Evaluation of Electrolyte Salts and Solvents for Na-Ion Batteries in Symmetric Cells

Abstract

The cycle life of NaCrO2 in NaPF6 and NaTFSI in PC and EC/DEC (1/2 v/v) electrolytes were studied in half-cells and symmetric cells. Half-cells were found to be not useful for evaluating the cycling performance due to impedance growth at the Na foil electrode. Symmetric cells containing no Na foil were found to be more useful when used for cycling performance evaluation instead of half-cells. NaCrO2 in 1M NaTFSI in PC and NaPF6 in EC/DEC electrolytes performed well in symmetric cells, having coulombic efficiencies of 99.94% and greater. We believe that these are the highest coulombic efficiencies reported for Na-ion battery materials. In contrast, half-cell performance did not reflect the cycling performance of NaCrO2 and had no correlation to the symmetric cell performance. This illustrates the importance of symmetric cells for the evaluation of Na-ion battery materials. In addition to these findings, it was found that 1M NaTFSI salt solutions caused significant corrosion of the Al current collector, which could be suppressed by additions of NaPF6.