On the Capacity and Cycle Stability of Na3MnCO3PO4 - a High Capacity, Multi-Electron Transfer Redox Cathode Material for Sodium Ion Batteries

Na₃MnCO₃PO₄ has been predicted via ab initio calculations (Hautier, et al. 2011) to have a high specific capacity of 191 mAh/g, owing to its potential to deliver two-electron transfer reactions per formula via Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox reactions. This study demonstrates, for the first time, that Na₃MnCO₃PO₄ can indeed display a specific capacity of 176.7 mAh/g experimentally, reaching 92.5% of its theoretical. The low electronic conductivity is found to be the limiting factor for the previously observed low specific capacities for Na₃MnCO₃PO₄. Furthermore, the charge/discharge cycle life of the cathode is found to depend on the carbon concentration. The capacity retention improves with increasing the carbon concentration. This finding has been attributed to the improved uniformity in the participation of most of Na₃MnCO₃PO₄ particles in redox reactions. With a specific capacity as high as 176.7 mAh/g, a clearly identified direction to enhance the capacity retention, and low cost of Na sources, Na₃MnCO₃PO₄ has a great potential to be a viable cathode material for Na-ion batteries for large-scale energy storage.