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

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-VOPO4 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-VOPO4 results in the irreversible structural phase transition to a new, expanded orthorhombic Na2VOPO4 phase, which can be reversibly cycled to an orthorhombic NaVOPO4. However, further extraction of Na ions from this new orthorhombic NaVOPO4 phase could not be realized even after charging to 4.3 V vs Na+/Na0.