Improvement of Electrochemical Stability of Layered Oxide Electrodes in Na-Ion Batteries Via Control of Chemical Composition in the Interlayer Region

Abstract

Layered transition metal compounds with expanded interlayer region, stabilized by structural water, often show high initial capacities but suffer from rapid capacity decay and poor rate capability in Na-ion batteries. High-temperature annealing, accompanied by phase transformation with the formation of more dense atomic structures, has been shown to improve electrochemical stability. However, the capacity of annealed materials decreases compared to their original forms. Here, we for the first time demonstrate that low-temperature annealing (260oC under vacuum) can be used to achieve enhanced electrochemical stability of high capacity Na-preintercalated bilayered vanadium oxide (δ-NaxV2O5·nH2O) nanobelts, while preserving its open layered structure with expanded interlayer region available for insertion and diffusion of a large number of electrochemically cycled Na+ ions. The mechanism of the thermally induced interlayer water loss is discussed. The improved capacity retention exhibited by low-temperature vacuum annealed δ-NaxV2O5·nH2O nanobelts is attributed to the partial removal of the structural water from the interlayer region, formation of additional bonds within the V-O bilayers, and increased stacking order of V-O bilayers. Additionally, we will show the effect of the chemical pre-intercalation of alkali (Li+, Na+, K+) and alkali-earth (Mg2+ and Ca2+) ions in the interlayer region of the bilayered vanadium oxide on electrochemical stability in Na-ion batteries. By altering the nature of the chemically preintercalated ion, interlayer spacing of the synthesized δ-MxV2O5 (M = Li, Na, K, Mg, Ca) materials was varied between 9.62 and 13.40 Å. We show that the interlayer spacing increases with the increase of the hydrated ion radius. The ion (Li+, K+, Mg2+, Ca2+) stabilization effect was investigated in Na-ion cells, with Na-preintercalated phase, δ-NaxV2O5, serving as a reference material. Our analyses indicate that cyclability and rate performance of the δ-MxV2O5 improves with increasing interlayer spacing and decreasing the ionic radius of chemically preintercalated ion. The highest initial capacity, greatest capacity retention, and highest capacity retention at higher current rates were exhibited by Mg- and Li-stabilized δ-V2O5. Low-temperature vacuum annealing ion stabilization achieved via chemical pre-intercalation synthesis approach are proposed as efficient strategies to advance electrochemical stability of the growing family of hydrated transition metal compounds used as electrodes in beyond lithium-ion batteries.