Thermally driven metastable solid-solution Li_0.5FePO_4 in nanosized particles and its phase separation behaviors

摘要

Nanosized LiFePO_4 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 LiFePO_4 particles, the metastable solid-solution phase was prepared by thermal treatment with a chemically delithiated nanosized Li_0.5FePO_4 sample. Thermal treatment makes all the nanosized particles transform easily to the metastable solid-solution phase because of the large thermal energy while an electrochemical reaction does not. The phase separation behavior of the metastable solid-solution sample (Li_0.5FePO_4) was investigated under various kinetic conditions to understand critical factors affecting the phase separation behavior of nanosized LiFePO_4 particles. The main findings in this study are as follows. The first finding is that the depressed phase separation behavior of the metastable phase may originate from the nanoparticle effect, in which the formation of a second phase inside a nanosized particle is not energetically favored because of the large interfacial energy. Therefore, phase separation in nanosized particles occurs between particles rather than inside a particle. If there was no over-potential, such as in the relaxed pellet experiment or in the relaxed electrode experiment in the electrolyte, the metastable phase was quite stable showing no phase separation behavior even though efficient pathways for lithium ions and electrons were well developed. The second finding is that the phase separation behavior of the metastable phase actually depends on the over-potential. Under open circuit voltage (OCV) conditions, the metastable phase started to exhibit a slight structural change during a long relaxation time, about ten days. The slow change of the metastable phase may be due to the low driving force, less than 10 mV, which comes from the energetic difference between the two-phase state and the metastable phase. This indicates that the phase separation behavior may require a large over-potential. When a large over-potential was applied using an external current, phase separation of the metastable phase was achieved, indicating that the phase separation behavior may be related to activation processes. Furthermore, the requirement for a large over-potential indirectly shows that the spinodal decomposition is depressed in nanosized particles. Considering that phase separation in nanosized particles occurs between particles, the surface charge transfer reaction can be a limited reaction for achieving phase separation because it is an activated process and governed by the over-potential. Considering the understanding obtained from the phase separation behavior of the metastable phase, the phase transition behavior of nanosized LiFePO_4 particles during charging/discharging can proceed via the metastable phase because there is no spinodal decomposition behavior in nanosized particles and the metastable phase is quite stable.