Topochemical behavior of ferrocene embedded in V₂O₅·nH₂O with weak hydrogen bonding enhancing ammonium-ion storage

Aqueous ammonium ion batteries (AAIBs) are garnering increasing attention due to their utilization of abundant resources, cost-effectiveness, safety, and unique energy storage mechanism. The pursuit of high-performance cathode materials has become a pressing issue. In this study, we propose and synthesize ferrocene-embedded hydrated vanadium pentoxide (Fer/VOH) for implementation in AAIBs. The inclusion of ferrocene serves to expand the interlayer spacing, mitigate interlayer forces, and introduce the electron-rich environment characteristic of ferrocene. This augmentation facilitates the creation of additional oxygen vacancies, substantially enhancing the capacity and efficiency of ammonium ion storage. Notably, our investigation reveals that the incorporation of ferrocene attenuates the hydrogen bonding interactions associated with ammonium ions, rendering them more amenable to the interlayer embedding and release processes. Building upon these advantages, Fer/VOH exhibits a specific capacity of 313 mAh/g at a current density of 0.2 A/g, representing the highest reported performance among vanadium oxides utilized in AAIBs to date. Even after 2000 charge/discharge cycles at a current density of 2 A/g, Fer/VOH maintains a reversible specific capacity of 89 mAh/g, with a capacity retention rate of 54.8%. This study confirms the viability of Fer/VOH as a cathode material for AAIBs and offers a novel approach to enhancing the electrical conductivity and diminishing the hydrogen bonding forces in vanadium oxide intercalation through the embedding of electron-rich species and positronic groups.