IntroductionLithium-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.MethodsThis 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 used Γ-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 discussionThis 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 fluorine groups at the molecule heads react with lithium to form LiF as molecular coverage increases, significantly reducing the system formation energy and thereby enhancing the stability and density of the interphase layer. At a high coverage, the fluorine atoms at the 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 the F₁ system), substantially inhibiting interfacial structural relaxation. The DOS analysis indicates that in the LiF layer formed by the F₃ system, Li–F orbitals overlap significantly at –7.5 eV, indicating a strong ionic bonding. The interfacial electrostatic potential is lower 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 interphase layer formed by trifluorinated molecules is more stable based on the AIMD simulations with a thickness of 6.2 Å and a lithium ion diffusion coefficient of 4.76×10^–6 cm²/s, indicating a superior lithium ion transport performance. In contrast, the interphase layer formed by monofluorinated molecules has a lithium ion diffusion coefficient of 1.37×10^–5 cm²/s, showing a relatively inferior lithium ion transport performance.ConclusionsThis study used the DFT and AIMD simulations to systematically explore the interaction processes of self-assembled molecules with varying fluorine contents on lithium metal surfaces, and investigate the regulatory effects of fluorine-terminated groups on the performance of the organic-inorganic composite artificial interphase layer. The introduction of fluorine-terminated groups optimized the electronic structure of lithium metal, promoted lithium ion diffusion, and effectively inhibited electron transmission to the organic molecular layer, thereby enhancing an interphase layer stability. Increasing fluorine content 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 trifluorinated self-assembled molecules had thicker structures, higher lithium ion diffusion coefficients, and exhibited a superior stability. This research elucidated the regulatory mechanisms of fluorine-terminated self-assembled molecules on lithium metal anode interphase layers in the atomic scale, providing a theoretical foundation for the design of high-performance lithium anode interphase layers based on molecular engineering, and extended the application prospects of self-assembly technology in the field of energy storage.