The anode-free cells have emerged due to the need to maximize Li metal batteries' energy density. However, anode-free Li batteries suffer from short cycle life because of the lack of Li inventory at the anode. Freshly deposited Li metal anodes usually take hundreds of cycles to reach initial SEI stabilization optimum coulombic efficiency (CE) due to initial SEI stabilization and electrode activation. The anode-free cell design requires high Li metal CE over the whole cycling life, particularly during the initial activation cycles. A holistic approach to electrolyte design, mechanism understanding, and battery engineering is needed to fulfill these requirements. Here, we present a mechanistic study on lithium plating and stripping from next generation electrolytes.

We conducted dynamic impedance spectroscopy (dEIS) to probe the formation and evolution of the SEI during Li plating and stripping on copper current collectors. dEIS superimposes a multisine waveform atop the dc stimulus signal, as shown by the lightly shaded curves in Fig 1 (top left). We applied a sliding window FFT protocol that takes the complex ratio of the measured potential and current signals obtained from Fig 1 (top right) and transforms it into complex impedance. We will discuss two Li platting systems, Lipon (an amorphous ceramic) and a liquid electrolyte with stabilizing additives. We observed drastic changes in the cells' impedance during plating and stripping, Figure 1 (bottom plots). We will show how the passivation layer's impedance continues to evolve during Li cycling and accounts for a significant amount of the overall cell resistance.

The US Department of Energy’s Energy Efficiency and Renewable Energy Vehicles Technologies Office provided funding for this work under the US-German Cooperation on Energy Storage: Lithium-Solid-Electrolyte Interfaces program.