Design of prototype for a solid-state lithium battery with LiFePO4 cathode and nanocomposite polymer gel electrolyte is developed. The concept of an asymmetric solid-state electrolyte was used for better compatibility of the solid electrolyte/electrode interface. According to the concept, a silica-nanoparticle-based transition layer is used facing the lithium anode; a liquid electrolyte layer; facing the cathode. The composition of the liquid electrolyte is optimized. 1 M lithium bis-trifluoromethanesulfonyl imide solution in a dioxolane/dimethoxyethane mixture (2 : 1) is shown to be the best electrolyte for the forming of a transition ion-conducting layer between the nanocomposite polymer gel electrolyte and the LiFePO4 cathode; 1 M LiBF4 in gamma-butyrolactonee, for the synthesis of a cross-linked polymer gel electrolyte, as an inert liquid medium for the radical-polymerization reaction. Comparative tests of Li/LiFePO4 battery prototypes showed the maximal capacity of the LiFePO4 cathode to be as large as 170 mA h g–1 when using a nanocomposite polymer gel electrolyte based on polyethylene glycol diacrylate with 6 wt % SiO2 with asymmetric interface.