electrolyte may be 0.01% by mass or more.

[0071] The content of the organosilicon compound in the nonaqueous electrolyte used in the production of a nonaqueous electrolyte secondary battery may be 0.01% by mass or more, 0.1% by mass or more, or 0.3% by mass or more. The content of the organosilicon compound in the nonaqueous electrolyte used in the production of a nonaqueous electrolyte secondary battery may be, for example, 1.5% by mass or less, 1% by mass or less, or 0.5% by mass or less. These lower and upper limits can be combined in any desired manner.

[0072] The contents of the oxalate compound and the organosilicon compound in the non-aqueous electrolyte can be determined, for example, using gas chromatography under the following conditions: Instrument used: GC-2010 Plus, manufactured by Shimadzu Corporation; Column: HP-1 (film thickness 1 μm, inner diameter 0.32 mm, length 60 m), manufactured by J&W; Column temperature: heated from 50° C. to 90° C. at a heating rate of 5° C. / min, maintained at 90° C. for 15 minutes, then heated from 90° C. to 250° C. at a heating rate of 10° C. / min, and maintained at 250° C. for 15 minutes; Split ratio: 1 / 50; Linear velocity: 30.0 cm / sec; Injection port temperature: 270° C.; Injection volume: 1 μL; Detector: FID 290° C. (sens. 101).

[0073] In the nonaqueous electrolyte, the mass ratio of the organosilicon compound to the oxalate compound (=organosilicon compound / oxalate compound) 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 composition of the coating formed on the particle surface of the composite oxide NMA is well balanced. In other words, a coating is formed that has excellent ionic conductivity and is highly effective in suppressing metal elution and suppressing an increase in internal resistance during repeated charge and discharge.

[0074] (Non-aqueous Solvent) Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates 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 chain 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 type of non-aqueous solvent or a combination of two or more types.

[0075] (Lithium Salt) Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include 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(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The non-aqueous electrolyte may contain one type of lithium salt or a combination of two or more types of lithium salts.

[0076] The concentration of the lithium salt (lithium salt other than the oxalate compound) in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0077] The non-aqueous electrolyte may further contain other additives, such as at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate.

[0078] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0079] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a laminated electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0080] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery will be described as an example of the nonaqueous electrolyte secondary battery according to the present disclosure with reference to FIG.

[0081] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. 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. In other words, 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 is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0082] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0083] [Example]

[0084] Examples 1 to 7 and Comparative Examples 1 to 6 Non-aqueous electrolyte secondary batteries were fabricated and evaluated according to the following procedure. (1) Fabrication of 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 an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm ) on both sides of the aluminum foil. ３ ) was formed to obtain a positive electrode.

[0085] The positive electrode active material particles were prepared as follows: An aqueous solution was prepared by dissolving nickel sulfate, aluminum sulfate, and, as needed, 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 relationship between the ratio of Ni and each metal element was the value shown in Table 1.

[0086] 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 hydroxide. The hydroxide was recovered by filtration, washed with water, and dried. The dried product was calcined at 500°C for 8 hours in a nitrogen atmosphere to obtain a composite oxide.

[0087] The obtained composite oxide and lithium hydroxide were mixed so that the atomic ratio of Li to the total of Ni, Co, Mn, and Al was 1:1. The mixture was fired in an oxygen atmosphere using an electric furnace by heating from room temperature to 650°C at a heating rate of 2.0°C / min. Thereafter, the mixture was fired by heating from 650°C to 750°C at a heating rate of 0.5°C / min. The fired product was washed with water and dried to obtain composite oxide NMA (positive electrode active material particles).

[0088] (2) Preparation of 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, a sodium salt of CMC (CMC-Na), SBR, and water were mixed in a predetermined mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil, which served as a negative electrode current collector. The coating was dried and then rolled to form a negative electrode mixture layer on both sides of the copper foil.

[0089] (3) Preparation of non-aqueous electrolyte A non-aqueous electrolyte (electrolytic solution) was prepared by dissolving LiPF6 and, if necessary, an oxalate compound (first component) and an organosilicon compound (second component) shown in Table 1 in a mixed solvent of EC and EMC (EC:EMC = 3:7 (volume ratio)). The concentration of LiPF6 in the electrolytic solution was 1.0 mol / L. The concentrations (initial concentrations) of the first component and the second component in the prepared non-aqueous electrolyte were the values ​​(mass %) shown in Table 1.

[0090] (4) Fabrication of a Non-Aqueous Electrolyte Secondary Battery An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to fabricate a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and the non-aqueous electrolyte was injected. The exterior body was then sealed to fabricate a non-aqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, portions of the positive electrode lead and the negative electrode lead were exposed to the outside from the exterior body.

[0091] (5) Evaluation The nonaqueous electrolyte secondary batteries obtained in the Examples and Comparative Examples were evaluated as follows. (a) Initial DC Resistance (DCIR) In a temperature environment of 25°C, the battery was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It. Next, the battery was discharged at a constant current of 0.3 It for 100 minutes to bring the state of charge (SOC) to 50%.

[0092] The battery with an SOC of 50% was discharged for 10 seconds at current values ​​of 0 A, 0.1 A, 0.5 A, and 1.0 A, and the voltage value was measured. The relationship between the discharge current value and the voltage value after 10 seconds was approximated to a straight line by the least squares method, and the DCIR (initial DCIR) was calculated from the absolute value of the slope.

[0093] (b) Charge-Discharge Cycle Test At an ambient temperature of 45°C, the battery was charged at a constant current of 0.5 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 It. Next, the battery was discharged at a constant current of 0.5 It until the voltage reached 3.0 V. This charge and discharge constituted one cycle, and was repeated 200 times.

[0094] (c) DCIR Increase Rate (ΔDCIR) The DCIR (DCIR at 200th cycle) was calculated in the same manner as in (a) above, except that the battery after 200 cycles of charge / discharge in the charge / discharge cycle test in (b) above was used. 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 200th cycle - initial DCIR) / initial DCIR} × 100

[0095] (d) Capacity Retention Rate (MR) In the charge-discharge cycle test (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 index of cycle characteristics: Capacity retention rate (%) = (discharge capacity at the 200th cycle / discharge capacity at the first cycle) × 100

[0096] The evaluation results are shown in Table 1. In Table 1, E1 to E7 represent Examples 1 to 7, and C1 to C6 represent Comparative Examples 1 to 6. Regarding the first component (oxalate compound) in Table 1, A1 is lithium difluorobisoxalate phosphate, A2 is lithium tetrafluorooxalate phosphate, and A3 is lithium bisoxalate borate. Regarding the second component (organosilicon compound) in Table 1, B1 is tetravinylsilane, B2 is tetraallylsilane, B3 is dimethyldivinylsilane, B4 is tetramethylsilane, and B5 is tetraethylsilane. Note that B4 and B5 are not compounds represented by general formula (2).

[0097]

[0098] When the composite oxide NMA containing no Co was used (C4), the MR decreased by 2.0% (86.5% to 84.5%) and the ΔDCIR increased by 1.8% (22.4% to 24.2%) compared to when the composite oxide containing relatively more Co was used (C1).

[0099] In E1 and C4, the Co-free composite oxide NMA was used as the positive electrode active material, and A1 was added to the non-aqueous electrolyte in E1, while A1 was not added to the non-aqueous electrolyte in C4. Compared to C4, E1 showed a significant increase in MR (84.5% to 88.3%) of 3.8% and a significant decrease in ΔDCIR (19.2% to 24.2%) of 5.0%.

[0100] In contrast, in samples C1 and C2, a composite oxide containing relatively large amounts of Co was used as the positive electrode active material, and in sample C2, Al was added to the nonaqueous electrolyte, while in sample C1, Al was not added to the nonaqueous electrolyte. Compared to sample C1, in sample C2, the MR increased by only 0.8% (86.5% to 87.3%) and the ΔDCIR decreased by only 2.2% (22.4% to 20.2%).

[0101] In E1, which used the composite oxide NMA containing no Co, the effect of adding A1 to the non-aqueous electrolyte was more pronounced than in C2, which used a composite oxide containing a relatively large amount of Co.

[0102] In E1 and E2, the Co-free composite oxide NMA was used as the positive electrode active material, and A1 was added to the non-aqueous electrolyte in E1, while A1 and B1 were added to the non-aqueous electrolyte in E2. Compared to E1, E2 showed a significant increase in MR by 1.5% (88.3% to 89.8%) and a further decrease in ΔDCIR (19.2% to 17.2%).

[0103] On the other hand, in C4 and C5, the composite oxide NMA containing no Co was used as the positive electrode active material, and in C4, neither A1 nor B1 was added to the non-aqueous electrolyte, while in C5, B1 was added to the non-aqueous electrolyte but A1 was not added. In C5, the MR increased by only 0.4% compared to C4 (84.5% → 84.9%).

[0104] When the Co-free composite oxide NMA was used as the positive electrode active material, the effect of suppressing the deterioration of cycle characteristics was more pronounced by adding B1 together with A1 to the non-aqueous electrolyte.

[0105] In E3, in which A1 and B2 were added to the non-aqueous electrolyte, the MR was further increased (88.3% to 89.1%) and the ΔDCIR was further reduced (19.2% to 18.5%) compared to E1.

[0106] C3, in which B4 was added to the non-aqueous electrolyte, showed a lower MR and an increased ΔDCIR compared to C1, in which B4 was not added to the non-aqueous electrolyte. C6, in which B5 was added to the non-aqueous electrolyte, showed a lower MR and an increased ΔDCIR compared to C4, in which B5 was not added to the non-aqueous electrolyte.

[0107] In E4 and E6, the Co-free composite oxide NMA was used as the positive electrode active material, and A2 was added to the non-aqueous electrolyte in E4, while A3 was added to the non-aqueous electrolyte in E6. In E4 and E6, as in E1, the MR increased significantly and the ΔDCIR decreased significantly.

[0108] In E5, in which A2 and B1 were added to the non-aqueous electrolyte, the MR was further increased and the ΔDCIR was further reduced compared to E4. In E7, in which A3 and B3 were added to the non-aqueous electrolyte, the MR was further increased and the ΔDCIR was further reduced compared to E6.

[0109] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, etc. Furthermore, the nonaqueous electrolyte secondary battery has high capacity and excellent cycle characteristics, making it suitable for in-vehicle use. However, the uses of the nonaqueous electrolyte secondary battery are not limited to these.

[0110] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0111] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. 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 including Ni, Mn, and Al, The ratios of Ni, Mn and Al to 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 and When the lithium transition metal composite oxide contains Co, the ratio of Co to the metal elements other than Li is 1.5 atomic % or less, the non-aqueous electrolyte contains an oxalate compound, The nonaqueous electrolyte secondary battery, wherein the oxalate compound contains a lithium cation and an anion of an oxalate complex.

2. The lithium transition metal composite oxide has the following formula: Li α Ni （１－ｘ１－ｘ２－ｙ－ｚ） Co ｘ１ Mn ｘ２ Al ｙ Me ｚ O ２＋β In the formula: ０．９５≦α≦１．０５、 0.685≦1-x1-x2-y-z≦0.95, 0≦x1≦0.015, 0<x2≦0.1, 0<y≦0.1, 0≦z≦0.1, and －０．０５≦β≦０．０５ Fulfilling 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein Me is an element other than Li, Ni, Mn, Al, Co, and oxygen.

3. 3. The nonaqueous electrolyte secondary battery according to claim 2, wherein the element Me 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.

4. The oxalate compound is represented by the general formula (1): 【Chemistry 1】 In the general formula (1), M is P or B; When M is P, n is an integer from 1 to 3, and m=6−2n is satisfied; When M is B, n is 1 or 2, and m=4−2n is satisfied; The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3.

5. 5. The nonaqueous electrolyte secondary battery according to claim 4, wherein the oxalate compound includes at least one selected from the group consisting of lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium trisoxalate phosphate, lithium bisoxalate borate, and lithium difluorooxalate borate.

6. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the oxalate compound in the nonaqueous electrolyte is 3 mass % or less.

7. The non-aqueous electrolyte may further comprise a compound represented by the general formula (2): 【Chemistry 2】 In the general formula (2), R １ ~R ４ are each independently an alkyl group, an alkenyl group, an alkynyl group, or an alkoxy group, and R １ ~R ４ At least one of is an alkenyl or alkynyl group; The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3.

8. 8. The nonaqueous electrolyte secondary battery in accordance with claim 7, wherein the organosilicon compound comprises at least one selected from the group consisting of tetravinylsilane, tetraallylsilane, and dimethyldivinylsilane.

9. 8. The nonaqueous electrolyte secondary battery in accordance with claim 7, wherein the content of the organosilicon compound in the nonaqueous electrolyte is 1.5 mass % or less.

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