Ultrasound-assisted synthesis of sodium powder as electrode additive to improve cycling performance of sodium-ion batteries

Excessive solid electrolyte interphase (SEI) buildup in the formation cycles or later cycles consumes electrolyte, depletes alkaline ion availability, and increases cell polarization; the reduction in the amount of available alkaline ions upon cycling often causes low capacity and poor capacity retention in full cells. To compensate the Na loss from SEI formation, we have developed a sodium-powder based pre-sodiation technique that could potentially be applied to both anode and cathode materials with minimal modification to conventional battery making process. The synthesis of sodium powder is accomplished via ultrasonic dispersion of molten sodium metal in mineral oil. When suspended in hexane, the sodium powder can be easily applied onto electrodes as pre-sodiation additive. In the half cell study with glucose derived carbon (GC1100), pre-sodiation decreases the initial open circuit cell potential (∼1 V drop) and reduces the first cycle irreversible Coulombic efficiency (from 19.3% to 8%). In the full cell study with GC1100 and NaCrO2, pre-sodiation leads to ∼10% improvement in cycling capacity and ∼5% increase in energy density. Decreased cell polarization is also observed in pre-sodiated cells.