Solid oxide cells (SOCs) are core technologies for sustainable energy transition, operating reversibly as solid oxide fuel cells (SOFCs) to convert chemical energy from fuels like hydrogen and methane into electricity, and as solid oxide electrolysis cells (SOECs) to store renewable energy via the valorization of carbon dioxide and water. Their performance depends on a multi-layer structure comprising anode, cathode, electrolyte, and interconnect. The conventional SOCs rely on high-temperature operation mainly due to the insufficient ionic conductivity of thick electrolyte layers at low temperatures, leading to excessive cell impedance. Reducing operating temperature is critical for cutting costs, via enabling the use of low-cost metal interconnects, and mitigating performance degradation, but this requires the fabrication of thin, dense electrolyte films to compensate for reduced conductivity and protective coatings for Cr-containing interconnects to prevent cathode poisoning. This review represents key thin-film fabrication technologies for SOCs (focusing on electrolytes and protective coatings), compares their strengths, limitations, and scalability, and outlines future research directions.

Thin-film technologies for SOCs are categorized into vapor deposition (i.e., chemical vapor deposition, CVD, and physical vapor deposition, PVD) and liquid precursor coating (i.e., sol-solution processes and colloid-slurry processes) based on phase transition pathways and energy input methods. Vapor deposition technologies mainly include chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD-based technologies form films through the reaction or decomposition of gaseous precursors, featuring good compositional uniformity and low-temperature film formation. Their derivative technologies realize the preparation of electrolyte films at medium and low temperatures, some of these technologies can prepare dense electrolytes but suffer from low growth efficiency, while others combining spray and flame synthesis can significantly optimize the electrode-electrolyte interface performance and reduce cell polarization impedance. Atomic Layer Deposition (ALD) achieves atomic-level thickness control through pulsed precursor supply, and when used for electrode modification or interlayer preparation, it can effectively enhance the performance of low-temperature batteries and improve stability. PVD-based technologies form films through physical processes in vacuum or low-pressure environments, and can prepare low-defect electrolyte films or interconnect protective coatings on low-temperature substrates, effectively solving the problem of chromium volatilization, some technologies can accurately deposit multi-component stoichiometric films, and the prepared batteries show excellent long-term stability. Plasma spraying technology can realize direct film formation without sintering, and the density of electrolytes can be significantly improved after optimization, but it is necessary to solve the defect problems during film formation.

Liquid precursor coating technologies are divided into sol-solution coating processes and colloid-slurry forming processes, both of which have the characteristics of low equipment cost and simple operation. Sol-solution processes include spin coating, dip coating, spray pyrolysis, and electrostatic spray deposition. Dense electrolyte films can be prepared through multiple coatings and subsequent treatments, and the thickness of functional layers can be accurately controlled to effectively optimize the interface conduction performance of batteries. Among them, electrostatic spray deposition combines high-voltage electric field and pyrolysis, enabling film formation at lower temperatures and improving electrode polarization characteristics. Colloid-slurry processes, mainly screen printing and tape casting, are the mainstream technologies for large-scale production. Screen printing forms films via scraping slurry, and the preparation temperature of functional layers can be effectively reduced and the battery performance can be improved through process optimization. Tape casting technology and various derivative technologies can produce wide ceramic tapes, which realize the mass production of high-performance batteries and construct gradient porous structures to further optimize mass transfer and interface bonding inside batteries.

A comprehensive comparison of these technologies reveals clear trade-offs. Low-cost, scalable options such as screen printing, tape casting, spin coating, and dip coating are preferred for industrialization due to simple equipment and low material costs, but they require optimization of slurry formulations and sintering processes to minimize defects like cracks and pores. High-performance, high-cost technologies (i.e., ALD, PLD, and low-pressure plasma spraying) deliver superior film density and composition control, but they are constrained by slow deposition rates, high equipment investment, or complex parameter tuning, limiting their use to specialized applications like ultra-thin electrolytes. Emerging technologies such as 3D printing and laser-assisted manufacturing show a promising potential for fabricating complex structures and simplifying co-sintering, but they lack the technical maturity for large-scale SOC production.