Mechanistic understanding of Li metal anode processes in a model 3D conductive host based on vertically aligned carbon nanofibers

Abstract

The use of three dimensional (3D) conductive hosts is a promising strategy to alleviate dendrite formation in lithium metal anodes. However, the fundamental issues associated with the 3D hosts, such as formation of excessive solid electrolyte interphase (SEI) and complicated Li deposition morphologies, have not been well studied. This study focuses on understanding these critical issues using vertically aligned carbon nanofiber (VACNF) array as a model 3D host. Various characterization tools are employed to elucidate the morphology and composition of electrodeposited Li (e-Li) and its associated SEI. The e-Li on VACNFs exhibits two different morphologies – a dominant micron-scale columnar infiltrative Li and a small amount of nanoscale coaxial Li sheath on VACNFs in the unfilled interstitial area between infiltrative grains. During Li intercalation, an organic-rich loose SEI film forms on the top of VACNF array and an inorganic-rich SEI sheath forms around individual VACNFs. Li plating transforms them into inorganic dominant SEIs directly in contact with the e-Li surface. The SEI skins on the infiltrative Li remain loosely attached to the VACNF array after stripping Li and irreversibly accumulate during Li plating/stripping cycling, while the coaxial SEI on the nanoscale Li remains as a stable elastic sheath. The loose SEI skins cause electrolyte consumption and increased heterogeneity, eventually leading to cell failure.