Electrochemically driven phase transformation in energy storage compounds

Nanoscale lithium transition metal phosphate olivines have become commercially important as positive electrode materials in a new generation of lithium-ion batteries. Not surprisingly, many energy storage compounds undergo phase transitions in-situ, including the production of metastable phases. Unique to this environment is the frequent application of electrical over- and underpotentials, which are the electrical analogs to undercooling and superheating. So far, overpotential effects on phase stability and transformation mechanisms have not been studied in detail. Here we use synchrotron X-ray diffraction performed in-situ during potentiostatic and galvanostatic cycling, combined with phase-field modeling, to reveal a remarkable dependence of phase transition pathways on overpotential in the model olivine Lii. ,FePO 4. For a sample of particle size -113 nm, at both low (e.g., <20 mV) and high (e.g., >75 mV) overpotentials, a crystal-to-crystal olivine transformation is preferred, whereas at intermediate overpotentials a crystalline-to-amorphous phase transition dominates. As particle sizes decrease to the nanoscale, amorphization is further emphasized. Moreover, in the LiipxFei. yMnyPO 4 (y=0. 1, 0.4) system, the phase transition behavior is ovepotential dependent, and the crystallization of the amorphous phase is overpotential driven. An extensive nonequilibrium solid solution has been observed upon galvanostatic discharge. The misfit strain between two end members determines the reaction type as well as the phase transformation rate. High rate capability is expected in Lii.xFe1.yMnyPO 4 when the misfit is adequately tuned by Mn content. In addition, the discrepancy in phase compositions between dynamic and equilibrium states can be resolved by inter-crystallite ion diffusion among phases, e.g. ion diffusion between amorphous and crystalline phases.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2011

Author dc:creator

• Gao, Yuhua

Advisor dc:contributor.advisor

• Yet-Ming Chiang.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/69790

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/69790