With the rapid development of equipment, smart textiles, electric vehicles, and renewable energy generators in flexible electronics, the electrochemical energy storage devices have attracted extensive attention. How to obtain an efficient, stable, and safe energy storage system has become an urgent problem to be solved at present. As a new type of energy storage device, the super-capacitor can provide a high power density and a long service life (> 100000 cycle), so it has become the key research direction in the field of energy storage. However, the application of a super-capacitor is limited by its low energy density. In recent years, the application of the laser direct writing technology to the preparation of energy storage electrodes has become a new research hotspot. Laser-induced graphene (LIG) electrode based on polyimide (PI) films has gradually attracted extensive attention due to its advantages of simple preparation and strong expandability, but its further application is limited by its low energy density.

In order to improve the electrochemical performance of LIG electrodes, the influence of laser power and scanning speed on the carbonization effect of the PI film is first studied, and the appropriate processing parameters of LIG electrodes are determined. Then, the RuCl₃ crystal is sprayed onto the surface of the PI film by ultrasonic spraying, and the LIG/RuO₂ composite electrode is prepared by the femtosecond laser direct writing technology. Finally, the super-capacitor is assembled. By means of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Raman spectrum, and X-ray photoelectron spectroscopy (XPS), the micro-nano structure and composition of the electrode are analyzed, and the influence of interdigital spacing on the electrode capacitance is studied. By comparing the electrochemical performances of the LIG/RuO₂ electrode and LIG electrode, the influence of the combination of RuO₂ and LIG on the performance of the super-capacitor is clarified.

Through the single-line scanning experiment, it can be found that the line resistance per unit length decreases with the increase of laser power and the decrease of scanning line speed. Because the laser energy deposited per unit area increases with the increase of laser power and the decrease of scanning line speed, the carbonization degree and depth deepen and the line resistance decreases. The SEM and EDS characterizations show that the electrode surface is a neat laser ablation groove, and the surface structure is still relatively flat. The inner walls of the grooves are all porous structures, which are porous carbon net structures formed after laser-induced carbonization of the PI film. The EDS analysis of the flocculent structure in the LIG/RuO₂ composite electrode shows that the main component is C, and the atomic fraction ratio of Ru to Cl is 7.73. Therefore, it can be judged that RuO₂ is generated in the flocculent structure by removing a small amount of RuCl₃ residues (Fig. 4). The EDS mapping analysis results on the surface of LIG/RuO₂ composite electrode are basically the same as those in flocculent structure, but the Ru content decreases, which may be due to the fact that the thermal deposition on the surface is less than that inside the electrode, and the generation of RuO₂ is decreased (Fig. 5). The Raman spectral analysis and the XPS analysis of the LIG electrode and LIG/RuO₂ composite electrode show that the electrode has a good carbonization degree, and the tetravalent Ru in the XPS peak splitting curve indicates the existence of RuO₂ (Fig. 6). Comparing the cycle voltammetry (CV) curves under different interdigital spacings, it can be found that with the decrease of interdigital spacing, the electrode capacitance gradually increases (Fig. 7). Except for the electrode with a 200 μm interdigital spacing, it can be found that the specific area capacitance of the LIG/RuO₂ composite electrode is more than 4.5 times that of the LIG electrode (Fig. 8). Through different voltage sweeping CV tests, it can be found that the rate performance of the LIG/RuO₂ composite electrode is obviously better than that of the LIG electrode (Fig. 9). In addition, the galvanostatic charge-discharge (GCD) test and the electrochemical impedance spectroscopy (EIS) test of the LIG/RuO₂ composite electrode show that it has reversible charge-discharge characteristics and obvious pseudo-capacitance characteristics (Fig. 10).

Through the femtosecond laser direct writing technology, the RuCl₃ crystals on the surface of the PI film are transformed into RuO₂ nanoparticles and anchored to the surface of porous carbon mesh structure while preparing the LIG electrode, thus the one-step preparation of LIG/RuO₂ composite electrode is realized. The SEM and EDS show that RuO₂ is formed in the micro-nano structure on the electrode surface after femtosecond laser irradiation, and the Raman spectral test shows that the LIG electrode has a good carbonization effect. With the decrease of interdigital spacing, the specific area capacitance of the electrode gradually increases. Compared with the unmodified LIG electrode, the LIG/RuO₂ composite electrode shows good rate performance and ion reaction kinetics. At a voltage sweeping speed of 10 mV/s, the specific area capacitance of the LIG/RuO₂ composite electrode super-capacitor is 4.9 mF/cm², 4.85 times that of the LIG electrode, and the energy density of 0.173 μW·h/cm² at a current density of 0.1 mA/cm² is superior. In addition, the constant current charge-discharge test and the impedance spectrum show that the LIG/RuO₂ composite electrode has good charge-discharge performance and conductivity. This laser direct writing technology does not need any mask design or auxiliary gases, so it is suitable for simple and scalable preparation of planar miniature super-capacitors.