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

Abstract

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 ionconducting 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 LiNi 0.8 Co 0.1 Mn 0.1 O 2 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.