Free-standing manganese oxide on flexible graphene films as advanced electrodes for stable, high energy-density solid‐state zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are regarded as the next-generation energy storage devices because they provide an abundant zinc source, reliable safety, eco-friendliness, and high specific capacity. In particular, the ZIBs are desirable because they can overcome the disadvantages of conventional lithium ion batteries (LIBs) such as poor safety and high cost. Nevertheless, the practical application of aqueous ZIBs remains hindered by their vulnerable cathodes which lead to low energy density, fast capacity fading, and unstable long-term stability. Herein, for the first time, free-standing manganese oxide (MnO₂) on a flexible graphene film is reported as an advanced cathode for aqueous ZIBs that is suitable for practical application. The resultant novel electrode shows a superior electrochemical performance in the electrolyte solution, including a high energy density of 396 W h kg⁻¹ at a power density of 90 W kg⁻¹, and a good capacity retention of 82.7% after up to 300 cycles. More interestingly, the solid-state ZIBs incorporating a gel electrolyte exhibit good electrochemical performance, remarkable safety, flexibility and waterproof properties, thus indicating that this promising candidate can provide new insights into flexible and wearable energy storage devices.