Positive electrode for secondary battery, and secondary battery

Abstract

This positive electrode for a secondary battery includes a positive electrode mixture including a positive electrode active material, a positive electrode conductive agent, and a binding agent. The positive electrode active material includes a lithium-containing composite oxide having a layered structure. The conductive agent includes a carbon material. The binding agent includes a first binding agent and a second binding agent different from the first binding agent, the first binding agent being polyacrylonitrile, the polyacrylonitrile including modified polyacrylonitrile with at least one functional group selected from the group consisting of acid groups, hydroxy groups, and derivatives thereof.

Description

Positive electrode for secondary batteries and secondary batteries Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2024-231285, filed with the Japan Patent Office on 26 December 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to a positive electrode for a secondary battery and a secondary battery.

[0003] Secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles due to their high output and high energy density. Composite oxides of lithium and transition metals (e.g., cobalt) are used as the positive electrode active material for lithium-ion secondary batteries.

[0004] In recent years, in order to improve the energy density and durability of lithium-ion batteries, attempts have been made to increase the amount of positive electrode active material per unit area by using positive electrode active material with a large charge / discharge capacity in the positive electrode, or by compressing a large amount of positive electrode active material.

[0005] As a positive electrode active material for secondary batteries, lithium-nickel composite oxides, in which all or part of the Co in lithium-cobalt composite oxide is replaced with Ni, are considered promising because they can achieve high capacity. In lithium-transition metal composite oxides, the higher the ratio of Ni to metals other than lithium, the higher the capacity that can be achieved.

[0006] Patent Document 1 describes a lithium-ion secondary battery, Li ｙ Ni （１－ｘ） M ｘ It has been proposed that the positive electrode contains a lithium-nickel composite oxide and carbon nanotubes, and that the ratio a / b, which is the ratio of the average length a of the carbon nanotubes to the average particle size b of the primary particles of the lithium-nickel composite oxide, be 0.5 or higher.

[0007] International Publication No. 2018 / 051667

[0008] On the other hand, lithium transition metal composite oxides generally have relatively high resistance, and positive electrodes containing lithium transition metal composite oxides as the positive electrode active material tend to have high resistance.

[0009] Furthermore, the positive electrode typically contains a fluorinated polymer such as polyvinylidene fluoride (PVdF) as a binder. While PVdF is electrochemically stable and has high binding properties with the positive electrode active material, it can increase internal resistance, leading to a decrease in cycle retention and potentially degrading the performance of the secondary battery. Using a binder other than PVdF can suppress the decrease in cycle retention, but because it has lower binding properties compared to PVdF, the adhesion of the positive electrode mixture tends to decrease. This can cause the positive electrode mixture to easily detach from the positive electrode current collector, potentially increasing the defect rate in the production process and making the secondary battery more susceptible to degradation during charging and discharging.

[0010] In view of the foregoing, one aspect of the present disclosure relates to a positive electrode for a secondary battery, comprising a positive electrode mixture comprising a positive electrode active material, a conductive agent, and a binder, wherein the positive electrode active material comprises a lithium-containing composite oxide having a layered structure, the conductive agent comprises a carbon material, and the binder comprises a first binder and a second binder different from the first binder, the first binder is polyacrylonitrile, and the polyacrylonitrile comprises a modified polyacrylonitrile having at least one functional group selected from the group consisting of acid groups, hydroxyl groups, and derivatives thereof.

[0011] Another aspect of this disclosure relates to a secondary battery comprising the above-mentioned positive electrode for a secondary battery, a separator, a negative electrode facing the positive electrode for a secondary battery via the separator, and an electrolyte.

[0012] According to this disclosure, the adhesion of the positive electrode mixture to the positive electrode can be improved, thereby improving the cycle life of the secondary battery. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0013] This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present disclosure.

[0014] The embodiments of 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". In the following description, when lower and upper limits are given as examples for numerical values ​​of specific physical properties or conditions, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0015] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0016] A positive electrode for a secondary battery according to one embodiment of the present disclosure (hereinafter sometimes simply referred to as "the positive electrode for a secondary battery according to this embodiment" or "positive electrode (PE)") comprises a positive electrode mixture comprising a positive electrode active material, a conductive agent, and a binder. The positive electrode active material comprises a lithium-containing composite oxide having a layered structure. The conductive agent comprises a carbon material. The binder comprises a first binder and a second binder different from the first binder. The first binder is polyacrylonitrile (PAN). The polyacrylonitrile comprises modified polyacrylonitrile having at least one functional group selected from the group consisting of acid groups, hydroxyl groups, and derivatives thereof.

[0017] The first binder, containing modified polyacrylonitrile, effectively enhances the adhesion between particles of the positive electrode active material and the positive electrode current collector by allowing the functional groups to bind or adsorb to the lithium-containing composite oxide, which is the positive electrode active material, and by improving the adhesion between the positive electrode active material and the positive electrode current collector. As a result, the particles of the positive electrode active material adhere firmly to each other via the first binder, and the peeling of the composite material from the positive electrode current collector is suppressed.

[0018] Polyacrylonitrile is a polymer containing a monomer unit U represented by [—CH ２ —CH(CN)—]. The modified polyacrylonitrile can be a polyacrylonitrile in which one or more functional groups other than the CN group are added or substituted in a part of the above monomer unit. The functional group may be a hydrophilic group such as an acid group or a hydroxy group. The acid group may be a carboxylic acid group, a sulfonic acid group, or a phosphoric acid group. The functional group may be at least one selected from the group consisting of a carboxylic acid group, a hydroxy group, a sulfonic acid group, a phosphoric acid group, and derivatives thereof. Among these, a hydroxy group, a sulfonic acid group, a phosphoric acid group, and derivatives thereof are preferable, and a hydroxy group, a sulfonic acid group, and derivatives thereof are more preferable. The functional group may be a derivative of these hydrophilic groups. The derivatives include salts of anions (anions of acid groups or alkoxides) from which hydrogen ions have been eliminated from these hydrophilic groups and cations (such as metal ions), and esters with these hydrophilic groups.

[0019] Polyacrylonitrile or modified polyacrylonitrile may be a copolymer with the above monomer unit U ＡＮ (and the monomer unit U after modification ＡＮ２ ), and another monomer unit different from the monomer units U ＡＮ and U ＡＮ２ . In that case, it is sufficient that 50% or more of the number of monomer units constituting the polymer is the monomer unit U ＡＮ or U ＡＮ２ , and it may be 80% or more or 90% or more of the above monomer unit U ＡＮ or U ＡＮ２ . 100% of the monomer units constituting the polymer may be the monomer units U ＡＮ and U ＡＮ２ .

[0020] ​​In order to strengthen the adhesion between the particles of the positive electrode active material and to suppress the peeling of the positive electrode mixture from the positive electrode current collector, the modification rate of the modified polyacrylonitrile may be, for example, 5% or more, and preferably 10% or more or 11% or more. Here, the modification rate of the modified polyacrylonitrile is the monomer unit U ＡＮ and U ＡＮ２ This refers to the ratio of the number of functional groups to the total number of other elements. The denaturation rate can be evaluated using the XPS method with chemical modification.

[0021] In the XPS method using chemical modification, the functional groups in modified polyacrylonitrile are chemically modified with a specific compound, and the proportion of functional groups present in the modified polyacrylonitrile is calculated by measuring the peaks originating from the chemically modified compound using XPS. Examples of compounds used for chemical modification include F compounds such as trifluoroacetic anhydride (TFAA) or trifluoroethanol (TFE). If the functional group is a hydroxyl group (OH group), for example, the functional group is reacted with trifluoroacetic anhydride, and the OH group is converted to O(CO)CF through an esterification reaction. ３ Substitution is performed on the group. If the functional group is a carboxylic acid group ((CO)OH group), for example, the carboxylic acid group is reacted with trifluoroethanol and esterified to form (CO)OCH. ２ CF ３ Substitute into the base. XPS method, CF ３ The degradation rate can be calculated by quantifying the peak intensity originating from the bonding of fluorine atoms.

[0022] The binder adheres to the surface of the positive electrode active material particles, as well as to the surface of the conductive carbon material. However, when the binder adheres to the surface of the conductive material, the electrical connectivity between the conductive material and the active material particles decreases, which can contribute to an increase in the resistance of the positive electrode.

[0023] Among binders, fluorine-containing polymers, such as PVdF, are electrochemically stable and have high bonding properties with the positive electrode active material. However, they also tend to adhere to the surface of carbon materials, which increases the resistance of the conductive network mediated by the carbon material, thus increasing the resistance of the positive electrode. In particular, when carbon nanotubes are used as the carbon material, the adhesion of the binder to the surface of the carbon nanotubes can negate the positive electrode resistance reduction effect based on the high conductivity and excellent conductive network formation ability of carbon nanotubes, making it difficult to achieve low internal resistance.

[0024] In contrast, polyacrylonitrile or modified polyacrylonitrile, used as the first binder, has many cyano groups (CN groups), resulting in low affinity with carbon materials (especially carbon nanotubes) and difficulty in adhering to the surface of carbon materials. Therefore, by using polyacrylonitrile, the conductivity and conductive network formation ability of the carbon material are effectively utilized, reducing positive electrode resistance. In particular, by using polyacrylonitrile as a binder in combination with carbon nanotubes as a conductive carbon material, the high conductivity and excellent conductive network formation ability of the carbon material carbon nanotubes are effectively utilized, significantly reducing positive electrode resistance. As a result, this contributes to improving the cycle maintenance rate of secondary batteries.

[0025] On the other hand, the higher the degree of modification of the modified polyacrylonitrile, the fewer cyano groups there are, and the easier it becomes for the modified polyacrylonitrile to adhere to the surface of the carbon material. In order to suppress the adhesion of polyacrylonitrile to the surface of the carbon material and to effectively exert low resistance through a conductive network mediated by the carbon material, the modification rate of the modified polyacrylonitrile may be 50% or less, and is preferably 20% or less or 17% or less.

[0026] The denaturation rate of the modified polyacrylonitrile may be 5% to 50%, 10% to 50%, or 10% to 20%. Preferably, the denaturation rate may be 11% to 20%, and more preferably 11% to 17%.

[0027] The binder includes a first binder and a second binder different from the first binder. The second binder has high affinity for carbon materials (especially carbon nanotubes). The first binder mainly adheres to the positive electrode active material, improving the adhesion between particles of the positive electrode active material. The second binder adheres not only to the positive electrode active material but also to conductive carbon materials, improving the adhesion between the carbon material and the particles of the positive electrode active material, and contributing to the formation of a conductive network. By combining the first and second binders, it is possible to suppress peeling of the positive electrode mixture and achieve both a high cycle retention rate.

[0028] In terms of electrochemical stability, the second binder may be a fluorine-containing polymer. The fluorine-containing polymer may contain at least one selected from the group consisting of monomer units derived from vinylidene fluoride (VdF), monomer units derived from hexafluoropropylene (HFP), and monomer units derived from tetrafluoroethylene (TFE). In particular, from the viewpoint of electrochemical stability and the stability of the cathode slurry, it is preferable that the fluorine-containing polymer contains at least monomer units derived from VdF. The fluorine-containing polymer may contain at least one selected from the group consisting of polyvinylidene fluoride (PVdF) and copolymers containing monomer units derived from vinylidene fluoride (VdF). The copolymer may be a block copolymer or a random copolymer.

[0029] A copolymer containing monomer units derived from VdF may also include a copolymer of VdF and a fluorine-containing monomer other than VdF. That is, the copolymer contains monomer units derived from VdF U １ And, fluorine-containing monomer unit U that does not originate from VdF ２ It may also include the following. The fluorine-containing monomer other than VdF may include at least one selected from the group consisting of HFP, TFE, trifluoroethylene, and trifluoroethylene chloride. In particular, from the viewpoint of ensuring the flexibility of the positive electrode plate, the fluorine-containing monomer other than VdF may be HFP. In the copolymer, the monomer unit U derived from VdF １ Fluorine-containing monomer unit U not derived from VdF２ Molar ratio (U ２ / U １ For example, the value may be 0.01 or more and 0.5 or less, or 0.05 or more and 0.3 or less.

[0030] The mass ratio C1 / C2 of the content of the second binder (polyacrylonitrile) to the content C1 of the first binder (polyacrylonitrile) in the positive electrode mixture may be in the range of 3 / 7 to 7 / 3. When the mass ratio C1 / C2 is within this range, the effect of improving the cycle maintenance rate of the secondary battery is enhanced.

[0031] The weight-average molecular weight of polyacrylonitrile may be 100,000 or more, 150,000 or more, or 300,000 or more. The weight-average molecular weight of polyacrylonitrile may be 600,000 or less, or 450,000 or less.

[0032] The carbon material used as a conductive agent may include carbon nanotubes. Carbon nanotubes have a very small fiber diameter (nano-sized) and an extremely large aspect ratio (ratio of fiber length to outer diameter). Therefore, the contact between carbon nanotubes interposed between active materials, and between active materials and current collectors, is linear rather than point contact. As a result, the highly conductive carbon nanotubes form linear conductive paths between active materials and between active materials and current collectors, and also form linear contact points with the current collector, thereby improving current collection performance. Furthermore, even when the positive electrode mixture layer is rolled after coating, gaps that can hold the electrolyte are secured between the fibers, thus maintaining high fluidity.

[0033] Multiple types of carbon nanotubes with different fiber lengths may be used in combination. Including carbon nanotubes with different fiber lengths in the cathode mixture allows for the formation of a complex three-dimensional conductive network with the cathode active material, further reducing cathode resistance. The presence of multiple types of carbon nanotubes with different fiber lengths in the cathode mixture can be confirmed by determining the distribution of carbon nanotube fiber lengths.

[0034] When the cathode mixture contains multiple types of carbon nanotubes with different fiber lengths, the fiber length distribution may have multiple peaks. In the fiber length distribution, there may be two peaks: one for a first fiber length and another for a second fiber length different from the first. In the fiber length distribution, for example, there may be a first peak in the range of 0.1 to 10 μm and a second peak in the range of 50 to 1000 μm.

[0035] The fiber length and distribution of carbon nanotubes can be determined from a sample obtained by extracting only the positive electrode mixture (positive electrode active material layer) from a discharged secondary battery. Specifically, first, the discharged secondary battery is disassembled and the positive electrode is extracted. Next, the positive electrode is washed with an organic solvent, then vacuum-dried, and finally, the mixture layer is peeled off to obtain the sample.

[0036] The average fiber length of carbon nanotubes is determined by image analysis using a scanning electron microscope (SEM). This is calculated by randomly selecting 100 carbon nanotubes, measuring their lengths, and then taking the arithmetic mean. The length refers to the length of the carbon nanotube when it is stretched in a straight line.

[0037] The diameter of carbon nanotubes can be determined by image analysis using a transmission electron microscope (TEM). The average diameter of carbon nanotubes can be measured by the following method: First, 100 carbon nanotubes are randomly selected, and the diameter (outer diameter) at any one point on each is measured. Then, the diameter is obtained by taking the arithmetic mean of the measured diameters.

[0038] Furthermore, carbon nanotubes can be separated by grinding the sample after exfoliation, dispersing the ground sample in a dispersion medium such as water or alcohol, and centrifuging it. In addition, the ratio of binder components other than the positive electrode active material and positive electrode conductive agent components can be calculated by performing thermal analysis such as TG-DTA on the sample. By performing micro-Raman spectroscopy on the cross-section of the positive electrode mixture layer, carbon species such as carbon nanotubes and carbon black can be identified, and their proportions can be calculated from thermal analysis such as TG-DTA on the exfoliated sample.

[0039] Carbon nanotubes are generally classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). SWCNTs, DWCNTs, and carbon nanotubes with three layers may be collectively referred to as 1-3 layered CNTs. Carbon nanotubes with three or fewer layers (1-3 layered CNTs) are preferred because a large effect can be obtained with a small amount, single-walled or double-walled carbon nanotubes are more preferred, and single-walled carbon nanotubes are the most preferred. The proportion of 1-3 layered CNTs to the total number of carbon nanotubes may be 20% or more, 30% or more, 50% or more, 70% or more, 80% or more, 90% or more, or 95% or more.

[0040] 1-3 layer CNTs can achieve the linear contact described above by simply adding a small amount to the positive electrode mixture layer, reducing resistance at the positive electrode while ensuring a gap that can hold the electrolyte within the positive electrode mixture layer. Furthermore, because 1-3 layer CNTs occupy a small volume, they do not easily hinder the flow of the electrolyte at the active material secondary particle interface. In addition, since a sufficient reduction in positive electrode resistance can be obtained with only a small amount of addition, it is easy to increase the content ratio of positive electrode active material in the positive electrode mixture layer, thereby realizing high-capacity secondary batteries.

[0041] The carbon material used as the positive electrode conductive agent may include particulate conductive carbon materials such as carbon black. Examples of carbon black include furnace black and acetylene black. The carbon material used as the positive electrode conductive agent may further include sheet-like carbon materials such as graphene and graphite, rod-like carbon materials such as VGCF (vapor-grown carbon fiber), and carbon fibers other than carbon nanotubes.

[0042] It is preferable to mix particulate conductive carbon material with carbon nanotubes in the positive electrode mixture. By mixing carbon nanotubes, which are fibrous conductive material, and carbon black, which is particulate conductive material, within the positive electrode mixture, numerous conductive paths are formed between adjacent positive electrode active materials and between the positive electrode active materials and the positive electrode current collector, thereby reducing resistance at the positive electrode.

[0043] The content ratio of the carbon material as a positive electrode conductive agent in the total positive electrode mixture is, for example, 0.0001% by mass or more and 2% by mass or less, and may be 0.0005% by mass or more and 2% by mass or less, or 0.0001% by mass or more and 1% by mass or less, or 0.0005% by mass or more and 1% by mass or less.

[0044] The carbon nanotube content in the total carbon material may be, for example, 0.01% by mass or more, and may be 0.05% by mass or more. The carbon nanotube content in the total carbon material may be 0.01% by mass or more and 100% by mass or less, 0.01% by mass or more and 50% by mass or less, or 0.01% by mass or more and 30% by mass or less.

[0045] The carbon nanotube content in the total cathode mixture is, for example, 0.0001% by mass or more, and may be 0.0005% by mass or more. The carbon nanotube content in the total cathode mixture layer may be 2.0% by mass or less, or 1.0% by mass or less.

[0046] The average fiber diameter of the carbon nanotubes may be 3 nm or less, preferably 1.7 nm or less. When the average fiber diameter is within the above range, a dense conductive network can be formed in the positive electrode mixture layer by the carbon nanotubes, thereby reducing the positive electrode resistance. The average fiber diameter of the carbon nanotubes may be 0.1 nm to 3 nm, 0.3 nm to 3 nm, 0.1 nm to 1.7 nm, or 0.3 nm to 1.7 nm.

[0047] As a binder other than polyacrylonitrile (second binder), a fluorine-containing polymer is preferred, but nitrile rubber, polyvinylidones (PVP), etc. may also be used because they provide good dispersibility of carbon nanotubes when preparing a cathode slurry containing the cathode mixture. Examples of nitrile rubber include copolymers of monomers containing acrylonitrile and diene (e.g., butadiene). Examples of nitrile rubber include acrylonitrile rubber such as nitrile-butadiene rubber (NBR) and hydrogenated nitrile-butadiene rubber (HNBR). Among these, hydrogenated nitrile-butadiene rubber (HNBR) is the most preferred.

[0048] Polyvinylpyrrolidones are at least one selected from the group consisting of polyvinylpyrrolidone and polyvinylpyrrolidone derivatives. Examples of polyvinylpyrrolidone derivatives include polymers in which the hydrogen atoms of polyvinylpyrrolidone are substituted with other substituents, such as alkylated polyvinylpyrrolidone. Polyvinylpyrrolidones may be used alone, or copolymers of vinylpyrrolidone and other monomolecules may be used. Examples of other monomolecules include styrene-based and vinyl acetate-based monomolecules. Polyvinylpyrrolidones may also be fluorine-containing polymers in which the hydrogen atoms of polyvinylpyrrolidone are substituted with fluorine atoms.

[0049] A secondary battery according to one embodiment of the present disclosure (hereinafter sometimes simply referred to as "the secondary battery according to this embodiment" or "secondary battery (S)") comprises the above-described positive electrode (PE), a separator, a negative electrode facing the positive electrode (PE) via the separator, and an electrolyte. The positive electrode (PE) has, for example, a positive electrode current collector and a positive electrode mixture layer containing the above-described positive electrode mixture, the positive electrode mixture layer being disposed on at least the surface of the positive electrode current collector. The secondary battery (S) comprises, for example, an outer casing (battery case), and the positive electrode, negative electrode, separator, and electrolyte are disposed inside the outer casing.

[0050] The shape of the secondary battery (S) is not limited and may be cylindrical, rectangular, coin-shaped, button-shaped, pouch-shaped, etc. The battery case is selected according to the shape of the secondary battery.

[0051] The secondary battery (S) may be a non-aqueous electrolyte secondary battery.

[0052] Below, an example of a secondary battery according to this embodiment and examples of its components are described. Note that for components not characteristic of this disclosure, known components may be used.

[0053] (Positive Electrode) The positive electrode (PE) typically comprises a positive electrode current collector and a positive electrode active material layer (positive electrode mixture layer) formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by coating a positive electrode slurry, obtained by dispersing the positive electrode mixture in a dispersion medium, onto the surface of the positive electrode current collector and drying it. The dried coating may be rolled if necessary. The positive electrode mixture contains a positive electrode active material as an essential component and a carbon material as a positive electrode conductive agent. The positive electrode mixture contains modified polyacrylonitrile as a binder (first binder) and a binder other than polyacrylonitrile (second binder).

[0054] (Positive Electrode Active Material) A lithium metal composite oxide can be used as the positive electrode active material. The lithium metal composite oxide may be a composite oxide having a layered structure (for example, a rock salt crystal structure) containing lithium and a transition metal. From the viewpoint of obtaining high capacity, high capacity can be achieved by making the proportion of Ni among the metal elements other than Li in the lithium metal composite oxide 50 atomic percent or more. 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 metal 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 (x≧0.85) or 90 atomic percent or more (x≧0.9). For example, it is desirable that the proportion of Ni among the metal elements other than Li be 95 atomic percent or less (x≦0.95). When limiting the range, these upper and lower limits can be combined arbitrarily.

[0055] More specifically, lithium metal composite oxides are, for example, Li ａ Ni ｘ M １－ｘ O ２(However, 0 < a ≤ 1.2, 0.5 ≤ x < 1, and M includes at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B.) This may be a lithium-nickel composite oxide represented as such. In particular, M is at least one element selected from the group consisting of Co, Al, and Mn. １ It is preferable that it contains M, which is at least one element selected from the group consisting of Ca and Sr. ２ It may include.

[0056] In lithium-nickel composite oxides, element M １ This contributes to stabilizing the crystal structure of composite oxides with a high Ni content. However, from the viewpoint of reducing manufacturing costs, a lower Co content is preferable. Lithium-nickel composite oxides with a low Co content (the proportion of Co to metal elements other than Li is 5 atomic percent or less) or no Co at all contain element M １ It may also contain Mn and Al. From the viewpoint of crystal structure stability, it is preferable to include Al as M.

[0057] In lithium-nickel composite oxides, element M ２ This stabilizes the surface energy of the particle surface of the composite oxide. Element M ２ This stabilizes the crystal structure on the surface of the composite oxide even when a large amount of Li is extracted during charging, and suppresses the change in the crystal structure to one that makes reversible intercalation and release of lithium difficult.

[0058] The lithium-nickel composite oxide described above can have its capacity increased as the Ni ratio x increases, allowing more lithium ions to be extracted from the lithium-nickel composite oxide during charging. From the viewpoint of obtaining high capacity, the Ni ratio x in the lithium-nickel composite oxide is 0.5 or higher, and may also be 0.85 or higher (x ≥ 0.85), or 0.9 or higher (x ≥ 0.9).

[0059] In terms of reducing manufacturing costs, a lower cobalt ratio in lithium metal composite oxides is preferable. It is desirable that lithium metal composite oxides either do not contain Co, or that the proportion of Co among the metal elements other than Li in the lithium metal composite oxide is 5 atomic percent or less. More specifically, the lithium-containing composite oxide is the above-mentioned lithium-nickel composite oxide, where Li ａ Ni ｘ Co ｙ M ３ １－ｘ―ｙ O ２ (However, M ３ (where is element M, excluding Co, and satisfies 0.5 ≤ x < 1 and 0 ≤ y ≤ 0.05.)

[0060] The lithium metal composite oxide described above tends to have higher resistance as the Ni ratio increases and the Co ratio decreases, which tends to reduce the adhesion between the positive electrode composite layer containing the lithium metal composite oxide and the positive electrode current collector. However, by including modified polyacrylonitrile as a binder, the adhesion between the positive electrode composite layer and the positive electrode current collector is improved, enabling a high cycle retention rate.

[0061] The elemental content of lithium metal composite oxides can be measured using inductively coupled plasma atomic emission spectrometers (ICP-AES), electron probe microanalyzers (EPMA), or energy dispersive X-ray spectrometers (EDX).

[0062] As the positive electrode active material, the above lithium-nickel composite oxide may be used in combination with other lithium metal composite oxides. Other lithium metal composite oxides include, for example, Li ａ CoO ２ Li ａ NiO ２ Li ａ MnO ２ Li ａ Co ｂ１ Ni１－ｂ１ O ２ Li ａ Co ｂ１ Ni １－ｂ１ O ｃ Li ａ Co ｂ２ M １－ｂ２ O ｃ Li ａ Ni １－ｂ１ M ｂ１ O ｃ Li ａ Mn ２ O ４ Li ａ Mn ２－ｂ２ Ni ｂ２ O ４、 Li ａ Mn ２－ｂ２ M ｂ２ O ４、 LiGPO ４、 Li ２ GPO ４ F is an example. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Cu, Zn, Al, Cr, Pb, Sb, Zr, Nb, W, and B. G includes at least one transition element (for example, at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0 ≤ a ≤ 1.2, 0.5 ≤ b1 ≤ 0.9, 0 ≤ b2 ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0063] The content ratio of the above lithium-nickel composite oxide to the total of the above lithium-nickel composite oxide and other lithium metal composite oxides may be 50% by mass, or it may be 70% by mass or more, 80% by mass or more, or 90% by mass or more. The above lithium-nickel composite oxide alone may be used as the positive electrode active material and included in the positive electrode mixture.

[0064] A metal compound may be present on the surface of the lithium metal composite oxide. The metal compound contains, for example, at least one metal element selected from the group consisting of Mg, Ca, Sr, Ba, Ti, W, Zr, Al, and Nb. Examples of the metal compound include oxides, hydroxides, carbonates, sulfates, nitrates, and phosphates of these metal elements. More specifically, examples of the compound containing Mg include MgO, Mg(OH) ２ , and MgCO ３ . Examples of the compound containing Ca include CaO, Ca(OH) ２ , and CaCO ３ . Examples of the compound containing Sr include SrO, Sr(OH) ２ , and SrCO ３ . Examples of the compound containing Ba include BaO, Ba(OH) ２ , and BaCO ３ . Examples of the compound containing Ti include TiO ２ , Ti(OH) ４ , and Ti(CO ３ ) ２ . Examples of the compound containing W include WO ３ . Examples of the compound containing Zr include ZrO ２ , Zr(OH) ４ , Zr(CO ３ ) ２ , and Zr(SO ４ ) ２ ·4H ２ O. Examples of the compound containing Al include Al ２ O ３ . Examples of the compound containing Nb include Nb ２ O5. Further, the metal compound may be a composite compound containing a plurality of these metal elements, such as composite oxides such as SrAlO ４ , CaAlO ４ .

[0065] Nonmetallic compounds may be present on the surface of lithium metal composite oxides. The nonmetallic compound may include, for example, at least one nonmetallic element selected from the group consisting of B, N, F, P, and S. Examples of compounds containing B include H. ３ BO ３ LiBO ２ Li ３ BO ３ Li ２ B ４ O ７ Examples include: A compound containing N is LiNO3. ３、 LiNO ２ Examples include LiF and Li ２ SiF ６ LiCF ３ SO ３ Examples include: Li ３－ｘ H ｘ PO ４ Examples include (0 ≤ x ≤ 3). Examples of compounds containing S include Li ２ SO ４ Li ２ S, Li ３ PS ４ Examples can be given.

[0066] The above-mentioned metal and non-metal compounds stabilize the crystal structure of the lithium metal composite oxide even when a large amount of lithium is extracted during charging, by covering at least a portion of the surface of the lithium metal composite oxide, thereby suppressing a change in the crystal structure to one that makes reversible intercalation and release of lithium difficult. This improves the initial efficiency of the secondary battery and suppresses a decrease in the cycle maintenance rate. The metal and / or non-metal compounds can be attached to the surface of the lithium metal composite oxide, for example, by mixing the raw material powder of the metal and / or non-metal compounds with the lithium metal composite oxide at any point during or after the manufacturing of the lithium metal composite oxide. Examples of raw materials for the metal compounds include oxides, hydroxides, and carbonates of metal elements.

[0067] More specifically, as a Mg raw material, Mg(OH)２ Examples include MgCO2. Ca raw materials include Ca(OH) ２ CaO, CaCO ３ CaSO ４ Ca(NO ３ ) ２ CaCl ２ CaAlO ４ Examples include Sr(OH) ２ , Sr(OH) ２ 8H ２ O, SrO, SrCO ３ , SrSO ４ , Sr(NO ３ ) ２ , SrCl ２ , SrAlO ４ Examples include Ba(OH) ２ Examples include BaCO2. As for Ti raw materials, Ti(OH) ４ , and Ti(CO ３ ) ２ Examples include tungsten oxide (WO) as a raw material for W. ３ ), lithium tungstate (Li ２ WO ４ Li ４ WO ５ Li ６ W ２ O ９ Examples include: ) etc. As the W raw material, a solution containing W may be used. As the Zr raw material, Zr(OH) ４ , ZrO ２ , Zr(CO ３ ) ２ , Zr(SO ４ ) ２ 4H ２ Examples include O. Also, as an Al raw material, Al ２ O ３ Al(OH) ３ Al ２ (SO ４ ) ３ Other materials may be used, but Al derived from lithium transition metal composite oxides may also be used. As for the Nb raw material, Nb ２ O ５ , Nb ２ O5・nH２ Examples include O. B raw materials include H. ３ BO ３ Li ３ BO ３ Li ２ B ４ O ７ Examples include LiNOx. ３ Examples include Li ３－ｘ H ｘ PO ４ Examples include (0 ≤ x ≤ 3). Examples of sulfonic acid compounds such as lithium methanesulfonate, lithium ethanesulfonate, lithium propanesulfonate, sodium methanesulfonate, magnesium methanesulfonate, and sodium fluoromethanesulfonate can be used. These compounds may be used after being crushed to change the particle size as appropriate or by adjusting the moisture content, including the hydrate.

[0068] These metallic and nonmetallic compounds attached to the lithium metal composite oxide may, in the rechargeable battery after manufacture, diffuse into the lithium metal composite oxide as the metallic and / or nonmetallic elements contained in these compounds diffuse during charging and discharging. As a result, the metallic and / or nonmetallic elements may be included in element M in the compositional formula of the lithium-nickel composite oxide described above.

[0069] (Positive Electrode Conductive) The positive electrode (PE) contains a carbon material as a positive electrode conductive agent. The carbon material includes carbon nanotubes. Carbon nanotubes are carbon fibers with a small fiber diameter in the nanoscale and an extremely large aspect ratio (ratio of fiber length to outer diameter). Carbon fibers with a large aspect ratio result in linear contact between active materials and between active materials and current collectors, rather than point contact. Highly conductive carbon fibers are interposed between active material particles, forming linear contact areas with the particles. As a result, the highly conductive carbon nanotubes form linear conductive paths between active materials and between active materials and current collectors, and also form linear contact areas with the current collectors, thereby improving current collection performance. In particular, 1-3 layer CNTs with three or fewer layers can reduce the volume they occupy within the positive electrode mixture layer, making it possible to increase the proportion of positive electrode active material in the mixture layer while maintaining high conductivity in the positive electrode mixture layer, thus facilitating high capacity. 1-3 layer CNTs are preferably 2-layer or single-layer CNTs, with single-layer CNTs being the most preferred.

[0070] The average fiber length of the carbon nanotubes is, for example, 0.4 μm or more. In this case, even when the volume of the negative electrode active material changes significantly due to charging and discharging, linear contact with the carbon nanotube fibers is maintained in accordance with the volume change, and the electrical connection with the positive electrode active material can be maintained. When the average fiber length of the carbon nanotubes is 0.4 μm or more, current collection failure is significantly suppressed. The average fiber length may be 0.8 μm or more, and 1.0 μm or more is preferred.

[0071] On the other hand, as the average fiber length of carbon nanotubes increases, they tend to aggregate more easily, leading to poor dispersion and increased viscosity of the slurry during slurry preparation. Furthermore, the longer the average fiber length, the higher the carbon nanotube content in the mixture layer becomes when securing the required number of carbon nanotubes, making it difficult to achieve high volume. To achieve high volume, and to facilitate the preparation of a slurry in which carbon nanotubes are dispersed together with the active material, as well as to suppress the increase in slurry viscosity, the average fiber length of the carbon nanotubes may be 8 μm or less. An average fiber length of 4 μm or less is preferable.

[0072] The average fiber length of the carbon nanotubes may be 0.4 μm to 40 μm, or 0.6 μm to 35 μm or 1.0 μm to 30 μm.

[0073] The average fiber diameter of the carbon nanotubes may be 0.5 nm or larger, 0.7 nm or larger, or 1 nm or larger. On the other hand, the larger the average fiber diameter of the carbon nanotubes, the fewer carbon nanotubes are contained in the composite layer when the carbon nanotube content is the same, making it difficult to suppress current collection failure. To suppress current collection failure, the carbon nanotube content should be increased, but the higher the carbon nanotube content in the slurry, the more easily the carbon nanotubes aggregate in the slurry, making it difficult to disperse them uniformly, and the viscosity of the slurry also tends to increase. To facilitate the preparation of a slurry in which carbon nanotubes are dispersed together with the active material, and to suppress the increase in slurry viscosity, the average fiber diameter of the carbon nanotubes may be 20 nm or less. By setting the average fiber diameter to 20 nm or less, current collection failure is significantly suppressed, and a cathode that is easy to manufacture can be obtained. The average fiber diameter of the carbon nanotubes is preferably 4 nm or less, and more preferably 3 nm or less.

[0074] Among the carbon nanotubes included as positive electrode conductive materials, single-walled carbon nanotubes (WYNAs) have an ideal one-dimensional structure in terms of electron conductivity, allowing electrons to conduct freely in the axial direction of the WYNA. Therefore, in WYNAs, electrons conduct without crossing layers, and as a result, it is possible to improve conductivity by including WYNAs as a positive electrode conductive material. On the other hand, as the number of layers increases, interlayer interactions exist during electron conduction, which can reduce conductivity. In multilayered carbon nanotubes (WYNAs), interactions exist between layers, so electrons need to traverse layers, which can result in reduced conductivity.

[0075] As a positive electrode conductive agent, conductive carbon materials other than carbon nanotubes may be used in mixture with carbon nanotubes. Examples of conductive carbon materials other than carbon nanotubes include at least one selected from the group consisting of amorphous carbon and carbon fibers. Amorphous carbon includes hard carbon and soft carbon. Examples of soft carbon include particulate conductive carbon materials such as carbon black, and carbon black includes acetylene black, Ketjen black, and furnace black. Carbon fibers include rod-shaped carbon materials such as VGCF, sheet-shaped carbon materials such as graphene, and carbon fibers. Multiple types of these materials may be combined and used as a positive electrode conductive agent.

[0076] Examples of carbon nanotubes include carbon nanofibers. Since various types of carbon nanotubes are commercially available, commercially available ones may be used. Alternatively, carbon nanotubes may be synthesized using known synthesis methods.

[0077] As a binder other than polyacrylonitrile (second binder), in addition to the nitrile rubber and polyvinylidone mentioned above, the same as those exemplified in the negative electrode described later may be used.

[0078] (Positive electrode current collector) The shape and thickness of the positive electrode current collector can be selected from the same shapes and ranges as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0079] (Negative Electrode) The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer (negative electrode mixture layer) disposed on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by coating a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, onto the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary. The negative electrode mixture contains the negative electrode active material as an essential component and may contain other components besides the negative electrode active material as needed. Examples of other components include binders, conductive agents, and thickeners. These other components may be components that are known to be used in secondary batteries.

[0080] (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 carbon materials and alloying materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Examples of alloying materials include those containing at least one metal capable of forming an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide and tin oxide, which are formed by bonding these with oxygen, may also be used.

[0081] The negative electrode active material preferably contains a material containing the element silicon (hereinafter sometimes referred to as "Si-containing material"), and more preferably contains graphite and the Si-containing material. The content ratio of the Si-containing material to the total negative electrode active material may be 5% by mass or more.

[0082] Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Known graphite used as a negative electrode active material may also be used. Other carbon materials may be included in the negative electrode active material. The carbon material may be used alone or in combination of two or more. Among the carbon materials, graphite is preferred because it exhibits excellent charge-discharge stability and has low irreversible capacity.

[0083] Note that graphite refers to a material in which a graphite-type crystal structure is well-developed, and generally refers to the average interplanar spacing d of the (002) plane, which is measured by X-ray diffraction. ００２ This refers to carbon materials with a wavelength of 0.340 nm or less.

[0084] Si-containing materials include elemental silicon, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed within a lithium-ion conductive phase (matrix). Examples of silicon oxides include SiO ＸParticles are an example. X is, for example, 0.5 ≤ X < 2, and may also be 0.5 ≤ X < 1.6 or 0.8 ≤ X ≤ 1.6. As the lithium ion conducting phase, at least one selected from the group consisting of silicon oxide phase, silicate phase, and carbon phase can be used. The main component of the silicon oxide phase (for example, 95 to 100% by mass) may be silicon dioxide. Among these, a composite material composed of a silicate phase and a silicon phase dispersed in the silicate phase is preferred because it has high capacity and low irreversible capacity.

[0085] The silicate phase may include, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the long-period periodic table. Examples of Group 1 and Group 2 elements of the long-period periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements that may be included include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a silicate phase containing lithium (hereinafter also referred to as the lithium silicate phase) is preferred because it has a small irreversible capacity and high initial charge-discharge efficiency.

[0086] The lithium silicate phase may be any oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase is given by the formula: Li ２ｚ SiO ２＋ｚ The composition may be represented by (0 < z < 2). Preferably, z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. Examples of elements other than Li, Si, and O that may be included in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0087] The average particle size (volume-based median diameter D50) of the composite particles, which consist of a silicate phase and silicon particles dispersed in the silicate phase (silicon phase), may be in the range of 1 μm to 25 μm (for example, 4 μm to 15 μm). In this range, stress due to volume changes of the composite particles during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics. Furthermore, the surface area of ​​the composite particles becomes appropriate, and capacity reduction due to side reactions with non-aqueous electrolytes is suppressed.

[0088] The crystallite size of silicon particles dispersed within the silicate phase is, for example, 10 nm or larger. The silicon particles have a particulate phase of elemental silicon (Si). When the crystallite size of the silicon particles is 10 nm or larger, the surface area of ​​the silicon particles can be kept small, making it less likely for the silicon particles to degrade, which is accompanied by the generation of irreversible capacitance. The crystallite size of the silicon particles is calculated using Scherrer's formula from the full width at half maximum of the diffraction peak attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0089] The average particle size of the silicon particles may preferably be 500 nm or less (more preferably 200 nm or less, and even more preferably 50 nm or less) before the first charge. After the first charge, the average particle size of the silicon particles is preferably 400 nm or less (more preferably 100 nm or less). By miniaturizing the silicon particles, the volume change during charging and discharging is reduced, and the structural stability of the second particle is further improved. The average particle size of the silicon particles is determined by observing the cross-section of the composite particles using a SEM or TEM and averaging the longest diameter of 100 or more silicon particles in the cross-sectional image.

[0090] The content of silicon particles (elementary Si) in the composite particles is preferably in the range of 20% to 95% by mass (for example, 35% to 75% by mass) from the viewpoint of increasing capacity and improving cycle characteristics. Within this range, lithium ion diffusion is also good, making it easier to obtain excellent loading characteristics. Furthermore, the surface area of ​​silicon particles that is not covered by the lithium silicate phase is reduced, and side reactions between the non-aqueous electrolyte and silicon particles are suppressed.

[0091] The composite particles may include a conductive material covering at least a portion of their surface. Since the lithium silicate phase has poor electronic conductivity, the conductivity of the composite particles tends to be low. By covering the surface with a conductive material, the conductivity can be dramatically increased. Preferably, the conductive layer is thin enough not to substantially affect the average particle size of the composite particles. For example, from the viewpoint of ensuring conductivity and lithium ion diffusion, the thickness of the conductive layer may be in the range of 1 nm to 200 nm (e.g., 5 nm to 100 nm).

[0092] The carbon phase may consist, for example, of amorphous carbon with low crystallinity. Amorphous carbon may be hard carbon, soft carbon, or something else. Amorphous carbon is generally defined by the average interplanar spacing d of the (002) plane, as measured by X-ray diffraction. ００２ This refers to carbon materials with a wavelength exceeding 0.34 nm.

[0093] Examples of Si-containing materials include SiO Ｘ At least one particle selected from the group consisting of a first particle containing silicon oxide represented by the formula (0.5 ≤ X < 1.6), a second particle containing a silicate phase and a silicon phase dispersed in the lithium silicate phase, and a third particle containing a carbon phase and a silicon phase dispersed in the carbon phase may be used. The Si-containing material may contain multiple types of particles selected from the group consisting of the first particle, the second particle, and the third particle. For example, the Si-containing material may consist of two types of particles selected from these, or it may contain all three types of particles. Specifically, the Si-containing material may contain the first particle and the second particle, or the first particle and the third particle, or the second particle and the third particle. Alternatively, the Si-containing material may contain all of the first, second, and third particles. The Si-containing material is preferably used as a negative electrode active material in combination with graphite.

[0094] The second and third particles each have a so-called sea-island structure. The silicon particles (islands) in the second and third particles are dispersed in a matrix (sea) of silicate and carbon phases, respectively, and covered by an ion-conducting phase (silicate and carbon phases). In the sea-island structure, contact between the silicon particles and the electrolyte is limited, thus suppressing side reactions. In addition, the stress generated by the expansion and contraction of the silicon particles is relieved by the lithium ion-conducting phase matrix.

[0095] The composition and component content of the second and third particles can be analyzed by the method described in International Publication No. 2018 / 179969.

[0096] The content of each element in a Si-containing material may be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the Si-containing material is dissolved in a heated acid solution, the carbon in the solution residue is removed by filtration, and then the obtained filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated.

[0097] The average particle size of the graphite (graphite particles) included as the active material in the negative electrode may be 13 μm or more and 25 μm or less. Preferably, the average particle size of the graphite is larger than the average particle size of the Si-containing material. With this configuration, voids are formed between the relatively large graphite particles, and the Si-containing material particles are easily accommodated in these voids. Therefore, it is easier to increase the packing density of the active material in the negative electrode and obtain a higher capacity negative electrode. In addition, the Si-containing material particles present in the voids contribute to maintaining electronic contact between the graphite particles. On the other hand, even if the Si-containing material particles present in the voids expand and contract, expansion and contraction of the entire negative electrode are unlikely to occur, thus reducing degradation due to charge-discharge cycles.

[0098] When the negative electrode active material contains graphite and Si-containing material, the Si-containing material content relative to the total of graphite and Si-containing material may be 5 to 50% by mass. In this case, a higher capacity can be achieved compared to when the negative electrode active material is graphite alone. The Si-containing material content relative to the total of graphite and Si-containing material is preferably 6 to 20% by mass.

[0099] The graphite content in the negative electrode active material may be in the range of 50 to 99% by mass. However, if the particles of the Si-containing material contain graphite on the surface and / or internally, that graphite is not included in the above graphite content. The graphite content refers to the graphite content not present in the Si-containing material.

[0100] The negative electrode active material may contain active materials other than graphite and Si-containing materials. Examples of other active materials include carbonaceous materials other than graphite, such as easily graphitizable carbon (soft carbon) and difficult-to-graphitize carbon (hard carbon).

[0101] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but for example, it can be 1 to 50 μm, or 5 to 30 μm.

[0102] Examples of dispersion media used in preparing the negative electrode slurry include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or mixed solvents thereof. The ratio of components in the negative electrode mixture can be adjusted by changing the mixing ratio of the materials in the negative electrode mixture.

[0103] Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives and salts. Examples of conductive agents include conductive carbon materials, fluorinated carbon, and organic conductive materials. Examples of thickeners include carboxymethylcellulose (CMC) and polyvinyl alcohol. These components may be used individually or in combination of two or more materials.

[0104] (Electrolytes) The electrolyte can be an electrolyte solution containing 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.

[0105] Non-aqueous solvents are used as solvents. Examples of non-aqueous 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), 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 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.

[0106] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0107] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0108] Examples of chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether, ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and the like.

[0109] These solvents may be fluorinated solvents in which some of the hydrogen atoms are replaced by fluorine atoms. As the fluorinated solvent, fluoroethylene carbonate (FEC) may be used because it readily forms a stable solid electrolyte film and easily improves the high-rate cycle maintenance rate.

[0110] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO2). ４ LiAlCl ４ LiB １０ Cl １０ (e.g.), lithium salts of fluorine-containing acids (LiPF) ６ LiPF ２ O ２ LiBF ４ LiSbF ６ LiAsF ６ LiCF ３ SO ３ LiCF ３ CO ２ (etc.), lithium salts of fluorine-containing acidimides (LiN(FSO) ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO２ ), LiN(C ２ F ５ SO ２ ) ２ Lithium halides (LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more. Among these, LiN (FSO) is particularly good due to its excellent ion dissociation properties. ２ ) ２ (LFSI) may be used.

[0111] The concentration of lithium salt 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.

[0112] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0113] (Separator) A separator is interposed between the positive electrode and the negative electrode. 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. In addition, aramid fibers may be used to increase mechanical strength.

[0114] 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.

[0115] 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 secondary battery 1 shown in Figure 1 includes a bottomed rectangular battery case 11, an electrode group 10 housed within the battery case 11, and a non-aqueous electrolyte (not shown). 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 core and then removing the core.

[0116] One end of the negative electrode lead 15 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of the 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 the negative electrode terminal 13 provided on the sealing plate 12. A gasket 16 is placed between the sealing plate 12 and the negative electrode terminal 13 to insulate them. The other end of the positive electrode lead 14 is connected to the sealing plate 12 and electrically connected to the battery case 11, which also serves as the positive electrode terminal. A resin frame 18 is placed on top of the electrode group 10. The frame 18 isolates the electrode group 10 from the sealing plate 12 and also isolates the negative electrode lead 15 from the battery case 11. The opening of the battery case 11 is sealed by the sealing plate 12. An injection hole 17a is formed in the sealing plate 12. The electrolyte is injected into the battery case 11 through the injection hole 17a. After that, the injection hole 17a is sealed by the seal 17.

[0117] (Note) The above description of embodiments discloses the following technologies. (Technology 1) A positive electrode for a secondary battery comprising a positive electrode mixture comprising a positive electrode active material, a positive electrode conductive agent, and a binder, wherein the positive electrode active material comprises a lithium-containing composite oxide having a layered structure, the conductive agent comprises a carbon material, the binder comprises a first binder and a second binder different from the first binder, the first binder is polyacrylonitrile, and the polyacrylonitrile comprises a modified polyacrylonitrile having at least one functional group selected from the group consisting of acid groups, hydroxyl groups, and derivatives thereof. (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the modification rate of the modified polyacrylonitrile is 10% or more. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, wherein the modified polyacrylonitrile has at least one functional group selected from the group consisting of carboxylic acid groups, hydroxyl groups, sulfonic acid groups, phosphoric acid groups, and derivatives thereof. (Technology 4) The positive electrode for a secondary battery according to Technology 3, wherein the modified polyacrylonitrile has at least one functional group selected from the group consisting of a hydroxyl group, a sulfonic acid group, a phosphoric acid group, and derivatives thereof. (Technology 5) The positive electrode for a secondary battery according to any one of Technology 1 to 4, wherein the second binder comprises a fluorine-containing polymer. (Technology 6) The positive electrode for a secondary battery according to any one of Technology 1 to 5, wherein the mass ratio C1 / C2 of the content of the second binder to the content C1 of the first binder in the positive electrode mixture is in the range of 3 / 7 to 7 / 3. (Technology 7) The positive electrode for a secondary battery according to any one of Technology 1 to 6, wherein the weight-average molecular weight of the polyacrylonitrile is 100,000 or more. (Technology 8) The positive electrode for a secondary battery according to any one of Technology 1 to 7, wherein the carbon material comprises carbon nanotubes. (Technology 9) The positive electrode for a secondary battery according to Technology 8, wherein the fiber length distribution of the carbon nanotube has two peaks at a first fiber length and a second fiber length different from the first fiber length. (Technology 10) The positive electrode for a secondary battery according to Technology 8 or 9, wherein the carbon nanotube comprises 1-3 layer carbon nanotubes. (Technology 11) The positive electrode for a secondary battery according to any one of Technologies 1 to 10, wherein the carbon material comprises carbon black.(Technical 12) The positive electrode for a secondary battery according to any one of Technical 1 to 11, wherein the positive electrode active material includes a lithium-nickel composite oxide in which 50 atomic percent or more of the metals other than lithium is nickel. (Technical 13) The lithium-nickel composite oxide is of the formula Li. ａ Ni ｘ M １－ｘ O ２ (wherein 0 < a ≤ 1.2, 0.5 ≤ x ≤ 1), and M is at least one element selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B, as described in Technical 12. (Technical 14) In the above formula, M is at least one element selected from the group consisting of Co, Al, and Mn. １ and at least one element M selected from the group consisting of Ca and Sr. ２ (Technology 15) A positive electrode for a secondary battery according to Technology 13, including the above. (Technology 16) A positive electrode for a secondary battery according to any one of Technology 1 to 15, wherein the surface of the positive electrode active material is covered with a metal compound, and the metal compound contains at least one metal element selected from the group consisting of Mg, Ca, Sr, Ba, Ti, W, Zr, Al, and Nb. (Technology 17) A positive electrode for a secondary battery according to any one of Technology 1 to 16, wherein the surface of the positive electrode active material is covered with a nonmetal compound, and the nonmetal compound contains at least one nonmetal element selected from the group consisting of B, N, F, P, and S.

[0118] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0119] 《Example 1》 (1) Preparation of the negative electrode A negative electrode slurry was prepared by mixing the negative electrode active material, sodium polyacrylate (PAA-Na), sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), and water in a predetermined mass ratio. A mixture of graphite and Si-containing material was used as the negative electrode active material. The mixing ratio of graphite and Si-containing material in the negative electrode active material was set to a mass ratio of graphite:Si-containing material = 95:5.

[0120] As a Si-containing material, composite particles having a sea-island structure in which the silicon phase is dispersed in the lithium silicate phase were prepared by the following method. First, silicon dioxide and lithium carbonate were mixed so that the atomic ratio: Si / Li was 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain the formula: Li ２ Si ２ O ５ A lithium silicate represented by [formula] was obtained. The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0121] Next, the obtained lithium silicate and raw silicon (3N, average particle size 10 μm) were mixed in a 50:50 mass ratio. The mixture was filled into a pot (made of stainless steel, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 stainless steel balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was ground in an inert atmosphere at 200 rpm for 50 hours. Next, the powdered mixture was removed in an inert atmosphere and fired at 800°C for 4 hours under pressure from a hot press in an inert atmosphere to obtain a sintered body (master particle) of the mixture.

[0122] Subsequently, the sintered body was crushed, passed through a 40 μm mesh, and mixed with coal pitch (JFE Chemical Corporation, MCP250). The mixture was then fired in an inert atmosphere at 800°C to form a conductive layer on the surface of the crushed particles by coating them with conductive carbon. The amount of conductive layer coating was 5% by mass of the total mass of the crushed particles. Then, using a sieve, second particles with an average particle size of 5 μm and a conductive layer were obtained.

[0123] Next, a coating film was formed on the surface of the copper foil (negative electrode current collector) by applying the negative electrode slurry. After drying the coating film, it was rolled. In this way, negative electrode active material layers were formed on both sides of the copper foil. The mixing ratio of the negative electrode active material, sodium polyacrylate, sodium carboxymethylcellulose, and styrene-butadiene rubber in the negative electrode slurry was set to a mass ratio of negative electrode active material:PAA-Na:CMC-Na:SBR = 100:0.5:1:1.

[0124] (2) Preparation of the positive electrode As the positive electrode active material, LiNi ０．８８ Co ０．０６ Mn ０．０６ O ２ The following materials were used. The positive electrode active material, the carbon material which is the positive electrode conductive agent, the first binder, and the second binder were mixed in a predetermined mass ratio, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare the positive electrode slurry. Acetylene black (AB) was used as the positive electrode conductive agent (carbon material). Modified polyacrylonitrile (weight-average molecular weight after modification treatment of 450,000) was used as the first binder. PVdF was used as the second binder. Modified polyacrylonitrile was obtained by modifying polyacrylonitrile (PAN) with a carboxylic acid group. The modification rate of the modified polyacrylonitrile was set to 5%.

[0125] The ratio of the positive electrode active material, positive electrode conductive agent (carbon material), and binder (total of the first and second binders) in the positive electrode slurry was set to a mass ratio of positive electrode active material:positive electrode conductive agent:binder = 100:1:1. The mixing ratio of modified polyacrylonitrile and PVdF in the binder was set to a mass ratio of modified polyacrylonitrile:PVdF = 5:5.

[0126] Next, a positive electrode slurry was applied to the surface of an aluminum foil, which served as the positive electrode current collector. After the coating was dried, the foil was rolled to form a positive electrode mixture layer on both sides of the aluminum foil. The density of the positive electrode mixture layer after drying was 3.66 g / cm³. ３ The material was rolled to achieve the following result. The amount of coating for the positive electrode mixture layer was 260 g / m². ２ That's what I decided.

[0127] (3) Preparation of the electrolyte A mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 was to be mixed with fluoroethylene carbonate (FEC) at a concentration of 3% by mass relative to the mixed solvent, and then LiPF6 was added as a lithium salt to prepare the electrolyte. The concentration of LiPF6 in the non-aqueous electrolyte was 1.3 mol / liter.

[0128] (4) Fabrication of the secondary battery Lead tabs were attached to each electrode. Next, the positive and negative electrodes were wound in a spiral shape via a separator so that the leads were located on the outermost part. The electrode group was thus fabricated. Next, the electrode group was inserted into an outer casing made of laminate film with aluminum foil as a barrier layer and vacuum dried. Next, a non-aqueous electrolyte was injected into the outer casing and the opening of the outer casing was sealed. In this way, secondary battery A1 according to Example 1 was completed.

[0129] (5) Evaluation [Evaluation 1: Peel Strength] The peel strength of the positive electrode mixture layer from the positive electrode current collector was measured by a 90-degree peel test in accordance with JIS K 6854-1. The positive electrode mixture layer was peeled off the positive electrode current collector at a nearly constant speed, and the average peel force was determined from the relationship between the distance traveled (length peeled off) and the force required to peel it off (peeling force). The average peel force was evaluated as the peel strength.

[0130] [Evaluation 2: Cycle Capacity Retention Rate] The completed batteries were placed in a 25°C environment, and charge-discharge cycle tests were performed under the following charge-discharge conditions. The rest period between charging and discharging was set to 10 minutes.

[0131] (Charging 1) Constant current charging was performed with a current of 0.5C until the voltage reached 4.2V, and then constant voltage charging was performed with a constant voltage of 4.2V until the current reached 0.05C. (Discharging 1) Constant current discharge was performed with a current of 0.5C until the voltage reached 2.5V.

[0132] The above charge 1 and discharge 1 cycles were repeated 500 times. The ratio (percentage) of the discharge capacity at cycle 500 to the discharge capacity at cycle 1 was calculated as the cycle capacity maintenance rate R (%).

[0133] Examples 2 and 3: In the preparation of the positive electrode, the modification rate of the modified polyacrylonitrile used as the first binder was changed from Example 1 to 11% in Example 2 and 17% in Example 3.

[0134] Aside from the above, secondary battery A2 according to Example 2 and secondary battery A3 according to Example 3 were completed and evaluated in the same manner as in Example 1.

[0135] Example 4: In the preparation of the positive electrode, modified polyacrylonitrile (PAN) obtained by denaturing with hydroxyl groups (weight-average molecular weight after denaturation: 450,000) was used as the first binder. The denaturation rate of the modified polyacrylonitrile was set to 5%. Otherwise, secondary battery A4 according to Example 4 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0136] Example 5: In the preparation of the positive electrode, modified polyacrylonitrile (PAN) obtained by modifying it with a sulfonic acid group (weight-average molecular weight after modification: 450,000) was used as the first binder. The modification rate of the modified polyacrylonitrile was set to 5%. Otherwise, secondary battery A5 according to Example 5 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0137] Example 6: In the preparation of the positive electrode, modified polyacrylonitrile (PAN) obtained by modifying it with phosphate groups (weight-average molecular weight after modification: 450,000) was used as the first binder. The modification rate of the modified polyacrylonitrile was set to 5%. Otherwise, secondary battery A6 according to Example 6 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0138] Example 7: In the preparation of the positive electrode, the modification rate of the modified polyacrylonitrile was changed from 5% in Example 4 to 11%. Otherwise, the secondary battery A7 according to Example 7 was completed and evaluated in the same manner as in Example 4.

[0139] Examples 8 and 9: In preparing the positive electrode, the mixing ratio of modified polyacrylonitrile (first binder) and PVdF (second binder) was changed from Example 4. In Example 8, the mass ratio of modified polyacrylonitrile:PVdF was 3:7, and in Example 9, the mass ratio of modified polyacrylonitrile:PVdF was 7:3.

[0140] Aside from the above, secondary battery A8 according to Example 8 and secondary battery A9 according to Example 9 were completed and evaluated in the same manner as in Example 4.

[0141] Example 10: In the preparation of the positive electrode, modified polyacrylonitrile (PAN) obtained by denaturing with hydroxyl groups (weight-average molecular weight after denaturation: 100,000) was used as the first binder. The denaturation rate of the modified polyacrylonitrile was set to 5%. Otherwise, the secondary battery A10 according to Example 10 was completed and evaluated in the same manner as in Example 4.

[0142] Example 11: In the preparation of the positive electrode, multi-walled carbon nanotubes (MWCNTs) (average fiber length 5 μm, average fiber diameter 10 nm) were used instead of AB as the positive electrode conductive agent (carbon material). The ratio of the positive electrode active material, conductive agent (carbon material), and binder (total of the first and second binders) in the positive electrode slurry was set to a mass ratio of positive electrode active material:conductive agent:binder = 100:0.5:1.

[0143] Aside from the above, the secondary battery A11 according to Example 11 was completed and evaluated in the same manner as in Example 4.

[0144] Example 12: In the preparation of the positive electrode, MWCNTs and single-walled carbon nanotubes (average fiber length 50 μm, average fiber diameter 1.5 nm) were mixed in a mass ratio of 90:10 as the positive electrode conductive agent (carbon material). The ratio of the positive electrode active material, positive electrode conductive agent (carbon material), and binder (total of the first and second binders) in the positive electrode slurry was set to a mass ratio of positive electrode active material: positive electrode conductive agent: binder = 100:0.5:1.

[0145] Aside from the above, the secondary battery A12 according to Example 12 was completed and evaluated in the same manner as in Example 4.

[0146] Example 13: In the preparation of the positive electrode, acetylene black (AB) and single-walled carbon nanotubes (average fiber length 50 μm, average fiber diameter 1.5 nm) were mixed in a mass ratio of 90:10 as the positive electrode conductive agent (carbon material). The ratio of the positive electrode active material, conductive agent (carbon material), and binder (total of the first and second binders) in the positive electrode slurry was set to a mass ratio of positive electrode active material: positive electrode conductive agent: binder = 100:0.5:1.

[0147] Aside from the above, the secondary battery A13 according to Example 13 was completed and evaluated in the same manner as in Example 4.

[0148] Example 14: Lithium oxide and metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O) and calcium hydroxide (Ca(OH) ２ ) are mixed and fired so that the total amount of Li, Ni, Co, and Mn and the molar ratio of Ca is 1.03:1:0.0025, and LiNi ０．８８ Co ０．０６ Mn ０．０６ O ２ A lithium metal composite oxide was obtained in which the surface was covered with a Ca compound. A secondary battery A14 according to Example 14 was completed and evaluated in the same manner as in Example 4, except that this lithium metal composite oxide was used as the positive electrode active material.

[0149] Example 15: Lithium oxide and metal oxide (Ni ０．８８ Co ０．０６ Mn ０．０６ O) and strontium hydroxide (Sr(OH) ２ ) are mixed and fired so that the total amount of Li, Ni, Co, and Mn and the molar ratio of Sr is 1.03:1:0.0025. ０．８８ Co ０．０６ Mn ０．０６ O ２A lithium metal composite oxide was obtained in which the surface was covered with an Sr compound. A secondary battery A15 according to Example 15 was completed and evaluated in the same manner as in Example 4, except that this lithium metal composite oxide was used as the positive electrode active material.

[0150] <Comparative Example 1> In the preparation of the positive electrode, the second binder was not used, and only modified polyacrylonitrile was used as the first binder to obtain the positive electrode slurry. The secondary battery B1 according to Comparative Example 1 was completed in the same manner as in Example 4 and evaluated in the same manner.

[0151] <Comparative Example 2> In the preparation of the positive electrode, modified polyacrylonitrile was not used as the first binder, and only PVdF was used as the second binder. Other than this, the secondary battery B2 according to Comparative Example 2 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0152] <Comparative Example 3> In the preparation of the positive electrode, unmodified polyacrylonitrile (PAN) (weight-average molecular weight 450,000) was used as the first binder. Otherwise, the secondary battery B3 according to Comparative Example 3 was completed and evaluated in the same manner as in Example 1.

[0153] Example 16: In Example 1, the second binder was changed to hydrogenated nitrile-butadiene rubber (HNBR). Otherwise, the secondary battery A16 according to Example 16 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0154] Comparative Example 4: In Comparative Example 4, modified polyacrylonitrile was not used as the first binder, and only hydrogenated nitrile-butadiene rubber (HNBR) was used as the second binder. Otherwise, the secondary battery B4 according to Comparative Example 4 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0155] The evaluation results are shown in Table 1. In Table 1, peel strength and cycle retention rate are shown as relative values, with the result for battery B1 in Comparative Example 1 set to 100. As shown in Table 1, batteries B1 and B2, which used polyacrylonitrile or PVdF alone as a binder, had low cycle retention rates. On the other hand, battery B3, which used a combination of unmodified polyacrylonitrile and PVdF, achieved a higher cycle retention rate than batteries B1 and B2, but its peel strength was significantly lower than that of battery B1, falling below 95, which is the peel strength required for mass production.

[0156] In contrast, in batteries A1 to A16, by using a combination of modified polyacrylonitrile as the first binder and a second binder, the decrease in the peel strength of the positive electrode mixture layer was suppressed, and a high cycle retention rate was achieved while maintaining a peel strength suitable for mass production.

[0157]

[0158] The secondary battery described herein can provide a secondary battery with low internal resistance. The secondary battery described herein is useful for small consumer applications, power storage devices, and as the main power source for electric vehicles.

[0159] 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.

[0160] 1: Non-aqueous electrolyte secondary battery, 3: Positive 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: Injection hole, 18: Frame

Claims

1. A positive electrode for a secondary battery comprising a positive electrode mixture comprising a positive electrode active material, a positive electrode conductive agent, and a binder, wherein the positive electrode active material comprises a lithium-containing composite oxide having a layered structure, the positive electrode conductive agent comprises a carbon material, the binder comprises a first binder and a second binder different from the first binder, the first binder is polyacrylonitrile, and the polyacrylonitrile comprises a modified polyacrylonitrile having at least one functional group selected from the group consisting of acid groups, hydroxyl groups, and derivatives thereof.

2. The positive electrode for a secondary battery according to claim 1, wherein the modification rate of the modified polyacrylonitrile is 10% or more.

3. The positive electrode for a secondary battery according to claim 1, wherein the modified polyacrylonitrile has at least one functional group selected from the group consisting of a carboxylic acid group, a hydroxyl group, a sulfonic acid group, a phosphate group, and derivatives thereof.

4. The positive electrode for a secondary battery according to claim 3, wherein the modified polyacrylonitrile has at least one functional group selected from the group consisting of a hydroxyl group, a sulfonic acid group, a phosphate group, and derivatives thereof.

5. The positive electrode for a secondary battery according to claim 1, wherein the second binder comprises a fluorine-containing polymer.

6. The positive electrode for a secondary battery according to claim 1, wherein the mass ratio C1 / C2 of the content of the second binder to the content C1 of the first binder in the positive electrode mixture is in the range of 3 / 7 to 7 / 3.

7. The positive electrode for a secondary battery according to claim 1, wherein the weight-average molecular weight of the polyacrylonitrile is 100,000 or more.

8. The positive electrode for a secondary battery according to claim 1, wherein the carbon material includes carbon nanotubes.

9. The positive electrode for a secondary battery according to claim 8, wherein the fiber length distribution of the carbon nanotubes has two peaks, one at a first fiber length and the other at a second fiber length different from the first fiber length.

10. The positive electrode for a secondary battery according to claim 8, wherein the carbon nanotube comprises a 1-3 layer carbon nanotube.

11. The positive electrode for a secondary battery according to any one of claims 1 to 10, wherein the carbon material comprises carbon black.

12. The positive electrode for a secondary battery according to any one of claims 1 to 10, wherein the positive electrode active material comprises a lithium-nickel composite oxide in which 50 atomic percent or more of the metal other than lithium is nickel.

13. The lithium-nickel composite oxide is of the formula Li ａ Ni ｘ M １－ｘ O ２ The positive electrode for a secondary battery according to claim 12, wherein M is expressed as (where 0 < a ≤ 1.2, 0.5 ≤ x ≤ 1) and M includes at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Ca, and B.

14. In the above formula, M is at least one element selected from the group consisting of Co, Al, and Mn. １ and at least one element M selected from the group consisting of Ca and Sr. ２ A positive electrode for a secondary battery according to claim 13, including the above.

15. The positive electrode for a secondary battery according to any one of claims 1 to 10, wherein the surface of the positive electrode active material is covered with a metal compound, and the metal compound comprises at least one metal element selected from the group consisting of Mg, Ca, Sr, Ba, Ti, W, Zr, Al, and Nb.

16. The positive electrode for a secondary battery according to any one of claims 1 to 10, wherein the surface of the positive electrode active material is covered with a nonmetallic compound, and the nonmetallic compound contains at least one nonmetallic element selected from the group consisting of B, N, F, P, and S.

17. A secondary battery comprising: a positive electrode for a secondary battery as described in claim 1; a separator; a negative electrode facing the positive electrode for a secondary battery via the separator; and an electrolyte.

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