Negative electrode for lithium secondary cell and lithium secondary cell
A negative electrode for a lithium secondary cell having a collector composed of an electroconductive metal foil and, provided on the surface thereof, an active material layer containing active material particles containing silicon and/or a silicon alloy and a binder, characterized in that the binder has mechanical characteristics of a tensile strength of 50 N/mm 2 or more, an elongation at break of 10% or more, a strain energy density of 2.5×10 −3 J/mm 3 or more and a coefficient of elasticity of 10000 N/mm 2 or less, and preferably characterized in that the collector has mechanical characteristics of a tensile strength of 80 N/mm 2 or more, a proportional limit 30 N/mm 2 or more, an elongation at break of 1.0% or more and an elastic elongation limit of 0.03% or more.
1. A negative electrode for a rechargeable lithium battery which includes a conductive metal foil current collector and an active material layer provided on a surface of the current collector and comprising a polyimide binder and particles of active material containing silicon and/or a silicon alloy;
said negative electrode characterized in that said current collector has mechanical properties of at least 80 N/mm 2 tensile strength, at least 30 N/mm 2 proportional limit, at least 1.0% elongation at break and at least 0.03% elastic elongation limit and said binder has mechanical properties of at least 50 N/mm 2 tensile strength, at least 10% elongation at break, at least 2.5 ×10 −3 J/mm 3 strain energy density and up to 10,000 N/mm 2 elastic modulus;
after provision of said active material layer on the surface of said current collector, heat treatment being carried out at a temperature higher than a glass transition temperature and lower than a decomposition temperature of said binder to improve adhesion of the binder to the current collector.
2. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said mechanical properties are imparted to the current collector by a thermal history of the sintering treatment.
3. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said mechanical properties are imparted to said current collector by subjecting the current collector to a heat treatment before said active material layer is provided on the surface of the current collector.
4. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that a thickness X of said active material layer, a thickness Y and a surface roughness Ra of said current collector satisfy the relationships 5Y≧X and 250Ra≧X.
5. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said surface of the current collector that carries the active material layer thereon has a surface roughness Ra of at least 0.2 μm.
6. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said current collector comprises a copper foil, a copper alloy foil or a metal foil having a copper or copper alloy surface layer.
7. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said current collector comprises an electrolytic copper foil, an electrolytic copper alloy foil or a metal foil having an electrolytic copper or copper alloy surface layer.
8. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said binder has said mechanical properties after a thermal history of said heat treatment.
9. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said binder has a linear expansion coefficient of 0.1×10 −5 −30×10 −5 ° C −2 .
10. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said binder has a glass transition temperature of up to 450° C.
11. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said active material particles have a mean particle diameter of up to 10 μm.
12. The negative electrode for a rechargeable lithium battery as recited in claim 1 , characterized in that said active material layer includes a conductive powder.
13. A rechargeable lithium battery including the negative electrode as recited in claim 1 , a positive electrode containing a positive electrode material and a nonaqueous electrolyte.