High-voltage structural evolution and its kinetic consequences for the Na4MnV(PO4)3 sodium-ion battery cathode material

The recently demonstrated possibility of high-voltage extraction of more than 2 Na ions per Na4MnV(PO4)3 formula unit is considered as a possible way to increase the specific energy of this material, which would open up prospects for its application in sodium-ion batteries. However, for the Na4MnV(PO4)3 material and for a wide group of related NASICON-structured materials, high-voltage Na extraction was demonstrated to lead to a rapid decrease in capacity and a sharp increase in hysteresis between the charge and discharge curves. In this work, we use electrochemical and structural analysis to elucidate the reasons for the degradation observed after deep desodiation of Na4MnV(PO4)3. X-ray diffraction analysis of the electrodes after cycling demonstrates that a phase transformation is initiated when the anodic potential limit exceeds 3.8 V, which results in the formation of distorted NASICON-type phases and gradual amorphization. X-ray absorption spectroscopy studies suggest that the observed detrimental phase transitions might be triggered by the changes in the local environment of oxidized vanadium, which destabilize the structure. We thus demonstrate that the degradation of the Na4MnV(PO4)3 material is of thermodynamic origin, with its consequences being reflected in a dramatically decreased de/intercalation rates and a rapid deterioration in the material performance.