New insights into lithium-ion battery failure mechanism

Date:
August 25, 2020
Source:
University of Cambridge
Summary:
Researchers have identified a potential new degradation mechanism
for electric vehicle batteries - a key step to designing effective
methods to improve battery lifespan.



FULL STORY ========================================================================== Researchers have identified a potential new degradation mechanism for
electric vehicle batteries -- a key step to designing effective methods
to improve battery lifespan.


==========================================================================
The researchers, from the Universities of Cambridge and Liverpool,
and the Diamond Light Source, have identified one of the reasons why state-of-the-art 'nickel-rich' battery materials become fatigued, and
can no longer be fully charged after prolonged use.

Their results, reported in the journal Nature Materials, open the door
to the development of new strategies to improve battery lifespans.

As part of efforts to combat climate change, many countries have
announced ambitious plans to replace petrol or diesel vehicles with
electric vehicles (EVs) by 2050 or earlier.

The lithium-ion batteries used by EVs are likely to dominate the EV market
for the foreseeable future, and nickel-rich lithium transition-metal
oxides are the state-of-the-art choice for the positive electrode,
or cathode, in these batteries.

Currently, most EV batteries contain significant amounts of cobalt in
their cathode materials. However, cobalt can cause severe environmental
damage, so researchers have been looking to replace it with nickel,
which also offers higher practical capacities than cobalt. However,
nickel-rich materials degrade much faster than existing technology and
require additional study to be commercially viable for applications such
as EVs.

"Unlike consumable electronics which typically have lifetimes of only a
few years, vehicles are expected to last much longer and therefore it
is essential to increase the lifetime of an EV battery," said Dr Chao
Xu from Cambridge's Department of Chemistry, and the first author of
the article. "That's why a comprehensive, in-depth understanding of how
they work and why they fail over a long time is crucial to improving
their performance." To monitor the changes of the battery materials
in real time over several months of battery testing, the researchers
used laser technology to design a new coin cell, also known as button
cell. "This design offers a new possibility of studying degradation
mechanisms over a long period of cycling for many battery chemistries,"
said Xu. During the study, the researchers found that a proportion
of the cathode material becomes fatigued after repetitive charging
and discharging of the cell, and the amount of the fatigued material
increases as the cycling continues.

Xu and his colleagues dived deep into the structure of the material
at the atomic scale to seek answers as to why such fatigue process
occurs. "In order to fully function, battery materials need to expand
and shrink as the lithium ions move in and out," said Xu. "However,
after prolonged use, we found that the atoms at the surface of the
material had rearranged to form new structures that are no longer able
to store energy." What's worse is that these areas of reconstructed
surface apparently act as stakes that pin the rest of the material in
place and prevent it from the contraction which is required to reach
the fully charged state. As a result, the lithium remains stuck in the
lattice and this fatigued material can hold less charge.

With this knowledge, the researchers are now seeking effective
countermeasures, such as protective coatings and functional electrolyte additives, to mitigate this degradation process and extend the lifetime
of such batteries.


========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
story is licensed under a Creative_Commons_License. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Chao Xu, Katharina Ma"rker, Juhan Lee, Amoghavarsha Mahadevegowda,
Philip
J. Reeves, Sarah J. Day, Matthias F. Groh, Steffen P. Emge,
Caterina Ducati, B. Layla Mehdi, Chiu C. Tang, Clare P. Grey. Bulk
fatigue induced by surface reconstruction in layered Ni-rich
cathodes for Li-ion batteries. Nature Materials, 2020; DOI:
10.1038/s41563-020-0767-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110721.htm

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