A Group of Cyclic Siloxane and Silazane Polymer Films as Nanoscale Electrolytes for Microbattery Architectures

Abstract

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...

iCVD Cyclic Polysiloxane and Polysilazane as Nanoscale Thin-Film Electrolyte: Synthesis and Properties

“…The synthesis process and chemical properties of these polymers were systematically analyzed. Our results show iCVD polysiloxane and polysilazane films can be conformally, smoothly ( R RMS < 1 nm), and uniformly coated onto substrates with high‐aspect‐ratio, such as electrode arrays used in 3D Li + batteries . The transparent nature and CA property of these films suggest their potential in other areas, such as encapsulation of optical devices and surface engineering.…”

“…Therefore these polymer films can be potentially applied as electrolytes in solid‐state Li + batteries. Due to words limitation, the integration of Li + and relevant analysis, including electrochemical properties of these polymers were introduced in ref . In here, we only include a key property—ionic conductivity in Table , which were measured by EIS.…”

“…Among all polymer films in the table, PV4D4 shows the highest ionic conductivity. Density functional theory (DFT) calculations indicated that Li + binding is more energetically favorable in V4D4 rings than that in V3D3 rings . The more ionic nature of SiO bonds compared to SiN bonds suggests a more favorable binding sites for Li + .…”

CVD Polymers for Devices and Device Fabrication

“…a) Schematic of crystalline PFDA domains with MAA proton conduction channels; [58] b) top: thermal shrinkage of PE separators with and without iCVD coatings, bottom: water contact angle on separator with and without iCVD coatings; [64] c) SEM cross section of conformal iCVD pV3D3 coated on C@SiO 2 fiber; [53] d) DFT simulation of V4D4 monomer electron densities. Blue volumes indicate areas of negative charge; [80] e) False color TEM of iCVD pV4D4 on spinel particle supplied by Veronica Augustyn of the Manthiram group at the University of Texas-Austin. Coating highlighted in light purple f) Left: TEM of ACNT coated with PEDOT Right: close up of single nanotube coated with PEDOT with schematic overlay of nanotube and volumetric charge storage within PEDOT.…”

“…d) Reproduced with permission. [80] Copyright 2015, American Chemical Society. f) Reproduced with permission.…”