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GM and University of Michigan Developing Method for Direct In-Situ Measurement of Li+ Transport Rates in a Li-ion Cell; Insights into Cell Degradation

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The measurement method is based on the differences in color of electrode material after intercalation of different amounts of lithium. Shown is an example of graphite from Maire (2008). Click to enlarge.

Researchers from General Motors R&D Center and the University of Michigan are developing a method for the direct in situ measurement of lithium ion (Li+) transport rates in a functioning Li-ion cell. Steve Harris from GM, the lead researcher on the project, presented a paper on the method at the 215th biannual meeting of the Electrochemical Society (ECS 215) this week in San Francisco (and is continuing on to present the work at institutions such as Caltech).

Harris’s goal for the project is to try to understand degradation mechanisms of lithium-ion cells. Degradation of cell capacity over time and in extreme temperatures or duty cycles is a problem for any battery application, but for electric drive vehicle applications the problem is particularly acute, given requirements for long calendar life and a stable contribution to vehicle power and energy.

When I started reading degradation literature, it seemed pretty daunting. There are so many different kinds of degradation that seem to have nothing to do with each other...I was looking for an organizing principle that would help guide me, and here’s what I came up with: in a working lithium-ion battery, the only thing that should be happening is that the lithium should be going back and forth. So if you have a battery that is not working, that must mean...that the lithium just isn’t getting to the right place at the right time. That immediately tells me, that if I’m going to study degradation, I should start with lithium transport.

—Steve Harris

Transport can be looked at from the macro-, meso- and nanoscales, Harris said. While there is a great deal of information on degradation at the macro- and nanoscales, there is hardly any experimental work on transport measurements made at the mesoscale (particle or sub-electrode). Harris and his co-workers are seeking to provide additional meso-scale information to model degradation.

Traditionally, the understanding of degradation is drawn indirectly from experimental studies that track current or voltage, or post-mortem analyses. Both of these have the drawback, Harris said, of making indirect measurements, or inferring what might have happened. Harris and team are seeking to make direct in-situ measurement of Li+ transport—i.e., time-dependent maps of lithium insertion and extraction.

The approach relies on the correlation between color and state of charge of some materials. In the experiment, Harris used graphite. This technique allows viewing the spatial distribution of the lithium inserted in the solid phase with a resolution on the order of 10 microns. This allows the researchers to explore important processes during normal operation, the better to understand how processes become abnormal.

The GM and U-M researchers are not trying to measure transport coefficients; rather, they are seeking to understand how transport coefficients change with factors such as state of charge, temperature, degradation, and spatial variation, as well as to understand how transport relates to specific properties of electrodes and electrolytes.

Also we want to understand how morphology affects transport..there is morphology at different scales. You can ask, What about morphology inside a particle? Here I just sort of sliced open some carbon stuff. What I was struck by is that there is an awful lot of holes. This is not a solid, there is several percent at least porosity in this thing. Wouldn’t it be interesting if electrolyte could get inside these particles? If they can, then because the liquid phase diffusion is five orders of magnitude faster than solid phase diffusion, it could make the transport...more complicated than we thought.

—Steve Harris

Harris’ paper was one of a series from GM researchers at the ECS meeting.

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