Impact of Sodium Metal Plating on Cycling Performance of Layered Oxide/Hard Carbon Sodium-ion Pouch Cells with Different Voltage Cut-offs

This study investigates the cycling performance and failure mechanisms of sodium-ion pouch cells with layered NaCa_0.03[Mn_0.39Fe_0.31Ni_0.22Zn_0.08]O₂ positive electrodes and hard carbon negative electrodes. Charge/discharge cycling between different lower and upper cut-off voltages at C/5 and 40 °C showed better capacity retention, lower voltage polarization, and less gassing when the upper cut-off voltage was limited to 3.80 V. Electrodes harvested from pouch cells after cycling were reassembled in symmetric coin cells to reveal the origin of voltage polarization by electrochemical impedance spectroscopy. The negative electrode charge transfer resistance dominated the full cell impedance and increased considerably after 100 cycles at 40 °C with standard alkyl carbonate electrolyte. The positive electrode impedance was less significant but increased dramatically when the full cell voltage was 4.00 V. Furthermore, ultra-high precision coulometry used for the in situ detection of sodium plating at 40 °C, revealed significant plating at charging rates greater C/2. Based on this failure analysis, long-lived sodium-ion cells with 97% capacity retention after 450 cycles at 40 °C could be realized by selecting appropriate voltage cut-offs, C-rates, and effective electrolyte additives that lowered the cell resistance and suppressed gas generation.