The integration of polymer electrolytes with artificial solid-electrolyte interphase (SEI) layers represents a significant advancement in stabilizing lithium metal anodes for next-generation batteries. Lithium metal anodes suffer from dendrite growth, parasitic reactions, and poor cycling stability due to their high reactivity and uneven lithium deposition. Conventional polymer electrolytes alone cannot sufficiently suppress these issues, necessitating engineered interfacial layers that combine the flexibility of polymers with the stability of inorganic components. Among these, lithium fluoride (LiF)-rich artificial SEI layers have emerged as particularly effective due to their high mechanical strength, electrochemical stability, and ability to promote uniform lithium-ion flux.

Polymer electrolytes offer inherent advantages over liquid electrolytes, including reduced flammability, better mechanical properties, and compatibility with lithium metal anodes. However, their lower ionic conductivity and insufficient interfacial stability limit their practical application. To address these shortcomings, researchers have developed in-situ formation methods for artificial SEI layers that integrate seamlessly with polymer matrices. These methods often involve the reaction of polymer-bound functional groups with lithium metal or the decomposition of fluorine-containing precursors to form LiF-rich interfaces. The in-situ approach ensures conformal coverage and strong adhesion to the lithium surface, preventing delamination during cycling.

One effective strategy involves the use of fluorinated polymers that decompose upon contact with lithium metal to form LiF. For example, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) blended with lithium salts can generate a LiF-rich layer when cast onto lithium metal. The fluorine atoms in the polymer backbone react with lithium to form LiF, while the remaining polymer matrix maintains ionic conductivity. This dual functionality enhances interfacial stability while preserving electrolyte performance. The resulting SEI layer exhibits high interfacial energy, which promotes uniform lithium deposition and reduces dendrite penetration.

Multi-layer architectures further improve the performance of polymer electrolytes by combining distinct functional layers. A typical design may include a soft polymer layer adjacent to the lithium anode to ensure good contact, followed by a rigid LiF-rich layer to block dendrites, and finally a high-conductivity polymer layer to facilitate ion transport. This graded structure balances mechanical robustness with electrochemical performance. For instance, a trilayer electrolyte composed of a polyethylene oxide (PEO) layer, a LiF-rich intermediate layer, and a ceramic-reinforced outer layer has demonstrated stable cycling over 500 hours in symmetric Li cells at 0.2 mA/cm². The PEO ensures interfacial compatibility, the LiF layer suppresses dendrites, and the ceramic reinforcement enhances mechanical strength.

In-situ electrochemical polymerization is another promising method for creating artificial SEI layers. By applying a voltage to a monomer-containing polymer electrolyte, a dense polymer film can be electrochemically deposited onto the lithium surface. Incorporating fluorine-containing monomers, such as fluoroethylene carbonate (FEC), results in LiF formation within the polymer matrix. This approach allows precise control over SEI composition and thickness, enabling optimization of mechanical and transport properties. Studies have shown that such electrochemically formed SEI layers can achieve high Coulombic efficiencies (>98%) and extended cycle life in lithium metal batteries.

The mechanical properties of artificial SEI layers are critical for long-term stability. LiF-rich layers typically exhibit high elastic moduli (>50 GPa), which are necessary to resist lithium dendrite penetration. However, excessive rigidity can lead to cracking during lithium stripping and plating. To mitigate this, composite SEI layers incorporating flexible polymer binders have been developed. These composites maintain high mechanical strength while accommodating volume changes. For example, a hybrid SEI composed of LiF nanoparticles dispersed in a polyacrylic acid (PAA) matrix has shown improved toughness and adhesion, enabling stable cycling at higher current densities (1 mA/cm²).

Ionic conductivity remains a challenge in polymer-based systems, particularly at room temperature. The addition of LiF-rich layers must not significantly impede ion transport. Strategies to address this include nanostructuring the SEI layer to create fast ion-conduction pathways or doping with lithium salts to enhance conductivity. For instance, introducing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into the SEI layer has been shown to improve ionic conductivity while maintaining electrochemical stability. The optimal balance between SEI thickness and conductivity is crucial; too thick a layer increases resistance, while too thin a layer may fail to prevent dendrite growth.

The electrochemical stability window of polymer electrolytes with artificial SEI layers must be compatible with high-voltage cathodes. LiF-rich interfaces exhibit excellent stability against oxidation, enabling operation above 4 V versus Li/Li⁺. However, the polymer matrix itself must also resist degradation. Crosslinked polymer networks, such as those formed by ultraviolet (UV) curing, have demonstrated improved oxidative stability compared to linear polymers. When combined with a LiF-rich SEI, these systems enable stable cycling in full cells with high-voltage cathodes like lithium nickel manganese cobalt oxide (NMC).

Scalability and manufacturability are essential considerations for practical deployment. Solution-casting and roll-to-roll processing are compatible with polymer electrolyte and artificial SEI layer fabrication. In-situ formation methods that do not require additional processing steps are particularly attractive for large-scale production. For example, thermally induced phase separation can be used to create porous polymer electrolytes that subsequently react with lithium to form LiF-rich interfaces in a single step. This approach simplifies manufacturing while maintaining performance.

Long-term cycling stability has been demonstrated in several systems incorporating polymer electrolytes and artificial SEI layers. Symmetric Li cells with optimized interfaces have achieved over 1000 cycles at moderate current densities, while full cells with high-capacity cathodes retain more than 80% of their initial capacity after 300 cycles. These results highlight the potential of this approach to enable practical lithium metal batteries.

Future developments may focus on further optimizing the composition and structure of artificial SEI layers. Advanced characterization techniques, such as cryogenic electron microscopy and X-ray photoelectron spectroscopy, provide insights into the dynamic evolution of these interfaces during cycling. Machine learning could aid in the design of multi-component SEI layers with tailored properties. Continued progress in polymer chemistry and interfacial engineering will be crucial for realizing the full potential of lithium metal anodes in next-generation energy storage systems.