Influence of Formation Temperature on Cycling Stability of Sodium-Ion Cells: A Case Study of Na3V2(PO4)2F3 | HC Cells

Abstract

Sodium-ion batteries are cheaper and attractive alternatives to lithium-ion batteries, particularly for low energy applications.1 Moreover, the Na+ cation, with its larger ionic radius and lower charge density than Li+, exhibits a weaker interaction within the ionic framework of electrode materials, as well as in solution, resulting in the higher power-rate capability of sodium materials. Hence, sodium-ion batteries based on Na3V2(PO4)2F3 (NVPF) | hard carbon (HC) and Prussian blue analogs (PBA) I HC chemistries are primarily promoted for their high power applications (nearly 80% charging of the cell within 10min). Of particular interest is the NVPF|HC system, due to its high energy density (~120 Wh/kg achieved in 18650 cell level) and high structural stability of NVPF material for long cycling.Our studies, together with others published in recent years, show that the electrode-electrolyte interphase (EEI) formed in the Na-ion cell is not stable as it partly dissolves during rest or discharge periods, and re-forms when recharging the cell.2 This dissolution and reformation cycle can lead to poor cycling stability and an increase in EEI thickness. A thicker EEI results in a higher impedance, which decreases the performance of the cell at high charge/discharge rates. Thus, different strategies have been explored to modify the EEI properties and enhance its stability, mainly focused on the incorporation of SEI-forming additives like vinylene carbonate or fluoroethylene carbonate.3 These additives could eventually stabilize the interphase but with the cost of increasing the interfacial impedance, hence affecting the power rate capability of the Na-ion cells, as mentioned before. Thus, different strategies are needed that allow for the formation of a stable interphase displaying high ionic conductivity (less resistance).Here, we show that it is possible to achieve a stable, ionically conducting interphase modulating the temperature during formation cycles. First, we demonstrated that the nature and chemical composition of the interphase changes as a function of the temperature applied during the formation cycles. The temperature of the cell is expected to play a role in the dissolution or precipitation of different chemical components, which helps us to tune the formation protocol, producing a stable and conducting interphase with or without electrolyte additives. The cells subjected to the optimized formation protocol exhibit low impedance, high power rate capability and long cycling stability. The results presented in this study benefit in great measure the future commercialization of NVPF|HC cells, but also are expected to be of value for other sodium cells based on HC anodes, by providing a framework to optimize the EEI formation and characteristics. References Tarascon, J.-M. Na-ion versus Li-ion Batteries: Complementarity Rather than Competitiveness. Joule 4, 1616–1620 (2020).Ma, L. A., Mogensen, R., Naylor, A. J. & Younesi, R. Solid Electrolyte Interphase in Na-ion batteries. in Na-ion Batteries 243–264 (John Wiley & Sons, Ltd, 2021). doi:10.1002/9781119818069.ch6.Eshetu, G. G. et al. Electrolyte Additives for Room-Temperature, Sodium-Based, Rechargeable Batteries. Chem. – Asian J. 13, 2770–2780 (2018).