F. Leroux

Nanostructured materials for energy storage

Abstract Traditional electrode materials for lithium-ion storage cells are based on materials which have both mixed electron and ion transport (for Li+). They are typically crystalline layered structures such as metal oxides that have high redox potentials, and act as positive electrodes; and graphitic carbons capable of reversible uptake of Li at low potentials which act as negative electrodes. Recently, however, nanostructured solid state materials, which are comprised of two or more compositional or structural phases, have been considered. This new area has been particularly exploited in the area of negative electrode design, where the intimate mix of components at the nanoscale permits and enhances Li reversibility. It also include cathode materials where materials that function on the basis of intergrowth structures (internal composites) have been found to be beneficial; and insulating materials where the limitations to electron transport must be overcome by judicious design of nanostructured composites. The research trends and future prospects are discussed.

Poly(pyrrole) and poly(thiophene)/vanadium oxide interleaved nanocomposites: positive electrodes for lithium batteries

Abstract Lithium insertion has been examined in a series of conductive polymer-V 2 O 5 nanocomposites that have a structure comprised of layers of polymer chains interleaved with inorganic oxide lamellae. Poly(pyrrole), [PPY]; poly(aniline) [PANI]; poly(thiophene), [PTH] and its derivatives constituted the polymer component; PTH was prepared from the monomers bithiophene, terthiophene, 3-methylthiophene and 2,5-dimethylthiophene. Compositions of the corresponding nanocomposites were [PANI] 0.4 V 2 O 5 , [PPY] x V 2 O 5 ( x ≈0.4, 0.9), and [PTH] x V 2 O 5 ( x ≈0.3–0.8). We find that for modified [PANI] 0.4 V 2 O 5 , polymer incorporation results in better reversibility, and increased Li capacity in the nanocomposite compared to the xerogel. For PPY and PTH nanocomposites, the electrochemical response is highly dependent on the preparation method, nature of the polymer, and its location. Reversible Li insertion was maximized in the case of PTH when it was prepared from 3-methyl or terthiophene as the monomers, suggesting that chain conjugation length and polymer order area important factors. In some of these materials, the Li insertion capacity can be increased by 40% by subjecting the electrode to an initial charge step.

Synthesis and characterization of polypyrrole/vanadium pentoxide nanocomposite aerogels

Vanadium pentoxide/polypyrrole aerogel (ARG) composites have been synthesized by sol–gel routes, and investigated as cathode materials in Li batteries. The primary method utilized simultaneous polymerisation of pyrrole and vanadium alkoxide precursors. Hydrolysis of VO(OC3H7)3 using pyrrole–water–acetone mixtures yielded monolithic green–black gels with polypyrrole/V ratios ranging from 0.15 to 1.0. Supercritical drying yielded high surface (150–257 m2 g–1) aerogels with densities between 0.1 and 0.2 g cm–3 , that were of sufficient mechanical integrity to allow them to be cut without fracturing. TEM studies of the ARGs show that they are comprised of fibers similar to that of V2O5 ARGs, but with a significantly shorter chain length. The interaction between the polypyrrole (PPy) and V2O5 aerogel in the nanocomposites was probed using IR spectroscopy. Our results suggest that the inorganic and organic components strongly interact during the initial stages, thus perhaps impeding the vanadium condensation process. Hence, the PPy/V2O5 nanocomposites exhibited lower electrical conductivity with increased polypyrrole content. The addition of (NH4)2S2O8 as an oxidizing agent improved the conductivity of the nanocomposites. The deleterious effect of the conductive polymer on the bulk conductivity does not necessarily affect the electrochemical properties of these materials. Nanocomposite materials that were subjected to post-oxidative treatment show enhanced Li insertion capacity compared to the pristine ARG. The physical properties of these ‘nanocomposite aerogels’ are different from ‘microcomposites’ prepared by an alternate route, in which the oxide gel is formed in the presence of a dispersion of preformed micrometer-sized polypyrrole particles.

Uptake of lithium by layered molybdenum oxide and its tin exchanged derivatives: high volumetric capacity materials

Abstract The materials A 0.25 MoO 3 (A=Na, Li, Sn), prepared by a ‘chimie douce’ route, are a promising alternative as anode materials in Li ion batteries. These materials present large reversible charge capacities, greater than 900 mAh/g, with a good capacity retention on cycling. At least 65% of the charge capacity (600 mAh/g) is maintained under 1.5 V vs. Li. The gravimetric capacities, on the order of 4000 mAh/cm 3 , are three to four times greater than for high capacity carbon materials and twice that of Sn oxide-based glasses. A mild heat treatment and an appropriate discharge cut-off potential stabilizes the cycling behavior. A discharge cut-off of 5 mV is associated with a large polarization, and fading charge retention, probably related to the oxygen diffusion process into the highly sub-stoichiometric oxide during the charge sweep. Conversely, raising the charge potential to 200 mV may conserve the oxygen environment surrounding the Mo centre to some degree, thus facilitating oxygen migration during charge. The irreversible capacity and the high average potential in charge are the major drawbacks in these systems. By utilizing the exchange capability of the interlayers ions, Sn can be incorporated into the material, thus lowering the average charge potential but at the expense of capacity fading. Finally, a catalytic effect of the carbon black in these composite electrodes via an interface effect is present, which must be accounted for by methods other than simple subtraction of the carbon contribution to the total capacity by mass fraction.

3-volt manganese dioxide: the amorphous alternative

Abstract New alkaline manganese dioxides were prepared via a single step soft chemistry route, by varying the preparation conditions to adjust the Mn oxidation state. Li capacities as high as 0.85 Li per manganese atom were achieved for some formulations. The lithiated materials, under an initial charge, display a topotactic type reaction within the 2–4.5 V voltage range during the Li (de-)intercalation process, in comparison to the discontinuity in the 3–4 V plateau regions for LiMn 2 O 4 spinel cycling. A slight conversion to the spinel response, however, was observed for materials heated at 300 °C. Samples heated at lower temperatures do not exhibit the spinel conversion and provide, in most cases, an appreciable electrochemical response at high cycling rates. This is exemplified by Li 0.37 K 0.01 MnO 2.01 , with a specific capacity of 160 mAh g −1 at C/1.8 (85 mA g −1 ).

Vanadium Oxide/Polypyrrole Aerogel Nanocomposites.

Vanadium pentoxide/polypyrrole aerogel (ARG) nanocomposites were prepared by hydrolysis of VO(OC{sub 3}H{sub 7}){sub 3} using pyrrole/water/acetone mixtures. Monolithic green-black gels with polypyrrole/V ratios ranging from 0.15 to 1.0 resulted from simultaneously polymerization of the pyrrole and vanadium alkoxide precursors. Supercritical drying yielded high surface (150--200 m{sup 2}/g) aerogels, of sufficient mechanical integrity to allow them to be cut without fracturing. TEM studies of the aerogels show that they are comprised of fibers similar to that of V{sub 2}O{sub 5} ARG`s, but with a much shorter chain length. Evidence from IR that the inorganic and organic components strongly interact leads them to propose that this impedes the vanadium condensation process. The result is ARG`s that exhibit decreased electronic conductivity with increasing polymer content. Despite the unexpected deleterious effect of the conductive polymer on the bulk conductivity, at low polymer content, the nanocomposite materials show enhanced electrochemical properties for Li insertion compared to the pristine aerogel.

Polymer-Oxide Anode Materials

The adaptable layer structure of molybdenum trioxide was exploited to insert the amino derivative form of the conductive polymer poly(para-phenylene) (PPPNH{sub 2}) within the van der Waals gap. Two polymer insertion routes were designed that yield novel PPPNH{sub 2}-MoO{sub 3} materials of different composition. Characterization of these materials using powder XRD, thermal analysis, and FTIR spectroscopy shows insertion of the polymer has occurred. The properties of the nanocomposites for low potential electrochemical lithium insertion were compared to those of the sodium molybdenum bronze using the materials as cathodes in conventional lithium cells. Initial results indicate the specific charge capacity and irreversibility during the first charge are effected by polymer content whereas polarization is not.

Low Potential Li Insertion in Transition Metal Oxides

Low-potential Li insertion materials comprised of molybdenum oxides (A x MoO 3 ) have been prepared by a “chimie douce” route. Li insertion below 200 mV is associated with dramatic transformation of the structure, leading to a material which displays good cyclability with a high reversible specific capacity of 940 mA/g in the voltage window 3.0–0.005V (volumetric capacity of 4000 mAh/cc), albeit with notable polarization on charge. The structural and compositional changes on discharge to 200 mV have been studied by a combination of XRD, and XAS. The interlayer ions have also been exchanged for Sn, and the electrochemical characteristics of these materials are compared with the alkali derivatives.

Electrochemical Li insertion in lamellar (birnessite) and tunnel manganese oxides (todorokite)

A comparison of Li insertion in manganese oxide phases with a tunnel (todorokite) framework, its two-dimensional layered precursor (birnessite/buserite), and Li-exchanged materials are presented. The results outline the effect of the MnO{sub 6} octahedral arrangement and framework composition on the electrochemical response. The interlayer cations in the lamellar materials are exchangeable for Li, giving rise to a lithiated birnessite that displays a sustainable capacity of 125 mAh/g. For todorokite, molten salt exchange using LiNO{sub 3} results in displacement of water from the tunnels, and incorporation of additional Li into the structure. Some of this Li is extractable during charge, resulting in a reversible capacity of 172 mAh/g in the voltage window 4.2--2.0V.