How to Model a Lithium Battery – Impedance Spectroscopy Video Tutorial

How to Model a Lithium Battery

This video tutorial from Solartron Analytical shows you how to test a lithium battery, including:

  • Measurement
  • Modelling
  • Fitting
  • Applications – what you can then do with the data acquired

Blue Scientific is the exclusive distributor for Solartron Analytical potentiostats and electrochemical testing instruments in the UK and Ireland. If you have any questions or would like a quote, please get in touch.

 Contact us on 01223 422 269 or info@blue-scientific.com

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Instrumentation

The potentiostat used in the video is the Modulab XM ECS from Solartron Analytical (Ametek). This modular electrochemical test system is individually calibrated for your application, for high accuracy whether you are measuring ultra-low, micro-ohm impedance cells (such as latest generation batteries and fuel cells) or characterising ultra-high impedance corrosion coatings.

The batteries tested in this example are two standard Lithium batteries.

Measurement

  • Connect the battery in 4-term mode – as demonstrated in the video.
  • In the software, choose Galvanostatic Impedance.
  • Start with 30mA amplitude. If there is too much noise, raise this amount, or try lower if it is good quality.
  • Frequency range: 10kHz – 50mHz

Modelling

Standard circuit model for Li-Ion batteries:

Lithium battery model

L = cell inductance
R_ESR = equivalent series resistance
R_SEI = Solid Electrolyte Interface
R_CT = Charge transfer resistance
C_PE/SEI and DL = Constant Phase Element
W = Warburg Diffusion Impedance

Fitting

Simply select the frequency range you wish to fit, and all free variables are calculated automatically by the ModuLab software.

Applications – What you can do with the Data

Plot the state of charge (SoC) using the equivalent circuit analysis.

Compare the impedance of solid electrolyte interface and charge transfer resistance change with SoC.

Study Warburg impedance state of charge correlation at 50mHz

The magnitude change is in the order to 500mOhm – so the change is well measurable.

State of health measurement

This example uses a LiFEPO4 (Lithium Ion Phosphate) battery, which is a type of cell used when safety is critical. The impedance was recorded at 60% SoC as a function of cycle number. This SoC was used because the change in the battery’s behaviour is can be observed clearly  best seen at 60%, compared with lower depths of discharge. EIS recorded at different cycling intervals.

State of charge versus cycle number

Plotting the state of charge against the cycle number shows how the charge transfer impedance changes with the number of cycles. Analysis could be simplified to single frequency measurement, which would reduce test time and require simpler equipment. This technique would be suitable for in-situ device testing. In this example, battery capacity tests showed that if charge transfer resistance is larger than 0.15mOhms, battery capacity was smaller than 50% of initial value.

More Information

Read more about battery testing with EIS in our article. If you have any questions about your particular application, potentiostats or EIS, please get in touch:

 Contact us on 01223 422 269 or info@blue-scientific.com

 View potentiostat range