Calcium-ion batteries have been considered attractive candidates for large-scale energy storage applications due to their natural abundance and low redox potential of Ca²⁺/Ca. However, current calcium ion technology is still hampered by the lack of high-capacity and long-life electrode materials to accommodate the large Ca²⁺ (1.00 Å). Herein, an amorphous vanadium structure induced by Mo doping and in-situ electrochemical activation is reported as a high-rate anode material for calcium ion batteries. The doping of Mo could destroy the lattice stability of VS₄ material, enhancing the flexibility of the structure. The following electrochemical activation further converted the material into sulfide and oxides co-dominated composite (defined as MoVSO), which serves as an active material for the storage of Ca²⁺ during cycling. Consequently, this amorphous vanadium structure exhibits excellent rate capability, achieving discharge capacities of 306.7 and 149.2 mAh g⁻¹ at 5 and 50 A g⁻¹ and an ultra-long cycle life of 2000 cycles with 91.2% capacity retention. These values represent the highest level to date reported for calcium ion batteries. The mechanism studies show that the material undergoes a partial phase transition process to derive MoVSO. This work unveiled the calcium storage mechanism of vanadium sulfide in aqueous electrolytes and accelerated the development of high-performance aqueous calcium ion batteries.

Aqueous zinc ion batteries (AZIBs) are a promising energy storage technology due to their cost-effectiveness and safety. Organic materials with sustainable and designable structures are of great interest as AZIBs cathodes. However, small molecules in organic cathode materials face dissolution problems and suboptimal cycle life, whereas large molecules suffer from a low theoretical capacity due to their inert carbon skeletons. Here, we designed two covalent organic framework (COF) materials (benzoquinoxaline benzoquinone-based COF (BB-COF) and triquinoxalinylene benzoquinone-based COF (TB-COF)) with the same structure and number of energy storage groups to investigate the correlation between the densities of active sites and electrochemical performance. We conclude that the electrochemical behavior of organic conjugate-based energy storage materials lacks a linear correlation with active site quantity. Adjusting active site densities is crucial for material advancement. BB-COF and TB-COF with dual active sites (C=O and C=N) exhibit distinct characteristics. TB-COF, which has dense active groups, shows a high initial capacity (222 mAh g⁻¹). Conversely, BB-COF, which features a large conjugated ring diameter, presents superior rate performance and enduring cycle stability. It even maintains stable cycling for 2000 cycles at −40 ℃. In-situ electrochemical quartz crystal microbalance tests reveal the energy storage mechanism of BB-COF, in which H⁺ storage is followed by Zn²⁺ storage.