The Anode/Electrolyte Interface

The anode/electrolyte interphase (the SEI) plays a key role in lithium-metal, lithiumalloy and lithium-ion batteries. Close to the lithium side, it consists of fully reduced (thermodynamically stable) anions such as F^-, O^2-,S^2-, and other elements such as As, B, C (or their lithiated compounds). The equivalent volumes of both LiF and Li₂0 are too small (9.84 and 7.43mL, respectively) to provide adequate corrosion protection for lithium metal. Thus a second layer of Li₂C0₃ (equivalent volume, 17.5mL) or other organic materials is required to cover the first layer in order to provide this protection. The outer part of the SEI (near the solution) consists of partially reduced materials such as polyolefins, poly-THF, Li₂CO₃, LiRCo₃, ROLi, LiOH, and LiF, LiCl, Li₂O, etc. Often, polymers are the inajor constituent of the outer part of the SEI. It has been shown that the rate constants of the reactions of solvated electrons with electrolyte and solvent components (and impurities) are a good measure of the stability of these substances towards lithium. Use of the rate constants (k_e) for these reactions is suggested as a tool for the selection of electrolyte components. Good correlation was found between k_e and SEI formation voltage and composition.