Understanding Lithium Plating from Solid and Liquid Electrolytes Using Dynamic Impedance Spectroscopy

A potential strategy for increasing energy density of lithium batteries is to implement the so-called anode-free design. Here, the lithium metal anode is solely sourced from the cathode, traveling through the electrolyte. However, such Li batteries suffer from short cycle life due to lithium loss from passivation layer formation (solid-electrolyte interphase), which require multiple cycles to stabilize. Anode-free batteries are required to have high Li metal coulombic efficiencies 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.<br/><br/>We used dynamic electrochemical 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 applied charge and discharge current. We applied a sliding window fast Fourier transform protocol that converts the complex ratio of the measured potential and current signals 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. 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. This knowledge enables us to predict the relative length of cycle life without long term continual measurements.<br/><br/>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.