Optical properties of ZnO deposited by atomic layer deposition (ALD) on Si nanowires

International audienceIn this work, we report proof-of-concept results on the synthesis of Si core/ ZnO shell nanowires (SiNWs/ZnO) by combining nanosphere lithography (NSL), metal assisted chemical etching (MACE) and atomic layer deposition (ALD). The structural properties of the SiNWs/ZnO nanostructures prepared were investigated by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies. The X-ray diffraction analysis revealed that all samples have a hexagonal wurtzite structure. The grain sizes are found to be in the range of 7-14 nm. The optical properties of the samples were investigated using reflectance and photoluminescence spectroscopy. The study of photoluminescence (PL) spectra of SiNWs/ZnO samples showed the domination of defect emission bands, pointing to deviations of the stoichiometry of the prepared 3D ZnO nanostructures. Reduction of the PL intensity of the SiNWs/ZnO with the increase of SiNWs etching time was observed, depicting an advanced light scattering with the increase of the nanowire length. These results open up new prospects for the design of electronic and sensing devices

Nanoarchitectured Array Electrodes for Rechargeable Lithium- and Sodium-Ion Batteries

Rechargeable ion batteries have contributed immensely to shaping the modern world and been seriously considered for the efficient storage and utilization of intermittent renewable energies. To fulfill their potential in the future market, superior battery performance of high capacity, great rate capability, and long lifespan is undoubtedly required. In the past decade, along with discovering new electrode materials, the focus has been shifting more and more toward rational electrode designs because the performance is intimately connected to the electrode architectures, particularly their designs at the nanoscale that can alleviate the reliance on the materials' intrinsic nature. The utilization of nanoarchitectured arrays in the design of electrodes has been proven to significantly improve the battery performance. A comprehensive summary of the structural features and fabrications of the nanoarchitectured array electrodes is provided, and some of the latest achievements in the area of both lithium‐ and sodium‐ion batteries are highlighted. Finally, future challenges and opportunities that would allow further development of such advanced electrode configuration are discussed

Origin of non-SEI related coulombic efficiency loss in carbons tested against Na and Li

Partially ordered but not graphitized carbons are widely employed for sodium and lithium ion battery (NIB and LIB) anodes, either in their pure form or as a secondary supporting phase for oxides, sulfides and insertion electrodes. These "pseudographitic" materials ubiquitously display a poor initial coulombic efficiency (CE), which has been historically attributed to solid electrolyte interface (SEI) formation on their large surface areas (up to 3c2500 m2 g-1). Here we identify the other sources CE loss by examining a pseudographitic carbon with a state-of-the-art capacity (>350 mA h g-1 for NIB, >800 mA h g-1 for LIB), but with a purposely designed low surface area (14.5 m2 g-1) that disqualifies SEI from having a substantial role. During the initial several (<5) cycles both Na and Li are irreversibly trapped in the bulk, with the associated CE loss occurring at higher desodiation/delithiation voltages. We measure a progressively increasing graphene interlayer spacing and a progressively increasing Raman G band intensity, indicating that the charge carriers become trapped not only at the graphene defects but also between the graphene planes hence causing them to both dilate and order. For the case of Li, we also unambiguously detected irreversible metal underpotential deposition ("nanoplating") within the nanopores at roughly below 0.2 V. It is expected that in conventional high surface area carbons these mechanisms will be a major contributor to CE loss in parallel to classic SEI formation. Key implications to emerge from these findings are that improvements in early cycling CE may be achieved by synthesizing pseudographitic carbons with lower levels of trapping defects, but that for LIBs the large cycle 1 CE loss may be unavoidable if highly porous structures are utilized.Peer reviewed: YesNRC publication: Ye

Nanocrystalline anatase TiO2: a new anode material for rechargeable sodium ion batteries

Anatase TiO2 nanocrystals were successfully employed as anodes for rechargeable Na-ion batteries for the first time. The mesoporous electrodes exhibited a highly stable reversible charge storage capacity of 3c150 mA h g-\ub9 over 100 cycles, and were able to withstand high rate cycling, fully recovering this capacity after being cycled at rates as high as 2 A g-\ub9.Peer reviewed: YesNRC publication: Ye

ALD TiO2 coated silicon nanowires for lithium ion battery anodes with enhanced cycling stability and coulombic efficiency

We demonstrate that silicon nanowire (SiNW) Li-ion battery anodes that are conformally coated with TiO2 using atomic layer deposition (ALD) show a remarkable performance improvement. The coulombic efficiency is increased to 3c99%, among the highest ever reported for SiNWs, as compared to 95% for the baseline uncoated samples. The capacity retention after 100 cycles for the nanocomposite is twice as high as that of the baseline at 0.1 C (60% vs. 30%), and more than three times higher at 5 C (34% vs. 10%). We also demonstrate that the microstructure of the coatings is critically important for achieving this effect. Titanium dioxide coatings with an as-deposited anatase structure are nowhere near as effective as amorphous ones, the latter proving much more resistant to delamination from the SiNW core. We use TEM to demonstrate that upon lithiation the amorphous coating develops a highly dispersed nanostructure comprised of crystalline LiTiO2 and a secondary amorphous phase. Electron energy loss spectroscopy (EELS) combined with bulk and surface analytical techniques are employed to highlight the passivating effect of TiO2, which results in significantly fewer cycling-induced electrolyte decomposition products as compared to the bare nanowires. \ua9 2013 The Owner Societies.Peer reviewed: YesNRC publication: Ye

High-density sodium and lithium ion battery anodes from banana peels

Banana peel pseudographite (BPPG) offers superb dual functionality for sodium ion battery (NIB) and lithium ion battery (LIB) anodes. The materials possess low surface areas (19-217 m2 g-1) and a relatively high electrode packing density (0.75 g cm-3 vs 3c1 g cm -3 for graphite). Tested against Na, BPPG delivers a gravimetric (and volumetric) capacity of 355 mAh g-1 (by active material 3c700 mAh cm-3, by electrode volume 3c270 mAh cm-3) after 10 cycles at 50 mA g-1. A nearly flat 3c200 mAh g-1 plateau that is below 0.1 V and a minimal charge/discharge voltage hysteresis make BPPG a direct electrochemical analogue to graphite but with Na. A charge capacity of 221 mAh g-1 at 500 mA g-1 is degraded by 7% after 600 cycles, while a capacity of 336 mAh g-1 at 100 mAg -1 is degraded by 11% after 300 cycles, in both cases with 3c100% cycling Coulombic efficiency. For LIB applications BPPG offers a gravimetric (volumetric) capacity of 1090 mAh g-1 (by material 3c2200 mAh cm-3, by electrode 3c900 mAh cm-3) at 50 mA g -1. The reason that BPPG works so well for both NIBs and LIBs is that it uniquely contains three essential features: (a) dilated intergraphene spacing for Na intercalation at low voltages; (b) highly accessible near-surface nanopores for Li metal filling at low voltages; and (c) substantial defect content in the graphene planes for Li adsorption at higher voltages. The <0.1 V charge storage mechanism is fundamentally different for Na versus for Li. A combination of XRD and XPS demonstrates highly reversible Na intercalation rather than metal underpotential deposition. By contrast, the same analysis proves the presence of metallic Li in the pores, with intercalation being much less pronounced.Peer reviewed: YesNRC publication: Ye

Si nanotubes ALD coated with TiO2, TiN or Al2O 3 as high performance lithium ion battery anodes

Silicon based hollow nanostructures are receiving significant scientific attention as potential high energy density anodes for lithium ion batteries. However their cycling performance still requires further improvement. Here we explore the use of atomic layer deposition (ALD) of TiO2, TiN and Al2O3 on the inner, the outer, or both surfaces of hollow Si nanotubes (SiNTs) for improving their cycling performance. We demonstrate that all three materials enhance the cycling performance, with optimum performance being achieved for SiNTs conformally coated on both sides with 1.5 nm of Li active TiO2. Substantial improvements are achieved in the cycling capacity retention (1700 mA h g-1vs. 1287 mA h g-1 for the uncoated baseline, after 200 cycles at 0.2 C), steady-state coulombic efficiency ( 3c100% vs. 97-98%), and high rate capability (capacity retention of 50% vs. 20%, going from 0.2 C to 5 C). TEM and other analytical techniques are employed to provide new insight into the lithiation cycling-induced failure mechanisms that turn out to be intimately linked to the microstructure and the location of these layers.Peer reviewed: YesNRC publication: Ye

High-Density Sodium and Lithium Ion Battery Anodes from Banana Peels

Banana peel pseudographite (BPPG) offers superb dual functionality for sodium ion battery (NIB) and lithium ion battery (LIB) anodes. The materials possess low surface areas (19–217 m² g^–1) and a relatively high electrode packing density (0.75 g cm^–3 <i>vs</i> ∼1 g cm^–3 for graphite). Tested against Na, BPPG delivers a gravimetric (and volumetric) capacity of 355 mAh g^–1 (by active material ∼700 mAh cm^–3, by electrode volume ∼270 mAh cm^–3) after 10 cycles at 50 mA g^–1. A nearly flat ∼200 mAh g^–1 plateau that is below 0.1 V and a minimal charge/discharge voltage hysteresis make BPPG a direct electrochemical analogue to graphite but with Na. A charge capacity of 221 mAh g^–1 at 500 mA g^–1 is degraded by 7% after 600 cycles, while a capacity of 336 mAh g^–1 at 100 mAg^–1 is degraded by 11% after 300 cycles, in both cases with ∼100% cycling Coulombic efficiency. For LIB applications BPPG offers a gravimetric (volumetric) capacity of 1090 mAh g^–1 (by material ∼2200 mAh cm^–3, by electrode ∼900 mAh cm^–3) at 50 mA g^–1. The reason that BPPG works so well for both NIBs and LIBs is that it uniquely contains three essential features: (a) dilated intergraphene spacing for Na intercalation at low voltages; (b) highly accessible near-surface nanopores for Li metal filling at low voltages; and (c) substantial defect content in the graphene planes for Li adsorption at higher voltages. The <0.1 V charge storage mechanism is fundamentally different for Na <i>versus</i> for Li. A combination of XRD and XPS demonstrates highly reversible Na intercalation rather than metal underpotential deposition. By contrast, the same analysis proves the presence of metallic Li in the pores, with intercalation being much less pronounced

Carbon Nanosheet Frameworks Derived from Peat Moss as High Performance Sodium Ion Battery Anodes

We demonstrate that peat moss, a wild plant that covers 3% of the earth’s surface, serves as an ideal precursor to create sodium ion battery (NIB) anodes with some of the most attractive electrochemical properties ever reported for carbonaceous materials. By inheriting the unique cellular structure of peat moss leaves, the resultant materials are composed of three-dimensional macroporous interconnected networks of carbon nanosheets (as thin as 60 nm). The peat moss tissue is highly cross-linked, being rich in lignin and hemicellulose, suppressing the nucleation of equilibrium graphite even at 1100 °C. Rather, the carbons form highly ordered pseudographitic arrays with substantially larger intergraphene spacing (0.388 nm) than graphite (<i>c</i>/2 = 0.3354 nm). XRD analysis demonstrates that this allows for significant Na intercalation to occur even below 0.2 V <i>vs</i> Na/Na⁺. By also incorporating a mild (300 °C) air activation step, we introduce hierarchical micro- and mesoporosity that tremendously improves the high rate performance through facile electrolyte access and further reduced Na ion diffusion distances. The optimized structures (carbonization at 1100 °C + activation) result in a stable cycling capacity of 298 mAh g^–1 (after 10 cycles, 50 mA g^–1), with ∼150 mAh g^–1 of charge accumulating between 0.1 and 0.001 V with negligible voltage hysteresis in that region, nearly 100% cycling Coulombic efficiency, and superb cycling retention and high rate capacity (255 mAh g^–1 at the 210th cycle, stable capacity of 203 mAh g^–1 at 500 mA g^–1)