Flexible aqueous Ca-ion full battery with super-flat discharge voltage plateau

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.