Lithium metal anodes represent a critical pathway toward achieving high-energy-density batteries, but their practical implementation faces persistent challenges related to dendrite formation and unstable solid-electrolyte interphase (SEI) layers. Recent advances in artificial SEI engineering demonstrate that tailored interfacial layers can significantly improve cycling stability by homogenizing lithium-ion flux and suppressing parasitic reactions. Among the most promising approaches are lithium nitride (Li3N) and lithium fluoride (LiF)-rich films, which exhibit high ionic conductivity and mechanical robustness while minimizing interfacial resistance.
The formation of a stable SEI is crucial for preventing dendritic growth, which arises from uneven lithium deposition during cycling. Conventional SEI layers formed in situ from organic electrolytes are inherently heterogeneous, leading to localized current hotspots and eventual cell failure. Artificial SEI layers address this by creating a uniform ion transport pathway with controlled composition and thickness. Li3N, for instance, possesses an exceptionally high lithium-ion conductivity exceeding 1 × 10⁻³ S/cm at room temperature, facilitating rapid ion diffusion. Meanwhile, LiF-rich films provide chemical inertness and high interfacial energy, effectively blocking electrolyte decomposition.
Deposition techniques play a pivotal role in determining the performance of artificial SEI layers. Chemical vapor deposition (CVD) enables precise control over film stoichiometry and thickness, producing conformal coatings that adhere strongly to the lithium metal surface. Studies show that CVD-grown Li3N layers as thin as 50 nanometers can reduce interfacial resistance to below 5 Ω·cm², a significant improvement over native SEI layers. Electrochemical pretreatment is another effective method, where lithium electrodes are cycled in carefully formulated electrolytes to induce the formation of LiF-rich interfaces. For example, electrolytes containing fluoroethylene carbonate (FEC) or lithium bis(fluorosulfonyl)imide (LiFSI) promote the in situ generation of LiF, which integrates seamlessly with the lithium surface.
The impact of these artificial SEI layers on battery performance has been rigorously evaluated in high-energy-density cell prototypes. In symmetric Li|Li cells, Li3N-modified electrodes exhibit stable cycling for over 1000 hours at a current density of 1 mA/cm², with minimal voltage hysteresis. Full cells pairing lithium metal anodes with high-nickel layered oxide cathodes (NMC811) demonstrate capacity retention exceeding 80% after 300 cycles when protected by LiF-rich interphases. These results highlight the critical role of interfacial engineering in extending cycle life.
Long-term stability data further support the viability of artificial SEI strategies. Accelerated aging tests under elevated temperatures (60°C) reveal that cells with Li3N coatings maintain 90% of their initial capacity after 200 cycles, whereas control cells degrade rapidly due to SEI breakdown. Post-mortem analysis confirms that the artificial layers remain intact, with no evidence of lithium dendrite penetration. Similarly, LiF-rich interfaces show remarkable resilience against mechanical stress, maintaining structural integrity even under high stack pressure (10 MPa).
Interfacial resistance remains a key metric for evaluating artificial SEI effectiveness. Electrochemical impedance spectroscopy (EIS) measurements indicate that optimized Li3N and LiF films reduce charge transfer resistance by an order of magnitude compared to conventional SEI. This improvement is attributed to the elimination of resistive decomposition products and the establishment of a defect-free ion transport network. Furthermore, the compatibility of these films with various liquid and solid electrolytes broadens their applicability across different battery chemistries.
Despite these advances, challenges persist in scaling artificial SEI deposition techniques for mass production. CVD, while precise, requires sophisticated equipment and controlled environments, increasing manufacturing costs. Electrochemical pretreatment offers a more scalable alternative but demands stringent electrolyte purity to avoid unintended side reactions. Future research is expected to focus on hybrid approaches combining vapor-phase and solution-based methods to balance performance and scalability.
In conclusion, artificial SEI layers based on Li3N and LiF represent a transformative approach to stabilizing lithium metal anodes. By homogenizing ion transport and mitigating interfacial degradation, these engineered interphases enable high-energy-density batteries with extended cycle life and improved safety. Continued refinement of deposition techniques and deeper understanding of structure-property relationships will further accelerate their adoption in next-generation energy storage systems.