A Phase-Transition-Free Sodium Vanadium Phosphate Cathode via Medium-Entropy Engineering for Superior Sodium Ion Batteries.

NaV(PO), based on multi-electron reactions between V/V/V, is a promising cathode material for SIBs. However, its practical application is hampered by the inferior conductivity, large barrier of V/V, and stepwise phase transition. Herein, these issues are addressed by constructing a medium-entropy material (NaVTiAlCrMnNi(PO), ME-NVP) with strong ME─O bond and highly occupied Na2 sites. Benefiting from the medium-entropy effect, ME-NVP manifests a phase-transition-free reaction mechanism, two reversible plateaus at 3.4 (V/V) and 4.0 V (V/V), and small volume change (2%) during Na insertion/extraction processes, as confirmed by comprehensive in/ex situ characterizations. Moreover, kinetics analysis illuminates the superior Na diffusion ability of ME-NVP. Thus, the ME-NVP cathode realizes remarkable rate capability of 67 mA h g at 50C and a long-term lifespan over 10 000 cycles (capacity retention of 81.3%). Theoretical calculations further illustrate that the weak binding of Na ion in the channel is responsible for the rapid Na diffusion, accounting for the superior reaction kinetics. Moreover, rigid MEO octahedral and feasible rearrangement of Na ions can suppress the phase transition, thus endowing an ultrastable ME-NVP cathode. This work highlights the significant role of medium-entropy engineering in advancing the output voltage, cycling stability, and rate capability of polyanionic cathodes.