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Wyznaczenie parametrów elektrochemicznych nowych elektrod krzemowo-litowych
| dc.abstract.en | Research on lithium-ion batteries began in the 1960s. In 1976, first attempts were made to intercalate the Li + cations into the graphite matrix from the organic electrolyte. In 1980, lithium intercalations with a mixture of molten LiCl-KCl salts were successfully completed. These works have made a significant contribution to the understanding of lithium ion intercalation processes and were responsible for the commercialization of Li-ion cells by Sony in 1991. Since then, the technology of lithium-ion batteries has been constantly developed. This is evidenced by the constantly growing number of published scientific papers. There are many reasons for the fast increase in interest in Li-ion cell technology. One of them is constantly growing public demand for ever smaller and lighter electrochemical energy sources. Li-ion batteries are currently used in almost every area of our life. They can be found in mobile devices (telephones, tablets, notebooks), construction equipment, radio and television, toys, voltage backup systems as well as in hybrid (HEV) and electric (EV) cars. The last mentioned applications constitute the main share in the overall production of these cells. The development of lithium-ion batteries is carried out in a multithreaded manner. At the same time, new anode and cathode materials as well as substances forming the electrolyte are tested. A multiple parameters affecting the final properties of the cell makes choosing the right materials complicated process. The interaction of the individual components of the cell causes difficulties in determining their impact on the operation of the entire battery. When developing new compounds included in the cell, one should not forget about the applicative nature of the research. Synthesized compounds must be characterized by low price, high purity and short preparation time. The synthesis procedure itself must also be capable of transferring to an industrial scale. Lithium-ion cells are currently one of the most expensive electrochemical energy sources. Despite the capacity and specific power exceeding every other electrochemical energy sources, their high price is often a parameter determining the use of older technologies, such as lead-acid batteries. The silicon in the Li-ion cell may form an alloy with a Li3.75Si structure. This allows it to achieve a specific capacity of 3590 mAh/g - the largest of all known anodic conversion materials. The volume change in the following process is 297%. High volume change is the main reason of poor mechanical and electrical stability of siliconbased electrodes. Despite very high specific capacity, none of the commercial cells utilize silicon as the negative electrode active material. High degradation rate is the main reason responsible for that situation. Better understanding of the electrochemical and mechanical processes that occurs during the silicon lithiation is necessary for development of stable and durable silicon-based anodes. The presented thesis is focused on silicon electrodes research. The electrodes worked as a negative electrode in a lithium-ion cell. The research involved two types of electrodes. The first of them contained a pre-commercial silicon alloy material made by 3M Company. I researched these electrodes as part of the SITNBAT project, which brings together a number of foreign scientific and commercial institutions from the lithium-ion cell industry. The second group of electrodes were self-made electrodes containing silicon nanoparticles. The most important goal of my research was to determine a series of electrochemical parameters of silicon electrodes within the mentioned SINTBAT project. The most important parameters determined included the diffusion coefficient of lithium ions in the active material, resistance of the SEI layer, charge transfer resistance, and electron resistance of the electrode mass. I determined the diffusion coefficient of lithium in the silicon alloy using two analytical methods - cyclic voltammetry and electrochemical impedance spectroscopy. The results of both measurement methods used show a good correlation level Changes in the diffusion coefficient determined by the EIS method confirm the single-phase mechanism of lithiation and deletion of the silicon alloy. At the same time, they had proven the creation of so-called dense phase, in the initial stage of silicon lithiation and prove the presence of a very fast, uninterrupted diffusion of lithium ion in the dense phase volume. Calculations of the ion transfer resistance through the SEI passivation layer formed on the silicon electrode showed that the value of this parameter is very high in the first work cycle of the electrode, which implies the need of a low density charging / discharging current use. The recorded SEI resistance value decreased in subsequent measurement cycles because of the reorganization of the layered structure of the passivation layer. The conducted research also proved the dependence of SEI resistance from the level of the active material lithiation. This dependence was attributed to the mechanical degradation and subsequent restoration of the passivation layer due to changes in the specific volume of silicon. The dependence of the charge transfer resistance relative to the lithium level in the active material structure allowed for linking this parameter with the mechanical structure of the electrode. The determined value of the charge transfer resistance increased as the active material was delithiated. This dependence resulted from a reduction in the volume of silicon grains in this process and a deterioration of their electrical contact with the electrode mass. The registered relationships allow for direct observation of the cell mechanical structure degradation by electrochemical impedance spectroscopy analysis. An important achievement during the performed research was the determination in the electroactive surface changes during active material lithiation. The dependence determined by measurement of impedance spectroscopy conducted with imposed, simultaneous DC current flow allowed to track changes in the electroactive electrodes surface, depending on the level of lithiation. The conducted research proved the dependence of changes in the electrode surface from its macrostructure. Thanks to the assessment of the range of surface contraction in the initial stage of lithiation, it became possible to determine the extent of silicon, where the changes in its volume do not cause mechanical degradation of the electrode structure. The presented procedure is the only available solution that, makes possible to determine the changes of the electroactive surface “in-situ” during the cell operation. This may contribute to an increased accuracy in determination of other important electrochemical parameters. During the second part of the experiments I focused on the determination of conditions that improve the commercial-beneficent properties of silicon. The most important parameters affecting the commercial value of the material include specific capacity, cyclic stability and cycle efficiency. In the first stage of the discussed research I determined the dependence of the electrode preparation parameters for the final cell stability. The results of these tests have proven that the high pressure of electrode calendaring improves all of the important parameters discussed. This effect results from the increase of the mechanical stiffness of the electrode structure and the reduction of the reaction front movement speed In the next stage of the work I prepared a modified current collector to improve the cyclic stability of silicon electrodes. The discussed collector was modified by the mechanical or electrochemical real surface area incensement that limits the unfavorable active material delamination during electrode cycling. The developed method of electrochemical modification of the standard current collector surface allows for a significant improvement of the cyclic stability of silicon electrodes. In a further stage of the research, I checked the influence of different types of binder material and the oxidation level of the silicon surface on the electrode stability. The results of these studies have shown that simple chemical treatment of silicon surface oxidation allows for great improvement in its cyclic stability during operation in the lithium-ion cell. I attributed a positive effect to the increase in the number of pseudo-ester bonds between the modified silicon surface and the binder polymer. The developed procedure of electrode mass bonding with the in-situ generated silica gel allowed to improve the cyclic resistance and can completely change the approach to the silicon electrode manufacturing procedure. During the research presented in following dissertation I determined a number of basic electrochemical parameters of silicon electrodes such as diffusion coefficient, charge transfer resistance, SEI layer resistance, electroactive electrode surface and connect these parameters values with active material structure. That enabled a better understanding of the mechanisms of silicon lithiation in the lithium-ion cell. The research discussed in the dissertation successfully combines the basic research as well as application (commercial) research aimed at accelerating the commercialization of silicon as the active material of a negative lithium-ion cell electrode. |
| dc.affiliation.department | Wydział Chemii |
| dc.contributor.author | Ratyński, Maciej |
| dc.date.accessioned | 2019-09-10T11:56:41Z |
| dc.date.available | 2019-09-10T11:56:41Z |
| dc.date.defence | 2019-09-20 |
| dc.date.issued | 2019-09-10 |
| dc.description.additional | Link archiwalny https://depotuw.ceon.pl/handle/item/3493 |
| dc.description.promoter | Czerwiński, Andrzej |
| dc.description.promoter | Hamankiewicz, Bartosz |
| dc.identifier.uri | https://repozytorium.uw.edu.pl//handle/item/3493 |
| dc.language.iso | en |
| dc.rights | ClosedAccess |
| dc.subject.en | surface modification |
| dc.subject.en | charge transfer resistance |
| dc.subject.en | SEI layer |
| dc.subject.en | diffusion coefficient |
| dc.subject.en | electrochemistry |
| dc.subject.en | anode |
| dc.subject.en | graphite |
| dc.subject.en | silicon |
| dc.subject.en | lithium-ion |
| dc.subject.en | Li-ion |
| dc.subject.en | battery |
| dc.subject.pl | modyfikacja powierzchni |
| dc.subject.pl | opór przeniesienia ładunku |
| dc.subject.pl | warstwa SEI |
| dc.subject.pl | współczynnik dyfuzji |
| dc.subject.pl | elektrochemia |
| dc.subject.pl | anoda |
| dc.subject.pl | grafit |
| dc.subject.pl | krzem |
| dc.subject.pl | litowo-jonowe |
| dc.subject.pl | Li-jon |
| dc.subject.pl | ogniwo |
| dc.title | Wyznaczenie parametrów elektrochemicznych nowych elektrod krzemowo-litowych |
| dc.title.alternative | Determination of electrochemical parameters of new silicon-lithium electrodes |
| dc.type | DoctoralThesis |
| dspace.entity.type | Publication |