The method of thermodynamic simulation was used to study chemical interactions in the cathode half-cell of lithium-ion battery, which contained LiCoO2 as cathode active material and a LiPF6 solution as the electrolyte. It was shown that in the temperature range 298–400K in thermodynamic equilibrium state a layer of solid products of chemical reaction formed on the cathode/electrolyte interface. The layer predominantly consisted of LiF and LiPO3 in the molar ratio about 2:1 (corresponding to the volume ratio 2:3). In equilibrium state also some soluble interaction products formed, namely: CoF2 (considerable quantity), POF3 and PF5. The concentration of the later two substances was small, but it increased with increase of the temperature. Furthermore, in equilibrium state oxygen gas formed as a product of chemical interactions in cathode half-cell. Thermodynamic characteristics of Co2O3 and LiCoO2 compounds were determined using a set of calculation methods.