Unravelling the ion transport and the interphase properties of a mixed olivine cathode for Na-ion battery

Abstract

The replacement of Li by Na in an analogue battery to the commercial Li-ion one appears a sustainable strategy to overcome the several concerns triggered by the increased demand for the electrochemical energy storage. However, the apparently simple change of the alkali metal represents a challenging step which requires notable and dedicated studies. Therefore, we investigate herein the features of a NaFe0.6Mn0.4PO4 cathode with triphylite structure achieved from the conversion of a LiFe0.6Mn0.4PO4 olivine for application in Na-ion battery. The work initially characterizes the structure, morphology and performances in sodium cell of NaFe0.6Mn0.4PO4, achieving a maximum capacity exceeding 100 mAh g−1 at a temperature of 55 °C, and adequate rate capability, and suitable retention confirmed by ex-situ measurements. Subsequently, the study compares in parallel key parameters of the NaFe0.6Mn0.4PO4 and LiFe0.6Mn0.4PO4 such as Na+/Li+ ions diffusion, interfacial characteristics, and reaction mechanism in Na/Li cells using various electrochemical techniques. The data reveal that relatively limited modification of NaFe0.6Mn0.4PO4 chemistry, structure and morphology compared to LiFe0.6Mn0.4PO4 greatly impact the reaction mechanism, kinetics and electrochemical features. These changes are ascribed to the different physical and chemical features of the two compounds, the slower mobility of Na+ with respect to Li+ , and a more resistive electrode/electrolyte interphase of sodium compared with lithium. Relevantly, the study reveals analogue trends of the charge transfer resistance and the ion diffusion coefficient in NaFe0.6Mn0.4PO4 and LiFe0.6Mn0.4PO4 during the electrochemical process in half-cell. Hence, the NaFe0.6Mn0.4PO4 achieved herein is suggested as a possible candidate for application in a low-cost, efficient, and environmentally friendly Na-ion battery.