Surface passivation and structural stabilization of Ni-rich and Co-free single-crystal cathodes to improve high-voltage performance in sulfide solid-state batteries

Layered Ni-rich and Co-free oxides, such as LiNi$_x$Mn$_{1−x}$O$_2$ (with x ≥ 0.5), are attracting much interest as cathode materials for solid-state lithium batteries (SSLBs) due to their high energy density and relatively low cost. However, the absence of cobalt exacerbates structural instabilities, particularly at high cutoff potentials, leading to rapid capacity decay. Here, we identify Li‒Al‒O$_x$ as the thermodynamically most stable interface with the thiophosphate solid electrolyte Li$_6$PS$_5$Cl based on high-throughput screening of LiM$_x$O$_y$ compounds. The theoretical findings are incorporated into a dual-modification strategy for single-crystalline LiNi$_{0.8}$Mn$_{0.2}$O$_2$ (NM), in which Al$^{3+}$ is introduced to increase both bulk and surface/interface stability. The resulting LiNi$_{0.79}$Mn$_{0.20}$Al$_{0.01}$O$_2$ (NMA) exhibits fewer side reactions and chemo-mechanical degradation upon cycling to 4.5 V vs. Li$^+$/Li. In fact, composite cathodes using NMA achieved a capacity retention of ∼90 % after 200 cycles at 0.2C, far exceeding the ∼54 % for NM. Through a combination of electrochemical tests, in situ and ex situ characterization, and theoretical calculations, we demonstrate that the improved performance results from the synergistic effects of stabilizing the bulk structure and passivating the surface. ... mehrOverall, this work provides a rational design strategy for developing advanced high-voltage (Co-free) cathodes for SSLB applications.