A method of forming an electrolyte solution involves combining ammonium tetrafluoroborate and a quaternary ammonium halide in a liquid solvent to form a quaternary ammonium tetrafluoroborate and an ammonium halide. The ammonium halide precipitate is removed from the solvent to form an electrolyte solution. The reactants can be added step-wise to the solvent, and the method can include using a stoichiometric excess of the ammonium tetrafluoroborate to form a substantially halide ion-free electrolyte solution. Filtration can be done at low temperatures to reduce the amount of excess bromide in the resulting electrolyte.