Synthesis, characterization, and degradation study of Mn-based phosphate frameworks (Na3MnTi(PO4)3, Na3MnPO4CO3, Na4Mn3(PO4)2P2O7) as aqueous Na-ion battery positive electrodes

Aqueous electrolyte Na-ion batteries based on abundant materials offer one of the attractive alternatives to replace conventional Li-ion batteries in the field of stationary electricity storage. However, the search for and development of stable, low-cost and compatible positive electrode materials remains a significant challenge. Mn-based materials are deemed to be among the most suitable due to their abundance, environmental compatibility and suitable electrochemical properties. In this work, three different phosphate and mixed phosphate framework compounds Na3MnTi(PO4)3, Na3MnPO4CO3, and Na4Mn3(PO4)2P2O7 are successfully prepared and comprehensively characterized as potential aqueous Na-ion positive electrode materials using a wide range of methods such as impedance spectroscopy, cyclic voltammetry, galvanostatic charge/discharge cycling. The degradation of these materials is studied by rotating ring-disc electrode technique. The results clearly show that charge capacity loss observed in these materials is related to the electrochemical and electrochemically-induced chemical dissolution of manganese from the electrode. The results obtained using a combination of different methods also indicate that a significant part of Mn is electrochemically inactive or inaccessible in such Mn-based phosphate frameworks. The fraction of electrochemically inactive Mn also tends to increase during the electrochemical operation, limiting the performance of these materials as potential battery electrodes in aqueous electrolytes.