Nanoscale (10−50 nm) thin films of cyclic siloxane and silazane polymers were synthesized by initiated chemical vapor deposition (iCVD). We have previously demonstrated that the non-line-of-sight iCVD synthesis process can create uniform conformal coverage of these films over complex nonplanar surfaces. This work will introduce the protocols used to convert these dielectric polymer films into ionic conductors at room temperature. The excellent thickness and morphological stability of these films will be demonstrated along with experiments that determine the ion content in the films. Finally, computational calculations will be used to elucidate the chemical nature of the ion doping and transport processes. These nanoscale, conformal, ionically conducting polymer thin films are attractive as a novel class of nanoscale electrolytes for emerging miniaturized or microbattery architectures such as three-dimensional (3D) batteries which combine high energy (due to high surface area) and power density (due to short ionic transport lengths) within small areal footprints.
■ INTRODUCTIONMicroelectromechanical systems (MEMS) have established high-volume markets for multiple commercial products over the past 20 years and possess further potential for widespread application in sensors, microfluidics, wireless communications, and optics. 1−4 However, MEMS devices require miniaturized or microbatteries with higher areal energy densities (>100 J/cm 2 ) than current planar lithium-ion (Li + ) batteries can provide (<5 J/cm 2 ). 5,6 Three-dimensional or 3D battery architectures comprising of high surface area, nonplanar electrode structures (e.g., interdigitated rods, cylinder, plates, and microporous networks) instead of the traditional planar electrodes can address this challenge by increasing areal energy density, while also maintaining the short ion-transport distances necessary for high power densities. 5−7 A key obstacle to the development of such 3D battery architectures is the synthesis of a suitable electrolyte, which must be conformal, pinhole-free, electronically insulating, and ionically conducting. 5 Unlike liquid electrolytes, conformal, solid-state electrolytes that uniformly "shrink-wrap" nonplanar electrode structures can minimize the volume devoted to the electrochemically inactive electrolyte and thus increase energy density to levels suitable for powering autonomous MEMS devices. 6 Liquid electrolytes further suffer from surface tension and dewetting effects, which lead to "soft" shorts that can create undesirable current shunting pathways in a battery. 8 Finally, solid-state electrolytes with thicknesses on the nanoscale can provide significantly shorter ionic transport times compared to their micron scale or liquid state counterparts. 6,9 The need for nanoscale thickness uniformity and conformality over the complex geometries of 3D battery electrodes has presented one of the most significant design challenges for the realization of 3D batteries.We recently demonstrated that initiated chemical vapor depositio...