Electrochemical Behavior of Drawn Thin-Film Vitreous Lithium Metaphosphate

Lithium metaphosphate (LiPO3) thin-film glassy solid-state electrolyte (GSSE) ribbons with thicknesses varying from 35 to 800 μm have been drawn for the first time through softening and drawing of a cast and annealed preform of LiPO3 glass. The short-range order structure and composition of the thin-film glasses were confirmed to be identical to the bulk glass LiPO3 using Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) was used to investigate the Li ionic conductivity of the thin-film GSSE samples and was essentially the same as that of the bulk LiPO3 glass. A model of area specific resistance (ASR) as a function of film thickness and temperature was generated and compared to resistances determined through equivalent circuit fitting of EIS generated Nyquist plots. The generated model compared well to the experimentally determined values. Symmetric cell cycling was conducted on the 50 μm thick samples to determine the cycling behavior and the durability of the electrolyte under applied voltage and sustained current. Evidence of the electrochemical stability of the LiPO3 thin-film GSSE against lithium metal was observed in the symmetric cell cycling as nearly perfectly flat and ohmic behavior cycling plateaus were found over a range of current densities. Prior to shorting, a critical current density of 65 μA/cm2 was obtained at 90 °C with a direct current (dc) Li ionic conductivity of 10–6.5 [Ω·cm]−1. While these current densities are low, this is the first report of a thin-film GSSE drawn into thicknesses of that required for high performance all-solid-state lithium batteries, and the proven stability and durability of this oxide GSSE is strong evidence that drawn thin-film GSSE materials, especially those with higher conductivities such as the well-studied sulfide glasses, are viable materials from which to create thin, easily processable solid-state electrolytes (SSEs) and warrant further research.