In-situ TEM revisiting NH4V4O10 to unveil the unknown sodium storage mechanism as an anode material

Abstract The recent discovery of a fast-charging vanadium-based disordered rock-salt anode for lithium-ion batteries (Nature, 2020, 585, 63−67) has rekindled great interest to screen possible anode candidates from the existing cathodes by studying their underexplored low-voltage ion storage behavior, particularly for vanadium-based compounds. Among them, layered NH4V4O10 (NVO) material is typically known as an intercalation-type cathode material, with large interlayer spacing for facile ion intercalation; however, to date, there is no investigation of its utilization as a potential anode material. Here, the vanadium redox and structural evolution of NVO nanobelts (NBs) under an anode voltage window (0.01−3.0 V vs. Na+/Na) are carefully studied by in-situ transmission electron microscopy (TEM) and electrochemical measurements. By in-situ TEM tracking the full sodiation process in real-time, a stepwise Na-storage reaction mechanism is revealed, initiating with the interlaminar intercalation of Na ions accompanied with the appearance of NaxNVO phase and ending with the conversion reaction with the final formation of V2O3 phase. While upon desodiation, the V2O3 phase can only be oxidized to VO2 phase, rather than the original NVO phase. Afterward, a reversible conversion reaction between VO2 and V2O3 phases is established upon the subsequent (de)sodiation cycles, which delivers a reversible cycling capacity of 148 mAh g–1 at 1 C, as verified by electrochemical measurements. The in-situ observation also witnesses the emergence of nanopores in NBs that may alleviate significant structural strain and contribute to the long-term cycling stability during the following (de)sodiation cycles. This work has validated for the first time the practicability of NVO as an anode material in sodium-ion batteries and afforded a paradigm of revisiting existing cathodes to explore their possible anode utilization.