B. Reeja-Jayan

Nanoscale, conformal polysiloxane thin film electrolytes for three-dimensional battery architectures

We report the development of nanoscale (10–40 nm), conformal thin film electrolytes realized by doping lithium ions (Li+) into poly-(tetravinyltetramethylcyclotetrasiloxane) (PV4D4) films, which were synthesized by initiated chemical vapor deposition (iCVD). This is the first time nanoscale films with siloxane ring moieties, which are excellent electrical insulators, have been demonstrated as room temperature ionic conductors. The films exhibit minimal changes in morphology and thickness during lithiation and are also demonstrated to be easily scalable over large areas. We show that the conformal nature of the iCVD polymerization process realizes complete coverage of nanostructured electrodes like nanowires by a uniform, continuous, and pinhole-free thin film, making the polysiloxane films attractive as a novel class of nanoscale electrolytes for the emerging field of three-dimensional (3D) batteries.

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

A group of crosslinked cyclic siloxane (Si-O) and silazane (Si-N) polymers are synthesized via solvent-free initiated chemical vapor deposition (iCVD). Notably, this is the first report of cyclic polysilazanes synthesized via the gas-phase iCVD method. The deposited nanoscale thin films are thermally stable and chemically inert. By iCVD, they can uniformly and conformally cover nonplanar surfaces having complex geometry. Although polysiloxanes are traditionally utilized as dielectric materials and insulators, our research shows these cyclic organosilicon polymers can conduct lithium ions (Li(+) ) at room temperature. The conformal coating and the room temperature ionic conductivity make these cyclic organosilicon polymers attractive for use as thin-film electrolytes in solid-state batteries. Also, their synthesis process and properties have been systemically studied and discussed.

Unlocking the structure of mixed amorphous-crystalline ceramic oxide films synthesized under low temperature electromagnetic excitation

The promise of using electromagnetic (EM) fields for low temperature materials synthesis is limited by our ability to structurally characterize these materials, which are often nanocrystalline or amorphous. Here we demonstrate that synchrotron X-ray radiation coupled with the recently developed thin film pair distribution function (tfPDF) analysis yields quantitative information about mixtures of crystalline and non-crystalline materials synthesized under EM excitation, which represents a new direction to study the chemical reactions and lattice ordering induced by EM fields. Our experiments demonstrate for the first time that ceramic oxide films of titanium dioxide (TiO2) grown under microwave radiation (MWR) exposure contain a different phase composition and increased crystallinity compared to TiO2 grown at similar temperatures without EM fields. Specifically, the field-assisted TiO2 is composed of a mixed-phase structure consisting of long-range anatase TiO2 phase with short-range amorphous components, while furnace-grown materials are amorphous with local ordering most resembling the brookite phase of TiO2. The disordered component of MWR-grown TiO2 results in a slightly narrower energy band gap relative to fully crystalline anatase, indicating enhanced light absorption in the visible spectrum. The impact of EM field-influenced atomic structure on resultant material properties creates the opportunity to utilize MWR-assisted synthesis as a novel method for rapid, single-step, low temperature synthesis of mixed ordered-disordered materials for potential use in photocatalysis, thermoelectrics, or lithium ion batteries.

Thermal conductivity of poly(3,4-ethylenedioxythiophene) films engineered by oxidative chemical vapor deposition (oCVD)

Oxidative chemical vapor deposition (oCVD) is a versatile technique that can simultaneously tailor properties (e.g., electrical, thermal conductivity) and morphology of polymer films at the nanoscale. In this work, we report the thermal conductivity of nanoscale oCVD grown poly(3,4-ethylenedioxythiophene) (PEDOT) films for the first time. Measurements as low as 0.16 W m−1 K−1 are obtained at room temperature for PEDOT films with thicknesses ranging from 50–100 nm. These values are lower than those for solution processed PEDOT films doped with the solubilizing agent PSS (polystyrene sulfonate). The thermal conductivity of oCVD grown PEDOT films show no clear dependence on electrical conductivity, which ranges from 1 S cm−1 to 30 S cm−1. It is suspected that at these electrical conductivities, the electronic contribution to the thermal conductivity is extremely small and that phonon transport is dominant. Our findings suggest that CVD polymerization is a promising route towards engineering polymer films that combine low thermal conductivity with relatively high electrical conductivity values.

Surface Engineering of a LiMn2O4 Electrode Using Nanoscale Polymer Thin Films via Chemical Vapor Deposition Polymerization

Surface engineering is a critical technique for improving the performance of lithium-ion batteries (LIBs). Here, we introduce a novel vapor-based technique, namely, chemical vapor deposition polymerization, that can engineer nanoscale polymer thin films with controllable thickness and composition on the surface of battery electrodes. This technique enables us to, for the first time, systematically compare the effects of a conducting poly(3,4-ethylenedioxythiophene) (PEDOT) polymer and an insulating poly(divinylbenzene) (PDVB) polymer on the performance of a LiMn2O4 electrode in LIBs. Our results show that conducting PEDOT coatings improve both the rate and the cycling performance of LiMn2O4 electrodes, whereas insulating PDVB coatings have little effect on these performances. The PEDOT coating increases 10 C rate capacity by 83% at 25 °C (from 23 to 42 mA h/g) and by 30% at 50 °C (from 64 to 83 mA h/g). Furthermore, the PEDOT coating extends the high-temperature (50 °C) cycling life of LiMn2O4 by over 60%. A model is developed, which can precisely describe the capacity degradation exhibited by the different types of cells, based on the aging mechanisms of Mn dissolution and solid-electrolyte interphase growth. Results from X-ray photoelectron spectroscopy suggest that chemical or coordination bonds form between Mn in LiMn2O4 and O and S in the PEDOT film. These bonds stabilize the surface of LiMn2O4 and thus improve the cycling performance. In contrast, no bonds form between Mn and the elements in the PDVB film. We further demonstrate that this vapor-based technique can be extended to other cathodes for advanced LIBs.