Polymer-in-salt electrolytes were introduced three decades ago as an innovative solution to the challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. Despite significant progress, the approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. This research offers a promising solution by integrating cationic poly(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. A stable polymer-in-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol % was achieved, providing a valuable opportunity for the in-depth understanding of these electrolytes at an extremely high salt concentration. This work explicates how increased salt concentration affects coordination structures, glass transitions, ionic conductivity, and the decoupling and coupling of ion transport from structural dynamics in a polymer electrolyte, ultimately enhancing electrolyte performance. These findings provide significant knowledge advancement in the field, guiding the future design of polymer-in-salt electrolytes.

A suitable interface between the electrode and electrolyte is crucial in achieving highly stable electrochemical performance for Li‐ion batteries, as facile ionic transport is required. Intriguing research and development have recently been conducted to form a stable interface between the electrode and electrolyte. Therefore, it is essential to investigate emerging knowledge and contextualize it. The nanoengineering of the electrode‐electrolyte interface has been actively researched at the electrode/electrolyte and interphase levels. This review presents and summarizes some recent advances aimed at nanoengineering approaches to build a more stable electrode‐electrolyte interface and assess the impact of each approach adopted. Furthermore, future perspectives on the feasibility and practicality of each approach will also be reviewed in detail. Finally, this review aids in projecting a more sustainable research pathway for a nanoengineered interphase design between electrode and electrolyte, which is pivotal for high‐performance, thermally stable Li‐ion batteries.

All‐solid‐state batteries (ASSBs) using organic ionic plastic crystals (OIPCs) are promising candidates to overcome the inherent safety issues of lithium‐ion batteries (LIBs). Although OIPCs have excellent process applicability in the roll‐to‐roll electrode fabrication process, their application as solid electrolytes incorporated in composite electrodes has yet to be demonstrated in detail. Herein, we denote the positive effect of the N ‐ethyl‐ N ‐methylpyrrolidinium bis(fluorosulfonyl)imide ([C ₂ mpyr][FSI]) incorporated within a composite graphite anode on the charge rate capability and cycle life. The highest charge capacity ratio (the charge capacity at 2C vs. that measured at 0.1C) was measured for the composite anode with an OIPC composite ratio of 50 wt % (89.5 %, 295.7 mAh/g at 2C charge), almost the same as that of the graphite anode with a liquid electrolyte (85.7 %, 295.9 mAh/g at 2C charge). More favorable lithium‐ion conduction pathways were resolved for the anode with a higher OIPC composite ratio, whereas an excessive amount of OIPC reduced the long‐term cyclability. The most stable discharge capacity retention was obtained for 30 wt % OIPC composite (257.4 mAh/g at the 100th discharge), which showed no signs of discharge capacity fading within 100 cycles. The lithiation/delithiation process of the solid‐state graphite‐[C ₂ mpyr][FSI] composite anode was evaluated to be stable and reversible. In addition, the incorporated OIPC composite enhanced the electrolyte/electrode and electrode/current collector contacts. This work highlights multiple advantageous functions of the OIPC in a composite graphite anode, which will broaden our horizons for the use of OIPC composites in ASSBs.