Progression of Solid Electrolyte Interphase Formation on Hydrogenated Amorphous Silicon Anodes for Lithium-Ion Batteries

Silicon anodes based on an alloy reaction with lithium have a large theoretical specific capacity making them an appealing candidate for use in lithium-ion batteries. A major factor influencing the power cyclability and cycle life of the battery is the formation of the solid electrolyte interphase (SEI) layer. In this work, the progression of SEI formation on hydrogenated amorphous Si (a-Si:H) anodes is determined as a function of applied electrochemical potential during the first charging cycle by combining cyclic voltammetry measurements with detailed surface chemical analysis. During this first lithiation cycle, the SET layer begins to form at 1.8 V by decomposition of the LiPF6 electrolyte to LiF, LixPFy, and PFy. The SEI layer, with LiF as the major species, continues to form upon further charging and forms a nonuniform layer on the surface of the electrode. At 0.4 V the Li atoms begin to penetrate the a-Si:H network, and upon full charging at 0.0 V, the anode itself is comprised in part by Si-Li, Si-F, and a network of F-Si-Li-n. During the second lithiation cycle, Li causes significant scission of the Si-Si bonds resulting in the formation of high concentrations of LixSi.