A semi-solid state (SSS) electrolyte with high ionic conductivity of 2.6 × 10 S cm is designed to avoid problems of interfacial side reactions brought by typical liquid carbonate electrolyte in Prussian blue-based sodium-ion batteries, as well as Na dendrite growth. The solidification mechanism is related to FEC polymerization triggered by 5 wt% AlCl Lewis acid addition in pristine liquid electrolyte. The SSS electrolyte enables the intrinsic high-reversibility of rhombohedral Prussian blue analogue (r-PBA) cathode, and ultra-long lifetime of 3000 and 4000 cycles at 1 and 2 C, and high rate capacities of 121 mAh g at 1 C and 88 mAh g at 10 C are achieved. The improved cyclability and rate capability are ascribed to interface reinforcement between r-PBA and electrolyte due to the protection of formed poly(vinylene carbonate) on r-PBA. This work provides valuable insights into the growing significance of interface stability for rhombohedral structure in Prussian blue analogue.