Reversible Multi-Electron Storage Enabled by Na5V(PO4)2F2 for Rechargeable Magnesium Batteries

Abstract

Rechargeable magnesium batteries (RMB) are one of the utmost promising post-lithium energy storage technologies due to their high theoretical energy density, affordable low cost, and inherent safety with moisture and air. Nonetheless, the research of RMB has been limited due to the low power and reversible energy densities of available cathode materials. Herein, we report a new Mg battery cathode of trigonal Na5V(PO4)2F2 (t­-NVPF) which performs 136 mA h g−1 reversible capacity realizing multi-electron storage through the V4+/V3+ and V5+/V4+ redox couples. After the first reversible cycle MgNa3V(PO4)2F2 is formed. The geometry optimization and energy calculations on the systems MgxNa3V(PO4)2F2 were carried within the density functional theory (DFT) demonstrating Mg insertion can fill the Na6, Na8, and Na9 sites, and evidenced a 1.5 V difference for the same redox couple comparing with the experimental results. Some facets of its crystal and local structure are determined by XRD, EPR, XPS, and 31P and 51V solid-state NMR spectroscopy. The cells cycled 2.5 – 0.2 V vs. Mg2+/Mg and at low current densities exhibited diminished polarization. The average cell potential is 1.4 V, entailing an energy density of 190 W h kg-1­ at the materials’ level. This piece of work provides an effective strategy for designing multi-electron storage for high-energy rechargeable Mg batteries, but more efforts are required to overcome high polarization during charge-discharge cycles which are commonly found in Mg batteries.