摘要
基于分子自组装方法在金属锂负极界面构筑功能性人工界面层,可以定向调控固体电解质界面(SEI)性能,进而优化金属锂负极的稳定性和安全性。本工作结合密度泛函理论和从头算分子动力学模拟方法,系统研究了头部不同含氟量的自组装分子与金属锂表面相互作用和结构演化,并重点探讨了分子头部氟端基结构对界面有机–无机复合人工界面层性能的调控作用。研究发现,随着自组装分子覆盖度的增加,分子头部端基F与金属锂发生氟化反应并形成LiF,降低了体系形成能,使得高覆盖度下界面有机–无机复合人工界面层的稳定性和致密性得到提升。态密度等电子结构分析表明,氟化物的引入改善了金属锂的局域电子结构,促进了锂离子在界面上的嵌入和扩散,并限制了电子向有机分子层的传输。从头算分子动力学模拟揭示,头部含三氟的分子相比于含一氟的分子,动态演化形成的人工界面层更厚、更均匀、氟配位数更高、锂扩散系数更大,表现出更优的稳定性。静电势分析表明,三氟分子形成的人工界面层具有更低的电势,热力学上更有利于锂离子输运。因此,通过自组装分子头部含氟量的调控能有效促进金属锂负极界面SEI层的稳定性和优化锂离子输运性能,这为设计锂负极界面高性能人工界面层提供了很好的理论依据。
Introduction Lithium-metal batteries(LMBs),as next-generation high-energy-density battery technologies,have attracted considerable attention due to their theoretical energy density(i.e.,approximately 3860 mA·h/g),which far exceeds that of conventional lithium-ion batteries(LIBs).This makes LMBs highly promising for applications in electric vehicles,portable electronic devices,and other fields.Compared to conventional LIBs with a theoretical capacity of 372 mA·h/g for graphite anodes,LMBs can offer a longer battery life or a higher energy output,thus addressing the energy density bottleneck faced by the existing energy storage technologies.As a result,LMBs are widely regarded as an important direction for the future development of battery technologies.However,despite the significant advantages of LMBs in terms of energy density,the use of lithium metal anodes still has some challenges.Some issues like dendrite growth and instability of the solid electrolyte interphase(SEI)severely restrict their practical application.It is thus critical to suppress lithium dendrite growth and improve the stability of SEI.Although the existing studies propose the use of artificial interphase layers to optimize the stability and safety of lithium metal anodes,there is a lack of systematic theoretical guidance,particularly in terms of how to regulate the composition and structure of the interphase layer.This study was to construct functional artificial interphase layers on the lithium metal anode surface by molecular self-assembly techniques.This study also regulated the SEI performance via utilizing self-assembled molecules with varying fluorine contents,thus enhancing the stability and safety of lithium metal anodes and providing a theoretical basis for future interface layer design in lithium metal batteries.Methods This study used the density functional theory(DFT)and ab initio molecular dynamics(AIMD)simulations to systematically investigate the interaction and structural evolution of self-assembled molecules with different fluorine contents on lithium metal surfaces.The focus was to clarify how fluorine-terminated molecular structures could affect the performance of the organic-inorganic composite artificial interphase layer.The calculations were performed by a Vienna Ab-initio Simulation Package(VASP)based on the density functional theory.The VASP employs the projector-augmented wave(PAW)method and the Perdew-Burke-Ernzerhof(PBE)functional within the generalized gradient approximation(GGA).All the calculations usedΓ-point centered Brillouin zone sampling to ensure accuracy.The PAW method was used for electron-ion interactions with an energy cutoff of 450 eV.Convergence criteria for energy and force were set at 1×10^(–4) eV and 0.05 eV/Å,respectively.The van der Waals interactions were determined by the DFT-D3 method.The density of states(DOS),charge differential density,and Bader charge distribution were calculated to characterize an interfacial electronic behavior.Organic-inorganic composite interphase layers were constructed by the molecular models with varying fluorine termini(i.e.,F1:C9H19F,F3:C9H17F3),and the molecular adsorption behavior was analyzed by coverage gradient models to evaluate the adsorption stability through formation energy calculations.The AIMD simulations were conducted to explore the dynamic behavior of the interphase layer.The 20 ps simulations were performed at 300 K(Nosé-Hoover thermostat)with one time step of 1 fs.Lithium ion diffusion coefficients were calculated via mean square displacement(MSD),and the structural characteristics of the interphase layer were analyzed via radial distribution functions(RDF)and coordination numbers.Results and discussion This study systematically reveals the regulatory mechanisms of fluorine-terminated self-assembled molecules on lithium metal anode interphase layers based on theoretical calculations and dynamic simulations. On the Li(100)lithium metal surface, higher fluorine content molecules (F3:C9H17F3) exhibit stronger interfacial binding capabilities. The fluorinegroups at the molecule heads react with lithium to form LiF as molecular coverage increases, significantly reducing the systemformation energy and thereby enhancing the stability and density of the interphase layer. At a high coverage, the fluorine atoms atthe heads of F3 molecules reactwith lithium to form an amorphous LiF layer with a thickness of 6.2 Å (compared to 3.8 Å for theF1 system), substantially inhibiting interfacial structural relaxation. The DOS analysis indicates that in the LiF layer formed by theF3 system, Li–F orbitals overlap significantly at –7.5 eV, indicating a strong ionic bonding. The interfacial electrostatic potential islower for the F3 system (–4.87 eV), and the thicker LiF layer formed by F3 molecules results in a a wider negative potential region,and more conducive to lithium ion intercalation/extraction. Lithium ion diffusion dynamics analysis shows that the interphaselayer formed by trifluorinated molecules is more stable based on the AIMD simulations with a thickness of 6.2 Å and a lithium iondiffusion coefficient of 4.76×10^(–6) cm^(2)/s, indicating a superior lithium ion transport performance. In contrast, the interphase layerformed by monofluorinated molecules has a lithium ion diffusion coefficient of 1.37×10^(–5) cm^(2)/s, showing a relatively inferiorlithium ion transport performance.Conclusions This study used the DFT and AIMD simulations to systematically explore the interaction processes ofself-assembled molecules with varying fluorine contents on lithium metal surfaces, and investigate the regulatory effects offluorine-terminated groups on the performance of the organic-inorganic composite artificial interphase layer. The introduction offluorine-terminated groups optimized the electronic structure of lithium metal, promoted lithium ion diffusion, and effectivelyinhibited electron transmission to the organic molecular layer, thereby enhancing an interphase layer stability. Increasing fluorinecontent enhanced the ionic nature of Li–F bonds, significantly reducing lithium vacancy formation energy and bonding strength,and improving lithium ion migration capability in the interphase layer. The interphase layers formed by trifluorinatedself-assembled molecules had thicker structures, higher lithium ion diffusion coefficients, and exhibited a superior stability. Thisresearch elucidated the regulatory mechanisms of fluorine-terminated self-assembled molecules on lithium metal anode interphaselayers in the atomic scale, providing a theoretical foundation for the design of high-performance lithium anode interphase layersbased on molecular engineering, and extended the application prospects of self-assembly technology in the field of energy storage.
作者
高文霞
方诗豪
毛鑫
刘畅
喻学锋
彭超
GAO Wenxia;FANG Shihao;MAO Xin;LIU Chang;YU Xuefeng;PENG Chao(Shenzhen Institute of Advanced Technology,Chinese Academy of Sciences,Shenzhen 518055,Guangdong,China;University of Chinese Academy of Sciences,Beijing 101408,China;Hubei Xingfa Chemicals Group Co.Ltd.,Yichang 443711,Hubei,China;Hubei Three Gorges Laboratory,Yichang 443007,Hubei,China)
出处
《硅酸盐学报》
北大核心
2025年第7期1902-1911,共10页
Journal of The Chinese Ceramic Society
基金
国家自然科学基金(52203303)
中国科学院未来伙伴关系网络专项(321GJHZ2023189FN)
深圳市国际合作研究项目(GJHZ20220913142812025)。
关键词
金属锂负极
分子自组装
人工界面层
氟化锂
密度泛函理论
lithium metal anode
molecular self-assembly
artificial interface layer
lithium fluoride
density functional theory
