Mario Valvo

The impact of size effects on the electrochemical behaviour of Cu2O-coated Cu nanopillars for advanced Li-ion microbatteries

The generation of a distribution of nanoparticles upon conversion reaction of thin Cu2O layers is demonstrated to produce a wide electrochemical potential window, as well as a distinctive capacity increase in large area three-dimensional electrodes. Cu nanopillars with a 10–15 nm Cu2O coating containing traces of nanocrystalline Fe2O3 yield capacities up to 0.265 mA h cm−2 (at 61 mA g−1), excellent cycling for more than 300 cycles and an electroactive potential window larger than 2 V, due to the size effects caused by the various Cu/Cu2O nanoparticles formed during conversion/deconversion. These 3D Li-ion battery electrodes based on electrodeposited Cu nanopillars spontaneously coated with a Cu2O layer are compatible with current densities of 16 A g−1 (i.e. 61 C rates) after aerosol-assisted infiltration with an iron acetate solution followed by low-temperature pyrolysis. The capacity of the composite material increases by 67% during 390 cycles due to the growth of the electroactive area during the electrochemical milling of Cu2O forced by its repeated conversion/de-conversion. The latter generates a distribution of nanoparticles with different sizes and redox potentials, which explains the broad potential window, as well as the significant capacity contribution from double layer charging. These 3D electrodes should be well-suited for Li-ion microbatteries and Li-ion batteries in general, since they combine high capacities per footprint area with excellent power capabilities. More importantly, such electrodes grant access to fundamental understanding of the electrochemical behaviour of these active materials providing new insights into both conversion mechanisms and nanostructured interfaces more in general.

Silicon-based nanocomposite for advanced thin film anodes in lithium-ion batteries

This work describes the preparation and the characterization of Si-based nano-composite anodes. The samples are prepared by a unique combination of two techniques: Laser Assisted Chemical Vapor Pyrolysis and Electrospray Deposition. The former is used to synthesize the active material, while the latter is employed for the deposition of thin electrode layers onto stainless steel supports. The silicon nano-particles characterization indicates a well-defined crystalline structure and a homogeneous, spherical-like morphology. The electrochemical measurements performed using the silicon-based electrode in the lithium cell show a maximum specific capacity of the order of 1200 mA h g−1 and a good rate capability. The initial irreversible capacity associated with this class of materials is strongly reduced by preliminary surface treatment. The morphology changes upon cycling are minimal and no extended fractures are observed for the cycled electrodes, thus finally indicating the validity of our silicon based electrode as an anode for advanced lithium-ion batteries.

Sustainable Materials for Sustainable Energy Storage: Organic Na Electrodes

In this review, we summarize research efforts to realize Na-based organic materials for novel battery chemistries. Na is a more abundant element than Li, thereby contributing to less costly materials with limited to no geopolitical constraints while organic electrode materials harvested from biomass resources provide the possibility of achieving renewable battery components with low environmental impact during processing and recycling. Together, this can form the basis for truly sustainable electrochemical energy storage. We explore the efforts made on electrode materials of organic salts, primarily carbonyl compounds but also Schiff bases, unsaturated compounds, nitroxides and polymers. Moreover, sodiated carbonaceous materials derived from biomasses and waste products are surveyed. As a conclusion to the review, some shortcomings of the currently investigated materials are highlighted together with the major limitations for future development in this field. Finally, routes to move forward in this direction are suggested.

Electrochemical elaboration of electrodes and electrolytes for 3D structured batteries

The challenges associated with the fabrication of three-dimensional (3D) electrode and electrolyte materials for Li-ion batteries are discussed. The basic issues for achieving a solid 3D cell foundation, which can simultaneously offer sufficient electronic conductivity to enable stable cycling, as well as enough compatibility with the incorporation of complementary components, have been addressed. Various electrochemical strategies for elaborating such systems are discussed and critically examined. Several current collector systems are presented including electrochemically prepared Cu and Al nanorods and commercial aperiodic carbon structures. Further electrochemical coating approaches then provide a direct method for the deposition of thin layers of active materials successfully demonstrated here as coatings on both 3D metal structures and commercially available 3D-structured carbon substrates. Enhanced capacities per foot print area are demonstrated for a number of 3D electrode materials, namely polyaniline on reticulated vitreous carbon, Cu2O on copper nanorods and TiO2 on Al nanorods. The crucial points for achieving a thin conformal coating of the corresponding 3D electrode structures with solid polymer electrolytes are also carefully analysed and discussed. In this context electro-polymerisation is proposed as a viable route to form thin electrolyte layers with promising characteristics. The high versatility of electro-polymerisation in combination with the various structures and methodologies adopted here represents a further step towards the development of cost-effective 3D microbattery devices.

Passivation Layer and Cathodic Redox Reactions in Sodium‐Ion Batteries Probed by HAXPES

The cathode material P2-Nax Co2/3 Mn2/9 Ni1/9 O2, which could be used in Na-ion batteries, was investigated through synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Nondestructive analysis was made through the electrode/electrolyte interface of the first electrochemical cycle to ensure access to information not only on the active material, but also on the passivation layer formed at the electrode surface and referred to as the solid permeable interface (SPI). This investigation clearly shows the role of the SPI and the complexity of the redox reactions. Cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; all are in the 4+ state at the end of charging. Reduction to Co(3+), Ni(3+), and Mn(3+) occurs upon discharging and, at low potential, there is partial reversible reduction to Co(2+) and Ni(2+). A thin layer of Na2 CO3 and NaF covers the pristine electrode and reversible dissolution/reformation of these compounds is observed during the first cycle. The salt degradation products in the SPI show a dependence on potential. Phosphates mainly form at the end of the charging cycle (4.5 V), whereas fluorophosphates are produced at the end of discharging (2.0 V).

Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate

Copper hexacyanoferrate, Cu(II)[Fe(III)(CN)6]2/3·nH2O, was synthesized, and varied amounts of K(+) ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written as Cu(II)[Fe(III)(CN)6]2/3·nH2O + 2x/3K(+) + 2x/3e(-) ↔ K2x/3Cu(II)[Fe(II)xFe(III)1-x(CN)6]2/3·nH2O. Infrared, Raman, and Mössbauer spectroscopy studies show that Fe(III) is continuously reduced to Fe(II) with increasing x, accompanied by a decrease of the a-axis of the cubic Fm3̅m unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction, ∼20% of the Fe(III), has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of ∼26 wt % upon heating to 180 °C, above which the structure starts to decompose. The crystal structures of Cu(II)[Fe(III)(CN)6]2/3·nH2O and K2/3Cu[Fe(CN)6]2/3·nH2O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN)6 groups are vacant, and the octahedron around Cu(II) is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the -Cu-N-C-Fe- framework. The K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K(+) ions.

3-D binder-free graphene foam as cathode for high capacity Li-O2 batteries

To provide energy densities higher than those of conventional Li-ion batteries, a Li–O2 battery requires a cathode with high surface area to host large amounts of discharge product Li2O2. Therefore ...

Surface Layer Evolution on Graphite During Electrochemical Sodium-tetraglyme Co-intercalation

One obstacle in sodium ion batteries is the lack of suitable anode materials. As recently shown, the most common anode material of the state of the art lithium ion batteries, graphite, can be used for sodium ion storage as well, if ether-based electrolyte solvents are used. These solvents cointercalate with the sodium ions leading to the highly reversible formation of ternary graphite intercalation compounds (t-GIC). In order for the solvent cointercalation to work efficiently, it is expected that only a very thin surface layer forms during electrochemical cycling. In this article, we therefore present the first dedicated study of the surface layer evolution on t-GICs using soft X-ray photoelectron spectroscopy. This technique with its inherent high surface sensitivity and low probing depth is an ideal tool to study the underlying interfacial reactions during the sodiation and desodiation of graphite. In this report, we apply this approach to graphite composite electrodes cycled in Na half cells with a 1 M sodium bis(fluorosulfonyl)imide/tetraethylene glycol dimethyl ether (NaFSI/TEG-DME) electrolyte. We have found a surface layer on the cycled electrodes, mainly composed of salt decomposition products and hydrocarbons, in line with irreversible capacity losses observed in the electrochemical cycling. Although this surface layer does not seem to block cointercalation completely, it seems to affect its efficiency resulting in a low Coulombic efficiency of the studied battery system.

Electrochemical fabrication and characterization of Cu/Cu2O multi-layered micro and nanorods in Li-ion batteries

Electrodes composed of freestanding nano- and microrods composed of stacked layers of copper and cuprous oxide have been fabricated using a straightforward one-step template-assisted pulsed galvanostatic electrodeposition approach. The approach provided precise control of the thickness of each individual layer of the high-aspect-ratio rods as was verified by SEM, EDS, XRD, TEM and EELS measurements. Rods with diameters of 80, 200 and 1000 nm were deposited and the influence of the template pore size on the structure and electrochemical performance of the conversion reaction based electrodes in lithium-ion batteries was investigated. The multi-layered Cu2O/Cu nano- and microrod electrodes exhibited a potential window of more than 2 V, which was ascribed to the presence of a distribution of Cu2O (and Cu, respectively) nanoparticles with different sizes and redox potentials. As approximately the same areal capacity was obtained independent of the diameter of the multi-layered rods the results demonstrate the presence of an electroactive Cu2O layer with a thickness defined by the time domain of the measurements. It is also demonstrated that while the areal capacity of the electrodes decreased dramatically when the scan rate was increased from 0.1 to 2 mV s(-1), the capacity remained practically constant when the scan rate was further increased to 100 mV s(-1). This behaviour can be explained by assuming that the capacity is limited by the lithium ion diffusion rate though the Cu2O layer generated during the oxidation step. The electrochemical performance of present type of 3-D multi-layered rods provides new insights into the lithiation and delithiation reactions taking place for conversion reaction materials such as Cu2O.

Electrocatalytic Water Oxidation Promoted by 3 D Nanoarchitectured Turbostratic δ-MnOx on Carbon Nanotubes

The development of manganese-based water oxidation electrocatalysts is desirable for the production of solar fuels, as manganese is earth-abundant, inexpensive, non-toxic, and has been employed by the Photosystem II in nature for a billion years. Herein, we directly constructed a 3 D nanoarchitectured turbostratic δ-MnOx on carbon nanotube-modified nickel foam (MnOx /CNT/NF) by electrodeposition and a subsequent annealing process. The MnOx /CNT/NF electrode gives a benchmark catalytic current density (10 mA cm-2 ) at an overpotential (η) of 270 mV under alkaline conditions. A steady current density of 19 mA cm-2 is obtained during electrolysis at 1.53 V for 1.0 h. To the best of our knowledge, this work represents the most efficient manganese-oxide-based water oxidation electrode and demonstrates that manganese oxides, as a structural and functional model of oxygen-evolving complex (OEC) in Photosystem II, can also become comparable to those of most Ni- and Co-based catalysts.