Investigations on Lithium Deposition on Pristine and Aged Lithium Metal Electrodes with an Optically Accessible Cell

Summary:

An in situ optically accessible cell was used to investigate the lithium deposition behaviour on lithium metal electrodes (Li0) as a function of the contact time between the electrodes and electrolyte (LP30). Three types of lithium electrodes were investigated: (1) Li-PRISTINE, pristine lithium foil, (2) Li-ART.-AGED, artificially-aged Li-PRISTINE foil exposed for 1 h to N2 at 25 °C, (3) Li-TIME-AGED, pristine lithium stored for several years in an Ar-filled glovebox.

In the custom-designed symmetrical cell, the electrodes are face-to-face orientated. After cell assembly, the cell is filled with electrolyte and a defined electrochemical test protocol is carried out: (Buffer Time + WAIT1), EIS1, CC, EIS2, WAIT2, EIS3, WAIT3, EIS4. The combination of Buffer Time and WAIT1 represents the period between the initial lithium-electrolyte-contact and the first electrochemical impedance spectroscopy (EIS) measurement. EIS1, EIS2, EIS3 and EIS4 represent EIS measurements (500 kHz – 10 mHz). CC represents a constant current phase with a current density of 1 mA/cm² for 3000 s. This work aims to contribute to a better understanding of how the lithium deposition behaviour is influenced by different initial waiting times (3 h, 8 h, 24 h and 48 h) and differently treated lithium metal electrodes.

Different combinations of lithium electrode types and initial waiting times resulted in a large variety of lithium deposition morphologies and properties. The findings are catalogued in terms of morphology, surface texture and colour.

The recorded EIS data is presented as Nyquist plots. Trends resulting from the various steps of the electrochemical test protocol pointed out. The EIS data are further fitted to an electrical equivalent circuit model. Exemplarily, the ZARC,2-resistances are presented.

The morphology of the lithium depositions seems to depend on the surface condition. Different types of morphologies (forms, structures, textures and colours) were identified. We point out, that it is difficult to generally address lithium depositions as „dendrites”. Long-time storing of lithium metal under inert gas conditions (Ar) apparently cannot prevent aging of the material. Trace amounts of moisture and atmospheric gases will alter the properties.