Chemomechanical Pairing of Alloy Anodes and Solid-State Electrolytes

Alloy anodes present promising alternatives to alkali metals in solid-state batteries but still face morphological instability upon cycling. Unlike conventional batteries using liquid electrolytes, interfacial evolution between solid-state electrolytes and alloy anodes is determined by interfacial electrochemistry and mechanics. Here, we adapt a classical chemomechanical model for Li metal to apply to alloy anodes. This allows generalizing a principle, namely, the hard and soft electrolytes and alloy anodes pairing principle, to guide improving morphological stability. Specifically, “hard” (high-shear-modulus) ceramic electrolytes should be paired with “harder” alloys, while “soft” (low-shear-modulus) polymer electrolytes favor “softer” alloys. We examine the chemomechanical properties of several Li–M alloys (M = Al, Mg, In, Sn, and Sb). Consistent with the principle, the “harder” Li–Sn anode exhibits a flattened morphology with the “hard” Li₆PS₅Cl electrolyte after cycling. Conversely, the “softer” Li–In anode evolves extremely rough, indicating Li–In dendrite formation. Our work underscores the significance of tuning alloy anode mechanical properties, incorporating well-established rules in traditional metallurgy.