First-principles density functional calculation of electrochemical stability of fast Li ion conducting garnet-type oxides

Abstract

The research and development of rechargeable all-ceramic lithium batteries are vital to realize their considerable advantages over existing commercial lithium ion batteries in terms of size, energy density, and safety. A key part of such effort is the development of solid-state electrolyte materials with high Li⁺ conductivity and good electrochemical stability; lithium-containing oxides with a garnet-type structure are known to satisfy the requirements to achieve both features. Using first-principles density functional theory (DFT), we investigated the electrochemical stability of garnet-type Li_xLa₃M₂O₁₂ (M = Ti, Zr, Nb, Ta, Sb, Bi; x = 5 or 7) materials against Li metal. We found that the electrochemical stability of such materials depends on their composition and structure. The electrochemical stability against Li metal was improved when a cation M was chosen with a low effective nuclear charge, that is, with a high screening constant for an unoccupied orbital. In fact, both our computational and experimental results show that Li₇La₃Zr₂O₁₂ and Li₅La₃Ta₂O₁₂ are inert to Li metal. In addition, the linkage of MO₆ octahedra in the crystal structure affects the electrochemical stability. For example, perovskite-type La_1/3TaO₃ was found, both experimentally and computationally, to react with Li metal owing to the corner-sharing MO₆ octahedral network of La_1/3TaO₃, even though it has the same constituent elements as garnet-type Li₅La₃Ta₂O₁₂ (which is inert to Li metal and features isolated TaO₆ octahedra).