The Road Towards Planar Microbatteries and Micro-Supercapacitors: From 2D to 3D Device Geometries.

The rapid development and further modularization of miniaturized and self-powered electronic systems have substantially stimulated the urgent demand for microscale electrochemical energy storage devices, e.g., microbatteries (MBs) and micro-supercapacitors (MSCs). Recently, planar MBs and MSCs, composed of isolated thin-film microelectrodes with extremely short ionic diffusion path and free of separator on a single substrate, have become particularly attractive because they can be directly integrated with microelectronic devices on the same side of one single substrate to act as a standalone microsized power source or complement miniaturized energy-harvesting units. The development of and recent advances in planar MBs and MSCs from the fundamentals and design principle to the fabrication methods of 2D and 3D planar microdevices in both in-plane and stacked geometries are highlighted. Additonally, a comprehensive analysis of the primary aspects that eventually affect the performance metrics of microscale energy storage devices, such as electrode materials, electrolyte, device architecture, and microfabrication techniques are presented. The technical challenges and prospective solutions for high-energy-density planar MBs and MSCs with multifunctionalities are proposed.