A free-standing sulfone-based solid-state electrolyte mitigating the release of crystal water from the Prussian blue cathode for high voltage potassium batteries

Prussian blue (PB) and its analogues are emerging cathode materials for potassium batteries due to their 3D open framework allowing facile ion diffusion. However, the crystal water, an inherent component of PB structure, can be released into the electrolytes especially during high voltage operation, causing detrimental side reactions and rapid degradation of potassium batteries. To address this challenge, we present a free-standing sulfone-based solid-state electrolyte (FSSE) comprising potassium bis(fluorosulfonyl)imide, dimethyl sulfone, and UV-cured trimethylolpropane ethoxylate triacrylate network. The FSSE effectively prevents the release of crystal water from PB cathode while exhibiting suitable electrolyte properties including decent ionic conductivity (2.2 × 10−4 S cm−1 at 25 °C), exceptional oxidative stability (~4.8 V vs. K+/K), and intimate contact with porous composite PB electrodes. K||PB cells utilizing FSSE demonstrate superior cyclability compared to a liquid carbonate electrolyte (74.6% vs 53.3 % capacity retention over 200 cycles). The solid-state electrolyte possessing the ability to block crystal water provides a promising avenue for exploring diverse PB cathode materials for high-voltage potassium batteries.