(Invited) Dendritic Lithium Electrodeposition: A New Mechanistic Perspective Investigating Li⁺ Transport Limitations Imposed By the SEI

High-energy density batteries are essential for powering future electric vehicles and electric aircraft. The Li-metal anode offers the highest theoretical specific capacity among practically available anode materials. One major obstacle in the development of secondary Li-metal batteries is the dendritic electrodeposition of Li during the battery charging process. In this contribution, we report on our investigations of the onset time of dendritic growth during galvanostatic electrodeposition of Li metal. Using a combination of electrochemical and optical microscopy techniques, we observe that Li dendrites initiate at a time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. The dendrite onset time (τonset) is shown to increase with increasing temperature and decrease with increasing current density and initial solid electrolyte interphase (SEI) thickness. These observations guide the development of an analytical transport model wherein the Li+ concentration available for plating decreases gradually within the SEI as it becomes thicker during electrodeposition. At t = τonset, the mobile Li+ concentration at the Li–SEI interface approaches zero. Due to this Li+ concentration depletion, surface roughness on the Li electrode amplifies, eventually producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for Li plating. Model predictions of how τonset varies with current density, initial SEI thickness, and temperature are found to be in qualitative agreement with experimental observations. The analytical model explains mechanistically how transport limitations within the SEI control the onset time of dendrites during Li electrodeposition. Our results highlight the critical need for SEI materials that suppress or eliminate solid-state Li+ concentration gradients, which are responsible for dendrite initiation during Li-metal battery charging.