Patent ID: 8628873

Claim:
A lithium ion secondary battery with an alloy based negative electrode containing a separator, wherein the separator has a dynamic friction coefficient in a machine direction and/or in a transverse direction of at least one surface is 0.1 or more and 0.4 or less, and wherein the dynamic friction coefficient is measured for a sample with a sample size of a width of 50 mm and a length in a measurement direction of 200 mm for three times in each of the machine direction and/or transverse direction by using a friction tester KES-SE manufactured by KATO TECH Co., Ltd. on the measuring condition that there is: a load of 50 g; a contactor area of 10 (10=100 mm 2 (20 piano wires made of 0.5 mmφ hard stainless steel SUS304 wound without any space between and overlapping with each other)); a contactor forwarding speed of 1 mm/sec; a tensile force of 6 kPa; a temperature of 25° C.; and a humidity of 50%; and the average of the resultant values was determined, and wherein the alloy based negative electrode comprises at least one of a tin based alloy element and a silicon based alloy element which can occlude and release lithium as a negative electrode active substance.