Understanding Rapid Intercalation Materials One Parameter at a Time

Demand for fast, energy-dense storage drives the research into nanoscale intercalation materials. Nanomaterials accelerate kinetics and can modify reaction path thermodynamics, intercalant solubility, and reversibility. The discovery of intercalation pseudocapacitance has opened questions about their fundamental operating principles. For example, are their capacitor-like current responses caused by storing energy in special near-surface regions or rather is this response due to normal intercalation limited by a slower faradaic surface-reaction? This review highlights emerging methods combining tailored nanomaterials with the process of elimination to disambiguate cause-and-effect at the nanoscale. This method is applied to multiple intercalation pseudocapacitive materials showing that the timescales exhibiting surface-limited kinetics depended on the total intercalation length scale. These trends are inconsistent with the near-surface perspective. A revised current-model without assuming special near-surface storage fits experimental data better across wide timescales. This model, combined with tailored nanomaterials and the process of elimination, can isolate material-specific effects such as how amorphization/defect-tailoring modifies both insertion and diffusion kinetics. Avenues for both faster intercalation pseudocapacitance and increased energy density are discussed. A relaxation time argument is suggested to explain the continuum between battery-like and pseudocapacitive behaviors. Future directions include synthetic methods emphasizing tailored defects and analytical methods that minimize assumptions

Faster Intercalation Pseudocapacitance Enabled by Adjustable Amorphous Titania where Tunable Isomorphic Architectures Reveal Accelerated Lithium Diffusivity

Intercalation pseudocapacitance is a faradaic electrochemical phenomenon with high power and energy densities, combining the attractive features of capacitors and batteries, respectively. Intercalation pseudocapacitive responses exhibit surface-limited kinetics by definition, without restriction from the collective of diffusion-based processes. The surface-limited threshold (SLT) corresponds to the maximum voltage sweep rate (vSLT) exhibiting a predominantly surface-limited current response prior to the onset of diffusion-limitations. Prior studies showed increased lithium diffusivity for amorphous titania compared to anatase. Going beyond prior binary comparisons, here a continuum of amorphous titania configurations were prepared using a series of calcination temperatures to tailor both amorphous character and content. The corresponding amorphous-phase vSLT increased monotonically by 317 % with lowered calcination temperatures. Subsequent isomorphic comparisons varying a single transport parameter at a time identified solid-state lithium diffusion as the dominant diffusive constraint. Thus, performance improvements were linked to increasing the lithium diffusivity of the amorphous phase with decreased calcination temperature. This remarkably enabled 95 % capacity retention (483±17 C/g) with 30 s of delithiation (120 C equivalent). These results highlight how isomorphic sample series can reveal previously unidentified trends by reducing ambiguity and reiterate the potential of amorphization to realize increased performance in known materials

Enhanced Structural Control of Soft-Templated Mesoporous Inorganic Thin Films by Inert Processing Conditions

Mesoporous thin films are widely used for applications in need of high surface area and efficient mass and charge transport properties. A well-established fabrication process involves the supramolecular assembly of organic molecules (e.g., block copolymers and surfactants) with inorganic materials obtained by sol-gel chemistry. Typically, subsequent calcination in air removes the organic template and reveals the porous inorganic network. A significant challenge for such coatings is the anisotropic shrinkage due to the volume contraction related to solvent evaporation, inorganic condensation, and template removal, affecting the final porosity as well as pore shape, size, arrangement, and accessibility. Here, we show that a two-step calcination process, composed of high-temperature treatment in argon followed by air calcination, is an effective fabrication strategy to reduce film contraction and enhance structural control of mesoporous thin films. Crucially, the formation of a transient carbonaceous scaffold enables the inorganic matrix to fully condense before template removal. The resulting mesoporous films retain a higher porosity as well as bigger pores with extended porous order. Such films present favorable characteristics for mass transport of large molecules. This is demonstrated for lysozyme adsorption into the mesoporous thin films as an example of enzyme storage

Self-Cleaning Antireflective Optical Coatings

Low-cost antireflection coatings (ARCs) on large optical surfaces are an ingredient-technology for high-performance solar cells. While nanoporous thin films that meet the zero-reflectance conditions on transparent substrates can be cheaply manufactured, their suitability for outdoor applications is limited by the lack of robustness and cleanability. Here, we present a simple method for the manufacture of robust self-cleaning ARCs. Our strategy relies on the self-assembly of a block-copolymer in combination with silica-based sol–gel chemistry and preformed TiO2 nanocrystals. The spontaneous dense packing of copolymer micelles followed by a condensation reaction results in an inverse opal-type silica morphology that is loaded with TiO2 photocatalytic hot-spots. The very low volume fraction of the inorganic network allows the optimization of the antireflecting properties of the porous ARC despite the high refractive index of the embedded photocatalytic TiO2 nanocrystals. The resulting ARCs combine high optical and self-cleaning performance and can be deposited onto flexible plastic substrates

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)

Block Sequence Directed Materials: Functional And Ordered Nanocomposites Derived From Block Copolymer Coassembly

Nanocomposite materials with ordered structures are critical for the advancement of numerous fields ranging from microelectronics to energy conversion and storage. However, there are few techniques for controlling the necessary nanoscale morphologies and compositions which are compatible with affordable, large-scale manufacturing. The coassembly of block copolymers with inorganic materials provides such a route to achieve controlled nanomaterials, but such examples have generally resulted in mesoporous single-component materials. In this thesis it is shown that the general challenge to achieve multifunctional nanocomposites directly from block copolymer coassembly may be surmounted by designing novel block copolymers where each block has the design intent to result in a functional component of the resulting nanocomposites. Such a method would enable block sequence directed materials (BSDM), where a sequence of three or more chemically unique polymer blocks direct the spatial arrangement and interface definitions of multiple functional materials. Towards this end, four examples are provided. First, a diblock copolymer poly(ethylene oxide-b-acrylonitrile) is demonstrated to enable direct synthesis of nanocomposites composed of crystalline titania and partially-graphitic carbon. Second, this method is expanded by adding a third chemically unique block to form PAN-b-PEO-b-PPO-b-PEO-b-PAN where now the use of three chemically distinct polymer blocks enabled control over each of the three final components: partiallygraphitic carbon, crystalline transition metal oxide, and porosity. Although these nanocomposites only possessed short-range order, tuning of the individual block lengths and block fractions resulted in control over the three components. Third, it is shown that highly-ordered, multi-ply nanocomposites can result from the coassembly of poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock terpolymers. Tuning the ratio of nanoparticles to ISO enabled access to four unique morphologies and the selection of quasi-1D, 2D, or 3D pathways. Fourth, it is shown that an ordered 3D network morphology which is chiral (non-centrosymmetric) can result from the coassembly of an ISO with a particular composition. Such non-centrosymmetric nanostructures are necessary to enable macroscopic polarization for piezoelectric, pyroelectric, and second-order nonlinear optical properties in amorphous materials. Thus through these four examples, it is demonstrated that the tuning of the polymer-oxide coassembled systems enables control over both nanocomposite composition and morphology

Single-Variable Porous Nanomaterial Series From Polymer Structure-Directing Agents

Block polymer structure-directing agents (SDA) enable the production of porous nanoscale materials. Most strategies rely upon polymer equilibration where diverse morphologies are realized in porous functional materials. This review details how solvent selectivity determines the polymer SDA behaviors, spanning from bulk-type to solution-type. Equilibrating behavior of either type, however, obscures nanostructure cause-and-effect since the resulting sample series convolve multiple spatial variations. Solution-type SDA behaviors include both dynamic and persistent micelles. Persistent micelle templates (PMT) use high solvent selectivity for kinetic entrapment. PMTs enable independent wall thickness control with demonstrated 2 Å precision alterations. Unimodal PMT pore size distributions have spanned from 11.8 to 109 nm and multimodal pore sizes up to 290 nm. The PMT method is simple to validate with diffraction models and is feasible in any laboratory. Finally, recent energy device publications enabled by PMT are reviewed where tailored nanomaterials provide a unique perspective to unambiguously identify nanostructure–property–performance relationships