Magnetography: A novel Characterization Tool for Li-Ion-Batteries
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Magnetography: A novel Characterization Tool for Li-Ion-Batteries Timm Bergholz, Theodor Nuñez, Jürgen Wackerl, Carsten Korte and Detlef Stolten1 1
Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Electrochemical Process Engineering (IEK-3), Leo-Brandt Straße 1, 52425 Jülich, Germany ABSTRACT The application of magnetography as a novel method to determine the state of charge (SoC) of commercial Li-ion Batteries is reported. The method is non-invasive and nondestructive and suitable to be applied during normal operation. It is based on spatially resolved measurement of the magnetic field B, induced by the changing current flow during cycling. A standardized measurement setup and procedure for conventional AMR-sensors has been developed, offering high reproducibility (~0.1%) and the chance to characterize the different spatial components of the magnetic field (Bx, By, Bz). The percentage deviation of the B-distributions for different SoCs for a given current load reveals significant differences. A change of B of up to 20% between SoCs of 90% and 10% is found. The influence of current density at different SoC reveals a constant magnetic susceptibility at low SoC and a field dependent at high SoC. Both effects are attributed to the change of the magnetic properties upon varying the amount of intercalated lithium in the transition metal (LixNi1/3Co1/3Mn1/3O2) based intercalation cathode. The method can be used to provide an additional parameter for SoCestimation to battery management systems. INTRODUCTION The cathode active material of Li-ion Batteries (LiBs) usually consists of transition metal based oxides (e.g. LiCoO2, LiMn2O4), polyanion-containing compounds (e.g. LiFePO4) or related materials [1]. These materials have the ability to reversibly intercalate Li+ at a high redox potential by changing the valence state of the transition metal. Graphitic carbon is the most important anode active material [2]. These cathode and anode materials are sensitive to overcharge and deep discharge [3]. Therefore an electronic battery management system is necessary, which controls the charge and discharge currents as well as the present SoC and the remaining total capacity (state of health) [4]. In actual battery management systems the SoC determination is based on the measurement of the open circuit voltage (OCV) and the integration of the charge and discharge current over time [5]. Many experimental methods, which are applicable for a direct determination of the SoC by measuring specific properties of the active components as a function of the Li stoichiometry, are cost intensive or require a complex manipulation of the cell. Experimental studies can be found on the in-situ application of X-Ray Diffraction (XRD) and X-Ray Absorption Spectroscopy (XAS) by using a conventional or a synchrotron radiation source, Impedance Spectroscopy (EIS), dilatometric characterization, IR-Imaging or Raman Spectroscopy [6]. The spatial magnetographic characterization has already been applied for the characterization of
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