Kinetic-Control of Block Copolymer Micelles for Tunable Nanomaterials Towards Energy Devices

Nano-scale inorganic porous materials are crucial for numerous applications ranging from electronics to energy conversion. The ability to control the morphology and the key architectural parameters of nanomaterials largely determines the transport characteristics and performance of such devices. But, there is very little understanding and development to tune these key architectural parameters such as inorganic wall-thickness and pore diameter. Even though there are numerous examples available on the fabrication of porous nanomaterials via block copolymer co-assembly, there is limited understanding on independent control over key architectural parameters. Because of that the major focus of this thesis is to explore and develop tunable nanomaterials with block copolymer micelles that could use to fabricate nano-optimize energy storage devices with high energy and fast rate capabilities. Towards this end, tunability of the key architectural parameters was studied and processing guidelines and solution conditions that enable nanostructures with tunable wall-thickness and pore diameters are presented. First, a new block copolymer poly (ethylene oxide-b-hexyl acrylate) (PEO-b-PHA) structure directing agent (SDA) was synthesized and used to demonstrate the tunability of the inorganic wall-thickness. Specifically, the use of a polymer with a high Flory-Huggins effective interaction parameter, χ, and appropriate solution conditions lead to the kinetic entrapment of micelles to produce persistent micelles which were used as templates to fabricate tunable isomorphic architectures. The use of different inorganic loadings with persistent micelles resulted in different wall-thicknesses with constant pore size. The processing guidelines for persistent micellar templating (PMT) were elaborated using mesoporous Nb2O5 that was thermally stable at 600 oC giving access to crystalline materials. Overall, this method provides a simple and a predictable path to produce porous nanomaterials with tunable wall thickness. Second, these kinetically entrapped block copolymer micelles resulted from a single block copolymer were re-arranged into different micelle sizes to achieve a range of pore dimeters. This target is particularly challenging since the rate of single-chain exchange and micelle fusion/fission reactions are hindered by the large thermodynamic barrier for rearrangement in these kinetically trapped micelle systems. The rate of the chain exchange reactions in block copolymer micelles will increase due to the production of solution-air interface as reported in literature. Here, we used ultrasonic cavitation for rapid interface production that accelerates micelle growth by an order of magnitude over agitation via vortexing. This extremely simple but powerful modification can be used as an unique handle to tune the pore diameter of nanomaterials by achieve a range of micelle sizes from a single block copolymer. Third, the PMT derived macroporous isomorphic architectures of Nb2O5 were used to show the potential application in understanding the operative mechanism of energy storage at different length scale. This sort of model systems could use to systematically investigate the optimal length scale of ion and electron diffusion to produce nano-optimize porous electrode systems. In this study T-Nb2O5 was selected as the electrode material due to its well-known pseudocapacitive behavior which has ability to combine both high energy densities and high-power densities into one materials. The overall goal of this thesis is to contribute towards the assembling the nano-porous electrodes into most effective architecture to achieve next generation high energy and power density devices. Therefore, this work demonstrated that the careful tuning of the block copolymer micelles in solution paves a simple and versatile path to obtain tunable nanomaterials which could be highly useful in nanotechnologies such as advanced energy conversion and storage devices

Amorphization of Pseudocapacitive T−nb\u3csub\u3e2\u3c/sub\u3eo\u3csub\u3e5\u3c/sub\u3e Accelerates Lithium Diffusivity as Revealed Using Tunable Isomorphic Architectures

Intercalationpseudocapacitancecan combinecapacitor-likepower densitieswith battery-likeenergy densities.Such surface-limitedbehaviorrequiresrapid diffusionwhere amorphizationcan increasesolid-statediffusivity.Here intercalationpseudoca-pacitivematerialswith tailoredextentsof amorphizationin T-Nb2O5are first reported.Amorphizationwas characterizedwithWAXS, XPS, XAFS, and EPR which suggesteda peroxide-rich(O22) surface that was consistentwith DFT predictions.A seriesof tunableisomorphicarchitecturesenabledcomparisonswhileindependentlyvaryingtransportparameters.Throughprocessof elimination,solid-statelithium diffusionwas identifiedas thedominantdiffusive-constraintdictatingthe maximumvoltagesweep rate for surface-limitedkinetics(vSLT), termed the Surface-LimitedThreshold(SLT). ThevSLTincreasedwith amorphizationhoweverstable cycling requiredcrystallineT-Nb2O5. A current-responsemodel using series-impedanceswell-matchedtheseobservations.This perspectiverevealedthat amorphizationof T-Nb2O5enhancedsolid-statediffusionby 12.2% and increasedsurface-limitationsby 17.0% (stablesamples).This approachenabledretaining95% lithiationcapacityat ~800mVs1(1,600C-rate equivalent)