Electrochemomechanical fatigue and fracture in electrode and electrolyte materials for Li-Ion batteries

In Li-ion batteries (LIBs), electrochemically driven dimensional changes in the electrodes lead to mechanical stress buildup during operation. Electrochemomechanical fatigue refers to both mechanical degradation (fracture) and the associated chemical degradation that is exacerbated by fracture as a result of this stress, accumulated during repeated electrochemical cycling. Such fracture can have serious consequences for the performance of LIBs over time in terms of capacity loss, growth of electrochemical impedance, and in all-solid-state batteries (ASSBs) even failure via short-circuiting. To better understand and predict mechanisms for electrochemically-induced fracture, we measured elastic, plastic, and fracture properties of electrode and solid electrolyte materials, focusing especially on sulfide electrolytes for ASSBs. We found that these electrolytes are extremely brittle and therefore vulnerable to fracture-assisted internal electrical shorting, an issue that currently limits commercialization of ASSBs. We built on these results with finite element modeling of electrolyte fracture in ASSBs, thus finding a strong dependence of fracture conditions on both electrolyte fracture toughness and plastic behavior of lithium metal. Using these results, we constructed electrochemomechanical failure maps to establish how microstructure, processing, and mechanical properties influence electrolyte fracture. We also studied how electrochemically induced fracture in turn affects battery performance, particularly for electrode materials. We implemented controlled fracture events in Li[subscript X]Mn₂O₄ cells employing liquid electrolytes and lithium anodes, and used acoustic emissions monitoring to confirm the timing of the fractures. We then used electrochemical impedance spectroscopy based on a distribution of relaxation times analysis method to isolate the fracture-based mechanisms leading to impedance growth, thereby observing sudden increases in electronic contact resistance concurrent with crack formation within the active particles. We also observed an increased rate of capacity fade following each fracture event, consistent with increased exposure of electrode surfaces to liquid electrolyte that promotes active material dissolution. Thus, within this thesis, we address complementary aspects of electrochemomechanical fatigue: how electrochemical changes promote fracture in electrodes and solid electrolytes, and how this fracture in turn affects electrochemical performance of LIB devices.

Department dc:contributor.department

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

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2018

Author dc:creator

• McGrogan, Frank Patrick, IV

Advisor dc:contributor.advisor

• Krystyn J. Van Vliet.

• Materials Science and Engineering.

Statement dc:rights

• MIT theses are protected by copyright. They may be viewed, downloaded, or printed from this source but further reproduction or distribution in any format is prohibited without written 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/1721.1/120187

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/120187