Quantitative Analysis of Residual Lithium in Lithium-ion Battery (LiB) Cathode Materials using Optical Spectroscopy

Abstract

As the use of electric vehicles and various mobile devices becomes common and the importance of eco-friendly energy is emphasized, technologies related to secondary batteries are also growing rapidly. Lithium-ion batteries (LiBs) have contributed decisively to the spread of the mobile and IT devices due to their advantages such as high energy density, operating voltage, and long life-time, and the demand for them is also progressively increasing. Among various components constituting a lithium-ion battery, cathode materials account for the largest proportion in the material cost of the battery and are very important factors which determine the capacity and life-time of the battery. Accordingly, researches on the cathode materials for making a low-cost, high-capacity lithium-ion batteries have been actively conducted. Cathode active materials of lithium-ion batteries are lithium (Li) oxides, which are produced by a calcination reaction between precursors and lithium compounds.

The lithium components that did not participate in the calcination reaction of the cathode active materials remain on the surface of the cathode active material in the form of lithium carbonate (Li2CO3) or lithium hydroxide (LiOH). This is called residual lithium. Residual lithium does not participate in charging and discharging in a battery, resulting in a decrease in capacity of the lithium-ion battery, and causes many side effects such as explosion due to gas generation or difficulty in manufacturing an electrode. Therefore, the residual lithium is one of the important specification items for producing or developing the cathode active materials. In order to measure the amount of residual lithium remaining in the cathode active material of the lithium-ion battery, Warder titration method has been generally used. Using this method, the lithium carbonate and lithium hydroxide can be measured in one titration, but it takes a long time to prepare the sample and is difficult to automate. In particular, as the production of the lithium-ion batteries increases and the development of new cathode active materials becomes active, the demand for a fast and automated method for measuring residual lithium has increased.

In this study, the amount of residual lithium remaining in the cathode active material of lithium-ion batteries was quantified using optical spectroscopy. First, Laser-Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy were used to obtain spectra emitted by atoms and molecules in the cathode active materials. These spectrometers are commonly used in Chemometrics, and have recently been actively used in researches on secondary battery materials. In LIBS, the spectrum emitted by carbon (C) and hydrogen (H) was of great interest, and in Raman spectroscopy Stokes scattering was measured due to the energy change of lithium carbonate and lithium hydroxide. The optical spectra obtained from the cathode active material samples were analyzed to find the correlation with the residual lithium measured by the titration method, and a model for predicting the residual lithium from the optical spectrum was proposed. In addition, various multivariate analysis algorithms and signal processing algorithms were applied to improve the prediction performance of residual lithium. If the residual lithium is quantified using optical spectroscopy, the sample preparation time can be drastically reduced and the analysis procedure can be simplified and accelerated. In addition, in-situ analysis or automated material analysis may be possible on the cathode materials production line, which is expected to contribute to the production and development of lithium-ion batteries.