Negative electrode for electrochemical element and a lithium ion secondary battery
The inventive negative electrode for the electrochemical element includes a negative electrode composition layer provided on at least one surface of a current collector made of a metal foil; in which the negative electrode composition layer includes material S comprising Si and a hardly-graphitizable carbon material as a negative electrode active material; in which assuming that the whole amount of the negative electrode active material contained in the negative electrode composition layer is 100 mass %, a ratio of the material S is 50 to 90 mass % and a ratio of the hardly-graphitizable carbon materials is 10 to 50 mass %. The inventive lithium ion secondary battery includes a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte liquid, and the negative electrode is constituted by the negative electrode for the electrochemical element of the present invention.
1. A lithium ion secondary battery, comprising:
a positive electrode;
a negative electrode comprising a negative electrode composition layer comprising a negative electrode active material, the negative electrode composition layer provided on at least one surface of a current collector made of a metal foil;
a separator provided between the positive electrode and the negative electrode; and
a nonaqueous electrolyte liquid,
wherein 100 mass % of the negative electrode active material contained in the negative electrode composition layer consists of:
50-80 mass % of a composite material S consisting of Si and a first carbon material;
20-50 mass % of a second carbon material of hardly-graphitizable carbon materials; and
optionally, graphite
wherein the composite material S and the second carbon material are provided separately before mixing the composite material S with the second carbon material,
wherein the lithium ion secondary battery has a charge discharge cycle capacity maintenance rate of 90 to 95%.
2. The lithium ion secondary battery according to claim 1 , wherein the hardly-graphitizable carbon material has d 002 of 0.34 to 0.42 nm in an X-ray diffraction.
3. The lithium ion secondary battery according to claim 1 , wherein SiO x (0.5≤x≤1.5) is included as the material S.
4. The lithium ion secondary battery according to claim 3 , wherein the SiO x constitutes a complex with a carbon material.
5. The lithium ion secondary battery according to claim 1 , wherein at least a part of the negative electrode active material contained in the negative electrode composition layer has been doped with Li ions.
6. The lithium ion secondary battery according to claim 1 , wherein the charge discharge cycle capacity maintenance rate is measure by a method comprising:
Step A of charging the lithium ion secondary battery by applying a constant current charge at a current value of 0.5 C to reach 4.4 V, followed by applying a constant voltage of 4.4 V to reach a current value of 0.02 C, followed by discharging the lithium ion secondary battery at a constant current of 0.2 C to reach 2.0 V, thereby obtaining a discharge capacity A;
Step B of conducing a cycle of charge and discharge with respect to the lithium ion secondary battery after the Step A, wherein the cycle comprises one cycle comprising:
charging the lithium ion secondary battery at a constant current at a current value of 1 C to reach 4.4 V;
then charging the lithium ion secondary battery at a constant voltage of 4.4 V to reach a current value of 0.05 C;
then discharging the lithium ion secondary battery at a current value of 1 C to reach a voltage of 2.0 V;
wherein said one cycle is repeated 300 times;
wherein the lithium ion secondary battery after the Step B is charged by applying a constant current charge at a current value of 0.5 C to reach 4.4 V, followed by applying a constant voltage of 4.4 V to reach a current value of 0.02 C, followed by discharging the lithium ion secondary battery at a constant current of 0.2 C to reach 2.0 V, thereby obtaining a discharge capacity B;
wherein the capacity maintenance rate is obtained by a formula below:
(capacity maintenance rate)=(discharge capacity B)/(discharge capacity A)×100.
7. The lithium ion secondary battery according to claim 6 , wherein at least a part of the negative electrode active material contained in the negative electrode composition layer has been doped with Li ions.