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

Abstract

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.

Interfacial Model Deciphering High‐Voltage Electrolytes for High Energy Density, High Safety, and Fast‐Charging Lithium‐Ion Batteries

“…The newly designed high‐voltage electrolyte is composed of EMC and MA solvents, in which EMC has a good oxidative stability ( Figure a) [ 19 ] and MA has a low freezing point and a high ionic conductivity (Figure 1b). [ 20 ] While EMC can guarantee high electrolyte stability at high voltage, MA can overcome the disadvantages of EMC (i.e., low ionic conductivity and dielectric constant) [ 21 ] to enhance the ionic conductivity for greater rate capabilities, especially in low‐temperature conditions (Figure 1c). As a paradigm, the graphite || NCM622 battery employing this high voltage electrolyte (i.e., EMC/MA = 7/3 v/v, E/M73) demonstrates a high initial Coulombic efficiency (ICE) of 88.9% and a capacity of 201.2 mAh g −1 , which is close to that when employing EMC electrolyte (89.7%, 202.3 mAh g −1 ) but much higher than that when employing MA electrolyte (72.3%, 189 mAh g −1 ) at a high voltage of 4.45 V (Figure 1d, see battery configuration in Figure S1, Supporting Information).…”

Prospects for lithium-ion batteries and beyond—a 2030 vision

“…Higher loadings can also be achieved by increasing the active layer thicknesses, decreasing the binder fraction, and decreasing the porosity. All of these require increased electrolyte (ionic) transport to maintain rate capability, an area of active research already for fast-charging battery technologies 8 . The transport properties and molecular-scale structures of new solution chemistries (e.g., new solvent systems, highly concentrated salts) are becoming increasingly understood 9,10 .…”

A Review of Existing and Emerging Methods for Lithium Detection and Characterization in Li‐Ion and Li‐Metal Batteries

“…Developing an accurate and computationally efficient lithium plating/stripping sub-model within the standard Newman pseudo 2D (P2D) electrochemical model is a key step in optimizing fast charging for EV batteries and developing robust, practical techniques for on-board Li detection. Numerous strategies have been reported in the literature for mitigating Li plating during fast charging of high energy density cells such as: improved electrolytes, [149,150] elevated temperature operation, [151][152][153] advanced electrode architectures, [154][155][156] electrode coatings, [157,158] and modifying active graphite material. [159,160] A well-parameterized P2D model with accurate Li sub-model enables understanding of limitations resulting in Li plating such as amount resulting from insufficient lithiation kinetics, solid-phase ion transport, and ion transport within the electrolyte phase.…”