Application notes 15 min read

How to check and correct non-stationary EIS measurements using EC-Lab® (battery cell) – Battery – AN69/2

Latest updated: August 31, 2021

Abstract
This application note presents the various tools available in EC-Lab® that can be used to check and correct the non-stationarity of your EIS measurements. Using the proprietary Non-Stationary Distorsion indicator and the instantaneous impedance analysis tool Z Inst, users can ensure that EIS measurements are correctly interpreted and fitted. This application note shows the use of such tools on data obtained on an LFP battery under discharge.

 

Introduction

For valid Electrochemical Impedance Spectroscopy (EIS) measurements, the system under investigation should be linear, stable, causal, and stationary [1,2]. In this note, the term “stationarity” comprises steady-state and time-invariance.

 

Steady-state is the state of a system after its transient state. For example, an R/C circuit submitted to a potential or current step is in a transient state and sees its response change after a certain amount of time has passed.

 

Time-variance refers to a system where the parameters defining its transfer function change with time. The chemical composition of a battery cell’s electrodes changes during charging or discharging, which affects its impedance.

 

The two properties may be difficult to separate.

 

EIS measurements during continuous charge or discharge, or in operando, are used to inspect a battery under operation [3-8]. The problem with such cases is understanding which impedance data can be considered valid, or moreover, which frequencies data should be considered erroneous because of the non-stationarity of the system.

 

In this second application note, we will present various tools implemented in EC-Lab® to help check and correct the non-stationarity of the measurements. These tools will be applied to data obtained on a discharging battery.

 

Lithium-ion battery battery cell discharge EIS Non-stationary 4D impedance NSD modelling analysis

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