Studies On Performance Decay Of Lithium Ion Cells And Electrode Interfaces

Development of new and renewable energy, as well as new energy vehicles is an efficient solution to the problems of energy sustainable utilization and environment pollution. In order to store and utilize these new energies in large-scale, batteries with high performance are required. Among several types of secondary batteries, Li-ion battery is concerned firstly owe to its favorable properties of high working voltage, high power and energy density and no memory effect. However, Li-ion battery suffers performance decay and even safety problem during the period of the initial charge, storage and cycle, which will shorten battery life. Consequently, it would lead to increase battery cost and impede its wide application, especially in the transportation field.In this thesis, storage behaviors of one type of Li-ion cells based on LiNi1/3Mn1/3Co1/3O2/LiPF6-EC:DEC:PC/artificial graphite under various storage condition were investigated using impedance spectroscopy (EIS), XRD, ICP, FTIR, SEM, EDS, DSC, etc. With aims of these analysis tools, effects of storage conditions on chemical and physical properties of electrodes and separators were studied. It was found that activation energy of cell capacity loss was in the range of28.8-35.8kJ/mol and that there was linear relationship between capacity loss and square root of storage time. Low temperature impedes the interfacial side reactions, capacity loss and growth of impedance of a cell. The capacity loss of an aged cell after storage for28weeks was only4.1%at0℃,39.4%lower than60℃. Gas and EIS analysis results showed that the interfacial reaction on graphite dominated the side reactions of the aged cells. The main gases including CO、CH4、C2H6detected in a swelling aged cell are caused by the reduction of electrolyte at graphite interface. High temperature does not alter structure of graphite, but surface morphology of graphite and composition of solid electrolyte interphase (SEI) film, which leads to increase the decompose temperature of SEI film. The effects of state of charge (SOC) and particle size of graphite on storage performance of Li-ion cells at elevated temperature were also studied.Changes of potentials and impedance of electrodes were in-situ monitored with the aid of reference electrode in3-electrode cells during storage. It was observed that the interfacial impedance of graphite electrode decreased firstly and then increased with the prolonged storage for both of LiNi1/3Mn1/3Co1/3O2/artificial graphite and LiCoO2/mixed graphite cells, indicating that the competition between the damage and growth of SEI film occurs during storage. Yet, the interfacial impedance of positive electrode increased with the storage. During storage, the potentials of negative electrode gradually increased due to the de-lithiation of LiC6, whereas the negative electrode gradually dropped due to Li+re-intercalation into LixNi1/3Mn1/3Co1/3or LixCoO2. It was also found that the impedance changes of both electrodes during cycle were similar with high-temperature storage.Mechanisms of reversible and irreversible capacity loss of Li-ion cells have been discussed. Experiment results showed that most of reversible loss of capacity for an aged cell after storage could be regained after recharging. There are two main sources to regain reversibly lost capacity or lithium ions during storage. Some reversibly lost lithium ions react with electrolyte solvent to become metastable lithium alkyl complex (as a part of SEI film), which can be cyclable again after recharging. On the other hand, some lithium ions of SEI film release in electrolyte due to the dissolution of SEI film and re-intercalation into LixNi1/3Mn1/3Co1/3or LixCoO2crystal driven by Li+concentration gradient. The irreversible capacity loss of Li-ion cells during cycle was also discussed.The effects of initial charge conditions on SEI film properties and electrochemical performance of Li-ion cells have been discussed. Some metastable lithium complex can be transformed to the stable one by increasing formation temperature, which can enhance the chemical stability of SEI film. The irreversible capacity loss of an aged cell after storage at60℃for10weeks or after300cycles (at～1C) at room temperature reduced ca.8%after increase the formation temperature from25℃to60℃. However, under the same temperature, the formation current density within the range of0.044-1.077mA/cm2hardly impacted on the performance of Li-ion cell.As for LiCoO2/LiPF6-EC:DEC:EMC/Mixed graphite Li-ion cell, the amount of lost active Li+during cell storage differs among various types of graphite particles due to their difference of crystal structure and specific surface area, resulting in formation of a concentration cell of (Li1-xC6)/(Li1-x-yC6)(x≠y) in the mixed graphite electrode. The concentration cell can induce inductive phenomena in impedance spectra of the mixed graphite electrode at lower frequency range. It was shown that in the fully charged celll, the activation of lithium ion transfer at graphite/electrolyte interface was64kJ/mol,24kJ/mol more than that of lithium ion transfer at LiCoO2/electrolyte interface. Because the graphite electrode dominates interfacial charge transfer resistance and its activation energy, lithium trends to be plating on the graphite, causing irreversible capacity loss for a cell during a charge process at low temperatures. By reducing both of them, it will be beneficial in improving the low-temperature and the high rate cycle performance of Li-ion cell.