Kyung Yoon Chung

Investigation of structural fatigue in spinel electrodes using in situ laser probe beam deflection technique

The presence of the tetragonal phase at the surface of a LiMn 2 O 4 particle due to a Jahn-Teller effect was previously reported to be one of the reasons for capacity fade observed during cycling of Li//Li x Mn 2 O 4 in the 4 V range. This study reports on the evidence of the onset of a Jahn-Teller effect above 3 V at the surface of LiMn 2 O 4 particle of thin-film electrodes using in situ laser probe beam deflection technique. The thin film electrode was prepared by electrostatic spray deposition technique. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously, and the potential range responsible for the Jahn-Teller distortion in the 4 V range was determined by differentiating the deflectograms against time. We observed three sets of peaks in both the cathodic and anodic scan of the differential cyclic deflectograms and the magnitudes of these are changed with the scan rate. In the absence of Jahn-Teller distortion, the shape of the differential deflectograms would be identical to that of cyclic voltammograms Analysis of the differential strain curves obtained from the cyclic deflectograms and the cyclic voltammograms indicates that one of the cathodic peaks of the differential cyclic deflectograms is due to the onset of the Jahn-Teller effect and one of the anodic peaks is due to the relaxation of the tetragonal phase, formed in the previous cathodic scan due to the Jahn-Teller effect, to cubic phase. The Jahn-Teller effect during the cathodic scan is observed to occur around 3.95 ∼ 4.0 V depending on the scan rates, i.e., at slower scan rates the cathodic peak of the differential strain curves became smaller. This could be ascribed to the suppression of the Jahn-Teller effect at near equilibrium conditions.

Onset Mechanism of Jahn-Teller Distortion in 4 V LiMn2O4 and Its Suppression by LiM0.05Mn1.95O4 (M = Co , Ni) Coating

There have been several causes for the capacity fading of LiMn 2 O 4 reported by many authors, which include Mn dissolution into the electrolyte, and the onset of a Jahn-Teller distortion. One of the main remedies suggestedto combat those causes has been doping with other transition-metal ions. However, most of the deterioration reactions take place on the LiMn 2 O 4 surface. In this study, LiMn 2 O 4 was coated with a doped-spinel oxide to combat the causes of capacity fading in LiMn 2 O 4 , which modifies only the surface of the electrodes, and its effect was diagnosed using the bending beam method (BBM). The doped-spinel coated LiMn 2 O 4 showed an excellent cycle performance. From the BBM data, it was confirmed that this excellent performance was largely due to the suppression of the Jahn-Teller distortion, which takes place at the end of the cathodic sweep in the 4 V range. Furthermore, the mechanism for the onset and suppression of Jahn-Teller distortion in 4 V LiMn 2 O 4 is proposed based on the strain variations of the stoichiometric, doped, and doped-spinel coated spinel electrodes.

Electronic Structure of the Electrochemically Delithiated Li1-xFePO4 Electrodes Investigated by P K-edge X-ray Absorption Spectroscopy

P K-edge X-ray absorption spectroscopy (XAS) has been carried out to investigate the electronic structure of the electrochemically delithiated Li{sub 1-x}FePO{sub 4} for Li rechargeable batteries. The gradual shift of main edge features to higher energy side showed that P-O bonds become less covalent during delithiation due to the more covalent Fe{sup 3+}-O bonds via the inductive effect. A principal component analysis of P K-edge XAS spectra of the electrochemically delithiated Li{sub 1-x}FePO{sub 4} reveals that this set of spectra can be well represented by two primary components, in good agreement with a first-order phase transition involving the LiFePO{sub 4} and FePO{sub 4} phases. From the observation of pre-edge peaks, it is concluded that electrochemical delithiation of Li{sub 1-x}FePO{sub 4} results in the hybridization of P 3p states with the Fe 3d states.

Suppression of Structural Fatigue by Doping in Spinel Electrode Probed by In Situ Bending Beam Method

dResearch Center for Energy Conversion and Storage, Seoul 151-744, Korea The onset of a Jahn-Teller effect at the surface of LiMn2O4 particles during cycling in the 4 V range was previously reported to be one of the causes for the capacity fading. Furthermore, it has been reported that the Jahn-Teller effect in the 4 V range may be suppressed by the substitution of the Mn ions by either Li or other transition metal ions. However, no direct evidence has yet been reported. This study provides evidence for the onset of a Jahn-Teller effect in thin film 4 V LiMn2O4 and its suppression caused by substituting the Mn ions with Co 31 and Ni 21 ions using in situ bending beam method ~BBM!. The deflectograms are measured simultaneously with galvanostatic charge/discharge or cyclic voltammograms, and the onset of the Jahn-Teller effect is investigated by means of the differential strain peak which is observed at around 3.90-3.95 V during cyclic voltammetry, and the slope variation observed in the strain curves during galvanostatic charge/discharge. The suppression of the Jahn-Teller effect in the doped spinel leads to the magnitude of the differential strain peak resulting from the Jahn-Teller effect being reduced in comparison with the other two pairs of peaks, which correspond to the current peaks of the cyclic voltammogram.

Nickel-metal hydride and lithium-ion batteries for hev applications: performance, limitations and recommendations for improvement

Nickel-metal hydride and lithium-ion batteries are the two contenders for hybrid electric vehicle HEV applications. Early entrants such as the Honda Insight and the Toyota Prius use nickel-metal hydride batteries. Lithium-ion batteries with either modified LiNiO2 or LiMn2O4 spinel cathodes are being considered for the next generation HEV. The main advantage of metal hydride batteries is that there is a built in electrochemical mechanism for balancing cells in series and accommodating overcharge. Other advantages are safety and ease of scale-up to large sizes. The major drawback is limited cycle life due to pulverization and corrosion of the metal hydride electrodes. Lithium-ion batteries with the required power for HEV can be designed and built. However, the batteries loose power capability, particularly when cycled or stored at elevated temperatures. This appears to be mostly due to the buildup of electrolyte decomposition products on the surface of the cathodes.