Flashlight-Induced Ultrafast, Scalable Surface Activation of Highly-Loaded Graphite Composite Anode
Authors: Woo Kyoohee , Choi Su Hyun , Kwon Sin , Jang Sohui , Kim Hyuntae , Yang Wooseok 
Published: December 2024

Numerous substantial challenges persist in achieving widespread and pragmatic applications of LIBs, including further improvements in energy and power density, safety, and cost reduction. One of the many efforts to address them is to increase the loading level of electrodes. This can not only improve energy density per unit area, but also dramatically lower the cost of the battery pack by reducing the stacking number of cells and the use of inactive components such as current collectors and separators. However, contrary to the expectation of such a positive effect, it had been shown that increasing the loading level, i.e., the thickness of the electrodes often resulted in a degradation of the electrochemical properties of the cell. For a thick electrode, a large amount of solvent is volatilized when drying, causing more binder to migrate to the electrode surface, and the paths that the lithium ions and electrons travel become longer and more tortuous. These lead to inhibition of lithium ion diffusion and the accumulation of a solid electrolyte interface (SEI) layer and dead lithium at the interface between the electrode/electrolyte, and consequent degradation of cell performance such as rate capability.
Several emerging materials with enhanced electronic and ionic conductivity are being developed to overcome the performance degradation arising from thickened electrode, but their practical use is hampered by the long time required to verify mass production and performance reliability. Another approach is to build thick electrodes into a three-dimensional (3D) porous structure that is effective for ion and electron transfer. Specially, anisotropic structure with vertical microchannels serving as the high transport pathway had known to enhance the electrolyte permeability into the electrode and alleviate the resistance to ion migration at the electrolyte-electrode interface. This promoted lithium ion diffusion within thick electrode, allowing enhanced rate capability and high volumetric capacity. resulting in improvement of rate capability, and magnetic templating or laser ablation are commonly used to create them. However, for these technologies to be implemented in the battery manufacturing industry, there are still issues that need to be solved, such as a complex and time-consuming manufacturing process, incompatibility with roll-to-roll process and significant loss of active material due to structuring. Therefore, it is important to develop more effective strategy to overcome the limitations in thick electrodes.
From this perspective, in this study, we explored a new and efficient way to address the challenges posed by highly-loaded graphite composite anode using a flash annealing process that is roll-to-roll compatible and capable of large-area processing. When a flashlight is irradiated on the surface of a high-energy-density electrode for less than a second, the light energy absorbed by the graphite active material and conductive carbon additives can be converted to thermal energy. At this time, the surrounding binder can undergo an instantaneous carbonization reaction by the heat, which eventually can form into a porous carbon nanostructure. In addition, some of the weakly bound active material can be cleaned away from the surface, creating pores and bumps on the electrode surface. These changes of electrode surface derived from the interaction with the flashlight can contribute to easier penetration of the electrolyte, increased reaction surface area and improved conductivity, which can lower the transfer resistance of electrons and lithium ions in a thick anode. To confirm these effects, the structural and chemical changes on the electrode surface after irradiation with flashes of different energies were closely analyzed, and the improvement in electrochemical properties such as performance stability and rate capability was confirmed by implementing flash-treated highly loaded graphite anode over 3.6 mAh/cm² into half-cell. Functionalization of the electrode surface using flashlight is not only effective in improving the electrochemical performance degradation caused by the adoption of thick electrodes, but also has the advantage of being compatible with the roll-to-roll process which is mainly adopted by the battery manufacturing industry, as it can be performed in milliseconds on a large area. Therefore, it is expected to become a new growth engine for the battery manufacturing industry in the near future.