Anode interface modification of lithium metal batteries: Benefiting from functional additives and conformal coatings

Lithium (Li) metal is regarded as a “Holy Grail” anode for next-generation rechargeable batteries due to its ultrahigh theoretical specific capacity and extremely low redox potential. Lithium metal batteries (LMBs) with high energy density have received extensive research attentions and been considered to be one of the most promising candidates for the post lithium-ion battery era. However, the intrinsic properties of Li metal hinder the practical application of LMBs, including severe safety concern and poor cycling stability. The closely related problems include: (1) The growth of Li dendrites to penetrate the polymer separator and cause a short circuit of cell; (2) the infinite volume expansion and contraction during Li plating and stripping processes to cause the continuous exposure of fresh Li and consumption of electrolyte; (3) the accumulation of electrochemically inert dead Li so as to increase the cell impedance and decease the Coulombic efficiency. Therefore, it is essential to address the above issues in order to achieve the safe and stable Li metal anode. In the past years, some models about the growth of Li dendrites have been proposed, and they can help us understand the dendrite growth mechanisms and guide the direction toward the design of prospective anodes. And some strategies such as anode structural design, solid-state electrolyte, separator decoration, artificial solid-state electrolyte interphase (SEI), additives and so on have been demonstrated to enable an effective suppression or alleviation of the growth of Li dendrites. Herein, we highlight the strategies of functional additives and conformal coating, especially their impacts on the composition and structure of the electrode/electrolyte interface, and thus on the homogenization of the Li-ion flux and formation of the uniform deposition of Li. Small amounts of additives can react with Li to regulate the properties of SEI, or disperse in the electrolyte as an ion transport channel to modulate the distribution of Li-ion flux. The construction of a conformal coating with rich lithiophilic sites on the Li metal anode or current collector can also tailor the Li ion plating behavior, and the binding energy between lithiophilic groups and Li atoms is obtained through calculation to prove the role of lithiophilic sites. In addition, conformal coating of Li metal can also improve its air stability, which is beneficial to the future commercial application of LMBs. Furthermore, the fluorine-rich components and robust inorganic moieties that may be formed based on both the above strategies are discussed, so as to obtain reinforced SEI with low resistance. We aim to illustrate the design ideas based on the recent works in this field and provide inspiration for the future research on Li metal anode.