# Low‐Concentration Electrolytes Based on Weakly Coordinating Anions for Applications in Lithium‐Ion‐Batteries and Lithium‐Metal‐Batteries

## TL;DR

Low-concentration electrolytes with weakly coordinating anions in fluorinated solvents show high conductivity and stability for lithium-ion and lithium-metal batteries.

## Contribution

The study introduces new low-concentration electrolytes with weakly coordinating anions that outperform standard electrolytes in battery performance and stability.

## Key findings

- LCEs with aluminate and gallate anions achieved up to 5.0 mS cm−1 conductivity and electrochemical stability up to 4.5 V.
- LCEs enabled stable cycling over 300 cycles in lithium-ion batteries and improved performance in lithium-metal batteries with thin anodes.
- Aluminate LCEs showed three-to-fourfold higher lithium ion mobility compared to standard carbonate-based electrolytes.

## Abstract

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.

Big is beautiful? The performance of 0.2 M low concentration lithium electrolytes with large weakly coordinating anions [Al{OC(CF3)3}4]− and [Ga(C2F5)4]− in low viscosity ortho‐difluorobenzene solution is tested in comparison to standard 1.0 M as well as diluted 0.2 M carbonate‐based electrolytes with Li[PF6] conducting salt. Lithium ion / lithium metal batteries were tested symmetric, half, and full cells.