Li3PO4 Coated Li[Ni0.75Co0.1Mn0.15]O2 Cathode for All-Solid-State Batteries Based on Sulfide Electrolyte

Surface coating of cathodes is an essential process for all-solid-state batteries (ASSBs) based on sulfide electrolytes as it efficiently suppresses interfacial reactions between oxide cathodes and sulfide electrolytes. Based on computational cal-culations, Li 3 PO 4 has been suggested as a promising coating material because of its higher stability with sulfides and its optimal ionic conductivity. However, it has hardly been applied to the coating of ASSBs due to the absence of a suitable coating process, including the selection of source material that is compatible with ASSBs. In this study, polyphosphoric acid (PPA) and (NH 4 ) 2 HPO 4 were used as source materials for preparing a Li 3 PO 4 coating for ASSBs, and the properties of the coating layer and coated cathodes were compared. The Li 3 PO 4 layer fabricated using the (NH 4 ) 2 HPO 4 source was rough and inhomogeneous, which is not suitable for the protection of the cathodes. Moreover, the water-based coating solution with the (NH 4 ) 2 HPO 4 source can deteriorate the electrochemical performance of high-Ni cathodes that are vulnerable to water. In contrast, when an alcohol-based solvent was used, the PPA source enabled the formation of a thin and homogeneous coating layer on the cathode surface. As a consequence, the ASSBs containing the Li 3 PO 4 -coated cathode prepared by the PPA source exhibited significantly enhanced discharge and rate capabilities compared to ASSBs containing a pristine cathode or Li 3 PO 4 -coated cathode prepared by the (NH 4 ) 2 HPO source.