Anode-Free Sodium Ion Batteries: Effect of Pressure on Sodium Plating on Copper

Abstract

Sodium ion anode-free battery technology puts extra emphasis on high plating and stripping efficiency of sodium metal at the anode surface, due to the lack of an excess of the transporting ion. Recent studies have shown the importance of strict control of water content in enabling efficient plating and stripping of sodium on copper in the 1M NaPF6 in diglyme electrolyte[1]. We have achieved promising results at coin cell level in this configuration: 99.9% coulombic efficiency over 400 cycles, low nucleation overpotential of 40mV and 10mV hysteresis at 0.2mA cm-2, without dendrite formation. To demonstrate the scalability of the anode-free concept, we attempted plating and stripping of sodium metal on copper foils with areas that are 10-fold higher (>10 cm2) than those previously achievable in coin cells (~1cm2) using a split pouch cell configuration. Furthermore, we investigate the effect of pressure at this scale by using two methods. The initial method involves increasing spacer thickness, which acts to compress a wave spring within the split pouch cell. These experiments demonstrate that the plating and stripping voltage hysteresis can be decreased from 125mV -> 40mV -> 10mV, with increasing spacer thickness on the order 3mm -> 3.5mm -> 4mm. The hysteresis observed in coin cells is then matched when 4mm of spacers is used in split pouch cells, indicating a similar pressure applied in these two cell formats. Voltage instabilities are also reduced by increasing the spacer thickness, indicating the formation an increasingly stable solid electrolyte interface (SEI). Secondly, the optimal pressure for the Na||Cu half-cell is investigated by using a pressure calibration unit to accurately measure the applied force within a compressive jig.