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 carbon material that is a positive electrode conductive agent, and a binding agent. The positive electrode active material includes a lithium-containing composite oxide having a layered structure in which 50 at% or more of metals other than lithium are nickel. The carbon material includes carbon nanotubes. 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.

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. 2024-231286, 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 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 Brochure

[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. As a result, the increased internal resistance of the secondary battery leads to a decrease in rapid charging performance and charge / discharge efficiency, thus degrading the overall performance of the secondary battery. In particular, the increased internal resistance tends to reduce the cycle maintenance rate under high load.

[0009] Lithium-nickel composite oxides tend to have particularly high resistance among lithium transition metal composite oxides, resulting in higher positive electrode resistance and a more pronounced problem of reduced secondary battery performance due to increased internal resistance.

[0010] 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 bonding properties with the positive electrode material, it tends to increase internal resistance, which can easily lead to a decrease in the performance of the secondary battery.

[0011] 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 including a positive electrode active material, a carbon material which is a positive electrode conductive agent, and a binder, wherein the positive electrode active material has a layered structure and includes a lithium-containing composite oxide in which 50 atomic percent or more of the metals other than lithium are nickel, the carbon material includes carbon nanotubes, and the binder includes a first binder and a second binder different from the first binder, wherein the first binder is polyacrylonitrile.

[0012] 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.

[0013] According to this disclosure, the internal resistance of secondary batteries can be reduced. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of its 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.

[0014] This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present disclosure.

[0015] 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.

[0016] 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.

[0017] 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 including a positive electrode active material, a carbon material which is a positive electrode conductive agent, and a binder. The positive electrode active material includes a lithium-containing composite oxide having a layered structure and in which 50 atomic percent or more of the metals other than lithium are nickel. The carbon material includes carbon nanotubes (CNTs). The binder includes a first binder and a second binder which is different from the first binder. The first binder is polyacrylonitrile (PAN).

[0018] The binder enhances the bonding between particles of the positive electrode active material. It also enhances the adhesion between the positive electrode active material and the positive electrode current collector. The binder adheres to the surface of the particles of the positive electrode active material, as well as to the surface of the carbon material that constitutes the positive electrode conductive agent. However, when the binder adheres to the surface of the positive electrode conductive agent, the electrical connectivity between the positive electrode conductive agent and the active material particles decreases, which can contribute to an increase in the internal resistance of the positive electrode.

[0019] 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 fluorine-containing polymers 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.

[0020] In contrast, polyacrylonitrile, 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 high conductivity and excellent conductive network formation ability of carbon nanotubes can be effectively utilized, and positive electrode resistance can be significantly reduced. Furthermore, since cyano groups have a high affinity for nickel, when used with a positive electrode active material containing a lithium-containing composite oxide in which 50 atomic percent or more of the metals other than lithium are nickel, adhesion to the carbon material is further suppressed, and positive electrode resistance is reduced. Moreover, as will be described later, the presence of a metallic or non-metallic compound on the surface of the positive electrode active material increases the affinity between the positive electrode active material and the cyano groups, further reducing positive electrode resistance.

[0021] In positive electrode active materials, when the ratio of nickel to non-lithium metals in a lithium-containing composite oxide is 50 atomic percent or more, the electronic conductivity of the lithium-containing composite oxide is low, and the internal resistance of the positive electrode tends to increase. By using carbon nanotubes as the positive electrode conductive agent and combining them with polyacrylonitrile as the first binder, the increase in the internal resistance of the positive electrode can be significantly suppressed when using a lithium-containing composite oxide with a high nickel ratio, thereby suppressing the degradation of secondary battery performance.

[0022] The binder includes, in addition to the first binder, a second binder different from the first binder. The second binder has a high affinity for carbon materials (particularly, carbon nanotubes). The first binder mainly adheres to the positive electrode active material and improves the adhesion between the particles of the positive electrode active material. The second binder adheres not only to the positive electrode active material but also to the conductive carbon material, improves the adhesion between the carbon material and the particles of the positive electrode active material, and contributes to the formation of a conductive network.

[0023] In terms of being electrochemically stable, the second binder may be a fluorine-containing polymer. The fluorine-containing polymer may include 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). Among them, from the viewpoints of electrochemical stability and stability of the positive electrode slurry, the fluorine-containing polymer preferably includes at least monomer units derived from VdF. The fluorine-containing polymer may include at least one selected from the group consisting of polyvinylidene fluoride (PVdF) and copolymers containing monomer units derived from VdF. The copolymer may be a block copolymer or a random copolymer.

[0024] The copolymer containing monomer units derived from VdF may include a copolymer of VdF and a fluorine-containing monomer other than VdF. That is, the copolymer includes monomer unit U １ derived from VdF and fluorine-containing monomer unit U ２ not derived from VdF. Fluorine-containing monomers other than VdF may include at least one selected from the group consisting of HFP, TFE, trifluoroethylene, and chlorotrifluoroethylene. In the copolymer, the molar ratio (U １ / U ２ ) of the fluorine-containing monomer unit U ２ not derived from VdF to the monomer unit U １ derived from VdF may be, for example, 0.01 or more and 0.5 or less, or 0.05 or more and 0.3 or less.

[0025] 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 suppressing the increase in the internal resistance of the positive electrode is enhanced, and the deterioration of the secondary battery's performance can be suppressed.

[0026] Polyacrylonitrile is [-CH ２ The polymer contains monomer units represented by [-CH(CN)-]. Polyacrylonitrile is suitable because it has high affinity for active materials and does not over-cover carbon nanotubes, thus providing a sufficient effect of reducing resistance. It is acceptable if 90% or more of the monomer units constituting the polymer are the above monomer units, or if 95% or more or 99% or more are the above monomer units. It is also possible that 100% of the monomer units constituting the polymer are the above monomer units.

[0027] Polyacrylonitrile may be modified polyacrylonitrile.

[0028] 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.

[0029] Carbon nanotubes have extremely small fiber diameters (nano-sized) and a very 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 rolling the positive electrode mixture layer after coating, gaps that can hold the electrolyte are secured between the fibers, thus maintaining high liquid flowability.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] The carbon material used as the positive electrode conductive agent may include particulate conductive carbon materials such as carbon black in addition to carbon nanotubes. By mixing carbon nanotubes, which are fibrous conductive materials, and carbon black, which is a particulate conductive material, within the positive electrode mixture layer, 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. 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The secondary battery (S) may be a non-aqueous electrolyte secondary battery.

[0048] Below, an example of a secondary battery according to this embodiment and examples of its components are described. Note that known components may be used for components that are not characteristic of this disclosure.

[0049] (Positive Electrode) The positive electrode (PE) typically includes 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 includes a positive electrode active material as an essential component, a carbon material as a positive electrode conductive agent, and carbon nanotubes as the carbon material. The positive electrode mixture includes polyacrylonitrile as a binder (first binder) and a binder other than polyacrylonitrile (second binder). Furthermore, the positive electrode mixture may include other positive electrode conductive agents as optional components, such as carbon nanotubes.

[0050] (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, or 90 atomic percent or more. For example, it is desirable that the proportion of Ni among the metal elements other than Li be 95 atomic percent or less. When limiting the range, these upper and lower limits can be combined arbitrarily.

[0051] 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.

[0052] 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.

[0053] In lithium-nickel composite oxides, element M ２This 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.

[0054] 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.)

[0055] The lithium metal composite oxides described above tend to have higher resistance as the Ni ratio increases and the Co ratio decreases, resulting in higher resistance at the positive electrode. However, by incorporating carbon nanotubes into the positive electrode conductive agent, the resistance at the positive electrode can be significantly reduced, enabling both high capacity and a high cycle maintenance rate under high load conditions.

[0056] 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).

[0057] 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 Ni １－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.

[0058] 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.

[0059] A metal compound may be present on the surface of the lithium metal composite oxide. The metal compound may include, 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 metal compounds include oxides, hydroxides, carbonates, sulfates, nitrates, and phosphates of these metal elements. More specifically, compounds containing Mg include MgO and Mg(OH) ２ , and MgCO ３ Examples include CaO and Ca(OH) ２ , and CaCO ３ Examples include: SrO, Sr(OH) ２ , and SrCO ３ Examples include: BaO, Ba(OH) ２ , and BaCO ３ Examples include: A compound containing Ti is TiO ２ , Ti(OH) ４ , and Ti(CO ３ ) ２ Examples include WO ３ Examples include ZrO ２ , Zr(OH) ４ , Zr(CO ３ ) ２ , and Zr(SO ４ ) ２ 4H ２ O can be used as an example. Compounds containing Al include Al ２ O ３can be exemplified. As the compound containing Nb, NbO5 can be exemplified. Further, the metal compound may be a composite compound containing a plurality of types of these metal elements, such as composite oxides such as SrAlO ４ , CaAlO ４ and the like.

[0060] A non-metal compound may be present on the surface of the lithium metal composite oxide. The non-metal compound contains at least one non-metal element selected from the group consisting of, for example, B, N, F, P, and S. As the compound containing B, H ３ BO ３ , Li ３ BO ３ , LiBO ２ , Li ２ B ４ O ７ can be exemplified. As the compound containing N, LiNO ３、 LiNO ２ can be exemplified. As the compound containing F, LiF, Li ２ SiF ６ , LiCF ３ SO ３ can be exemplified. As the compound containing P, Li ３－ｘ H ｘ PO ４ (0 ≤ x ≤ 3) can be exemplified. As the compound containing S, Li ２ SO ４ , Li ２ S, Li ３ PS ４ can be exemplified.

[0061] 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.

[0062] 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 W２ 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, NbO ５ , NbO5・nH ２ Examples include O. As for P raw materials, Li ３－ｘ H ｘ PO ４ Examples include (0 ≤ x ≤ 3). Examples of S raw materials include sulfonic acid compounds such as lithium methanesulfonate, lithium ethanesulfonate, lithium propanesulfonate, sodium methanesulfonate, magnesium methanesulfonate, and sodium fluoromethanesulfonate. Examples of B raw materials include H ３ BO ３ Li ３ BO ３ Li ２ B ４ O ７ These are some examples. These compounds may also be used after being crushed to change the particle size as appropriate or by adjusting the water content, including the hydrate.

[0063] 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.

[0064] (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.

[0065] 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, the conductivity of the positive electrode mixture is significantly improved. The average fiber length may be 0.8 μm or more, and 1.0 μm or more is preferred.

[0066] 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, and the more likely the conductivity of the positive electrode composite is to decrease. To improve the conductivity of the positive electrode composite, the carbon nanotube content should be increased, but the higher the carbon nanotube content in the slurry, the more likely the carbon nanotubes are to aggregate in the slurry, making uniform dispersion difficult, 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, a positive electrode with significantly improved current collection performance and easy manufacturing can be obtained. The average fiber diameter of the carbon nanotubes is preferably 4 nm or less, and more preferably 3 nm or less.

[0067] 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.

[0068] As a positive electrode conductive agent, conductive carbon materials other than carbon nanotubes may be used in mixture with carbon nanotubes. Conductive carbon materials other than carbon nanotubes include particulate carbon materials such as acetylene black, Ketjen black, and furnace black; rod-shaped carbon materials such as VGCF; sheet-shaped carbon materials such as graphene; and fibrous carbon materials such as carbon fiber. Multiple types of these materials may be combined and used as a positive electrode conductive agent.

[0069] 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.

[0070] (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.

[0071] (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.

[0072] (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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] Si-containing materials include elemental Si, 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 Ｘ Particles 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.

[0077] 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.

[0078] 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).

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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).

[0084] 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.

[0085] Examples of Si-containing materials include SiO ＸAt 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.

[0086] 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.

[0087] The composition and component content of the second and third particles can be analyzed by the method described in International Publication No. 2018 / 179969.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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).

[0093] 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.

[0094] 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.

[0095] 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.

[0096] (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.

[0097] 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.

[0098] Other non-aqueous solvents include cyclic ethers, linear ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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 LiBF４ 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 SO ２ ), 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.

[0103] 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.

[0104] The electrolyte may contain other known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0105] (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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] (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 carbon material which is a positive electrode conductive agent, and a binder, wherein the positive electrode active material comprises a lithium-containing composite oxide having a layered structure and in which 50 atomic percent or more of the metals other than lithium are nickel, the carbon material comprises carbon nanotubes, and the binder comprises a first binder and a second binder different from the first binder, wherein the first binder is polyacrylonitrile. (Technology 2) The positive electrode for a secondary battery according to Technology 1, wherein the second binder comprises a fluorine-containing polymer. (Technology 3) The positive electrode for a secondary battery according to Technology 1 or 2, 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 4) The positive electrode for a secondary battery according to any one of Techniques 1 to 3, wherein the weight-average molecular weight of the polyacrylonitrile is 100,000 or more. (Technology 5) The positive electrode for a secondary battery according to any one of Techniques 1 to 4, 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 6) The positive electrode for a secondary battery according to any one of Techniques 1 to 5, wherein the carbon material further comprises carbon black. (Technology 7) The positive electrode for a secondary battery according to any one of Techniques 1 to 6, wherein the carbon nanotube comprises 1-3 layer carbon nanotubes. (Technology 8) The positive electrode active material is of the formula Li ａ Ni ｘ M １－ｘ O ２ A positive electrode for a secondary battery according to any one of the Techniques 1 to 7, comprising a lithium metal composite oxide represented by (wherein 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). (Technology 9) 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 10) A positive electrode for a secondary battery according to Technology 8, including the above. (Technology 11) A positive electrode for a secondary battery according to any one of Technology 1 to 9, 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 12) A positive electrode for a secondary battery according to any one of Technology 1 to 9, 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.

[0110] 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.

[0111] 《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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] (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 was 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. The positive electrode conductive agent (carbon material) used was multi-walled carbon nanotube (MWCNT) (average fiber length 5 μm, average fiber diameter 10 nm). The first binder used was polyacrylonitrile (PAN) (weight-average molecular weight 450,000). The second binder used was PVdF.

[0117] 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. The mixing ratio of polyacrylonitrile and PVdF in the binder was set to a mass ratio of polyacrylonitrile:PVdF = 5:5.

[0118] 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.

[0119] (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.

[0120] (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.

[0121] (5) Evaluation (Internal Resistance) The completed battery was placed in an environment of 25°C and charged with a constant current of 0.5C until the voltage reached 4.2V. Then, it was charged with a constant voltage of 4.2V until the current reached 0.05C. After that, it was discharged with a constant current of 0.5C until the voltage reached 2.5V.

[0122] The above charge-discharge process was repeated for two cycles. After the second discharge cycle, the battery was charged with a constant current of 0.5C at 25°C until it reached 50% of its charge capacity from the second cycle, resulting in a battery with a State of Charge (SOC) of 50%.

[0123] A constant current discharge was performed on a battery with a state of charge (SOC) of 50% at a current I of 0.5C. The difference ΔV between the voltage immediately before the start of discharge and the voltage 10 seconds after the start of discharge was calculated, and the DC resistance value (Ω) expressed by dividing ΔV by the current I (ΔV / I) was evaluated as the internal resistance r of the battery.

[0124] Examples 2 and 3: In Example 1, the mixing ratio of polyacrylonitrile and PVdF in the binder was changed. In Example 2, the mass ratio of polyacrylonitrile:PVdF was 3:7, and in Example 3, the mass ratio of polyacrylonitrile:PVdF was 7:3.

[0125] 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.

[0126] Examples 4 and 5: In Example 1, the molecular weight of the first binder, polyacrylonitrile, was changed. In Example 2, polyacrylonitrile with a weight-average molecular weight of 100,000 was used. In Example 3, polyacrylonitrile with a weight-average molecular weight of 150,000 was used.

[0127] Aside from the above, secondary battery A4 according to Example 4 and secondary battery A5 according to Example 5 were completed and evaluated in the same manner as in Example 1.

[0128] Example 6: As the positive electrode conductive agent (carbon material), MWCNTs (average fiber length 5 μm, average fiber diameter 10 nm) 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 and used. The ratio of positive electrode active material, positive electrode conductive agent (carbon material), and binder (total of the first binder and the second binder) 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.

[0129] Aside from the above, the secondary battery A6 according to Example 6 was completed and evaluated in the same manner as in Example 1.

[0130] Example 7: As the positive electrode conductive agent (carbon material), 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 and used. The ratio of 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.

[0131] Aside from the above, the secondary battery A7 according to Example 7 was completed and evaluated in the same manner as in Example 1.

[0132] Example 8: 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 A8 according to Example 8 was completed and evaluated in the same manner as in Example 1, except that this lithium metal composite oxide was used as the positive electrode active material.

[0133] Example 9: 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 A9 according to Example 9 was completed and evaluated in the same manner as in Example 1, except that this lithium metal composite oxide was used as the positive electrode active material.

[0134] Comparative Example 1: In Comparative Example 1, the second binder was omitted, and only polyacrylonitrile was used as the first binder to obtain a positive electrode slurry. The secondary battery B1 according to Comparative Example 1 was completed and evaluated in the same manner as in Example 1.

[0135] Comparative Example 2: In Comparative Example 2, polyacrylonitrile was not used as the first binder, and only PVdF was used as the second binder. Otherwise, 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.

[0136] 《Comparative Example 3》 In Comparative Example 2, only acetylene black (AB) was used as the positive electrode conductive agent (carbon material). The ratio of 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. The secondary battery B3 according to Comparative Example 3 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0137] 《Comparative Example 4》 In Example 1, only acetylene black (AB) was used as the positive electrode conductive agent (carbon material). The ratio of positive electrode active material, positive electrode conductive agent (carbon material), and binder (total of the first binder and the second binder) 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. 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.

[0138] Example 10: In Example 1, the second binder was changed to hydrogenated nitrile-butadiene rubber (HNBR). Otherwise, the secondary battery A10 according to Example 10 was completed in the same manner as in Example 1 and evaluated in the same manner.

[0139] Comparative Example 5: In Comparative Example 5, 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 B5 according to Comparative Example 5 was completed and evaluated in the same manner as in Example 1.

[0140] The evaluation results are shown in Table 1. In Table 1, the internal resistance r is shown as a relative value with the result for battery B1 in Comparative Example 1 set to 100. As shown in Table 1, the increase in internal resistance r is suppressed in batteries A1 to A10. This is thought to be because batteries A1 to A10 were able to maintain low resistance at the positive electrode by using a combination of polyacrylonitrile (PAN) as the first binder, the second binder, and carbon nanotubes as the positive electrode conductive agent.

[0141]

[0142] 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.

[0143] 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.

[0144] 1: Non-aqueous electrolyte secondary battery, 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 containing a positive electrode active material, a carbon material which is a positive electrode conductive agent, and a binder, wherein the positive electrode active material has a layered structure and contains a lithium-containing composite oxide in which 50 atomic percent or more of the metals other than lithium are nickel, the carbon material contains carbon nanotubes, and the binder comprises a first binder and a second binder different from the first binder, wherein the first binder is polyacrylonitrile.

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

3. 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.

4. 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.

5. The positive electrode for a secondary battery according to claim 1, wherein the distribution of the fiber lengths of the carbon nanotubes has two peaks: one for a first fiber length and another for a second fiber length different from the first fiber length.

6. The positive electrode for a secondary battery according to claim 1, wherein the carbon material further comprises carbon black.

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

8. The positive electrode active material is of formula Li ａ Ni ｘ M １－ｘ O ２ A positive electrode for a secondary battery according to claim 1, comprising a lithium metal composite oxide represented as (wherein 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).

9. 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 8, including the above.

10. The positive electrode for a secondary battery according to claim 1, 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.

11. The positive electrode for a secondary battery according to claim 1, 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.

12. A secondary battery comprising: a positive electrode for a secondary battery according to any one of claims 1 to 11; a separator; a negative electrode facing the positive electrode for a secondary battery via the separator; and an electrolyte.

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