LiPO2F2, an electrolyte salt additive for batteries, is manufactured by the reaction of POF3, PF5 or mixtures thereof, with Li3PO4 forming a reaction mixture comprising LiPO2F2. When POF3 is applied, the reaction mixture which contains essentially only LiPO2F2 is preferably extracted from the reaction mixture with a solvent which also is applicable as solvent for lithium ion batteries. If PF5 is applied, then, depending on the molar ratio of PF5 and Li3PO4, the reaction mixture also contains LiF and/or LiPF6. To isolate pure LiPO2F2 from LiF, the reaction mixture containing essentially only LiPO2F2 and LiF may for example, be extracted with dimethoxyethane, acetone, dimethyl carbonate or propylene carbonate. To isolate pure LiPO2F2 from LiPF6, the reaction mixture containing essentially only these constituents is preferably extracted with a solvent which also is applicable as solvent for the LiPF6 in lithium ion batteries to dissolve and remove LiPF6.