ИСТИНА

NASICON-type Na3V2(PO4)3 cathode materials are considered as promising candidates for high-performance Na-ion batteries due to extremely long cyclic stability and an outstanding ability to operate at high (dis)charge rates. However, its cost-effectiveness can still be improved using transition metals cheaper than vanadium, which could also increase the energy density compared to that in Na3V2(PO4)3. The replacement of V with Mn and Cr in Na3V2(PO4)3 lowers the cost of the materials and enhances the operation voltage. Substitution of V by electrochemically inactive Mg and Sc allows studying vanadium redox processes without contributions from these metals (Figure 1). Figure 1 Comparison of galvanostatic charge curves of NASICON compounds with different composition. When the voltage window above 3.8 V is utilized, Mn2+ substitution enhances the energy density of Na3+xMnxV2-x(PO4)3 compounds for up to 10% compared with the Na3V2(PO4)3. The evolution of the transition metal oxidation states in Na4MnV(PO4)3 during charge and discharge was studied by X-ray absorption spectroscopy (XAS) in ex situ and operando regimes using an electrochemical cell with X-ray transparent windows. In order to link electrochemical features with the phase transformations in NASICON samples operando X-ray powder diffraction was carried out (Figure 2). The crystal structure changes on charge and discharge were studied as well as the electrode performance in narrow (2.5-3.8 V), wide (2.5-4.5 V) and extra wide (1.0-4.5 V) potential windows. Figure 2 Schematic illustration of the Na+ deintercalation regimes from Na3+xMnxV2-x(PO4)3 (0≤x≤1). The results indicate that in case of narrow voltage window the Na+ (de)intercalation reaction is reversible, however for the V-substituted compounds charging above 3.8 V increases energy density, but leads to the loss of reversibility and capacity fade. However, for some compounds an overdischarge below 2.5 V suppresses the capacity fade [1]. The experimental results indicate the benefits of vanadium-substituted compounds and ability to outperform the unsubstituted materials in terms of rate-capability. Therefore they should be preferred for high-power applications. Acknowledgements This work was supported by the Russian Science Foundation (Grant No. 17-73-30006-P). References [1] Perfilyeva, T.I., Drozhzhin, O.A., Alekseeva, A.M., Zakharkin, M.V., Mironov, A.V., Mikheev, I.V., Bobyleva, Z.V., Marenko, A.P., Marikutsa, A.V., Abakumov, A.M., Antipov, E.V. // Journal of The Electrochemical Society 2021, 168, p.110550