L. Sánchez

Synthesis and characterization of high-temperature hexagonal P2-Na0.6 MnO2 and its electrochemical behaviour as cathode in sodium cells

A layered sodium manganese oxide, Na0.6MnO2, stable at temperatures above 800 °C, was synthesized by using a sol–gel method that employs Mn(acac)3 (ac = acetylacetonate), Na2CO3 and propionic acid to form the resin framework. This layered bronze possesses a hexagonal, P2-type structure, in which the distortion associated with Mn3+ is hardly perceptible. It reacts slowly, though reversibly, with atmospheric moisture, which causes the interlayer spacing in the structure to increase by ca. 2.5 A, through intercalation of water molecules into the interlayer gap occupied by Na+ ions. The anhydrous material was tested as a cathode in sodium cells. Although the electrochemical intercalation of Na+ occurs in two steps, the host retains its main structural features, with a slight tendency in the interlayer spacing to contract as the sodium content increases. The similarity between the discharge and charge profiles of the first cycles reveals a quasi-reversible nature in the intercalation process and that the cell can deliver a constant specific capacity of ca. 140 A h kg−1 at 0.1 mA cm−2 when cycled in a voltage window of 3.8–2.0 V. However, the continuous strains and distortions resulting from the insertion and extraction of Na+ ions cause the host structure to gradually collapse and yield an amorphous material after the first eight cycles. This leads to a progressive reduction of the cell capacity, irrespective of the specific voltage window used.

Synthesis and Characterization of Nanometric Iron and Iron-Titanium Oxides by Mechanical Milling: Electrochemical Properties as Anodic Materials in Lithium Cells

Nanometric mixed iron-titanium oxides were prepared by mechanical milling with a view to determining their ability to act as anodic materials in lithium cells. At a TiO2/Fe2O3 mole ratio of 0.4, a solid-state reaction occurs that leads to the formation of Fe5TiO8, which possesses a spinel-like structure; at lower ratios, however, the structure retains the hematite framework. Li/g-Fe2O3 cells exhibit poor electrochemical reversibility; by contrast, Ti-containing electrodes possess improved cycling properties. Changes in the electrodes upon cycling were examined by X-ray photoelectron spectroscopy (XPS). XPS data confirm the participation of electrolyte in the electrochemical reaction and the different type of electrochemical reversibility exhibited by samples. Both processes were influenced by the presence of titanium. Titanium dioxide, in the presence of iron oxides, seems to be inactive to the electrochemical process. Based on the step potential electrochemical spectroscopy (SPES) curves and photoelectron spectra obtained, the presence of Ti increases the reversibility of the redox reactions undergone by the electrolyte during discharge/charge processes. The increased active-material/electrolyte/inactive-material interaction which is reported here offers new perspectives for the use of well-known transition oxides as anode materials in Li-ion batteries.

Use of Li–M–Mn–O [M=Co, Cr, Ti] spinels prepared by a sol-gel method as cathodes in high-voltage lithium batteries

Abstract Doped spinels of formula LiM 0.2 Mn 1.8 O 4 (M=Cr, Co) and LiTi 0.19 Mn 1.76 O 4 were prepared by using a sol-gel method involving Mn(acac) 3 , Cr(acac) 3 , [Ti(acac) 3 ] 2 [TiCl 6 ] and Li 2 CO 3 as precursors and propionic acid as chelating agent. On firing at 600°C, the gels gave normal spinel phases of a high purity. The Co and Cr spinels consisted of very uniformly shaped microcrystals. The three doped spinels were tested as cathodes in 4 V lithium cells. The best performance was exhibited by the Co-doped spinel, followed by the Cr-doped spinel. By contrast, LiTi 0.79 Mn 1.76 O 4 exhibited significant capacity fading upon cycling. All lithium in the Co- and Cr-doped manganese spinels can be extracted by charging the cells above 5 V. Under these conditions, the cell based on the Cr-doped spinel provided the best electrochemical performance. Ionic size and ligand field stabilization energies were considered in explaining the structural stability of the spinels, which has a direct effect on cathode performance.

Electrochemical properties of lead oxide films obtained by spray pyrolysis as negative electrodes for lithium secondary batteries

Lead(II) oxides in bulk and thin film form were assessed as electrodes for lithium rechargeable batteries. Films were prepared by spray pyrolysis of aqueous solutions of Pb(CH3–COO)2·2H2O and deposited onto lead substrates at 175°C. Films heated at 250°C were found to consist of well-crystallized tetragonal PbO and evolve to the orthorhombic polymorph with prolonged heating. Cycling of the cells at a current density of 0.25 mA/cm2 over the range 1.0–0.0 V led to the formation of various LiyPb alloys. Cells made from bulk oxides, whether tetragonal or orthorhombic, were found to exhibit poor performance (their capacity rapidly faded with cycling). By contrast, PbO film electrodes exhibited reversible capacity above 500 mA h/g beyond 40 cycles. The lead substrate must thus appreciably influence the electrochemical properties of the cell by facilitating adhesion of LiyPb microcrystals to its surface, thereby favoring alloying/de-alloying processes.

Electrodeposition of Cu2 O : An Excellent Method for Obtaining Films of Controlled Morphology and Good Performance in Li-Ion Batteries

Highly uniform films of pure electrodeposited Cu 2 O in variable particle size (micronic and submicronic), morphology, and thickness (0.3-13 μm) depending on the applied potential and the amount of electrical charge transferred were examined as working electrodes in lithium cells to check their suitability as anodes for Li-ion cells. The thinnest film studied delivered a specific capacity close to the theoretical value; such a capacity decreased with increasing film thickness and particle size. All films except the thickest exhibited excellent capacity retention upon extensive cycling irrespective of particle size. On similar thickness, the discharge capacity increased with decreasing particle size.

Lead-based systems as suitable anode materials for Li-ion batteries

Three lead-based materials formed by PbO2, PbO and Pb as main phases were prepared by following different synthetic procedures and tested as anodic materials in Li-ion batteries by using potentiostatic and galvanostatic methods. While the reduction of Pb(IV) to Pb(II) takes place in a single step, that of Pb(II) to Pb is a complex process involving several steps. Both reduction reactions are irreversible. Lead, whether electrochemically or chemically formed, undergoes an electrochemical reaction with lithium that over the 1.0 � /0.0 V potential range yields Lix Pb alloys (0 5/x 5/4.4). The anodic and cathodic potentiostatic curves exhibit various signals that account for: (i) the formation of different intermediates with variable lithium contents; (ii) the reversibility of the alloying/de-alloying processes; (iii) the increase in complexity of such processes as the oxidation state of lead in them decreases. This results in capacity fading with cycling, particularly in the samples having Pb as the main component. One way of avoiding the capacity loss on cycling involves depositing the active material on lead sheets from spraying suspensions. These coatings exhibit a good capacity retention, which can be ascribed to the formation of a Lix Pb layer at the active material/substrate interface that facilitates electron and ion transfer across the electrode. # 2002 Elsevier Science Ltd. All rights reserved.

Anchoring Si nanoparticles to carbon nanofibers: an efficient procedure for improving Si performance in Li batteries

Carbon fibers obtained by pyrolysis of tailored resorcinol/formaldehyde polymer particles were used to anchor Si nanoparticles at their surface. The carbonization process, carried out at 1000 °C under nitrogen, induced strong interactions between Si particles and the carbon matrix through a thick amorphous silicon oxide layer as revealed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Therefore, the actual composition of the composites was Si/SiOx/C (fibers). Component contents were determined from thermogravimetric measurements (TG) made under oxygen. The composites delivered specific capacities as high as 2500 mA h gSi−1 at rather high current densities (500 mA gsi−1) and exhibited good capacity retention on cycling. By contrast, a mixture of pristine Si nanoparticles and carbon nanofibers performed considerably worse, their capacity fading abruptly with cycling. The improved performance of composites is ascribed to a combination of the properties of the amorphous SiOx layer, and the texture and morphological properties of carbon, increasing the electrode conductivity and buffering Si expansion and shrinkage during Li insertion and deinsertion.

Mechanochemical synthesis of Sn1 − xMoxO2 anode materials for Li-ion batteries

Sn1 − xMoxO2 mixed oxides of low crystallinity have been synthesized by mechanical milling of the starting elements in an air atmosphere at room temperature and investigated as electrode materials for lithium batteries. The oxides were characterized by X-ray diffraction, infrared spectroscopy and X-ray photoelectron spectroscopy. The results suggest that the Mo-doped samples are solid solutions with a cassiterite-type structure and Mo in the tetravalent oxidation state. Significant amounts of amorphous silica over the range 12–23% by weight as determined by energy dispersive analysis (EDX), and originating from the agate jar and balls of milling apparatus used for the experiments, were also detected. The compounds were tested as electrodes in secondary lithium batteries. The addition of Mo has two favourable effects, namely: (i) it increases the discharge capacity; and (ii) it improves capacity retention in cells cycled between 1.0 and 0.0 V. The formation of a Li–Mo–O oxide conductive matrix during the electrochemical insertion of lithium may account for this enhanced performance. In this reasoning, the silica present in the samples was assumed to play a minor role based on the stability of the Si–O bond.

Sol–gel derived Li–V–Mn–O spinels as cathodes for rechargeable lithium batteries

a ´´ ´ ´ b ´´ ´ ´ Abstract Nearly stoichiometric ternary Li-V-Mn-O spinels were prepared by a sol-gel method using propionic acid to form the resin framework, and subsequent calcination of the gel at 6008C. The resulting material was tested as cathode over the voltage range 5.2-2.0 V in lithium cells. The profiles of the discharge-charge processes were found to be surprisingly similar to that recently reported for thin-film LiMn O. The results of X-ray photoelectron spectroscopy (XPS) and 24 elemental analysis are consistent with an oxidation state 1 5 for V and with 1 3 and 1 4 for Mn. Although the spinel preserves its basic framework across the whole potential window, significant capacity fading is observed along the 5.0 and 2.0 V plateaus, the latter effect being associated with a reversible cubic-tetragonal phase transition. The best electrochemical performance of the cell was observed in the potential window 4.6-2.5 V. Over this voltage range, the cell maintains an acceptable capacity (105 Ah / K) upon extensive cycling.

Use of amorphous tin-oxide films obtained by spray pyrolysis as electrodes in lithium batteries

Abstract Amorphous tin-oxide films were prepared by spray pyrolysis of SnCl 2 ·2H 2 O mixed with CH 3 –COOH and deposited onto a stainless steel substrate at mild temperatures (350°C). The films grown were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX). Also, they were tested as electrodes in lithium rechargeable batteries. The XPS results suggest that the substrate is thoroughly coated and that the films are composed mainly of SnO and SnO 2 . These films exhibit good charge–discharge properties over more than 100 cycles. Heating at 600°C causes significant changes in their surface composition, in the virtual disappearance of the tin component and in the presence of oxygen-bound Fe. Under these conditions, the reversible capacity dramatically fades and the cell behaves similarly to that made from uncoated substrate.