Balancing Salt Dissociation and Mechanical Robustness in Polymer-in-Salt Solid Electrolytes via a Quaternized Polymer Matrix

Abstract

Polymer-in-salt solid electrolytes (PISSEs) have been regarded as a promising strategy to overcome the low ionic conductivity of solid polymer electrolytes (SPEs). However, developing PISSEs remains challenging due to the intrinsic trade-off between accommodating high lithium salt fractions (>50 wt.%) with sufficient dissociation and maintaining structural stability. Herein, we rationally design a polymer matrix, denoted as FBN, that combines mechanical robustness with enhanced lithium salt dissociation capability by incorporating a rigid main-chain and side-chain-tethered quaternary ammonium moieties. The prepared FBN provides superior toughness, enabling FBN-based PISSEs to form intimate contact with electrodes and stabilize electrode–electrolyte interfaces. In addition, the positive charges in mechanically robust FBN allow lithium bis(fluorosulfonyl)imide (LiFSI) to be sufficiently dissociated up to 80 wt.%, improving the concentration of free Li⁺ and compositional uniformity of the FBN-based PISSEs. As such, the FBN demonstrates high ionic conductivity (4.84 × 10⁻⁴ S cm⁻¹) and enables superior Li⁺ transport behavior during cycles. In particular, the abundant LiFSI content in FBN-based PISSEs facilitates the formation of an inorganic-rich solid electrolyte interphase (SEI) layer, improving electrochemical performances (167.5 mAh g⁻¹ at a 0.5 C after the 100^th cycle) in lithium metal batteries (LMBs) by forming favorable electrode–electrolyte interfaces. This work presents a design strategy of polymer matrices for SPEs containing high lithium salt concentration from the perspective of simultaneously promoting lithium salt dissociation and improving mechanical robustness.