Pore Collapse and Regrowth in Lithiated Si Anodes Determined By in-Operando Neutron Reflectometry

Structure and composition changes of an 11 nm thick amorphous silicon (a-Si) thin film anode, capped with a 4 nm thick alumina film are measured, in operando, by neutron reflectometry (NR) and electrochemical impedance spectroscopy in a lithium half-cell.  NR data are analyzed to quantify the a-Si thickness and composition at various states of charge over six cycles. The a-Si anode expands and contracts upon lithiation and delithiation, respectively, while maintaining its integrity and low interfacial roughness (£1.6 nm) throughout the cycling.  The apparently non-linear expansion of the a-Si layer volume versus lithium content agrees with findings from previous thin-film a-Si anode studies.  However, a proposed pore collapse and regrowth (PCRG) mechanism explains apparently minimal volume expansion of the film as a whole for low Li content in terms of linear expansion of the solid domains within the initially porous amorphous Si. In the PCRG model, the porosity is first consumed by the expansion of solid domains upon lithiation, after which the film as a whole expands linearly with Li content with a proportionality factor found to be 8.48 cm³/mol-Li, which is similar to that of crystalline Si.  Porosity is reversibly reestablished at 5-28% upon delithiation. Data also show that the alumina protective layer on the a-Si film functions as an effective artificial solid electrolyte interphase (SEI), which maintains its structural integrity, low interfacial roughness, and a relatively small transport resistance. No additional spontaneously-formed SEI is observed in this study.