Lithium-ion rechargeable battery

Abstract

To provide a positive electrode mixture material that can suppress the deterioration of the discharge rate characteristics of a lithium secondary battery having a relatively thick positive electrode mixture material layer.SOLUTION: A positive electrode mixture material according to the present disclosure includes a positive electrode active material composed of a lithium transition metal oxide having a layered crystal structure, a binder, and a conductive assistant. The lithium transition metal oxide contains nickel, cobalt, and manganese. The binder contains polyvinylidene fluoride. The conductive assistant contains carbon nanotubes. The amount of lithium hydroxide contained in the positive electrode active material is 0.15 mass% or more and 0.35 mass% or less with respect to the total amount of the positive electrode active material.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This disclosure relates to cathode composite materials and lithium secondary batteries. [Background technology]

[0002] Lithium-ion batteries using non-aqueous electrolytes (hereinafter also referred to as "lithium-ion batteries") are used in information and communication technologies (e.g., personal computers, smartphones, etc.), in vehicles, and for energy storage.

[0003] Patent Document 1 discloses a method for manufacturing a positive electrode for a non-aqueous electrolyte lithium secondary battery. The manufacturing method comprises the steps of: preparing a positive electrode mixture paste using a positive electrode active material, polyvinylidene fluoride, acetic anhydride, and a solvent; coating the positive electrode mixture paste onto a positive electrode current collector; and drying the coated positive electrode mixture paste. The mass ratio of LiOH contained in the positive electrode active material to polyvinylidene fluoride is 0.040 to 0.075. The amount of acetic anhydride relative to the total amount of solid components when preparing the positive electrode mixture paste is 0.01% by mass to 0.2% by mass. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-164960 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The migration involves a relatively thick cathode composite layer (with a basis weight of 38 mg / cm²). 2This may occur when forming the positive electrode composite layer described above. "Migration" refers to the phenomenon in which the binder (e.g., polyvinylidene fluoride, etc.) and conductive additive rise to the surface of the coated material when the positive electrode composite paste is dried. If migration occurs, the ion diffusion properties of the resulting positive electrode composite layer may decrease. As a result, the discharge rate characteristics of the lithium secondary battery may decrease.

[0006] This disclosure is made in light of the circumstances described above. One embodiment of this disclosure aims to solve the problem of a lithium secondary battery having a relatively thick cathode composite layer, and provides a cathode composite material and a lithium secondary battery that can suppress the decrease in the discharge rate characteristics of the lithium secondary battery. [Means for solving the problem]

[0007] The following embodiments are included as means for solving the above problems. <1> It contains a positive electrode active material composed of a lithium transition metal oxide having a layered crystalline structure, a binder, and a conductive additive. The lithium transition metal oxide includes nickel, cobalt, and manganese. The aforementioned binder contains polyvinylidene fluoride, The aforementioned conductive additive includes carbon nanotubes, A positive electrode composite material in which the amount of lithium hydroxide contained in the positive electrode active material is 0.15% by mass or more and 0.35% by mass or less relative to the total amount of the positive electrode active material. <2> It comprises a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode is the <1> A lithium secondary battery having a positive electrode composite layer containing the positive electrode composite material described above. <3> The basis weight of the positive electrode composite layer is 35 mg / cm². 2 ~46 mg / cm³ 2 The above <2> Lithium secondary batteries as described above. <4> The basis weight of the positive electrode composite layer is 35 mg / cm². 2 ~40 mg / cm³ 2The lithium secondary battery according to <3>. [Advantages of the Invention]

[0008] According to the present disclosure, a positive electrode composite material and a lithium secondary battery capable of suppressing a decrease in discharge rate characteristics of a lithium secondary battery having a relatively thick positive electrode composite material layer are provided. [Brief Description of the Drawings]

[0009] [Figure 1] FIG. 1 is a graph of the zeta potential difference ΔV with respect to the amount of LiOH contained in the positive electrode active materials of Reference Examples 1 to 14. [Figure 2] FIG. 2 is a graph of the 1C discharge rate with respect to the amount of lithium hydroxide contained in the positive electrode active materials of Examples 1 to 9 and Comparative Examples 1 to 4. [Figure 3] FIG. 3 is a graph of the 1C discharge rate with respect to the drying temperature of the coating film of the positive electrode composite paste. [Embodiments for Carrying Out the Invention]

[0010] In the present disclosure, a numerical range indicated by using "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range of other stepwise descriptions. In the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the amount of each component means the total amount of a plurality of substances when there are a plurality of substances corresponding to each component, unless otherwise specified. In the present disclosure, the term "step" includes not only an independent step but also the term even when it cannot be clearly distinguished from other steps as long as the intended purpose of the step is achieved.

[0011] (1) Positive electrode composite material The positive electrode composite material of this disclosure contains a positive electrode active material composed of a lithium transition metal oxide having a layered crystalline structure, a binder, and a conductive additive. The lithium transition metal oxide includes nickel (Ni), cobalt (Co), and manganese (Mn). The binder includes polyvinylidene fluoride (hereinafter also referred to as "PVDF"). The conductive additive includes carbon nanotubes (hereinafter also referred to as "CNT"). The amount of lithium hydroxide contained in the positive electrode active material (hereinafter simply referred to as "LiOH amount") is 0.15% by mass or more and 0.35% by mass or less, relative to the total amount of the positive electrode active material.

[0012] In this disclosure, "positive electrode composite material" refers to the solid component of the positive electrode composite material layer contained in the positive electrode of a lithium secondary battery. The lithium secondary battery may be a battery comprising a solid electrolyte or a battery comprising a non-aqueous electrolyte. "Lithium transition metal oxide" refers to a compound that can reversibly intercept and release lithium ions and contains LiOH. Lithium transition metal oxide may also be represented as LiMO2. M includes Ni, Co, and Mn, and may further include at least one selected from the group consisting of V, Cr, Fe, Cu, Zr, Nb, Mo, Ru, Pd, Ag, Hf, Ta, W, Ir, Pt, Au, and Pb. The "layered crystal structure of lithium transition metal oxides" refers to a crystal structure in which lithium layers and transition metal layers containing nickel, cobalt, and manganese are arranged alternately. "Positive electrode active material" refers to a powder whose main component is lithium transition metal oxide particles (hereinafter also referred to as "positive electrode active material particles"). "Main component" means that the proportion of lithium transition metal oxide particles is 50% by mass or more of the total amount of positive electrode active material. The proportion of lithium transition metal oxide particles may be 100% by mass of the total amount of positive electrode active material.

[0013] Because the positive electrode composite material of this disclosure has the above configuration, a relatively thick positive electrode composite material layer (for example, with a basis weight of 38 mg / cm²) is formed. 2 This can suppress the decrease in the discharge rate characteristics of a lithium secondary battery having the above-mentioned positive electrode composite layer. This effect is presumed to be due to the following reasons, but is not limited thereto. In the present disclosure, the amount of LiOH is 0.15% by mass or more and 0.35% by mass or less. Thereby, when drying a relatively thick coating of the positive electrode active material paste containing the positive electrode active material of the present disclosure, PVDF is likely to adhere to the surface of the particles of the positive electrode active material. CNT has a high affinity for PVDF. CNT is likely to adhere to the PVDF attached to the particles of the positive electrode active material. Therefore, migration is unlikely to occur. As a result, it is presumed that the positive electrode active material of the present disclosure can suppress a decrease in the discharge rate characteristics of a lithium secondary battery having a relatively thick positive electrode active material layer.

[0014] (1.1) Positive electrode active material The positive electrode active material is composed of a lithium transition metal oxide. The lithium transition metal oxide contains nickel, cobalt, and manganese and may be represented by the following formula (I). Formula (I): LiNi x Co y Mn z O2 In formula (I), the relationships of 0.5 ≦ x < 1, 0 < y, 0 < z, and x + y + z = 1 are satisfied. x may be 0.60 or more, may be 0.80 or more, or may be 0.90 or more. The positive electrode active material may be composed of one type of lithium transition metal oxide or may be composed of at least two types of lithium transition metal oxides.

[0015] The amount of LiOH contained in the positive electrode active material is 0.15% by mass to 0.35% by mass with respect to the total amount of the positive electrode active material, may be 0.19% by mass to 0.35% by mass, may be 0.23% by mass to 0.35% by mass, or may be 0.31% by mass to 0.35% by mass. The method for measuring the amount of LiOH is the same as the method described in the examples.

[0016] The average particle size of the positive electrode active material may be 1 μm to 20 μm, or 5 μm to 15 μm. The "average particle size" refers to the particle size (median diameter) corresponding to the cumulative frequency of 50% by volume from the smaller particle size side in the volume-based particle size distribution based on laser diffraction and light scattering methods.

[0017] The content of the positive electrode active material may be more than 50% by mass of the total amount of the positive electrode composite material, or it may be 80% by mass or more and 97% by mass or less.

[0018] (1.2) Binding agent The binder may include PVDF, and may also include other binders different from PVDF. The binder may be PVDF. Examples of other binders include carboxymethylcellulose, rubber-based binders (e.g., butadiene rubber, hydrogenated butadiene rubber, etc.), fluoride-based binders (e.g., polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE), etc.), polyolefin-based thermoplastic resins (e.g., polyethylene, polypropylene, polystyrene, etc.), imide-based resins (e.g., polyimide, polyamide-imide, etc.), amide-based resins (e.g., polyamide, etc.), acrylic-based resins (e.g., polymethyl acrylate, polyethyl acrylate, etc.), methacrylic-based resins (e.g., polymethyl methacrylate, polyethyl methacrylate, etc.), etc.

[0019] The binder content may be 0.1% to 10% by mass relative to the total amount of the positive electrode composite material.

[0020] (1.3) Conductive additives The conductive additive may contain CNTs and further contain carbon materials. The conductive additive may also be CNTs. The CNTs have a shape in which graphene sheets are rolled into single-walled or multi-walled tubular shapes. The CNTs include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes (hereinafter also referred to as "MWCNTs"). Examples of carbon materials include carbon black (e.g., acetylene black, furnace black, Ketjen black, etc.), coke, graphite, etc.

[0021] The content of the conductive additive may be 0.1% to 10% by mass relative to the total amount of the positive electrode composite material.

[0022] (2) Lithium secondary batteries The lithium secondary battery of this disclosure comprises a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode composite layer containing the positive electrode composite of this disclosure. As a result, the lithium secondary battery of this disclosure can suppress a decrease in discharge rate characteristics even if the thickness of the positive electrode composite layer is relatively thick.

[0023] The structure of a lithium secondary battery is not particularly limited, and examples include wound type and stacked type. In the wound type, a straight-rubber-shaped electrode body in which a negative electrode, separator, and positive electrode are stacked in that order is wound up. In the stacked type, a single-wafer-shaped electrode body in which a negative electrode, separator, and positive electrode are stacked in that order is stacked.

[0024] (2.1) Positive electrode The positive electrode has a positive electrode composite layer and may further have a positive electrode current collector (e.g., aluminum foil). The positive electrode composite layer is laminated on at least one main surface of the positive electrode current collector. The positive electrode composite layer includes the positive electrode composite of the present disclosure.

[0025] The basis weight of the aforementioned positive electrode composite layer is 35 mg / cm². 2 ~46 mg / cm³ 2 This is preferable. This further suppresses the decrease in the discharge rate characteristics of the lithium secondary battery.

[0026] The basis weight of the aforementioned positive electrode composite layer is 35 mg / cm². 2 ~40 mg / cm³ 2 This is more preferable. This further suppresses the decrease in the discharge rate characteristics of the lithium secondary battery.

[0027] (2.2) Negative electrode The negative electrode has a negative electrode current collector and may or may not have a negative electrode composite layer.

[0028] If the negative electrode does not have a negative electrode composite layer, the negative electrode current collector includes a main surface on which lithium metal is deposited during charging. Specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode current collector during charging, causing lithium metal to deposit. The deposited lithium metal dissolves as lithium ions in the non-aqueous electrolyte during discharge. The lithium ions contained in the non-aqueous electrolyte may be at least one of ions derived from the lithium salt described later, and ions supplied from the positive electrode active material during charging.

[0029] If the negative electrode has a negative electrode composite layer, the negative electrode composite layer is laminated on at least one main surface of the negative electrode current collector (e.g., copper foil). The negative electrode composite layer contains a negative electrode layer active material (e.g., carbon (e.g., natural graphite, artificial graphite), a compound that can be alloyed with lithium (e.g., silicon, tin, etc.)) capable of intercalating and releasing charge carriers. The negative electrode composite layer may further contain, if necessary, a conductive additive (e.g., acetylene black), a binder (e.g., polyvinylidene fluoride), an electrolyte support salt (lithium salt) to enhance ionic conductivity, a polymer electrolyte, or an additive (e.g., trifluoropropylene carbonate). The negative electrode may have a known configuration.

[0030] (2.3) Separator The separator maintains the distance between the positive and negative electrodes to prevent contact short circuits and allows lithium ions to pass through. Examples of separators include porous resin sheets or nonwoven fabrics. Examples of porous resin sheets include polyolefins (polypropylene, polyethylene, etc.). Examples of nonwoven fabrics include polypropylene, polyethylene terephthalate, methylcellulose, etc. The separator may also have a known configuration.

[0031] (2.4) Non-aqueous electrolyte The non-aqueous electrolyte may contain a non-aqueous solvent and a lithium salt. Examples of lithium salts include LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2. Examples of non-aqueous solvents include cyclic carbonates (e.g., ethylene carbonate), linear carbonates (e.g., dimethyl carbonate, ethyl methyl carbonate), cyclic esters (e.g., γ-butyllactone, γ-valerolactone), linear esters (e.g., methyl formate, methyl acetate), and ethers (e.g., dimethoxyethane, ethoxymethoxyethane). The non-aqueous electrolyte may also contain additives (e.g., vinylene carbonate, lithium bis(oxalato)borate, etc.).

[0032] (2.5) Case Lithium-ion batteries typically have a case. The case houses the positive electrode, negative electrode, separator, and non-aqueous electrolyte. The case is not particularly limited and may include laminate film (e.g., aluminum sheet), battery cans (e.g., cylindrical, prismatic, coin-shaped, etc.), etc. [Examples]

[0033] The present disclosure will be described in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0034] [1] Calculation of LiOH content of positive electrode active material The amount of LiOH in the positive electrode active material was calculated using the neutralization titration method. Specifically, 10 g of positive electrode active material was mixed with 100 mL of pure water and stirred for 1 minute to obtain a dispersion. The dispersion was filtered by suction to obtain a filtrate. 25 μL of hydrochloric acid was added dropwise to the filtrate every 30 seconds for titration. By the neutralization titration method, the amount of OH in lithium hydroxide (LiOH) was determined to be... - The quantity was determined. OH - and lithium ions (Li + ) molar ratio (OH - moles / Li + Using the fact that the moles of OH are 1 / 1, - From the amount of Li+ The amount (amount of lithium derived from LiOH) was calculated. - The amount and Li + The mass of LiOH was calculated using the amount of [the specified quantity]. The ratio of the mass of LiOH to the total amount of positive electrode active material (i.e., "amount of LiOH") was calculated.

[0035] When the positive electrode active material consists of only one type (positive electrode active material A), the amount of LiOH in positive electrode active material A was defined as "the amount of LiOH in the positive electrode active material." When the positive electrode active material consists of two types (positive electrode active material A and positive electrode active material B), the value calculated by the following formula (1) was defined as the "LiOH content of the positive electrode active material." In formula (1), "mixing ratio of positive electrode active material A" refers to the ratio of the mass of positive electrode active material A to the total amount of positive electrode active material. "Mixing ratio of positive electrode active material B" refers to the ratio of the mass of positive electrode active material B to the total amount of positive electrode active material.

[0036] Equation (1): LiOH content of positive electrode active material = LiOH content of positive electrode active material A × blending ratio of positive electrode active material A + LiOH content of positive electrode active material B × blending ratio of positive electrode active material B

[0037] [2] Relationship between the amount of LiOH in the positive electrode active material and the zeta potential The positive electrode active materials shown in Table 1 were prepared. The positive electrode active material and N-methylpyrrolidone (NMP) (solvent for the positive electrode paste) were mixed to obtain the first solution. The solid content concentration NV (Non-Volatile) of the positive electrode active material in the first solution was 75% by mass. The positive electrode active material, polyvinylidene fluoride (PVDF) (binding agent for the positive electrode paste) and N-methylpyrrolidone (NMP) (solvent for the positive electrode paste) were mixed to obtain the second solution. The solid content concentration NV (Non-Volatile) of the positive electrode active material in the second solution was 75% by mass. The PVDF content was 1.4% by mass relative to the total of 100% by mass of the positive electrode active material and PVDF. The zeta potential V1 of the first solution and the zeta potential V2 of the second solution were measured by ultrasonic attenuation zeta potential measurement. The zeta potential difference ΔV (mV) (absolute value) was calculated using the following formula (2). The calculation results are shown in Figure 1 and Table 1.

[0038] Equation (2): Zeta potential difference ΔV(mV) = |Zeta potential of solution 1 V1 - Zeta potential of solution 2 V2|

[0039] [Table 1]

[0040] A "single particle" refers to a particle that possesses the continuity of a single crystal (i.e., a primary particle). A "polycrystalline" refers to a particle that does not possess the continuity of a single crystal. Specifically, a "polycrystalline" refers to a secondary particle formed by the sintering of primary particles.

[0041] As shown in Table 1, it was found that the zeta potential difference ΔV tends to be smaller as the amount of LiOH in the positive electrode active material increases. PVDF has fluorine atoms (F) with high electronegativity. Therefore, when PVDF is adsorbed onto the surface of the particles of the positive electrode active material, the repulsion between the particles of the positive electrode active material increases, and the zeta potential of the second solution increases. When PVDF exists alone without adhering to the positive electrode active material, the repulsion between the particles of the positive electrode active material is small. Therefore, when the amount of LiOH is high, PVDF does not easily adhere to the surface of the active material, and the zeta potential difference is small. As a result, migration is more likely to occur. CNTs and PVDF have a high affinity for each other. Therefore, when PVDF is present on the surface of the particles of the positive electrode active material, CNTs also tend to adhere to the surface of the particles of the positive electrode active material.

[0042] As shown in Table 1, the amount of LiOH in the positive electrode active material generally tends to be higher as the Ni ratio of the active material is higher. Due to differences in the synthesis methods of the positive electrode active materials, the positive electrode active materials are generally classified into powders with a high content of primary particles (single particle type) and powders with a high content of secondary particles (polycrystalline type). When the positive electrode active material is of the polycrystalline type, the amount of LiOH in the positive electrode active material tends to be high. When the Ni ratio of the positive electrode active material is high and the positive electrode active material is of the polycrystalline type, the amount of LiOH in the positive electrode active material tends to be high. If the amount of LiOH in the positive electrode active material is too high (more than 0.35% of the LiOH amount), it becomes difficult for PVDF to adhere to the surface of the particles of the positive electrode active material. As a result, migration is likely to occur. If there is too much LiOH on the surface of the particles of the positive electrode active material, PVDF is difficult to adhere to the surface of the particles of the positive electrode active material.

[0043] [3] Examples and Comparative Examples [3.1] Examples 1 to 7 and Comparative Examples 1 to 4 The positive electrode active materials A and B shown in Table 2 were prepared. The composition of each of the positive electrode active materials A and B was LiNi x Co y Mn z O2 (0.60 ≦ x < 1, 0 < y, 0 < z, x + y + z = 1). As the positive electrode active material, a powder obtained by mixing the positive electrode active materials A and B at the ratios shown in Table 2 was used.

[0044] The positive electrode active material shown in Table 2, carbon nanotubes (MWCNT) as a conductive aid, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a solvent were mixed to prepare a positive electrode composite paste. The mass ratio of the positive electrode active material, MWCNT, and PVDF (positive electrode active material: MWCNT: PVDF) was 97.8:0.7:1.4. The positive electrode composite paste was applied to an aluminum foil (thickness: 30 μm) with a doctor blade to form a coating. The coating was dried under the conditions of 90°C to 125°C for 30 minutes. The dried coating was pressed by a roll press so that the electrode density became 3.2 g / cc. Thereby, a positive electrode was obtained.

[0045] A three-layer porous resin sheet (thickness: 16 μm) was prepared as the separator. The porous resin sheet consists of a polypropylene (PP) layer, a polyethylene (PE) layer, and another PP layer, laminated in that order. Li metal was prepared as the negative electrode.

[0046] The positive electrode, separator, and negative electrode were stacked in this order and housed in a battery case having an electrolyte injection port. A non-aqueous electrolyte was supplied through the injection port of the battery case, and the port was sealed to fabricate a coin cell (lithium secondary battery). The non-aqueous electrolyte contained a mixed solvent, LiPF6 as a supporting salt, and vinylene carbonate (VC) as an additive. The mixed solvent contained ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The volume ratio of EC, DMC, and EMC (EC:DMC:EMC) was 30:40:30. LiPF6 was dissolved at a concentration of 1.1 mol / L. The VC content was 1% by mass relative to the total amount of the non-aqueous electrolyte.

[0047] [3.2] Activation of lithium secondary batteries The initial charging method was a constant current-constant voltage method. Constant current (CC) charging was performed at a current value of 0.1C until the voltage reached 4.25V. Constant voltage charging was then performed for 3 hours. Finally, the lithium secondary battery was discharged at a constant current value of 0.1C until the voltage reached 2.5V to activate it.

[0048] [3.3] 1C discharge maintenance rate of lithium secondary batteries The charging and discharging were performed using a constant current method. The current rate was set to 0.1C or 1.0C. Specifically, in [2.2] above, the total discharge capacity (hereinafter also referred to as "0.1C-CC discharge capacity") was measured when the battery was discharged from 4.25V to 2.5V at a current of 0.1C. Next, constant current (CC) charging was performed up to 4.25V at a current of 1C, constant voltage charging was performed until the constant voltage charging time was 3 hours, and then the battery was discharged down to 2.5V at a current of 1C using the constant current method. The total discharge capacity at this time (hereinafter also referred to as "1C-CC discharge capacity") was measured. The "1C discharge maintenance charge rate (%)" was calculated using the following formula (3). The calculation results are shown in Figure 2 and Table 2. The acceptable range for the 1C discharge maintenance charge rate is 73% or higher.

[0049] Formula (3): 1C discharge maintenance charge rate (%) = (0.1C-CC discharge capacity / 1.0C-CC discharge capacity) × 100

[0050] [3.4] Results [Table 2]

[0051] Even if the LiOH content is greater than 0.35%, if the positive electrode base weight is thin (for example, if the positive electrode base weight is 38 mg / cm³), 2 If the value is less than [value missing], the 1C discharge maintenance rate tends to be high. When the positive electrode basis weight increases (the thickness of the positive electrode increases), ion diffusion within the positive electrode becomes the rate-limiting factor. Therefore, the effect of migration becomes apparent, and the decrease in discharge rate tends to be greater.

[0052] [3.4.1] Comparative Example 1 to Comparative Example 4 In Comparative Examples 1 to 4, the LiOH content of the positive electrode active material was not within the range of 0.15% to 0.35%. Therefore, the positive electrode basis weight was 38 mg / cm³. 2 In the above tests, the 1C discharge maintenance rate was not 73% or higher. These results indicate that the cathode composite materials of Comparative Examples 1 to 4 are not cathode composite materials that can suppress the decrease in discharge rate characteristics of lithium secondary batteries having a relatively thick cathode composite layer.

[0053] [3.4.2] Examples 1 to 7 In Examples 1 to 7, the LiOH content of the positive electrode active material was in the range of 0.15% to 0.35%. Therefore, the positive electrode basis weight was 38 mg / cm³. 2 In the above tests, the 1C discharge maintenance rate was 73% or higher. These results indicate that the cathode composite materials of Examples 1 to 7 are cathode composite materials that can suppress the decrease in discharge rate characteristics of lithium secondary batteries having a relatively thick cathode composite layer.

[0054] [3.5] Drying temperature of the coating film It was found that migration is less likely to occur by adjusting the LiOH content of the positive electrode active material to within the range of 0.15% to 0.35%. Therefore, it is possible to increase the drying temperature of the coating film when preparing the positive electrode composite layer. Positive electrodes were prepared with different coating film drying temperatures for two configurations: one with a LiOH content of 0.35% and another with a LiOH content of 0.37%, and the 1C discharge maintenance rate was calculated. The results are shown in Figure 3.

[0055] As shown in Figure 3, when the LiOH content of the positive electrode active material was 0.35%, the 1C discharge maintenance rate remained high even when the drying temperature was increased. When the LiOH content of the positive electrode active material was 0.37%, increasing the drying temperature resulted in a greater degree of migration, and the 1C discharge maintenance rate was low.

Claims

[Claim 1] A device comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, The positive electrode has a positive electrode composite layer containing a positive electrode composite material, The positive electrode composite material contains a positive electrode active material composed of a lithium transition metal oxide having a layered crystalline structure, a binder, and a conductive additive. The lithium transition metal oxide comprises nickel, cobalt, and manganese, and is represented by the following formula (I): The aforementioned binder contains polyvinylidene fluoride, The aforementioned conductive additive includes carbon nanotubes, The amount of lithium hydroxide contained in the positive electrode active material is 0.19% by mass or more and 0.35% by mass or less relative to the total amount of the positive electrode active material. A lithium secondary battery in which the basis weight of the positive electrode composite layer is 35 mg / cm² to 38 mg / cm². Formula (I): LiNix Co y Mn z O 2 In equation (I), the relationships 0.6 ≤ x < 1, 0 < y, 0 < z, and x + y + z = 1 are satisfied.

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