Constructing δ-MnO2 nano-flower microspheres for high-voltage and long-life aqueous ammonium-ion battery

Rechargeable ammonium-ion batteries (AAIBs) can be used in large-scale energy storage applications. An important factor for achieving high voltage and long life in AAIBs is developing appropriate cathode materials. Herein, the δ-MnO2-x (x = 1, 2) cathodes with nano-flower microspheres structure were designed and synthesized, besides, the corresponding electrochemical performances of NH4+-ion storage were systemically analyzed. Results indicated that the δ-MnO2-2 cathode delivered high discharge capacity of 217.6 mAh g−1 at 0.1 A g−1, outstanding rate performance as well as ultra-durable cycling stability with capacity retention of 92 % after 1000 cycles at 1.0 A g−1. Ex-situ characterizations revealed that the primary mechanism for NH4+-ion storage involves reversible NH4+-ion insertion and extraction into δ-MnO2-2 host's layered structure, which is accompanied by an unintermittent process of hydrogen bond formation and breaking. Furthermore, the δ-MnO2-2//UP aqueous ammonium ion full battery was successfully constructed and delivered the voltage window as high as 2.0 V, an ultra-long service life, and 47.8 % capacity retention over 20,000 cycles at 5.0 A g−1. The present study provided the new direction and impetus for designing and developing high-performance cathode materials for AAIBs.