Increasing the energy density of sodium-ion battery systems requires highvoltage cathode materials to compensate for the inherently higher redox potential of the Na/Na+ couple compared to the Li/Li+ couple (difference of 0.3 V). Distinct from the high-voltage (>4.2 V) operation using late transition metals (Co-3+Co-/(2+) or Ni3+/Ni2+) in previously reported polyanionic compounds, here we identify the record-high Cr4+/Cr3+ (3d(2)/3d(3)) redox potential in Na3-xCr2(PO4)(2)F-3 (0 < x < 1) at 4.7 V vs Na/Na+. Despite higher d-band position at early 3d transition metal with smaller effective nuclear charge compared to late transition metals, Cr 3d(t(2g)) in less than half-filled state possesses no energy level increments arising from crystal field splitting and intra-orbital Coulombic penalty, leading to extremely high voltages of the Cr4+/Cr3+ (3d(2)/3d(3)) redox couple.

<title>Abstract</title>Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis &gt;0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O₂²⁻ dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na_{2-<italic>x</italic>}Mn₃O₇, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O^−• was identified in Na_{2-<italic>x</italic>}Mn₃O₇. Computations reveal that O^−• is thermodynamically favorable over the peroxide-like O₂²⁻ dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

<p>NASICON-type Na₂CrTi(PO₄)₃ offers a stable redox reaction of Cr⁴⁺/Cr³⁺ at 4.5 V <italic>vs.</italic> Na/Na⁺.</p>