The invention relates to a method of operating a zinc-bromine battery, especially at a high temperature, comprising adding 1-n-butyl-2-methyl-pyridinium bromide to the electrolyte of said battery, and charging or discharging said cell. Also provided is the use of 1-n-butyl-2-methyl-pyridinium bromide as an additive in a zinc-bromine battery operating at a temperature above 30° C., and an aqueous concentrate with high content of 1-n-butyl-2-methyl-pyridinium bromide.