Research Progress on Artificial Protective Films for Lithium Metal Anodes

In the early 1990s, Sony launched the first commercial lithium ion battery (LIB), which achieved great success in energy storage systems. The current commercially used insertion anode, graphite, is approaching its capacity limit (similar to 372 mAh.g(-1)), and is inadequate to satisfy the ever-increasing energy demand for power grids and large-scale energy storage systems. In order to address this challenge, lithium metal anodes have been the focus of considerable research effort in recent years, and are regarded as the most promising anode materials because of their extremely high theoretical capacity (3860 mAh.mg(-1)), lowest electrode potential (-3.04 V vs. standard hydrogen electrode), and low density (0.534 g.cm(-3)). For example, the theoretical energy densities of lithium-sulfur batteries and lithium-air batteries are as high as 2567 and 3505 Wh.kg(-1), respectively. However, the uncontrollable dendrite growth during cycling leads to low coulombic efficiency and puncture of the separator, causing a short circuit or even explosion of the battery, thereby seriously hindering the development of the lithium metal anode. Many solutions have been proposed to inhibit dendrite growth, including the use of electrolyte additives, solid electrolytes, and artificial protective films. During charging and discharging, the solid electrolyte interphase (SEI) plays an important role in lithium metal anodes. However, the infinite volume changes of the electrode during plating/stripping processes result in breakage of the SEI film, which continuously consumes the electrolyte and lithium metal. Designing an artificial interface on the surface of lithium metal anodes has been considered as a simple and efficient strategy to control lithium deposition behavior, and is achieved by precoating a protective layer on the surface of lithium metal. An ideal artificial protective film should possess high ionic conductivity, chemical stability, and excellent mechanical strength, in order to prevent side reactions between lithium metal and the electrolyte and realize dendrite-free lithium metal anodes with a long cycle life and high coulombic efficiencies. In this paper, the research progress on artificial protective films for lithium metal anodes in recent years is reviewed. Further, the structural characteristics and preparation methods of various protective films are introduced in detail, including polymer protective films, inorganic protective films, organic-inorganic composite protective films, and alloy protective films. The mechanisms of various protective films toward the suppression of dendrite growth are summarized. Existing challenges and future research directions are also proposed, which together provide a reference for promoting the use of lithium metal in high-energy batteries.