Wednesday 5/20/09
I’m still reading literature about impedance and lithium ion batteries. There’s a surprising amount of material on this field lying around. Today, Mohit actually sat down with me for a while and gave me an overview of everything I was reading, and explained a lot of it to me. I started looking at Nyquist plots, which are plots of the real vs imaginary parts of impedance, which are plots that I will be using a lot in my experiments. It’s really cool how with different combinations of capacitors and resistors different shapes of plots are made based on the impedances of those circuit elements. For example, the Nyquist plot of an in-parallel combination of a resistor and capacitor is a semicircle. An in-series combination of resistor and capacitor is a straight, vertical line. Mohit and I took some time and went through and examined different derivations of the Nyquist plot, and he demonstrated the results to me using real circuit elements. Mohit also had me make some Lithium symmetric cells with sample polymer that he had made. It takes a while to make them, and certain parts of the process are really annoying. Lithium is pretty sticky, so it’s hard positioning it just right over the polymer so that none of it hangs over the edge and shorts with the other Lithium electrode. Takes about 20-30 minutes to make one cell. Afterwards, I hooked it up to the measuring instrument (note: ask what it is called) and took Nyquist impedance plots of it, and as expected, based on what I have learned from lectures and literature, the graph looked normal and showed that the cell I had built resembled a resistor and capacitor in parallel in series with another resistor and capacitor combination in parallel. Awesome. Actually got to get my hands dirty already.
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