Enabling high-performance Na4MnV(PO4)3 cathode via synergetic strategy of carbon encapsulation and nanoengineering

NASICON-type materials with three-dimensional ionic migration pathways are proposed as the most important cathode candidates for sodium ion batteries. To promote the real applications, Na4MnV(PO4)3 (NMVP) with low cost and high energy density is highly recommended by partial substitution of V with earth-abundant active element Mn. Nevertheless, NMVP still encounters the problems of poor electrical conductivity and fair reversibility of Mn2+/Mn3+ redox couple, resulting in unsatisfactory cycling stability. Herein, carbon-encapsulated NMVP nanoroads are synthesized by a facile solid-state reaction in molten hydrocarbon media. The synergistic effects of carbon encapsulation and nanoengineering enable high electronic conductivity and large electrolyte-accessible surface areas of NMVP/C cathode, as well as highly reversible phase transitions upon repeated Na+ insertion/extraction. Thus, the NMVP/C nanoroads deliver excellent electrochemical performances, including high reversible capacity (112.3 mA h g−1 at 0.02 A g−1), excellent rate capability (58.3 mA h g−1 at 5 A g−1), and good cycling stability (85.1% of capacity retention over 1200 cycles at 1 A g−1). Moreover, the storage mechanism of Na+ is investigated by ex situ X-ray diffraction, and the fast ionic/electronic transport kinetics are revealed by combining electrochemical methods with density functional theory (DFT) calculations.