Understanding Charge Acceptance Limitations During Extreme Fast‐Charging of Li‐ion Batteries

Extreme fast charging in lithium‐ion batteries is limited by their poor charge acceptance at high C‐rates. We use overpotential component analysis to decouple ohmic, kinetic and transport voltage losses during fast charging. The concentration gradients are identified as the primary culprit in poor charge acceptance, so we study carbonate electrolytes that utilize acetonitrile, a low viscosity, high dielectric liquid, as a co‐solvent to improve ion transport. Our results indicate acetonitrile co‐solvent successfully reduces fast charging concentration gradients and therefore improves charge acceptance at high C‐rates, particularly when used in combination with interface‐stabilizing additives. Molecular dynamics simulations show that ACN enhances Li + transport through a combination of reduced bulk viscosity and increased solvation‐shell lability, with shorter solvent residence times and faster coordination exchange producing transport gains beyond those expected from viscosity reduction alone. Finally, we use three‐electrode cells to deconvolute overpotential at each electrode, and the specific LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) electrode architecture and loading used in our study is found to have much higher overpotentials than our graphite electrode. In particular, the significant differences in NMC811 concentration gradients between electrolytes with or without acetonitrile indicate that the design of electrode architecture is crucial for extreme fast charging applications.