Ammonium Vanadium Bronze as a Potassium-Ion Battery Cathode with High Rate Capability and Cyclability

Abstract

K-ion batteries (KIBs) are a promising alternative to lithium-ion batteries. Despite the rapid development of KIB anodes, cathodes have not developed to the same extent due to the sluggish kinetics of K-ion intercalation. Here, ammonium vanadium bronze NH4V4O10 (NVO) is proposed as a potential KIB cathode material. The as-synthesized NVO features a large interlayer spacing of 9.8 Å and self-assembled flower-like architecture. The cathode delivers a high capacity of 136 mAh g−1 (50 mA g−1) and a decay rate of 0.02% per cycle over 200 cycles in the range of 1–3.8 V. It retains 94% capacity (80 mAh g−1) after 200 cycles in the range of 2–3.8 V. Moreover, it exhibits fast rate capability by delivering 51 mAh g−1 at a rate as high as 3 A g−1 (2–3.8 V), being 90% of the capacity at 0.1 A g−1. Electrochemical mechanism studies suggest that K-ion storage in NVO is a topotactic process, where transition between V4+ and V5+ occurs. They also show that prevention of deammoniation at a higher voltage toward 4.2 V is critical for the structural stability of NVO. This work may stimulate future exploitation of vanadium oxides in KIBs and more insights into the mechanisms of K-ion storage.