Electrochemical Studies on Degradation Mechanisms of Electrode Materials in Lithium-Ion Batteries

Li-ion batteries are the primary power source in small consumer electronics, and they are becoming common in larger systems, such as (hybrid) electric vehicles and stationary energy storage, as well. In the large-scale applications, especially the safety and reliability requirements are strict, and the gradual degradation of Li-ion cell materials over time and use should be prevented. The aim of this thesis was to study the aging of Li-ion battery electrode materials and to investigate different strategies to improve their stabilities. The aging mechanisms in commercial graphite/LiFePO4 battery cells were studied at different temperatures. A post-mortem analysis of the aged cells revealed that the cell performance decline originated from side reactions at the graphite negative electrode. Furthermore, cycling at low temperatures was found to promote surface and intergranular cracking of the graphite particles, which likely originated from mechanical stress caused by large Li concentration gradients within the particles. In addition to the graphite negative electrode, the positive electrode materials can be a major source of cell aging. While commercial LiFePO4 was shown to be stable towards prolonged cycling, the most commonly used transition metal oxides (LiMO2, M=Co, Ni, Mn) are known to degrade especially at high potentials. In this work, small amounts of Mg and Ti co-doping (<1 mol-%) were shown to improve the electrochemical performance and to increase the durability of LiCoO2 by suppressing the impedance increase during cycling. On the other hand, Li overstoichiometry was found to be detrimental to the performance of LiCoO2 when cycled at relatively low potentials. To replace the expensive and often toxic transition metal oxides, organic electrode materials such as conducting polymers have been studied extensively, but a poor cycling stability restricts their use in commercial applications. In the present work, the addition of carbon nanotubes as a support for a common conducting polymer, polyaniline, was shown to improve its cycling stability. The carbon nanotube support inhibited mechanical changes of the electrode, and suppressed the deterioration of the electrical properties of the composite electrodes during cycling. Cell design also affects the aging of Li-ion batteries. Side reactions related to the faradaic Li-intercalation reaction are a major source of aging in Li-ion cells. By replacing one of the Li-intercalation electrodes with a capacitor-type electrode showing a purely capacitive charge-storage mechanism, the aging rate can be reduced. In this work, the preparation of this type of Li-ion capacitor cells was considered, and the electrode balancing, pre-lithiation of the intercalation electrode and different material combinations were considered.