Rethinking Multifunction in Three Dimensions for Miniaturizing Electrical Energy Storage

The miniaturization of batteries has not kept pace with the size scaling achieved with CMOS electronics. The reduced area available on microscale devices requires that batteries reduce their areal footprint commensurately. For traditional battery designs with twodimensional (2D) electrode geometries, the areal footprint limitation is problematic as the total amount of stored energy for 2D batteries decreases and the maximum current output falls because of the small electrode area. The 2D battery design inherently imposes a compromise between energy density and power density for footprint-limited applications. This paper reviews the emerging area of three-dimensional batteries. These architectures represent a new approach for miniaturized power sources that are purposely designed to maintain small footprint areas and yet provide sufficient power and energy density to operate the autonomous devices described above. Although relatively few results have been reported on functional and practical 3D batteries, the active research efforts on designs, fabrication methods, and materials portend a promising future for this field.