Unlocking the Excitation Mechanism of Unpaired Electrons of High-Valent Vanadium Ions in a NASICON Cathode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries (LIBs) due to their stable cycling performance, low cost, and abundance of sodium resources. Among cathodes of SIBs, sodium superionic conductors (NASCIONs) have garnered significant attention due to their unique 3D framework, high thermal stability, and high ionic conductivity. Activation of multiple electron transfer in NASICON materials is crucial for improving energy density, but the activation mechanism of the high-valent V-platform, especially V⁴⁺/V⁵⁺ redox, is currently understudied. To this end, we have synthesized Na₃Cr_<i>x</i>V_2-<i>x</i>(PO₄)₃ (<i>x</i> = 0, 0.25, 0.5, 0.75, 1) cathode materials with controlled Cr doping ratios. When meticulously tuning the Cr doping levels to <i>x</i> = 0.5, the specific capacity can be strikingly optimized with a high plateau at around 4 V (vs Na⁺/Na) and a higher capacity retention of 89.1% after 2500 cycles. The electron paramagnetic resonance (EPR) and theoretical calculations show that the spin angular momentum of unpaired electrons leads to spin polarization and their magnetic moment results in electron spin-nuclear spin coupling. Therefore, the overall magnetic moment of the material is increased after chromium doping. Meanwhile, the unpaired electrons filling the orbitals leads to the hybridized metal p, d, and f orbitals, which can reduce the V band gap and in turn lower the energy barrier for electron migration, promoting the V⁴⁺/V⁵⁺ redox coupling in Na₃Cr_<i>x</i>V_1-<i>x</i>(PO₄)₃. This discovery refines the doping strategy for vanadium-based cathode materials and facilitates the understanding of multielectron reactions in SIBs.