Abstract:

Na metal-based coin-type half-cells are widely used to evaluate the electrochemical performance of electrode materials. This work presents the limitations of Na half-cells when evaluating the low-temperature performance of electrode materials in a commercial ester electrolyte. Due to the high interface and charge transfer resistance of Na metal at low temperatures, a large deposition/stripping overpotential was reached. This interferes with the evaluation of the low-temperature performance. When the Na||hard carbon (HC) half battery was charged/discharged at 0.2C (1C = 300 mA/g) at -20 ℃, the change in potential of Na metal is as high as 0.94 V. The specific capacity of the HC electrode material is only 21.1 mAh/g, thus an inaccurate evaluation of the electrochemical performance is likely. Herein, a Na₁₅Sn₄@Na composite electrode was used to evaluate the performance of electrode materials at low temperature. The electrode potential of the composite is the same than that of the Na metal. At -20 ℃, the deposition/stripping overpotential of the Na₁₅Sn₄@Na||Na₁₅Sn₄@Na cell is only 0.09 V at 0.1^{mA/cm2, much smaller than that of the Na metal electrode (0.96 V). In the Na₁₅Sn₄@Na||HC half-cell, the HC anode exhibits a high specific capacity of 100.8mAh/gbefore Na metal deposition at -20 ℃, much higher than that of the Na||HC half-cell (21.1 mAh/g), indicating that the Na₁₅Sn₄@Na-based half-cell would allow for a more accurate evaluation of the low-temperature performance of electrode materials. This work provides an experimental basis for accurate assessment of the low-temperature electrochemical performance.}

Key words: Na metal, Na battery, low temperature, electrochemistry, half-cell