Abstract
Lithium metal is considered as one of the most promising anode materials for high-energy-density rechargeable batteries. However, uncontrolled dendrite growth, the unstable interface between lithium metal anode and electrolyte, and infinite volume change are major obstacles in their practical applications. Constructing a solid electrolyte interphase (SEI) with high strength, good stability, and desirable flexibility is one of the most promising approaches to mitigate the volume expansion of lithium anode and induce the uniform deposition of lithium for dendrite-free anode. Herein, we summarize the advances of SEI modification from the aspects of in-situ (adding electrolyte additives) and ex-situ (constructing artificial SEI) methods. The ideal SEI on lithium anode can effectively suppress the lithium dendrite growth and volume change of lithium metal anode. In the future study, the modification of SEI should focus on the suppression of side reactions between active lithium metal and electrolyte and the formation of dead lithium, which is quite significant to reduce the consumption of active lithium anode and electrolyte for safe and long-life high energy lithium metal batteries. Constructing an excellent SEI is a robust strategy to achieve the highly stable lithium metal anode for practical application of lithium metal batteries.
Introduction
Lithium metal has become an ideal anode material for next-generation high-energy-density lithium-ion batteries due to its high specific capacity (3,860 mAh g−1), low reduction potential (−3.040 V vs. standard hydrogen electrode) and low density (0.534 g cm−3) (Jung et al., ). However, uncontrollable dendrite formation, large volume expansion during the lithium deposition and the unstable SEI have caused serious safety problems and rapid capacity fading of the lithium metal batteries (Lin et al., ), which have restricted the application of lithium metal anode. Researchers have proposed some methods to solve these problems such as designing a three-dimensional (3D) current collector to induce uniform lithium deposition and mitigate the volume change of the lithium anode (Yun et al., ), constructing coating layer on separator to control lithium dendrite growth (Liu Y. et al., ), improving the stability and mechanical strength of the SEI on the surface of lithium anode to inhibit the growth of dendrites (Gao et al., ), and employing non-flammable solid electrolyte to block Li dendrites and mitigating the safety hazards (Lu et al., ; Zhu et al., ; Pang et al., ).
Among them, constructing an SEI with high strength, good stability, and sufficient flexibility on the surface of lithium anode can fundamentally suppress lithium dendrite growth, and effectively reduce the sacrifice in energy density of the battery. However, the SEI formed naturally is relatively brittle and unstable, which is easily damaged due to the mechanical stress caused by the rapid volume change during lithium deposition and dissolution, resulting in the uneven lithium ion flux and formation of lithium dendrites. In addition, repeated cracking and formation of the SEI during battery cycling will consume a large amount of electrolyte and active lithium anode, resulting in decrease of the coulombic efficiency and drying up of the electrolyte (Li N. W. et al., ). Therefore, it is important to protect lithium anode by introducing additives in the electrolyte to form an ideal SEI in situ, or by coating an artificial SEI through ex-situ physical and chemical methods. This perspective briefly summarizes the research progress of in-situ SEI and artificially constructed ex-situ SEI on protection of lithium metal anode in recent years and prospects the development of this method in the future.
Advances in Constructing SEI Film
Constructing an ideal SEI film with desired properties is an effective strategy to protect lithium metal from dendrites growth via in-situ and ex-situ methods, as summarized in Table 1. This section will focus on the recent advances of SEI modification for stable lithium metal anode.
Table 1
| Material | Thickness | Li | Li or Li | Cu cell | Full cell | References | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Current density (mA cm−2) | Area capacity (mAh cm−2) | Cycle number/time | CE (%) | Cathode material | Rate (C) | Cycle number (cycles) | Initial & retainer capacity (mAh g−1) | |||
| FEC additive | - | 0.5 | 0.5 | 100 cycles | 95 | LiNi0.5Co0.2Mn0.3O2 | 1 | 100 | 154/100.1 | Zhang et al., |
| LiNO3/PVDF-HFP gel film | 18 μm | 1 | 1 | 200 cycles | 98.1 | - | - | - | - | Liu et al., |
| CsPF6 additives | - | - | - | - | - | Li4Ti5O12 | 1 | 660 | - | Ding et al., |
| Pinhole-free-Li3N layer | - | - | - | - | - | Li4Ti5O12 | 1 | 500 | 160/159 | Li Y. et al., |
| Cu3N nanowires printed Li | 3 μm | 5 | 5 | 100 h | - | LiCoO2 | 0.5 | 300 | - | Lee et al., |
| Li4Ti5O12 | 4 | 1000 | -/125 | |||||||
| Conformal LiF layer | 40 nm | 1 | 1 | 200 cycles | - | S | 0.5 | 100 | -/1000 | Lin et al., |
| LiF/Sn/Sn–Li alloy hybrid layer | 25 μm | 1 | 1 | 850 h | - | LiNi1/3Co1/3Mn1/3O2 | 1 | 150 | 130.86/ 104.71 | Pathak et al., |
| Gradient ZnO-CNT | 20 μm | 10 | 1 | 100 h | - | - | - | - | - | Zhang et al., |
| LiPAA | 20 nm | 1 | - | 250 h | - | LiFePO4 | - | 500 | 140/116 | Li N. W. et al., |
| Dynamically cross-linked polymer | - | 1 | 1 | 120 cycles | 97 | LiFePO4 | - | 50 | 142.1/142.1 | Liu K. et al., |
| LiPEO–UPy | 70 nm | 5 | 10 | 1000 h | - | LiNi0.6Co0.2Mn0.2O2 | 1 | 200 | 148.2/124.8 | Wang et al., |
| LiF/PVDF-HFP | 12 μm | 2 | 1 | 200 h | - | LiFePO4 | 0.5 | 250 | 150/120 | Xu et al., |
| LLZTO/Li-Nafion | 5 μm | 1 | 0.5 | 200 h | - | LiFePO4 | 1 | 150 | 135/120 | Xu et al., |
| PTMEG-Li/Sn alloy | <10 μm | 1 | 1 | 1000 h | - | S | 0.5 | 300 | –/766.3 | Jiang et al., |
| LiFePO4 | 0.5 | 300 | - | |||||||
The comparison of electrochemical performance of Li metal anode with different artificial SEI.
In-situ Formed SEI
The ideal SEI should have high ionic conductivity, near-insulated electronic conductivity, proper thickness, dense structure and high elastic modulus to suppress dendrite penetration. The application of additives in electrolyte is one of the most facile ways to improve the stability of the SEI because it can directly affect the physical and chemical properties of the SEI, change the interface environment and finally improve the lithium deposition behavior (Tao et al., ). Additives can be divided into two types which are film-forming additives and Li+ plating additives according to the formation mechanism of SEI. The film-forming additive is a self-sacrificing additive that preferentially in-situ reacts with lithium metal to form a stable SEI in the initial cycle of the battery (Li et al., ). The Li+ plating additives would not react with lithium metal but form an electrostatic shield or “cloud” around the initial growth tip and induce uniform deposition of lithium, which dissipates once the applied voltage is removed, thus eliminating its consumption upon repeated cycling (Ding et al., ).
Vinyl carbonate (VC) and fluoroethylene carbonate (FEC) have been widely used as electrolyte additives to improve the properties of the SEI. The FEC can be reduced to form LiF-rich SEI on the surface of lithium metal, which can facilitate the lithium ion transportation and effectively inhibit the growth of lithium dendrites (Zhang et al., ). Due to the presence of an unsaturated double bond, VC is easily reduced on lithium metal through ring-opening polymerization to form a gel-like film on the lithium anode, which can also greatly suppress side reactions between lithium metal and the electrolyte and improving the coulombic efficiency (Ota et al., ). Therefore, additives of VC and FEC have been widely used in lithium metal batteries.
In addition to some organic solvent additives, inorganic additives such as LiNO3 (Liu et al., ), KNO3 (Jia et al., ), AlI3 (Ma et al., ), MgCl2 (Ouyang et al., ), and InX3 (X = Cl, F, Br) (Pang et al., ) have also been extensively studied to improve the performance of lithium metal anode. Liu et al. () designed a gel polymer film composed of LiNO3 and PVDF-HFP attached on lithium sheet to overcome the poor solubility of LiNO3 in carbonate electrolyte. The dissolved LiNO3 can continuously replenish the consumed by lithium anode to form an SEI rich in LiNxOy, Li3N, and Li2O, which induces the deposition of lithium from dendritic to spherical (see Figure 1A). This strategy achieved a high Coulombic efficiency (CE) of ~98.1% for more than 200 cycles at a current density of 1 mA cm−2 and a capacity of 1 mAh cm−2. Due to the excellent properties for protecting lithium metal anode, the LiNO3 additive has been widely applied in lithium metal batteries and Li-S batteries.
Figure 1
Most electrolyte additives are self-sacrificing additives that require continuous consumption to maintain the integrity of the SEI. As a result, the thickness of SEI may increase, which easily leads to the rate and cycling performance decay of lithium metal batteries. In contrast, the Li+ plating additives can induce the uniform deposition of lithium without consumption. For instance, Ding et al. (
Ex-situ Constructed SEI
The use of electrolyte additives to in situ form a dense SEI during the activation of battery is a simple and effective way for suppressing lithium dendrites. However, it is difficult to precisely regulate the parameters of the SEI such as components, thickness and strength. Therefore, researchers have proposed the ex-situ methods such as artificially constructing inorganic SEI, organic SEI and organic/inorganic composite SEI before the assembly of battery to protect the lithium anode. From the perspective of the preparation methods of artificial SEI, there are two ways to construct the ex-situ SEI on Li metal anode: the chemical method and physical method. Among them, the chemical method is relatively complicated, but it can regulate the composition and thickness of the protective layer by adjusting the type of reactant, reaction time, concentration, and other parameters according to designer's requirements. The current physical coating methods such as spin-coating, chemical vapor deposition, atomic layer deposition, and magnetron sputtering which much depend on the process engineering to achieve a dense and thin protective layer, is easier to control the parameters of the ex-situ SEI precisely.
Ex-situ Inorganic SEI
A common artificial inorganic SEI composition is Li3N (Alpen,
Figure 2

(A) Schematic diagram of lithium surface with a pinhole-free and tightly connected Li3N film (Li Y. et al.,
Carbon materials are also widely used for building artificial SEI due to their unique properties and structures such as excellent chemical and electrochemical stability, strong mechanical strength and adjustable nanostructures. Zhang et al. (
Ex-situ Organic SEI
Although the inorganic SEI can protect lithium metal to a certain extent, it is too brittle and cannot buffer the volume change of lithium metal during cycling. Unlike rigid inorganic materials, polymer with good flexibility can adapt to the volume change of lithium metal, which is an effective material for artificial SEI. Li N. W. et al. (
Ex-situ Organic/Inorganic Composite SEI
Organic SEI is promising in protecting lithium metal due to its diversity and controllability. However, polymers play a limited role in suppressing lithium dendrites due to their low mechanical strength. To combine the advantages of both inorganic and organic based SEI, researchers have proposed an organic/inorganic composite SEI, in which the inorganic component provides sufficient mechanical strength to suppress lithium dendrites, and the organic component gives the protective layer a certain flexibility to mitigate the volume change of lithium anode. Xu et al. (
Perspectives
Constructing a high ion conductivity, high elastic modulus and uniform SEI on the surface of lithium metal by in-situ or ex-situ method is an effective strategy to protect the lithium anode. The in-situ formation of SEI is easily achieved by the reduction or deposition of electrolyte additives, but it is difficult to monitor the formation or evolution of the SEI due to the dynamic operating environment of batteries. The ex-situ construction of an artificial SEI on lithium metal by physical or chemical methods is relatively complicated, but the parameters such as composition, thickness, and morphology of the SEI can be precisely controlled. Although important progress has been made in the research of lithium metal protective layer, there still exist some deficiencies. Firstly, the structure, composition, optimization method and Li ion diffusion mechanism of the SEI need to be further studied, and the dynamic change process of the SEI needs analyzing in situ through advanced characterization such as in-situ atomic force microscopy (AFM), in-situ transmission electron microscopy (TEM) and in-situ X-ray absorption spectroscopy (XAS). Therefore, the role of each component in the SEI layer can be clearly identified for optimizing the SEI design. Secondly, the current design of the SEI is only suitable for limited operating environment (such as moderate temperature, charging and discharging currents and capacity), therefore, the SEI that can adapt to the extremely harsh conditions according to the changing operating environment (such as high temperature, huge pulse charging and discharging currents and capacity) is quite crucial for the commercial application of lithium anode. Finally, most research of protecting lithium metal with SEI is based on some ideal conditions, such as excessive lithium anode and electrolyte, which are far from practical application. In the future, the constructed SEI on lithium anode not only should effectively suppress the lithium dendrite growth and volume change, but also suppress the side reactions between active lithium metal and electrolyte and dead lithium formation, which is quite significant to reduce the consumptions of active lithium anode and electrolyte for safe high energy lithium metal batteries. The constructing of an excellent SEI opens a new way for the practical application of lithium metal anode and batteries.
Statements
Data availability statement
All datasets generated for this study are included in the article/supplementary material.
Author contributions
JY and LZ wrote the manuscript. YHua, YHu, and LC revised the manuscript. Y-BH designed the framework of the manuscript, and revised the manuscript.
Funding
This work was supported by the National Natural Science Foundation of China (51672156), Local Innovative Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01N111), Guangdong Province Technical Plan Project (2017B010119001 and 2017B090907005), and Shenzhen Technical Plan Project (JCYJ20170412170706047 and JCYJ20170817161221958).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
lithium metal anode, lithium dendrite, solid electrolyte interphase, in-situ SEI, ex-situ SEI
Citation
Yu J, Zhao L, Huang Y, Hu Y, Chen L and He Y-B (2020) Progress and Perspective of Constructing Solid Electrolyte Interphase on Stable Lithium Metal Anode. Front. Mater. 7:71. doi: 10.3389/fmats.2020.00071
Received
12 February 2020
Accepted
09 March 2020
Published
25 March 2020
Volume
7 - 2020
Edited by
Cheng Zhong, Tianjin University, China
Reviewed by
Shiyou Zheng, University of Shanghai for Science and Technology, China; Ziying Zhang, Shanghai University of Engineering Sciences, China
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© 2020 Yu, Zhao, Huang, Hu, Chen and He.
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*Correspondence: Yan-Bing He he.yanbing@sz.tsinghua.edu.cn
This article was submitted to Energy Materials, a section of the journal Frontiers in Materials
†These authors have contributed equally to this work
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