Role of Electrolyte in Stabilizing the Solid Electrolyte Interface of Hard Carbon As an Anode for Sodium-Ion Batteries

Hard carbon (HC) is an attractive anode material for sodium-ion batteries (NIBs) because it can be made from biomass or plastic, has a high capacity of ~300 mAh/g, and an extremely low average voltage near 0V. When fabricated with cathodes free of Li, Ni, and Co, NIBs using HC anodes are a promising approach for grid storage because they are energy-dense, sustainable, and have a low cost of energy. In order for these batteries to become a practical long-life solution, the issues of low first coulombic efficiency and poor rate performance of HC in conventional carbonate electrolytes need to be addressed. Two mechanisms that potentially cause these issues to occur are changes to the HC structure affecting sodium storage and the formation of the solid electrolyte interface (SEI). The proper selection of electrolyte is critical for controlling these mechanisms. To observe and characterize these mechanisms, we compared HC using a conventional carbonate electrolyte, 1M NaPF₆ in propylene carbonate (PC), with a high performing glyme based electrolyte, 1M NaBF₄ in tetraethylene glycol dimethyl ether (TEGDME). Compared to the poor rate capability of HC in PC electrolyte, HC in TEGDME electrolyte shows an excellent rate capability of ~265 mAh/g at C/20 and ~260 mAh/g at C/3. Titration gas chromatography and Raman spectroscopy were used to show that the HC with either electrolyte had the same sodium storage mechanism with no "stuck" sodium in the HC structure after the first cycle. These observations suggest that SEI formation is responsible for the irreversible capacity differences. The SEI formation was explored with scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). The PC electrolyte's SEI is found to be thick, fluffy, irregular and composed of polyesters, alkyl carbonates, Na_xPF_yO_z, and NaF. The TEGDME electrolyte's SEI is found to be thin, conformal, uniform, and composed of polyethers, sodium alkoxides, Na_xBF_yO_z, and NaF. Additionally, the PC electrolyte's SEI is rate dependent with more unstable products formed at high rates whereas the TEGDME electrolyte's SEI is not rate dependent. These findings show that HC anode's rate capability is controlled by the quality of the SEI formed by the electrolyte rather than the HC material structure. This model of rate and electrolyte dependent SEI indicates that SEI engineering is a pathway to improve HC as an anode for long-life grid-level NIBs.

Figure 1