Hierarchical cathode constructed by carbon coated Na 3.5 VMn 0.5 Cr 0.5 (PO 4 ) 3 nanoparticles on rGO for high-capacity and long-cycle life sodium storage

Mn-substituted NASICON-type Na 3+ x Mn x V 2− x (PO 4 ) 3 compounds have been extensively studied as desirable cathode materials for sodium-ion batteries (SIBs) due to their higher operating voltage, low cost and weak biological toxicity compared with Na 3 V 2 (PO 4 ) 3 . However, they present limited reversible capacity and cycling properties originating from the low electronic conductivity and irreversible phase transition caused by the Jahn–Teller active Mn 3+ . Herein, the electronic conductivity and structural stability of the material are ameliorated by constructing a double-carbon-layer hierarchical structure and substitution of Mn by Cr. As expected, the unique hierarchical Na 3.5 VMn 0.5 Cr 0.5 (PO 4 ) 3 @C/rGO electrode demonstrates excellent sodium-ion storage properties, consisting of a 2.4-electron redox reaction, high energy density (472 W h kg −1 ), cycling performance with 94.7% capacity retention after 1600 cycles at 10C, along with a retention of 81% after 8000 cycles at 20C. Moreover, the full cell based on the Na 3.5 VMn 0.5 Cr 0.5 (PO 4 ) 3 @C/rGO cathode and hard carbon anode demonstrates a reversible capacity of 119 mA h g −1 with an energy density of 405.8 W h kg −1 at 0.2C, and a high capacity retention ratio of 94.6% at 1C after 200 cycles. Such dual-carbon hierarchical engineering will facilitate the application of NASICON-type cathodes in sodium ion batteries for grid-scale energy storage systems.