In-Built Curable High-Salt-Concentrated Elastomeric Polymer Electrolytes for Integrated All-Solid-State Lithium Batteries and Ion Conduction

The elastomeric polymer-in-salt electrolytes (PISEs) comprising a polyether backbone and pendant cyclic carbonates have been developed through a mild and clean carbonate–amine reaction. The pendant cyclic carbonate moieties, analogous to the classic carbonate liquid electrolytes, can host lithium salts to a high concentration on the one hand; on the other hand, they can function as reactive sites toward diamine cross-linkers. This simple and efficient approach, free of extra additives and residuals, facilitates the in situ integration of the polymer electrolyte and the cathode in which the PISE also serves as an ion-conductive binder. The resulting in-built flexible PISE elastomers exhibit enhanced mechanical stability, favorable interfacial compatibility, and a high lithium-ion transference number of up to 0.72. Moreover, with the evolution of solvation structures of lithium ions encompassing their coordination with TFSI counterions and different moieties of polymer electrolytes, the particular molecular conformations of TFSI counterions have been revealed at the salt concentrations from dilute to nondilute regimes by spectral analysis. It is found that lithium ions interact preferentially with the ether segments of the polymer electrolytes at the low-salt-content regime, while nearly all of the cyclic carbonates as well as TFSI counterions participate in the coordination of lithium ions at the high-salt-content regime. The population of the cis-conformation of TFSI– rises with respect to the trans-conformation as the salt concentration increases. Ion conduction mediated by solvating polymer chains and clustering of counterions is proposed to dictate the enhanced ionic conductivity and lithium-ion transference number in this polymer-in-salt system. This integrated composite cathode/electrolyte strategy enables a compact interfacial contact and thus facilitates ion conduction across the boundary, eventually leading to all-solid-state LiFePO4/Li cells with a prolonged cycling stability and a high cathode areal capacity at ambient temperature. The present work provides a facile approach to tailor the polymer composite structure for flexible solid batteries with an optimal electrochemical performance.