Electrochemically-induced phase transition in olivine type cathode materials
Author(s)Xiang, Kai, Ph. D. Massachusetts Institute of Technology
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
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Phase transitions are commonly observed in ion storage compounds when being used in rechargeable batteries and thus, the phase behavior of ion storage compounds as electrode active materials has significant impact on battery performance. This thesis aims to understand the interplay between materials structure, phase behavior and battery performance. The effects of operating conditions, especially overpotential and temperature, on phase behavior and battery performance are also investigated. Using olivine-type phosphates (i.e. phospho-olivines) with varying composition and particle size as model system, strain accommodation mechanism within single nanoparticles (Chapter 2 to 3) and mesoscale kinetics of nanoparticle aggregates (Chapter 4 to 5) during electrochemically-induced phase transition have been systematically investigated. In the first part, phospho-olivines with varying transformation strain, from 0 - 3vol% for LiMnyFe1-yPO4 (LMFP, y<0.5), 5vol% for LiFePO4 (LFP), to 17vol% for NaFePO4 (NFP), have been studied using operando Powder X-ray Diffraction (PXD), among other methods. While small transformation strain as in LMFP is accommodated and even avoided by formation of metastable solid solution, large transformation strain as in NFP is mitigated by formation and dissolution of intermediate amorphous phase. This novel mechanism to accommodate large transformation strain may pave the way of utilizing battery materials that deem not working otherwise. In the second part, potentiostatic studies are conducted and a model modified from Avrami model is developed to quantitatively describe phase transformation progresses. The phase transition of LMFP and LFP nanoparticle aggregates is found to follow a nucleation and growth process while the growth is governed by lithium ion diffusion. Based on analysis using the modified Avrami model, more instantaneous nucleation and facile growth tend to occur when transformation strain is small (intermediate Mn content and/or small particle size), overpotential is high and/or temperature is high. And instantaneous nucleation and facile growth improve the rate capability of batteries. The relationship between phase behavior and material structure as well as operating conditions is attributed to: 1) decreasing transformation strain reduces energy barrier for both nucleation and growth; 2) increasing overpotential and temperature boost the electrochemical driving force for phase transition and promote more instantaneous nucleation and facile growth.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 159-165).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering.; Massachusetts Institute of Technology. Department of Materials Science and Engineering
Massachusetts Institute of Technology
Materials Science and Engineering.