From Solid-Solution MXene to Cr-Substituted Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 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.