Positive electrode active material

Abstract

To provide a positive electrode active material that can suppress the generation of gas due to decomposition of an electrolytic medium compared to conventional positive electrode active materials.SOLUTION: A positive electrode active material is used in a positive electrode portion of a lithium ion secondary battery and includes a first oxide active material and a second oxide active material, the first oxide active material is a lithium-manganese-based oxide having a spinel-type crystal structure, and the second oxide active material is a lithium-manganese-based oxide composed of the same elements as the first oxide active material and having a lithium-excess layered crystal structure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material. [Background technology]

[0002] BACKGROUND ART Lithium nickel manganese oxide (hereinafter also referred to as LNMO) has been known as a positive electrode active material for lithium ion secondary batteries (for example, Patent Document 1).

[0003] LNMO has an operating voltage of 4.7 V based on the lithium metal deposition potential, which is higher than conventional lithium insertion materials used as positive electrode active materials (for example, lithium cobalt oxide, which has a voltage of 4 V), and is therefore expected to contribute to achieving higher energy density. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-051987 Summary of the Invention [Problem to be solved by the invention]

[0005] In lithium-ion secondary batteries, the crystal structure of the active material changes during the insertion and desorption of Li ions, and distortion occurs within the crystal structure, which can lead to instability.

[0006] In such cases, transition metal ions that constitute the crystalline structure of the positive electrode active material are eluted, and in the vicinity of the positive electrode, the bond between the transition metal and oxygen is broken, causing oxygen to be released, resulting in the generation of oxygen gas or reaction with the electrolytic medium to generate carbon dioxide gas. In particular, in lithium ion secondary batteries that use a nonaqueous electrolyte as the electrolytic medium and LNMO as the positive electrode active material, the operating voltage is high and the reaction proceeds in an oxidizing atmosphere near the positive electrode, which causes the problem of significant gas generation.

[0007] Therefore, an object of the present invention is to provide a positive electrode active material that can suppress gas generation due to decomposition of the electrolytic medium compared to conventional materials. [Means for solving the problem]

[0008] One aspect of the present invention for solving the above-mentioned problems is a cathode active material used in a cathode portion of a lithium ion secondary battery, the cathode active material comprising a first oxide active material and a second oxide active material, wherein the first oxide active material is a lithium-manganese-based oxide having a spinel-type crystal structure, and the second oxide active material is a lithium-manganese-based oxide composed of the same elements as the first oxide active material and having a lithium-excess layered crystal structure.

[0009] According to this aspect, the generation of gas due to decomposition of the electrolytic medium can be suppressed compared to the conventional method.

[0010] In a preferred aspect, the first oxide active material is Li 1+x M y Mn 2-x-y It is a lithium manganese oxide represented by O4 (0≦x<0.2, 0≦y≦0.8, M is at least one selected from the group consisting of Ni, Co, Fe, and Ti).

[0011] In a preferred aspect, in the first oxide active material, the M is Ni.

[0012] In a preferred aspect, the second oxide active material is Li 1+a M b Mn 1-b O 2+2a (0.2≦a<1, 0.1≦b<0.5, M is at least one selected from the group consisting of Ni, Co, Fe, and Ti).

[0013] In a preferred aspect, the second oxide active material has a value of a greater than 0.5.

[0014] In a preferred aspect, in the second oxide active material, the M is Ni.

[0015] In a preferred aspect, the ratio of the second oxide active material to the first oxide active material is 0.5% or more and 10% or less.

[0016] One aspect of the present invention is a positive electrode active material for use in a positive electrode portion of a lithium ion secondary battery, comprising: The positive electrode active material has a first oxide active material and a second oxide active material, wherein the first oxide active material is a lithium manganese-based oxide having a spinel-type crystal structure, and the second oxide active material is a lithium manganese-based oxide that is in a polymorphic relationship with the first oxide active material.

[0017] According to this aspect, the generation of gas due to decomposition of the electrolytic medium can be suppressed compared to the conventional method.

[0018] The above aspects may be made dependent on each other, or some of the configurations may be quoted or substituted for each other, as long as they are included in the technical scope of the present invention. [Effects of the Invention]

[0019] The positive electrode active material of the present invention can suppress gas generation due to decomposition of the electrolytic medium compared to conventional materials. [Brief explanation of the drawings]

[0020] [Figure 1] 1A and 1B are explanatory diagrams conceptually illustrating a lithium ion secondary battery according to a first embodiment of the present invention, in which (a) is a perspective view of the lithium ion secondary battery, and (b) is a cross-sectional view of (a). DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described in detail.

[0022] A lithium ion secondary battery 1 (hereinafter also simply referred to as secondary battery 1) according to a first embodiment of the present invention includes an electrode stack 2, an electrolytic medium 3, and an outer casing 5, as shown in FIG. 1, and most of the electrode stack 2 and the electrolytic medium 3 are housed and sealed within the outer casing 5.

[0023] <Electrode laminate 2> As shown in FIG. 1(b), the electrode stack 2 has a plurality of positive electrode sections 20, a plurality of negative electrode sections 21, a plurality of separators 22, and electrode extraction members 23, 24, and is provided with a battery section 25 in which the positive electrode sections 20, the separators 22, and the negative electrode sections 21 are stacked in this order. In the electrode stack 2 of this embodiment, the electrode parts 20, 21 and the separators 22 are arranged alternately, such as separator 22 / ··· / separator 22 / negative electrode part 21 / separator 22 / positive electrode part 20 / ··· / separator 22, and the separators 22 are arranged on the outermost sides in the stacking direction.

[0024] (positive electrode part 20) The positive electrode part 20 is an intercalation electrode in which a positive electrode active material layer 31 is laminated on at least one main surface of a positive electrode current collector 30, and is capable of inserting and extracting Li ions. In the positive electrode part 20 of this embodiment, positive electrode active material layers 31 are laminated on both main surfaces of a positive electrode current collector 30, as shown in FIG. 1(b).

[0025] (negative electrode part 21) The negative electrode section 21 is an intercalation electrode in which a negative electrode active material layer 41 is laminated on at least one main surface of a negative electrode current collector 40, and is capable of inserting and extracting Li ions. In the negative electrode section 21 of this embodiment, negative electrode active material layers 41 are laminated on both main surfaces of a negative electrode current collector 40, as shown in FIG. 1(b).

[0026] (Current collectors 30, 40) The current collectors 30 and 40 are electrically conductive plate-like or film-like bodies. The current collectors 30 and 40 are not particularly limited as long as they are electrically conductive, and for example, metals such as aluminum and alloys thereof can be used.

[0027] (Cathode active material layer 31) The positive electrode active material layer 31 includes a positive electrode active material, a conductive additive, and a binder. The average thickness of the positive electrode active material layer 31 is preferably 10 μm or more and 200 μm or less from the viewpoint of ensuring the electric capacity and the output density.

[0028] (Cathode active material) The positive electrode active material contains at least a first oxide active material and a second oxide active material.

[0029] The first oxide active material is a lithium ion conductive active material, and is a high potential positive electrode active material that operates at a higher potential than the second oxide active material. The first oxide active material has an average potential for lithium desorption and insertion relative to the deposition potential of Li (vs. Li + / Li) is preferably 4.5 V or more and 5.0 V or less. That is, the first oxide active material preferably has an operating potential of 4.5 V or more and 5.0 V or less relative to lithium metal as a single substance. The potential (hereinafter referred to as voltage) of the lithium ion insertion and desorption reaction (vs. Li + / Li) can be determined, for example, by measuring the charge-discharge characteristics of a half cell with a working electrode using the first oxide active material and a lithium metal counter electrode, and reading the voltage values ​​at the start and end of the plateau. If there are two or more plateaus, the plateau with the lowest voltage value is 4.5 V (vs. Li + / Li) or higher, and the highest voltage plateau is 5.0V (vs. Li + / Li) or less is sufficient.

[0030] The first oxide active material is a lithium manganese-based oxide having a spinel-type crystal structure, and is preferably a lithium manganese-based oxide represented by the following formula (1). Li 1+x M y Mn 2-x-y O4···Eq.(1) In the formula (1), 0 ≦ x < 0.2, 0 ≦ y ≦ 0.8, and M is at least one selected from the group consisting of Ni, Co, Fe, and Ti.

[0031] In the first oxide active material, in the formula (1), it is more preferable that M is Ni. That is, the first oxide active material is 1+x Ni y Mn 2-x-y It is more preferable that it is a lithium manganese oxide represented by O4.

[0032] The second oxide active material is a lithium ion conductive active material and is a high-capacity positive electrode active material with a higher capacity than the first oxide active material. The second oxide active material is composed of the same elements as the first oxide active material and has a crystal structure with a lithium-excess layered structure. That is, the second oxide active material is in a polymorphic relationship with the first oxide active material.

[0033] From the viewpoint of suppressing the generation of gas caused by the decomposition of the electrolytic medium 3, the second oxide active material is preferably a lithium manganese oxide represented by the following formula (2). Li 1+a M b Mn 1-b O 2+2a ··· Formula (2) In the formula (2), 0.2 ≦ a < 1, 0.1 ≦ b < 0.5, and M is at least one selected from the group consisting of Ni, Co, Fe, and Ti.

[0034] In the second oxide active material, in the formula (2), it is more preferable to satisfy 0.5 < a. In the second oxide active material, in the formula (2), it is more preferable to satisfy a ≦ 0.8. In the second oxide active material, in the formula (2), it is more preferable that M is Ni. That is, the second oxide active material is 1+a Ni b Mn 1-b O 2+2a It is more preferable that it is a lithium manganese oxide represented by.

[0035] The ratio of the second oxide active material to the first oxide active material is preferably 0.5% or more, and more preferably 1% or more. The ratio of the second oxide active material to the first oxide active material is preferably 10% or less, more preferably 7% or less, and even more preferably 5% or less.

[0036] (Negative electrode active material layer 41) The negative electrode active material layer 41 includes a negative electrode active material, a conductive additive, and a binder.

[0037] (Negative electrode active material) The negative electrode active material is not particularly limited as long as it is capable of inserting and extracting Li ions, and for example, titanium compounds such as lithium titanate can be used.

[0038] (Conductive additive) The conductive additive used in the positive electrode active material layer 31 and / or the negative electrode active material layer 41 is not particularly limited, but is preferably a carbon material. The carbon material is preferably at least one selected from natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. The amount of the conductive additive contained in the positive electrode active material layer 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the positive electrode active material. The amount of the conductive additive contained in the negative electrode active material layer 41 is preferably 1 part by weight or more and 30 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. Within the above range, the conductivity of the active material layers 31, 41 is ensured, while the adhesiveness to the binder is maintained, and sufficient adhesiveness to the current collectors 30, 40 can be obtained.

[0039] (binder) The binder used in the positive electrode active material layer 31 and / or the negative electrode active material layer 41 is not particularly limited, but for both of the active material layers 31 and 41, for example, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. The amount of binder contained in the positive electrode active material layer 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the positive electrode active material. The amount of binder contained in the negative electrode active material layer 41 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the negative electrode active material. Within the above range, the adhesiveness between the active material and the conductive additive in the active material layers 31, 41 can be maintained, and sufficient adhesiveness with the current collectors 30, 40 can be obtained.

[0040] (Separator 22) The separator 22 is not particularly limited as long as it can be placed between the positive electrode active material layer 31 and the negative electrode active material layer 41, has insulating properties, and has a structure capable of containing the electrolytic medium 3. Examples of the separator 22 include nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and woven fabrics, nonwoven fabrics, and microporous membranes made of a combination of two or more of these materials.

[0041] (Electrode extraction members 23, 24) The positive electrode lead-out member 23 is electrically connected to at least one of the positive electrode sections 20 constituting the electrode stack 2 within the exterior package 5, and is a positive electrode terminal that extends inside and outside the exterior package 5. The negative electrode lead-out member 24 is electrically connected to at least one of the negative electrode portions 21 constituting the electrode stack 2 within the exterior package 5, and is a negative electrode terminal that extends inside and outside the exterior package 5. The electrode extraction members 23, 24 are electrically conductive plate-like bodies, and are not particularly limited as long as they are electrically conductive. For example, metals such as aluminum or alloys thereof can be used for the electrode extraction members 23, 24.

[0042] <Electrolytic medium 3> The electrolytic medium 3 is not particularly limited as long as it has lithium ion conductivity, but examples thereof include a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent, etc. The electrolytic medium 3 may be in a solid state, or may be a solid electrolyte. The electrolytic medium 3 of this embodiment uses a non-aqueous electrolyte, a part of which is impregnated into the separator 22 .

[0043] <Exterior body 5> As shown in FIG. 1(b), the exterior body 5 has an internal space, and accommodates and seals the electrode stack 2 and the electrolytic medium 3 in the internal space. The exterior body 5 is not particularly limited as long as it is chemically stable with respect to the electrolytic medium 3 and has water vapor barrier properties. The exterior body 5 may be, for example, a can made primarily of aluminum, iron, or stainless steel, or a laminate film containing a laminate resin. The exterior body 5 of this embodiment is made up of a first exterior film 50 and a second exterior film 51, as shown in FIG. The first exterior film 50 covers one side of the electrode laminate 2 in the stacking direction of the battery section 25 of the electrode laminate 2, and the second exterior film 51 covers the other side of the electrode laminate 2 in the stacking direction of the battery section 25 of the electrode laminate 2. The exterior films 50, 51 are adhered so as to surround the battery section 25 of the electrode stack 2 when viewed in plan.

[0044] In the secondary battery 1 of this embodiment, the positive electrode active material comprises a first oxide active material made of a lithium-manganese-based oxide having a spinel-type crystal structure, and a second oxide active material made of a lithium-manganese-based oxide that is composed of the same elements as the first oxide active material and has a lithium-excess layered crystal structure, which makes it possible to suppress gas generation due to decomposition of the electrolytic medium 3 compared to conventional batteries.

[0045] In the secondary battery 1 of this embodiment, the positive electrode active material includes a first oxide active material made of a lithium-manganese-based oxide having a spinel-type crystal structure, and a second oxide active material made of a lithium-manganese-based oxide that has a polymorphic relationship with the first oxide active material, which makes it possible to suppress gas generation due to decomposition of the electrolytic medium 3 compared to conventional batteries.

[0046] In the above-described embodiment, the positive electrode active material includes two types of oxide active materials that are in a polymorphic relationship, but the present invention is not limited to this. The positive electrode active material may include three or more types of oxide active materials that are in a polymorphic relationship.

[0047] In the above-described embodiment, the secondary battery 1 has two positive electrode parts 20 as shown in FIG. 1 , but the present invention is not limited to this, and the secondary battery 1 may have one positive electrode part 20, or may have three or more positive electrode parts 20. Similarly, in the above-described embodiment, the secondary battery 1 has two negative electrode parts 21, but the present invention is not limited to this, and the secondary battery 1 may have one negative electrode part 21, or may have three or more negative electrode parts 21. Similarly, in the above-described embodiment, the secondary battery 1 has five separators 22, but the present invention is not limited to this, and the secondary battery 1 may have one separator 22, or may have three or more separators 22.

[0048] In the above-described embodiment, the separator 22 is located on the outermost side of the electrode stack 2 in the stacking direction of the battery section 25 of the secondary battery 1, but the present invention is not limited to this. The electrode sections 20 and 21 may be located on the outermost side of the electrode stack 2.

[0049] In the above-described embodiment, the secondary battery 1 is a laminated secondary battery in which the electrode stack 2 is sandwiched between two exterior films 50, 51, but the present invention is not limited to this and may be a cylindrical secondary battery, a rectangular secondary battery, or a pin-type secondary battery.

[0050] In the above-described embodiments, each component can be freely substituted or added between the respective embodiments as long as it falls within the technical scope of the present invention. [Example]

[0051] The present invention will be specifically described below with reference to examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0052] Example 1 (i) Preparation of the positive electrode (a) Preparation of the first oxide active material A Ni / Mn intermediate (manufactured by Tosoh Corporation, Ni:Mn=1:3, 20 g) and lithium carbonate (4.12 g) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder. Next, the mixed powder was filled into an alumina firing container and heated in a muffle furnace (Yamato Scientific Co., Ltd., FP411) in an air atmosphere at 900°C for 12 hours, and then at 750°C for 24 hours. After that, it was allowed to cool naturally in the furnace, and the resulting powder was dissolved in water. 0.5 Mn 1.5 A first oxide active material having a spinel-type crystal structure represented by O4 was obtained.

[0053] (b) Preparation of the second oxide active material The Ni / Mn intermediate (Ni:Mn=1:3, 20 g) and lithium carbonate (14.04 g) were weighed and mixed in an automatic mortar for 60 minutes to form a mixed powder. Next, the mixed powder was filled into an alumina firing container and heated in a muffle furnace at 900°C in an air atmosphere for 12 hours. After that, it was allowed to cool naturally in the furnace. 1.75 Ni 0.25 Mn 0.75 O 3.5 A second oxide active material was obtained, which has a lithium-excess layered crystal structure represented by the following formula:

[0054] (c) Assembling the positive electrode The positive electrode active material was a mixture of the first oxide active material obtained by (a) and the second oxide active material obtained by (b) in a ratio of 99:1. A mixture containing acetylene black as a conductive additive and polyvinylidene fluoride (PVdF) as a binder in solid concentrations of 90 parts by weight, 6 parts by weight, and 4 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The binder used was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid content of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, which will be described later.

[0055] The slurry was applied to a 15 μm thick aluminum foil, dried in an oven at 120° C., and then further dried in vacuum at 170° C. to prepare a positive electrode.

[0056] (ii) Preparation of the negative electrode The negative electrode active material was spinel-type lithium titanate (Li4Ti5O 12 , hereinafter referred to as LTO) was used. A mixture containing 100 parts by weight of LTO, acetylene black as a conductive additive, and 5 parts by weight of polyvinylidene fluoride (PVdF) as a binder, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry. The binder was prepared as an NMP solution with a solids concentration of 5% by weight, and further NMP was added to adjust the viscosity to facilitate coating, as described below.

[0057] The slurry was applied to a 20 μm thick aluminum foil and then dried in an oven at 120° C. This operation was performed on both sides of the aluminum foil, and then the foil was further dried in a vacuum at 170° C. to prepare a negative electrode part.

[0058] (iii) Preparation of evaluation cells An evaluation cell was produced using the positive electrode part and negative electrode part produced in (i) and (ii) above and a 20 μm polypropylene separator in the following manner. First, the positive electrode part and the negative electrode part were dried under reduced pressure at 80° C. for 12 hours. Next, 15 positive electrode parts and 16 negative electrode parts were stacked in the order of negative electrode part / separator / positive electrode part / separator, with the outermost layers being separators in both cases. Next, aluminum tabs were vibration-welded to the positive and negative electrode portions at both ends. Two sheets of aluminum laminate film were prepared as exterior materials, and a recess for the battery portion and a recess for the gas collection portion were formed by pressing, after which the electrode stack was placed inside. The outer periphery, leaving a space for injecting the non-aqueous electrolyte, was heat-sealed at 180°C for 7 seconds, and LiPF6 was dissolved in a solvent prepared by mixing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70 to give a concentration of 1 mol / L from the unsealed portion. After the non-aqueous electrolyte was injected, the unsealed portion was heat-sealed at 180°C for 7 seconds while the pressure was reduced. The resulting battery was charged at a constant current equivalent to 0.2 C until the battery voltage reached a cut-off voltage of 3.4 V, and then the charging was stopped. Thereafter, the battery was left to stand in an environment of 60°C for 24 hours, and then subjected to constant current discharge at a current value equivalent to 0.2 C. Discharge was stopped when the battery voltage reached 2.5 V. After the discharge was stopped, the gas trapped in the gas trap was removed and the cell was resealed.

[0059] Example 2 In the above (c), Example 2 was prepared in the same manner as Example 1, except that the positive electrode active material was a mixture of the first oxide active material and the second oxide active material in a ratio of 97:3.

[0060] Example 3 In the above (c), Example 3 was prepared in the same manner as Example 1, except that the positive electrode active material was a mixture of the first oxide active material and the second oxide active material in a ratio of 95:5.

[0061] Example 4 This was designated as Example 4, and was the same as Example 1 except that in the above (b), the amount of lithium carbonate was changed to 12.03 g.

[0062] (Comparative Example 1) This was designated Comparative Example 1, and was the same as Example 1, except that in (b) above, the Ni / Mn intermediate was changed to a Co / Mn intermediate (Co:Mn=1:3, 20 g) and the amount of lithium carbonate was changed to 12.03 g.

[0063] (Comparative Example 2) This was designated Comparative Example 2, and was the same as Example 1 except that in the above (c), the second oxide active material was not added.

[0064] (Initial capacity measurement) The evaluation cells of Examples 1 to 4 and Comparative Examples 1 and 2 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and subjected to constant current charging at a current value equivalent to 0.2 C in an environment of 25°C until the battery voltage reached a cut-off voltage of 3.4 V, and then charging was stopped. Subsequently, constant current discharge was carried out at a current value equivalent to 0.2 C, and the discharge was stopped when the battery voltage reached 2.5 V, which was taken as the initial capacity.

[0065] (Gas generation rate measurement) The amount of gas generated before and after the cycle performance evaluation was evaluated using the Archimedes method, that is, the buoyancy of the evaluation cell. The evaluation was performed as follows. First, the weights of the evaluation cells of Examples 1 to 4 and Comparative Examples 1 and 2 were measured using an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage, product number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. This buoyancy is due to the density of water (1.0 g / cm 3 ) to calculate the volumes of the evaluation cells of Examples 1 to 4 and Comparative Examples 1 and 2. The amounts of gas generated in the evaluation cells of Examples 1 to 4 and Comparative Examples 1 and 2 after 100 cycles of cycle performance evaluation were calculated by comparing the volumes before and after the cycle performance evaluation of 100 cycles, which will be described later.

[0066] (Cycle characteristics evaluation) The evaluation cells of Examples 1 to 4 and Comparative Examples 1 and 2 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and cycle operation was carried out. In an environment of 60°C, constant current charging was carried out at a current value equivalent to 1.0 C until the battery voltage reached a cut-off voltage of 3.4 V, and then charging was stopped. Subsequently, constant current discharge was performed at a current value equivalent to 1.0 C, and the discharge was stopped when the battery voltage reached 2.5 V. This constituted one cycle, and charge and discharge were repeated.

[0067] Table 1 shows the results of the initial capacity measurement and the gas generation amount measurement for Examples 1 to 4 and Comparative Examples 1 and 2.

[0068] [Table 1]

[0069] In Examples 1 to 4, which contained a first oxide active material and a second oxide active material that are polymorphic, the amount of gas generated after 100 cycles was 1.5 ml / Ah or less in all cases, as shown in Table 1, and the amount of gas generated after 100 cycles was significantly reduced compared to Comparative Example 2, which did not contain the second oxide active material. Although both Example 2 and Comparative Example 1 contain a first oxide active material composed of a lithium-manganese-based oxide having a spinel-type crystal structure and a second oxide active material composed of a lithium-manganese-based oxide whose crystal structure is a lithium-excess layered structure, the amount of gas generated after 100 cycles was significantly different. That is, in Example 2, in which the first oxide active material and the second oxide active material are in a polymorphic relationship, the amount of gas generated after 100 cycles was significantly reduced compared to Comparative Example 1, in which the first oxide active material was isomorphously substituted with a second oxide active material.

[0070] This indicates that gas generation can be suppressed by including lithium-manganese oxides, which are in a polymorphic relationship, as the first oxide active material and the second oxide active material as the positive electrode active material.

[0071] In addition, in Examples 1 to 3, the amount of gas generated after 100 cycles was 0.5 ml / Ah or less, which was significantly reduced compared to Example 4, in which the amount of lithium carbonate added was reduced compared to Examples 1 to 3, and the amount of dissolved lithium was reduced. That is, the amount of gas generated was reduced by including more excess lithium ions in the second oxide active material relative to the layered rock salt type crystal structure. This suggests that the distortion of the crystal structure has an effect on suppressing gas generation.

[0072] In Examples 1 to 3, the amount of gas generated decreased as the amount of the second oxide active material added increased, which revealed that the second oxide active material mainly contributed to suppressing the amount of gas generated.

[0073] Furthermore, in Examples 1 to 4, the initial capacity did not change significantly compared to Comparative Examples 1 and 2, as shown in Table 1.

[0074] From the above results, it is believed that the amount of gas generated was suppressed for the following reasons. The second oxide active material undergoes a gradual structural change to the first oxide active material as the amount of lithium and oxygen in the structure changes through repeated charge and discharge. During the charging process, the lithium released from the second oxide active material exists in the electrolyte as lithium ions, and at the same time, the nickel and manganese in the structure are oxidized to maintain charge balance. However, nickel is easily reduced when it is trivalent or higher, so it reacts with the negatively charged decomposition products of the electrolyte that are generated during the discharging process, and oxygen is taken into the active material, thereby suppressing the generation of gas.

[0075] As described above, it has been found that by constructing a positive electrode active material from a first oxide active material and a second oxide active material that are in a polymorphic relationship, in particular a first oxide active material that is composed of a lithium-manganese-based oxide having a spinel-type crystal structure, and a second oxide active material that is composed of the same elements as the first oxide active material and is composed of a lithium-manganese-based oxide whose crystal structure has a lithium-excess layered structure, it is possible to suppress the amount of gas generation while maintaining the initial capacity, compared to when a lithium-manganese-based oxide having a spinel-type crystal structure is used alone as the positive electrode active material. [Explanation of symbols]

[0076] 1. Lithium-ion secondary battery 20 Positive electrode 31 Positive electrode active material layer 41 Negative electrode active material layer

Claims

1. A positive electrode active material used in a positive electrode portion of a lithium ion secondary battery, a first oxide active material and a second oxide active material; the first oxide active material is a lithium manganese-based oxide having a spinel-type crystal structure, The second oxide active material is a positive electrode active material that is composed of the same elements as the first oxide active material and is a lithium-manganese oxide having a lithium-excess layered crystal structure.

2. The first oxide active material is Li １＋ｘ M ｙ Mn ２－ｘ－ｙ O ４ 2. The positive electrode active material according to claim 1, which is a lithium manganese-based oxide represented by the formula (1), where 0≦x<0.2, 0≦y≦0.8, and M is at least one element selected from the group consisting of Ni, Co, Fe, and Ti.

3. The positive electrode active material according to claim 2 , wherein the M in the first oxide active material is Ni.

4. The second oxide active material is Li １＋ａ M ｂ Mn １－ｂ O ２+２ａ 4. The positive electrode active material according to claim 1, which is a lithium-manganese-based oxide represented by the formula (1) (wherein 0.2≦a<1, 0.1≦b<0.5, and M is at least one element selected from the group consisting of Ni, Co, Fe, and Ti).

5. The positive electrode active material of claim 4 , wherein the second oxide active material has a value of a greater than 0.

5.

6. The positive electrode active material according to claim 4 , wherein in the second oxide active material, M is Ni.

7. 4. The positive electrode active material according to claim 1, wherein the ratio of the second oxide active material to the first oxide active material is 0.5% or more and 10% or less.

8. A positive electrode active material used in a positive electrode portion of a lithium ion secondary battery, a first oxide active material and a second oxide active material; the first oxide active material is a lithium manganese-based oxide having a spinel-type crystal structure, The positive electrode active material, wherein the second oxide active material is a lithium-manganese-based oxide that is in a polymorphic relationship with the first oxide active material.

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