Safe electrolyte for long-cycling alkali-ion batteries

Safety is essential to battery sustainability, particularly considering that flammable organic molecules still dominate the electrolyte formulation. For a single electrolyte chemistry, improving its safety is often at the expense of cost and the battery’s electrochemical performance. Here we show an electrolyte that breaks this trade-off with combined flame retardancy, cost advantage and excellent cycling performance in both potassium-ion and lithium-ion batteries. Our rational design is to tame the flammability of a commonly used glyme solvent by introducing a fluorinated liquid and a non-polar solvent, known on the market as Novec 7300 coolant fluid and Daikin-T5216, respectively. The formulated electrolyte with excellent chemical and thermal stability proves non-flammable and works in a wide working temperature range of −75 to 80 °C. When assembled in potassium metal batteries, the K||K cell sustains cycling for >12 months, and the K||graphite cell retains 93% of its initial capacity after 2,400 cycles. Even in a 18650 Li-ion cell under harsh conditions ( N / P = 1.08, E / C = 3.0 g Ah −1 ), the capacity retention is as high as 96.7% after cycling more than 200 times. Together with low cost, the current formulation suggests new space in electrolyte design where nearly all factors that matter to the sustainability of batteries can be well balanced.