Daniel P. Abraham

Surface changes on LiNi0.8Co0.2O2 particles during testing of high-power lithium-ion cells

LiNi0.8Co0.2O2 particles from high-power lithium-ion cells were examined to determine material changes that result from accelerated aging tests. X-ray absorption spectroscopy (XAS) and transmission electron microscope (TEM) data indicated a LixNi1−xO-type layer on the particle surfaces. The greater thickness on particles from high-power fade cells indicate that these surface layers are a significant contributor to cathode impedance rise observed during cell tests.

ZrO2- and Li2ZrO3-stabilized spinel and layered electrodes for lithium batteries

Strategies for countering the solubility of LiMn{sub 2}O{sub 4} (spinel) electrodes at 500 {sup o}C and for suppressing the reactivity of layered LiMO{sub 2} (M = Co, Ni, Mn, Li) electrodes at high potentials are discussed. Surface treatment of LiMn{sub 2}O{sub 4} with colloidal zirconia (ZrO{sub 2}) dramatically improves the cycling stability of the spinel electrode at 50 {sup o}C in Li/LiMn{sub 2}O{sub 4} cells. ZrO{sub 2}-coated LiMn{sub 0.5}Ni{sub 0.5}O{sub 2} electrodes provide a superior capacity and cycling stability to uncoated electrodes when charged to a high potential (4.6 V vs Li{sup 0}). The use of Li{sub 2}ZrO{sub 3}, which is structurally more compatible with spinel and layered electrodes than ZrO{sub 2} and which can act as a Li{sup +}-ion conductor, has been evaluated in composite 0.03Li{sub 2}ZrO{sub 3} - 0.97LiMn{sub 0.5}Ni{sub 0.5}O{sub 2} electrodes; glassy Li{sub x}ZrO{sub 2 + x/2} (0

Microscopy and spectroscopy of lithium nickel oxide-based particles used in high power lithium-ion cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk LiNi{sub 0.8}Co{sub 0.2}O{sub 2} powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a <5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R{bar 3}m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.

Local Structure of Layered Oxide Electrode Materials for Lithium‐Ion Batteries

Li-ion batteries are promising candidates for electrical energy storage in applications ranging from portable electronics to hybrid and electric vehicles. In this context, layered compounds in the Li(1+delta)(TM(x)Mn(1-x))(1-delta)O(2) family (TM = transition metal) have received much attention due to their high capacity and stability. In this Research News article we describe recent advances on structural characterization of Li-ion electrode materials using state-of-the-art electron microscopy. Direct evidence of the monoclinic nature of Li(2)MnO(3) has been provided. It has been demonstrated that differences in Z-contrast imaging between Li(2)MnO(3) and LiTMO(2) may be used to screen samples for phase separation in the 10-100 nm scale.

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.

Hydrogen-enhanced localization of plasticity in an austenitic stainless steel

Microscopic observations and the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves in a previous article. The hydrogen-free specimens showed minute yield points during static strain aging, while the hydrogen-containing specimens demonstrated “preyield microstrain. ” Thermal activation analysis of the strain rate change and stress relaxation plots led to the conclusion that the activation area for dislocation motion is decreased by hydrogen. Microstructural examination with the scanning electron microscope (SEM) revealed extensive strain localization, while transmission electron microscopy (TEM) studies showed microtwinning and austenite faulting in hydrogenated specimens tested at room temperature. The relation of hydrogen-induced changes in plastic deformation to hydrogen embrittlement is discussed.

Examining the Solid Electrolyte Interphase on Binder-Free Graphite Electrodes

The solid electrode interphase (SEI) on graphite electrodes is important to the performance, calendar life, and safety characteristics of lithium-ion cells. This article examines the SEI formed on binder-free graphite electrodes prepared by electrophoretic deposition. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis data were obtained on electrodes cycled in cells containing four electrolytes comprising ethylene carbonate: ethylmethyl carbonate (3:7 by weight) solvent and 1.2 M LiPF{sub 6}, 1 M LiF{sub 2}BC{sub 2}O{sub 4}, 1 M LiBF{sub 4}, or 0.7 M LiB(C{sub 2}O{sub 4}){sub 2} salt. Our observations suggest that, in addition to solvent reduction, the reduction of electrolyte salts plays an important role in SEI formation. Mechanisms to account for the formation of these SEI constituents are included in the article.

The effect of hydrogen on the yield and flow stress of an austenitic stainless steel

Tensile tests on 310s stainless steel foils, with and without hydrogen, were conducted at temperatures from 77 to 295 K and strain rates from 10-3 to 10-6/s. Cathodic charging at elevated temperatures and at very low current densities was used to produce homogeneous solid solutions of hydrogen in this material. The yield stress and flow stress were found to increase with hydrogen content. Discontinuous yielding was observed at room temperature for specimens with hydrogen contents greater than 5 at. pct. The ductility, as measured by the strain to failure, was not critically dependent on hydrogen concentration at 77 and 295 K but was reduced at intermediate temperatures. The changes in mechanical behavior are discussed in terms of hydrogen-dislocation interactions.

Investigating phase transformation in the Li1.2Co0.1Mn0.55Ni0.15O2 lithium-ion battery cathode during high-voltage hold (4.5 V) via magnetic, X-ray diffraction and electron microscopy studies

This study is the first that provides evidence of phase transformation in a Li-rich Li1.2Co0.1Mn0.55Ni0.15O2 cathode material for lithium-ion batteries (LIBs) during constant voltage charging. Diffraction and magnetic measurement techniques were successfully implemented to investigate the structural transformation in this cathode material during holding a half-cell at 4.5 V in a charged state. The results from X-ray diffraction showed a decrease in c-lattice parameters during high-voltage hold. Magnetic data revealed an increase in average effective magnetic moments of transition metal (TM) ions at constant voltage corresponding to a change in electronic states of TM ions. Analysis showed the reduction of Ni4+ to Ni2+, which was attributed to charge compensation due to oxygen loss. The appearance of the strong {100} forbidden reflection in the single-crystal selected area electron diffraction (SAED) data was attributed to migration of transition metal ions to the octahedral vacancy sites in the lithium layer during high-voltage hold, which was in agreement with the magnetization results. After prolonged hold at 4.5 V, high-resolution transmission electron microscopy (TEM) images along with SAED results showed the presence of spinel phases in the particles, indicating a layered to spinel like phase transformation at constant voltage in agreement with the magnetic data. The results obtained from these magnetic and diffraction studies furnish the fundamental understanding of the structural transformation pathways in Li-rich cathodes at constant voltage and will be instrumental for modifying the parent structure to achieve greater stability.

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.