Energy storage technologies have changed our lifestyles. The devices from portable smart electronics to the huge electric vehicles push the rapid developments of batteries. However, the energy densities of lithium ion batteries have touched the bottlenecks owing to the low capacities of the cathode and anode. Compared with the world-wide commercial graphite anode, the Li metal anode has attracted much attention due to its high theoretical specific capacity (3860 mAh g-1) and low standard potential (-3.04 V vs. SHE). However, the practical application of Li metal anode encounters severe challenges: 1) growth of dendrites resulted from the uncontrollable reduction and deposition of lithium ions with undesired solvation shell; 2) repeated crack and repairment of the solid electrolyte interphase (SEI), consuming the limited electrolyte; 3) the large volumetric changes during plating/stripping process, resulting in crack of electrode. These complicated issues together deteriorate the battery performance and even cause serious safety problems.
Prof. Hongzhen Lin's research group in Suzhou Institute of Nano-tech & Nano-bionics (SINANO) has engaged in developing robust lithium metal anodes for advanced batteries. In their previous reports, Dr. Jian Wang in Lin's group designed the dual-structured SEI artificial layer derived from the active ionic liquid (Pry13FSI) to inhibit the growth of lithium dendrites and found the functional SEI is capable of resisting the solvent corrosion (Adv. Funct. Mater. 2021, 31, 2007434). Successively, Dr. Wang further constructed a rapid ion diffusion alloy layer to modulate the lithium ion/atom behaviors across the electrolyte/anode interface, and the artificial layer shows strong ability of persisting in highly moist air surroundings (Adv. Funct. Mater. 2022, 31, 2110468).
Figure 1. The interphase engineering for inhibiting Li dendrite formation.
With deep understanding from a fundamental view, the Li plating process can be divided into four steps: i) the migration of the solvated Li+ (a complex of Li+-solvent molecules) in the electrolyte; ii) the desolvation of Li+-solvents at the electrode/electrolyte interface; iii) the formation of Li(0) atom after coupling with external electron; iv) the diffusion and nucleation of the formed Li(0). These steps are restricted by the barriers of the solvated Li+ or generated Li(0). Inspired by the former researches in conversion-based sulfur cathodes in which defect-rich catalysts are explored to reduce the reaction barriers and to improve the utilization of sulfur species(Chem. Eng. J. 2020, 128072; ACS Appl Mater Interfaces 2020, 12,12727; J. Mater. Chem. A 2020, 8, 14769), it is of great interest to use highly active electrocatalysts to adjust the kinetic behavior of lithium ions/atoms for solving the tough problem of uncontrollable plating and dendrite formation at the anode side.
From the perspective of surface-interface functionalization, the catalytic ability can be further improved by regulating the catalyst center-supporter interactions. Specifically, in a recent collaboration work of Dr. Wang and Prof. Lin with Dr. Jing Zhang and Prof. Yincai You, the application of Schottky defects to regulate the 4f-center electronic structure of cerium oxide (SDMECO@HINC) is proposed for providing a large number of active sites to enhance its catalytic activity. The SDMECO@HINC shows excellent electrocatalytic activity in accelerating the Li ion desolvation and the Li atom diffusion kinetics, smoothing Li plating with dendrite-free morphology (Adv. Sci. 2022, 2202244).
Figure 2. The schematic illustration of 4f-electron state modulation in realizing dendrite-free morphology.
In details, as displayed in Figure 3, the 4f electronic structure in CeO2 has the characteristics of tunability and orbital hybridization, inducing the disorder of the electronic states by introducing Schottky defects. Subsequently, the 4f electronic state is significantly recovered after interaction with Li atoms via charger transfer, which is a key factor for the uniform trapping of Li atoms during the initial nucleation process. In addition, the adsorption energy increases with the intensity of electronic manipulation together with the higher concentration of Schottky defect, which is more favorable for capturing Li atom and smooth plating.
Figure 3. Theoretical simulation of the interaction between Schottky Defects in Cerium Oxide and lithium atoms.
Due to the rearrangement of the electronic structure of the 4f center of Ce, the generated numerous active sites enhance the catalytic activity of SDMECO@HINC and thus reduces the nucleation barrier of Li atoms down to 11 mV. During subsequent cycling, SDMECO@HINC enables faster diffusion of Li atoms and more uniform deposition with a stable overpotential of ~13 mV for 1200 h without dendrite formation. At the same time, the SDMECO@HINC catalytic layer could significantly improve the Coulombic efficiency up to about 98%. With the Scanning Electron Microscopy, the surface of Li metal after SDMECO@HINC modification is uniform and smooth without obvious volume change in contrast to the obvious crack in pristine Li electrode. The catalytic desolvation of the SDMECO@HINC modified layer is further verified by interface-sensitive in situ and frequency spectroscopy (SFG), which effectively promotes the generation of free Li+ from solvated Li+ for uniform Li deposition. As summarized in Figure 4, the principal mechanism of SDMECO@HINC is well depicted.
Figure 4. Modulation mechanism of SDMECO@HINC on the kinetics of lithium ions for smooth plating.
In their previous works on sulfur cathodes, Lin’s group has found that metal single-atom catalysts and defect catalysts can control the kinetic behavior of lithium ions (Energy Storage Mater 2019, 18, 246; Energy Storage Mater. 2020, 28, 375; ChemSusChem 2020, 13, 3404; Energy. Environ. Mater. 2021, DOI: 10.1002/eem2.12250; Adv. Energy Sus. Res. 2022, 2100187 ). Based on the gained knowledge, one would expect that metal single-atom catalysts on nanocarbons can help to provide abundant binding sites with proper lithiophilicility to modulate the diffusion kinetics of lithium, and to guide the uniform deposition of lithium without dendrite formation. However, apart from the wide preparation method of using nitrogen-doped nanocarbons to support SACs, there still lacks of simple and effective methods for anchoring and stabilizing high-concentration SACs.
For the first time, a strategy of using defect sites to anchor metal single atoms (SAC-in-Defect) is developed by Lin's group (Figure 5). Briefly, highly active metal single atoms are anchored in cationic defect compounds through hydrothermal and heating treatments, as revealed by STEM-HAADF and XAS. As the modulation promoter, lithium ion diffusion kinetic is propelled by the abundant metal single atom sites. In the later cycles, the SAC active sites can guide the uniform nucleation of Li with a lower energy barrier and promote the dendrite-free process on Li metal surfaces. Cycled at 1 mA cm-2, the prepared pouch battery with a sulfur loading of 5.4 mg cm-2 delivers an areal capacity of 3.78 mA cm-2, shedding light on the future for practical application (Nano Lett. 2021, 21, 3245).
Figure 5. The schematic description of “SAC-in-Defect” catalyst in regulating lithium behaviors.
The above works successfully applied the state-of-the-art catalyst design strategies, such as defect engineering and atomic dispersion of active sites, in decreasing the energy barriers and propelling the lithium kinetics of ionic/atomic diffusion, desolvation, and transfer. The concept that these physical processes can be “catalyzed” in a similar way as a typical chemical reaction will greatly broaden the application range of electrocatalysts in secondary batteries.
Tuning 4f-center Electron Structure by Schottky Defects for Catalyzing Li Diffusion to Achieve Long-term Dendrite-free Lithium Metal Battery, Jing Zhang, Rong He, Quan Zhuang, Xinjun Ma, Caiyin You, Qianqian Hao, Linge Li, Shuang Cheng, Li Lei, Bo Deng, Xifei Li, Hongzhen Lin, and Jian Wang, Adv. Sci. 2022, 20210468. DOI: 10.1002/advs.202202244
Long-Life Dendrite-Free Lithium Metal Electrode Achieved by Constructing a Single Metal Atom Anchored in a Diffusion Modulator Layer, Jian Wang;Jing Zhang;Shuang Cheng;Jin Yang;Yonglan Xi;Xingang Hou;Qingbo Xiao;Hongzhen Lin, Nano Lett. 2021, 21, 3245-3253.
More details are available in the group website: www.hzlin.cn
Dr. Hongzhen Lin，a full professor in i-Lab of Suzhou Institute of Nano-tech and Nano-bionics, CAS, leads a research group in developing advanced electrochemical energy materials and batteries via novel catalytic strategies, and revealing the related interfacial mechanisms with in-situ sum frequency generation spectroscopy. He has published over 100 works on top journals such as Nat. Commun.、Sci. Adv.、JACS、Nano Lett.、Adv. Funct. Mater.、Angew.Chem. Int. Ed.、Adv. Sci.、Nano Energy、Energy Storage Mater.、J. Phys. Chem. Lett., as communicating or first author.
Dr. Jian Wang, granted by Humboldt Scholar fellowship, works in the Helmholtz Institute Ulm, KIT, in Germany. His research interest mainly focuses on interdisciplinary between electrocatalysis and second conversion-based batteries. He is devoted to exploring the battery mechanism by in-situ/operando methods such as in-situ XAS, Raman and SFG spectroscopies as well. He has published over 23 papers on the top journal of Nano Lett., Energy Storage Mater., Adv. Funct. Mater., Adv. Sci, Nano Energy, Energy Environ. Mater., Chem. Eng. J., J. Mater. Chem. A, ChemSusChem, J. Power Sources and ACS Appl. Mater. Interface as the first or corresponding author.