Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery using the same

Abstract

This positive electrode for a non-aqueous electrolyte secondary battery comprises: a positive-electrode current collector; and a positive-electrode mixture layer that is disposed on the surface of the positive-electrode current collector. The positive-electrode mixture layer contains a positive-electrode active substance, a conductor material, and a binder. The binder contains a polyvinylpyrrolidone and a cellulose derivative.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are widely used in consumer and automotive applications due to their high power output and high energy density. In recent years, there has been a demand for even greater durability and energy density in non-aqueous electrolyte secondary batteries. One method known to achieve this is to use electrodes made by loading a large amount of positive electrode active material with a large charge / discharge capacity per unit area and compressing it under high pressure. The more positive electrode active material is loaded per unit area, the more likely the migration of conductive materials and binders will occur during the manufacturing process of the positive electrode. As a result, the adhesion of the positive electrode active material may decrease, and the energy density may decrease.

[0003] On the other hand, one known method for improving the performance of non-aqueous electrolyte secondary batteries is to enhance the conductivity of the positive electrode mixture layer. For example, adding conductive materials such as carbon nanotubes to the positive electrode mixture has been a conventional practice. When using conductive materials that tend to aggregate, such as carbon nanotubes, it is important to uniformly disperse the conductive material within the positive electrode mixture layer.

[0004] Patent Document 1 (Japanese Patent Publication No. 2020-19705) describes a dispersion liquid used in the manufacture of electrodes, comprising "bundle-type carbon nanotubes, a dispersion medium, and a partially hydrogenated nitrile rubber having a residual double bond (RDB) value of 0.5 to 40% by weight calculated by the following mathematical formula 1, wherein the dispersed particle size of the carbon nanotubes is the particle size distribution D 50 A carbon nanotube dispersion characterized by having a particle size of 3 to 10 μm. [Mathematical formula 1] RDB (weight %) = BD weight / (BD weight + HBD weight) × 100. In the above mathematical formula 1, BD means structural units derived from conjugated dienes, and HBD means structural units derived from hydrogenated conjugated dienes. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-19705 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Currently, there is a need for further improvement in the properties of cathodes using conductive materials such as carbon nanotubes. In this context, one of the objectives of this disclosure is to provide a cathode that can be obtained for non-aqueous electrolyte secondary batteries with higher properties, and a non-aqueous electrolyte secondary battery using the same. [Means for solving the problem]

[0007] One aspect of this disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery. The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, wherein the positive electrode mixture layer comprises a positive electrode active material, a conductive material, and a binder, the binder comprising polyvinylpyrrolidones and cellulose derivatives.

[0008] Another aspect of this disclosure relates to a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery includes a positive electrode for a non-aqueous electrolyte secondary battery as described in this disclosure. [Effects of the Invention]

[0009] According to this disclosure, it is possible to realize a non-aqueous electrolyte secondary battery with high performance characteristics. While novel features of the present invention are described in the appended claims, the present invention, both in terms of its structure and content, will be better understood by the following detailed description in conjunction with the drawings, in conjunction with other objects and features of the present invention. [Brief explanation of the drawing]

[0010] [Figure 1]This is a schematic perspective view showing a portion of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0011] The embodiments relating to this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B".

[0012] (Positive electrode for non-aqueous electrolyte secondary batteries) The positive electrode of this embodiment is a positive electrode for a secondary battery and includes a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a conductive material, and a binder. The binder includes polyvinylpyrrolidones and cellulose derivatives. The positive electrode mixture layer may optionally contain other components (e.g., thickeners).

[0013] Traditionally, polyvinylpyrrolidone and other materials have been used as additives in the positive electrode mixture layer of secondary batteries. However, there are countless types of additives, and the effects of their combinations have not been sufficiently studied. Furthermore, it is difficult for those skilled in the art to predict what effects will be obtained when additives are combined. If the effects of additive combinations could be predicted, the optimal combination of additives could be determined without experimentation. However, in reality, predicting the effects of additive combinations is difficult. As a result of our research, the inventors of this application have discovered a new combination that yields a specific effect. Specifically, the inventors of this application have found that by using a binder containing polyvinylpyrrolidones and cellulose derivatives, it is possible to manufacture secondary batteries with high performance, as shown in the examples. This disclosure is based on this new finding.

[0014] (Binding agent) Examples of the cellulose derivatives used as the binder include alkyl celluloses such as methyl cellulose, hydroxyalkyl celluloses, and their alkali metal salts. Examples of the alkali metals forming the alkali metal salts include potassium and sodium. Among these, methyl cellulose, ethyl cellulose, and hydroxypropyl methyl cellulose are preferred. The weight average molecular weight of the cellulose derivative may be in the range of 1,000 to 1,000,000 (for example, in the range of 10,000 to 1,000,000). In terms of enhancing the effects of the configuration of the present disclosure, the weight average molecular weight of the cellulose derivative may be in the range of 10,000 to 200,000.

[0015] The polyvinylpyrrolidones are at least one selected from the group consisting of polyvinylpyrrolidone and polyvinylpyrrolidone derivatives. Examples of the polyvinylpyrrolidone derivatives include polymers in which the hydrogen atoms of polyvinylpyrrolidone are substituted with other substituents, such as alkylated polyvinylpyrrolidone. As the polyvinylpyrrolidones, only polyvinylpyrrolidone may be used, or a copolymer of vinylpyrrolidone and other single molecules may be used. Examples of the other single molecules include styrene-based and vinyl acetate-based single molecules.

[0016] The weight average molecular weight of the polyvinylpyrrolidones may be in the range of 1,000 to 2,000,000. In terms of enhancing the effects of the configuration of the present disclosure, the weight average molecular weight of the polyvinylpyrrolidones may be in the range of 5,000 to 1,000,000.

[0017] In the positive electrode active material layer, the amount of the cellulose derivative with respect to 100 parts by mass of the polyvinylpyrrolidones is preferably in the range of 30 to 400 parts by mass (for example, in the range of 100 to 400 parts by mass or 300 to 400 parts by mass). By setting the amount in the range of 100 to 400 parts by mass, as shown in the examples, particularly high effects can be obtained.

[0018] The binder may further contain nitrile rubber. Examples of nitrile rubber include copolymers of monomers containing acrylonitrile and a diene (e.g., butadiene). Examples of nitrile rubber include acrylonitrile rubber such as nitrile-butadiene rubber (NBR) and hydrogenated nitrile-butadiene rubber (H-NBR). By including polyvinylpyrrolidones, cellulose derivatives, and nitrile rubber in the binder, particularly high effects can be obtained, as shown in the examples.

[0019] In the positive electrode mixture layer, the amount of nitrile rubber per 100 parts by mass of polyvinylpyrrolidone is preferably in the range of 30 to 500 parts by mass (for example, in the range of 100 to 300 parts by mass). By setting this amount in the range of 100 to 300 parts by mass, particularly high effects can be obtained, as shown in the examples.

[0020] There are no particular limitations on the weight-average molecular weight of the nitrile rubber; it should be within the range suitable for use as a binder in non-aqueous electrolyte secondary batteries.

[0021] The binder may further contain polyvinylidene fluoride. The inclusion of polyvinylpyrrolidones, cellulose derivatives, nitrile rubber, and polyvinylidene fluoride in the binder significantly influences the suppression of binder migration due to steric hindrance between molecular chains, resulting in improved bonding strength. Furthermore, the chemically stable cellulose derivatives and nitrile rubber suppress side reactions with the lithium component contained in the positive electrode active material, thereby suppressing the decrease in bonding strength due to migration. In particular, when a lithium-containing composite oxide with a high Ni ratio is used as the positive electrode active material, side reactions with the lithium component generally increase; therefore, a high synergistic effect can be obtained by including polyvinylpyrrolidones, cellulose derivatives, nitrile rubber, and polyvinylidene fluoride. In the positive electrode mixture layer, the amount of polyvinylidene fluoride per 100 parts by mass of polyvinylpyrrolidones may be in the range of 50 to 5000 parts by mass (for example, in the range of 200 to 2000 parts by mass).

[0022] The binder may include other binders besides those mentioned above. Known binders may be used for such other binders. However, the proportion of other binders to the total binders should be small, for example, 10% by mass or less.

[0023] In the positive electrode mixture layer, the amount of binder per 100 parts by mass of positive electrode active material may be in the range of 0.1 to 2 parts by mass.

[0024] (Conductive material) Examples of conductive materials include conductive materials containing carbon. Examples of conductive materials containing carbon may include conductive carbon particles such as carbon black, graphene, carbon nanofibers, and fibrous conductive carbon materials such as carbon nanotubes, with carbon nanotubes being preferred. Fibrous conductive carbon materials may be referred to as "carbon fibers" below. Since carbon nanotubes are preferred carbon fibers, in the following description, carbon fibers can be read as carbon nanotubes. The proportion of carbon nanotubes in all conductive materials is, for example, 50% by mass or more, and is preferably in the range of 66 to 100% by mass (for example, in the range of 80 to 100% by mass or 90 to 100%).

[0025] As mentioned above, the preferred conductive material is carbon nanotubes. When carbon nanotubes are used as the conductive material, particularly high effectiveness can be obtained by using a binder containing polyvinylpyrrolidones and cellulose derivatives. Specifically, the adhesion and discharge capacity of the positive electrode mixture layer can be improved, and the internal resistance of the battery can be reduced.

[0026] In the dispersion of carbon nanotubes, polyvinylpyrrolidones have some effect in improving dispersibility due to their good wettability to carbon nanotubes, but their low steric hindrance presents challenges in long-term dispersion stability. On the other hand, by using polyvinylpyrrolidones in combination with cellulose derivatives, the following unique effects can be obtained. For cellulose derivatives, which exhibit high steric hindrance and long-term dispersion stability but have low affinity for aprotic polar solvents, polyvinylpyrrolidones not only function as a dispersant but also as a dispersion aid for cellulose derivatives, making it possible to achieve both good dispersibility and long-term dispersion stability.

[0027] Carbon nanotubes are tiny carbon fibers with a fiber diameter in the nanoscale. When carbon nanotubes are used as a conductive material, even a small amount can reduce the resistance of the positive electrode mixture layer.

[0028] The average fiber length of carbon nanotubes may be 1 μm or more. In this case, the aspect ratio (ratio of fiber length to outer diameter) of the carbon nanotubes, which are carbon fibers, becomes extremely large. Carbon fibers with a large aspect ratio make linear contact with the active material and current collector, rather than point contact. The highly conductive carbon fibers are interposed between the particles of the positive electrode active material, forming linear contact points with the particles, thereby improving the DC resistance (DCR) of the battery.

[0029] In addition, since carbon fibers occupy only a small volume within the positive electrode mixture layer, the proportion of positive electrode active material in the positive electrode mixture layer can be increased. Furthermore, as mentioned above, the drawbacks (increased resistance) that occur when the positive electrode mixture layer is made thicker or compressed can be suppressed by using carbon fibers. Therefore, by using carbon fibers, it is possible to make the positive electrode mixture layer thicker or more compressed. On the other hand, it was found that further increasing the capacity in this way leads to problems such as a decrease in the adhesion of the positive electrode mixture layer, an increase in positive electrode active material that does not contribute to the discharge capacity, and an increase in internal resistance. As a result of the investigation, the inventors found that these problems can be solved by a special combination of binders.

[0030] The carbon fiber content is, for example, 1 part by mass or less per 100 parts by mass of positive electrode active material. The carbon fiber content may be 0.01 parts by mass or more and 1 part by mass or 0.02 parts by mass or more and 0.5 parts by mass per 100 parts by mass of positive electrode active material. The above carbon fiber content is a value based on the mass of the positive electrode active material in the discharged state.

[0031] Here, the average fiber length of carbon fibers is determined by image analysis using a scanning electron microscope (SEM). The average fiber length of carbon fibers can be determined, for example, by arbitrarily selecting several carbon fibers (e.g., about 100), measuring their lengths, and then taking the arithmetic mean. Furthermore, fiber length refers to the length of a carbon fiber when it is stretched in a straight line.

[0032] The average fiber diameter (outer diameter) of carbon fibers is, for example, 20 nm or less, but may also be 15 nm or less. Here, the average fiber diameter of carbon fibers is determined by image analysis using a transmission electron microscope (TEM). The average fiber diameter of carbon fibers can be determined, for example, by arbitrarily selecting several carbon fibers (e.g., 100 fibers), measuring their fiber diameters, and taking the arithmetic mean of these measurements. Furthermore, fiber diameter refers to the length in the direction perpendicular to the fiber length direction.

[0033] The proportion of positive electrode active material in the positive electrode mixture layer can be determined using a mixture sample. The mixture sample is obtained by disassembling a discharged secondary battery, washing the positive electrode with an organic solvent, vacuum drying it, and then peeling off only the positive electrode mixture layer. By performing thermal analysis such as TG-DTA on the mixture sample, the ratio of binder components and conductive material components other than the positive electrode active material can be calculated. If the binder components and conductive material components contain multiple types of carbon materials, the proportion of carbon fibers can be calculated by performing micro-Raman spectroscopy on a cross-section of the positive electrode mixture layer.

[0034] Carbon nanotubes may be single-walled, double-walled, or multi-walled, and may consist of at least two of these configurations. Carbon nanotubes with an average fiber diameter of 20 nm or less are preferred because a large effect can be obtained with a small amount. The average fiber length of the carbon nanotubes is preferably 1 μm or more from the viewpoint of ensuring electron conduction inside the positive electrode. On the other hand, there is no upper limit to the fiber length if they are properly arranged inside the positive electrode, but considering that the particle diameter of the positive electrode active material is generally between 1 μm and 20 μm, a length of a similar length is considered appropriate. That is, the average fiber length of the carbon nanotubes may be, for example, between 1 μm and 20 μm.

[0035] The positive electrode mixture layer of the positive electrode in this embodiment satisfies the following condition (1). The positive electrode mixture layer preferably satisfies at least one of (2) to (8). (2) to (8) can be combined in any way. (1) The binder includes polyvinylpyrrolidones and cellulose derivatives. (2) The mass ratio of the components contained in the binder may be in the range of polyvinylpyrrolidones:cellulose derivatives:acrylonitrile rubber:polyvinylidene fluoride = 100:30 to 400:0 to 3000:0 to 5000. The ratio of other components to polyvinylpyrrolidones may be changed to the range described above. (3) Examples of cellulose derivatives include methylcellulose, ethylcellulose, and hydroxypropylmethylcellulose. (4) The binder includes acrylonitrile rubber. Examples of acrylonitrile rubber include NBR and H-NBR. (5) The binder includes polyvinylidene fluoride and polytetrafluoroethylene. (6) The conductive material includes carbon materials such as carbon nanotubes, carbon black, and graphene. The proportion of carbon nanotubes in the conductive material may also be within the range described above. The mass ratio of the total of the binder to the conductive material (or the mass ratio of the total of the binder to the carbon nanotubes) may be in the range of binder total:conductive material = 100:1 to 100. (7) The ratio (mass ratio) of the positive electrode active material, the binder, and the conductive material is in the range of positive electrode active material:binder:conductive material = 100:0.1 to 2:0.01 to 1. (8) The polyvinylpyrrolidones are polyvinylpyrrolidone.

[0036] (Positive electrode active material) Examples of the positive electrode active material include composite oxides containing lithium and transition metals. In this specification, such composite oxides may be referred to as "lithium-containing composite oxides". The lithium-containing composite oxide may have a layered structure (for example, a rock salt-type crystal structure). The positive electrode active material has the composition formula Li y Ni x M 1-x O2 (where 0.8 ≤ x ≤ 1, 0 < y ≤ 1.2, and M contains at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B).) It is preferably a lithium-containing composite oxide represented by M is preferably at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B. Among them, M preferably contains at least one selected from the group consisting of Co, Mn, Al, and Fe. In the above example, M is typically a metal element. From the viewpoint of the stability of the crystal structure, M may contain Al. The value of y indicating the composition ratio of lithium increases and 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.).

[0037] Here, from the viewpoint of obtaining high capacity, it is desirable that the proportion of Ni among the metal elements other than Li in the lithium-containing composite oxide be 80 atomic percent or more. The proportion of Ni among the metal elements other than Li may be 85 atomic percent or more, or 90 atomic percent or more. For example, in the above compositional formula, x may satisfy 0.85 ≤ x ≤ 1. The proportion of Ni among the metal elements other than Li is preferably 95 atomic percent or less. These lower and upper limits can be combined arbitrarily.

[0038] At least one element selected from the group consisting of Sr and Ca may be unevenly distributed near the surface of the positive electrode active material. That is, at least a portion of the surface of the particles of the positive electrode active material may be covered with a surface modification layer, and the surface modification layer may contain at least one element selected from the group consisting of Sr and Ca. The positive electrode active material usually exists in the positive electrode mixture layer in the form of secondary particles formed by the aggregation of primary particles of a composite oxide. In this case, the surface modification layer may cover at least a portion of the surface of the primary particles. The surface modification layer can be formed on the surface of the positive electrode active material particles (e.g., primary particles) by substituting some of the metal elements other than lithium that constitute the composite oxide with at least one of Sr and Ca. A method for producing positive electrode active material particles with such surface modification will be described later.

[0039] The higher the nickel ratio in the composite oxide, the more unstable the surface structure of the active material becomes during charging, and the release of Li makes it easier for the crystal structure to change into one that makes it difficult to intercept and release Li ions.

[0040] In particular, in composite oxides with a high nickel content, a nickel oxide (NiO) layer is easily formed on the surface of the positive electrode active material. The NiO layer has a structure that makes it difficult to intercept and release Li ions, and if it forms thickly, the resistance at the positive electrode may increase, leading to a decrease in cycle characteristics. In addition, as the NiO layer forms, the metals that make up the composite oxide may dissolve into the electrolyte and precipitate at the negative electrode, which may affect the durability of the secondary battery.

[0041] Furthermore, when moisture is contained in the positive electrode or the electrolyte, it may react with lithium salts such as LiPF6 contained in the electrolyte to generate HF (hydrogen fluoride). The generated HF promotes the elution of the metal constituting the composite oxide and degrades the cycle characteristics.

[0042] In the composite oxide, nickel exists in trivalent or tetravalent state and contributes to charge and discharge, but the stable state as an element is divalent. The reaction to become divalent is an irreversible reaction. Once it becomes divalent, it no longer contributes to charge and discharge and cannot return to the trivalent or tetravalent oxidation state inside the battery. Trivalent or tetravalent nickel can be reduced to divalent by reaction with water or the like. Among them, trivalent or tetravalent nickel is strongly reduced by HF and easily becomes divalent. Therefore, a composite oxide with a large nickel ratio is likely to promote the degradation of cycle characteristics due to cation elution and structural collapse in the presence of HF.

[0043] The lithium-containing composite oxide may be a lithium-containing composite oxide represented by the composition formula Li a Ni x M2 1-x O 2-δ (However, 0 < a ≤ 1.2, 0.8 ≤ x < 1, 0 ≤ δ ≤ 0.05, and M2 includes at least one selected from the group consisting of Mn, Fe, Ti, Si, Nb, Zr, Mo, Co, Al, Zn, Sr, and Ca.) In order to obtain a high capacity, the Ni ratio x in the above composition formula may be 0.85 or more (x ≥ 0.85).

[0044] In the above composition formula, M2 may include at least one element selected from the group consisting of Mn, Fe, Ti, Si, Nb, Zr, Mo, Co, Al, and Zn, and at least one element A selected from the group consisting of Sr and Ca. When at least one of Sr and Ca (for example, Sr and / or Ca) is contained in the composite oxide, it is considered that the surface structure of the composite oxide is stabilized, metal elution is suppressed, and the cycle characteristics can be maintained high even in an environment where HF is likely to be generated.

[0045] The composite oxide may contain element A (one selected from the group consisting of Sr and Ca, for example, Sr and / or Ca). Element A stabilizes the crystal structure of the composite oxide surface even when a large amount of Li is extracted due to charging. Element A only needs to be present on the surface of the composite oxide. A surface layer containing element A may be formed on the surface of the composite oxide.

[0046] Element A is preferably present on the particle surface of the composite oxide and in its vicinity (for example, in the near-surface region within 30 nm from the particle surface). Composite oxide particles are usually secondary particles formed by the aggregation of multiple primary particles, and element A may be present in a higher concentration on the surface and near the surface of the primary particles, including the surface of the secondary particles, than in the center of the primary particles. In other words, element A is unevenly distributed on the surface and near the surface of the primary particles of the composite oxide, and the content of element A per unit volume may be higher on the surface than inside the primary particles. The distribution of Ca in the composite oxide can be analyzed by TEM-EDX, etc.

[0047] From the viewpoint of improving cycle characteristics, the content of element A in the composite oxide may be 0.01 mol% or more, 0.015 mol% or more, or 0.03 mol% or more relative to the total number of moles of metal elements excluding Li. On the other hand, from the viewpoint of suppressing the decrease in capacity due to the increase in resistance on the surface of the composite oxide, the content of element A in the composite oxide may be 2 mol% or less, 1.7 mol% or less, or 1.5 mol% or less relative to the total number of moles of metal elements excluding Li.

[0048] The lithium-containing composite oxide described above can increase its capacity as the Ni ratio x increases, allowing more lithium ions to be extracted from the composite oxide during charging. However, charging tends to destabilize the crystal structure, and repeated charging and discharging can easily lead to a change in crystal structure (deactivation) that makes reversible intercalation and release of lithium ions difficult. As a result, the cycle performance tends to deteriorate.

[0049] In particular, when a lithium salt containing fluorine (e.g., LiPF6) is used as the solute in the electrolyte, if water is present or generated in the electrolyte, HF may be produced by the following reaction. In addition, fluorine components contained in fluorinated solvents such as fluoroethylene carbonate (FEC) or thickeners and binders in the electrolyte may decompose in a strongly alkaline environment, also generating HF. LiPF6 + H2O → LiPOF4 + 2HF

[0050] The generated HF promotes the dissolution of metals other than lithium in the composite oxide, and deactivates the crystal structure on the surface of the composite oxide, making reversible intercalation and release of lithium ions difficult. As a result, this leads to a further deterioration of the cycle characteristics.

[0051] However, by using the positive electrode additive described above, the leaching of metal due to the generated HF can be suppressed, thereby preventing a decrease in cycle characteristics. Furthermore, if element A is present on at least the surface of the composite oxide, the surface of the composite oxide is stabilized. Due to these synergistic effects, cycle characteristics can be significantly improved even when a lithium-containing composite oxide with a Ni ratio x of 0.8 or higher is used as the positive electrode active material. Therefore, a secondary battery with excellent cycle characteristics and high energy density can be realized.

[0052] From the viewpoint of obtaining high capacity, the Ni ratio x in the lithium-containing composite oxide may be 0.85 or higher, or 0.9 or higher.

[0053] A lithium-containing composite oxide containing element A on at least its surface can be produced by a manufacturing method comprising: (i) obtaining a lithium-containing composite oxide containing any metal element M0 other than Ni, Sr, and Ca; (ii) mixing the lithium-containing composite oxide obtained in step (i) with a lithium compound and a compound containing at least one element A selected from the group consisting of Sr and Ca to obtain a mixture; and (iii) calcining the mixture. The metal element M0 is a metal element constituting the lithium-containing composite oxide.

[0054] In step (i), for example, an alkaline solution such as sodium hydroxide is added dropwise while stirring a solution of a metal salt containing Ni and the metal element M0 to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (coprecipitation) a composite hydroxide containing Ni and the metal element M0. Subsequently, the composite hydroxide is calcined to obtain a composite oxide containing Ni and the metal element M0. The calcination temperature is not particularly limited, but for example, it is 300°C to 600°C.

[0055] In step (ii), the composite oxide obtained in step (i), a lithium compound, and a compound containing element A are mixed to obtain a mixture. The mixing ratio of the composite oxide and the lithium compound is adjusted, for example, so that the molar ratio of metal elements excluding Li to Li is in the range of 1:0.98 to 1:1.1. Examples of lithium compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of compounds containing element A include inorganic compounds such as oxides, chlorides, and hydroxides containing element A.

[0056] In step (iii), the mixture obtained in step (ii) is calcined at a predetermined temperature and time to obtain a composite oxide. Step (iii) may be, for example, a calcination step under an oxygen stream.

[0057] The ratio of element A to the total of Ni and metallic element M0 in the final composite oxide is adjusted, for example, by controlling the mixing ratio of the raw materials in step (ii), the firing temperature and time in step (iii), etc.

[0058] The composite oxide obtained in step (iii) is washed with water and subjected to solid-liquid separation to obtain a lithium-containing composite oxide powder, for example, with a water content of 3 to 8% by mass. The mixed powder of the composite oxide is heat-treated at a low temperature of 150 to 280°C under vacuum. A heat treatment temperature of 150 to 210°C is more preferable. By heat-treating the mixed powder at a low temperature and under vacuum, the generation of cation mixing is suppressed.

[0059] The elemental content of lithium-containing composite oxides can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), electron probe microanalyzer (EPMA), or energy dispersive X-ray spectroscopy (EDX).

[0060] The lithium-containing composite oxide described above can extract more lithium ions during charging as its Ni ratio increases (i.e., as the value of x in the above compositional formula increases), thereby increasing its capacity. However, the Ni in lithium-containing composite oxides with increased capacity tends to have a higher valence state. Therefore, the crystal structure tends to become unstable, especially in a fully charged state, and repeated charging and discharging can easily cause the surface of the active material particles to change (become inactive) into a crystal structure that makes reversible intercalation and release of lithium ions difficult. As a result, the cycle characteristics tend to deteriorate. In particular, when the amount of positive electrode active material per unit area is increased, the flow of lithium ions and / or electrons is easily hindered during the charge-discharge reaction, and unevenness in the charge-discharge reaction is likely to occur. When unevenness occurs in the charge-discharge reaction, deactivation of the crystal structure progresses in some regions where the amount of lithium ion extraction is large due to excessive charging, which can lead to a deterioration in cycle characteristics.

[0061] However, in the positive electrode for secondary batteries of this embodiment, the resistance of the positive electrode mixture layer can be reduced. Therefore, even when a lithium-containing composite oxide with a large Ni ratio x is used as the active material, high cycle characteristics can be maintained. Thus, a secondary battery with excellent cycle characteristics and high energy density can be realized.

[0062] 1 m of positive electrode mixture layer (1 layer) 2The mass per sheet may be 200g or more, and preferably 250g or more. By setting the mass to 250g or more, it is possible to increase the capacity of the lithium-ion battery. As described above, the positive electrode plate according to this disclosure can suppress the disadvantages caused by increasing the mass. The mass can be increased by making the positive electrode mixture layer thicker or increasing the density of the positive electrode mixture layer.

[0063] There are no particular limitations on the thickness of the positive electrode mixture layer, but it may be in the range of 50 μm to 250 μm. According to this embodiment, even if the positive electrode mixture layer is thickened, the increase in internal resistance can be suppressed.

[0064] (Positive electrode current collector) The shape and thickness of the positive electrode current collector can be selected according to the application and can be selected to correspond to the shape and thickness of the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0065] An example of the configuration of a non-aqueous electrolyte secondary battery relating to this disclosure is described below. The secondary battery includes a positive electrode relating to this disclosure. The secondary battery may include, for example, a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator as follows.

[0066] (positive electrode) As described above, the positive electrode used is the positive electrode according to this disclosure. The positive electrode mixture layer may be formed, for example, by the following method. First, a positive electrode slurry is prepared by dispersing the materials for the positive electrode mixture layer (positive electrode active material, conductive material, binder, and other optional components as needed) in a dispersion medium. Next, the positive electrode slurry can be applied to the surface of the positive electrode current collector to form a coating film, and then the coating film can be dried to form the positive electrode mixture layer. The dried coating film may be rolled as needed. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces. The positive electrode mixture layer may contain thickeners and the like as optional components. Known materials may be used for these optional components. The ratio of components contained in the positive electrode slurry is reflected in the ratio of components in the positive electrode mixture layer. Therefore, the ratio of components in the positive electrode mixture layer can be changed by changing the ratio of components contained in the positive electrode slurry.

[0067] (Negative electrode) The negative electrode contains a negative electrode active material. The negative electrode typically includes a negative electrode current collector and a layered negative electrode mixture (hereinafter referred to as the negative electrode mixture layer) disposed on the negative electrode current collector. The negative electrode mixture layer can be formed by dispersing the components of the negative electrode mixture in a dispersion medium to create a negative electrode slurry, which is then applied to the surface of the negative electrode current collector and dried. The dried coating may be rolled if necessary.

[0068] The negative electrode mixture layer may contain a negative electrode active material as an essential component and optionally contain binders, thickeners, conductive materials, etc. The negative electrode mixture layer may use a known negative electrode used in the negative electrode of a non-aqueous electrolyte secondary battery.

[0069] (Negative electrode active material) As the negative electrode active material, metallic lithium, lithium alloys, etc., may be used, but a material capable of electrochemically intercalating and releasing lithium ions is preferably used. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode active material may contain Si-containing materials or may be made of Si-containing materials alone. The negative electrode may contain one type of negative electrode active material or a combination of two or more types.

[0070] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). A single carbonaceous material may be used, or two or more may be used in combination. Among these, graphite is preferred as the carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0071] Si-containing materials include elemental Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which the silicon phase is dispersed within the lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO2. x Particles are an example. x may be, for example, 0.5 ≤ x < 2, and also 0.8 ≤ x ≤ 1.6. As the lithium ion conducting phase, at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase may be used.

[0072] For example, the materials exemplified for the positive electrode can be used as binders, thickeners, conductive materials, and dispersion media for the negative electrode slurry.

[0073] For example, a metal foil may be used as the negative electrode current collector. The negative electrode current collector may be porous. Examples of materials 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 for example, it may be 1 to 50 μm, or 5 to 30 μm.

[0074] (Non-aqueous electrolytes) Non-aqueous electrolytes (non-aqueous electrolyte solutions) contain a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that undergoes ion dissociation in the electrolyte solution. The solute may include, for example, a lithium salt. Components of the electrolyte solution other than the solvent and solute are additives. Various additives may be included in the electrolyte solution.

[0075] Any known material can be used as a solvent. Examples of solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of linear carbonate esters 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 linear carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). Non-aqueous solvents may be used individually or in combination of two or more.

[0076] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0077] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0078] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0079] (Separator) A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and possesses appropriate mechanical strength and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. Polyolefins such as polypropylene and polyethylene are preferred as the material of the separator.

[0080] One example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, is housed together with the non-aqueous electrolyte in an outer casing. However, it is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0081] Figure 1 is a schematic perspective view showing a portion of a rectangular non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. The battery comprises a bottomed rectangular battery case 4, an electrode group 1 and a non-aqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a seal 8 after injection. [Examples]

[0082] The present disclosure will be described below in detail based on examples, but the present disclosure is not limited to the following examples.

[0083] In this example, multiple non-aqueous electrolyte secondary batteries were fabricated by changing the type and amount of binder, and their characteristics were evaluated. <Batteries A1-A6 and C1-C2> [Fabrication of the negative electrode] A silicon composite material and graphite were mixed in a mass ratio of silicon composite material:graphite = 5:95 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (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 a copper foil, which was to be used as the negative electrode current collector. After the coating was dried, it was rolled to form negative electrode mixture layers on both sides of the copper foil.

[0084] [Fabrication of the positive electrode] First, positive electrode slurries SA1-SA6 and SC1-SC2 were prepared by mixing the positive electrode active material, carbon nanotubes (conductive material), binder, and N-methyl-2-pyrrolidone (NMP) in a predetermined mass ratio. The binder used was the one shown in Table 1. The positive electrode active material had the composition formula LiNi 0.90 Co 0.05 Al 0.05 A lithium-containing composite oxide represented by O2 was used. The average fiber length and average fiber diameter of the carbon nanotubes were set to 1 μm and 10 nm, respectively. The amount of carbon nanotubes added was 0.5 parts by mass per 100 parts by mass of positive electrode active material. The total amount of binder was 1 part by mass per 100 parts by mass of positive electrode active material. Next, the positive electrode slurry was applied to the surface of the aluminum foil, which was the positive electrode current collector, to form a coating film. After drying the coating film, it was rolled to form positive electrode mixture layers on both sides of the aluminum foil. One layer of positive electrode mixture formed on one side of the aluminum foil, 1 m 2 The mass per unit was assumed to be 250g. That is, 1m² of two positive electrode mixture layers formed on both sides of the aluminum foil. 2The mass per unit was set to 500g. In this way, positive electrodes PA1 to PA6 and PC1 to PC2 were fabricated.

[0085] [Preparation of electrolyte solution] An electrolyte 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 was 1.0 mol / L.

[0086] [Manufacturing of secondary batteries] An electrode group was fabricated by attaching lead tabs to each electrode and spirally winding the positive and negative electrodes via a separator so that the leads were located on the outermost periphery. The positive electrodes PA1-PA6 and PC1-PC2 described above were used. Next, the electrode group was inserted into an outer casing made of laminate film with aluminum foil as a barrier layer, vacuum-dried at 105°C for 2 hours, then a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed. A secondary battery was fabricated in this manner.

[0087] Positive electrodes PA1-PA6 and PC1-PC2 were fabricated using the method described above, and the adhesion of the positive electrode mixture layer was evaluated. Adhesion was assessed by evaluating the bonding strength between the aluminum foil and the positive electrode mixture using the method described in JIS 6854-1. To allow for comparison between samples, electrodes of the same size were prepared and measured.

[0088] The settling properties of the cathode slurry were evaluated using centrifugal sedimentation. The cathode slurry prepared as described above was subjected to centrifugal separation at a rotation speed of 1000 rpm for 1 hour. Using the sample after centrifugation, the supernatant and the settled portion were dried separately, and the solid content was evaluated. The difference between these two was used as the settling property value.

[0089] Furthermore, the discharge capacity and DC resistance (DCR) were measured for the secondary batteries prepared using the above method. Table 1 shows the type and ratio of binders used in batteries A1-A6 and C1-C2, as well as the evaluation results for the positive electrode slurry, positive electrode, and secondary battery. Batteries C1 and C2 are comparative examples. The evaluation results in Table 1 are relative values ​​with the evaluation results for positive electrode slurry SA1, positive electrode PA1, and battery A1 set to 100%.

[0090] [Table 1]

[0091] Table 1 shows that the sedimentation rate of the slurry indicates lower sedimentation (i.e., higher dispersibility and dispersion stability) as the numerical value decreases. Adhesion indicates higher adhesion as the numerical value increases. When using conductive materials that tend to aggregate, such as carbon nanotubes, the dispersibility of the materials in the slurry tends to decrease. Low dispersibility prevents the formation of a uniform cathode composite layer, leading to reduced battery performance. Furthermore, low dispersibility can make it difficult to form the cathode composite layer. In particular, productivity decreases significantly when the sedimentation rate exceeds 120%.

[0092] As shown in Table 1, the dispersibility of slurries A1 to A6 using binders containing PVP and cellulose derivatives was significantly higher than that of slurries C1 and C2. In particular, the dispersibility of positive electrode slurries SA2 to SA6, in which the amount of cellulose derivative per 100 parts by mass of PVP was in the range of 100 to 400 parts by mass (for example, in the range of 300 to 400 parts by mass), was greatly improved. Furthermore, batteries A2 to A6 using positive electrode slurries SA2 to SA6 showed greatly improved adhesion of the positive electrode mixture layer and improved battery characteristics.

[0093] In particular, when polyvinylpyrrolidone (polyvinylpyrrolidones), cellulose derivatives, and H-NBR (nitrile rubber) were used as binders, the dispersibility of the positive electrode slurry, the adhesion of the positive electrode mixture layer, and the battery characteristics were all significantly improved.

[0094] As described above, it is clear that the properties are improved by the synergistic effect of polyvinylpyrrolidone (polyvinylpyrrolidone derivatives) and hydroxypropyl methylcellulose (cellulose derivative). This effect is thought to be particularly high in combinations of polyvinylpyrrolidone, cellulose derivatives, and H-NBR (or combinations of these with PVDF). [Industrial applicability]

[0095] This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and can be used in a non-aqueous electrolyte secondary battery. The secondary battery according to this disclosure can be used in a variety of applications, and is preferably used as a main power source for mobile communication devices, portable electronic devices, etc. Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0096] 1: Electrode group, 2: Positive lead, 3: Negative lead, 4: Battery case, 5: Sealing plate, 6: Negative terminal, 7: Gasket, 8: Sealing plug

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector, The positive electrode mixture layer comprises a positive electrode active material, a conductive material, and a binder. The binder comprises polyvinylpyrrolidones and cellulose derivatives, The positive electrode active material includes a lithium-containing composite oxide containing nickel, The cellulose derivative is at least one of alkylcellulose, hydroxyalkylcellulose, and alkali metal salts thereof, in a positive electrode for a non-aqueous electrolyte secondary battery.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of the cellulose derivative per 100 parts by mass of the polyvinylpyrrolidones is in the range of 30 to 400 parts by mass.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the amount of the cellulose derivative per 100 parts by mass of the polyvinylpyrrolidones is in the range of 100 to 400 parts by mass.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the binder further comprises a nitrile rubber.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 4, wherein the amount of nitrile rubber per 100 parts by mass of polyvinylpyrrolidone is in the range of 100 to 300 parts by mass.

6. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the binder further comprises polyvinylidene fluoride.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the conductive material includes carbon nanotubes.

8. The lithium-containing composite oxide has the compositional formula Li ｙ Ni ｘ M １－ｘ O ２ A lithium-containing composite oxide represented by the formula (wherein 0.8 ≤ x ≤ 1, 0 < y ≤ 1.2, and M comprises at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B), is a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7.

9. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 8, wherein the x in the composition formula satisfies 0.85 ≤ x ≤ 1.

10. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein at least one element selected from the group consisting of Sr and Ca is unevenly distributed near the surface of the positive electrode active material.

11. 1 m of the positive electrode mixture layer ２ A positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 10, wherein the mass per unit is 250 g or more.

12. A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 11.

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