Scalable Fabrication of 3D Cu-CNT Composite Anodes for High-Performance Lithium-Metal Batteries

The quest for higher energy density in lithium-ion batteries (LIBs) has been a driving force behind extensive research efforts in recent years. Lithium-ion batteries have become the preferred technology for various applications, including consumer electronics and electric vehicles, due to their high energy density. However, the demand for even higher energy densities persists. One promising approach to achieve this goal is to replace traditional graphite anodes with lithium metal, which boasts a high specific capacity and low redox potential. However, the adoption of lithium metal anodes has been hindered by challenges such as dendrite formation, solid-electrolyte interface (SEI) instability, and volume expansion during cycling. This editorial discusses a recent breakthrough in addressing these challenges by utilizing 3D copper-carbon nanotube (Cu-CNT) composites as anodes for lithium-metal batteries.

Lithium metal anodes hold immense potential due to their high specific capacity (theoretically 3860 mAh g–1) and low redox potential. However, practical issues have limited their commercialization. The main challenges associated with lithium-metal anodes include dendrite formation where over time, lithium dendrites can grow on the anode’s surface, causing the formation of so-called “dead lithium” and leading to short circuits within the battery cell. Additionally, SEI Instability, the continuous formation and dissolution of lithium during cycling results in the growth of the SEI, which consumes lithium inventory and compromises battery performance. Moreover, lithium-metal anodes undergo significant volume changes during cycling, which poses design challenges for cell manufacturers and can lead to mechanical failure.

Efforts to suppress dendrite formation have involved various approaches, including the use of electrolyte additives, control of mechanical pressure on the electrodes, and the development of 3D porous anodes. Electrolyte additives such as cesium ions (Cs⁺), rubidium ions (Rb⁺), boron nitride (BN), and lithium nitrate (LiNO₃) have been explored to stabilize the SEI. However, achieving stable SEIs at high areal loadings and high current densities remains challenging. Controlling mechanical pressure applied to the electrodes has also been studied, taking into account factors like electrolyte infiltration and mechanical stability. This approach aims to balance the forces acting on the anode to reduce dendrite growth. The third approach involves the use of 3D porous anodes, which provide a higher surface area for the lithium plating process. This lowers the current density per surface area and reduces the risk of dendrite formation. Copper (Cu) has been a logical choice of material for these 3D structures due to its high electrical conductivity and electrochemical stability. Several techniques have been proposed to create 3D porous Cu structures, but these methods are often complex and challenging to scale up.

In a recent study led by Professor Michael De Volder from the Department of Engineering at the University of Cambridge presented a scalable electrodeposition method for creating Cu foams with suitable pore structures for Li-metal plating and stripping. The research work is now published in the peer-reviewed Journal ACS NANO.

The electrodeposition process involves using Cu foil as the electrode substrate and an aqueous CuSO4 electrolyte solution containing oxidized multiwall carbon nanotubes (CNTs). This process results in the formation of 3D Cu-CNT composites with an open pore structure where Cu coats the CNTs. Importantly, the CNTs do not create excessive surface area that could lead to further SEI formation. The height of these composites can be controlled by varying the electrodeposition conditions, making the process inherently scalable. The researchers conducted experiments to evaluate the performance of these 3D Cu-CNT composites as Li-metal anodes. Impressively, the proposed composites exhibited outstanding cycle life with high Coulombic efficiencies (CEs) of 99% for up to 800 cycles in a half-cell configuration, using an LiTFSI-based electrolyte. This result represents a significant advancement in the field, with the highest reported CE for a host material for a Li-metal anode in such an electrolyte. The research team  conducted various characterization techniques to analyze the fabricated 3D Cu-CNT composites. SEM imaging revealed the morphology of the composites after electrodeposition, showing an intricate microstructure with Cu coating the CNTs. XRD analysis confirmed the presence of both Cu and CNTs within the composite. They also studied the morphological changes during Li plating and stripping. Optical microscopy images showed uniform Li plating on the 3D Cu-CNT anode, with needle-shaped Li protrusions observed after further Li deposition. Stripping the Li back removed some dendrites, crucial for maintaining a good CE.

The thickness and porosity of the deposited Cu foam could be controlled by adjusting the electrodeposition time. Longer electrodeposition times resulted in increased mass loading and thickness. The porosity of the samples was characterized using the Barrett–Joyner–Halenda and Brunauer–Emmett–Teller methods. Notably, the thickest electrodes demonstrated the highest cycle stability, aligning with the hypothesis that a larger surface area is advantageous. Moreover, the mechanical stability of the Cu-CNT composites allowed for calendering, which deforms the foam plastically without fracturing it. Calendering was used to adjust the electrode porosity and thickness, further demonstrating the versatility of these materials. However, excessive calendering was found to be detrimental to cycling stability.

To evaluate the practicality of the 3D Cu-CNT anodes, symmetrical Li-Cu-CNT//Li-Cu-CNT cells were cycled. These cells exhibited stable cycling performance for a remarkable 140 cycles, highlighting the potential of these anodes for practical applications. Additionally, the anodes were tested in full cells with lithium iron phosphate (LiFePO₄) cathodes, showing a promising capacity retention of 73% after 20 cycles.

In conclusion, the study by Professor Michael De Volder and his team represents a significant step forward in the development of lithium-metal anodes for high-performance batteries. The scalable electrodeposition method for creating 3D Cu-CNT composites addresses critical challenges associated with dendrite formation, SEI instability, and volume expansion. The exceptional cycle life and Coulombic efficiencies achieved in half-cell configurations are highly promising, indicating the potential for practical applications in advanced batteries. The ability to control the thickness and porosity of the composites through electrodeposition and calendering provides a valuable degree of customization for specific battery requirements. Moreover, the successful performance of these anodes in symmetrical cells and full cells suggests their compatibility with practical battery architectures.