Understanding two-phase reaction processes in electrodes for Li-ion batteries

Abstract

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 LiFePO$_{4}$ 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 TiO$_{2}$ and LiVPO$_{4}$F.

In this dissertation, LiFePO$_{4}$ 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 LiFePO$_{4}$ was examined via a cation exchange process between $^{6}$Li and $^{7}$Li 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 LiFePO$_{4}$ nanoparticles was investigated by $\textit{in situ}$ synchrotron X-ray powder diffraction (XRD). At high cycling rates, the transition between LiFePO$_{4}$ ($\textit{Pnma}$) and FePO$_{4}$ ($\textit{Pnma}$) 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 LiFePO$_{4}$.

To explore the availability of the non-equilibrium continuous phase transition path in other materials, the (de)lithiation processes of anatase TiO$_{2}$ and LiVPO$_{4}$ at high cycling rates were also examined with $\textit{in situ}$ synchrotron XRD. Phase separation was found to occur, even at high rates, for transitions TiO$_{2}$ ($\textit{I41/amd}$)$\rightarrow$ Li$_{0.5}$TiO$_{2}$ ($\textit{Imma}$) and Li$_{0.67}$VPO$_{4}$ ($\textit{P-1}$) $\rightarrow$ VPO$_{4}$ ($\textit{C2/c}$), where the two end member phases adopt different, albeit group-subgroup related, symmetries. As with LiFePO$_{4}$, a continuous phase transition was observed during the high rate cycling of LiVPO$_{4}$F ($\textit{P-1}$) $\rightarrow$ Li$_{0.67}$VPO$_{4}$F ($\textit{P-1}$), where both phases adopt the same symmetry.