Researchers must repeatedly ask themselves, compared to what? How do the results generated in the lab compare with those acquired by others? How do theoretical calculations compare with experimental data?
Answering these kinds of questions is particularly decisive for scientists and developers of lithium-ion batteries. Invented forty years ago, lithium-ion batteries now run most portable electronics like power tools and laptops. They are also being advanced to match the high energy storage demands for powering electric grids and electric vehicles. New designs with varied compositions of electrode and electrolyte — the two main battery components — are regularly coming online.
Evaluating whether innovation in electrolyte material or electrode is really an improvement necessitates comparing it to other test outcomes. However, there is no “one size fits all” standard for battery testing. Approaches for testing batteries can differ extensively.
Industrial engineers and researchers from governmental and academic labs often devise their own procedures for characterizing lithium-ion batteries based on the battery technology’s intended application. This makes the comparison of any technological innovations extremely complicated.
Ira Bloom, Battery Researcher, Argonne National Laboratory
A team from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, University of Warwick, OVO Energy, Hawaii National Energy Institute, and Jaguar Land Rover has studied the literature on the different methods used worldwide to define the performance of lithium-ion batteries so as to offer insight on best practices.
Usually, battery researchers use three factors to describe electrochemical performance: open-circuit voltage, capacity, and resistance. The open-circuit voltage is the voltage available from a battery without any current flow. It signifies the maximum voltage of the battery. Capacity is a measure of the maximum charge stored in a battery. The resistance is the degree to which the component materials hinder electric current flow, resulting in a voltage drop.
The issue is that, based on battery application, scientists may measure these factors under various test conditions (rate of discharge, temperature, state of charge, etc.), and thereby achieve a different battery operating life. Battery resistance, for instance, can be measured with a direct current or with an alternating current.
“It’s complicated,” observes Anup Barai, a primary investigator and senior research fellow at the University of Warwick. “The appropriateness of a test depends on what the investigator is studying. Our review provides guidance on the most appropriate test method for a given situation.” Therefore, the team has formulated an easy-to-use table comparing eight test techniques, including the key equipment required, the information generated, and the benefits and downsides for each.
“Our hope,” Bloom adds, “is that our results may one day lead to more reliably comparable methods for testing lithium-ion batteries tailored to different applications.”
The study titled “A comparison of methodologies for the non-invasive characterisation of commercial Li-ion cells,” can be found in the online version of the journal Progress in Energy and Combustion Science.
Besides Bloom and Barai, the study team included Kotub Uddin, OVO Energy; Matthieu Dubarry, University of Hawaii at Mānoa; Limhi Somerville, Jaguar Land Rover; and Andrew McGordon and Paul Jennings, University of Warwick.
This study was supported by a number of sponsors, including Innovate UK, the UK Engineering and Physical Sciences Research Council, the state of Hawaii, the Asia Pacific Research Initiative for Sustainable Energy Systems, and the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.