Investigations into the Superionic Glass Phase of Li4PS4I for Improving Stability of High-Loading All-Solid-State Batteries

In recent years, investigations into improving the performance of bulk-type solid-state batteries (SSBs) have attracted much attention. This is due, in part, to the fact that they offer an opportunity to outperform the present Li-ion battery technology in terms of energy density. Ni-rich Li1+x(Ni1−y−zCoyMnz)1−xO2 (NCM) and lithium-thiophosphate-based solid electrolytes appear to be a promising material combination for application at the cathode side. Here, we report about exploratory investigations into the 1.5Li2S/0.5P2S5/LiI phase system and demonstrate that a glassy solid electrolyte has more than an order of magnitude higher room-temperature ionic conductivity than the crystalline counterpart, tetragonal Li4PS4I with the P4/nmm space group (∼1.3 versus ∼0.2 mS cm−1). In addition, preliminary results show that usage of the glassy 1.5Li2S–0.5P2S5–LiI in pellet stack SSB cells with an NCM622 (60% Ni content) cathode and a Li4Ti5O12 anode leads to enhanced capacity retention when compared to the frequently employed argyrodite Li6PS5Cl solid electrolyte. This indicates that, apart from interfacial instabilities, the stiffness (modulus) of the solid electrolyte and associated mechanical effects may also impact significantly the long-term performance. Moreover, SSB cells with the glassy 1.5Li2S–0.5P2S5–LiI and high-loading cathode (∼22 mgNCM622 cm−2) manufactured using a slurry-casting process are found to cycle stably for 200 cycles at C/5 rate and 45 °C, with areal capacities in excess of 3 mA h cm−2.