A Study on Phase Transition Mechanism of Olivine LiFePO4 Particles in Composite Electrode

Abstract

Olivine-type bulk LiFePO4 has been recognized as one of the most promising cathode materials for rechargeable Li batteries. Its advantages include high capacity, high stability, nontoxicity, and low cost, except high power capability. Applications of lithium-ion batteries, including automotive applications, require fast kinetics and high conductivity of ions and electrons. Unfortunately, LiFePO4 has the electronic structure of an insulator, an entirely unsatisfactory situation if it is to be used as a battery electrode. However, nanosized LiFePO4 particles easily show a fast electrochemical response that can be achieved via a non-equilibrium pathway. To understand this intriguing phase transition behavior in nanosized LiFePO4 particles, the metastable solid-solution phase was prepared by thermal treatment with a chemically delithiated nanosized Li0.5FePO4 sample. The phase separation behavior of the metastable solid-solution sample (Li0.5FePO4) was investigated under various kinetic conditions to understand critical factors affecting the phase separation behavior of nanosized LiFePO4 particles. And to understand how a phase transformation pathway affects the electrochemical properties. We prepared the solid-solution sample that experience the solid-solution phase and then may undergo solid-solution phase transformation, and the two-phase sample that do not experience and then can undergo typical two-phase transformation. And their electrochemical characteristics were compared under various conditions. Multi-particle electrode of LiFePO4 was designed to extend the scope of investigation, we explored the delithiation behaviors in multi-particle electrode with different particle sizes. For this purpose, we developed a simple centrifugation method to separate particles in a chemically-delithiated Li0.49FePO4 with respect to their size. And the small particles got more FePO4 phase than large particle at the middle of delithiation. The inhomogeneous delithiation of particles could be also observed in the electrode. Finally, we proposed bimodal particle LiFePO4 by mixing small and large particles for the clear understanding of size dependent delithiation. The small particles react first on mixed particle LiFePO4 at the middle of delithiation, even though high over-potential and high temperature was applied during delithiation. The results of chemical and electrochemical delithiation showed inhomogeneous and sequential delithiation. We can achieve high power characteristic of LiFePO4 using kinetically activated bulk LiFePO4 particles. These are important in practice, because nanosized LiFePO4 is showing excellent electrochemical performance, but it is too expensive for commercialization. In this thesis, the main finding is as follows: The metastable solid-solution phase was quite stable and showed nonspontaneous reaction. Phase separation in nanosized particles occurs between particles, and the surface charge transfer reaction may be an important factor affecting the phase separation. The fast electrochemical response in LiFePO4 nanoparticles can be ascribed to different phase transformation pathways such as the solid-solution one. Intrinsic particle size distribution of multi-particle LiFePO4 electrode imposes inhomogeneous delithiation of particles even with high over-potential and high temperature. That inhomogeneous delithiation may lead to a sequential phase transformation during charging. The bimodal particle LiFePO4 strategy to improve thermodynamic properties reveals high power characteristic of bulk particles.