A separator for a battery in lithium. The separator is a thick membrane, of around 60 to 120 .mu.m in polyvinylidene fluoride copolymer, which has considerable porosity, of around 50 to 90%, with pores of size between 1 and 10 .mu.m. With these characteristics, it is possible to optimize the operation of a battery with an electrode in, e.g., Li.sub.4Ti.sub.5O.sub.12, in particular for rapid charges and discharges, in other words in power applications.