Suppressing Unfavorable Interfacial Reactions Using Polyanionic Oxides as Efficient Buffer Layers: Low-Cost Li3PO4 Coatings for Sulfide-Electrolyte-Based All-Solid-State Batteries

The poor electrochemical performance of all solid-state batteries (ASSBs) that use sulfide electrolytes can be attributed to undesirable side reactions at the cathode/sulfide-electrolyte interface; this issue can be addressed via surface coating. Ternary oxides such as LiNbO₃ and Li₂ZrO₃ are generally used as coating materials because of their high chemical stabilities and ionic conductivities. However, their relative high cost discourages their use in mass production. In this study, Li₃PO₄ was introduced as a coating material for ASSBs, because phosphates possess good chemical stabilities and ionic conductivities. Phosphates also prevent the exchange of S^2- and O^2- in the electrolyte and cathode and, thus, inhibit interfacial side reactions caused by ionic exchange, because they contain the same anion (O^2-) and cation (P⁵⁺) species as those present in the cathode and sulfide electrolyte, respectively. Furthermore, the Li₃PO₄ coatings can be prepared using low-cost source materials such as polyphosphoric acid and lithium acetate. We investigated the electrochemical performance of the Li₃PO₄-coated cathodes and found that the Li₃PO₄ coating significantly improved the discharge capacities, rate capabilities, and cyclic performances of the all-solid-state cell. While the discharge capacity of the pristine cathode was ∼181 mAh·g^-1, that of 0.15 wt % Li₃PO₄-coated cathode was ∼194-195 mAh·g^-1. And the capacity retention of the Li₃PO₄-coated cathode over 50 cycles was much superior (∼84-85%) to that of the pristine sample (∼72%). Simultaneously, the Li₃PO₄ coating reduced the side reactions and interdiffusion at the cathode/sulfide-electrolyte interfaces. The results of this study demonstrate the potential of low-cost polyanionic oxides, such as Li₃PO₄, as commercial coating materials for ASSBs.