Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

The techniques of initiated chemical vapor deposition (iCVD) and oxidative chemical vapor deposition (oCVD) enable the fabrication of chemically well‐defined thin polymeric films on complex objects with micro‐ and nano‐scale features. By depositing polymers from the vapor phase, many wetting and solution effects are avoided, and conformal films can be created. In iCVD, a variant of hot filament CVD, the deposition rate is enhanced and chemical functionalities of the polymers' constituents are maintained by including a thermally labile initiator in the feed stream. Due to the low energy required when using an initiator, delicate substrates can be coated. In oCVD, infusible, electrically conductive films are formed directly on the substrate of interest as the oxidant and monomer are introduced into the reactor simultaneously. This Feature Article provides an overview of the work that has been done to develop iCVD and oCVD into platform technologies. Relevant background, fundamentals, and applications will be discussed.

The synthesis, processing, and performance of a low‐cost monolithic battery electrode, produced entirely of natural and renewable resources, are reported. This anode material exhibits tunable electrochemical performance suitable for both high power and high energy applications. A synthesis method that directly results in electrically interconnected three‐dimensional architectures is presented, where the carbon framework functions as current collector and lithium insertion material, eliminating the extra mass and expense of inactive materials in conventional designs. Fibrous carbon electrode materials are produced from solvent extracted lignin using scalable melt processing technology and thermal conversion methods. The resulting free‐standing electrodes exhibit comparable electrochemical performance to commercial carbon‐based anodes at a fraction of the materials and processing costs. Compositional and electrochemical characterization shows that carbonized lignin has a disordered nano‐crystalline microstructure. The carbonized mats cycle reversibly in conventional aprotic organic electrolytes with Coulombic efficiencies over 99.9%. Moreover, lignin carbon fibers carbonized at 2000 °C can cycle reversibly in 1 m LiPF6 in propylene carbonate.

For transportation applications, the safety of lithium ion batteries is an important problem demanding further progress. A promising approach is the replacement of flammable liquid electrolytes with non-flammable solid electrolytes. Solid electrolytes can also block dendrites, solving another important problem in Li metal batteries. Solid lithium electrolytes have been studied extensively and many classes of promising materials have been identified, including sulfides, oxides, ceramics, inorganic glasses, and polymers. Several reviews thoroughly discuss these classes of materials. [1] Recently, the Li 7 La 3 Zr 2 O 12 (LLZO) ceramic electrolyte with a garnet crystal structure has emerged as a promising solid electrolyte material. Favorable properties include high ionic conductivity (ca. 4 10 À4 S cm À1 at 25 8C), low electronic conductivity (less than or equal to 10 À8 S cm À1 ), electrochemical stability with Li metal, and thermal and chemical stability. [2] Stability with Li metal is an especially important feature, which combined with the high lithium conductivities, sets this material apart from other solid Li-ion conductors.The total conductivity of polycrystalline oxide conductors is a function of both lattice and grain boundary contributions. Thus, it is important to have knowledge of both the lattice and grain boundary behavior. Several groups report consistent total Li + conductivities for cubic polycrystalline LLZO samples; [2b, 3] however, there is less clarity on the relative contributions of the grains and grain boundaries. For instance, Murugan et al. show comparable grain and grain boundary resistances over a limited temperature range. [2b] In other studies, it is difficult to resolve impedance responses due to grains and grain boundaries. [3c,e] Given the importance of this material, it is critical that these conduction processes be thoroughly characterized. This paper reports thorough characterization of LLZO by broadband impedance spectroscopy. Impedance spectroscopy was performed at temperatures from À100 to + 60 8C with the frequency of the applied sinusoidal voltage varied from 10 9 to 10 À2 Hz. This frequency range is two to three orders magnitude broader than other reports, and the temperature window is larger as well. By controlling these two experimental variables, the contributions to total conductivity have been elucidated.LLZO is synthesized by hot isotactic pressing, yielding dense membranes with high-fidelity grain boundaries as previously reported. [3e] Aluminum is intentionally doped into the crystal lattice to stabilize the cubic phase; the nominal composition is Li 6.28 Al 0.24 La 3 Zr 2 O 12. The XRD diffraction pattern for this composition is presented in Figure 1. The absence of doublet peak reflections is indicative of the pure phase cubic structure. [3b] A SEM micrograph of the LLZO fracture surface is provided in the Figure 1 inset. The solid-state reaction and hot isotactic pressing densification protocols yielded dense LLZO membranes with few observable voids. The...

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Using neutron reflectometry, we have determined the thickness and scattering length density profile of the electrode-electrolyte interface for the high-voltage cathode LiMn(1.5)Ni(0.5)O4 in situ at open circuit voltage and fully delithiated. Upon exposure to a liquid electrolyte, a thin 3.3 nm Li-rich interface forms due to the ordering of the electrolyte on the cathode surface. This interface changes in composition, as evident by an increase in the scattering length density of the new layer, with charging as the condensed layer evolves from being lithium rich to one containing a much higher concentration of F from the LiPF6 salt. These results show the surface chemistry evolves as a function of the potential.

A new nanoscale sensing concept for the detection of nitroaromatic explosives is described. The design consists of nitroaromatic‐selective polymeric layers deposited inside microfabricated trenches. As the layers are exposed to nitroaromatic vapors, they swell and contact each other to close an electrical circuit. The nitroaromatic selective polymer, poly(4‐vinylpyridine) (P4VP), is deposited in the trenches using initiated chemical vapor deposition (iCVD). P4VP is characterized for the first time as a selective layer for the absorption of nitroaromatic vapors. The Flory–Huggins equation is used to model the swelling response to nitroaromatic vapors. The Flory–Huggins interaction parameter for the P4VP–nitrobenzene system at 40 °C is 0.71 and 0.25 for P4VP–4‐nitrotoluene at 60 °C. Sensing of nitrobenzene vapors is demonstrated in a prototype device, while techniques to improve the performance of the design in terms of response time and sensitivities are described. Modeling shows that concentration and mass limits of detection of 0.95 ppb and 3 fg, respectively, can be achieved.