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Title: Understanding two-phase reaction processes in electrodes for Li-ion batteries
Author: Liu, Hao
ISNI:       0000 0004 6057 3265
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2015
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The occurrence of a phase separation, which induces substantial structural rearrangements and large volume changes, is generally considered to limit the high rate application of any battery electrode material. Contrary to this perception, nanoparticulate LiFePO₄ exhibits exceptionally high rates although the large Li miscibility gap in the Li-composition phase diagram dictates that delithiation takes place by a kinetically limited nucleation and growth process. It remains controversial as to whether the delithiation process is fundamentally different than expected from thermodynamics. This dissertation is set out to resolve this controversy and explore the implications in the (de)lithiation process of other phase separating electrode materials, such as TiO₂ and LiVPO₄F. In this dissertation, LiFePO₄ is treated as the model compound that exemplifies the issues of Li diffusion and phase transitions in phase separating electrode, where a second phase is formed upon Li extraction/insertion. Li diffusion in LiFePO₄ was examined via a cation exchange process between ⁶Li and ⁷Li ions. The results indicate a single-file diffusion for Li along the diffusion channel, yet the Li diffusion was found to be rapid enough to allow for fast delithiation. The phase transition process of LiFePO₄ nanoparticles was investigated by in situ synchrotron X-ray powder diffraction (XRD). At high cycling rates, the transition between LiFePO₄ (Pnma) and FePO₄ (Pnma) was found to proceed continuously via metastable solid solution phases, instead of a phase separation. Phase transition through this facile non-equilibrium path is thought to be essential in realising the high rate capability of nanoparticulate LiFePO₄. To explore the availability of the non-equilibrium continuous phase transition path in other materials, the (de)lithiation processes of anatase TiO₂ and LiVPO₄ at high cycling rates were also examined with in situ synchrotron XRD. Phase separation was found to occur, even at high rates, for transitions TiO₂ (I41/amd) → Li₀.₅TiO₂ (Imma) and Li₀.₆₇VPO₄ (P-1) → VPO₄ (C2/c), where the two end member phases adopt different, albeit group-subgroup related, symmetries. As with LiFePO₄, a continuous phase transition was observed during the high rate cycling of LiVPO₄F (P-1) → Li₀.₆₇VPO₄F (P-1), where both phases adopt the same symmetry.
Supervisor: Grey, Clare Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Li ions ; Li ion batteries ; Electrodes ; Delithiation ; Two-phase reaction processes