Battery life for wearable electronic devices could be improved

Date:
August 25, 2020
Source:
University of Warwick
Summary:
Researchers have found that asymmetric stresses within electrodes
used in certain wearable electronic devices provides an important
clue as to how to improve the durability and lifespan of these
batteries.



FULL STORY ========================================================================== Researchers in WMG and the Department of Physics at the University of
Warwick have found that asymmetric stresses within electrodes used in
certain wearable electronic devices provides an important clue as to
how to improve the durability and lifespan of these batteries.


========================================================================== Batteries for medical applications and wearable devices continue to evolve
in size and shape, with miniaturisation of Li-ion technologies becoming increasingly popular. However, as the size of the battery shrinks,
the fabrication process for composite electrodes and the use of liquid electrolyte is becoming a processing challenge for microfabrication
using conventional approaches.

Lithium cobalt oxide LiCoO2 (LCO) has remained a common choice of
cathode for these small formats due to its high voltage platform and
energy density.

However, following the initial reported performance benefits of LCO, it
is known that LCO cells have large impedance issues due to the growth of
high surface layer resistance and charge transfer resistance. This can
affect how efficiently the battery charges and discharges. There are
also ethical and health considerations around the use of the element
cobalt. The increasing impedance was thought to be attributable to the
growth of a surface layer on both the anode (solid electrolyte interface,
SEI) and cathode (cathode electrolyte interface, CEI) due to the reaction between the electrodes and the electrolyte.

However, in the paper "Ageing analysis and asymmetric stress
considerations for small format cylindrical cells for wearable
electronic devices" published recently in the Journal of Power Sources,
the University of Warwick's WMG and Physics department researchers
disassembled these cells. They have found that and the condition of
the cathode and anode varied greatly after 500 cycles, as a function of
which side of the current collector it was on.

The inward facing cathode (under compression) when rolled into a
jelly-roll, develops significant signs of coating delamination from the aluminium foil. On the outward facing cathode side (under tension),
however, only a partial delamination was evident and the coating was transferred unto the separator. By contrast, severe delamination was
observed on both sides of the anode coating.

The inward facing anode side (under compression) showed almost no coating
still adherent to the copper foil, compared to the outward facing anode
side (under tension). Likewise, the delaminated coating had become
adhered to the separator during operation.

Dr Mel Loveridge from WMG, University of Warwick comments: "It is
interesting to note that, for both the cathode and the anode, the
delamination is more severe on the electrode coating side that would have
been subjected to compression stress, rather than tensile strain. This
can be further explained by considering the asymmetric forces in place on either side of double side coated electrodes." The research team also
carried out electrochemical testing, X-ray photoelectron spectroscopy
(XPS), X-ray computed tomography (XCT) and scanning electron microscopy
SEM), to reveal the battery's structural features and changes. They
found that it maintains 82% cell capacity after 500 continuous charging
and discharging, after which it shows severe delamination due to high
bending stress exerted on the cell components. However this seemingly
has minimum impact on the electrochemical performance if the coating
is sufficiently compressed in the jelly roll with a good electrical
contact. After ageing, the surface layers continue to grow, with more
LiF found on the cathode and anode.

Their research opens up exciting areas in battery manufacturing to
address winding issues for cylindrical cells (especially miniaturised
formats). For example, highlighting the need to understand whether there
is merit in varying the coating properties on each side of double-sided
coating for wound cylindrical cells, in order to improve the mechanical resilience of coatings that have asymmetric stresses exerted on them.


========================================================================== Story Source: Materials provided by University_of_Warwick. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. C.C. Tan, M. Walker, G. Remy, N. Kourra, F. Maddar, S. Dixon, M.

Williams, M.J. Loveridge. Ageing analysis and asymmetric stress
considerations for small format cylindrical cells for wearable
electronic devices. Journal of Power Sources, 2020; 472: 228626 DOI:
10.1016/ j.jpowsour.2020.228626 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110654.htm

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