The formation process is a critical manufacturing step for battery cells, involving the initial charging and discharging. This process, particularly the electrolyte composition and initial cycles, has a significant, irreversible impact on the cell’s performance over its entire lifespan. Although manufacturers typically do not disclose their formation methods and electrolyte compositions, this research aims to investigate the effects of electrolytes on these processes in commercially available, unformed sodium-ion cells, resulting in a broader investigation than has been published in the scientific community to date.

In this study, dry 150 mAh sodium-ion cells were formed using NaPF6-based electrolytes. The formation procedure was kept constant throughout the work to improve the comparability between electrolyte compositions. The cyclic solvents tested include propylene carbonate and ethylene carbonate, while linear carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and ethyl propionate were investigated. Fluoroethylene carbonate was used both as an additive and a solvent. The additives under investigation were dioxathiolandioxide and propanesultone.

After formation, the gas composition for each electrolyte combination was measured using GC-MS. Electrochemical impedance spectroscopy (EIS) was employed to determine the ohmic and charge transfer resistance at 25°C. Additionally, the float currents were analysed in the temperature range from 25°C to 40°C. Subsequently, most of the cells were cycled at C/3 rates at 10°C, 25°C, and 40°C, with rapid performance tests conducted every 50 cycles to assess capacity, quasi open-circuit voltage (qOCV), pulse resistance, and impedance.

In general, a strong correlation was observed between the float currents and cyclic ageing behaviour. The gas compositions displayed significant qualitative and quantitative differences across all electrolyte combinations. Initial EIS measurements were able to identify the least effective electrolytes. The lifetime of the cells at 40°C was primarily limited by excessive gas formation rather than capacity retention for the best-performing electrolytes. At 10°C, some electrolytes that initially appeared promising demonstrated only mediocre ageing behaviour compared to alternatives. X-ray spectroscopy and post-mortem analysis were used to further understand the ageing behaviour, focusing on sodium plating, gassing, and SEI (solid electrolyte interphase) formation.

In summary, promising electrolyte combinations have been discovered, which could significantly boost the advancement of sodium-ion batteries.