Electrolyte Design for Self-Forming and Self-Healing SEI Layer for Li Metal Anodes

Abstract

The driving range and cost of electric vehicles is severely limited by the energy density of the state of art Li-ion batteries. The energy density of Li-ion batteries can be improved by replacing graphite anodes with lithium metal anode. The US Department of Energy targets for lithium metal batteries are cost < 100 $/kWh, gravimetric energy density > 500 Wh/kg, volumetric density > 800 Wh/L and have a charging time of 15 min. Lithium metal anodes suffer from dendrite/mossy formation and low cycling efficiency. It has been shown that forming stable SEI layers on top of lithium metal anodes can stop dendrite formation.[i],[ii] Electrolyte components play a crucial role in formation of stable SEI and also the SEI morphology, as the SEI components primarily comprise of decomposition products of the solvent, the salt and the additives in the electrolyte. In this work, we will discuss a new approach of forming a stable SEI layer with sufficient ionic conductivity and mechanical strength to suppress dendrites . The battery under cycling conditions will unavoidably form cracks in the SEI layer. Our approach is to provide a persistent source for self-healing of these cracks formed in the self-formed SEI during battery cycling. This is achieved by choosing appropriate solvents, salt anions and additives in the battery, which will form the same SEI layer during a forming step and during cycling of the battery. Using the appropriate solvent, salt and additives in the desired proportion leads to a spontaneously formed SEI layer comprised of specific components such as LiF, LiOH, Li2O, Li2CO3, Li2SO3, Li2S. The design principles associated with the selection of the electrolyte additives and salt components will be discussed. The effectiveness of the different SEI components will also be compared. We will quantify the amount of electrolyte components required for self-healing during entire cycle life. An important consideration is that the dissolved species in the electrolyte should also be stable to against high voltage Li-ion battery cathodes. Thus the high voltage stability of the electrolytes will also be shown. We will also explore the ion conduction pathways in SEI through the interfaces formed by the different SEI components. We will also discuss formation of stable SEI in the context of thin lithium foils (< 20 mm) and anode free batteries. Lastly the effect of the current collector in the context of anode free batteries will be discussed. [i] Xin-Bing Cheng, Rui Zhang, Chen-Zi Zhao, and Qiang Zhang. Toward safe lithium metal anode in rechargeable batteries: a review. Chemical reviews, 117(15):10403–10473, 2017. [ii] Dingchang Lin, Yayuan Liu, and Yi Cui. Reviving the lithium metal anode for high-energy batteries. Nature nanotechnology, 12(3):194, 2017.