An equimolar mixture of fluoroborate salts: LiBF4 and lithium difluoro(oxalate)borate LiBF2(C2O4) (MIX-LiFBs) was obtained from a simple one-step reaction of lithium oxalate and boron fluoride. Voltamperommetry shows that the salt obtained is stable in the potential range of 4.9V. Impedance measurements of liquid electrolytes involving imidazolium ionic liquid and aliphatic carbonates have been carried out, which show the highest ionic conductivity of the order of 10−3 Scm−1 (and low activation energy of 0.14eV) when using carbonates as the solvent. The mixture of fluoroborate salts MIX-LiFBs used as a component of solid polymer electrolytes provides much higher ionic conductivity values at high salt concentrations in “polymer-in-salt systems”. The conductivity of solid polymer electrolytes was considerably increased by adding a low-molecular-weight organic plasticizer.