In Operando Measurement of the Lithium-Ion Concentration in the Battery Electrolyte to Study Degradation Modes

Abstract

In operando measurements of the lithium-ion concentration inside a lithium-ion battery (LiB) using a fiber-optic sensor are demonstrated. The concentration changes in the electrolyte are correlated to the incremental capacity response of the battery during charge and discharge. The goal is to study battery degradation based on the dynamic and state-of-charge dependent concentration of lithium ions in the electrolyte.Batteries lose capacity over time, both due to charging/discharging and to shelf ageing (calendar ageing). Extending the lifetime of a battery directly reduces its environmental footprint. This ageing is caused either by i) unwanted chemical reactions, reducing the number of available lithium-ions or increasing the impedance, or ii) by physical changes that reduces the available lithium storage capacity of the electrodes. Both mechanisms affect the spatial distribution of lithium ions.Ageing has traditionally been studied through monitoring the operational performance (e.g., coulombic capacity, internal resistance), or through post-mortem investigations. Other advanced methods, such as thermal neutron imaging or X-ray tomography has also been used to estimate the internal composition of the battery through image analysis [1,2]. This study presents live concentration measurements using an optical fiber placed between the electrodes. The intrinsic properties of optical fibers (e.g., small, chemically inert, electrically insulating) make them ideal for internal sensing in the chemically harsh interior of a battery.The fiber is placed in the electrolyte between the electrodes, see Fig. 1a. Because the reflected intensity depends on the concentration of lithium ions (Fig. 1b), we are able to demonstrate and analyze how the concentration changes during cycling (Fig. 1c). Both single-cycle and long-term effects are investigated. Information about how the lithium concentration changes during cycling is valuable both as an input parameter in battery modelling, and to increase the understanding of battery ageing. For the latter, the lithium-concentration during cycling of batteries is evaluated in correlation with an incremental capacity analysis. Siegel, J.B.; Lin, X.; Stefanopoulou, A.G.; Hussey, D.S.; Jacobson, D.L.; Gorsich, D. Neutron Imaging of Lithium Concentration in LFP Pouch Cell Battery. J. Electrochem. Soc. 2011, 158, A523, doi:10.1149/1.3566341.Senyshyn, A.; Mühlbauer, M.J.; Dolotko, O.; Hofmann, M.; Ehrenberg, H. Homogeneity of lithium distribution in cylinder-type Li-ion batteries. Sci. Rep. 2015, 5, 1–9, doi:10.1038/srep18380. Figure 1