Comparison of LiTaO3 and LiNbO3 Surface Layers Prepared by Post- and Precursor-Based Coating Methods for Ni-Rich Cathodes of All-Solid-State Batteries

Surface coating is essential for the cathode materials applied in all-solid-state batteries (ASSBs) based on sulfide electrolytes because of the instability of the cathode/sulfide interface. In contrast with those for general lithium ion batteries (LIBs) using a liquid electrolyte, the coating materials for ASSBs require different functional properties such as high ionic conductivity, low reactivity with sulfide electrolytes, and low electronic conductivity. In addition to LiNbO₃, which is the most popular coating material for ASSBs, LiTaO₃ is another highly promising coating material, and both materials mostly satisfy these requirements. In this work, LiTaO₃ and LiNbO₃ were used to coat the surface of LiNi_0.82Co_0.12Mn_0.06O₂ cathodes for ASSBs. Further, the effects of two different coating methods, postcoating and precursor-based (PB) coating, were characterized and compared. The postcoating method simply forms a coating layer, whereas the PB coating method offers an additional doping effect owing to the diffusion of coating ions into the cathode structure. Surface coating considerably increased the capacity of the ASSB cathodes under all experimental conditions. With the same coating amount and method, the effect of the LiTaO₃ coating was similar or superior to that of the LiNbO₃ coating. Compared with the postcoating method, however, the PB coating method resulted in a superior rate capability and cyclic performance, which was mostly attributed to the doping effect of Ta or Nb. An X-ray photoelectron spectroscopy analysis confirmed that both the LiTaO₃ and LiNbO₃ coatings suppressed side reactions. Among the coatings we examined, the LiTaO₃ coating prepared by the PB method most effectively enhanced the electrochemical performance of the cathodes for sulfide electrolyte-based ASSBs.