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.

Examples of LiFePO4 composite electrodes are shown in which solid state and interfacial processes are not the principal rate limiting process during fast discharge. Rate dependence on electrode thickness, electrolyte concentration, lithium transference number, and dilution of the active material is explained by a simple salt diffusion model. A discharge to 25 % capacity (0.3 mA h) was obtained on a 40 micrometre thick electrode after only 4 s in an optimised electrolyte -aqueous Li2SO4. Publication of this paper is extrem ely urgent because it provid es an alternative m od el w ith evid ence for an alternative explanation for the recent results in a published in N ature by Kang and Ced er, concerning ultrafast d ischarge of LiFePO4. Several prom inent w orkers in the field , includ ing Ced er, are aw are of the w ork d escribed in the present m anuscript because I gave an outline of it at recent conferences. H ow ever, the significance of the w ork w as not fully appreciated until the Kang paper cam e out in March . Since then I have received countless m essages asking w here I published the m od el so they can reference the w ork. Of course, I have had to reply "com ing asap!". School of Chemistry

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