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Title: The effect of temperature on phase transformation mechanisms in electrodes for Li-ion batteries
Author: Meng, Wei
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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The effect of elevated temperatures on the phase transformation mechanisms in electrodes for lithium-ion batteries (LIBs) is an important but – to date – only less studied subject in battery research. In real-life applications, LIBs usually function at non-ambient conditions and especially increased temperatures give rise to safety concerns. This thesis focuses to gain deeper insights into the phase transformations at high temperatures (HTs) by tackling both the challenging hardware development of a HT in situ synchrotron X-ray diffraction (XRD) battery testing system as well as its application to study two important cathode materials: LiFePO4 and V6O13. This allows unprecedented insights into the structural changes and its influence on electrochemical performance at variable temperatures (VTs). LiFePO4 was investigated for various battery cycling rates and temperatures. Electrochemical cycling of LiFePO4 in the newly designed in situ XRD setup proved that the in situ XRD cells work from low to high cycling rates between 25 to 150oC. The current induced non-equilibrium solid solution metastable LiFePO4 phase, present at room temperature during high rate cycling, was found to be less pronounced at temperatures above 125oC. This is possibly due to faster Li-ion diffusion at HT, leading to faster phase separations in the solid solution phases. In a next step, V6O13, a promising cathode material for HT applications, especially for oil field applications, was tested using the in situ HT XRD setup. The material exhibits a very high capacity with a complex voltage profile. The underlying asymmetric discharge and charge phase transition mechanisms, which include a six-step discharge and five-step charge process, are unravelled by in situ XRD. The LixV6O13 unit cell expands sequentially in c, b, and a directions during discharge and reversibly contracts back during charge. The process is associated with a change of occupied lithium sites as well as charge ordering in LixV6O13. Density functional theory (DFT) calculations and nuclear magnetic resonance spectroscopy gave further insight into the electronic structures and preferred Li positions in the different structures formed upon cycling, particularly at high lithium contents. At HT, V6O13 exhibits an even greater capacity, as well as a more symmetric discharge and charge profile. Combining the results from the HT in situ XRD study and the DFT calculation, the most Li puckered phase was found to be able to open further along the b axis, with a new Li site getting (partially) occupied. The new Li site corresponds to more Li intercalation into the LixV6O13 structure and, therefore, more electrode charge storage capacity. The more symmetric discharge and charge process was attributed to the disappearance of phase 2 (present at room temperature for 1.7 < x ≤ 2.1 in LixV6O13) at HT.
Supervisor: Grey, Clare Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Battery ; Electrode ; Temperature ; Materials