Published our first paper on modeling nonlinear impedance (NLEIS) for lithium-ion batteries!
Lithium-ion batteries are complex electrochemical systems which store energy to power everything from our cell phones and electric vehicles to large grid scale energy storage. To improve how we study and develop new battery materials as well as monitor and control batteries in their daily operations, electrochemical researchers rely on diagnostic techniques for probing the internal state of the system.
Electrochemical impedance spectroscopy (EIS) is a commonly used tool for noninvasively analyzing many electrochemical systems. EIS works by applying a small perturbation in the current or voltage and measuring the resulting voltage or current. The relationship between the voltage and current (called the impedance) at different frequencies tells us about the different physical processes going on inside the system.
While EIS provides great insight and is useful in a lot of different systems, the use of small perturbations makes EIS an inherently linear technique. Linearizing the physics of the system can lead to degeneracy and loss of information which may be especially important in probing important nonlinear phenomena like side reactions (degradation).
A natural extension to EIS is nonlinear EIS (NLEIS) in which moderate amplitude perturbations are used to excite harmonics in the system response. In this paper we extend the pseudo two-dimensional (P2D) impedance model developed by Doyle et al.1 to include these higher harmonics.