Herein we report key developments on the scale-up of sodium ion anode free batteries through investigation of the effects of applied external pressure cell performance. Sodium ion anode free 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. We demonstrate excellent Na||Cu half-cell results in coin cell configuration, and the scalability of the anode-free concept is further demonstrated, by plating and stripping of sodium metal on copper foils 10-fold larger (>10 cm ²) than in other studies in coin cells (∼1 cm²). It is discovered that pressure is paramount in establishing dendrite free sodium deposition at this scale through investigating the half-cell cycling at 56–743 kPa. Achieving a low hysteresis in these large-area cells is found to only require moderate pressures (∼185 kPa). However, achieving a high cycle life required increasing the pressure to 743 kPa. It is only at these high pressures that non-dendritic sodium deposition is demonstrated due to a homogeneous plating distribution enabled by proper contact between electrodes, as confirmed by impedance measurements and optical imaging of the deposited sodium.

This study investigated the impact of pouch cell design on energy density, both volumetric and gravimetric, through the development of accurate 3D models of small-format (<5 Ah) pouch cells. Various configurations were analysed, considering material properties and extrapolating expected electrochemical performance from studies on Prussian white cathodes in coin and pouch cells. This approach allowed for a rapid assessment of several performance-influencing factors, including the number of layers in the pouch cell, cathode thickness, active material percentage, and electrolyte volume. The highest calculated energy density of small-format pouch cells was shown to be 282 Wh kg⁻¹ and 454 Wh L⁻¹, achieved in a 3 Ah, 20-layer pouch cell. The calculations were validated using sodium-ion anode-free pouch cells utilising a Prussian white cathode in single- and few-layer format pouch cells (<0.1 Ah) cycled under a low external pressure (~200 kPa).

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. References: [1] Chengtian Zhou et al 2021 J. Electrochem. Soc. 168 060532 Figure 1