In this paper, we analyse ion association, the formation of high ionic aggregates and phase separation in amorphous solvent-free polymer electrolytes based on a butadiene and acrylonitrile (PBAN) copolymer and lithium salts having different anion geometries -LiClO4, LiAsF6 and LiBr. The analysis is carried out on the basis of the results of quantum chemical calculations at the RTF + MP2/6-311G(star star) level of theory using the GAMESS (US) and Firefly software packages. It is established that the solubility of the salts under study in PBAN is strongly correlated with the dimensionality of the most stable high ionic aggregates {Li+X-}(m) (where X- = Br-, ClO4-, AsF6- and m = 1-6): the formation of 3D species causes salt precipitation, while the 1D and 2D species determine the high solubility of the salts. On the basis of thermodynamic considerations, we conclude that ion clustering in PBAN-based electrolytes can be attributed to the existence of metastable supersaturated solutions near the limit of salt solubility. These ion clusters can be considered as fluctuations in salt concentration stabilised through the coordination of nitrile groups being part of polynitriles.