Design principles for anode stable solid-state electrolytes

Abstract

Metal containing oxides and halides offer a wide range of design spaces for solid-state electrolytes. However, the stability of these electrolytes at the anode/electrolyte interface remains a concern. It is commonly believed that electrolytes containing metals, such as lithium perovskite and lithium metal halides, are inherently unstable at the anode. This issue of electrochemical stability has been somewhat overlooked amidst the surge of research focusing on the development of new superionic conductors. In this work, we employ a computational framework to uncover the underlying causes of anode instability in different electrolytes that contain metals. Our findings indicate that deficient compounds in lithium, such as lithium perovskite or lithium metal halide, tend to be unstable at the anode regardless of the metal chemistry involved. This instability is primarily due to their unsuitable lithium stoichiometry, positioning them in regions of the phase diagram where stronger decomposition phases prevail. Conversely, lithium-rich compounds, such as newly identified overlithiated disordered rocksalt-type electrolytes, exhibit significantly better anode stability. Additionally, our research also shows that while anode stability is generally less problematic for sodium metal oxide-based electrolytes in solid-state sodium-ion batteries, it remains a challenge for sodium metal halides. This work rationales the choice of metal species and structural phases as stoichiometry for more electrochemical stable solid-state electrolytes.