TEM uncovers why batteries fail


Rebecca Pool

Thursday, December 6, 2018 - 10:30
Image: TEM provides atomic-scale images of how Li dendrite contact affects LiCoO2 cathodes in rechargeable batteries.
US-based researchers have used in situ TEM to image how lithium dendrites damage lithium cobalt oxide thin film cathodes, used in rechargeable batteries, at the atomic scale. 
Using this approach, Yingge Du from Pacific Northwest National Laboratories and colleagues discovered an unexpected Li propagation pathway and detailed the reaction steps that lead to the cathode failure.
“Lithium dendrites are fine strands, like whiskers, that can come in contact with cathode materials and cause a chain of irreversible, spontaneous chemical reactions," highlights Du. "The reaction steps and intermediates revealed provide a clear failure mechanism for the LiCoO2 cathodes caused by Li dendrites, and may also offer insights into the over-discharge processes in cathodes.”
During a Li battery’s charge-discharge cycles, highly localised Li dendrites can form and degrade battery performance.
As Du writes in Small: "Non-uniform and highly localised Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries."
"However, the mechanisms of cathode failures upon contact with Li metal are far from clear," he adds.
To better understand how Li dendrites came into contact with the cathode materials and pinpoint the exact mechanism that leads to cathode failure, Du and colleagues first used pulsed-laser deposition to fabricate well-defined, epitaxial LiCoO2 thin films.
The films had controlled crystallographic orientations and served as model cathode materials.
A Li metal tip was then used to mimic the Li dendrite inside a TEM, and study its interaction with the LiCoO2.
Using advanced microscopy and spectroscopy techniques – including scanning transmission electron spectroscopy, nanobeam diffraction, and electron-energy loss spectroscopy – the researchers investigated the reactions with high spatial and temporal resolution.
In combination with density functional theory calculations, they were able to determine the reaction steps, intermediates, and final products.
Researchers imaged the structural and chemical evolutions of LiCoO2 cathodes upon Li dendrite contact at an atomic scale. A spontaneous, irreversible conversion reaction leads to the formation of Co metal and Li2O, with CoO as a metastable reaction intermediate. [PNNL]
According to the researchers, an unexpected Li diffusion direction perpendicular to the Li-containing planes was found, which tore the LiCoO2 crystal apart, generating grain boundaries and antiphase boundaries.
"[Analyses have] shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to expansion and pulverization of LiCoO2 and ending with the final reaction products of Li2O and Co metal," says Du. 
While Co metal and Li2O were found to be the final products of the full conversion reaction, CoO was identified as a metastable intermediate at the reaction front as a result of facile phase transition from LiCoO2.
Du and colleagues now intend to fabricate all-solid-state batteries using pulsed-laser deposition to better understand the ion transport processes across the well-defined interfaces.
"These fundamental insights are of general importance in mitigating Li dendrites related issues and guiding the design principle for more robust energy materials," says Du.
Research is published in Small.
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