Abstract
Lithium-sulfur (Li-S) batteries promise the next-generation energy storage. However, one of the issues facing Li-S batteries is the high reactivity of the Li metal, leading to serious interfacial side reactions; the other is the dendritic Li growth that can usually breach the solid electrolyte interphase (SEI) applied for mitigating side reactions. To address these issues, we propose the indium fluoride-based artificial solid electrolyte interphase with tunable surface properties. Over the initial charging, InF₃ nanoparticles coated on the Li anode surface can be lithiated to form a layer of nanoporous lithium/indium alloy (Li_xIn) encapsulating lithium fluoride (LiF) nanoparticles (3Li⁺+3e^-+InF₃→3LiF+In) as shown in Figs. 1(a, b). The tightly anchored Li_xIn layer well attaches LiF on the anode surface during Li stripping/plating, while LiF with a higher Young’s modulus physically suppresses dendritic Li growth. Both Li_xIn and LiF provide facile adsorption towards Li atoms as well as high Li⁺ conductivity, inducing uniform Li plating [1-4]. In addition, both materials are stable with corrosive electrolyte containing polysulfides and shield Li anode from side reactions. Tuning the SEI surface properties via adding other metal fluorides such as BiF₃ and AlF₃, we further show the possibility to further enhance the Young’s modulus and flexibility for the SEI. Applying the rationally designed artificial SEI, substantial enhancement in terms of Li stripping/plating stability (Fig. 1c) and Li-S battery cyclability is achieved even using LiNO₃-free electrolyte and the gas evolution arising from interfacial electrolyte decomposition is suppressed [5]. With enhanced mechanical strength and chemical stability, the metal fluoride-based artificial SEI paves a new way in developing durable and safer Li-S batteries.

Key words: Li metal; artificial solid electrolyte interphase; dendrite; side reaction; metal fluoride.

Acknowledgement

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R)

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