Kathryn A. Striebel

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.

Electrochemical analysis for cycle performance and capacity fading of a lithium-ion battery cycled at elevated temperature

Laboratory-size LiNi0.8Co0.15Al0.05O2/graphite lithium-ion pouch cells were cycled over 100% DOD at room temperature and 60 8 Ci n order to investigate high-temperature degradation mechanisms of this important technology. Capacity fade for the cell was correlated with that for the individual components, using electrochemical analysis of the electrodes and other diagnostic techniques. The high-temperature cell lost 65% of its initial capacity after 140 cycles at 60 8C compared to only a 4% loss for the cell cycled at room temperature. Cell ohmic impedance increased significantly with a elevated temperature cycling, resulting in some of loss of capacity at the C/2 rate. However, as determined with slow rate testing of the individual electrodes, the anode retained most of its original capacity, while the cathode lost 65%, even when cycled with a fresh source of lithium. Diagnostic evaluation of cell components including X-ray diffraction (XRD), Raman, CSAFM and suggest capacity loss occurs primarily due to a rise in the impedance of the cathode, especially at the end-of-charge. The impedance rise may be caused in part by a loss of the conductive carbon at the surface of the cathode and/or by an organic film on the surface of the cathode that becomes non-ionically conductive at low lithium content. Published by Elsevier Science B.V.

Electrochemical behavior of LiMn{sub 2}O{sub 4} and LiCoO{sub 2} thin films produced with pulsed laser deposition

Thin films of Li{sub x}Mn{sub 2}O{sub 4} and Li{sub x}CoO{sub 2} have been prepared by pulsed laser deposition on heated stainless steel substrates. These films have thicknesses from 0.2 to 1.5 {micro}m and are crystalline without postdeposition annealing. The films` electrochemical properties were studied with cyclic voltammetry, current pulse measurements, and galvanostatic charge/discharge techniques. Film capacity densities as high as 56 and 62 {micro}Ah/cm{sup 2}-{micro}m were measured for Li{sub x}Mn{sub 2}O{sub 4} and Li{sub x}CoO{sub 2}, respectively. Chemical diffusivities on the order of 2.5 {times} 10{sup {minus}11} and 1 {times} 10{sup {minus}10} were measured for Li{sub x}Mn{sub 2}O{sub 4} and Li{sub x}CoO{sub 2}, respectively. Some of the films were cycled electrochemically for up to 300 cycles against lithium metal in 1 M LiClO{sub 4}/propylene carbonate electrolyte, demonstrating the promise of pulsed laser deposition for the production of cathode films for rechargeable lithium microbatteries.

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.

Effect of Carbon Source as Additives in LiFePO4 as Positive Electrode for Lithium-Ion Batteries

The electrochemical properties of LiFePO4 cathodes with different carbon contents were studied to determine the role of carbon as conductive additive. LiFePO4 cathodes containing from 0 to 12% of conductive additive ~carbon black or mixture of carbon black and graphite! were cycled at different C rates. The capacity of the LiFePO4 cathode increased as conductive additive content increased. Carbon increased the utilization of active material and the electrical conductivity of electrode, but decreased volumetric capacity of electrode. This composition ~LiFePO4 with 3 wt % of carbon and 3 wt % of Graphite! is suitable for HEV application.

FTIR characterization of PEO + LiN(CF3SO2)2 electrolytes

Abstract FTIR transmission spectra of thin films of polyethylene oxide (PEO) + lithium trifluoromethanesulfonimide (LiN(CF3SO2)2, LiTFSI) mixtures have been obtained for ethylene oxide Li ratios from 64:1 to 2:1. The phase information from these spectra is compared with the reported phase diagrams based on thermal measurements. The infrared spectrum of LiTFSI is assigned by analogy with those of related compounds. Using the spectral subtraction technique, the effects of Li+ solvation on the PEO matrix and of ion pairing and aggregate formation on the TFSI anion are revealed. An explanation is offered for the variation in ionic conductivity for compositions within the “crystallinity gap”.

Comparison of LiFePO4 from Different Sources

The lithium iron phosphate chemistry is plagued by the poor conductivity and slow lithium diffusion in the solid phase. In order to alleviate these problems, various research groups have adopted different strategies including decreasing the particle sizes, increasing the carbon content, and adding dopants. In this study we obtained LiFePO4 electrodes from six different sources and used a combined model-experimental approach to compare the performance. Samples ranged from one with no carbon coating to one with 15 percent coating. In addition, particle sizes varied by as much as a order of magnitude between samples. The study detailed in this manuscript allows us to provide insight into the relative importance of the conductivity of the samples compared to the particle size, the impact of dopant on performance and ideas for making materials in order to maximize the power capability of this chemistry.

Cyclic voltammetry of pulsed laser deposited Li{sub x}Mn{sub 2}O{sub 4} thin films

Electrochemical properties of thin films of Li{sub x}Mn{sub 2}O{sub 4} spinel prepared by pulsed laser deposition were studied using constant current cycling and cyclic voltammetry. Films have been cycled more than 220 times with no significant capacity fading. The shape of the cyclic voltammogram is very sensitive to the composition and morphology of the film., The diffusion process for the Li{sub x}Mn{sub 2}O{sub 4} thin films with an excess of lithium (x > 1) appears to be slower than for lithium-deficient films. Films were subjected to overcharge (5 V vs Li/Li{sup +}) and overdischarge (2 V vs Li/Li{sup +}). Overcharge does not significantly affect the structure of the film. Overdischarge leads to changes in the shape of the cyclic voltammogram: (1) loss of resolution of the two oxidation peaks at 4.1 and 4.2 V which are the signatures of the spinel structure, and (2) loss of capacity. These changes in the electrochemical behavior may be correlated to the structural disorder associated with the phase transition when more than one lithium is intercalated in LiMn{sub 2}O{sub 4}. It is a reversible phenomenon.

Impedance studies of the thin film LiMn2O4/electrolyte interface

Room-temperature impedance measurements of a thin-film LiMn2O4/LiPF6-EC-DMC interface have been used to identify the spontaneous formation Li2Mn2O4 at the interface at room temperature at voltages of 3.7 and higher. The impedance of the LiMn2O4 films exhibited two time constants: at about 14 kHz and 60 to 200 Hz. The high frequency loop is dependent on film morphology and was attributed to the substrate/oxide interface. The low frequency behavior was dependent on both state-of-charge (SOC) and time at a given SOC. At full charge the impedance in this electrolyte was stable at room temperature over several days. At high lithium contents, film OCV and impedance tended to grow logarithmically with time, with lower rates for lower Mn3+ content in the film. The increased impedance was removed by oxidation of the film to 4.5V vs. Li/Li+. The observations are consistent with a reversible disproportionation of part of the LiMn2O4 into Li2Mn2O4 and a lithium-deficient spinel. With extended constant current cycling part of the Li2Mn2O4 degrades to the Mn2O3 and the process is no longer reversible.

FTIR Spectroscopy of Metal Oxide Insertion Electrodes A New Diagnostic Tool for Analysis of Capacity Fading in Secondary Cells

Fourier transform infrared spectroscopy (FTIR) has been applied to the study of oxide insertion compounds used in rechargeable lithium batteries. The mechanisms responsible for capacity fading during normal cycling of LiMn 2 O 4 cells in both the 3 and 4 V regions were determined by examination of spectra obtained from electrodes following 25 cycles at charge and discharge rates of C/6. In the 3 V region, electroactive material becomes electronically disconnected from the rest of the electrode due to fracture of the oxide particles and/or expansion and contraction of the active material during the cubic-to-tetragonal phase transformation. In the upper voltage region, the active electrode material is gradually converted to a lower voltage defect spinel phase via dissolution of manganese in the electrolyte.