High-rate formation protocol enables a high ionic conductivity SEI for sodium-ion batteries

The formation process for lithium-ion batteries has been comprehensively investigated, while the optimization protocols for sodium-ion batteries are barely discussed. The relationship between the formation procedure and the interfacial characteristics of the electrodes, especially the solid-electrolyte interphase (SEI) on the anode, is of significance to the cell performance. Herein, we examine three formation protocols within a fixed potential range in superconcentrated ionic liquid electrolytes, and successfully shorten the formation time by 38x while maintaining or even enhancing the cycling performance for the sodium-ion half cell. We also correlate the SEI structure with the electrochemical performance, demonstrating that high C-rate formation contributes to a highly conductive and thinner SEI layer, which facilitates the Na+ diffusion kinetics across the electrode/electrolyte boundaries. NMR also indicates a reduced amount of NaF based SEI products on the high C-rate electrodes, consistent with XPS results. This work provides an optimized formation protocol for NIBs that is cost-effective and time efficient.