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 NH 4 V 4 O 10 (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 V 4+ and V 5+ 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.