Carbon-coating-increased working voltage and energy density towards an advanced Na₃V₂(PO₄)₂F₃@C cathode in sodium-ion...

One main challenge for phosphate cathodes in sodium-ion batteries (SIBs) is to increase the working voltage and energy density to promote its practicability. Herein, an advanced Na₃V₂(PO₄)₂F₃@C cathode is prepared successfully for sodium-ion full cells. It is revealed that, carbon coating can not only enhance the electronic conductivity and electrode kinetics of Na₃V₂(PO₄)₂F₃@C and inhibit the growth of particles (i.e., shorten the Na⁺-migration path), but also unexpectedly for the first time adjust the dis-/charging plateaux at different voltage ranges to increase the mean voltage (from 3.59 to 3.71 V) and energy density (from 336.0 to 428.5 Wh kg^-1) of phosphate cathode material. As a result, when used as cathode for SIBs, the prepared Na₃V₂(PO₄)₂F₃@C delivers much improved electrochemical properties in terms of larger specifc capacity (115.9 vs. 93.5 mAh g^-1), more outstanding high-rate capability (e.g., 87.3 vs. 60.5 mAh g^-1 at 10 C), higher energy density, and better cycling performance, compared to pristine Na₃V₂(PO₄)₂F₃. Reasons for the enhanced electrochemical properties include ionicity enhancement of lattice induced by carbon coating, improved electrode kinetics and electronic conductivity, and high stability of lattice, which is elucidated clearly through the contrastive characterization and electrochemical studies. Moreover, excellent energy-storage performance in sodium-ion full cells further demonstrate the extremely high possibility of Na₃V₂(PO₄)₂F₃@C cathode for practical applications.