The Effect of State of Charge and Charge Rate on the Formation and growth Deposit Layer on the Anode Electrode of the Lithium Ion Battery
Type of Degreedissertation
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The lithium ion battery technology has for the past two decades received a lot of attention because of its high energy density and excellent cycle life compared to other battery chemistries such as lead acid and Ni-Cd. These attributes of the lithium ion battery have positioned it as the preferred portable energy source for most consumer appliances and for electric/hybrid electric vehicles. However, several reported battery failures during its operation, have raised some safety concerns. These failures of the lithium ion batteries are linked to the degradation of its components: electrodes, current collectors, separator and electrolyte. In particular, the carbon-based anode has been associated with many aging mechanisms. The formation of the solid electrolyte interphase (SEI) layer on the surface of the anode electrode prevents further electrolyte decomposition reaction, however, at certain battery operating parameters the SEI breakdown gives way to more electrolyte solvent and salt decomposition reactions to form several species that are non-uniform and electronically insulating on the anode electrode. The research described in this dissertation focuses on investigating the effect of battery potential and charge rate on the decomposition reaction on the anode electrode of a lithium ion polymer battery. This relationship is important for understanding how charging protocols are related to performance degradation. The investigation showed that at high potential and charge rate the metastable species ROCO2Li within the SEI layer decomposes into more stable compounds –Li2CO3 and LiF. This therefore created a defective SEI structure thereby exposing the graphite surface to more electrolyte decomposition reaction. The overall impedance of batteries increased, particularly the charge transfer resistance. This was ii attributed to deposit layer formed at the electrode/electrolyte interface which affected the lithium intercalation kinetics at the interface. A direct link between the capacity fade during cycling and the progressive deposit layer thickness growth resulting from side reaction at the anode was established. Analysis of the crystal structure of the graphite electrode showed an increasing amount of lithium residing in the graphite sheets as the batteries are aged at higher SOC. The “trapped” lithium in the crystal structure of the graphite led to reduction/isolation of recyclable lithium taking part in the electrochemical process. Cycling the batteries at high charge rate of 4C induce some stresses in the electrode matrix during the intercalation/de-intercalation process that led to loss and isolation of carbon particles from the current collector that could make these particles electrochemically inactive. At high potential, the depletion of the recyclable lithium via trapping of lithium in the crystal structure of the graphite, deposit layer formation, and the partial loss of graphite active materials were predominant regardless of the charge rate and these factors contributed to the high capacity loss of the lithium ion batteries.