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 S2- and O2- in the electrolyte and cathode and, thus, inhibit interfacial side reactions caused by ionic exchange, because they contain the same anion (O2-) 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.