Smarter Power for a Better Future
Authors: Golodnitsky Diana , Blanga Raymond , Cohen Elazar , Kosa Gabor , Kamir Yosi , Rosen Eran , Yufit Vladimir , Preezant Yevgeni 
Published: February 2020

Down-scaling in the microelectronic industry has far outpaced advances in small-scale electric-power supplies. The absence of on-board power is a hindrance to advances in many critical areas in which microelectromechanical systems (MEMS) are used. The problem of insufficient power from planar microbattery configurations has stimulated the search for three-dimensional microbatteries in which cheap and light micro-/nano-fabrication materials and techniques are used. Lithium and Li-ion batteries exhibit very high energy-density values, which are generally based on the performance of large cells with capacities of up to several ampere-hours. For microbatteries, the overall size and mass of the complete battery are determined by packaging and internal battery hardware, which cannot be miniaturized to the same extent as the electrochemical system. In addition, the rate and energy performance of current commercial batteries are limited by the two-dimensional (2D) bulk architecture of electrode materials, which possess relatively small electrode/electrolyte interfacial areas. Therefore, further improvements in advanced microbatteries are closely linked to the development of novel battery designs and materials.
One of the approaches to the achievement of significant cathode- and anode-volume gain and increased battery capacity, by a factor of up to 25-40, is based on the use of a high-aspect-ratio perforated, rather than continuous, silicon substrate, thereby utilizing the dead volume of the substrate[1-4]. The microbatteries with modified 3D-LiFePO4 cathodes ran for over 100 reversible cycles with an areal energy density of about 7.0 mWh/cm2 [4], which is at least one order of magnitude higher than that of the commercial flat thin-film microbatteries at the same C-rate. Despite greater performance characteristics, several problems still prevent commercialization and wide application of 3D on-perforated-silicon microbatteries. These include: the complexity and high cost of silicon perforation, the use of limited cathode materials, the difficulty in conformal coating of the complex-geometry cathode by the polymer membrane and its insufficient robustness, the use of liquid electrolytes and interfacial instability.
We present here, for the first time, the prototype of the all-solid-state rechargeable 3D-microbattery assembled on 3D-printed high-aspect-ratio perforated polymer substrates (3DMB-3DP) of different shapes and area gain (Fig.1a-d). The interconnected perpendicular channels formed through XYZ planes, as in the 3D-printed-polymer prototype, or interpenetrating network of metal or metal-coated current collectors, provide an area gain of the substrate which is 1.5 to 10 times that of the top-to-bottom perforated sample used by us in previous research. We wish to point out that the novel structure simplifies electrochemical insertion of consecutive battery layers which, in turn, enables fabrication of 3D microbatteries with an aspect ratio much higher than 10. The battery occupies a footprint area of only a few mm2, while its height may approach a few cm.
Our group has pioneered in the application of simple and inexpensive electrophoretic-deposition routes for the fabrication of all the thin-film active-material layers of the microbattery. Taking advantage of thin films, which conformally follow all the contours of the 3D-substrate and are composed of nanosize cathode and anode materials, like modified lithium iron phosphate and high-voltage spinels, lithium titanate or graphite, and original polymer-in-ceramic solid electrolyte (Fig.1e-g), enable the maintenance of high reversible specific capacity, long cycle life, and intrinsic safety of the microbattery.
Given high-performance, inexpensive 3D micropower sources, MEMS devices will completely change our lives, by introducing new microsensor arrays, micro-vehicles, and identification cards, memory backup, and biomedical micro-machines (pacemakers, defibrillators, neural stimulators, and drug-delivery systems).

References

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[2]. D. Golodnitsky, M. Nathan, V. Yufit, E. Strauss, T. Ripenbein, I. Shechtman, S. Menkin, L. Burstein, A. Gladkich, E. Peled. Solid State Ionics, 177, 2006, 26-32, 2811-2819
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[4]. H. Mazor, D. Golodnitsky, L. Burstein, A. Gladkich, E. Peled, Journal of Power Sources 198, 2012, 264-272
Fig.1 Photo images of 3D printed polymer (a, b, d) and metal substrates with bottom-up (a), interconnected perpendicular channels formed through XYZ planes (b, c), or interpenetrating network of current collectors (d). ESEM images of 3D perforated polymer substrate (b) coated by the successive layers of Ni current collector(e), LiFePO4 cathode(f) and LiAlO2-PEO based solid electrolyte (g)

Figure 1