Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises: a positive electrode; a negative electrode; and an electrolyte. The positive electrode contains a composite oxide including lithium, which is a first metal, and a second metal X other than the lithium. In the composite oxide, a second metal X includes Ni, Al, and Mn, and either does not contain Co or the atomic ratio Co / X indicating the proportion of Co in the second metal X is 0.02 or less. The electrolyte contains a chain carboxylic acid ester with a carbon number of two to four, and a cyclic compound having a cyclic structure comprising two oxygen atoms and three to five carbon atoms.

Description

[Technical Field]

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

[0002] Secondary batteries, especially lithium-ion secondary batteries, have high voltage and high energy density, and are therefore expected to be used in small consumer applications, power storage devices, and power sources for electric vehicles. To address the growing demand for longer battery life, it has been proposed to improve cycle characteristics by using a carboxylic acid ester compound as a solvent in the electrolyte (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-172120 Summary of the Invention

[0004] A composite oxide containing lithium and a metal (particularly, a transition metal) is used as a positive electrode active material for a lithium-ion secondary battery. Lithium cobalt oxide (LiCoO2) has traditionally been used as the composite oxide containing lithium and a transition metal.

[0005] On the other hand, in recent years, due to the rising price of cobalt caused by the increase in demand, there is a demand for the use of positive electrode active materials that contain as little Co as possible. In this regard, attempts have been made to use lithium-containing composite oxides in which the Co content is reduced by replacing part of the metal constituting the composite oxide with other metal elements. For example, attempts have been made to use lithium-containing composite oxides (NCAMs) in which part of the Co in lithium cobalt oxide is replaced with Ni, Al, and / or Mn.

[0006] In a battery using the above composite oxide NCAM in the positive electrode and adding an ester compound to the electrolyte, the cycle characteristics may be adversely reduced.

[0007] One aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode includes a composite oxide containing lithium as a first metal and a second metal X other than lithium, wherein the second metal X includes Ni, Al, and Mn, and the second metal X does not include Co or the atomic ratio of Co to the second metal X: Co / X is 0.02 or less, and the electrolyte includes a chain carboxylic acid ester having 2 to 4 carbon atoms and a cyclic compound having a ring structure formed of two oxygen atoms and 3 to 5 carbon atoms.

[0008] According to the present disclosure, a nonaqueous electrolyte secondary battery can be realized that can be manufactured at low cost and has excellent long-term cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0010] A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure has a positive electrode, a negative electrode, and an electrolyte solution. The positive electrode contains a composite oxide containing lithium as a first metal and a second metal X other than lithium. In the composite oxide, the second metal X contains Ni, Al, and Mn. Meanwhile, the second metal X does not contain Co, or the atomic ratio of Co to the second metal X, Co / X, is 0.02 or less. The electrolyte solution contains a chain carboxylic acid ester having 2 to 4 carbon atoms and a cyclic compound having a ring structure composed of two oxygen atoms and 3 to 5 carbon atoms.

[0011] The chain carboxylic acid ester described above, when incorporated into the solvent of the electrolyte, has the effect of improving cycle characteristics. In particular, an electrolyte containing methyl acetate (MA) has low viscosity, which allows the electrolyte to easily spread over the surfaces of the positive and negative electrode active materials, thereby suppressing unevenness in the charge-discharge reactions at the positive and negative electrodes. This effectively improves cycle characteristics. In particular, even when the amount of positive and negative electrode active materials applied is increased to increase the energy density and the active material layer is formed thick, the electrolyte can penetrate deep into the collector side of the active material layer. This makes it easy to achieve both high capacity and long life.

[0012] However, when a lithium-transition metal composite oxide containing Mn is used as the positive electrode active material, the inclusion of the chain carboxylic acid ester in the electrolyte may adversely affect cycle performance. This may be due to the oxidation or reduction of the chain carboxylic acid ester, which dissolves Mn into the electrolyte, causing the composite oxide to change (become inactive) to a crystalline structure that makes it difficult to reversibly store and release lithium ions. This may result in a decrease in capacity retention as the charge-discharge cycle progresses.

[0013] In general, carboxylic acid esters are easily oxidized and reduced. Methyl acetate, in particular, is easily oxidized and reduced in batteries. On the other hand, Mn is known to be easily dissolved into the electrolyte. In batteries, chain carboxylic acid esters exist in both oxidized and reduced states, and can reversibly change by donating or receiving electrons from or to the positive and negative electrode active materials.

[0014] When the oxidized chain carboxylic acid ester reaches the vicinity of the positive electrode surface, it may steal electrons from the positive electrode active material, causing a reaction that oxidizes the positive electrode active material. In this case, in a discharged state, the oxidized carboxylic acid ester may induce a normal charging reaction in which lithium ions are released from the positive electrode active material. However, in a fully charged state or a state close to full charge, there is almost no lithium available to be released from the positive electrode active material, so a reaction in which Mn elutes instead of lithium may be induced. This is thought to result in a decrease in capacity retention.

[0015] The cyclic compound acts to inhibit the reaction of (oxidized or reduced) chain carboxylic acid esters on the positive electrode surface. It is believed that the cyclic compound adsorbs to the surface of the positive electrode material and forms a protective layer that inhibits the reaction of (oxidized or reduced) chain carboxylic acid esters on the positive electrode surface (e.g., oxidation or reduction reaction of complex oxides). As a result, it is presumed that the inactivation of the positive electrode active material is suppressed and the decrease in capacity is also suppressed. In other words, the capacity retention rate is improved when the battery is repeatedly charged and discharged over a long period of time.

[0016] As described above, the cyclic compound has a ring structure composed of two oxygen atoms and 3 to 5 carbon atoms. That is, the cyclic compound has a five-membered ring structure composed of two oxygen atoms and three carbon atoms, a six-membered ring structure composed of two oxygen atoms and four carbon atoms, or a seven-membered ring structure composed of two oxygen atoms and five carbon atoms. The cyclic compound may be a derivative in which the hydrogen atoms bonded to the carbon atoms constituting the ring structure are substituted with other functional groups, for example, alkyl groups such as methyl groups and ethyl groups, or aromatic hydrocarbon groups such as benzyl groups. The cyclic compound may also be a dioxane having a six-membered ring structure.

[0017] The dioxane may be any of 1,2-dioxane, 1,3-dioxane, and 1,4-dioxane. Among these, 1,4-dioxane is preferred. 2-benzyl-1,3-dioxane may also be used as the dioxane. It is believed that the protective layer derived from dioxane can maintain a stable structure even at high temperatures due to the coordination of lithium ions to oxygen atoms in the dioxane.

[0018] Other cyclic compounds include, for example, 1,3-dioxolane and 4-methyl-1,3-dioxolane, each of which has a five-membered ring structure.

[0019] The content of the cyclic compound in the electrolytic solution may be 0.5% by mass or more relative to the mass of the electrolytic solution, which can significantly improve the capacity retention rate.

[0020] The content of the cyclic compound in the electrolyte solution may be, for example, 5% by mass or less relative to the mass of the electrolyte solution. By containing the cyclic compound in the electrolyte solution at 5% by mass or less, an increase in the resistance of the positive electrode surface due to the protective layer formed by the cyclic compound is suppressed. The content of the cyclic compound in the electrolyte solution may be 2% by mass or less, or 1.5% by mass or less relative to the mass of the electrolyte solution.

[0021] The cyclic compound can be consumed by repeated charge / discharge cycles. To maintain the effect of the cyclic compound even when the battery is repeatedly charged / discharged over a long period of time, it is necessary that the electrolyte before being poured into the battery or the electrolyte recovered from an early-used battery contains a sufficient amount of the cyclic compound. The electrolyte recovered from an early-used battery may contain, for example, 0.02 mass % or more of the cyclic compound relative to the mass of the electrolyte, and may also be 0.1 mass % or more.

[0022] When the cyclic compound is consumed, a coating (protective layer) derived from the cyclic compound is formed at least on the surface of the positive electrode. Even if the cyclic compound cannot be detected in the electrolyte in the battery, the present invention encompasses an embodiment in which at least the positive electrode has a coating derived from the cyclic compound on its surface.

[0023] The content of the cyclic compound in the electrolytic solution can be measured, for example, by using gas chromatography-mass spectrometry (GC-MS), nuclear magnetic resonance (NMR), ion chromatography, or the like for the electrolytic solution.

[0024] As the positive electrode active material, a composite oxide containing lithium as the first metal and a second metal X other than lithium is used. Specifically, the composite oxide is, for example, Li a Ni 1-x-y-z Co x Mn y M 1 z It may include a lithium-containing composite oxide represented by O2 (where 0 ≤ x ≤ 0.02, 0 < y ≤ 0.05, 0 < z ≤ 0.1). M 1 may contain Al. The second metal X may not contain Co (that is, even if x = 0). When Co is contained, the atomic ratio of Co in the second metal X is 0.02 or less (x ≤ 0.02). The lithium ratio a varies depending on charge and discharge, but in the fully discharged state or the initial state immediately after the production of the active material, for example, 0.95 ≤ a ≤ 1.2.

[0025] The smaller the cobalt ratio x, the easier it is for the composite oxide to change (inactivate) to a crystal structure in which reversible occlusion and release of lithium ions are difficult by repeated charge and discharge. In particular, when x ≤ 0.02, the inactivation of the composite oxide is remarkable, and the need to replace Co with Mn to stabilize the crystal structure increases. When Co is replaced with Mn, when a chain carboxylic acid ester is used as the solvent of the electrolytic solution, the above problem of reduction in the capacity retention rate occurs. In this case, the addition of the cyclic compound is effective.

[0026] The atomic ratio of Mn in the second metal X may be 0.02 or more and 0.05 or less (0.02≦y≦0.05). When y≧0.02, the structure of the positive electrode active material is easily stabilized when the Co ratio x in the composite oxide is 0.02 or less. On the other hand, when y≦0.05, the addition of a cyclic compound makes it easy to suppress a decrease in capacity retention rate even when the electrolyte contains a chain carboxylic acid ester.

[0027] Next, a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure will be described in detail. The nonaqueous electrolyte secondary battery includes, for example, the following negative electrode, positive electrode, and electrolyte solution.

[0028] [Negative electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0029] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. The negative electrode active material contains a material that electrochemically absorbs and releases lithium ions. Examples of the material that electrochemically absorbs and releases lithium ions include carbon materials and Si-containing materials. Examples of the Si-containing material include silicon oxide (SiO x : 0.5≦x≦1.5), and a composite material (LSX) containing a silicate phase and silicon particles dispersed within the silicate phase.

[0030] Examples of carbon materials include graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. Among them, graphite is preferred as it has excellent charge-discharge stability and low irreversible capacity. Graphite means a material having a graphite-type crystal structure and includes natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. The carbon material may be used alone or in combination of two or more kinds.

[0031] As the negative electrode current collector, a metal foil, a mesh body, a net body, a punching sheet, etc. are used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, etc.

[0032] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0033] The positive electrode mixture contains a positive electrode active material as an essential component and may contain, as optional components, a binder, a conductive agent, etc. The positive electrode active material includes a material that electrochemically intercalates and deintercalates lithium ions. As the positive electrode active material, a composite oxide having a layered rock salt structure similar to LiCoO2 and in which at least a part of Co is replaced with Ni, Al, and Mn can be used. The composite oxide may be, for example, a lithium-containing composite oxide represented by the above Li a Ni 1-x-y-z Co x Mn y M 1 z O2 (where 0 ≦ x ≦ 0.02, 0 < y ≦ 0.05, 0 < z ≦ 0.1). In the above chemical formula, M 1 may contain Al. The Co ratio may be substantially zero. That is, the composite oxide is a Co-free Li a Ni 1-y-z Mn y M1 z It may also be O2 (where 0 < y ≤ 0.05, 0 < z ≤ 0.1).

[0034] In terms of obtaining high capacity, the atomic ratio of Ni in metals other than lithium may be 0.8 or more and less than 1 (0 < x + y + z ≤ 0.2), and may also be 0.9 or more and less than 1 (0 < x + y + z ≤ 0.1). The atomic ratio of Ni may be 0.8 or more and 0.98 or less (0.02 ≤ x + y + z ≤ 0.2), and may also be 0.9 or more and 0.98 or less (0.02 ≤ x + y + z ≤ 0.1).

[0035] Element M 1 may contain elements other than Co, Ni, Al, and Mn. Examples of other elements include at least one selected from the group consisting of Na, Mg, Sc, Y, Fe, Cu, Zn, Cr, Pb, Sb, and B.

[0036] As the positive electrode current collector, for example, a metal foil is used, and examples of the material include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0037] As the binder for each electrode, resin materials such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resins; polyimide resins such as polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polyacrylate (for example, lithium polyacrylate), methyl polyacrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; rubber-like materials such as styrene-butadiene copolymer rubber (SBR), etc. can be exemplified. These may be used alone or in combination of two or more.

[0038] Examples of conductive agents include carbon blacks such as acetylene black, conductive fibers such as carbon fiber and metal fiber, carbon fluoride, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene derivatives. These may be used alone or in combination of two or more.

[0039] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, polyethers (polyalkylene oxides such as polyethylene oxide, etc.), etc. These may be used alone or in combination of two or more.

[0040] The dispersion medium is not particularly limited, but examples thereof include water, alcohol, and N-methyl-2-pyrrolidone (NMP).

[0041] [Electrolyte] The electrolyte typically contains a solvent, a solute dissolved in the solvent, and an additive. The solute is an electrolyte salt that ionically dissociates in the electrolyte, and may contain a lithium salt. The electrolyte may contain various additives. The total amount of the lithium salt and the solvent in the electrolyte preferably accounts for 90% by mass or more, and more preferably 95% by mass or more, of the electrolyte.

[0042] The solvent is a cyclic carbonate ester, a cyclic carboxylic acid ester, a chain carbonate ester, a chain carboxylic acid ester, or an electrolyte component that is liquid at 25°C and is contained in the electrolyte at 3% by mass or more. One or more solvents may be used in any combination. Components other than the solvent and solute are additives, and the cyclic compound can be classified as an additive.

[0043] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), and vinylethylene carbonate (VEC).

[0044] Examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0045] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0046] The solvent contains a chain carboxylic acid ester having 2 to 4 carbon atoms. Examples of such a chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, and methyl propionate. Among these, methyl acetate has low viscosity and high stability, and can improve the low-temperature characteristics of the battery. The content of the chain carboxylic acid ester in the electrolyte is preferably 10% by volume or more and 80% by volume or less, and more preferably 20% by volume or more and 45% by volume or less. In this case, the viscosity of the electrolyte can be sufficiently reduced, and the cycle characteristics can be easily improved.

[0047] The solvent may further contain a chain carboxylic acid ester other than those mentioned above (that is, a chain carboxylic acid ester having 5 or more carbon atoms).

[0048] Note that polymers that are solid by themselves at 25°C are not included in the electrolyte solution components even when their content in the electrolyte solution is 3% by mass or more. Such polymers function as a matrix that gels the electrolyte solution.

[0049] In addition to the above cyclic compounds, examples of the additives include carboxylic acids, alcohols, 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0050] The electrolyte may contain other salts in addition to the lithium salts already mentioned. Other salts include LiClO4, LiAlCl4, LiB 10 Cl 10 , LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, LiCl, LiBr, LiI, etc. One or more lithium salts may be used in any combination.

[0051] [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 insulation properties. The separator can 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.

[0052] 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 and a nonaqueous electrolyte are housed in an exterior body. Instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0053] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0054] The secondary battery 100 is a wound-type battery including a wound-type electrode group 40 and a non-aqueous electrolyte (not shown). The wound-type electrode group 40 includes a strip-shaped positive electrode 10, a strip-shaped negative electrode 20, and a separator 30. A positive electrode lead 13 is connected to the positive electrode 10, and a negative electrode lead 23 is connected to the negative electrode 20.

[0055] One longitudinal end of the positive electrode lead 13 is connected to the positive electrode 10, and the other longitudinal end is connected to a sealing plate 90. The sealing plate 90 is equipped with a positive electrode terminal 15. One end of the negative electrode lead 23 is connected to the negative electrode 20, and the other end is connected to the bottom of the battery case 70, which serves as the negative electrode terminal. The battery case 70 is a cylindrical battery can with a bottom, and is open at one longitudinal end, with the bottom of the other end serving as the negative electrode terminal. The battery case (battery can) 70 is made of metal, for example, iron. The inner surface of the iron battery case 70 is usually nickel-plated. An upper insulating ring 80 and a lower insulating ring 60, made of resin, are arranged above and below the wound electrode group 40, respectively.

[0056] In the illustrated example, a cylindrical nonaqueous electrolyte secondary battery having a wound electrode group has been described, but this embodiment is not limited to this case and can be applied to other types of nonaqueous electrolyte secondary batteries. The structure of the nonaqueous electrolyte secondary battery may be cylindrical, coin-shaped, button-shaped, or the like, having a metal battery case, or may be a laminate-type battery having a battery case made of a laminate sheet that is a laminate of a barrier layer and a resin sheet. The shape of the electrode group is also not particularly limited, and may be a laminate type.

[0057] The present invention will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0058] Example 1 [Preparation of LSX] Silicon dioxide and lithium carbonate were mixed to an atomic ratio of Si / Li of 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate represented by the formula: Li2Si2O5 (u=0.5). The obtained lithium silicate was then pulverized to an average particle size of 10 μm.

[0059] Lithium silicate (Li2Si2O5) with an average particle size of 10 μm was mixed with raw silicon (3N, average particle size of 10 μm) in a mass ratio of 45:55. The mixture was loaded into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm for 50 hours in an inert atmosphere.

[0060] Next, the powder mixture was taken out in an inert atmosphere and sintered at 800°C for 4 hours while applying pressure using a hot press in an inert atmosphere to obtain a sintered body (LSX) of the mixture.

[0061] The LSX was then crushed and passed through a 40 μm mesh. The resulting LSX particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation), and the mixture was fired at 800°C in an inert atmosphere to coat the surfaces of the LSX particles with conductive carbon to form a conductive layer. The amount of the conductive layer was 5% by mass based on the total mass of the LSX particles and the conductive layer. LSX particles with an average particle size of 5 μm and a conductive layer were then obtained using a sieve.

[0062] [Preparation of negative electrode] Conductive LSX particles and graphite were mixed in a mass ratio of 3:97 to form the negative electrode active material. The negative electrode active material, lithium polyacrylate, and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 97.5:1:1.5, water was added, and the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) to prepare a negative electrode slurry. The negative electrode slurry was then applied to the surface of copper foil, the coating was dried, and the foil was rolled to form a negative electrode mixture layer on both sides of the copper foil.

[0063] [Preparation of positive electrode] The positive electrode active material is a lithium-containing composite oxide (LiNi 0.9 Co 0.02 Al 0.03 Mn 0.05O2), acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil serving as a positive electrode current collector. The coating was dried and then rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0064] [Preparation of non-aqueous electrolyte] A nonaqueous electrolyte solution was prepared by adding LiPF6 (a lithium salt) and 1,4-dioxane to a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and methyl acetate (MA) in a volume ratio of 20:60:20. The concentration of LiPF6 in the nonaqueous electrolyte solution was 1.0 mol / L. The dioxane content was 1% by mass relative to the total mass of the nonaqueous electrolyte solution.

[0065] [Fabrication of non-aqueous electrolyte secondary battery] A lead tab was attached to each electrode, and the positive and negative electrodes were spirally wound with the separator interposed therebetween so that the leads were positioned at the outermost periphery to prepare an electrode assembly. The electrode assembly was inserted into a metal battery can and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte solution was poured into the can, and the opening of the battery can was sealed to obtain a nonaqueous electrolyte secondary battery A1.

[0066] <Comparative Example 1> In preparing the non-aqueous electrolyte, 1,4-dioxane was not added.

[0067] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, to obtain a battery B1.

[0068] <Comparative Example 2> The positive electrode active material is a lithium-containing composite oxide (LiNi 0.9 Co 0.07 Al 0.03 O2) was used.

[0069] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, to obtain Battery B2.

[0070] <Comparative Example 3> In preparing the non-aqueous electrolyte, 1,4-dioxane was not added.

[0071] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 2, to obtain Battery B3.

[0072] <Comparative Example 4> In preparing the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 20:80 was used.

[0073] Also, no 1,4-dioxane was added.

[0074] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 2, to obtain Battery B4.

[0075] <Example 2> In the production of the positive electrode, the positive electrode active material, lithium-containing composite oxide (LiNi 0.9 Al 0.05 Mn 0.05 O2), acetylene black, polyvinylidene fluoride, and N-methyl-2-pyrrolidone (NMP) were mixed in a predetermined mass ratio to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil serving as a positive electrode current collector. The coating was dried and then rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0076] A lead tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator between them so that the leads were positioned at the outermost periphery to prepare an electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte solution was poured into the exterior case, and the opening of the laminate exterior case was sealed.

[0077] Other than this, a non-aqueous electrolyte secondary battery A2 was obtained in the same manner as in Example 1.

[0078] <Comparative Example 5> In preparing the non-aqueous electrolyte, 1,4-dioxane was not added.

[0079] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, to obtain Battery B5.

[0080] <Comparative Example 6> In preparing the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 20:80 was used.

[0081] Also, no 1,4-dioxane was added.

[0082] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, to obtain Battery B6.

[0083] <Comparative Example 7> The positive electrode active material is a lithium-containing composite oxide (LiNi 0.9 Co 0.07 Al 0.03 O2) was used.

[0084] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, to obtain Battery B7.

[0085] <Comparative Example 8> In preparing the non-aqueous electrolyte, 1,4-dioxane was not added.

[0086] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 7, to obtain Battery B8.

[0087] <Comparative Example 9> In preparing the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 20:80 was used.

[0088] Also, no 1,4-dioxane was added.

[0089] Other than this, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 7, to obtain Battery B9.

[0090] [evaluation] (Initial charge / discharge) Each completed battery was placed in a 45°C environment and subjected to constant current charging at a current of 0.3 It until the voltage reached 4.2 V, and then constant voltage charging at a constant voltage of 4.2 V until the current reached 0.02 It. Subsequently, constant current discharging was performed at a current of 0.5 It until the voltage reached 2.5 V, and the initial discharge capacity C0 was calculated. Charging and discharging were performed in a 25°C environment.

[0091] (DCIR measurement) After the initial charge / discharge test, each battery was placed in a 45°C environment and 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 0.02 It. After a 2-hour break, the battery was discharged at a current of 0.5 It for 10 seconds, and the resistance was calculated from the voltage drop before and after, which was used as the initial DCIR at SOC = 100%.

[0092] (Capacity maintenance rate) The rest period between charge and discharge was 10 minutes, and charge and discharge were repeated under the above conditions at 45°C to determine the discharge capacity C1 at the 300th cycle. The ratio R1 = C1 / C0 of the discharge capacity C1 to the initial discharge capacity C0 was evaluated as the capacity retention rate.

[0093] Table 1 shows the evaluation results of the capacity retention rate R1 and initial DCIR for batteries A1, B1 to B4. Table 1 also shows the positive electrode active material used in each battery and whether or not methyl acetate (MA) and 1,4-dioxane (1,4-DOX) were contained in the electrolyte (and, if present, the percentage of each). The positive electrodes of batteries A1 and B1 contained 2% Co and 5% Mn, while the positive electrodes of batteries B2 to B4 contained 7% Co and no Mn.

[0094] Batteries B2 and B3 have lower initial DCIR than battery B4. This is due to the effect of replacing part of the electrolyte solvent with methyl acetate, which has a low viscosity. In fact, a comparison of batteries A2 and B5 with battery B6, and a comparison of batteries B7 and B8 with battery B9 in Table 2 below suggests that the effect of replacing the electrolyte solvent with methyl acetate is independent of the positive electrode composition. Therefore, replacing part of the electrolyte solvent with methyl acetate is important to reduce the initial DCIR.

[0095] Batteries A1 and B1 have a lower Co content in the positive electrode active material than batteries B2 to B4, and are therefore more advantageous than batteries B2 to B4 in terms of manufacturing costs. However, when comparing batteries B1 and B3, battery B1, which uses a lithium-containing composite oxide containing 5 atomic % of Mn as the positive electrode active material, has a lower capacity retention rate than battery B3. The reason for this is thought to be that the use of a solvent containing methyl acetate in the electrolyte makes it easier for Mn in the composite oxide to dissolve.

[0096] On the other hand, in Battery A1, the addition of 1,4-dioxane (1,4-DOX) to the electrolyte suppressed the decrease in capacity retention compared to Battery B1. Therefore, by including methyl acetate (MA) and 1,4-dioxane (1,4-DOX) in a positive electrode composition with a low Co ratio of about 2%, it became possible to lower the initial DCIR and maintain a high capacity retention at low cost.

[0097] Batteries B2 and B3 use a lithium-containing composite oxide that does not contain manganese as the positive electrode active material. In this case, there is no significant difference in the capacity retention rate whether or not 1,4-dioxane (1,4-DOX) is added. The suppression of the decrease in capacity retention rate by adding 1,4-dioxane (1,4-DOX) is an effect unique to NCAM.

[0098] Table 2 shows the evaluation results of the capacity retention rate R1 and initial DCIR for batteries A2, B5 to B9. As in Table 1, the table also shows the positive electrode active material used in each battery and whether or not methyl acetate (MA) and 1,4-dioxane (1,4-DOX) were present in the electrolyte (if present, the percentage of each). The positive electrodes of batteries A2 and B5 contained no Co and 5% Mn, while the positive electrodes of batteries B7 to B9 contained 7% Co and no Mn.

[0099] Comparison of the initial DCIRs of the batteries A2 and B5 to B9 reveals that the inclusion of methyl acetate (MA) has the effect of lowering the initial DCIR.

[0100] However, the capacity retention rate of battery B5 containing methyl acetate (MA) was lower than that of battery B6 without methyl acetate (MA). This is thought to be because methyl acetate (MA) oxidized by charge-discharge cycles reacts with the positive electrode active material, causing Mn elution.

[0101] However, the capacity retention of Battery A2, which contains both methyl acetate (MA) and 1,4-dioxane (1,4-DOX), is higher than that of Battery B5, which contains methyl acetate (MA) but no 1,4-dioxane (1,4-DOX). As with the case of the 2% Co and 5% Mn cathode shown in Table 1, adding 1,4-dioxane (1,4-DOX) to the electrolyte in Battery A2 suppressed the decline in capacity retention compared to Battery B1. This indicates that the addition of 1,4-dioxane (1,4-DOX) suppressed the decline in capacity retention even in the case of a Co-free cathode.

[0102] The initial DCIR of Battery A1 in Table 1 is slightly higher than that of Battery B1. Similarly, the initial DCIR of Battery A2 in Table 2 is slightly higher than that of Battery B5. This is thought to be due to the film formed by 1,4-dioxane (1,4-DOX) during the initial charge / discharge. The film formed by 1,4-dioxane (1,4-DOX) is thought to suppress Mn dissolution.

[0103] [Table 1]

[0104] [Table 2] [Industrial Applicability]

[0105] According to the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery having excellent long-term cycle characteristics. The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0106] 10 positive electrode 13 Positive lead 15 Positive terminal 20 negative electrode 23 Negative lead 30 Separator 40 Wound electrode group 60 Lower insulating ring 70 Battery Case 80 Upper insulating ring 90 Sealing plate 100 Secondary battery

Claims

1. a positive electrode, a negative electrode, and an electrolyte; the positive electrode has a layered rock salt structure and includes a composite oxide containing lithium as a first metal and X as a second metal other than lithium; In the composite oxide, the second metal X contains Ni, Al, and Mn, and the atomic ratio of Ni to the metals other than lithium is 0.8 or more and less than 1; the second metal X does not contain Co; the electrolytic solution contains a chain carboxylic acid ester having 2 to 4 carbon atoms and a cyclic compound having a ring structure composed of two oxygen atoms and 3 to 5 carbon atoms; The nonaqueous electrolyte secondary battery has an atomic ratio of Mn to the second metal X: Mn / X of 0.02 or more and 0.05 or less.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the chain carboxylic acid ester is methyl acetate.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the cyclic compound is 1,4-dioxane.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the cyclic compound relative to the mass of the electrolyte solution is 0.5 mass % or more.

5. 5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of said chain carboxylic acid ester in said electrolytic solution is 10% by volume or more.

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