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Dissolution and migration of manganese from cathode lead to severe capacity fading of Li2MnO4-carbon cells. Overcoming this major problem requires better understanding of the mechanism of manganese dissolution, migration, and deposition. Here, we apply a variety of advanced analytical methods to the study of LiMn2O4 cathodes cycled with different anodes. We provide solid evidence, for the first time, that oxidation state of manganese deposited on the anodes is +2, which differs from the results reported earlier. The results also indicates that a metathesis reaction between Mn(II) and some species on the solid-electrolyte interphase takes place during the deposition of Mn(II) on the anodes, rather than a reduction reaction that leads to the formation of metallic Mn otherwise, as speculated in earlier studies. The concentration of Mn deposited on the anode increases gradually with cycles; this trend is well correlated with anode’s rising impedance and capacity fading of the cell.

Magnesium-substituted Li{sub 4-x}Mg{sub x}Ti{sub 5}O{sub 12} spinel electrodes (0

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A novel functionalized electrolyte based on Li2B12F9H3 was investigated to improve the electrochemical and safety performance of lithium-ion cells using a lithium manganese oxide spinel positive electrode and a mesocarbon microbeads (MCMB) negative electrode. The test results showed that Li2B12F9H3 can act both as the lithium salt, like the conventional LiPF6, and as the redox shuttle for overcharge protection of lithium-ion cells. In addition, the performance of the lithium-ion cells was dramatically improved by the addition of lithium bis(oxalato)borate and tris(pentafluorophenyl)borane as electrolyte additives. With the help of the proposed additives, MCMB/Li1.156Mn1.844O4lithium ion cells maintained more than 85% capacity after 1200 cycles.

Redox shuttle is a promising mechanism for intrinsic overcharge protection in lithium-ion cells and batteries. Two lithium borate cluster salts are reported to function as both the main salt for a nonaqueous electrolyte and the redox shuttle for overcharge protection. Lithium borate cluster salts with a tunable redox potential are promising candidates for overcharge protection for most positive electrodes in state-of-the-art lithium-ion cells.

Low-temperature performance loss is a significant barrier to commercialization of lithium-ion cells in hybrid electric vehicles. Increased impedance, especially at temperatures below 0 C, reduces the cell pulse power performance required for cold engine starts, quick acceleration, or regenerative braking. Here we detail electrochemical impedance spectroscopy data on binder- and carbon-free layered-oxide and spinel-oxide electrodes, obtained over the +30 to ?30 C temperature range, in coin cells containing a lithium-preloaded Li{sub 4/3}Ti{sub 5/3}O{sub 4} composite (LTOc) counter electrode and a LiPF{sub 6}-bearing ethylene carbonate/ethyl methyl carbonate electrolyte. For all electrodes studied, the impedance increased with decreasing cell temperature; the increases observed in the midfrequency arc dwarfed the increases in ohmic resistance and diffusional impedance. Our data suggest that the movement of lithium ions across the electrochemical interface on the active material may have been increasingly hindered at lower temperatures, especially below 0 C. Low-temperature performance may be improved by modifying the electrolyte-active material interface (for example, through electrolyte composition changes). Increasing surface area of active particles (for example, through nanoparticle use) can lower the initial electrode impedance and lead to lower cell impedances at -30 C.

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.

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Lithium-ion cells for hybrid electric vehicle applications must deliver and accept current pulses at relatively high rates and over a range of temperatures. We examined the effect of test temperature from room temperature to 55°C on the performance of three electrode cells (LiNi 0.8 Co 015 Al 0.05 O 2 -based positive electrode, Li 4/3 Ti 5/3 O 4 -based negative electrode, and Li-Sn alloy reference electrode) by galvanostatic cycling and electrochemical impedance spectroscopy (EIS) measurements. Increasing the test temperature reduced cell impedance, which improved its power delivery capability. The higher test temperatures reduced the width of the midfrequency impedance arc in the positive-electrode EIS data but did not affect the low-frequency Warburg impedance portion. We discuss the impedance reductions and capacity gains from the higher temperature measurements in terms of the electrochemical processes occurring at the electrode/electrolyte interfaces.

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.

Magnesium-substituted Li{sub 4-x}Mg{sub x}Ti{sub 5}O{sub 12} spinel electrodes (0