A theoretical study on the role of ammonium ions in the double-layered V2O5 electrode

Abstract

Double-layered V₂O₅ and its analogues have received increasing attention as a proper cathode for Mg²⁺, Na⁺, Li⁺ ion batteries, even for ammonium ion batteries. Our theoretical research focuses on the effects of NH₄⁺ ions on the structural stability and the ion diffusion properties of double-layered V₂O₅. The elastic constant calculations indicate the NH₄⁺ and water contents have a dramatic influence on the stability of the electrode. When the ratio of H₂O and ammonia ions decreases to (NH₄)_0.125V₂O₅·0.125H₂O, double-layered bronze will transform into other phases. The predicted specific capacity for the redox process from (NH₄)_0.5V₂O₅·0.5H₂O to (NH₄)_0.125V₂O₅·0.125H₂O is 54.6 mA h g⁻¹, which agrees with the experimental value of 55.6 mA h g⁻¹. From the diffusion barrier calculations, it is found that the H₂O molecules can shield the polarization of NH₄⁺ and lower the diffusion barrier of NH₄⁺ ions. Furthermore, the migrations of common charge carriers in NH₄⁺ pre-intercalated V₂O₅ have also been studied, which implies that Li⁺, Zn²⁺, Na⁺, Mg²⁺ ions may move easily in the electrode with energy barriers lower than 525 meV. Our findings match well with the reported experimental results. A special structure of Mg₆NH₄V₈O₂₀ with a much higher Mg ion concentration has been reported. Our findings show that the theoretical specific density of Mg batteries based on NH₄⁺ pre-intercalated V₂O₅ can be improved to 431 mA h g⁻¹, which is 2.5 times larger than the reported values. This work highlights the effects of the ratio of NH₄⁺ and H₂O on double-layered V₂O₅ and provides insights into designing vanadium oxide based fast-diffusion multivalent ion conductors, which are suitable for battery applications.