High-capacity lithium-ion batteries are essential to the rapid development of electric vehicles. Typically, the capacities are improved by increasing the thicknesses of electrodes, but this leads to inefficient diffusion of lithium ions, particularly at high current rates. Laser texturing of three-dimensional (3D) structures can provide channels for lithium-ion diffusion in thick-film electrodes. However, laser texturing using conventional laser sources suffers from electrode material melting, leading to failure of the active material. This is accompanied by capacity loss or a reduced number of lithium-ion diffusion paths, which limits the improvements to high-rate performance. This study proposes a method of texturing using a green femtosecond laser with a wavelength of 515 nm for high-capacity NCM811 cathodes. The effects of the green femtosecond laser on NCM811 ablation and the enhancement of the laser-textured structure on the electrochemical performance are investigated.
A laser texturing experiment is conducted using a green femtosecond laser with a wavelength of 515 nm and pulse width of 800 fs. A scanning galvanometer is used to control the laser-scanning paths. Laser-textured structures with various structural features are fabricated in a slurry-coated NCM811 cathode with a thickness of 100 μm (Fig.1). The morphological evolution and phase constitution of the laser-textured structures are characterized by scanning electron microscopy and X-ray diffraction, respectively. The electrochemical performance is tested under a working current density of 0.1-3.0 C (1 C=180 mA·h/g) and a voltage range of 2.8-4.3 V.
The effects of the femtosecond laser parameters on the material removal of the NCM811 cathode are first investigated. With an increase in the energy density or a decrease in the scanning speed, the laser ablation depth and width gradually increase (Figs. 2 and 4). The laser ablation threshold for NCM811 is determined (Fig. 3), which provides a reference for selecting texturing parameters. The green femtosecond laser irradiation changes the morphology of the NCM811 cathode and has little effect on its phase constitution (Fig. 5), demonstrating that no material melting occurs during laser irradiation. Two laser-textured structures, that is, the line structure (Fig. 6 and Table 1) and grid structure (Fig. 8 and Table 2), are then fabricated with various feature sizes to identify the effects of structural features on electrode performance (Figs. 7 and 9). The results show that the grooves in the laser-textured structures increase the contact area between the active material and electrolyte and provide available channels for lithium-ion diffusion. The grid structure with a groove width and column width of 50 μm and 100 μm, respectively, shows superior high-rate performance with remaining specific gravimetric and areal capacities of 92 mA·h/g and 1.37 mA·h/cm2, respectively, at 3 C.
This work successfully improves the rate performance of a thick-film NCM811 cathode through green femtosecond laser texturing. The effects of laser parameters on the morphology and phase constitution of the textured cathode are studied to realize controllable and accurate texturing. Compared with the original NCM811 electrode, laser texturing significantly improves the rate performance of textured electrodes. In addition, compared with the line structure, the grid structure provides more channels with the identical feature sizes (i.e., groove and column widths) for electrolyte wetting and lithium-ion diffusion. This leads to significant improvements in both the specific gravimetric and areal capacities at high rates.
