The high reactivity of Li metal anodes towards liquid electrolytes leads to an unstable and accumulated solid electrolyte interphase (SEI) film, which results in dendrite growth and low Coulombic efficiency (CE). Lithium fluoride (LiF) coating is considered as a reliable and dense SEI film to protect the reactive anode, however, the chemistry to form uniform, conformal and high quality LiF protection layer on Lithium metals remains as a major challenge. Here we develop a simple solution method to obtain LiF coating on Li metal anodes. We have discoverd a chemical method to fabricate LiF coating via the in-situ reaction between metallic Li and polyvinylidene fluoride (PVDF)-dimethyl formamide (DMF) solution. Owing to the chemically and mechanically stable artificial SEI film, the LiF-coated Li anode delivers a better cycling performance than bare Li anode under various current densities in symmetrical cells. Stable cycling over 300 plating/striping cycles was achieved with LiF-coated Li electrodes under a high current density of 3 mA cm-2 . The LiF coating also effectively suppresses dendrite formation and reduces side reactions between the metallic Li and the carbonate-based electrolyte. Therefore, this simple and low-cost method may benefit the future applications of the next generation Li metal batteries.

Introduction

Research on rechargeable batteries has made great progress in the past few decades, especially the research of the lithium ion batteries (LIBs)[1-4]. However, the energy density of commercial LIBs has very limited room for improvement, which makes it difficult for LIBs to meet the increasing demands of electric vehicles and portable electronics [5-6]. Li metal batteries have potentials to realize a higher energy density owing to their high theoretical capacity (3860 mAh g-1 ) and the low electrochemical potential (−۳٫۰۴ V vs the standard hydrogen electrode) of Li metal anodes. Nevertheless, the high reactivity of metallic Li and the huge volume change during Li plating/striping cycles result in an unstable SEI film when using liquid organic electrolytes [7]. As a result, Li metal batteries suffer from persistent side reactions between anodes and electrolytes, Li dendrite growth, low CE, and severe consumption of the electrolyte [8]. Therefore, to improve the cycling performance of Li metal anodes and avoid safety issues caused by dendrite growth, a chemically and mechanically stable SEI film is necessary [9-14]. Great efforts have been made to solve the interfacial issues and construct a robust SEI film [15-20]. Engineering the liquid electrolytes is a straightforward method to enhance the stability of the SEI film. By adding additives, using high concentrated electrolytes and employing new type liquid electrolytes, the interfacial stability can be significantly improved [21-26]. A functional coating on Li metal anodes can also contribute to the enhancement of the interfacial stability, especially an artificial SEI layer. Guo’s group demonstrated that the Li3PO4 SEI layer by in situ reaction of polyphosphoric acid with Li metal could suppress dendrite growth and restrain side reactions between Li metal anodes and liquid electrolytes [27]. LiF is also an ideal SEI film owing to its high chemical stability, low solubility in carbonate based electrolytes and high shear modulus [28, 29]. Recently, Cui’s group demonstrated that the uniform LiF coating on Li metal anode could effectively prevent the corrosion reaction of carbonate electrolytes and enhance the performance of Li anodes [30].