Multilayer current collectors have emerged as a critical component in high-voltage battery systems, particularly for applications requiring high energy density, thermal stability, and long cycle life. Unlike single-material foils, these composite structures integrate multiple metal layers, each serving distinct functions to enhance electrochemical performance, mechanical integrity, and interfacial stability. Common configurations include nickel-aluminum-copper (Ni/Al/Cu) stacks, where each layer contributes to optimizing conductivity, corrosion resistance, and adhesion to active materials.
The primary role of a current collector is to facilitate electron transfer between the electrode material and the external circuit while maintaining minimal electrical resistance. In multilayer designs, the outer layers are typically selected for their compatibility with the electrode chemistry, while inner layers provide structural support and conductivity. For instance, in a Ni/Al/Cu stack, nickel serves as the outermost layer due to its excellent oxidation resistance and compatibility with high-voltage cathodes. Aluminum, positioned as an intermediate layer, offers a balance between lightweight properties and conductivity. Copper, often the innermost layer, provides superior bulk conductivity, ensuring efficient current distribution across the electrode.
Interfacial resistance between layers is a critical factor influencing the performance of multilayer current collectors. Poor interfacial contact can lead to increased resistivity, localized heating, and accelerated degradation. To mitigate this, manufacturers employ techniques such as roll bonding, diffusion annealing, or intermediate coatings to enhance adhesion and reduce contact resistance. Roll bonding involves mechanically pressing layers together under high pressure, creating a metallurgical bond without introducing additional resistive phases. Diffusion annealing further improves bonding by promoting atomic interdiffusion at the interfaces, though care must be taken to avoid forming brittle intermetallic compounds that could compromise mechanical flexibility.
Compatibility with high-nickel cathodes, such as NMC (LiNiMnCoO2) or NCA (LiNiCoAlO2), is another key consideration. High-nickel cathodes operate at elevated voltages, often exceeding 4.3 V versus lithium, which can accelerate oxidative degradation of conventional current collectors. Nickel’s inherent stability at high potentials makes it an ideal choice for the outer layer, as it resists corrosion and maintains low interfacial resistance with the cathode material. Additionally, the aluminum intermediate layer acts as a barrier to prevent copper diffusion, which could otherwise catalyze parasitic reactions at the cathode-electrolyte interface.
Thermal management is another advantage of multilayer designs. The differential thermal expansion coefficients of the constituent metals can be engineered to minimize warping or delamination during cycling. For example, aluminum’s intermediate coefficient of thermal expansion helps bridge the mismatch between nickel and copper, reducing stress accumulation during temperature fluctuations. This is particularly important in high-voltage batteries, where operational heat generation can exacerbate mechanical failures in single-layer foils.
Mechanical robustness is equally critical, especially for large-format cells or flexible battery applications. Multilayer current collectors exhibit superior tensile strength and fatigue resistance compared to single-material foils, owing to the composite effect of dissimilar metals. The nickel layer provides surface hardness, protecting against abrasion during electrode coating and calendering, while the copper core ensures ductility, preventing crack propagation under repeated bending or stacking pressures.
From a manufacturing perspective, multilayer current collectors introduce additional complexity in processing but offer long-term benefits in performance and reliability. Precision thickness control is essential to maintain optimal conductivity and weight distribution. A typical Ni/Al/Cu stack might have a thickness ratio of 1:3:1, though this can vary based on specific application requirements. Advanced deposition techniques, such as physical vapor deposition (PVD) or electroplating, enable precise layer formation with minimal defects, further enhancing interfacial quality.
In terms of electrochemical performance, multilayer current collectors contribute to reduced polarization and improved rate capability. The low bulk resistance of copper ensures efficient charge collection, while the nickel surface layer minimizes charge transfer resistance at the cathode interface. This is particularly beneficial for fast-charging applications, where inhomogeneous current distribution can lead to lithium plating or cathode cracking.
Despite these advantages, challenges remain in scaling multilayer current collectors for mass production. Cost considerations, particularly the price volatility of nickel and copper, can impact economic feasibility. Additionally, recycling multilayer structures requires specialized separation processes, adding complexity to end-of-life management. However, the trade-offs are often justified by the performance gains in high-voltage, high-energy-density systems.
Future developments in multilayer current collectors are likely to focus on further optimizing layer compositions and interfaces. Alternative materials, such as conductive ceramics or carbon-based interlayers, are being explored to reduce weight and enhance thermal conductivity. Advances in laser welding and ultrasonic bonding may also improve the manufacturability of these structures, enabling broader adoption in next-generation battery systems.
In summary, multilayer current collectors represent a sophisticated solution for high-voltage batteries, addressing the limitations of single-material foils through tailored layer functionalities, controlled interfacial resistance, and enhanced compatibility with advanced cathode chemistries. Their adoption is poised to grow as battery technologies continue to push the boundaries of energy density and operational voltage, underscoring their importance in the evolution of energy storage systems.