Nano-interface engineering in all-solid-state lithium metal batteries: Tailoring exposed crystal facets of epitaxially grown LiNi0.5Mn1.5O4 films

Understanding the solid electrolyte/cathode nano-interfacial kinetics is critical in designing advanced all-solid-state lithium metal batteries with improved performance and stability. However, the correlation between crystallographic features of cathodes and the solid nano-interface behaviour remains controversial due to the difficulty in eliminating the impact of other factors. Here, we systematically investigated the effect of exposed crystal facets of LiNi0.5Mn1.5O4 on the solid nano-interface using the substrate orientation-dependent epitaxial growth of thin films as a model study. (100), (110), and (111)-oriented Pt/MgO substrates were used to make selective high-quality epitaxial LiNi0.5Mn1.5O4 films with different {100}/{111}-exposed facet ratios. The atomic arrangement of the exposed facets was found to affect the electrochemical performance. Loosely packed {100} facets and densely packed {111} facets were beneficial for lithium ion diffusion and cycle stability, respectively. In particular, stable {111} facets effectively suppressed the dissolution and diffusion of transition metals at the solid nano-interface during charge-discharge, enabling a 99.6% retention after 100 cycles. In addition, this model study reveals that an amorphous cathode surface layer and a twin boundary inside the cathode are crystallographic origins that hinder the electrochemical performance of batteries. These findings suggest that crystallographic modifications of cathodes can be a key to improving the solid nano-interface.