Text

theses

Currently, cathode materials for Li-ion batteries are based on intercalation processes where, during charge and discharge processes, Li intercalates into the crystal lattice while maintaining the host crystal structure. More recently, new cathode materials have been introduced based on conversion reactions involving phase transformation and complete reduction of the host transition metal. In addition, conversion reactions involve two or more Li ions with a resulting much higher capacity than obtainable for intercalation materials. However, mechanism of phase transformation and cycling reversibility are at present still poorly understood. In this study transmission electron microscopy (TEM) techniques including selected area electron diffraction (SAED) pattern, annular dark field (ADF) STEM image, and electron energy loss spectroscopy (EELS) with nanoscale spatial resolution were used to study the phase evolution and structural changes of iron fluorides (FeFe2, FeO0.7F1.3, FeF3) after various discharge/charge cycles. Additionally, the changes of the Fe valence states upon cycling were determined using EELS by measuring the L3/L2 intensity ratio of Fe-L edge. The structural transformations of FeO0.7F1.3 during the first lithiation show that litiahation contains two regions. The first region, lithiation is an intercalation reaction iii with reduction of Fe3+ to Fe2+. The second region of lithiation involves a conversion reaction, with the formation of metallic Fe, LiF, and Li0.7Fe2+0.5O0.7F0.3 (rocksalt type) phases. The first delithiation process follows a different conversion reaction path compared to the first lithiation reaction involving the formation an amorphous rutile-type phase along with with the rocksalt-type phase. Interestingly, upon full recharge (delithiated electrode), the measured average Fe valence state returns back to its initial value of Fe2.7+. The growth of a solid electrolyte interphase (SEI) layer formation at the electrode/electrolyte interface is observed for the iron fluoride compounds (FeF2, FeF3, and FeOF) after cycling. The evolution of the SEI layer formation after cycling has been studied for the FeF2 samples in details by EELS and XPS. We observed the growth of SEI layer with cycle number, which mainly contained LiF and Li2CO3 compounds. Two degradation mechanisms are identified. First, the increase in the decomposition product layer after cycling inhibits complete reconversion process. Second, dissolution of Fe into the SEI layer after cycling which leads to the loss of active material.

ETD_5770

Ph.D.

Includes bibliographical references

by Mahsa Sina

doi:10.7282/T3K075WR

ETD doctoral

The author owns the copyright to this work.