Rational Design of Protective Film for Long-Life and Stable Lithium Metal Anode Via Molecular Layer Deposition
Authors: Sun Yipeng , Zhao Yang , Adair Keegan , Sham Tsun-Kong , Sun Andy Xueliang 
Published: May 2020

It is urgent to develop efficient energy storage system for the increasing energy demands of human society. Batteries with high energy density is one of the most promising solutions. Metallic Li anode is believed to be the most promising electrode candidate for high-energy-density batteries due to its ultrahigh capacity and low electrochemical potential [1]. However, it has still not been successfully commercialized because of several great challenges, including poor safety and dendrite growth in plating/stripping process. Highly reactive nature of Li metal anode towards organic electrolyte leads to severe side reactions and prompt the formation of solid electrolyte interphase (SEI). Although SEI can passivate the Li surface and serve as a protective layer on the electrode, it cannot prevent the dendrite growth due to its heterogeneous and unstable properties [2]. In recent years, there are large amount of reported works on stabilizing SEI for Li metal anode via in-situ or ex-situ approach. In-situ approach is realized by optimization of electrolyte using electrolyte additives or changing concentration of Li salt, while ex-situ approach is achieved by building protective layer, such as metal oxides, lithium salts, and organic molecules on Li before electrochemical cycling [3]. However, up to now it still remains very challenging to achieve conformal, stable, and ultrathin layer as ideal SEI for long-term electrochemical cycling.

Our group has been dedicated to build effective protective films for high performance Li metal anodes using Atomic layer deposition (ALD) or Molecular layer deposition (MLD). ALD is a widely used coating technique for the deposition of inorganic materials with great advantages including conformal coverage and accurate control over thickness in nanoscale. MLD can be employed to produce inorganic-organic hybrid or pure polymer coatings as an analogue technique of ALD. Therefore, MLD maintained the advantages of ALD and provides more advantages such as tunable composition and improved mechanical properties. Previously reported works from our group includes inorganic metal oxide and inorganic-organic metalcones synthesized by ALD/MLD for improved stability of Li metal anodes [4-5]. In this talk, we report a novel pure polymer film of MLD Polyurea (PU) as protective layer for Li-metal anodes for the first time [6]. The depth profile of PU layer on Li was illustrated by TOF-SIMS and the detailed surface chemistry was investigated by XPS for both the anodes before and after electrochemical cycling. Compared to bare Li, the polyurea coated Li shows highly improved cycle life and stability. Our results showed this electrically nonconductive PU film can effectively suppress the dendrite growth as a protective barrier and remain stable upon the repeated Li plating/stripping process. Meanwhile, the nitrogen-containing polar groups in PU can effectively regulate the Li-ion flux and lead to a uniform Li deposition. Owing to these advantages, the Li metal anode coated with PU layer enables greatly prolonged lifetime in the symmetric cells at different current densities and capacities. The full cells were tested using LiFePO4 (LFP) as the cathode and showed improved capacity retention and rate performance. Therefore, this work sheds new light on the design of protective layer for Li-metal anodes for high-energy-density next-generation batteries.

[1] D. Lin, Y. Cui, et al,
Nature Nanotechnology
, 2017, 12, 194-206

[2] X. Cheng, Q. Zhang, et al,
Chemical Reviews,
2017, 117, 10403-10473

[3] X. Cheng, Q. Zhang, et al,
Adv. Sci.,
2016, 3, 1500213

[4] Y. Zhao, X. Sun et al,
Small Methods,
2018, 2, 1700417

[5] K. R. Adair, X. Sun et al,
Angew. Chem. Int. Ed.,
2019, 58, 15797.

[6] Y. Sun, X. Sun, et al,
Advanced Materials
, 2019, 31, 1806541

Figure 1