Frank McLarnon

Effect of surface carbon structure on the electrochemical performance of LiFePO{sub 4}

The electrochemical performance of LiFePO 4 samples synthesized by sol-gel or solid-state routes varies considerably, although their physical characteristics are similar. Raman microprobe spectroscopic analysis indicated that the structure of the residual carbon present on the surfaces of the powders differs significantly and accounts for the performance variation. Higher utilization is associated with a larger ratio of sp 2 -coordinated carbon, which exhibits better electronic properties than disordered or sp 3 -coordinated carbonaceous materials. Incorporation of naphthalenetetracarboxylic dianhydride during synthesis results in a more graphitic carbon coating and improves utilization of LiFePO 4 in lithium cells, although the total carbon content is not necessarily higher than that of samples prepared without the additive. This result suggests that practical energy density need not be sacrificed for power density, provided that carbon coatings are optimized by carefully choosing additives.

Electrochemical performance of lithium/sulfur cells with three different polymer electrolytes

Abstract Charge and discharge characteristics of lithium/polymer electrolyte/sulfur cells are presented. Three different electrolytes were studied, and cells were operated at temperatures ranging from ambient to about 100°C. The effects of the sulfur electrode composition and cycling regimen on both the potential profiles and the capacity fade rate were investigated. Cells prepared with poly(ethylene oxide) (PEO) and operated at 90–100°C could be discharged to nearly the full theoretical 1672 mA h/g active material but with a high rate of capacity fade. Reducing the depth of discharge to 30% or less increased the cell lifetime. Room-temperature cells with poly(ethylene glycol) dimethyl ether could be discharged to about 45% utilization of the sulfur and showed a much lower capacity fade rate after the second cycle. Several possible explanations for the high rate of capacity fade and the effect of the depth of discharge on this rate are presented.

The Secondary Alkaline Zinc Electrode

Secondary batteries that use zinc electrodes typically exhibit short lifetimes, because of problems with zinc material redistribution and undesirable zinc morphologies that form during recharge. There has been a worldwide effort to develop a long-lived secondary alkaline zinc electrode, and marked improvements in cell lifetimes have resulted. This article reviews these efforts, paying particular attention to research and development during the period 1975-1990

Diagnostic Characterization of High Power Lithium-Ion Batteries for Use in Hybrid Electric Vehicles

A baseline cell chemistry was identified as a carbon anode, LiNi 0.8 Co 0.2 O 2 cathode, and diethyl carbonate-ethylene carbonate LiPF 6 electrolyte, and designed for high power applications. Nine 18650-size advanced technology development cells were tested under a variety of conditions. Selected diagnostic techniques such as synchrotron infrared microscopy, Raman spectroscopy, scanning electronic microscopy, atomic force microscopy, gas chromatography, etc., were used to characterize the anode, cathode, current collectors and electrolyte taken from these cells. The diagnostic results suggest that the four factors that contribute to the cell power loss are solid electrolyte interphase deterioration and nonuniformity on the anode; morphology changes, increase of impedance, and phase separation on the cathode; pitting corrosion on the cathode current collector; and decomposition of the LiPF 6 salt in the electrolyte at elevated temperature.

Surface Layer Formation on Thin-Film LiMn2 O 4 Electrodes at Elevated Temperatures

Thin-film LiMn 2 O 4 electrodes, which were exposed to pure dimethyl carbonate (DMC) or to a mixture of ethylene carbonate (EC) and DMC (1:1 by volume) containing 1 M LiPF 6 at elevated temperatures, were studied by using X-ray diffraction, current-sensing atomic force microscopy (CSAFM). cyclic voltammetry, ordinary Raman spectroscopy, and surface-enhanced Raman spectroscopy (SERS). Thin, electronically insulating surface layers were detected by CSAFM and SERS on all electrodes. The surface layer formed by exposure to DMC at 70°C was uniform and preserved the electrode structure, however, it led to complete electrode deactivation, probably due to loss of surface electronic conductivity and slower lithium-ion transport rates through the surface layer. A similar surface layer was formed when the electrodes were exposed to EC-DMC-LiPF 6 , however, the layer formed at 70°C did not prevent LiMn 2 O 4 decomposition and consequent electrode capacity loss In this case, manganese dissolution was observed, accompanied by the formation of λ-MnO 2 . SER spectra of the surface layers suggest that they were formed as a result of DMC decomposition at the LiMn 2 O 4 surface. The SER spectra displayed hands characteristic of Li-O-R, carbonate. and carboxyl groups. Derivatives of carbon-oxygen triple bonds or silver-carbon-oxygen groups, which are possibly a result of interactions between the surface layer and silver microparticles, were also detected.

Local-probe studies of degradation of composite LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} cathodes in high-power lithium-ion cells

High-power Li-ion cells tested at elevated temperatures showed a significant impedance rise, which was associated primarily with the LiNi 0 . 8 Co 0 . 1 5 Al 0 . 0 5 O 2 cathode. Raman microscopy mapping provided evidence that the surface composition ratio between LiNi 0 . 8 Co 0 . 1 5 Al 0 . 0 5 O 2 and carbon in the composite cathode increases upon cell aging and cycling. Current-sensing atomic force microscopy imaging of single grains of pristine LiNi 0 . 8 Co 0 . 1 5 Al 0 . 0 5 O 2 powder revealed poor residual electronic contact between submicrometer primary particles within LiNi 0 . 8 Co 0 . 1 5 Al 0 . 0 5 O 2 agglomerates. Carbon retreat or rearrangement that occurs during cell testing allows residual interparticle resistance to dominate cathode interfacial charge-transfer impedance and accounts for the observed cell power and capacity loss.

Microprobe study of the effect of Li intercalation on the structure of graphite

The structural stability of graphite electrodes in Li-ion cells, which were cycled at room temperature and 60 °C was investigated. We observed gradual structural degradation of the graphite, which was most pronounced on the electrode surface but also extended into the bulk of the electrode. Graphite particles close to the Cu current collector remained almost unchanged, whereas those close to the electrode/electrolyte interface suffered significant structural damage. Structural degradation of the graphite led to an increased anode surface reactivity vs. the electrolyte. A thick layer of inorganic products from side reactions was observed on the disordered carbon areas of the anode.

Direct Anodic Oxidation of Methanol on Supported Platinum/Ruthenium Catalyst in Aqueous Cesium Carbonate

Methanol electro-oxidation was carried out on porous gas diffusion electrodes in CS{sub 2}CO{sub 3} electrolytes at 100 to 140 C and ambient pressures. It was found that Pt-Ru bimetallic catalysts supported on graphitized carbon provided enhanced performance, compared to supported Pt catalyst. Performance curves, based on unit catalyst mass, for Pt-Ru at 1.20 C matched or exceeded previously reported performance data for supported Pt or Pt black in concentrated Cs{sub 2}CO{sub 3} electrolytes at 120 to 150 C at 8 atm, and for supported Pt-Ru in concentrated H{sub 3}PO{sub 4} electrolytes at 200 C. It was found that the polytetrafluoroethylene (PTFE) content in the reaction layer, and consequently the extent of wetting, had a marked effect on cell performance, and 20 to 30 weight percent PTFE was found to be optimal. Maximum cell performance was found at operating temperatures 10 to 15 C below the boiling point of the electrolyte. 59 refs.

Diagnostic Evaluation of Detrimental Phenomena in High-Power Lithium-Ion Batteries

A pouch-type lithium-ion cell, with graphite anode and LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} cathode, was cycled at C/2 over 100% depth of discharge (DOD) at ambient temperature. The LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} composite cathode was primarily responsible for the significant impedance rise and capacity fade observed in that cell. The processes that led to this impedance rise were assessed by investigating the cathode surface electronic conductance, surface structure, composition, and state of charge at the microscopic level with the use of local probe techniques. Raman microscopy mapping of the cathode surface provided evidence that the state of charge of individual LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} particles was non-uniform despite the deep discharge at the end of cell testing. Current-sensing atomic force microscopy imaging revealed that the cathode surface electronic conductance diminished significantly in the tested cells. Loss of contact of active material particles with the carbon matrix and thin film formation via electrolyte decomposition not only led to LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2} particle isolation and contributed to cathode interfacial charge-transfer impedance but also accounted for the observed cell power and capacity loss.

Characterization of SEI Layers on LiMn2O4 Cathodes with In Situ Spectroscopic Ellipsometry

In situ spectroscopic ellipsometry was employed to study the initial stage of SEI layer formation on thin-film LiMn{sub 2}O{sub 4} electrodes. It was found that the SEI layer formed immediately upon exposure of the electrode to EC/DMC (1:1 by vol) 1.0 M LiPF{sub 6} electrolyte. The SEI layer thickness then increased in proportion to a logarithmic function of elapsed time. In comparison, the SEI layer thickness on a cycled electrode increased in proportion to a linear function of the number of cycles.