Dennis W. Dees

Conductivity of porous Ni/ZrO/sub 2/-Y/sub 2/O/sub 3/ cermets

The conductivity of porous Ni/ZrO/sub 2-/Y/sub 2/O/sub 3/ cermets at 1000/sup 0/C was determined as a function of Ni content between 15 and 50 volume percent (v/o) of total solids for two different zirconia particle sizes (23 and 47 m/sup 2//g). Below Ni contents of 30 v/o, ionic conduction through the zirconia phase dominated. At 30 v/o Ni, a greater than three order of magnitude increase in the conductivity was observed, corresponding to a change in mechanism to electronic conduction through the Ni phase. The conductivity of cermets made with a Ni content greater than 30 v/o Ni was found to decrease with increasing temperature between 700/sup 0/ and 1000/sup 0/C. While the conductivity of the cermets with the larger particle size zirconia was higher by more than a factor of four, all the samples studied had the same activation energy, 5.7 +- 0.1 kJ/mol. The increase in conductivity with zirconia particle size is attributed to improved Ni particle-to-particle contact, resulting from the Ni phase being able to cover more completely the surface of the zirconia matrix where it resides.

Development of a high-power lithium-ion battery

Safety is a key concern for a high-power energy storage system such as will be required in a hybrid vehicle. Present lithium-ion technology, which uses a carbon/graphite negative electrode, lacks inherent safety for two main reasons: (1) carbon/graphite intercalates lithium at near lithium potential, and (2) there is no end-of-charge indicator in the voltage profile that can signal the onset of catastrophic oxygen evolution from the cathode (LiCoO{sub 2}). Our approach to solving these safety/life problems is to replace the graphite/carbon negative electrode with an electrode that exhibits stronger two-phase behavior further away from lithium potential, such as Li{sub 4}Ti{sub 5}O{sub 12}. Cycle-life and pulse-power capability data are presented in accordance with the Partnership for a New Generation of Vehicles (PNGV) test procedures, as well as a full-scale design based on a spreadsheet model.

Factors responsible for impedance rise in high power lithium ion batteries

Abstract High-power, 18,650 lithium-ion cells have been designed and fabricated in order to understand the factors limiting the calendar life of the lithium-ion system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a blend of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1:1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elevated temperatures, the cells experienced a significant impedance rise and loss of power. The fade rate of power in these cells was dependent of the state of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfacial resistance at the positive electrode was the main reason behind the impedance rise in the high power cell.

Structural and electrochemical studies of α-manganese dioxide (α-MnO2)

Abstract The structural and electrochemical properties of α -MnO 2 , prepared by acid digestion of Mn 2 O 3 , and its lithiated derivatives x Li 2 O · MnO 2 (0 ≤ x ≤ 0.25) have been investigated as insertion compounds in the search for new and viable cathode materials for rechargeable 3 V batteries. The α -MnO 2 product fabricated by this technique contains water within the large (2 × 2) channels of the structure; the water can be removed from the α -MnO 2 framework without degradation of the structure, and then at least partially replaced by Li 2 O (lithium oxide). The Li 2 O-doped α -MnO 2 electrodes, described generically as x Li 2 O · MnO 2 , stabilize the structure and provide higher capacities on cycling than the parent material. The structures of these α -MnO 2 -type electrode materials are described, and electrochemical data are presented for both liquid electrolyte and polymer electrolyte Li/ α -MnO 2 and Li/ x Li 2 O · MnO 2 cells.

Alternating Current Impedance Electrochemical Modeling of Lithium-Ion Positive Electrodes

Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois, USAAn electrochemical model was developed to describe alternating current ~ac! impedance experimental studies conducted onlithium-ion positive electrodes. The model includes differential mass and current balances for the positive electrode’s compositestructure, as well as details of the oxide-electrolyte interface. A number of specialized experiments were conducted to help definethe parameter set for the model. The electrochemical ac impedance model was used to examine aging effects associated with thepositive electrode.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1928169# All rights reserved.Manuscript submitted November 18, 2004; revised manuscript received January 19, 2005. Available electronically June 10, 2005.

The thermal stability of lithium-manganese-oxide spinel phases☆

The thermal stability of stoichiometric spinel phases in the system Li{sub 1+{delta}}Mn{sub 2{minus}{delta}}O{sub 4} (0 {le} {delta} {le} 0.33) has been investigated by high-temperature powder X-ray diffraction, differential thermal analysis, and thermogravimetric analysis. At elevated temperatures, the lithium-manganese-oxide spinels undergo phase changes by loss of oxygen and lithia (Li{sub 2}O). The data highlight the importance of temperature control when synthesizing lithium-manganese-oxide spinel compounds.

Electrochemical modeling of lithium polymer batteries

An electrochemical model for lithium polymer cells was developed and a parameter set for the model was measured using a series of laboratory experiments. Examples are supplied to demonstrate the capabilities of the electrochemical model to obtain the concentration, current, and potential distributions in lithium polymer cells under complex cycling protocols. The modeling results are used to identify processes that limit cell performance and for optimizing cell design. Extension of the electrochemical model to examine two-dimensional studies is also described.

Modeling thermal management of lithium-ion PNGV batteries

Batteries were designed with the aid of a computer modeling program to study the requirements of the thermal control system for meeting the goals set by the Partnership for a New Generation of Vehicles (PNGV). The battery designs were based upon the lithium-ion cell composition designated Gen-2 in the US Department of Energy Advanced Technology Development Program. The worst-case cooling requirement that would occur during prolonged aggressive driving was estimated to be 250 W or about 5 W per cell for a 48-cell battery. Rapid heating of the battery from a very low startup temperature is more difficult than cooling during driving. A dielectric transformer fluid is superior to air for both heating and cooling the battery. A dedicated refrigeration system for cooling the battery coolant would be helpful in maintaining low temperature during driving. The use of ample insulation would effectively slow the battery temperature rise when parking the vehicle in warm weather. Operating the battery at 10 °C during the first several years when the battery has excess power would extend the battery life.

Morphological Transitions on Lithium Metal Anodes

Coin cells were prepared using a metallic lithium anode, a Li{sub 4}Ti{sub 5}O{sub 12} cathode, and a 1.2 M LiPF{sub 6}/ethylene carbonate:ethyl methyl carbonate (30:70 wt %) electrolyte. The cells were cycled galvanostatically between 1 and 2 V vs Li/Li{sup +} (i=2.0 mA/cm{sup 2}) at a 2C rate. After a specific number of cycles, the cells were disassembled and the morphology of the lithium anode was characterized using scanning electron microscopy. It was observed that the surface morphology of the lithium metal electrode transitioned from a flat and smooth morphology to a microscopically rugged structure that shows three distinct layers: a top dendritic layer, an intermediate porous layer, and a residual metallic lithium layer. Morphological and electrochemical evidence points to the depletion of the electrolyte and the active metallic lithium that reacted to produce the porous layer as the most likely cause of cell failure under the conditions studied.

Electrochemical Modeling of Lithium-Ion Positive Electrodes during Hybrid Pulse Power Characterization Tests

An electrochemical model was developed to examine hybrid pulsed power characterization (HPPC) tests on the positive electrode of lithium-ion cells. By utilizing the same fundamental equations as in previous electrochemical impedance spectroscopy studies, this investigation serves as an extension of the earlier work and a comparison of the two techniques. The electrochemical model was used to examine performance characteristics and limitations for the positive electrode during HPPC tests. Parametric studies using the electrochemical model and focusing on the positive electrode thickness were employed to examine methods of slowing electrode aging and improving performance.