Boosting the ultrastable Li storage performance in electron-sponge-like polyoxovanadates by constructing inorganic 3D structures

Abstract

Polyoxometalates act as an electron sponge, processing multielectron redox reactions and acting as a fast ionic conductor. They show great potential as promising electrode materials for next-generation lithium ion batteries (LIBs). However, there are still some fundamental issues which should be solved before their application can be realized, such as determining the stable structural feature with reversible Li⁺ ion insertion/desertion. In this work, polyoxovanadates (POVs) based materials of K₄Na₂V₁₀O₂₈·nH₂O (KNaV₁₀) and Mg₂(NH₄)₂V₁₀O₂₈·nH₂O (MgV₁₀) have been prepared and used as the electrode material for a Li⁺ ion reservoir. The 10-core polyoxovanadate is demonstrated as a anionic building block and the 3D extended structure has been smartly tuned by counter cations. For MgV₁₀, a 1D tunnel with an approximate size of 3 Å × 10 Å was formed along the a axis by Mg²⁺ ions and [V₁₀O₂₈]⁶⁻ polyanions. The MgV₁₀ shows a higher capacity, cycling stability, and rate performance than that of KNaV₁₀ without tunnels. The capacity of MgV₁₀ is about 160 mA h g⁻¹ at a high discharge rate of 250 mA g⁻¹, while it is only 118 mA h g⁻¹ for KNaV₁₀. Even after 60 discharge/charge cycles at 50 mA g⁻¹, it displayed a capacity of 180 mA h g⁻¹. The 1D tunnel in MgV₁₀ facilitates the Li⁺ ion transport and provides spatial Li storage sites, which promotes the electrochemical performance in LIBs. Moreover, the Mg²⁺ ions remained stable during battery cycling and promoted the 3D structure stability. This work demonstrates promising guidelines for the structural design of POVs based materials for Li storage.