Importance of First Cycle Conditions on the Electrochemical Performance of Hard Carbon and Prussian White Based Sodium-Ion Batteries Using Fire Resistant, Fluorine-Free Electrolyte

Electrolytes based on sodium bis(oxolato)borate (NaBOB) in organophosphates (trimethyl phosphate and triethyl phosphate (TEP)) have shown promise in sodium-ion batteries when compared to conventional electrolytes in that they are fire resistant, fluorine-free and are of lower toxicity. However, these electrolytes tend to exhibit low initial Coulombic efficiency and high overpotentials. We have here demonstrated that NaBOB in TEP can be used in cells with near-commercial capacity loadings. Furthermore, we have shown that formation cycle conditions have a significant positive effect on the cell performance in these higher mass loading cells, and that modification of the formation cycle conditions can be used to increase the capacity retention, lower the overpotentials, and as such increase the rate capability. The viability of optimized formation protocols was also demonstrated in scaled up prototype cells. Formation cycling: In this article we present a solution to the poor cyclability of non-flammable sodium bis(oxolato)borate (NaBOB) in triethyl phosphate (TEP) electrolyte. By developing a formation cycle specific to this NaBOB in TEP electrolyte, we are able to cycle a 4.5 Ah full cell with high mass loading electrodes to beyond 900 cycles before reaching 80 % state of health.image