Electrolyte systems
based on binary mixtures of organic carbonate ester cosolvents have
limitations in ionic transport and thus limit extreme fast charge
(XFC) and high-rate cycling of energy dense lithium-ion cells with
thick electrodes (>80 μm per side) at ambient temperature
and below. Here, we present LiPF6 in methyl acetate (MA)
as an ester-based liquid electrolyte that offers substantial improvements
in ionic transport, doubling the conductivity of conventional electrolyte
systems. Density functional theory-based molecular dynamics (DFT-MD)
simulations give insights into the experimentally observed low solvation
number for lithium ions in MA solutions and show a solution system
with highly mobile, loosely bound ionic species. We show that MA-based
electrolytes with suitable additive formulas enable high cycling rates
and excellent low-temperature cycling performance in lithium-ion cell
designs with thick electrodes but come with a trade-off in lifetime
at elevated temperature. While there are inherent practical issues
with MA as an electrolyte solvent, including a low flash point (−10
°C) and lifetime penalties compared to state-of-the-art electrolytes,
this work demonstrates that excellent ionic transport in the electrolyte
can enable fast charging without the energy density sacrifice inherently
associated with specifically tailored electrodes. Further work in
electrolyte design, particularly in increasing ionic conductivity
without sacrificing stability, has the potential to enable XFC in
practical lithium-ion cell chemistries and cell designs.