When 9:00 AM - 11:00 AM May 07, 2015
Where 3158 H.H. Dow Building
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Real-time atomic-resolution probing of lithium ion intercalation in TiO2–related anodes using transmission electron microscopy


Sung Joo Kim
Thesis Defense

Xiaoqing Pan, advisor

 

Nanostructured TiO2 polymorphs such as rutile and bronze have been considered for lithium ion battery (LIB) application due to their chemical stability and accessibility. Despite their promising performances as anodes and coating materials, understanding of lithium ion behaviors in TiO2-related systems is still controversial, since most studies have relied on bulk characterization techniques which do not present local changes in morphology. Here, we employed in-situ transmission electron microscopy (TEM) and high-resolution scanning TEM (HRSTEM) to perform nano-scale structural studies of TiO2-related anodes upon Li intercalation.

 

The electrochemical study of a single-crystalline rutile TiO2 nanowire (NW) was conducted under high-resolution TEM aided by selected area electron diffraction. The result demonstrates the two-step lithiation accompanied by highly anisotropic volumetric expansion and phase transformation. 

  

An in-depth study of lithiation in bronze TiO2 (TiO2-B) had been possible by the growth of a high-quality TiO2-B thin film templated by a Ca-modified bronze phase, CaTi5O11 (Ca:TiO2-B). Various interfaces and defects in TiO2-B and Ca:TiO2-B thin films deposited on (100) and (110) SrTiO3 substrates were first studied under HRSTEM. The information obtained especially on (001) TiO2-B on a Ca:TiO2-B template was useful for the in-situ TEM study on the same material. Revealed by high-resolution observation of electrochemical lithiation into TiO2-B, many defects were induced by strain relaxation upon Li-induced TiO2-B lattice expansion. In fact, depending on Li intercalation direction into the structure, either high-symmetry structural transformation or plain shearing was generated. The observations were corroborated by post-mortem HRSTEM characterization and theoretical calculation. 

 

The capability of a TiO2 nanostructure as a coating material for a LIB anode was also investigated via potentiostatic lithiation of a rutile TiO2-coated Si NW. Lithium intercalation into this NW occurred locally by having each segment achieving full lithiation throughout the Li migration pathway. The TiO2 coating also influenced final lithiated morphology of a coated Si (Li15Si4) to be different from that of an uncoated. 

 

The results discussed in this thesis provide the in-depth knowledge on the Li ion dynamics in the two TiO2 polymorphs. The application of the in-situ high-resolution TEM technique can be expanded towards other polymorphs of various different structural forms.