Our world is going electric, and secondary batteries are leading the charge as they have penetrated the electric vehicle and large-scale grid markets. Yet, by nature, a battery's temperature changes during charging and discharging due to its inevitable internal resistance. The uneven battery pack temperature or excessive local temperature seriously threatens the safety and durability of batteries, especially in energy-dense batteries such as Li-ion and K-ion cells. Oddly, organic molecules dominate the electrolyte formulation in any battery type, although their high flammability exacerbates the thermal runaway risk. Moreover, battery safety concerns stemming from metallic dendrite growth, limited operating temperature range, and poor cycling life have restricted their development and applications. Because these issues directly or indirectly stem from the interactions between different electrodes and organic electrolytes, taking a different stance for electrolyte development has become imperative. The new electrolyte designs must simultaneously encompass the multi-faceted attributes of 1) non-flammability, 2) high thermal conductivity, 3) ability to function over a  wide temperature range, 4) high durability, and 5) compatibility with any battery chemistry. 

Most known non-flammable organic solvents containing fluorine and phosphorus are expensive and harmful to the environment, and it isn't easy to meet the above attributes simultaneously. Whether introducing inorganic elements into the electrolyte can truly meet the requisite attributes is still unknown. Fortunately, we started working with the environmentally friendly, fluorine-containing Novec 7300 (MME) fluid - a mature, safe, and affordable commercial coolant widely used in fire extinguishers, heat dissipation,  electronic testing, and cleaning applications. In this regard, attributes 1-3 above seem to be well resolved. Nevertheless, further studies were needed to determine whether this fluid could be tamed as a battery electrolyte.

In our recent work published in Nature Sustainability, we realized a universal safe electrolyte formulation for sustainable Li-ion and K-ion cells by combining 1 M imide salts with glyme, MME, and non-polar solvent Daikin-T5216. We demonstrated the uniqueness of MME fluorine molecules in appropriate coordination environments and their inherent thermal properties. The electrolyte promotes the formation of a favorable inorganics-dominated passivation layer to enable year-long cycling stability and embraces an excellent fire-extinguishing nature. Equally intriguingly, we found this electrolyte also shows outstanding operating capability across a wide temperature range from −75 to 80 °C. Cells with this electrolyte also possess high thermal conductivity and cleared the nail penetration tests without catching fire. Impressively, this electrolyte formulation uses commercially available ingredients and, with appropriate salts, can be adapted for different battery chemistries. With a competitive price, the Novec-Daikin solvent family, in combination with suitable salts, is expected to become the preferred industrial electrolyte. This work developed a sustainable electrolyte formulation to address safety and cyclability issues and paved the way for practical, environmentally friendly, sustainable cells.

To know more about this electrolyte design technology developed by Hunan University researchers, please refer to our recent publication in Nature Sustainability: Safe electrolyte for long-cycling alkali-ion batteries:  https://www.nature.com/articles/s41893-024-01275-0