Unraveling the Multielectron Redox Reactions of β-VOPO₄ for Sodium Ion Batteries

Abstract

Realizing multielectron redox reactions is a promising strategy to improve the specific capacity and energy density of rechargeable batteries. Although reversible two-electron redox has been demonstrated for a few cathode materials for Li-ion batteries, few examples are known for Na-ion batteries. Here, we have investigated the two-electron redox of β-VOPO 4 for Na-ion intercalation via a combination of X-ray diffraction and absorption spectroscopic methods to elucidate the evolution of crystal and electronic structures. The insertion of two Na ions per formula unit of β-VOPO 4 results in the irreversible structural phase transition to a new, expanded orthorhombic Na 2 VOPO 4 phase, which can be reversibly cycled to an orthorhombic NaVOPO 4 . However, further extraction of Na ions from this new orthorhombic NaVOPO 4 phase could not be realized even after charging to 4.3 V vs Na + /Na 0 .