The irreversible capacity during the first cycle is investigated at a carbon electrode in a Li-ion battery in 1 M LiPF6/EC:DEC (ethylene carbonate:1,2 diethyl carbonate; 1:1 volume ratio). Solvent decomposition is one of train causes of the initial irreversible capacity. A lithium phosphorus oxynitride (UPON) thin-film, which is a solid ionic conductor, is deposited on the surface of the carbon electrode by means of a radio frequency magnetron sputtering system. The effect of the UPON layer is examined with chronopotentiometry and cyclic voltammetry, as well as with scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD). The deposition of UPON on the surface of the carbon electrode results in a decrease in the initial irreversible capacity due to the suppression of solvent decomposition on the electrode surface.