Balancing Charged Groups in Protein Additives: A Key to Enhancing the Performance of Polymer-Based Electrolytes in Solid-State Lithium–Metal Batteries

Solid-state Li metal batteries equipped with polymer electrolytes are highly coveted for their flexibility and remarkable energy density; however, their advancement is hindered by limited ionic conductivity and poor interfacial stability. Herein, we report that protein additives can significantly improve the performance of poly(ethylene oxide)-based solid electrolytes and elucidate the ion-conducting mechanism by comparing proteins with different charged groups (CGs). Positive CGs can anchor anions in Li salts to increase ion transference number but also adsorb polymer chains resulting in a decrease in ionic conductivity. Negative CGs promote the dissociation of Li salts and Li+ conduction; however, it depends on the long-range protein chains. In comparison to the α-amylase with more negative CGs and bovine serum albumin (BSA) with more positive CGs, the casein with balanced groups enables the solid electrolyte to have a significantly higher Li+ transference number of 0.45 and superior mechanical properties. Furthermore, it can also promote uniform Li plating and stripping, as well as the formation of a stable solid electrode–electrolyte interphase. When paired with the LiNi0.8Co0.1Mn0.1O2 cathode, the batteries can still maintain a high specific capacity of 97.7 mA h g–1 after 200 cycles at a 1 C-rate, highlighting the efficacy of utilizing CGs-balanced proteins as additives in solid-state batteries.