Planar microscale electrochemical energy storage devices toward AI-integrated intelligent electronics
The rapid rise of artificial intelligence (AI)-integrated electronics, has created an urgent demand for microscale energy storage systems that are not only compact but also capable of intelligent interaction, rapid responsiveness, and seamless system-level integration. Traditional power sources struggle to meet the stringent requirements of miniaturized and multifunctional electronics, where device footprints shrink to the sub-centimeter or even millimeter scale while functionality expands toward adaptive sensing, and wireless communication. In this context, planar microscale electrochemical energy storage devices (PMESDs), including micro-supercapacitors (MSCs) and micro-batteries, have attracted significant attention as essential components of next-generation intelligent systems.1 Advanced materials such as laser-induced graphene and MXene have enabled MSCs with areal capacitances exceeding 10 mF/cm2, power densities up to 10 mW/cm2, and exceptional cycling stability (>10,000 cycles), making them highly suitable for integration into flexible displays, wearable sensors, and neural interfaces. Micro-batteries, offering higher energy densities, have demonstrated energy densities over 10 mWh/cm2 and volumetric densities approaching 100 mWh/cm3.2
Despite their progress, both MSCs and micro-batteries face persistent challenges in achieving energy-power trade-offs, conforming to unconventional form factors, and stable operation under mechanical conditions. Overcoming these barriers calls for innovations not only in materials and fabrication processes but also in architectural design, interface engineering, and intelligent energy management. This commentary highlights the current state and emerging opportunities in PMESDs, with a focus on enhancing their performance, and adaptability to meet the evolving demands of AI-driven microscale electronics.
Horizontal and multi-directional ion transport
The unique configuration of PMESDs featuring opposite microelectrodes on the same substrate results in ion electromigration predominantly parallel to the substrate (Figure 1A). This contrasts sharply with traditional sandwich-type energy storage devices, where ion transport occurs vertically, perpendicular to the substrate. Notably, the high transverse carrier mobility of two-dimensional (2D) materials aligns well with the horizontal electric field distribution between the microelectrodes of PMESDs. This alignment significantly enhances the charge storage along the plane of the 2D material-based microelectrodes, further improving overall performance. Beyond conventional horizontal electromigration, the geometric dimensions of microelectrodes, where both thickness and width are comparable, induce ion mass transfer behavior akin to spherical diffusion. This distinctive phenomenon underpins the multi-directional ion transfer mechanism observed at the microelectrode scale. For instance, in the case of two interdigital microelectrodes, four distinct ion transport pathways can be identified: two pathways involve ion migration from the side of one microelectrode to both the side and upper surface of the opposing microelectrode, while the remaining two facilitate ion exchange between the upper surfaces of the two microelectrodes. This multi-directional transport behavior stands in stark contrast to the single-direction mass transfer attached to plate-like diffusion mechanism typically observed in conventional sandwich configurations. As a result, the multi-directional ion dynamics substantially enhance the rate performance of PMESDs by reducing ion diffusion resistance and accelerating charge transport.
