Tailoring Chloride Solid Electrolytes for Reversible Redox

Solid-state electrolytes enable next-generation batteries that can theoretically deliver higher energy densities while improving device safety. However, when fabricating cathodes for all-solid-state batteries, solid-state electrolytes must be combined with the active materials in high weight fractions in order to achieve sufficient ionic percolation within the cathode composite. This requirement drastically hinders the practicality of solid-state batteries as the solid-state electrolyte is conventionally designed to be electrochemically inactive and is effectively electrochemical "dead weight", lowering both the gravimetric and volumetric energy density of the cell. In this work, a well-known solid-state electrolyte, Na₂ZrCl₆, is modified by aliovalent substitution of inactive Zr⁴⁺ cations with redox-active M⁵⁺ (M = Nb or Ta) cations to create a series of Na₂–_xM_xZr₁–_xCl₆ solid solutions that possess both high ionic conductivities and active sites for Na⁺ storage. The Na⁺ intercalation mechanisms of these solid-solution materials, in addition to those of the NaMCl₆ end-member materials, are elucidated in this work. It was discovered that both the niobium- and tantalum-containing chlorides exhibit rather high electrochemical potentials (2.2–2.8 V vs Na₉Sn₄), making them ideal catholytes to pair with commonly used oxide cathode materials like NaCrO₂. This synergistic pairing leads to a cathode composite with an 83–102% increase in energy density and 39–81% improvement in areal discharge capacity compared to a redox-innocent solid electrolyte. This approach highlights the benefits of designing and employing redox-active solid-state electrolytes that can reversibly intercalate charge-carrying cations, opening up a broad new avenue for solid-state electrolyte discovery and solid-state battery design.