Interface Strengthening of Composite Current Collectors for High-Safety Lithium-Ion Batteries

Abstract: The use of high-capacity ternary cathode materials for high-energy batteries can cause thermal runaway of lithium-ion batteries (LIBs), hindering their safe use and further development. Therefore, improving the energy density of LIBs while maintaining their safety is essential. Current collectors (CCs), which serve as the electron carrier during the electrochemical process, do not contribute to capacity and are regarded as "dead weight" to the cells. The use of composite CCs, which have a sandwich structure where a thin metal (e.g., Al and Cu) layer is deposited on both sides of polymer films, can reduce the weight of CCs owing to the use of the low-density insulating substrate and improve the safety of LIBs (evaluated by the nail penetration test). However, due to the weak interfacial adhesion between the substrate and metal coating layer, the composite CCs may easily delaminate in electrolytes during high-temperature immersion, which could not meet the requirement for the long-term stability. Herein, we introduced an oxide strengthening layer between the substrate (polyethylene terephthalate, PET) and Al layer. The objective of strengthening layer is to increase the interface binding force between the metal and polymer substrate by enhancing the mechanical interlocking effect between the layers and forming a stable chemical bond at the interface. This increased interface binding force effectively improved the electrolyte compatibility of composite CCs even at a high temperature of 85 °C. Based on the results of atomic force microscopy and X-ray photoelectron spectroscopy, we proposed a mechanism for the enhancement of both mechanical interlocking and chemical bonding. Additionally, the composite CCs possessed good mechanical properties that ensure their compatibility with conventional battery fabrication technologies. LIBs using composite CCs exhibited a comparable electrochemical performance to that of aluminum-CC-based (Al CCs) cells, but better performance in nail penetration test. After 280 cycles at 0.2 C, the cell showed high-capacity retention. Al-CC-based cells and PET-AlOx-Al-CC based cells remain 80.55% and 80.9% capacity retention respectively, which indicates the comparable performance. This shows that the composite CCs technology is fully adapted to the existing battery manufacturing technology, and has little influence on the electrochemical performance of LIBs. Specifically, cells with PET-AlOx-Al CCs easily passed the nail penetration test under 100% state of charge without an obvious temperature rise. Furthermore, the voltage of the punctured batteries remained at ~4 V and could still be charged and discharged. The composite CCs successfully prevented the internal short circuit and markedly improved the safety of LIBs during the nail penetration test. Our findings provide theoretical guidance and solutions for the industrialization of composite CCs.