Current collectors play a critical role in the performance and reliability of solid-state batteries. Unlike conventional lithium-ion batteries with liquid electrolytes, solid-state batteries employ solid electrolytes, which impose unique demands on current collectors. The interface between the current collector and the solid electrolyte must exhibit exceptional stability to prevent degradation, resist lithium dendrite penetration, and maintain low interfacial resistance over the battery’s lifetime. Material selection, surface engineering, and mechanical properties are key factors in addressing these challenges.

Material choices for current collectors in solid-state batteries are constrained by several factors, including chemical stability, electrical conductivity, and mechanical robustness. Stainless steel is a commonly used material due to its corrosion resistance, mechanical strength, and cost-effectiveness. However, stainless steel can form insulating oxides when exposed to certain solid electrolytes, increasing interfacial resistance. To mitigate this, surface treatments such as coatings or alloying are employed. For instance, gold or silver coatings can enhance interfacial conductivity while preventing undesirable reactions. Alternatively, aluminum is another candidate due to its lightweight and high conductivity, but it faces challenges with lithium alloying, which can lead to volume expansion and delamination.

Copper is widely used in conventional batteries but presents difficulties in solid-state systems. Copper reacts with lithium to form intermetallic compounds, which can degrade the interface and increase resistance. Moreover, copper does not inherently suppress lithium dendrite growth, a major concern in solid-state batteries. To address this, researchers have explored modifications such as nanostructured copper or copper alloys with elements like tin or magnesium, which improve interfacial stability and dendrite resistance.

Surface engineering is essential to optimize the interface between the current collector and the solid electrolyte. Rough or uneven surfaces can lead to poor contact, increasing interfacial resistance and promoting inhomogeneous lithium deposition. Polishing or electrochemical polishing can create smoother surfaces, improving adhesion and reducing voids. Another approach involves depositing thin interfacial layers, such as lithium-conducting oxides or polymers, to enhance compatibility. For example, a thin layer of lithium lanthanum zirconium oxide (LLZO) on a stainless-steel current collector can improve ionic contact while preventing chemical reactions.

Lithium dendrite penetration is a critical challenge in solid-state batteries. Dendrites can grow through solid electrolytes, causing short circuits and battery failure. The current collector’s mechanical properties influence dendrite suppression. Harder materials, such as nickel or titanium, can physically block dendrite propagation, but their high stiffness may lead to interfacial stress during cycling. Composite current collectors, combining a rigid outer layer with a softer inner layer, have been proposed to balance mechanical suppression and flexibility. Additionally, patterned or porous current collectors can redistribute lithium ion flux, reducing localized dendrite formation.

Interfacial resistance remains a significant hurdle. Poor contact between the current collector and solid electrolyte leads to high impedance, reducing energy efficiency and power output. Techniques such as thermal compression or spark plasma sintering are used to improve interfacial contact. These methods enhance adhesion without damaging the solid electrolyte’s microstructure. Another strategy involves using compliant interlayers, such as lithium metal or lithium alloys, which deform during cycling to maintain contact.

Temperature stability is another consideration. Solid-state batteries often operate at elevated temperatures to enhance ionic conductivity, but this can accelerate interfacial degradation. Current collectors must maintain structural integrity and minimal reactivity under thermal stress. Refractory metals like molybdenum or tungsten exhibit excellent high-temperature stability but are costly and heavy. Research is ongoing to develop cost-effective alternatives with similar thermal properties.

Long-term cycling performance depends on the current collector’s ability to withstand repeated lithium plating and stripping. Volume changes during cycling can cause mechanical fatigue, leading to delamination or cracking. Graded materials, where the composition gradually changes from the current collector to the solid electrolyte, can alleviate stress concentrations. Alternatively, flexible substrates like carbon-based materials or conductive polymers are being explored for their ability to accommodate volume changes without fracturing.

Manufacturing scalability is a practical concern. While lab-scale studies demonstrate promising materials and coatings, translating these to mass production requires cost-effective and reproducible methods. Roll-to-roll processing, sputtering, and electrodeposition are potential techniques for large-scale current collector fabrication. However, uniformity and defect control remain challenges, particularly for complex multilayer designs.

Environmental and regulatory factors also influence material choices. The use of precious metal coatings, such as gold or platinum, raises concerns about cost and resource availability. Researchers are investigating earth-abundant alternatives, such as carbon-coated metals or conductive ceramics, to achieve similar performance without relying on scarce materials. Additionally, recycling considerations are becoming increasingly important, favoring materials that can be easily separated and reused in end-of-life battery processing.

In summary, current collectors for solid-state batteries must meet stringent requirements to ensure interfacial stability, dendrite suppression, and long-term performance. Material selection, surface modifications, and mechanical design are critical areas of focus. Stainless steel, aluminum, and copper are common base materials, but their limitations necessitate advanced coatings or alloying. Surface engineering techniques improve contact and reduce resistance, while mechanical strategies address dendrite penetration and cycling stresses. Scalability and sustainability further guide the development of next-generation current collectors. Continued research and innovation in this area are essential to realizing the full potential of solid-state batteries.