From Solid-Solution MXene to Cr-Substituted Na₃V₂(PO₄)₃: Breaking the Symmetry of Sodium Ions for High-Voltage and Ultrahigh-Rate Cathode Performance

Stabilizing Na⁺ accessibility at high voltage and accelerating Na⁺ diffusivity are pressing issues to further enhance the energy density of the Na₃V₂(PO₄)₃ (NVP) cathode for sodium-ion batteries (SIBs). Herein, by taking a V/Cr solid-solution MXene as a precursor, a facile in-situ reactive transformation strategy to embed Cr-substituted NVP (NVCP) nanocrystals in a dual-carbon network is proposed. Particularly, the substituted Cr atom triggers the accessibility of additional Na⁺ in NVCP, which is demonstrated by an additional reversible redox plateau at 4.0 V even under extreme conditions. More importantly, the Cr atom alters the Na⁺ ordering at the Na2 sites with an additional intermediate phase formation during charging/discharging, thus reducing the energy barriers for Na⁺ migration. As a result, Na⁺ diffusivity in NVCP accelerates to 2-3 orders of magnitude higher than that of NVP. Eventually, the NVCP cathode exhibits extraordinarily high-rate capability (78 mA g^-1 at 200 C and 68975 W kg^-1), outstanding cycle stability (over 1500 cycles at 10 C), excellent low-temperature property, and full cell performance.