A flexible energy storage device with a glycerol-based gel electrolyte is provided. The flexible energy storage device can include a pair of electrodes separated by the gel electrolyte. The electrolytes can be in gel form, bendable and stretchable in a device. The gel electrolyte can include glycerol, redox-active molybdenum-containing ions, and a secondary ionic substance. The secondary ionic substance can include a salt. The gel electrolyte can have a density of 1.4 to 1.9 g/cm3 and an ionic conductivity of 2.3×10−4 to 3.2×10−4 Scm−1. The flexible energy storage device may retain greater than 95% of an unbent energy storage capacity when bent at an angle of 10 to 170°.