Mohamed Mohamedi

Amorphous tin oxide films: preparation and characterization as an anode active material for lithium ion batteries

Abstract Tin oxide thin films were prepared by the electrostatic spray deposition at 400°C, followed by annealing at 500°C in air. X-ray diffraction and X-ray photoelectron spectroscopy revealed that the resulting films were amorphous SnO2. The electrochemical properties of the SnO2 films with lithium were studied by in situ conductivity measurements using an interdigitated microarray electrode as well as by cyclic voltammetry, galvanostatic cycling measurements and ac-impedance spectroscopy in 1 M LiClO4/(PC+EC). The SnO2 film electrodes exhibited reversible capacities greater than 1300 mA h g−1 when cycled between 0.05 and 2.5 V at 0.2 mA cm−2. However, this capacity faded rapidly after repeated cycling. If the electrode was cycled between 0 and 1.0 V, a reversible capacity of 600 mA h g−1 was maintained for more than 100 cycles. In addition, a stable reversible capacity of about 500 mA h g−1 was obtained even at current density as high as 2 mA cm−2. Thus it is suggested that a higher potential than 1.5 V would cause reformation of tin oxide, which may destroy the stable nanocomposite structure of metallic tin and lithium. These arguments were supported by in situ conductivity measurements with microarray electrodes.

Kinetic Characterization of Single Particles of LiCoO2 by AC Impedance and Potential Step Methods

This is the first report of impedance technique run on single particle electrodes with the aim of clarifying its electronic and ionic transport properties. Measurements were successfully conducted on a particle of 15 μm diam resulting in impedance magnitude on the order of MΩ. The impedance spectra exhibited (i) one semicircle in the high frequency region, (ii) Warburg impedance in low frequencies, and finally, (iii) a limiting capacitance in the very low frequencies. The spectra were analyzed using a modified Randles-Ershler circuit, so that the reaction kinetics could be precisely evaluated. The charge transfer resistance decreased as the potential increased, whereas the double layer capacitance was almost invariant with the potential. Thus, the apparent chemical diffusion coefficient of lithium ions was determined to be to as function of electrode potential. These results are in agreement with those obtained by potential step chronoamperometry technique.

Porous-microelectrode study on Pt/C catalysts for methanol electrooxidation

AbstractWe havedeveloped a porous-microelectrode (PME) to investigate the electroactivity of catalyst particles for proton exchangemembrane fuel cells. The cavity at the tip of the PME was filled with Pt/C catalysts prepared by impregnation method. Cyclicvoltammograms (CVs) recorded in 1 N H 2 SO 4 aqueous solution revealed that the active area of the stacked catalysts exist not onlyat the surface but also inside of the stack. For methanol electrooxidation, 30 wt.% Pt/C exhibited the highest electroactivity, whereasthe 50 wt.% Pt/C showed extremely small current. The small current is considered as a result of a small active-surface area. Methanoloxidation peak potential shifted toward cathodic direction as Pt-loading decreased, which agrees well with the Pt-oxide formationpotential. The activation energy for methanol oxidation was assessed to be 449 / 3 kJ mol 1 for all Pt/C catalysts and Pt-discelectrode.# 2003 Elsevier Science Ltd. All rights reserved. Keywords: Porous-microelectrode; Direct methanol fuel cell; Pt-loading carbon; Methanol electrooxidation; Electroactivity

Kinetic Study of Li-Ion Extraction and Insertion at LiMn2 O 4 Single Particle Electrodes Using Potential Step and Impedance Methods

The kinetics of Li-ion extraction and insertion at LiMn 2 O 4 single particles (8-21 μm diam) were investigated by cyclic voltammetry, potential step chronoamperometry (PSCA), and electrochemical impedance spectroscopy (EIS) methods using a microelectrode technique. The EIS measurements in a frequency range from 110 kHz to 11 mHz were conducted successfully on a LiMn 2 O 4 single particle resulting in the magnitude of MΩ orders. The impedance spectra exhibited (i) a single semicircle in the high frequency region, (ii) a Warburg impedance in the low frequency region, and (iii) a limiting capacitance in the very low frequency region. The EIS spectra were fitted to a modified Randles-Ershler circuit, so that the reaction kinetics could be evaluated precisely. The dependences of the charge transfer resistance (R cl ) and the apparent diffusion coefficient of Li within the particle on the electrode potential were evaluated. Obtained values for D app were in the range of 10 -10 to 10 -6 cm2/s from EIS measurements, in fair agreement with those from PSCA results. Finally, the apparent chemical diffusion coefficients of Li-ion in the single crystal, thin-film, and single particle of LiMn 2 O 4 are compared.

Electrochemical investigation of LiNi0.5Mn1.5O4 thin film intercalation electrodes

This work provides kinetic and transport parameters of Li-ion during its extraction/insertion into thin film LiNi0.5Mn1.5O4 free of binder and conductive additive. Thin films of LiNi0.5Mn1.5O4 (0.2 μm thick) were prepared on electronically conductive gold substrate utilizing the electrostatic spray deposition technique. High purity LiNi0.5Mn1.5O4 thin film electrodes were observed with cyclic voltammetry, to exhibit very sharp peaks, high reversibility, and absence of the 4 V signal related to the Mn3+/Mn4+ redox couple. The electrode subjected to 100 CV cycles of charge/discharge delivered a capacity of 155 mAh g−1 on the first cycle and sustained a good cycling behavior while retaining 91% of the initial capacity after 50 cycles. Kinetics and mass-transport of Li-ion extraction at LiNi0.5Mn1.5O4 thin film electrode were investigated by means of electrochemical impedance spectroscopy. The apparent chemical diffusion coefficient (Dapp) value determined from EIS measurements changed depending on the electrode potential in the range of 10−10–10−12 cm2 s−1. The Dapp profile shows two minimums at the potential values close to the peak potentials of the corresponding cyclic voltammogram.

In situ Raman spectroscopic studies of LiNixMn2 − xO4 thin film cathode materials for lithium ion secondary batteries

Chemical states and structural changes accompanying the electrochemical Li extraction and insertion of LiNixMn2 − xO4 (0 < x < 0.5) thin films in LiBF4–EC–DMC solutions, studied by in situ Raman spectroscopy, are reported for the first time. Ex situ Raman measurements for the virgin electrodes revealed that the oxidation state of Ni in the pristine thin films was Ni2+. In situ Raman spectra of the thin films collected in the organic electrolyte during Li ion extraction and insertion in the potential range 3.4–5.0 V vs. Li/Li+ showed a new Raman band at 540 cm−1 appearing around 4.7 V, which is attributed to the Ni4+–O bond. In addition, from the in situ Raman spectral changes, it is suggested that Li ion extraction and insertion proceed as follows: the redox of Ni2+/3+ takes place in the potential range 4.4–4.7 V, and Ni3+/4+ in the 4.7–5.0 V range, while the redox at 3.8–4.4 V corresponds to Mn3+/4+. Furthermore, it was confirmed that these changes in the Raman spectra were reversible upon changing the electrode potential, and the Li ion extraction and insertion proceed in a reversible manner.

Structural and Morphological Modifications of a Nanosized 62 Atom Percent Sn-Ni Thin Film Anode during Reaction with Lithium

.There is a strong incentive to develop and characterize noncarbonaceous materials for use as negative electrodes that deliver capacities higher than carbon. In 1994, Fuji Film Co., Ltd., filed a patent for Sn oxides as a novel anode material for lithium ion batteries with a theoretical capacity exceeding that of carbon. 1 However, the tin oxide material showed high irreversible capacity during the first cycle, which has precluded its commercial success as an anode in lithium ion secondary batteries. The large irreversible capacity is caused by the reduction of the tin oxides and the formation of lithium oxide during the first cycle. 2 Nonetheless, the studies on tin oxide material demonstrated the possible performance given a tin-based negative electrode material. To mitigate the irreversible capacity associated with tin oxides, several attempts have been made to develop nonoxide tin materials with both high capacity and long cycle life. 3-26 From these studies, it may be seen that the selection of an adequate matrix that can accommodate the volume change of tin during cycling is crucial to a successful tin anode compound that can deliver both high capacity and long cycle life. Elements that are inactive against lithium are assumed to suppress the volume change effectively without appreciable irreversible capacity. Alloying Sn with elements such as Fe, 15-19 Cu, 20-23 Mn, 19,24 and Co, 19 has been investigated based on this assumption. Nickel is a typical element, which does not react with lithium and can be expected to serve as an adequate matrix for improving the cycleability of the electrode without high initial irreversible capacity. Studies on Sn-Ni alloys pre

In Situ Observation of LiNiO2 Single‐Particle Fracture during Li ‐ Ion Extraction and Insertion

Electrochemical lithium‐ion extraction/insertion properties of single particles were investigated by attaching a filament microelectrode to the particle in carbonate + propylene carbonate electrolyte. High‐resolution cyclic voltammograms and galvanostatic chronopotentiograms were recorded. In addition, we observed in situ particle fracture during charge‐discharge using an optical microscope equipped with a charge‐coupled device camera. We found that the particle fractures when it is polarized above vs. . This phenomenon was explained by the change in crystal parameters expected to occur for in . ©2000 The Electrochemical Society

ESD Fabricated Thin Films of Spinel LiMn2 O 4 for Lithium Microbatteries: I. Effects of Thickness

Electrochemical properties of thin films of LiMn 2 O 4 spinels of several thicknesses prepared by electrostatic spray deposition (ESD) were studied in 1 M LiCIO 4 /propylene carbonate solution using cyclic voltammetry and electrochemical impedance spectroscopy. The shapes of the cyclic voltammograms (CVs) is extremely affected by the scan rate. In general, at a constant film thickness, the slower the scan rates the sharper and narrower are the peaks. A progressive decrease in the charge-transfer resistance was always found with raising the film thickness and the applied potential Both the film thickness and the charge injection process effectively limit the region over which diffusion control is observed. The diffusion control domain shrinks with decreasing film thickness in agreement with the theoretical predictions. The diffusion coefficient of Li ions in the solid phase varied in a similar manner with the potential, irrespective of the film thickness. The diffusion coefficient exhibited a peak-shaped function of the electrodes potential with minima at potentials close to the peak potentials of the CV. Importantly, operating at a constant potential. the chemical diffusion coefficient decreases within the 1-0.5 μm range, and is almost constant for smaller thicknesses.

Microvoltammetry for cathode materials at elevated temperatures: electrochemical stability of single particles

Abstract The electrochemical stability of single particles of cathode materials (LiMn 2 O 4 , Li 1.10 Cr 0.048 Mn 1.852 O 4 , LiCoO 2 and LiNi 0.85 Co 0.15 O 2 ) was investigated by means of a microelectrode technique at 25°C and 50°C. The cycle stability was evaluated by multi-cyclic voltammetry. LiMn 2 O 4 showed good cycle stability in LiClO 4 /propylene carbonate (PC)+ethylene carbonate (EC) and LiBF 4 /PC+EC solutions even at 50°C. On the contrary, in LiPF 6 /PC+EC, significant capacity fading during charge–discharge was observed at 50°C. The cycle stability of LiMn 2 O 4 in the latter solution was improved by partial substitution of Mn by Cr and Li. Regarding LiCoO 2 , its cycle life in LiClO 4 /PC+EC at 50°C was unsatisfactory when the potential was scanned between 3.60 and 4.30 V. On the other hand, LiCoO 2 retained 90% of its capacity when the potential scan was limited to 4.00 V. LiNi 0.85 Co 0.15 O 2 showed similar trend at 50°C.