Ester-Based Electrolytes for Fast Charging of Energy Dense Lithium-Ion\nBatteries

Abstract

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