Modeling Particle Size Effects on Phase Stability and Transition Pathways in Nanosized Olivine Cathode Particles
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1100-JJ03-04
Modeling Particle Size Effects on Phase Stability and Transition Pathways in Nanosized Olivine Cathode Particles Ming Tang, Hsiao-Ying Huang, Nonglak Meethong, Yu-Hua Kao, W. Craig Carter, and YetMing Chiang Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139 ABSTRACT Recent experiments show that nanosized olivine LiFePO4 has different phase transition and solubility behavior than that of larger cathode particles. The possibility of metastable or globally stable amorphous phase in nanosized LiFePO4 particles during delithiation is considered in a diffuse-interface model. At a small enough particle size, a lithiated crystalline phase can undergo amorphization upon charging instead of transforming directly to the delithiated crystalline phase at nanoscale particle sizes. INTRODUCTION Olivine lithium transition metal phosphates are a promising class of cathode materials for future rechargeable batteries. Performance such as rate capability is greatly improved in nanoscale materials [1]. It has been shown that the phase transition behavior of the pure olivine LiFePO4 changes significantly with shrinking particle size. For example, there is a much reduced miscibility gap between FePO4 and LiFePO4 in nanosized particles compared to conventional particle sized cathodes [2]. Furthermore, several experimental observations indicate that amorphous phase may be present during the phase transformation in olivine cathodes upon delithiation [3], though amorphization may not appear at larger particle sizes. We hypothesize that partial or total amorphization results from reduced total surface energy. Amorphous/ liquid structures usually have lower specific surface energies than their crystalline counterparts which have lower volumetric free energies at low temperatures. This hypothesis has been verified for surface melting in metals and in ice [4](where a liquid-like film readily forms at surface below the bulk melting point), and for the formation of nanometer-thick solid amorphous films on crystalline oxide surfaces [5]. Although surface energy data are not available for amorphous FePO4, we estimate that the surface energy difference is about 0.3-1J/m2 between amorphous and crystalline phases, which is the value of its structural analog, SiO2 [6]. The total free energy of a particle includes both volumetric and surface parts. Because the surface-to-volume ratio increases with decreasing particle size, an amorphous phase that is metastable in bulk may become the thermodynamically stable structure below a critical particle size. Such phase conversion has been confirmed in nanocrystalline zirconia [7], where experiments show a phase-stability crossover from the bulk-stable monoclinic phase to meta-stable tetragonal and ultimately to an amorphous phase with increasing specific particle surface area (i.e., decreasing particle size). Because iron phosphate is a good glass former, the volumetric free energy difference between crystalline and amorphous FePO4 could be r
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