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.