Patent ID: 8557413
Filing Date: 2013-10-15
Classification: H01M,Y02E

Abstract:
1. A method of improving the cycle characteristics of a lithium secondary battery which includes (a) a negative electrode having an active material layer including silicon provided on a current collector comprising a metal which does not form an alloy with lithium, (b) a positive electrode, and (c) a nonaqueous electrolyte, comprising charging and discharging the lithium secondary battery within a range of state of charge (SOC) of negative electrode (a) where no peak corresponding to a compound of silicon and lithium is observed in an X-ray diffraction of negative electrode (a) using CuK α rays as the X-ray source, wherein said state of charge (SOC) of negative electrode (a) is calculated from the following steps: a step of forming a first negative electrode corresponding to negative electrode (a) by forming an active material layer including silicon on a current collector comprising a metal which does not form an alloy with lithium; a step of forming a first cell by using the first negative electrode as a working electrode and using a lithium metal as a counter electrode and as a reference electrode and by immersing the working electrode, the counter electrode, and the reference electrode in an electrolyte; a step of, regarding the first negative electrode, charging at two levels of constant current, which are 2 mA and 1 mA, to 0.20 V (vs. Li/Li+), then charging at a current of 0.4 mA to 0.00 V (vs. Li/Li+) using the first cell and setting capacity at an end of the charging as a state of charge (SOC) of the first negative electrode=100%; a step of forming a second negative electrode corresponding to negative electrode (a) under the same conditions as the conditions of forming the first negative electrode; a step of forming a second cell by using the second negative electrode as a working electrode and using a lithium metal as a counter electrode and as a reference electrode and by immersing the working electrode, the counter electrode, and the reference electrode in an electrolyte; a step of, regarding the second negative electrode, charging at two levels of constant current, which are 2 mA and 1 mA, to 0.20 V (vs. Li/Li+), charging at a current of 0.4 mA to a predetermined voltage level (vs. Li/Li+) using the second cell and setting capacity at the end of charging to the predetermined voltage level (vs. Li/Li+) as the capacity of the second negative electrode; and calculating a ratio of the capacity of the second negative electrode to the capacity of the first negative electrode in terms of percentage, and then setting the calculated percentage as said state of charge (SOC) of negative electrode (a).