The development of flexible energy storage devices has gained significant attention due to the increasing demand for wearable electronics and epidermal health monitoring systems. Among the various materials explored, laser-induced graphene (LIG) has emerged as a promising candidate for flexible supercapacitors due to its unique properties, including high conductivity, tunable porosity, and compatibility with polymer substrates. The synthesis of LIG involves the direct conversion of carbonaceous precursors, such as polyimide or other polymers, into porous graphene-like structures through laser irradiation. This process eliminates the need for traditional chemical vapor deposition or wet-chemical methods, enabling rapid and scalable fabrication.

The properties of LIG are highly dependent on lasing parameters, including laser power, scanning speed, pulse frequency, and wavelength. For instance, increasing laser power enhances the degree of carbonization, leading to higher electrical conductivity but may reduce porosity if excessive energy causes material ablation. Optimal power ranges between 5 to 10 W for CO2 lasers, balancing conductivity and structural integrity. Scanning speed influences the residence time of the laser on the substrate, with slower speeds (50–200 mm/s) yielding thicker and more conductive LIG due to prolonged exposure. Pulse frequency adjustments allow control over the density of defects and edge sites, which are critical for pseudocapacitive charge storage. Near-infrared lasers (1064 nm) are commonly used due to their ability to penetrate deeper into the polymer matrix, creating a three-dimensional porous network.

Porosity and conductivity are inversely related in LIG; higher porosity improves ion accessibility but may reduce electrical percolation pathways. A balance is achieved by modulating laser parameters to create hierarchical pore structures, including micropores (less than 2 nm) for ion adsorption and mesopores (2–50 nm) for rapid ion transport. The resulting LIG typically exhibits a specific surface area of 300–800 m²/g and sheet resistance values ranging from 10 to 50 Ω/sq, depending on processing conditions.

Ion transport in LIG-based supercapacitors is governed by both in-plane and through-plane diffusion mechanisms. In-plane transport occurs along the basal planes of graphene sheets, facilitated by the interconnected network of sp²-hybridized carbon. Through-plane transport relies on the penetration of electrolyte ions into the porous structure, which is influenced by pore alignment and tortuosity. Studies indicate that vertically aligned pores, achieved by controlled laser rastering patterns, enhance through-plane ion diffusion, reducing equivalent series resistance. In contrast, randomly oriented pores favor in-plane transport but may limit charge storage at high current densities. Electrochemical impedance spectroscopy reveals that through-plane-dominated LIG electrodes exhibit lower Warburg impedance, indicating faster ion kinetics.

Integration with solid-state electrolytes is critical for flexible supercapacitors to prevent leakage and ensure mechanical stability. Gel polymer electrolytes, such as polyvinyl alcohol (PVA)-H₂SO₄ or poly(ethylene oxide) (PEO)-based ionic liquids, are commonly employed due to their high ionic conductivity (10⁻³ to 10⁻² S/cm) and adhesion to LIG. The interface between LIG and the electrolyte is optimized by pre-treating the LIG surface with plasma or chemical functionalization to enhance wettability. Stretchable designs incorporate serpentine interconnects or kirigami-inspired patterns to accommodate mechanical deformation without compromising electrical performance. For instance, LIG supercapacitors embedded in polydimethylsiloxane (PDMS) matrices retain over 90% capacitance after 1000 stretching cycles at 30% strain.

Scalability and patterning resolution present challenges for LIG fabrication. Large-area production requires precise control of laser focal points to maintain uniformity, as defocusing can lead to inconsistent carbonization. Multi-pass lasing strategies improve homogeneity but increase processing time. Patterning resolution is limited by the laser spot size (typically 50–100 µm for CO₂ lasers), restricting feature sizes for micro-supercapacitors. Advancements in ultrafast lasers (picosecond or femtosecond pulses) may enable sub-micron resolution by reducing thermal diffusion effects.

Epidermal energy storage applications benefit from LIG’s lightweight and conformal properties. Supercapacitors fabricated on biocompatible substrates, such as medical-grade polyurethane, demonstrate stable performance under repetitive bending and twisting. The energy density of LIG-based devices ranges from 1 to 10 Wh/kg, with power densities exceeding 10 kW/kg, suitable for powering wearable sensors or transient biomedical devices. Encapsulation strategies using thin-film barriers (e.g., atomic layer deposition of Al₂O₃) protect the LIG from moisture and mechanical wear while maintaining flexibility.

Future directions include hybrid systems combining LIG with redox-active materials (e.g., MnO₂ or conductive polymers) to enhance capacitance via faradaic reactions. Machine learning-assisted optimization of laser parameters could further refine the trade-offs between porosity, conductivity, and mechanical robustness. The development of roll-to-roll laser processing systems will be pivotal for industrial-scale adoption, enabling continuous production of LIG-based energy storage devices.

In summary, laser-induced graphene offers a versatile platform for flexible supercapacitors, with tunable properties governed by lasing parameters. The interplay between in-plane and through-plane ion transport dictates electrochemical performance, while integration with solid-state electrolytes and stretchable designs ensures durability. Overcoming scalability and resolution challenges will unlock broader applications, particularly in epidermal electronics where conformal energy storage is essential.