Recently, multivalent metal-ion batteries have attracted considerable interests on the merits of their natural abundance and multi- electron redox property. However, the development of Ca-ion battery is still in their preliminary stage because of the lack of suitable electrode material. The Ca-storage performance of the existing materials is still unsatisfactory with low capacity, poor cyclic stability, as well as sloping discharge profiles, which cannot provide stable energy output. In this work, transition metal oxide Sn-doped In₂O₃ (ITO) has been explored as the aqueous Ca-ion battery anode, which could deliver a high discharge capacity of 71.2 mAh·g^-1 with an ultra-flat discharge voltage plateau. The Ca storage mechanism was revealed to be reversible conversion reaction based on ex-situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations. A flexible aqueous Ca-ion battery was subsequently assembled with zinc hexacyanoferrate (ZnHCF) cathode and ITO anode sandwiched by hydrogel electrolyte, which could deliver a high specific capacity of 75.3 mAh·g^-1 at 0.4 A·g^-1 with a flat output voltage plateau at around 0.8 V. The bendable and flexible Ca-ion battery with decent voltage output will pave the way for the energy storage devices towards practical applications in flexible and wearable electronics.

Safe and long lifespan batteries facilitate the development of portable electronics and electric vehicles. Owing to the low-cost, naturally abundance, and trivalent charge carrier of aluminum with the highest theoretical volumetric capacity, rechargeable aqueous aluminum-ion-based batteries are considered as promising next-generation secondary batteries. However, traditional electrolytes and frequent collapse of the host structure of electrode materials greatly jeopardize the cycle stability of the batteries. Here, we develop a novel hydrogel-based electrolyte coupled with stable layered intercalation electrodes for the first time to fabricate a highly safe and flexible rechargeable hybrid Al³⁺/H⁺ battery. The as-fabricated hybrid-ion battery (HIB) delivers a high specific capacity of 125 mAh∙g⁻¹ at 0.1 A∙g⁻¹ and exhibits an unprecedented super long-term cycling stability with no capacity fading over 10, 000 cycles at 2 A∙g⁻¹. In addition, the hydrogel-based electrolyte possesses smart function of thermoresponsive switching, which can effectively prevent thermal runaway for the batteries. The unprecedented long cycle stability, highly intrinsic safety as well as low-cost indicate that the flexible aqueous HIBs are promising for applications.