Flexible rechargeable batteries are urgently needed because of the rapid development of wearable electronics. However, most lithium-ion batteries fail to meet safety and stretchability requirements, which are critical factors to the practical application of such devices. Herein, a freestanding, stretchable, and self-healing polymer film that acts as a hydrogel electrolyte is developed for the preparation of flexible rechargeable batteries. The electrolyte film exhibits a high ionic conductivity of 1.15 mS cm⁻¹ at room temperature with a lean water content (4.62%). It also displays a wide electrochemical stability window of ∼ 2.2 V and a high stretchable ratio of > 300%. A stretchable full battery is then developed using the hydrogel electrolyte film. The full cell exhibits long cyclic stability with 73.2% capacity retention after 1,000 cycles at the current density of 1 A g⁻¹. Furthermore, the open circuit voltage of the full cell is maintained well when it is stretched by 50%–125% and even after cutting the cell. This study offers a strategy for designing high-performance hydrogel electrolytes, which have potential applications in wearable electronics.