Dr. Stefan
Seidlmayer and Dr. Petra Kudejová at the PGAA instrument
at FRM II – Photo:
Claudia Niiranen / TUM
Perpetual youth for batteries?
(November 17, 2015) A
key issue with lithium ion batteries is aging. It significantly reduces their
potential storage capacity. To date, very little is known about the causes of
the aging effects. Scientists from the Department of Technical Electrochemistry
and the Research Neutron Source FRM II at the Technical University of Munich
(TUM) have now come a step closer to identifying the causes in their latest
experiments.
Lithium ion batteries with graphite anodes are a relatively
new development. They were patented only in 1989 and have been deployed in
electrical devices since 1991. Since then, they have been a success worldwide
and do their service not only in small electrical devices but also in electric
cars, airplanes and even locomotives. In the future they will also serve as
intermediate storage with up to megawatt capacities.
Irmgard Buchberger
at the X-ray diffractometer
Photo: Andreas
Battenberg / TUM
Batteries with graphite anodes suffer their first
significant loss of capacity during the initial charging cycle, the formation
step. A battery loses up to ten percent of its capacity in the process. Each
additional charge-discharge cycle reduces storage capacity further, if only
insignificantly. Capacity is also lost through the mere storage of batteries –
especially above room temperature.
Physics has come up with a number of ideas about the nature
of these aging effects, but no one has yet found the definitive explanation for
them. TUM scientists at the Chair of Technical Electrochemistry and from the
FRM II have now come a good deal closer to closing this knowledge gap in their
latest experiments.
Detective work using X-rays and neutrons
In order to understand the aging mechanism and to uncover
the reasons behind them, TUM scientists combined electrochemical investigations
with measurement methodologies as diverse as X-ray diffraction, impedance
measurements and prompt gamma activation analysis (PGAA).
Material samples
of anode (grey) and cathode (black)
Photo: Andreas
Battenberg / TUM
They deployed these methodologies to analyze the behavior of
batteries with graphite anodes and nickel-manganese-cobalt cathodes, so-called
NMC cells, at various temperatures. NMC cells are popular in electromobility
since they have a large capacity and can theoretically handle charging voltages
up to just under five volts. However, above 4.4 volts aging effects increase
strongly.
Using X-ray diffraction, the scientists investigated the loss
of active lithium over multiple charging cycles. They used impedance
measurements to register the increasing resistance in the battery cells.
Neutron activation analysis ultimately facilitated the accurate determination
of extremely minute quantities of transition metals on the graphite electrodes.
Deposits of
decomposed electrolyte on the graphite particles
of the anode of a
NMC-lithium ion battery, charged with high voltage (4,6 V)
Image: Irmgard
Buchberger / TUM
Mechanisms of capacity reduction
The significant capacity loss in the formation step is
caused by the build-up of a pacifying layer on the anode. This consumes active
lithium, but also protects the electrolyte from decomposition at the anode.