Diagnostic of Solid-State Lithium Metal Batteries with the Sand Equation: A Symmetric Cell Electrochemical Characterization Method upon Plating
Authors: Costalin Mehdi , Barcha Cédric , Rousselot Steeve , Foran Gabrielle , Nicolle Paul , Prébé Arnaud , Ayme-Perrot David , Dolle Mickael 
Published: August 2024

Lithium Metal Anodes (LMAs) have generated significant interest for use in solid-state batteries (SSBs) due to their high theoretical specific capacity. Nonetheless, issues such as dendrite growth (1) and cell failure caused by lithium loss with solid polymer electrolytes (SPEs) (2) of decent ionic conductivities have hindered widespread commercialization. In this work, we report the electrochemical characterization of symmetric Li- SPE– Li cells using thermoplastic vulcanizate (TPV) electrolytes comprised of: PCL:HNBR LiTFSI. Full plating of the lithium metal (LiM) electrode was achieved at 100 μA.cm-2 during constant polarization in pressurized pouch cells. Complete polarization was confirmed through ex-situ analysis by scanning electron microscopy (SEM) with SEM images showing that no dendrites were formed at this current density. Furthermore, cell polarization performed at higher current densities allowed lithium diffusion in the TPV electrolyte to be calculated using the Sand Equation (3, 4). Lithium diffusion was found to be 1.7 x 10-8 cm2.s-1 at 60 °C, which was consistent with other published values (5). The calculated threshold current density (j*) of the corresponding symmetric Li cell was approximately 200 μA.cm-2, confirming the cell failures due to dendrite growth and short circuiting that were observed in the system above this current density. This j* provides valuable information regarding the design of a full cell as the operation current density would be driven by the Li-SPE interface limitation. To this end, 100 cycles were performed in Li-Li symmetric cells with the TPV electrolyte without failure at currents below j* with 0.1 mAh.cm-2 of charge being transferred per cycle, while a charge of 0.5 mAh.cm-2 resulted in rapid cell failure. These findings emphasize the need for current densities and charge references in symmetric Li-Li cells and cells employing solid electrolytes.

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