Effect of Processing Conditions of 75Li2S-25P2S5 Solid Electrolyte on its DC Electrochemical Behavior

The effect of processing conditions of the 75Li2S-25P2S5 (LPS) on the Li/LPS DC electrochemical stability was investigated. LPS was densified by compacting at room temperature, and hot pressed between 130-300 °C, 47 MPa. The relative density (80% for all samples) was not affected by the hot pressing temperature, which was likely due to insufficient bulk diffusion (at the pressure used within the temperature range selected) to promote densification. The highly conductive meta-stable crystalline phase (thio-LiSICON III analog) precipitated from the mother glass when hot pressing between 170 − 250 °C and the low conductivity Li3PS4 phase formed when hot pressing at 300 °C. The Li/LPS DC electrochemical stability was characterized as a function of current density between 0.01-1.0 mA cm−2. Two phenomena were observed; first, as the current density increased, deviation from Ohmic behavior was observed, manifested in an increase in output potential difference. Second, as the current density was further increased, a drop in cell potential was observed followed by an output potential difference instability; likely consistent with short-circuiting caused by Li metal propagation. The highest critical current density, at which short circuit occurs, resulted when hot pressing at 170 °C, reaching 1.0 mA cm−2. In general, obtaining LPS glass-ceramic through hot pressing increased the rate at which current can be passed through the cell. It is believed that further atomic/microstructure optimization can improve the critical current density of LPS.