Abstract The rapid development of wearable electronics promotes a high demand for flexible power sources. Flexible rechargeable batteries, as the stars of flexible energy storage and conversion systems, possess simultaneously high flexibility, high energy density, and dynamically stable output. However, energy density is often sacrificed largely for achieving high flexibility. In light of the exciting progress that has been achieved in flexible energy storage devices, an in-depth energy density assessment is urgently needed. This review begins with a brief introduction of the measurement method for energy density. Then state-of-the-art progress in the development of high energy flexible lithium-ion batteries, sodium-ion batteries and zinc-ion batteries are summarized and compared in detail. The strategies towards high energy density while keeping high flexibility are elucidated, including choosing active materials with high specific capacity or high voltage, minimizing the fraction of inactive components, structural engineering on both electrodes and batteries, developing solid-state electrolyte, etc. Critical challenges and important directions for future research are envisioned to close. This review aims to provide potential guidance for improving the energy density of flexible energy storage devices.