Succinonitrile, a molecular plastic crystal, can be modified to become gel by introducing the hydrogen bond (O–H⋯F) network upon addition of silica nanoparticles and 1-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄). The succinonitrile–BMI·BF₄–silica gel system is thermostable over a wide temperature range. The gel electrolyte based on this system containing I⁻ and I₃⁻ is also thermostable and highly conductive (6.6–18.2 mS cm⁻¹) in the measured temperature region (20–80 °C). The appropriate addition of silica nanoparticles and BMI·BF₄ has no adverse effects on the mobility capability of I⁻/I₃⁻ ions, and the absence of succinonitrile deteriorates the mobility capability. Importantly, dye-sensitized solar cells (DSSCs) with this gel electrolyte show solar-to-electric energy conversion efficiency of 5.0–5.3% over a wide temperature range (20–80 °C). Furthermore, the aging test reveals that the cell still maintains 93% of its initial value for the conversion efficiency after being stored at 60 °C for 1000 h, indicating an excellent long-time durability.