Hierarchical cathode constructed by carbon coated Na3.5VMn0.5Cr0.5(PO4)(3) nanoparticles on rGO for high-capacity and long-cycle life sodium storage

Mn-substituted NASICON-type Na3+xMnxV2-x(PO4)(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 Na3V2(PO4)(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 Mn3+. 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 Na3.5VMn0.5Cr0.5(PO4)(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 Na3.5VMn0.5Cr0.5(PO4)(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.