A composition containing: at least 99.75% by weight of bis(fluorosulfonyl)imide lithium salt; and acetic acid in a content by mass greater than 0 and less than or equal to 400 ppm. A process for preparing the composition, including a) step of preconcentrating a composition C1 including an organic solvent OS1, water and bis(fluorosulfonyl)imide salt, to give a composition C2 including: the lithium salt of bis(fluorosulfonyl)imide in a content ranging from 35% to 50% relative to the total weight of composition C2; water in a mass content of less than or equal to 500 ppm relative to the total mass of composition C2; said preconcentration step being performed at a temperature of less than or equal to 50° C.; b) a step of concentrating composition C2; c) an optional step of crystallizing the composition obtained in step b).