Nonaqueous electrolyte secondary battery

Abstract

A nonaqueous electrolyte secondary battery (10) is provided with a positive electrode (11), a negative electrode (12), and a nonaqueous electrolyte. A positive electrode mixture layer constituting the positive electrode (11) contains a positive electrode active material having a lithium-cobalt composite oxide having a layered crystal structure as a main component, and the lithium-cobalt composite oxide is a composite oxide represented by the general formula LiCo (1-w-x-y-z) AlwMgxTiyMnzO2 (in the formula, 0.010 < = w < = 0.013, 0.003 < = x < = 0.006, 0.0004 < = y < = 0.0006, and 0 < = z < = 0.0015). The nonaqueous electrolyte contains a lithium salt having a fluorosulfonyl group.

Description

Technical Field

[0001] This disclosure relates to non-aqueous electrolyte secondary batteries. Background Technology

[0002] The positive electrode active material, a key component of non-aqueous electrolyte secondary batteries, significantly impacts not only battery capacity but also charge-discharge cycle characteristics, thermal stability, and output characteristics. Therefore, extensive research has been conducted on positive electrode active materials. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery using the general-purpose LiNiCo... （1-x-y） M1 y O2 (where 0.65≤x≤0.90, 0.02≤y≤0.06, and M1 is at least one metallic element selected from Al, Mg, Ti, Mn, and Zr) represents a lithium nickel cobalt composite oxide, and the general formula LiCo （1-z） M2 z The lithium cobalt composite oxide represented by O2 (0.005≤z≤0.03, M2 is at least one metal element selected from Al, Mg, Ti, Mn, Zr) is used as the positive electrode active material.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2011-192541 Summary of the Invention

[0006] When using lithium cobalt composite oxide (hereinafter, sometimes referred to as "LCO") as the positive electrode active material in non-aqueous electrolyte secondary batteries, high capacity can be achieved by increasing the charge termination voltage. However, according to the results of the researchers of the present invention, in conventional batteries using LCO, increasing the charge termination voltage is accompanied by a greater decrease in charge and discharge capacity and a decrease in thermal stability.

[0007] The non-aqueous electrolyte secondary battery disclosed herein is characterized by comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode core and a positive electrode additive layer disposed on the positive electrode core. The positive electrode additive layer contains a positive electrode active material mainly composed of a lithium cobalt composite oxide having a layered crystal structure. The lithium cobalt composite oxide is of the general formula LiCo. （1-w-x-y-z） Al w Mg x Ti y Mn zThe composite oxide represented by O2 (where 0.010≤w≤0.013, 0.003≤x≤0.006, 0.0004≤y≤0.0006, 0≤z≤0.0015) and the non-aqueous electrolyte contained a lithium salt having a fluorosulfonyl group.

[0008] The non-aqueous electrolyte secondary battery disclosed herein can improve charge-discharge cycle characteristics and thermal stability while achieving high capacity. Attached Figure Description

[0009] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an implementation method. Detailed Implementation

[0010] In non-aqueous electrolyte secondary batteries using LCO-based composite oxides as the positive electrode active material, achieving high capacity by increasing the charge termination voltage while ensuring good charge-discharge cycle characteristics and thermal stability is an important challenge. However, increasing the charge termination voltage leads to the crystallization transformation or decomposition of LCO, releasing oxygen from the crystal structure and promoting the oxidative decomposition reaction between the positive electrode active material and the electrolyte. As a result, it is believed that the capacity decreases more significantly with charge-discharge cycles, and thermal stability decreases.

[0011] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that by using the general formula LiCo （1-w-x-y-z） Al w Mg x Ti y Mn z Using a specific composite oxide represented by O2 (where 0.010≤w≤0.013, 0.003≤x≤0.006, 0.0004≤y≤0.0006, 0≤z≤0.0015) as the positive electrode active material, and adding a lithium salt with a fluorosulfonyl group to the non-aqueous electrolyte, excellent cycle characteristics and thermal stability can be ensured even if the charging termination voltage of the battery is increased.

[0012] Adding Al, Mg, and Ti to LCO-based composite oxides suppresses crystal structure transformation or decomposition associated with increasing charging voltage. However, at high voltages, the decomposition of the non-aqueous electrolyte on the cathode surface becomes problematic. It is believed that lithium salts with fluorosulfonyl groups strengthen the protective film formed on the cathode surface, and through synergistic effects with the aforementioned cathode active materials, excellent cycle characteristics and thermal stability can be achieved.

[0013] Hereinafter, an example of an embodiment of the non-aqueous electrolyte secondary battery of the present disclosure will be described in detail with reference to the accompanying drawings. It should be noted that configurations formed by selectively combining the constituent elements of the various embodiments and modifications described below are included within the scope of this disclosure.

[0014] Hereinafter, a cylindrical battery is illustrated where a wound electrode 14 is housed in a bottomed cylindrical outer casing 16. However, the outer casing is not limited to a cylindrical outer casing; for example, it can also be a square outer casing (square battery) or a coin-shaped outer casing (coin-shaped battery). Additionally, it can be an outer casing made of a laminate containing a metal layer and a resin layer (pouch battery). Furthermore, the electrode is not limited to a wound type; it can also be a stacked type electrode consisting of multiple positive electrodes and multiple negative electrodes alternately stacked with a separator.

[0015] Figure 1 This is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, as an example of an implementation method. (See diagram below.) Figure 1 As shown, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte (not shown), and an outer container 16 for housing the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound into a spiral shape with the separator 13 in between. The outer container 16 is a bottomed cylindrical metal container with an opening on one axial side, and the opening of the outer container 16 is blocked by a sealing body 17. Hereinafter, for ease of explanation, the sealing body 17 side of the battery is referred to as the upper side, and the bottom side of the outer container 16 is referred to as the lower side.

[0016] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all strip-shaped elongated bodies, which are alternately stacked in the radial direction of the electrode body 14 by being wound into a spiral. To prevent lithium deposition, the negative electrode 12 is formed to be one size larger than the positive electrode 11. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the long side direction and the width direction (short side direction). The separator 13 is formed to be at least one size larger than the positive electrode 11, for example, two sheets are arranged to sandwich the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are respectively disposed above and below the electrode body 14. Figure 1In the example shown, the positive lead 20 extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative lead 21 extends towards the bottom of the outer can 16 through the outer side of the insulating plate 19. The positive lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the top plate, i.e., the cover 27, of the sealing body 17, which is electrically connected to the internal terminal plate 23, becomes the positive terminal. The negative lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 becomes the negative terminal.

[0018] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness of the battery interior. A groove 22, extending inward from a portion of the side of the outer can 16, supports the sealing body 17. The groove 22 is preferably formed in a ring shape along the circumference of the outer can 16, with its upper surface supporting the sealing body 17. The sealing body 17 is fixed to the upper part of the outer can 16 by the groove 22 and the open end of the outer can 16 that is tightly sealed to the sealing body 17.

[0019] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heating, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 upwards towards the cover 27, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0020] The charging termination voltage of the non-aqueous electrolyte secondary battery 10 is preferably 4.3V or higher. Based on the non-aqueous electrolyte secondary battery 10, it is possible to achieve high battery capacity while simultaneously achieving excellent cycle characteristics and thermal stability by increasing the charging termination voltage. There is no particular upper limit to the charging termination voltage; as an example, it is 4.8V.

[0021] The following is a detailed description of the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10.

[0022] [positive electrode]

[0023] The positive electrode 11 has a positive electrode core and a positive electrode additive layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal disposed on the surface. The positive electrode additive layer contains a positive electrode active material, a binder, and a conductive agent, and is preferably disposed on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by coating a slurry containing a positive electrode active material, a binder, and a conductive agent onto the positive electrode core, drying the coating, and then compressing it to form a positive electrode additive layer on both sides of the positive electrode core.

[0024] The positive electrode active material is mainly composed of lithium cobalt composite oxide with a layered crystal structure. The lithium cobalt composite oxide (hereinafter referred to as "LCO"), which constitutes the main component of the positive electrode active material, has the general formula LiCo. （1-w-x-y-z） Al w Mg x Ti y Mn z O2 (where 0.010≤w≤0.013, 0.003≤x≤0.006, 0.0004≤y≤0.0006, 0≤z≤0.0015) represents a composite oxide. LCO is a composite oxide formed by adding specified amounts of Al, Mg, and Ti to a composite oxide containing Li and Co as metal elements. The Co content in LCO is, for example, 97.8 mol% or more relative to the total moles of elements other than Li and O. Mn can be any component, but the improvement in cycle characteristics becomes more significant by adding Mn.

[0025] In this specification, the main component of the positive electrode active material refers to the component with the highest mass percentage in the materials constituting the positive electrode active material. The LCO content is preferably 60% by mass or more, more preferably 70% by mass or more, of the positive electrode active material. The positive electrode active material may substantially consist solely of LCO as shown in the above general formula. It should be noted that the composition of the positive electrode active material can be determined using an ICP emission spectrometer (Thermo Fisher Scientific, iCAP6300).

[0026] In the positive electrode composite layer, the positive electrode active material may include other lithium-containing composite oxides besides LCO as shown in the above general formula, to the extent that it does not impair the purpose of this disclosure. As an example of other lithium-containing composite oxides, a lithium-cobalt composite oxide that substantially does not contain Al, Mg, or Ti and is represented by the general formula LiCoO2 can be cited. Furthermore, composite oxides other than lithium-cobalt composite oxides may be used to the extent that it does not impair the purpose of this disclosure.

[0027] As described above, LCO exhibits a layered crystal structure. Specific examples include layered structures belonging to space group R-3m or space group C2 / m. The addition of Al, Mg, and Ti, particularly Al and Mg, is considered to significantly contribute to the stabilization of these crystal structures. The crystal structure of the composite oxide can be determined using a powder X-ray diffractometer (Rigaku Corporation, RINT2200, Cu-Kα source).

[0028] The median particle size (D50) of LCO in a volumetric reference is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 40 μm or less, and more preferably 15 μm or more and 30 μm or less. LCO is, for example, secondary particles formed by the aggregation of primary particles, and the D50 of LCO refers to the D50 of the secondary particles. D50 refers to the particle size in the volumetric particle size distribution where the cumulative frequency reaches 50% from the smallest particle size. The particle size distribution of the composite oxide can be measured using a laser diffraction-type particle size distribution measuring device (e.g., Microtrac BEL Co., Ltd., MT3000II) with water as the dispersion medium.

[0029] The Al content in LCO is 1.0 mol% to 1.3 mol% relative to the total molar amount of elements other than Li and O, more preferably 1.0 mol% to 1.2 mol%. If the Al content is within this range, the stability of the crystal structure is improved, as are the cycle characteristics and thermal stability of the battery. When the Al content is less than 1.0 mol% or more than 1.3 mol%, no improvement in cycle characteristics is achieved, and the capacity reduction becomes significant. It should be noted that even if the Al content is within the above range, the aforementioned improvement effects cannot be obtained without the addition of Mg and Ti.

[0030] The Mg content in LCO is between 0.3 mol% and 0.6 mol% relative to the total molar amount of elements other than Li and O. If the Mg addition is within this range, the stability of the crystal structure, cycling characteristics, and thermal stability are improved. When the Mg addition is less than 0.3 mol% or more than 0.6 mol%, no improvement in cycling characteristics is achieved; if the Mg addition becomes excessive, the capacity reduction becomes significant.

[0031] The Ti content in LCO is between 0.04 mol% and 0.06 mol% relative to the total molar amount of elements other than Li and O. It is believed that if the Ti addition is within this range, the material resistivity can be reduced, particularly improving cycle characteristics. When the Ti addition is less than 0.04 mol% or more than 0.06 mol%, no improvement in cycle characteristics is obtained; if the Ti addition becomes excessive, the capacity reduction becomes significant.

[0032] LCO also contains Mn, and the Mn content (z) in the general formula is preferably 0.0004 ≤ z ≤ 0.0015, more preferably 0.0010 ≤ z ≤ 0.0015. In particular, it is believed that if the amount of Mn added is 0.10 mol% or more and 0.15 mol% or less relative to the total moles of elements other than Li and O, the material resistivity can be effectively reduced. As a result, the improvement in the cycle characteristics and thermal stability of the battery becomes more significant, and the discharge characteristics at low temperatures are also improved. It should be noted that if the amount of Mn added becomes too much, for example, the cycle characteristics will decrease, and the capacity will also decrease significantly.

[0033] LCOs are manufactured, for example, through the following processes.

[0034] (1) Step 1 to obtain a composite hydroxide or composite oxide containing Co, Al, Mg, Ti and Mn

[0035] (2) The second step of mixing the composite hydroxide or composite oxide with a lithium compound and then firing it.

[0036] (3) The third step of washing and drying the fired product.

[0037] In the first step, for example, while stirring a solution containing metal salts of Co, Al, Mg, Ti, and Mn, an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side, thereby causing the composite hydroxide to precipitate (co-precipitate). It should be noted that, for example, in the first step, the composite hydroxide can be calcined at a lower temperature than in the second step.

[0038] In the second step, the composite hydroxide or composite oxide obtained in the first step is mixed with a lithium compound. Examples of lithium compounds include Li₂CO₃, LiOH, Li₂O₂, Li₂O, LiNO₃, LiNO₂, Li₂SO₄, LiOH·H₂O, LiH, and LiF. The mixture in the second step is calcined, for example, under an oxygen flow at a temperature of 500°C or higher and 900°C or lower. The calcination temperature is preferably 600°C or higher and 800°C or lower. The calcination can be performed in two or more stages with different calcination conditions.

[0039] In the third step, the calcined material is washed with water to remove residual Li and other impurities, and then dried. If necessary, the calcined material is pulverized and graded to adjust the D50 of LCO to the target range. The washed calcined material can be vacuum dried or dried in the atmosphere. An example drying temperature is above 150°C and below 250°C.

[0040] As described above, the positive electrode binder layer includes a binder and a conductive agent. Examples of binders included in the positive electrode binder layer include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins may also be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, or polyethylene oxide (PEO). The binder content is, for example, 0.5% by mass and 5% by mass relative to the positive electrode binder layer.

[0041] In the positive electrode mixture layer, examples of conductive agents include acetylene black, carbon black such as Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, and conductive whiskers. A single conductive agent can be used, or multiple agents can be used in combination. The content of the conductive agent is not particularly limited, but relative to the mass of the positive electrode mixture layer, it is, for example, 0.1% by mass or more and 5% by mass or less.

[0042] [negative electrode]

[0043] The negative electrode 12 has a negative electrode core and a negative electrode binder layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal disposed on the surface. Preferably, the negative electrode binder layer contains a negative electrode active material, a binder, and a conductive agent such as carbon nanotubes as needed, and is disposed on both sides of the negative electrode core. The negative electrode 12 can be manufactured, for example, by coating a negative electrode binder slurry containing a negative electrode active material and a binder onto the negative electrode core, drying the coating, and then compressing it to form a negative electrode binder layer on both sides of the negative electrode core.

[0044] In the negative electrode composite layer, the negative electrode active material typically includes a carbon material that reversibly absorbs and releases lithium ions. Suitable examples of carbon materials include natural graphite such as flake graphite, block graphite, and amorphous graphite, as well as artificial graphite such as blocky graphite (MAG) and graphitized mesophase carbon microspheres (MCMB). Alternatively, as the negative electrode active material, active materials containing at least one of elements alloyed with Li, such as Si or Sn, and materials containing those elements can be used. Suitable examples of such active materials include SiO2 phase, silicate phases such as lithium silicate, or Si-containing materials with Si particles dispersed in an amorphous carbon phase. Graphite and Si-containing materials can be used in combination as the negative electrode active material.

[0045] Similar to the case of the positive electrode 11, the binder contained in the negative electrode binder layer can also be fluoropolymers, PAN, polyimide, acrylic resins, polyolefins, etc., with styrene-butadiene rubber (SBR) being preferred. Furthermore, the negative electrode binder layer preferably also contains CMC or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc. It is suitable to use SBR in combination with CMC or its salts, or PAA or its salts.

[0046] [Septum]

[0047] The diaphragm 13 is a porous sheet with ion permeability and insulation properties. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Suitable materials for the diaphragm 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The diaphragm 13 can be a single-layer structure or a multi-layer structure. Additionally, a resin layer with high heat resistance, such as an aromatic polyamide resin, can be formed on the surface of the diaphragm 13.

[0048] A filler layer containing inorganic material can be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating a slurry containing the filler onto the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0049] [Non-aqueous electrolytes]

[0050] Non-aqueous electrolytes comprise a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include ethers, esters, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these solvents. The non-aqueous solvent may contain halogen-substituted derivatives formed by substituting at least a portion of the hydrogen atoms of these solvents with halogen atoms such as fluorine. Examples of halogen-substituted derivatives include fluorocyclic carbonates such as fluoroethylene carbonate (FEC), fluorochain carbonates, and fluorochain carboxylic acid esters such as methyl fluoropropionate (FMP). It should be noted that non-aqueous electrolytes are not limited to liquid electrolytes and can also be solid electrolytes.

[0051] The non-aqueous electrolyte contains a lithium salt with a fluorosulfonyl group as the electrolyte salt. It is believed that by using a lithium salt with a fluorosulfonyl group in conjunction with the aforementioned positive electrode active material, a good protective film is formed on the particle surface of the positive electrode active material, electrolyte side reactions on the particle surface are suppressed, and cycle characteristics and thermal stability are significantly improved. The content of the lithium salt with a fluorosulfonyl group relative to the non-aqueous electrolyte is preferably 0.05 M (mol / L) or more and 0.30 M or less, more preferably 0.07 M or more and 0.25 M or less, and particularly preferably 0.08 M or more and 0.20 M or less. If the content of this lithium salt is within this range, the improvement in cycle characteristics and thermal stability becomes more significant.

[0052] The lithium salt having a fluorosulfonyl group is preferably lithium fluorosulfonylimide. Examples of lithium fluorosulfonylimide include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(nonafluorobutyryl)imide, and lithium bis(pentafluoroethanesulfonyl)imide (LIBETI). Among these, at least one of LiFSI and lithium bis(trifluoromethanesulfonyl)imide is preferred, with LiFSI being particularly preferred.

[0053] Non-aqueous electrolytes may further contain other lithium salts. Examples of other lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), and LiPF6. 6-x (C) n F 2n＋1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 Examples of suitable materials include borates such as LiCl, LiBr, LiI, lithium chloroborane, lower aliphatic carboxylic acids, Li₂B₄O₇, and Li(B(C₂O₄)F₂). Among these, LiPF₆ is preferred based on its ionic conductivity and electrochemical stability.

[0054] Preferably, a lithium salt having a fluorosulfonyl group and a second lithium salt coexist in the non-aqueous electrolyte. In this case, a combination of LiFSI and LiPF6 is particularly preferred. When a second lithium salt is included, the concentration of the lithium salt having a fluorosulfonyl group (LiFSI) is also adjusted to, for example, the range described above. The concentration of LiFSI is preferably lower than the concentration of LiPF6, for example, 5% to 25% of the concentration of the electrolyte salt contained in the non-aqueous electrolyte. It should be noted that the non-aqueous electrolyte may contain additives such as vinylene carbonate (VC), ethylene sulfite (ES), cyclohexylbenzene (CHB), o-terphenyl (OTP), and propanesulfonyl lactone compounds.

[0055] As mentioned above, non-aqueous solvents can use ethers and esters. Examples of ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-eucalyptol, and crown ethers; and cyclic ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, and dihexyl ether. Ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and other chain ethers.

[0056] Examples of esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0057] The non-aqueous electrolyte preferably contains a carboxylic acid ester as a non-aqueous solvent. According to the research results of the inventors, the improvement in cycle characteristics becomes more significant when using a carboxylic acid ester in conjunction with the aforementioned positive electrode active material and a lithium salt having a fluorosulfonyl group. The content of the carboxylic acid ester is preferably 20% to 80% by volume of the non-aqueous electrolyte. The non-aqueous electrolyte may contain other non-aqueous solvents besides the carboxylic acid ester. Examples of other non-aqueous solvents include cyclic carbonates such as ethylene carbonate (EC) and chain carbonates such as ethyl methyl carbonate (EMC).

[0058] The carboxylic acid ester is preferably a chain carboxylic acid ester, more preferably a chain carboxylic acid ester with 10 or fewer carbon atoms. Examples of suitable carboxylic acid esters include propyl acetate (PA) and propyl propionate (PP). The non-aqueous electrolyte may include at least one of PA and PP. PP is preferred.

[0059] Example

[0060] The present disclosure is further illustrated below with experimental examples, but the present disclosure is not limited to these experimental examples.

[0061] <Experimental Example 1>

[0062] Synthesis of positive electrode active material

[0063] A composite hydroxide containing Co, Al, Mg, Ti, and Mn, obtained by co-precipitation, was mixed with lithium hydroxide to achieve a molar ratio of total metal elements to Li of 1.0:1.03. The mixture was calcined at 850°C for 20 hours under an oxygen flow. The calcined product was then washed with water and vacuum dried at 180°C for 2 hours, yielding a lithium-cobalt composite oxide with a layered crystal structure, containing specified amounts of Al, Mg, Ti, and Mn, and a D50 of approximately 26 μm. In Experimental Example 1, this lithium-cobalt composite oxide was used as the positive electrode active material. ICP confirmed the composition of the positive electrode active material to be LiCo. 0.9821 Al 0.010 Mg 0.006 Ti 0.0004 Mn 0.0015 O2. Table 1 shows the amount (mol%) of metal elements other than Li and Co.

[0064] [The production of the positive electrode]

[0065] The above-mentioned positive electrode active material, carbon black, and polyvinylidene fluoride were mixed at a solid content mass ratio of 97.6:1.2:1.2, and N-methyl-2-pyrrolidone (NMP) was used as the dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry was coated onto a positive electrode core formed of aluminum foil. After drying and compressing the coating, the positive electrode core was cut into specified electrode dimensions, resulting in a positive electrode with positive electrode slurry layers formed on both sides of the core. It should be noted that aluminum leads were welded to the exposed portion of the positive electrode core.

[0066] [Making the negative electrode]

[0067] Graphite was used as the negative electrode active material. A negative electrode slurry was prepared by mixing the negative electrode active material, sodium salt of CMC, and SBR dispersion at a solid content mass ratio of 97.8:1.2:1.2, using water as the dispersion medium. Next, the negative electrode slurry was coated onto a negative electrode core formed of copper foil. After drying and calendering the coating, the negative electrode core was cut into specified electrode dimensions, resulting in a negative electrode with negative electrode slurry layers formed on both sides of the core. It should be noted that nickel leads were soldered to the exposed portion of the negative electrode core.

[0068] [Preparation of non-aqueous electrolytes]

[0069] Non-aqueous electrolytes were prepared by dissolving LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI) in a solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propyl propionate (PP) in a volume ratio of 30:45:25 (25°C) at concentrations of 1.2 mol / L and 0.1 mol / L, respectively.

[0070] [Fabrication of Non-Aqueous Electrolyte Secondary Batteries]

[0071] The positive and negative electrodes, separated by a separator, are wound into a spiral shape, and then the wound is flattened to obtain the electrode body. The laminate is processed to form an outer casing, which houses the electrode body and non-aqueous electrolyte in an inactive atmosphere. Then, the internal pressure of the outer casing is reduced, allowing the electrolyte to permeate into the separator. The opening of the outer casing is then sealed, resulting in a non-aqueous electrolyte secondary battery with dimensions of 69 mm (length), 56 mm (width), 4.9 mm (thickness), and a rated capacity of 3150 mAh.

[0072] <Experimental Example 2>

[0073] A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Experimental Example 1, without adding carboxylic acid esters to the non-aqueous electrolyte.

[0074] <Experimental Example 3>

[0075] A non-aqueous electrolyte secondary battery was fabricated using a lithium cobalt composite oxide with the composition shown in Table 1 as the positive electrode active material, otherwise the same procedure as in Experimental Example 1.

[0076] <Experimental Examples 4 and 5>

[0077] The amount of propyl propionate (PP) added was changed to 40% or 60% by volume relative to the non-aqueous electrolyte, and the non-aqueous electrolyte secondary battery was prepared in the same manner as in Experimental Example 1.

[0078] <Experimental Example 6>

[0079] A non-aqueous electrolyte secondary battery was fabricated by adding propyl acetate (PA) instead of propyl propionate (PP), otherwise the procedure was the same as in Experimental Example 1.

[0080] <Experimental Example 7>

[0081] Using the lithium cobalt composite oxide with the composition shown in Table 1 as the positive electrode active material, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Experimental Example 2.

[0082] <Experimental Examples 8 and 9>

[0083] By changing the concentration of LiFSI to 0.05M or 0.3M, and otherwise operating in the same manner as in Experimental Example 2, a non-aqueous electrolyte secondary battery was fabricated.

[0084] <Experimental Examples 10-15>

[0085] Using the lithium cobalt composite oxide with the composition shown in Table 1 as the positive electrode active material, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Experimental Example 2.

[0086] <Experimental Example 16>

[0087] A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Experimental Example 7, without adding LiFSI to the non-aqueous electrolyte.

[0088] <Experimental Example 17>

[0089] A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Experimental Example 15, without adding LiFSI to the non-aqueous electrolyte.

[0090] For the non-aqueous electrolyte secondary batteries in the above experimental examples, their cycle characteristics and thermal stability (high-temperature charging and storage characteristics, heating test, and overcharge test) were evaluated under the following conditions. The evaluation results, along with the composition of the positive electrode active material and the amounts of carboxylic acid ester and LiFSI added, are shown in Table 1.

[0091] [Evaluation of Cyclic Characteristics]

[0092] The non-aqueous electrolyte secondary batteries in the above experimental examples were charged at a constant current of 0.7C at 23°C until the battery voltage reached 4.45V, and then charged at a constant voltage of 4.45V until the current reached 1 / 50C. Then, they were discharged at a constant current of 0.5C until the battery voltage reached 3.0V. This charge-discharge cycle was performed 500 times, and the capacity retention and swelling rate after the charge-discharge cycle were calculated based on the following formula.

[0093] Capacity retention rate = (Discharge capacity at 500th cycle / Discharge capacity at 1st cycle) × 100

[0094] Expansion rate = (Battery thickness after charge-discharge cycles / Battery thickness before charge-discharge cycles) × 100

[0095] [Evaluation of high-temperature charging and storage characteristics]

[0096] Each non-aqueous electrolyte secondary battery in the above experimental examples was charged at a constant current of 0.7C at 23°C until the battery voltage reached 4.45V, and then charged at a constant voltage of 4.45V until the current value reached 1 / 50C. The charged batteries were then left to stand at 60°C for 20 days, and the expansion rate after the storage test was calculated based on the following formula.

[0097] Expansion rate = (Battery thickness after storage test / Battery thickness before storage test) × 100

[0098] [Heating Test]

[0099] After charging each non-aqueous electrolyte secondary battery in the above experimental examples to a battery voltage of 4.50V, the temperature was increased from room temperature to 130°C at a rate of 5°C / minute to confirm whether there was a temperature rise. Batteries that did not show a confirmed temperature rise exhibited superior thermal stability compared to batteries that did (the same applies to overcharge tests).

[0100] 〇: No temperature rise

[0101] ×: Temperature rise

[0102] [Overcharge Test]

[0103] Each non-aqueous electrolyte secondary battery in the above experimental examples was charged at a constant current of 0.7C at a temperature of 23°C. The charging was stopped when the battery surface temperature reached 95°C, and then it was confirmed whether there was a temperature rise.

[0104] [Table 1]

[0105]

[0106] As shown in Table 1, the batteries in Experimental Examples 1-12 have higher capacity retention, better cycle characteristics, and better thermal stability compared to the batteries in Experimental Examples 13-17.

[0107] For example, as can be understood from Experimental Examples 2, 7 and 13-17, good battery performance can be obtained in cycling tests, storage tests, heating tests and overcharge tests by using the lithium cobalt composite oxide shown in the above general formula and a non-aqueous electrolyte containing LiFSI. These performance characteristics are significantly reduced when the lithium cobalt composite oxide does not contain any of Al, Mg, or Ti (refer to Experimental Examples 13-15) and when LiFSI is absent from the non-aqueous electrolyte (refer to Experimental Examples 16 and 17).

[0108] Furthermore, as can be seen from Experiments 1-6, adding carboxylic acid esters to non-aqueous electrolytes significantly improves cycling characteristics and high-temperature storage properties. Additionally, as shown in Experiments 1 and 3, adding Mn to lithium-cobalt composite oxides further enhances these improvements.

[0109] This disclosure is further illustrated by the following embodiments.

[0110] Configuration 1: A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode core and a positive electrode additive layer disposed on the positive electrode core. The positive electrode additive layer contains a positive electrode active material mainly composed of a lithium cobalt composite oxide having a layered crystal structure. The lithium cobalt composite oxide is of the general formula LiCo. （1-w-x-y-z） Al w Mg x Ti y Mn z The composite oxide represented by O2 (where 0.010≤w≤0.013, 0.003≤x≤0.006, 0.0004≤y≤0.0006, 0≤z≤0.0015) and the non-aqueous electrolyte contained a lithium salt having a fluorosulfonyl group.

[0111] Configuration 2: The non-aqueous electrolyte secondary battery as described in Configuration 1, wherein the lithium salt is lithium bis(fluorosulfonyl)imide.

[0112] Configuration 3: A non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein, relative to the non-aqueous electrolyte, the content of the lithium salt having a fluorosulfonyl group is 0.05M or more and 0.30M or less.

[0113] Configuration 4: A non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the non-aqueous electrolyte further comprises a carboxylic acid ester, and the content of the carboxylic acid ester is 20% or more and 80% or less of the volume of the non-aqueous electrolyte.

[0114] Composition 5: The non-aqueous electrolyte secondary battery according to Composition 4, wherein the carboxylic acid ester is at least one of propyl acetate and propyl propionate.

[0115] Configuration 6: A non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the lithium cobalt composite oxide contains Mn, and the content (z) of Mn in the above general formula is 0.0010≤z≤0.0015.

[0116] Explanation of reference numerals in the attached figures

[0117] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive lead, 21 Negative lead, 22 Inlet section, 23 Internal terminal block, 24 Lower valve body, 25 Insulating component, 26 Upper valve body, 27 Cover, 28 Gasket

Claims

1. A non-aqueous electrolyte secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode core and a positive electrode compound layer disposed on the positive electrode core. The positive electrode mixture layer contains a positive electrode active material mainly composed of lithium cobalt composite oxide with a layered crystal structure. The lithium-cobalt composite oxide is of the general formula LiCo. （1-w-x-y-z） Al w Mg x Ti y Mn z The composite oxide represented by O2, in the formula, 0.010≤w≤0.013, 0.003≤x≤0.006, 0.0004≤y≤0.0006, 0≤z≤0.0015, The non-aqueous electrolyte contains a lithium salt having a fluorosulfonyl group.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The lithium salt is lithium bis(fluorosulfonyl)imide.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The content of the lithium salt with fluorosulfonyl group is more than 0.05M and less than 0.30M relative to the non-aqueous electrolyte.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The non-aqueous electrolyte also contains carboxylic acid esters. The content of the carboxylic acid ester is more than 20% by volume and less than 80% by volume of the non-aqueous electrolyte.

5. The non-aqueous electrolyte secondary battery according to claim 4, wherein, The carboxylic acid ester is at least one of propyl acetate and propyl propionate.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein, The lithium-cobalt composite oxide contains Mn. The content z of Mn in the general formula is 0.0010≤z≤0.0015.

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