Polymer‐Based Solid‐State Batteries: Anion Synergistic Effect Activates Organic Cathode with High Voltage and High Energy Density

Abstract This study presents a solid‐state lithium battery that employs phenoxathiin (PHX), a high‐voltage organic cathode delivering exceptional high energy density. The PHX cathode is paired with a wood fiber‐reinforced poly (diallyldimethylammonium bis (trifluoromethanesulfonyl)imide) (PDADMATFSI)‐based polymer solid electrolyte (FPSE). Leveraging an anion synergistic effect from TFSI − anions within the FPSE, the system effectively activates both sulfur (S) and oxygen (O) redox sites in PHX, resulting in two pairs of redox peaks at the average potentials of 3.87 and 4.12 V versus Li + /Li with the first cycle achieve 181.3 mAh g −1 at 30 °C, 0.1C, and energy density exceeding 600 Wh g −1 . Remarkably, the battery demonstrates outstanding cycling stability, retaining a capacity of 74.6 mA h g −1 after 2800 cycles at 30 °C, corresponding to 56.1% capacity retention, thereby setting a new benchmark for longevity in this class. Molecular dynamics simulations and advanced characterization techniques clarify the electrolyte‐cathode interactions and redox mechanisms.