Soon-Ho Ahn

Electrochemical properties of LiCoO2-Coated LiMn2O4 prepared by solution-based chemical process

In order to enhance both the cycle stability at elevated temperatures and the rate capability of LiMn 2 O 4 , the surface of LiMn 2 O 4 was covered with fine LiCoO 2 particles prepared by a solution-based chemical process. The amount of LiCoO 2 measured with inductively coupled plasma was about 7 mol %. LiCoO 2 -coated LiMn 2 O 4 had an excellent cycle stability at 65°C compared to pure LiMn 2 O 4 . This can be explained by suppression of the Mn dissolution. The rate capability of LiCoO 2 -coated LiMn 2 O 4 improved significantly. The improved rate capability may be attributed to a decrease in the passivation film that acts as an electronic insulating layer and interparticle contact resistance. Consequently, it is proposed that the surface encapsulation of LiMn 2 O 4 with LiCoO 2 improves its rate capability and cycling stability at high temperatures.

Battery dimensional changes occurring during charge/discharge cycles—thin rectangular lithium ion and polymer cells

Abstract During cycles, the battery thickness changes for the three reasons—(i) expansion and contraction of host materials due to lithium intercalation, (ii) electrode volume increase caused by irreversible reaction deposits, and (iii) dead volume and pressure changes within the cell case depending on battery structure and construction. In this study we have identified and quantified those three reasons and related thickness increases employing commercially available thin prismatic lithium ion and polymer cells.

Development of high capacity, high rate lithium ion batteries utilizing metal fiber conductive additives

Abstract As lithium ion cells dominate the battery market, the performance improvement is an utmost concern among developers and researchers. Conductive additives are routinely employed to enhance electrode conductivity and capacity. Carbon particulates—graphite or carbon black powders—are conventional and popular choices as conductive fillers. However, percolation requirements of particles demand significant volumetric content of impalpable, and thereby high area conductive fillers. As might be expected, the electrode active surface area escalates unnecessarily, resulting in overall increase in reaction with electrolytes and organic solvents. The increased reactions usually manifest as an irreversible loss of anode capacity, gradual oxidation and consumption of electrolyte on the cathode—which causes capacity decline during cycling—and an increased threat to battery safety by gas evolution and exothermic solvent oxidation. In this work we have utilized high aspect ratio, flexible, micronic metal fibers as low active area and high conductivity additives. The metal fibers appear well dispersed within the electrode and to satisfy percolation requirements very efficiently at very low volumetric content compared to conventional carbon-based conductive additives. Results from 18650-type cells indicate significant enhancements in electrode capacity and high rate capability while the irreversible capacity loss is negligible.

Composite fiber structures for catalysts and electrodes

Abstract We have recently envisioned a process wherein fibers of various metals in the 0.5 to 15 μm diameter range are slurried in concert with cellulose fibers and various other materials in the form of particulates and/or fibers. The resulting slurry is cast via a wet-lay process into a sheet and dried to produce a free-standing sheet of ‘composite apper’. When the ‘preform’ sheet is sintered in hydrogen, the bulk of the cellulose is removed with the secondary fibers and/or particulates being entrapped by the sinter-locked network provided by the metal fibers. The resulting material is unique in that it allows the intimate contacting and combination of heretofore mutually exclusive materials and properties. Moreover, due to the ease of paper manufacture and processing, the resulting materials are relatively inexpensive and can be fabricated into a wide range of three-dimensional structures. Also, because cellulose is both a binder and a pore-former, structures combining high levels of active surface area and high void volume (i.e., low pressure drop) can be prepared as free-standing flow through monoliths.

Air Electrode: Identification of Intraelectrode Rate Phenomena via AC Impedance

In this study, we report here on the use of ac impedance spectroscopy to investigate intraelectrode processes occurring during oxygen reduction in a KOH half-cell. Investigated were novel composite electrode structures containing high surface area activated carbon fibers entrapped within a sinter-bonded matrix of 2 μm diam stainless steel fibers. Systematic variations of the metal fiber to carbon fiber loading in the composite and also the electrode thickness and void volume fraction allowed mechanistic discrimination and development of a detailed equivalent circuit model for the electrode. AC impedance was performed at various applied potentials and imposed current densities to determine the relative magnitude of the resistances distributed among kinetic, ohmic, and transport processes as a function of reaction rate. A conceptual model proposed earlier for the active reaction zone within the air electrode is discussed.

Fibrous metal–carbon composite structures as gas diffusion electrodes for use in alkaline electrolyte

The fabrication of novel fibre composite electrode structures and the performance assessments for oxygen reduction in alkaline electrolyte is reported. An array of 2μm diameter activated carbon fibres interlocked within a network of 2μm sinter-bonded metal fibres to form the composite structure was used. The resulting electrode structure is stable, highly conductive and can maintain void fraction exceeding 95%. Electrode physical properties including thickness, macroporosity, volume and mass fractions of constituent carbon and metal fibres have been controlled, characterized, and related to the electrode polarization in a KOH half cell. Comparisons have been made with a commercial Teflon-bonded gas diffusion electrode (GDE). It has been demonstrated that this novel method allows reproducible and low-cost fabrication of GDEs with the optimal balance between macropores for gas access, micropores for liquid access, and conductive paths for electron access.