Investigations into the superionic glass phase of Li4PS4I for improving the stability of high-loading all-solid-state batteries

Abstract

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 Li_1+x(Ni_1−y−zCo_yMn_z)_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 mg_NCM622 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.