Suppressing Unfavorable Interfacial Reactions Using Polyanionic Oxides as Efficient Buffer Layers: Low-Cost Li 3 PO 4 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 LiNbO 3 and Li 2 ZrO 3 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, Li 3 PO 4 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 (P 5+ ) species as those present in the cathode and sulfide electrolyte, respectively. Furthermore, the Li 3 PO 4 coatings can be prepared using low-cost source materials such as polyphosphoric acid and lithium acetate. We investigated the electrochemical performance of the Li 3 PO 4 -coated cathodes and found that the Li 3 PO 4 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 % Li 3 PO 4 -coated cathode was ∼194-195 mAh·g -1 . And the capacity retention of the Li 3 PO 4 -coated cathode over 50 cycles was much superior (∼84-85%) to that of the pristine sample (∼72%). Simultaneously, the Li 3 PO 4 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 Li 3 PO 4 , as commercial coating materials for ASSBs.