Protective coatings for lithium metal anodes: Recent progress and future perspectives

The demand for lithium batteries with energy densities beyond those of lithium-ion has driven the recent studies on lithium metal anode. High-efficiency electrochemical cycling of lithium requires improved lithium deposition morphology and reduced parasitic reactions between lithium and the liquid electrolyte. A protective layer on lithium metal is expected to reduce contact between lithium metal and the organic solvent, exert compressive mechanical force on the anode, and improve the selectivity and uniformity of lithium ion transport at the electrode surface. This review covers recent advancements in this topic. We first establish the design criteria for an effective coating followed by a brief description of the methods for depositing the layer, characterizing its structure and morphology, and evaluating its electrochemical performance. Our discussion of the literature is organized on resultant layer composition and corresponding ion conduction mechanisms. In the case of polymeric materials, the polarity difference between the polymer and electrolyte solvents determines the degree of swelling and selectivity of lithium ion transport. We conclude by advocating for the need of increased mechanistic study for the functioning mechanism, improved understanding of layer degradation, and demonstration of the protective function in realistic cell environment, namely lean electrolytes and coupled with appropriate cathodes.