3D microbatteries are proposed as a step change in the energy and power per footprint of surface mountable rechargeable batteries for microelectromechanical systems (MEMS) and other small electronic devices. Within a battery electrode, a 3D nanoarchitecture gives mesoporosity, increasing power by reducing the length of the diffusion path; in the separator region it can form the basis of a robust but porous solid, isolating the electrodes and immobilising an otherwise fluid electrolyte. 3D microarchitecture of the whole cell allows fabrication of interdigitated or interpenetrating networks that minimise the ionic path length between the electrodes in a thick cell. This article outlines the design principles for 3D microbatteries and estimates the geometrical and physical requirements of the materials. It then gives selected examples of recent progress in the techniques available for fabrication of 3D battery structures by successive deposition of electrodes, electrolytes and current collectors onto microstructured substrates by self-assembly methods.

The market for lithium batteries is undergoing a rapid expansion as new applications demand higher densities of energy and power storage. Simple theoretical estimates show that lithium and lithium ion cells can reach specific energies of 880 and 500 W h kg^–1 respectively. With an electrolyte conductivity above 3 × 10^–4 S cm^–1 and thickness below 0.01 cm, a power density of 300 W dm^–3 can be obtained without excessive energy losses. Diffusion in porous or polymer composite electrodes is enhanced by an interpenetrating electrolyte provided the electrode particles are small. Batteries using transition metal oxide positive electrodes and carbon negative electrodes are expected to give practical specific energies up to 180 W. h kg^–1 including packaging and other essential additional materials in the near future.