Review of modification strategies in emerging inorganic solid-state electrolytes for lithium, sodium, and potassium batteries

The design of solid-state electrolyte (SSE) entails controlling not only its bulk structure but also the internal (e.g., grain boundary and pore) and heterophase (e.g., anode-SSE reaction layer) interfaces. Moreover, defects are integral for establishing both the initial all-solid-state battery (ASSB) cell performance and its ultimate cycling lifetime. This topical review discusses 5 highly promising inorganic lithium-, sodium-, and potassium-based SSEs, providing case studies of strategies to enhance ionic conductivity, tune the internal and heterophase interfaces, and prevent dendrites and deleterious side reactions. The primary focus is on garnet LLZO (Li7La3Zr2O12), argyrodite (Li6AS5X, A = P, Sb and X = Cl, Br, I), Na3AB4 (A = P, Sb, As and B = S, Se), and NASICON-type electrolytes, with several exploratory potassium systems being presented as well. The article begins with a discussion of the superionic conduction mechanisms, which is assisted by defects, topology, correlation, and disorder. The discussion then shifts to the thermodynamic or kinetic phase stability of metastable SSEs, followed by the SSE internal interfaces (pores and grain boundaries), and finally the complex microstructure associated with the reactive boundaries between the SSE and the metal anode and the SSE and the high-voltage ceramic cathode. This review explores why SSE systems with highly promising bulk ionic conductivity values do not demonstrate comparable performance in full cells. A major cause is the cycling-induced formation of ionically insulating but electrically conducting new phases at both the anode and the cathode. For the SSEs that do not rapidly decompose when contacting the anode, site-specific variations in ionic conductivity/electrical resistivity lead to growth of metal dendrites. In each electrolyte class, creative fabrication strategies have been employed to minimize, but not fully eliminate, new phase formation and dendrites, creating ample opportunities for additional analytical and synthesis-focused research. The emphasis of the discussion is on the bulk doping strategies for SSEs, emerging simulation and characterization findings in relation to SSE performance and cell failure, and new synthesis approaches and cell architectures to improve cycling stability. Throughout the article, outstanding scientific questions are identified, and future research directions are proposed.