Applications Of Electron Spin Resonance And Mass Spectroscopy To The Study Of Batteries

Batteries are playing an increasingly important role in modern society. In the recent two decades, the research and development of high quality batteries has been driven harder than ever by social demands, especially the requirements of electrical vehicles and mobile electronic appliances. The effective approach of research and development of batteries is a combination of technological works with mechanism studies. The understanding of the structure-property relationship of battery materials and some fundamental processes occurring in batteries needs a variety of experimental techniques among which various spectroscopic techniques have produced unique and invaluable information. As a part of the efforts of the author's group in developing new spectroscopic techniques for electrochemistry, this thesis project was focused on the application of electron spin resonance (ESR) and mass spectroscopy (MS) to battery studies. Based on the previous works of the group, the ESR study of the thesis included improvement of data processing method and ESR studies of typical carbonaceous and non-carbonaceous materials for the negative electrode of lithium ion batteries. The MS work was preliminary, aiming at establishing a non-invasive technique to monitor the changes of gas phase inside a secondary battery during charge and discharge. The main achievements of the thesis are as follows.1, Improvement of data processingThe ESR spectra of the materials for the negative electrode of lithium ion batteries often consist of two or even three component signals and are further complicated by the so-called Dysonian line shape which is caused by incomplete penetration of microwave in the sample. Kramers-Kronig transformation was adopted to process Dysonian and partially Dysonian ESR spectra to obtain more precise g-value and secondary integration of the signal. Based on this approach, the complicated ESR spectra were successively simulated and relevant ESR parameterswere extracted.2, Deducing the density of electronic states at the Fermi level D(EF) for lithiated carbonsAccording to the theory of paramagnetism of conducting electrons the D(Ep) function was deduced from in-situ ESR measurements during the discharge of lithiated carbon, including a synthetic graphite, two different MCMB's and a natural graphite. The results were compared favorably with relevant theoretical and experimental curves for carbons reported by different researchers based on different approaches.3, Separating lithium intercalation capacity due to the band model mechanism from the total capacity of carbonsThe D(Ep) function deduced from ESR measurement was further processed to generate the discharge capacity due to the band model mechanism of intercalation. This capacity is called the Pauli-site-related capacity. The directly measured discharge curve was than decomposed into Pauli and non-Pauli components. It was revealed that the carbons containing more ordered microstructures (such as graphite) would show more Pauli capacity which is characterized by a discharge potential plateau below 0.25 V (vs. Li).4, ESR studies of lithiated SiA Lorentzian type ESR signal was found to increase rapidly with the degree of lithiation for the negative electrode composed of Si powder. This ESR signal showed an intensity independent of the temperature of ESR measurement and was attributed to the Pauli spins of conducting electrons. The D(EF) for lithiated Si was deduced the same way as for lithiated carbons and turned out to be two to three orders of magnitude smaller than that for carbons. Therefore, the major capacity of Si was describable by the density of non-Pauli states (Li-Si alloy) which was found to basically follow the Nernst equation.5, ESR studies of lithiated nano SnOPristine SnO showed no detectable ESR signal while lithiated SnO showed an ESR signal the intensity of which remained essentially unchanged during discharge (de-lithiation). The ESR intensity showed little temperature dependence and was assigned to the conducting electron in nano Sn which had been produced during cathodic lithiation of SnO but not lithiated owing to poor electronic contact. Therefore, the lithium storage of SnO was totally due to Li-Sn alloy formation and had nothing to do directly with the ESR signal.6, On-line MS detection of the gas leaked from small commercial batteriesAn experimental setup was established to monitor the changes of gas phase inside an alkaline battery during charge and discharge by detecting the gas leaked from the battery. An analysis was given for the characteristic time of the mass transport in the sampling system, pointing out the importance of sampling valve resistance and the volume of the gas chamber. The change of hydrogen partial pressure in Ni-MH batteries and the change of oxygen partial pressure in Ni-Cd batteries were successively sensed during charge and discharge processes, demonstrating the feasibility of the new technique. The new method appears promising as a non-invasive diagnostic technique for commercial small batteries. The problems of slow response and selective leakage should be resolved in the future and be taken into account in data interpretation.