Composition Dependence of Ionic Conductivity in LiSiPO(N) Thin-Film Electrolytes for Solid-State Batteries

The current commercial standard thin film electrolyte LiPON is the limiting factor for the further development of microbatteries due to its low Li+ ionic conductivity (2 × 10–6 S/cm). In order to produce more conductive electrolytes and elucidate the synthesis–properties interrelation for this system, we sputtered thin films from single-phase ceramic targets of composition Li3+xSixP1–xO4 under Ar and N2 atmospheres. The amorphous thin films produced under Ar (LiSiPO) are more conducting than the crystalline target materials (amorphization effect). Furthermore, the fact that the resulting amorphous films contain both phosphate and silicate building units (mixed-former effect) increases the conductivity to approximately the values of LiPON (10–6 S/cm). Reactive sputtering under N2 leads to oxynitride (LiSiPON) thin films with a maximum Li+ ionic conductivity of 2.06 × 10–5 S/cm (Ea = 0.45 eV), about 1 order of magnitude higher than LiPON, in accordance with previous works. These results are discussed in the context of available literature in order to elucidate the effect of Si:P and Li:(Si + P) compositional ratios on ionic conductivity. Finally, we expose a target-dependent effect of nonstoichiometric, Li-deficient depositions that is a current impediment to sputtering of highly Li+-conductive targets.