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

Na₃V₂(PO₄)₃, based on multi-electron reactions between V³⁺/V⁴⁺/V⁵⁺, is a promising cathode material for SIBs. However, its practical application is hampered by the inferior conductivity, large barrier of V⁴⁺/V⁵⁺, and stepwise phase transition. Herein, these issues are addressed by constructing a medium-entropy material (Na_3.2V_1.1Ti_0.2Al_0.2Cr_0.2Mn_0.2Ni_0.1(PO₄)₃, 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 (V³⁺/V⁴⁺) and 4.0 V (V⁴⁺/V⁵⁺), 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^-1 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 MEO₆ 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.