JP-7854663-B2 - Positive electrode for secondary batteries and secondary batteries

Inventors

• 宇賀 洋一郎
• 吉成 梨乃

Assignees

• パナソニックＩＰマネジメント株式会社

Dates

Publication Date

20260507

Application Date

20211028

Priority Date

20201030

Claims (6)

1. The device comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material and provided on the surface of the positive electrode current collector, The positive electrode active material has a layered structure and contains a lithium-containing composite oxide in which 80 atomic percent or more of the metals other than lithium is nickel. The positive electrode mixture layer contains a conductive additive containing carbon in a proportion of 1 part by mass or less per 100 parts by mass of the positive electrode active material. The material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less. The ratio Rm/Rc of the material resistance Rm (Ω·cm) of the positive electrode mixture layer to the interfacial resistance Rc (Ω· cm² ) between the positive electrode current collector and the positive electrode mixture layer is 200 or more. The aforementioned conductive additive includes carbon nanotubes, The fiber length of the carbon nanotube is 1 μm or more. A positive electrode for a secondary battery , wherein the carbon nanotube content in the positive electrode mixture layer is 0.01 parts by mass or more and 1 part by mass or less per 100 parts by mass of the positive electrode active material .
2. The positive electrode for a secondary battery according to claim 1 , wherein the ratio Rm/Rc is 500 or more.
3. The lithium-containing composite oxide is a lithium-nickel composite oxide represented by the chemical formula Li a Ni x M 1-x O 2 (where 0 < a ≤ 1.2, 0.8 ≤ x ≤ 1, and M includes at least one selected from the group consisting of Co, Al, Mn, Fe, Ti, Sr, Na, Mg, Ca, Sc, Y, Cu, Zn, Cr, and B), as described in claim 1 or 2 , for a secondary battery positive electrode.
4. The positive electrode for a secondary battery according to claim 3 , wherein x ≥ 0.85 in the chemical formula.
5. The positive electrode for a secondary battery according to any one of claims 1 to 4 , wherein the positive electrode mixture layer is provided on the surface of the positive electrode current collector in an amount of 250 g/m² or more .
6. A positive electrode for a secondary battery according to any one of claims 1 to 5 , A secondary battery comprising a separator, a negative electrode facing the positive electrode for the secondary battery via the separator, and an electrolyte.

Description

This disclosure relates to secondary batteries, and more particularly to improvements to the positive electrode used in secondary batteries. 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. The positive electrode active material for lithium-ion secondary batteries is a composite oxide of lithium and a transition metal (e.g., cobalt). Higher capacity can be achieved by replacing some of the cobalt with nickel. Furthermore, 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, and by compressing the large amount of positive electrode active material that is mounted. Patent Document 1 proposes using non-aggregated composite oxide particles having a compressive strength of 250 MPa or more as the positive electrode active material in a non-aqueous electrolyte secondary battery. This makes it possible to suppress the decrease in capacity retention rate and the increase in resistance during charge-discharge cycles, even when using a positive electrode active material containing composite oxide particles that include Ni, Co, and Li, and at least one of Mn and Al, where the ratio of Ni to the total number of moles of metal elements excluding Li is 50 mol% or more. Patent Document 2 proposes a lithium-ion secondary battery in which a lithium nickel composite oxide represented by Li y Ni (1-x) M x and carbon nanotubes are included in the positive electrode, and 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, is set to 0.5 or higher. International Publication No. 2019/026629 BrochureInternational Publication No. 2008/051667 Pamphlet This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present disclosure. The positive electrode for a secondary battery according to the embodiment of this disclosure comprises a positive electrode current collector and a positive electrode mixture layer containing a positive electrode active material and provided on the surface of the positive electrode current collector. The positive electrode active material has a layered structure and contains a lithium-containing composite oxide in which 80 atomic percent or more of the metals other than lithium are nickel. The positive electrode mixture layer contains a conductive additive containing carbon in a ratio of 1 part by mass or less per 100 parts by mass of the positive electrode active material. The material resistance Rm of the positive electrode mixture layer is 30 Ω·cm or less. The ratio Rm/Rc of the material resistance Rm (Ω·cm) of the positive electrode mixture layer to the interfacial resistance Rc (Ω· cm² ) between the positive electrode current collector and the positive electrode mixture layer is 200 or more. Here, material resistance Rm refers to the resistivity (Ω·cm) of the positive electrode mixture containing the positive electrode active material and conductive additives. Interface resistance Rc refers to the resistivity (Ω· cm² ) of the interface between the positive electrode mixture and the core (current collector) material, converted to a unit area of 1 cm² . Generally, a positive electrode mixture is formed by mixing a slurry containing materials such as positive electrode active material, conductive additives, binders, and solvents, applying it to a core such as metal foil, and drying it. During this drying process, the solvent moves to the outside of the mixture due to volatilization, and along with this movement, materials such as conductive additives also move away from the vicinity of the core. Therefore, it is thought that the interfacial resistance Rc increases as the probability of conductive additives being present near the core decreases. The phenomenon of conductive additives moving away from the vicinity of the core means that the conductive additives are unevenly distributed within the positive electrode mixture. This uneven distribution of conductive additives can hinder the movement of lithium ions and inhibit electron conduction between positive electrode active material particles, ultimately leading to non-uniformity in the charge-discharge reaction. In particular, when using a high-Ni positive electrode active material, a large charge-discharge capacity can be expected to improve battery capacity, but the amount of lithium released is also large, which destabilizes the crystal structure and can be modified, especially at the interface with the electrolyte, potentially worsening the char