Positive electrode for secondary battery and secondary battery

Abstract

This positive electrode for a secondary battery comprises a positive electrode current collector, and a positive electrode mixture layer provided on a surface of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, an electrically conductive agent, a polysaccharide, and a resin other than the polysaccharide. The polysaccharide covers at least part of a surfaces of the positive electrode active material, the electrically conductive agent, and the resin.

Description

[Technical Field]

[0001] The present disclosure relates to secondary batteries, and more particularly to positive electrodes used in secondary batteries. [Background technology]

[0002] Secondary batteries, especially lithium-ion secondary batteries, have high output and high energy density, and are therefore expected to be used in small consumer applications, power storage devices, and power sources for electric vehicles. A composite oxide of lithium and a transition metal (e.g., cobalt) is used as the positive electrode active material for lithium-ion secondary batteries. Replacing part of the cobalt with nickel can increase the capacity.

[0003] In lithium-ion secondary batteries, the positive electrode is usually manufactured by applying a slurry containing a positive electrode active material and a binder to the surface of a positive electrode current collector, followed by drying to form a positive electrode mixture layer. Non-Patent Document 1 claims that by using an aqueous guar gum solution as a binder solution and forming a positive electrode mixture layer using a slurry containing guar gum, durability can be improved even when a lithium-excess lithium-transition metal composite oxide is used as the positive electrode active material. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Shi-Gang Sun, Electrochimica Acta, 351, 136401 (2020) Summary of the Invention [Problem to be solved by the invention]

[0005] However, when preparing a slurry containing guar gum, depending on the alkalinity of the slurry containing the positive electrode active material, the guar gum may gel and become uncoatable, or even if it is coatable, the properties of the positive electrode may be deteriorated, resulting in reduced productivity. [Means for solving the problem]

[0006] In view of the above, one aspect of the present disclosure relates to a positive electrode for a secondary battery, including a positive electrode current collector and a positive electrode mixture layer provided on a surface of the positive electrode current collector, wherein the positive electrode mixture layer contains a positive electrode active material, a conductive agent, a polysaccharide, and a resin other than the polysaccharide, and the polysaccharide covers at least a portion of the surfaces of the positive electrode active material, the conductive agent, and the resin.

[0007] Another aspect of the present disclosure relates to a secondary battery including the above-mentioned positive electrode for secondary battery, a separator, a negative electrode facing the positive electrode for secondary battery with the separator interposed therebetween, and an electrolyte. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to stably produce a secondary battery having excellent durability and a positive electrode for the secondary battery used therein. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away; [Figure 2A] 10 is a graph showing the change in discharge capacity for each charge / discharge cycle for the secondary batteries of Example 3 and Comparative Example 2. [Figure 2B] 10 is a graph showing the change in average discharge voltage for each charge / discharge cycle for the secondary batteries of Example 3 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Positive electrodes for secondary batteries] A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a conductive agent, a polysaccharide, and a resin other than the polysaccharide. The polysaccharide covers at least a portion of the surfaces of the positive electrode active material, the conductive agent, and the resin.

[0011] The formation of a polysaccharide coating on the surface of the positive electrode active material stabilizes the crystallinity on the surface of the positive electrode active material. Therefore, a secondary battery manufactured using the positive electrode for a secondary battery maintains high crystallinity on the surface of the positive electrode active material, resulting in a secondary battery with low resistance and excellent durability. The polysaccharide coating can also be formed on the surface of the conductive agent and / or resin. This also suppresses electrolyte decomposition originating from the conductive agent or resin, resulting in a secondary battery with excellent durability.

[0012] The polysaccharide is preferably a water-soluble compound having a glycoside bond and having viscosity increasing properties when dissolved in water. Examples of polysaccharides include pectin, alginic acid, pullulan, mannan, xanthan gum, guar gum, starch, glycogen, chitin, dextran, agarose, carrageenan, heparin, hyaluronic acid, glucomannan, and gum arabic. ,to The polysaccharide derivatives may include at least one selected from the group consisting of Remel gum and derivatives thereof. The polysaccharide derivatives may include salt compounds, esterified products, etherified products, amidated products, etc. Among these, guar gum is preferably used. Guar gum may be a derivative in which at least some of the hydrogen atoms of its hydroxyl groups are substituted with alkyl groups or the like.

[0013] Resins other than polysaccharides may be included in the positive electrode mixture layer as a binder. Examples of the resin include at least one selected from the group consisting of fluororesins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (HFP); acrylic resins such as polymethyl acrylate and ethylene-methyl methacrylate copolymer; polyolefin resins; polyamide resins; polyimide resins; and rubber-like materials such as styrene-butadiene rubber (SBR) and acrylic rubber. The resin may be an organic compound that is more soluble in organic solvents than in water.

[0014] A positive electrode mixture layer in which the surfaces of a positive electrode active material, a conductive agent, and a resin are coated with a polysaccharide can be obtained by applying a slurry containing the positive electrode active material, the conductive agent, and the resin to the surface of a positive electrode current collector, drying the slurry, forming a polysaccharide-free positive electrode mixture layer on the positive electrode current collector, and then applying a dispersion in which the polysaccharide is dispersed in a dispersion medium (e.g., water) to the surface of the positive electrode mixture layer and drying the coating. By forming a polysaccharide coating as described above, the dispersion does not contain a strongly alkaline positive electrode active material, so the polysaccharide does not gel in the dispersion, and the dispersion can be easily applied. Therefore, a highly durable positive electrode for a secondary battery can be obtained stably without reducing productivity.

[0015] By applying the dispersion liquid, the polysaccharide penetrates into the interior of the positive electrode mixture layer through the voids in the porous positive electrode mixture layer, and the surfaces of the positive electrode active material, the conductive agent, and the resin can be covered with the polysaccharide throughout the entire depth of the positive electrode mixture layer. However, in this case, the concentration of the polysaccharide in the positive electrode mixture layer is not uniform in the depth direction, and the polysaccharide concentration can be unevenly distributed in the depth direction, such that the polysaccharide concentration is higher on the surface side of the positive electrode mixture layer (the side opposite the positive electrode current collector).

[0016] In the positive electrode mixture layer, the polysaccharide may be contained in an amount of, for example, 0.1% by mass to 0.5% by mass, and preferably 0.1% by mass to 0.25% by mass, based on the total amount of the positive electrode active material. When the positive electrode for a secondary battery contains the polysaccharide in the above concentration range, a secondary battery with excellent durability can be realized.

[0017] The polysaccharide content is determined from a sample obtained by removing only the positive electrode mixture layer from a discharged secondary battery. Specifically, the discharged secondary battery is first disassembled to remove the positive electrode. Next, the positive electrode is washed with an organic solvent and further vacuum-dried, and then only the positive electrode mixture layer is peeled off to obtain a sample. By subjecting the sample to thermal analysis such as TG-DTA, the ratio of polysaccharides, binder, and conductive material components other than the positive electrode active material can be calculated.

[0018] The positive electrode active material may include a lithium-containing composite oxide (hereinafter, sometimes referred to as "HN composite oxide") having a layered structure, in which 80 atomic % or more of the metal other than lithium is nickel. Using a lithium-containing composite oxide with a high Ni ratio as the positive electrode active material can achieve high capacity. However, the higher the Ni ratio of the lithium-containing composite oxide, the more Li is extracted from the positive electrode active material, which can modify the surface of the positive electrode active material and change the structure to one that makes it difficult to absorb and release Li. As a result, the movement of lithium ions is inhibited, and the deterioration of cycle characteristics is likely to become significant. However, by covering the surface of the HN composite oxide with a polysaccharide, the surface structure of the HN composite oxide is stabilized, which suppresses the deterioration of cycle characteristics and improves durability.

[0019] The positive electrode active material contains a lithium-transition metal composite oxide based on a layered rock salt structure. In the lithium-transition metal composite oxide, it may be a lithium-excess type lithium-transition metal composite oxide (hereinafter sometimes referred to as "Li-excess composite oxide") in which the atomic ratio of lithium to transition metal M: Li / M is 1 or more. The Li-excess composite oxide has a structure in which a part of the transition metal sites in the layered rock salt structure is substituted with Li. Similar to the HN composite oxide, the surface structure of the Li-excess composite oxide tends to become unstable with the release of Li, and it easily changes to a structure in which the occlusion and release of Li are difficult. As a result, the movement of lithium ions is inhibited, and the cycle characteristics are likely to deteriorate significantly. However, by covering the surface of the Li-excess composite oxide with a polysaccharide, the surface structure of the Li-excess composite oxide is stabilized, the deterioration of the cycle characteristics is suppressed, and the durability is improved. Furthermore, by covering the surface of the Li-excess composite oxide with a polysaccharide, the decrease in the battery voltage after charging accompanying repeated charge and discharge is also suppressed.

[0020] More specifically, the positive electrode active material may contain a lithium-nickel composite oxide represented by the chemical formula Li a Ni b M c O2. Here, M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B. In the case of the HN composite oxide, for example, 0 < a ≤ 1.2, 0.8 ≤ b ≤ 1, and b + c = 1 may be satisfied. In the case of the Li-excess composite oxide, for example, 0 < a ≤ 1.8, 0.5 ≤ b + c < 1, and 0.1 ≤ b ≤ 0.6 may be satisfied. The a value representing the molar ratio of Li varies with charge and discharge.

[0021] Also, as the positive electrode active material, in order to obtain a high capacity, the ratio of nickel to the metals other than lithium in the above lithium-containing composite oxide may be 85 atomic% or more. According to this embodiment, even when the Ni ratio of the positive electrode active material is increased in this way, high cycle characteristics can be obtained.

[0022] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery includes the above-described positive electrode for secondary battery, a separator, a negative electrode facing the positive electrode for secondary battery with the separator interposed therebetween, and an electrolyte solution.

[0023] [Positive electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode may be the same as the positive electrode for a secondary battery described above. The positive electrode mixture layer may be formed, for example, by coating the surface of the positive electrode current collector with a positive electrode slurry containing a positive electrode active material, a binder (resin), and the like, in which the positive electrode mixture is dispersed in a dispersion medium, and then drying the coating. A dispersion containing a polysaccharide is then applied to the coating of the positive electrode slurry, allowing the polysaccharide to penetrate into the voids in the positive electrode mixture layer, thereby covering the surfaces of the positive electrode active material, conductive agent, and binder with the polysaccharide. The coating of the positive electrode mixture layer after being coated with the polysaccharide may be rolled, if necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode slurry may contain a polysaccharide as long as it does not impair productivity. The polysaccharide contained in the positive electrode slurry may be the same as or different from the polysaccharide contained in the dispersion liquid.

[0024] The conductive agent may be a conductive material containing carbon. Carbon black such as acetylene black or ketjen black can be preferably used as the conductive material containing carbon. The conductive material containing carbon may contain carbon nanotubes. A conductive additive other than the conductive agent containing carbon may be contained in the positive electrode mixture layer. The positive electrode mixture layer may contain a thickener or the like as an optional component. Known materials can be used as the thickener and other conductive additive.

[0025] As the positive electrode active material, a lithium-containing composite oxide having a layered structure (for example, a rock salt crystal structure) containing lithium and a transition metal can be used. Specifically, the lithium-containing composite oxide can be, for example, Li a Ni b M 1-bIt may also be a lithium-nickel composite oxide represented by O2 (where 0 < a ≤ 1.2, 0.8 ≤ b ≤ 1, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B). Among these, it is preferable that M contains at least one selected from the group consisting of Co, Mn, Al, and Fe. From the perspective of the stability of the crystal structure, Al may be included as M. Note that the value of a indicating the molar ratio of lithium increases or decreases during charge and discharge. Specific examples of such composite oxides include lithium-nickel-cobalt-aluminum composite oxide (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.).

[0026] Here, from the perspective of obtaining a high capacity, the lithium transition metal composite oxide is preferably an HN composite oxide in which the proportion of Ni in the metal elements other than Li is 80 atomic% or more. The proportion of Ni in the metal elements other than Li may be 85 atomic% or more, or 90 atomic% or more. The proportion of Ni in the metal elements other than Li is preferably, for example, 95 atomic% or less. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0027] Co, Mn, and Al contribute to the stabilization of the crystal structure of the HN composite oxide with a high Ni content. However, from the perspective of reducing manufacturing costs, the lower the Co content, the more desirable. The HN composite oxide with a low Co content or without Co may contain Mn and Al.

[0028] The proportion of Co in the metal elements other than Li is preferably 20 atomic% or less, more preferably 10 atomic% or less or 5 atomic% or less, and it may not contain Co. From the perspective of the stability of the crystal structure of the HN composite oxide, it is desirable to contain 1 atomic% or more or 1.5 atomic% or more of Co.

[0029] The proportion of Mn in the metal elements other than Li may be 10 atomic % or less, or may be 5 atomic % or less. The proportion of Mn in the metal elements other than Li may be 1 atomic % or more, may be 3 atomic % or more, or may be 5 atomic % or more. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0030] The proportion of Al in the metal elements other than Li may be 10 atomic % or less, or may be 5 atomic % or less. The proportion of Al in the metal elements other than Li may be 1 atomic % or more, may be 3 atomic % or more, or may be 5 atomic % or more. When limiting the range, these upper and lower limits can be arbitrarily combined.

[0031] Element M may contain at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. Among them, when at least one selected from the group consisting of Nb, Sr, and Ca is contained in the HN composite oxide, it is considered that the surface structure of the composite oxide is stabilized, the resistance is reduced, and the elution of the metal is further suppressed. These elements are more effective when they are unevenly distributed in the vicinity of the particle surface of the composite oxide.

[0032] As another example of the positive electrode active material, in the above lithium-transition metal composite oxide having a rock salt-type crystal structure, a lithium-excess type lithium-transition metal composite oxide (Li-excess composite oxide) in which a part of the sites occupied by the transition metal is substituted with lithium may be used. Specifically, the Li-excess composite oxide is, for example, Li a Ni b M c O2 (where 0 < a ≤ 1.8, 0.5 ≤ b + c < 1, 0.1 ≤ b ≤ 0.6, and M contains at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B). Among them, it is preferable that M contains at least one selected from the group consisting of Co, Mn, Al, and Fe. From the viewpoint of the stability of the crystal structure, Al may be contained as M. The value of a indicating the molar ratio of lithium increases or decreases by charge and discharge.

[0033] Specific examples of Li-excess composite oxides include Li 1.16 Mn 0.5 Ni 0.34 Lithium-manganese-nickel composite oxide, represented by O2, Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2 and Li 1.16 Mn 0.5 Ni 0.17 Co 0.17 Examples include lithium-manganese-nickel-cobalt composite oxides represented by the following formula:

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

[0035] The HN composite oxide and the Li-excess composite oxide are, for example, secondary particles formed by aggregation of multiple primary particles. The particle size of the primary particles is, for example, 0.05 μm or more and 1 μm or less. The average particle size of the secondary particles of the composite oxide HN is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less.

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

[0037] Although the above-mentioned HN composite oxides and Li-excess composite oxides can provide high capacity, their crystal structures tend to become unstable, particularly in a fully charged state, and repeated charge and discharge cycles tend to change (deactivate) the surface of the active material particles to a crystal structure that makes it difficult to reversibly absorb and release lithium ions. As a result, cycle characteristics tend to deteriorate. However, by using the positive electrode for secondary batteries of this embodiment, the surfaces of the active material particles are coated with polysaccharides, so that high cycle characteristics can be maintained even when HN composite oxides and Li-excess composite oxides are used as the active material. Therefore, a secondary battery with excellent durability and high energy density can be realized.

[0038] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0039] [Negative electrode] The negative electrode contains a negative electrode active material. The negative electrode typically includes a negative electrode current collector and a layer of a negative electrode mixture (hereinafter referred to as a negative electrode mixture layer) held on the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the coating. The dried coating may be rolled, if necessary.

[0040] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0041] (Negative electrode active material) The negative electrode active material may be metallic lithium, a lithium alloy, or the like, but is preferably a material capable of electrochemically absorbing and releasing lithium ions. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode may contain one type of negative electrode active material or a combination of two or more types.

[0042] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination. Among these, graphite is preferred as the carbonaceous material because it has excellent charge / discharge stability and a small irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0043] Examples of Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which silicon phases are dispersed within a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO x The lithium ion conductive phase may be at least one selected from the group consisting of an SiO2 phase, a silicate phase, and a carbon phase.

[0044] As the binder, thickener, conductive agent, and dispersion medium used in the negative electrode slurry, for example, the materials exemplified for the positive electrode or known materials can be used.

[0045] The negative electrode current collector may be, for example, a metal foil. The negative electrode current collector may be porous. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0046] [Electrolyte] The electrolyte solution includes a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolyte solution. The solute may include, for example, a lithium salt. Components of the electrolyte solution other than the solvent and the solute are additives. The electrolyte solution may include various additives.

[0047] Known materials can be used as the solvent. Examples of the solvent that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0048] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0049] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0050] The electrolyte may contain other known additives, such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0051] [Separator] A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulation. 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 secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a non-aqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0053] FIG. 1 is a schematic perspective view, with a portion cut away, of a prismatic secondary battery according to an embodiment of the present disclosure. The secondary battery 1 shown in Fig. 1 includes a bottomed prismatic battery case 11, and an electrode group 10 and a nonaqueous electrolyte (not shown) housed within the battery case 11. The electrode group 10 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 10 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0054] One end of a negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of a positive electrode lead 14 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 15 is electrically connected to a negative electrode terminal 13 provided on the sealing plate 12. A gasket 16 is disposed between the sealing plate 12 and the negative electrode terminal 13 to insulate them from each other. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and is electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is disposed above the electrode group 10. The frame 18 separates the electrode group 10 from the sealing plate 12 and also separates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed with the sealing plate 12. The sealing plate 12 has a liquid injection hole 17a formed therein. The electrolyte is poured into the battery case 11 through the pouring hole 17a. The pouring hole 17a is then closed with the sealing plug 17.

[0055] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0056] Example 1 (1) Preparation of the negative electrode A silicon composite material and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil, which served as a negative electrode current collector. The coating was dried and then rolled to form a negative electrode mixture layer on both sides of the copper foil.

[0057] (2) Preparation of the positive electrode The positive electrode active material is a HN composite oxide, LiNi 0.8 Mn 0.2 O2 was used. 0.8 Mn 0.2 O2, acetylene black (AB) as a conductive additive, polyvinylidene fluoride (PVdF) as a binder (resin), and N-methyl-2-pyrrolidone (NMP) were mixed with LiNi 0.8 Mn 0.2The positive electrode slurry was prepared by mixing them in a predetermined mass ratio of O2:AB:PVdF = 92:5:3. Next, the positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried and then rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0058] A guar gum aqueous solution was prepared. The guar gum aqueous solution was applied to the surface of the positive electrode mixture layer, and the guar gum was allowed to penetrate into the positive electrode mixture layer. The resulting mixture was then dried to remove moisture, resulting in a positive electrode. In the guar gum aqueous solution, 0.25 parts by mass of guar gum was dissolved in water relative to 100 parts by mass of the positive electrode active material.

[0059] (3) Preparation of electrolyte An electrolyte solution was prepared by adding LiPF6 as a lithium salt to a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. The concentration of LiPF6 in the non-aqueous electrolyte solution was 1.0 mol / L.

[0060] (4) Fabrication of secondary batteries A lead tab was attached to each electrode, and the positive and negative electrodes were spirally wound with the separator interposed between them so that the leads were positioned at the outermost periphery to produce an electrode assembly. The electrode assembly was inserted into a laminate film exterior with an aluminum foil barrier layer, and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte solution was poured into the exterior, and the opening of the exterior was sealed to obtain secondary battery A1.

[0061] (5) Evaluation (Initial charge / discharge) The completed battery was placed in a 25°C environment and subjected to constant current charging at a current of 0.1 C until the voltage reached 4.5 V, and then constant voltage charging at a constant voltage of 4.5 V until the current reached 0.05 C. It was then subjected to constant current discharging at a current of 0.1 C until the voltage reached 2.5 V, and the initial capacity C0 (mAh) was calculated. Charging and discharging were performed in a 25°C environment. The initial capacity was divided by the weight of the positive electrode active material to calculate the capacity per weight (g).

[0062] (Charge / discharge 2) The battery was charged at a constant current of 0.2 C until the voltage reached 4.5 V, and then charged at a constant voltage of 4.5 V until the current reached 0.01 C. It was then discharged at a constant current of 0.2 C until the voltage reached 2.5 V, and the discharge capacity C (mAh) was calculated. Charging and discharging were performed at 25°C.

[0063] (durability) The rest period between charge and discharge was 20 minutes, and 24 charge / discharge cycles were repeated under the conditions shown in Charge / Discharge 2 above. Then, one charge / discharge cycle was performed under the same conditions as the initial charge / discharge, and the discharge capacity was confirmed. Then, 24 charge / discharge cycles were repeated under the conditions shown in Charge / Discharge 2 above, for a total of 50 charge / discharge cycles. The ratio R1 = C1 / C0 of the discharge capacity C1 at the 50th cycle to the initial discharge capacity C0 was used as the capacity retention rate, and R1 × 100 was used as an index of durability.

[0064] <Example 2> In preparing the positive electrode, 0.5 parts by mass of guar gum was dissolved in water relative to 100 parts by mass of the positive electrode active material to obtain an aqueous guar gum solution, which was then applied to the surface of the positive electrode mixture layer and dried to obtain a positive electrode. A secondary battery A2 was produced in the same manner as in Example 1 except for this, and was evaluated in the same manner.

[0065] <Comparative Example 1> In the preparation of the positive electrode, the guar gum aqueous solution was not applied to the surface of the positive electrode mixture layer. Other than this, a secondary battery B1 was produced in the same manner as in Example 1 and evaluated in the same manner.

[0066] The evaluation results of the initial capacity and durability of batteries A1, A2, and B1 are shown in Table 1. As can be seen from Table 1, batteries A1 and A2, which use a positive electrode coated with guar gum as a polysaccharide, have higher initial capacity and improved durability than battery B1, which uses a positive electrode without guar gum.

[0067] [Table 1]

[0068] Example 3 (1) Preparation of the positive electrode The positive electrode active material is a Li-rich composite oxide, Li 1.17 Ni 0.56 Mn 0.28 O2 was used. 1.17 Ni 0.56 Mn 0.28 O2, acetylene black (AB) as a conductive additive, polyvinylidene fluoride (PVdF) as a binder (resin), and N-methyl-2-pyrrolidone (NMP) were mixed in Li 1.17 Ni 0.56 Mn 0.28 The positive electrode slurry was prepared by mixing them in a predetermined mass ratio of O2:AB:PVdF = 92:5:3. Next, the positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried and then rolled to form a positive electrode mixture layer on both sides of the aluminum foil.

[0069] A guar gum aqueous solution was prepared. The guar gum aqueous solution was applied to the surface of the positive electrode mixture layer, and the guar gum was allowed to penetrate into the positive electrode mixture layer. The resulting mixture was then dried to remove moisture, resulting in a positive electrode. In the guar gum aqueous solution, 0.25 parts by mass of guar gum was dissolved in water relative to 100 parts by mass of the positive electrode active material.

[0070] A negative electrode was produced, an electrolyte solution was prepared, and a battery was assembled in the same manner as in Example 1, except that the positive electrode thus obtained was used, to obtain a secondary battery A3.

[0071] (2) Evaluation The initial capacity C0 and the capacity retention rate R1 were determined in the same manner as in Example 1 except that the charge cutoff voltage was changed to 4.7 V, and durability was evaluated.

[0072] (Output maintenance rate) Furthermore, the average discharge voltage was measured for each charge / discharge cycle. The ratio of the average discharge voltage V1 after 50 cycles to the average discharge voltage V0 after the first charge / discharge cycle (R2 = V1 / V0) was calculated as the voltage retention ratio. The product of the capacity retention ratio R1 and the voltage retention ratio R2 was calculated, and the output retention ratio was evaluated as R1 × R2 × 100.

[0073] Example 4 In preparing the positive electrode, 0.5 parts by mass of guar gum was dissolved in water relative to 100 parts by mass of the positive electrode active material to obtain an aqueous guar gum solution, which was then applied to the surface of the positive electrode mixture layer and dried to obtain a positive electrode. A secondary battery A4 was produced in the same manner as in Example 3 except for this, and evaluated in the same manner as in Example 3.

[0074] <Comparative Example 2> In the preparation of the positive electrode, the guar gum aqueous solution was not applied to the surface of the electrode mixture layer. A secondary battery B2 was produced in the same manner as in Example 3 except for this, and evaluated in the same manner as in Example 3.

[0075] Table 2 shows the evaluation results of the initial capacity, durability, and output retention rate of the batteries A3, A4, and B2. 2 As a result, in batteries A3 and A4, which used positive electrodes coated with guar gum as a polysaccharide, although a slight decrease in initial capacity was observed in battery A4, durability and output retention rate were improved.

[0076] Figure 2A shows the change in discharge capacity for each charge / discharge cycle for batteries A3 and B2. Figure 2B shows the change in average discharge voltage for each charge / discharge cycle for batteries A3 and B2. As shown in Figures 2A and 2B, battery A3, which uses a positive electrode coated with guar gum, shows a more suppressed decrease in discharge capacity with repeated charge / discharge cycles than battery B2, and also shows a more suppressed decrease in average discharge voltage.

[0077] [Table 2] [Industrial Applicability]

[0078] The secondary battery according to the present disclosure can provide a secondary battery with high capacity and excellent cycle characteristics. The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0079] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0080] 1: non-aqueous electrolyte secondary battery, 2: negative electrode, 10: electrode group, 11: battery case, 12: sealing plate, 13: negative electrode terminal, 14: positive electrode lead, 15: negative electrode lead, 16: gasket, 17: sealing plug, 17a: liquid injection hole, 18: frame

Claims

1. a positive electrode current collector; and a positive electrode mixture layer provided on a surface of the positive electrode current collector, the positive electrode mixture layer contains a positive electrode active material, a conductive agent, a polysaccharide, and a resin other than the polysaccharide, the polysaccharide covers at least a portion of the surfaces of the positive electrode active material, the conductive agent, and the resin over the entire depth direction of the positive electrode mixture layer, In the positive electrode mixture layer, the polysaccharide is distributed in a depth direction of the positive electrode mixture layer so that the concentration of the polysaccharide is higher on a surface side of the positive electrode mixture layer opposite to the positive electrode current collector, The positive electrode active material for a secondary battery includes at least one of a lithium-containing composite oxide having a layered structure and containing nickel at 80 atomic % or more of metals other than lithium, and a lithium-transition metal composite oxide having a layered rock salt structure as a base and having an atomic ratio of lithium to a transition metal M: Li / M of 1 or more.

2. 2. The positive electrode for secondary batteries according to claim 1, wherein the content of the polysaccharide in the positive electrode mixture layer is 0.1% by mass to 0.5% by mass with respect to the total amount of the positive electrode active material.

3. A positive electrode for a secondary battery as described in claim 1 or 2, wherein the polysaccharide comprises at least one selected from the group consisting of pectin, alginic acid, mannan, xanthan gum, guar gum, starch, glycogen, chitin, dextran, agarose, carrageenan, heparin, hyaluronic acid, glucomannan, gum arabic, tremel gum, and derivatives thereof.

4. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the polysaccharide includes guar gum.

5. The positive electrode for a secondary battery according to any one of claims 1 to 4, wherein the resin contains polyvinylidene fluoride (PVdF).

6. The positive electrode for a secondary battery according to any one of claims 1 to 5, A secondary battery comprising: a separator; a negative electrode facing the positive electrode for the secondary battery with the separator interposed therebetween; and an electrolyte.

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