Abstract
This article aims to describe the most important aspects to consider when using the concept of internal impedance in algorithms that focus on characterizing the degradation of lithium-ion (Li-ion) batteries. The first part of the article provides a literature review that will help the reader understand the concept of electrochemical impedance spectroscopy (EIS) and how Li-ion batteries can be represented through electrochemical or empirical models, in order to interpret the outcome of typical discharge and/or degradation tests on Li-ion batteries. The second part of the manuscript shows the obtained results of an accelerated degradation experiment performed under controlled conditions on a Li-ion cell. Results show that changes observed on the EIS test can be linked to battery degradation. This knowledge may be of great value when implementing algorithms aimed to predict the End-of-Life (EoL) of the battery in terms of temperature, voltage, and discharge current measurements. The purpose of this article is to introduce the reader to several types of Li-ion battery models, and show how the internal impedance of a Li-ion battery is a dynamic parameter that depends on different factors; and then, illustrate how the EIS can be used to obtain an equivalent circuit model and how the different electronic components vary with the use given to the battery.
Highlights
Since their invention, batteries have been used in a wide variety of applications that require an autonomous energy source, or just as a backup for normal operation
Ning and Popov (2004) propose a capacity fade model that includes the charge rate (CR), the depth of discharge (DOD), the end-of-charge voltage (EOCV) and the discharge rate (DR). These authors explain through the use of partial differential equations the flow of ions from the anode to the cathode or vice-versa. Their results show that the degradation process that occur on the electrodes and electrolyte are the reasons of why the impedance increases throughout the lifespan
The following results aim to illustrate how the degradation process can be studied through the use of electrochemical impedance spectroscopy (EIS) and the corresponding Nyquist plots that are generated with this method
Summary
Batteries have been used in a wide variety of applications that require an autonomous energy source, or just as a backup for normal operation. The SOH is an indicator associated with the long-term cycle life of batteries, since every time a battery is used its RUL decreases Both the SOC and the SOH are a function of several parameters, one of them being the internal impedance. Of the type of model that is used, it is important to note that the main purpose is to describe either how the battery cell discharges or degrades during the lifecycle All these effects are associated with changes in the internal impedance of the battery. The Warburg impedance affects the response at low frequencies The adjustment of these circuits allow the transformation of visual information obtained with the Nyquist plots, into parameters that evolve in time as the degradation process becomes more evident. This way it is possible to establish a correlation between parameter changes and the SOH of the battery
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