Interfacial engineering plays a critical role in improving the performance and stability of solid-state batteries by addressing the challenges of poor solid electrolyte-electrode contact. The interface between the solid electrolyte and electrodes often suffers from high resistance, mechanical instability, and chemical incompatibility, leading to inefficient ion transport and accelerated degradation. To mitigate these issues, researchers have developed various coatings and surface treatments that enhance interfacial adhesion, reduce impedance, and prevent undesirable side reactions.
One of the most widely studied approaches involves applying thin functional coatings to the electrode or solid electrolyte surfaces. These coatings serve multiple purposes, including improving wettability, preventing interfacial reactions, and facilitating lithium-ion transport. For instance, lithium-containing oxides such as LiNbO₃ and LiTaO₃ have been deposited as buffer layers on cathode materials to suppress interfacial decomposition. These coatings act as a barrier against chemical reactions while maintaining high ionic conductivity. Similarly, lithium phosphorus oxynitride (LiPON) has been employed as an interlayer to stabilize the interface between lithium metal anodes and sulfide-based solid electrolytes, reducing dendrite formation and impedance growth.
Another effective strategy is the use of surface treatments to modify the morphology or chemistry of the electrode or electrolyte. Plasma treatment, for example, has been utilized to enhance the surface energy of solid electrolytes, promoting better adhesion with electrodes. This method can introduce functional groups or create nanoscale roughness, both of which improve interfacial contact. Atomic layer deposition (ALD) is another precise technique for applying ultrathin coatings with controlled thickness and composition. ALD layers of Al₂O₃ or TiO₂ have been shown to reduce interfacial resistance and prevent side reactions in oxide-based solid electrolytes.
Mechanical pressing and sintering techniques are also employed to improve interfacial contact. By applying controlled pressure during cell assembly, voids and gaps at the interface can be minimized, leading to lower interfacial resistance. However, excessive pressure may induce mechanical stress or fracture in brittle solid electrolytes, necessitating optimization. Thermal sintering can further enhance contact by promoting interdiffusion or forming intermediate phases at the interface. For example, heating sulfide-based solid electrolytes with oxide cathodes can form a lithium-ion conductive interphase that bridges the two materials without significant resistance increase.
Chemical compatibility remains a major concern in interfacial engineering. Many solid electrolytes react with high-voltage cathodes or lithium metal anodes, forming resistive interphases that hinder performance. To address this, researchers have explored the use of chemically inert but ionically conductive interlayers. For instance, lithium borohydride (LiBH₄) has been investigated as a coating material due to its stability against lithium metal and ability to facilitate ion transport. Similarly, hybrid interfaces combining organic and inorganic components have been designed to balance flexibility and ionic conductivity while suppressing side reactions.
Recent advances in computational modeling have provided deeper insights into interfacial phenomena, guiding the design of better coatings and treatments. Simulations reveal that certain interfacial phases, though thermodynamically unstable, may persist kinetically and contribute to high resistance. By identifying these phases, researchers can tailor coatings to either prevent their formation or render them ionically conductive. Machine learning approaches are also being leveraged to predict optimal coating compositions and thicknesses for specific electrolyte-electrode combinations.
Despite significant progress, challenges remain in scaling these techniques for commercial production. Uniform coating deposition over large areas, cost-effective processing, and long-term stability under cycling conditions are key hurdles. Future work may focus on developing roll-to-roll coating processes or self-assembled interfacial layers that require minimal post-treatment. Additionally, in-situ characterization techniques will be crucial for understanding interfacial evolution during battery operation and refining engineering strategies accordingly.
In summary, interfacial engineering through coatings and surface treatments is essential for advancing solid-state battery technology. By improving contact, reducing resistance, and enhancing stability, these methods pave the way for higher energy density, longer cycle life, and safer energy storage solutions. Continued innovation in materials science and processing techniques will further optimize these interfaces, bringing solid-state batteries closer to widespread adoption.