Low-Concentration Electrolytes Based on Weakly Coordinating Anions for Applications in Lithium-Ion-Batteries and...

0.2 M Low Concentration Electrolytes (LCEs) for lithium-based batteries formed from lithium salts with very weakly coordinating anions, i.e., the aluminate Li[Al{OC(CF₃)₃}₄] and the gallate Li[Ga(C₂F₅)₄] in ortho-difluorobenzene (o-DFB), showed competitive conductivity to classical electrolytes of up to 5.0 mS cm^-1 at 25 °C combined with electrochemical stability at least up to 4.5 V vs. Li/Li⁺. Given that a stoichiometric amount of 2 equivalents dimethoxyethane (DME) per lithium ion (as Li⁺ complexing agent) and 2 wt.% fluoroethylene carbonate (as solid electrolyte interphase (SEI) former) were present in the LCEs, half and full-cell measurements confirmed stable LCE cycling over 300 cycles in Lithium-Ion-Batteries. Even at high currents (5C), the discharge retained two thirds of the practical 1C capacities of NMC622. By contrast, a LCE made from 0.2 M LiPF₆ in EC/EMC 3:7 solution already led at a 2C rate to cell death, while a simple switch of the conducting salt to 0.2 M Li[Al{OC(CF₃)₃}₄] led to stable cycling including rate tests for over 300 cycles and approached closely the values of the standard 1.0 M LiPF₆ electrolyte in EC/EMC 3:7 - attributed to the anions' stability. The performance of the aluminate LCE was further evaluated in symmetrical Li-Li cells and Lithium-Metal-Batteries containing 48 µm thin Lithium-Metal-Anodes (LMAs): LCEs improved the cell's lifetime by a factor of 3-6 at a current density of 1 mA cm^-2. Scanning electron microscope/energy-dispersive X-ray and potentiostatic electrochemical impedance spectroscopy measurements confirmed the exceptional stabilization of the LMAs by the aluminate LCE throughout the cycling, especially when combined with an artificial, adaptive and self-healing SEI based on Li[PO₂(OCH₂CF₃)₂]. The solvation structure of standard and LCEs was investigated by NMR spectroscopic diffusion measurements and quantum chemical calculations. A three-to-fourfold increased Li ion mobility was found in LCEs compared to the system with 0.2 M LiPF₆ in standard carbonate solution. The presence of stable and compact Li(DME)₂ ⁺ structures as moving ions was shown and the relevance of Li⁺ ions solvated with fluoro-ethylene carbonate or o-DFB for SEI-formation is discussed.