Safety and operational strategies of lithium-based batteries – dendrite and SEI growth on metallic lithium

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The aim of this work is to use the empirical findings and parameters obtained from the experimental procedures to model these processes on a physical-electrochemical basis. A prerequisite for the development of the mathematical model of the new energy storage devices with lithium metal electrodes is the experimental analysis characterizing the electrochemical and chemical properties of the lithium metal electrode and its behavior within the energy storage device. For the above mentioned reasons two project partners are working together:
> The fem Research Institute for Precious Metals and Metal Chemistry: Experimental characterization of the solid electrolyte interphase (SEI) and dendrite growth on metallic lithium
> The Fraunhofer Institute for Energy Economics and Energy System Technology IEE: Development of a mathematical model for an energy storage device with lithium metal electrodes

The growth of the SEI was investigated by electrochemical impedance spectroscopy (EIS) and the growth of lithium dendrites during cycling was determined using an optical in-situ test cell with a light microscope. For all the experiments, lithium metal foil was used as electrode material on both anode and cathode. 1 M lithium hexafluorophosphate (LiPF6) in a 1:1 solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as electrolyte for all sorts of experiments. A VSP- and SP-150-potentiostat were used with the EC-Lab-software to carry out the experiments. The experimental data of the EIS measurements were fitted with an equivalent circuit within the RelaxIS 3-software. The characterization of the SEI growth was carried out using a TSC Battery test cell, a Microcell HC device was used to apply a defined temperature to the test cell. The growth of lithium dendrites during cycling was determined using an optical in-situ test cell ECC-Opto-Std in side by side setup with a light microscope VHX 7000.

Characterization of the SEI growth
With the EIS measurements, the EIS spectra were observed.
To gather all the parameters of the ongoing processes on the electrodes, the observed data was fitted using an equivalent circuit.
The fitted data of the SEI resistance showed a significant rise over the first hours of investigation. After this period, the resistance was reaching a plateau and only a slight increase in SEI resistance was observed over the remaining period of the investigation.

Characterization of the Dendrite Growth
By using the light microscope, two different types of lithium growth was observed. At a current strength of 0.5 and 1.0 mA, the growth of mossy lithium was observed. At a current strength of 2.5 and 5.0 mA, dendrite growth was observed.

With the experiments performed, SEI growth and dendrite growth on metallic lithium were successfully investigated. In a next step, the generated data will be used to verify the modulation software developed by the project partner.

Acknowledgements
The present investigation IGF 21113 N was supported by funds of the German Federation of Industrial Research Associations (AiF) within the program to promote the Industrial Collective Research (IGF) from the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag.

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