Formation Accelerated Stress Test (FAST): A New Technique to Study Anode Passivation in Lithium-ion Cells using Differential Capacity, Low Salt Concentration and Low Temperature.

This study investigates the formation and properties of the solid electrolyte interface (SEI) in lithium-ion batteries under varying salt concentrations (0.4M, 0.8M, 1.2M) and low formation temperatures. This method was used as a formation accelerated stress test (FAST) to evaluate the impact of different solvent blends and additives on anode passivation using the reduction rate of ethyl methyl carbonate (EMC) seen in the differential capacity of cells containing them as a performance metric. Electrolytes containing a mixture of vinylene carbonate and ethylene sulfate showed superior passivation, while a 3:7 mixture of fluoroethylene carbonate and ethyl methyl carbonate proved to be the most effective at passivating. Furthermore, the low-viscosity solvents methyl acetate and acetonitrile significantly enhanced SEI passivation during formation at the anode due to lower graphite exfoliation. We also show that the differential capacity data correlate with long-term cycling performance for some of the chemistry studied here. These results provide valuable insights for the efficient design of a new formation accelerated stress test (FAST) that could accelerate the discovery of high-performance electrolytes through rapid monitoring of SEI passivation during formation.