Access to the full text of the published version may require a subscription. Rights © Tsinghua University Press and Springer-Verlag Berlin ABSTRACTThe performance of the lithium-ion cell is heavily dependent on the ability of the host electrodes to accommodate and release Li + ions from the local structure. While the choice of electrode materials may define parameters such as cell potential and capacity, the process of intercalation may be physically limited by the rate of solid-state Li + diffusion. Increased diffusion rates in lithium-ion electrodes may be achieved through a reduction in the diffusion path, accomplished by a scaling of the respective electrode dimensions.In addition, some electrodes may undergo large volume changes associated with charging and discharging, the strain of which, may be better accommodated through nanostructuring. Failure of the host to accommodate such volume changes may lead to pulverisation of the local structure and a rapid loss of capacity. In this review article, we seek to highlight a number of significant gains in the development of nanostructured lithium-ion battery architectures (both anode and cathode), as drivers of potential next-generation electrochemical energy storage devices.

While it is well-known that electrode conductivity has a critical impact on rateperformance in battery electrodes, this relationship has been quantified only by computer simulations. Here we investigate the relationship between electrode conductivity and rateperformance in Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) cathodes filled with various quantities of carbon black, single-walled carbon nanotubes and graphene. The electrode conductivity is always extremely anisotropic with the out-of-plane conductivity, which is most relevant to rate-performance, roughly ×1000 smaller than the in-plane conductivity. For all fillers the conductivity increases with filler loading although the nanotube-filled electrodes show by far the most rapid increase. Fitting capacity versus rate curves yielded the characteristic time associated with charge/discharge. This parameter increased linearly with the inverse of the out-of-plane conductivity, with all data points falling on the same master curve.Using a simple mechanistic model for the characteristic time, we develop an equation which matches the experimental data almost perfectly with no adjustable parameters. This implies that increasing the electrode conductivity improves the rate-performance by decreasing the RC charging time of the electrode. This model shows the effect of electrode resistance on the rateperformance to become negligible in almost all cases once the out-of-plane conductivity of the 2 electrode exceeds 1 S/m. Our results show that this can be achieved by including <1wt% singlewalled carbon nanotubes in the electrode.

The anodic behavior of highly doped ͑Ͼ10 18 cm −3 ͒ n-InP in aqueous KOH was investigated. Electrodes anodized in the absence of light in 2-5 mol dm −3 KOH at a constant potential of 0.5-0.75 V ͑SCE͒, or subjected to linear potential sweeps to potentials in this range, were shown to exhibit the formation of a nanoporous subsurface region. Both linear sweep voltammograms and current-time curves at constant potential showed a characteristic anodic peak, corresponding to formation of the nanoporous region. No porous region was formed during anodization in 1 mol dm −3 KOH. The nanoporous region was examined using transmission electron microscopy and found to have a thickness of some 1-3 m depending on the anodization conditions and to be located beneath a thin ͑typically ϳ40 nm͒, dense, near-surface layer. The pores varied in width from 25 to 75 nm and both the pore width and porous region thickness were found to decrease with increasing KOH concentration. The porosity was approximately 35%. The porous layer structure is shown to form by the localized penetration of surface pits into the InP, and the dense, near-surface layer is consistent with the effect of electron depletion at the surface of the semiconductor.

The early stages of nanoporous layer formation, under anodic conditions in the absence of light, were investigated for n-type InP with a carrier concentration of ϳ3 ϫ 10 18 cm −3 in 5 mol dm −3 KOH and a mechanism for the process is proposed. At potentials less than ϳ0.35 V, spectroscopic ellipsometry and transmission electron microscopy ͑TEM͒ showed a thin oxide film on the surface. Atomic force microscopy ͑AFM͒ of electrode surfaces showed no pitting below ϳ0.35 V but clearly showed etch pit formation in the range 0.4-0.53 V. The density of surface pits increased with time in both linear potential sweep and constant potential reaching a constant value at a time corresponding approximately to the current peak in linear sweep voltammograms and current-time curves at constant potential. TEM clearly showed individual nanoporous domains separated from the surface by a dense ϳ40 nm InP layer. It is concluded that each domain develops as a result of directionally preferential pore propagation from an individual surface pit which forms a channel through this near-surface layer. As they grow larger, domains meet, and the merging of multiple domains eventually leads to a continuous nanoporous sub-surface region.

Nanostructured surfaces are common in nature and exhibit properties such as antireflectivity (moth eyes), self-cleaning (lotus leaf), iridescent colors (butterfly wings), and water harvesting (desert beetles). We now understand such properties and can mimic some of these natural structures in the laboratory. However, these synthetic structures are limited since they are not easily mass produced over large areas due to the limited scalability of current technologies such as UV-lithography, the high cost of infrastructure, and the difficulty in nonplanar surfaces. Here, we report a solution process based on block copolymer (BCP) self-assembly to fabricate subwavelength structures on large areas of optical and curved surfaces with feature sizes and spacings designed to efficiently scatter visible light. Si nanopillars (SiNPs) with diameters of ∼115 ± 19 nm, periodicity of 180 ± 18 nm, and aspect ratio of 2-15 show a reduction in reflectivity by a factor of 100, <0.16% between 400 and 900 nm at an angle of incidence of 30°. Significantly, the reflectivity remains below 1.75% up to incident angles of 75°. Modeling the efficiency of a SiNP PV suggests a 24.6% increase in efficiency, representing a 3.52% (absolute) or 16.7% (relative) increase in electrical energy output from the PV system compared to AR-coated device.

Access to the full text of the published version may require a subscription. Rights © 2014 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form AbstractA critical aspect in the practical application and enhanced catalytic performance of shape controlled nanocrystals is their stability and morphology retention under ambient conditions.Changes to the morphology of shape-controlled Pd nanocrystals capped by PVP are assessed by TEM and surface oxidation was evaluated by X-ray photoelectron spectroscopy (XPS), over 12 months. Surface oxidation of PVP-capped Pd nanocrystals resulted in loss of edge and corner sites and transition to spherical morphologies. The shape stability of the nanocrystals was found to follow the trend cubic < cuboctahedra < octahedral ~ concave cubes. For low index planes, {111} surfaces are more resistant to oxidation compared to {100} facets, correlating with the surface free energy of the nanocrystals. Cubic and cuboctahedral nanocrystals transitioned to spherical particles while octahedral nanocrystals retained their morphology. The presence of high energy {110} facets were observed in the cubic nanocrystals which undergo surface reconstruction. The presence of surface defects such as stacking faults were also found to influence the rate of the structural changes.Concave cubic nanocrystals, which possess high index facets and surface energies were consistently found to display excellent morphology retention. The concave cubic 2 nanocrystals displayed superior shape stability and reduced oxidation compared to cubic and cuboctahedral nanocrystals. XPS analysis further determined that PVP capping ligands on different Pd surface facets strongly influences the morphological consistency. The stability of the concave cubes can be attributed to stronger chemisorption of PVP capping ligands to the high index plane making them less susceptible to oxidation.

Since the isolation of two-dimensional (2D) phosphorene, black phosphorus (BP) has gained popularity due to its high carrier mobility and tunable bandgap. Poor ambient stability of BP remains a key issue and impedes its use in electronic applications. Here we report a new stabilization strategy based on covalent functionalization of liquid exfoliated few-layer BP using aryl iodonium salts. Arylation of BP using iodonium salts enables covalent modification without inducing oxidation and alters the degradation chemistry of BP by inhibiting bridged oxygen formation through attachment to surface oxygen sites. In comparison, functionalization using aryl diazonium salts results in oxidation and aryl multilayer formation and does not adequately disrupt noncovalent solvent passivation. Aryl functionalization of BP using iodonium salts displays superior ambient stability compared to arylation using diazonium salts associated with greater covalent functionalization as characterized using X-ray photoelectron spectroscopy, scanning transmission electron microscopy, photoluminescence, and attenuated total reflectance infrared spectroscopy.

High performance thin film lithium batteries using structurally stable electrodeposited V2O5 inverse opal (IO) networks as cathodes provide high capacity and outstanding cycling capability and also were demonstrated on transparent conducting oxide current collectors. The superior electrochemical performance of the inverse opal structures was evaluated through galvanostatic and potentiodynamic cycling, and the IO thin film battery offers increased capacity retention compared to micron-scale bulk particles from improved mechanical stability and electrical contact to stainless steel or transparent conducting current collectors from bottom-up electrodeposition growth. Li(+) is inserted into planar and IO structures at different potentials, and correlated to a preferential exposure of insertion sites of the IO network to the electrolyte. Additionally, potentiodynamic testing quantified the portion of the capacity stored as surface bound capacitive charge. Raman scattering and XRD characterization showed how the IO allows swelling into the pore volume rather than away from the current collector. V2O5 IO coin cells offer high initial capacities, but capacity fading can occur with limited electrolyte. Finally, we demonstrate that a V2O5 IO thin film battery prepared on a transparent conducting current collector with excess electrolyte exhibits high capacities (∼200 mAh g(-1)) and outstanding capacity retention and rate capability.