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 NH 4 V 4 O 10 (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 Na x NVO phase and ending with the conversion reaction with the final formation of V 2 O 3 phase. While upon desodiation, the V 2 O 3 phase can only be oxidized to VO 2 phase, rather than the original NVO phase. Afterward, a reversible conversion reaction between VO 2 and V 2 O 3 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. • In-situ TEM study of anodic sodium storage mechanisms in NH 4 V 4 O 10 nanobelt is conducted. • NH 4 V 4 O 10 nanobelt undergoes stepwise phase transformation during the first sodiation process. • A stable and reversible conversion reaction is established during the repeated (de)sodiations. • Irreversible microstructure deterioration maybe account for the capacity decay. • Nanopores that appeared in microstructure might alleviate the structural strain during repeated cycles.