News Source & Photographer: Advanced Research Institute of Multidisciplinary Science

Editor: Yang Jing

The solid electrolyte-interfacial interphase (SEI) at the anode/electrolyte interface in lithium(Li) metal batteries repeatedly ruptures and regenerates during cycling, triggering the continuous reaction and consumption of active Li and electrolyte, which is the key factor leading to the rapid deterioration of battery cycle stability. Huang Jiaqi, Zhang Xueqiang in Advanced Research Institute of Multidisciplinary Science of Beijing Institute of Technology and their collaborators published a research paper entitled "Homogeneous and mechanically stable solid–electrolyte interphase enabled by trioxane-modulated electrolytes for Li metal batteries" in the journal Nature Energy. In this paper, a double-layer solid electrolyte interphase (SEI) was constructed in situ through the design of a trioxane-based electrolyte, and the uniformity and mechanical stability of the SEI were improved at the same time to achieve 130 stable cycles of a high energy density Li metal pouch cell (440 Wh kg⁻¹). The LiF-rich inner layer of the double-layer SEI enhances the uniformity of Liion transport, and the Li polyoxymethylene-rich outer layer enhances the mechanical stability of the SEI. The double-layer SEI can effectively inhibit the cracking and reconstruction of the SEI during the Li deposition/extraction process, reduce the active Li and electrolyte consumption, and significantly prolong the battery cycle life.

There is a growing demand for high energy density batteries for the sustainable development of electric vehicles, portable electronic devices, etc. . The energy density of graphite-based Li-ion batteries is approaching the upper limit of 350 Wh/kg, while the energy density of Li metal batteries can exceed 400 Wh/kg, which is a promising next-generation battery system. However, the short cycle life of Li metal batteries seriously hinders its practical application.

The rapid depletion of active Li and electrolyte is the main reason for the short cycle life of Li metal batteries. SEI acts as a nanoscale passivation layer between Li and electrolyte, which can terminate the continuous reaction between Li and electrolyte. However, SEI derived from common organic electrolytes is heterogeneous and has poor mechanical stabil- ity9,10. The heterogeneity of SEI results in spatially non-uniform Li-ion fluxes, further inducing uneven Li plating/stripping. Meanwhile, the poor mechanical stability of SEI facilitates its rupture due to the uneven stress distribution during the uneven Li plating/stripping, forming reactions between active Li and electrolytes again. It is therefore pivotal to simultaneously improve the homogeneity and mechanical stability of SEI to mitigate the incessant reactions between Li and electrolytes for stabilizing Li metal anodes.

Figure 1. Schematic diagram of the structural evolution of single-layer and tailored bilayer SEIs during Li plating.

The single-layer SEI is a LiFSI-derived LiF-rich SEI in a locally high salt, which is prone to fracture-regeneration during Li deposition. The double-layer SEI is composed of a LiF-rich inner layer and a highly mechanically stable LiPOM outer layer, which can alleviate the rupture-regeneration of SEI.

Figure 2. The solvation structure and reduction behaviour of DME- and TO-based electrolytes. a,c, Snapshots of the MD simulation boxes of DME- (a) and TO-based (c) electrolytes. b,d, Schematic illustration of the representative solvation sheath of Li ions in DME- (b) and TO-based (d) electrolytes. e,f, The radial distribution function (RDF) plots of Li–ODME, Li–OFSI−, Li–OHFE and Li‒OTO pairs in DME- (e) and TO-based electrolytes. (g) The calculated reduction potential of DME, TO and FSI− based on density functional theory.

The theoretical calculation in Figure 2 proves that TO can participate in the solvation sheath of Li ions. In addition, in both electrolytes, FSI⁻ and DME are involved in the solvation sheath of Li ions, which resembles the solvation structure of DME-based electrolyte, a typical electrolyte to form anion-derived SEI. The reduction potential of TO (0.58 V) was higher than that of DME (0.31 V); while both were significantly lower than FSI⁻ (1.57 V). Therefore, FSI⁻ is preferentially reduced over TO and DME to form the LiF-rich inner layer in the bilayer SEI, while TO is preferentially reduced over DME to generate LiPOM in the outer SEI layer.

Figure 3. The 3D nanostructure of SEI resolved by ToF-SIMS. a,b, The 3D views of LiF− (for LiF) and C2H2O‒ (for organic components) in the ToF-SIMS sputtered volumes of single/F SEI (a) and bilayer/P‒F SEI (b). c,d, The corresponding ToF-SIMS depth profiles for LiF− (c) and C2H2O‒ (d) in different SEIs. Normalized intensity is obtained through log (intensity) conversion.

In Fig. 3, LiF⁻ and C2H2O⁻ are the characteristic ion fragments of LiF and organic components, respectively. Both single-layer SEI and double-layer SEI contain abundant LiF, but the double-layer SEI contains more organic components than the single-layer SEI, implying that TO can decompose into LiPOM and participate in the formation of SEI. In addition, the organic components formed by TO decomposition are uniformly distributed on the surface of LiF, constituting a bilayer SEI. In contrast, the DME-based electrolyte only decomposes to form a LiF-containing monolayer SEI.

Figure 4. The formation process and mechanical stability of SEI resolved by in situ electrochemical AFM. a, Schematic illustration of the in situ electrochemical AFM cell. b, AFM topography images of HOPG at OCP (3.0 V). c–f, AFM topography images of single/F SEI (c,d) and bilayer/P‒F SEI (e,f) upon the cathodic scan of cyclic voltammetry. White arrows represent the scanning direction. g,h, The height image of single/F SEI (g) and bilayer/P‒F SEI (h). i, DMT modulus distribution of single/F SEI and bilayer/P‒F SEI.

Figure 4. In situ electrochemical atomic force microscopy (AFM) is employed to probe the dynamic formation and mechanical stability of SEI in real time. During the formation process of the single/F SEI, there are many tiny and irregular nanoparticles (about a dozen nanometres) below 1.11 V. These nanoparticles continued to grow up to 0.74 V (Fig. 4c). The granular sediments increase to tens of nano- metres and cover the surface of HOPG completely at 0.74 V (Fig. 4d). Meanwhile, there is no particle in other morphologies, , which meets the expectation of single-layer SEI. Based on the calculated and reported reduction potentials, these granular sediments are the decomposition products of FSI⁻. When it comes to bilayer SEI, nanoparticles of similar sizes and shapes to those for single/F SEI begin to appear below 1.06 V, a value again similar to that for single-layer SEI (Fig. 4e, 4f). However, at 0.95 V, two distinct particles can be identified: porous but interconnected deposits (marked by white circles) form over granular substances (marked by pink circles), as the interconnected sediments typically have higher height profiles in the AFM image. Thus, the formation process of the bilayer SEI was demonstrated by in situ AFM. The height of the bilayer SEI is only 10–22 nm, while the height of the single layer SEI is 25–35 nm, which indicates that the bilayer SEI has good spatial uniformity. In addition, the average DMT modulus of the bilayer SEI is 2.1 times that of the monolayer SEI (2.9 vs. 1.4 GPa), which indicates that the bilayer SEI can improve the mechanical stability of the SEI.

Figure 5. The effects of various SEIs on cycling stability of Li metal coin cells, Li morphology and interfacial kinetics. a, Cycling performance of Li | NCM523 coin cells with different SEIs at 0.4 C. b, Comparison of cycling performance of Li | NCM523 batteries with various electrolytes. c–f, Top-view and cross- sectional-view SEM images of Li metal anodes at the 1st and the 100th cycles with single/F SEI (c,e) and bilayer/P‒F SEI (d,f). g, Schematic illustration of anode evolution during cycling and the corresponding volume expansion after 100 cycles. X axis is broken to make a clear distinction in the anode thickness between before and after cycles. h, The evolution of interfacial resistance of Li | NCM523 coin cells after 100 and 200 cycles with single/F SEI and bilayer/P‒F SEI.

The effectiveness of a bilayer SEI was evaluated in a Li|NCM523 coin cell, including an ultrathin Li metal anode (50 μm) and a high-loading cathode (3.0 mAh cm⁻²). The cell with bilayer SEI exhibits a lifespan of 430 cycles, which is not only 115% longer than the cell with single-layer SEI (Fig. 5a). The double-layer SEI can effectively promote the uniform deposition of Li, reduce the generation of dead Li, and effectively suppress anode volume expansion of metallic Li. In addition, the double-layer SEI can effectively reduce the interfacial impedance of Li metal anodes and improve the interfacial stability of Li metal anodes.

Fig. 6 | The performance of 440 Wh kg⁻¹ Li metal pouch cells enabled by tailored bilayer SEI. a, Schematic illustration of a Li | NCM811 pouch cell of 440 Wh kg−1. b, The voltage profiles of pouch cells at the 1st and the 100th cycles. c, Cycling performance of Li | NCM811 pouch cell at 0.1 C charge/0.2 C discharge after two formation cycles at 0.05 C charge/discharge. The inset is the optical  image of the corresponding pouch cell. d, The state-of-the-art performance of high-energy-density (over 300 Wh kg⁻¹) Li metal pouch cells with more than 100 cycles in published literature and this work.

A 5.3Ah Li|NCM811 pouch battery can achieve a specific energy of 440 Wh kg⁻¹ and a capacity retention of 91.7% after 130 cycles. Furthermore, the performance of reported Li | NCM batteries is compared based on two criteria41–48, including the cell-level energy density >300 Wh kg⁻¹ and the cycle life >100 cycles, suggesting the effectiveness of bilayer/P–F SEI in improving the anode stability in practical Li metal batteries.

This paper proposes a nanoscale double-layer SEI design to simultaneously improve SEI uniformity and mechanical stability, alleviate SEI rupture and regeneration, and improve the cycle stability of Li metal anodes. The bilayer SEI is constructed in situ by electrolyte design, and the homogeneity is improved by the inner layer LiF and the mechanical stability is improved by the outer layer LiPOM. Through multiple characterizations and theoretical calculations, the SEI formation mechanism, compositional structure, homogeneity, and mechanical stability are revealed. The enhanced bilayer SEI uniformity and mechanical stability facilitate highly reversible Li deposition/stripping, enabling 130 cycles of 440 Wh kg^-1 metal Li pouch cells. Aiming at the problem of poor anode/electrolyte interface uniformity and poor mechanical stability in high-energy-density and long-cycling metal Li batteries, this work proposed a double-layer SEI structure design to alleviate the repeated rupture-regeneration of SEI. It is expected that this design principle can be applied to other emerging battery systems.

The above research work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, and the Beijing Natural Science Foundation.

Original information and link:

Qian-Kui Zhang+, Xue-Qiang Zhang+, Jing Wan, Nan Yao, Ting-Lu Song, Jin Xie, Li-Peng Hou, Ming-Yue Zhou, Xiang Chen, Bo-Quan Li, Rui Wen, Hong-Jie Peng, Qiang Zhang, Jia-Qi Huang*, Homogeneous and mechanically stable solid–electrolyte interphase enabled by trioxane-modulated electrolytes for Li metal batteries, Nat. Energy, 2023, doi: 10.1038/s41560-023-01275-y.

https://www.nature.com/articles/s41560-023-01275-y

Attached profile:

Huang Jiaqi, professor, doctoral supervisor, member of Jiusan Society, Beijing Institute of Technology Frontier Interdisciplinary Science Research Institute. Mainly carry out research on energy chemistry of high specific energy batteries. He has published more than 200 papers in journals such as Nat. Energy, Angew. Chem., J. Am. Chem. Soc., Adv. Mater., among which more than 70 papers are ESI highly cited papers with an h-factor of 110. Served as a council member of the Chinese Particle Society, a member of the Energy Chemistry Special Committee of the Chinese Chemical Society, an editorial board member of journals such as J. Energy Chem., InfoMat, and a young editorial board member of journals such as Chin. Chem. Lett., Green Energy Environ., and Energy Mater. Adv. He has won the 2016 Hou Debang Chemical Science and Technology Youth Award of the Chinese Chemical Industry Association, the 2018 Chinese Particle Society Youth Particle Science Award, the 2018 National Talent Plan, the 2020 Beijing Outstanding Youth Science Fund, the 2022 Particle Society First Prize in Natural Science, the 2022 17th China Youth Science and Technology Award Special Award, etc. From 2018 to 2022, he was continuously selected as the Clarivate Global Highly Cited Scientist.

Zhang Xueqiang, assistant professor of Frontier Interdisciplinary Research Institute of Beijing Institute of Technology, is mainly engaged in the research of solid-liquid interface electrochemical model and regulation in secondary batteries, focusing on high specific energy, long-cycle metal Li batteries, Li-sulfur batteries, etc. He successively obtained a bachelor's degree and a doctorate degree from the School of Chemical Engineering of Tianjin University and the Department of Chemical Engineering of Tsinghua University. As the first/corresponding author, he published more than 50 SCI papers in Nat Energy, Sci Adv, JACS, Angew Chem, Adv Mater, Chem, Joule, J Energy Chem, CCL and other journals, h-factor 46. Served as a youth editorial board member of Chinese Chemical Letters and an independent reviewer for journals such as Chem Soc Rev, JACS, Angew Chem, Adv Mater, Adv Energy Mater, J Energy Chem, etc. Received awards such as the Technology Innovation Award of the National Advanced Energy Storage Technology Innovation Challenge, and the Excellent Doctoral Dissertation of the Chinese Particle Society. In 2021 and 2022, he was selected as a Clarivate Global Highly Cited Scientist.