# Electrochemical Impedance Spectroscopy

## What is this?

Well, it’s a beginner’s guide to Electrochemical Impedance Spectroscopy, which is an electrochemical analysis technique often used to study batteries (and fuel cells, and other systems). It’s a technique I used often during my PhD, and I was a teacher for the lab part of this topic at the Southampton Electrochemistry Summer School for 3 years during this time – and now I occasionally teach it to PhD students at Uppsala University.

I was inspired to put these pages together because although I think there are some good resources for learning the theory out there, I don’t think there are many that are at a nice accessible level for, say, battery scientists – who often have a stronger background in materials chemistry than they do in electrochemistry. Nor do I think that there are many that cover the practical aspects of EIS well enough – there’s very little help out there for people who get the basic idea of EIS but are struggling to fit their data or aren’t sure how to set up their experiments. So I hope, eventually, that my own pages will be of some use to… somebody!

The pages here are a reworked version of my lecture material for this topic, mixed in with some simulation apps I made with R and Shiny. You can get started by following the links below. If you find this useful, or have some suggestions for future additions or changes, I’d be very happy to hear from you:

## Words of caution/encouragement

First off, I want to say that although I’ve been a user of impedance spectroscopy since 2008, on an applied level (battery characterisation) it still confuses the hell out of me on a regular basis. Learning the theory is one thing, but in-depth analysis, especially when it comes to equivalent circuit fitting, is hard – it requires a healthy degree of intuition, and usually needs very carefully designed experiments to get meaningful results… but sometimes it’s just a pain. I do think that maybe half the people in the battery field who have done some interpretation of impedance spectroscopy in peer-reviewed publications do not really understand what they’re doing with it.

But, don’t be discouraged(!). It’s not easy to learn, but in writing this I’ve done my best to keep the level as accessible as possible, to give examples, some little apps where you can do simulations to see how changing parameters in a model system changes things, and many more, to try and give you the best chance to build up that level of intuition.

I always finish my lectures with “Matt’s top tips for impedance spectroscopy”, to try to restore some of the enthusiasm and confidence I’ve just stolen from my poor students, and here they are:

#### Matt's top tips for impedance spectroscopy

• EIS can be ambigious, and a good equivalent circuit fit can be elusive (but don’t let that put you off)
• The fewer parameters an equivalent circuit has, the better
• Always check the fitted values make sense (and the errors are reasonable).
• Practice makes perfect!
• impedanceR: an R package for simulating equivalent circuits; a development of the page above.