Mechanisms of Ionic Diffusion and Stability of the Na4MnCr(PO4)3 Cathode

The NASICON-type polyanionic compounds are promising cathode materials for sodium-ion batteries (SIBs) due to their robust framework and high work voltage. Motivated by the recent synthesis of high-performance Na4MnCr(PO4)3(NMCP) [Zhang et al. Adv. Mater. 2020, 32, 1906348] that exhibits a reversible three-electron process with a high energy density of 566.5 Wh/kg, we provide an in-depth theoretical study on the underlying mechanisms of ion diffusion and stability for a better understanding of the experimental results. We self-consistently calculate the Hubbard U parameters for Mn and Cr in the NMCP system using the linear response approach and successfully reproduce the three voltage plateaus observed in the experiment. At the low voltage plateau, the Na+ ions diffuse with both concerted and stepwise migration mechanisms, and the corresponding energy barrier is 0.18 and 0.21 eV. The synergy of these two mechanisms results in fast diffusion kinetics for the Na ion in NMCP. Besides, the redox couples of Mn2+/Mn3+, Mn3+/Mn4+, and Cr3+/Cr4+ are confirmed theoretically in good agreement with the experiment. Despite the distinct changes of O-2p states during the charging/discharging process, the NASICON framework of NMCP withstands the formations of O2 or (O2)2–, thus exhibiting high stability. Especially, we have identified the locking effect of Na+ ions at low Na+ concentration due to the large site energy difference and weak concerted migration, which can be effectively modulated by enlarging the lattice constants to improve the performance of NMCP during cycling.