Solvent-Free Synthesis of the Polymer Electrolyte via Photo-Controlled Radical Polymerization: Toward Ultrafast In-Built Fabrication of Solid-State Batteries under Visible Light

Thin solid polymer electrolytes (SPEs) with good processability, improved room-temperature ionic conductivity, and better interfacial compatibility are urgently needed to develop solid-state batteries without safety and leakage issues. In-built electrolyte polymerization has emerged as a novel and effective platform to obtain such electrolytes. However, existing in-built methods usually involve heat, UV, γ irradiation, and so forth to initiate the polymerization and often require the addition of solvents to avoid the concentrated active propagating species, which inevitably afford solvent residues that persist in the electrolyte matrix, leading to complex SPE preparation processes, safety hazards, and side reactions with the electrodes. Herein, a simple solvent-free preparation of the poly(mPEGAA)-based electrolyte film was achieved via the photo-controlled radical polymerization under visible light irradiation via an in-built manner, which resulted in 99% monomer conversion within 5 min to obtain the polymer electrolytes with a controlled molecular weight distribution. Thanks to the mild and green conditions, a thin, solvent-free, and cross-linked SPE electrolyte film was obtained efficiently yet in a well-regulated manner, which gave rise to good interfacial compatibility and an improved room-temperature ionic conductivity of 1.5 × 10–4 S cm–1 at 25 °C. As-prepared solid-state LiFePO4|Li batteries based on the in-built thin SPE exhibited a high discharge areal capacity of 1.7 mA h cm–2 (164.6 mA h g–1) at an ambient temperature. Furthermore, the system displayed lithium dendrite suppression behavior and good long-term charge–discharge cycling in the Li symmetric battery for over 270 h, representing enhanced stability and capacities compared with ex-built systems.