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

Abstract

Mn-substituted NASICON-type Na_3+xMn_xV_2−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 Na_3.5VMn_0.5Cr_0.5(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 Na_3.5VMn_0.5Cr_0.5(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.