Effects of composition and structure on the performance of tin/graphene-containing carbon nanofibers for Li-ion anodes

Abstract

Tin is a promising replacement for graphite anodes in Li-ion batteries (994 mA h g⁻¹ for Sn vs. 372 mA h g⁻¹ for graphite), but suffers from particle pulverization upon lithiation that causes capacity fade. Herein, thermally reduced graphene oxide-containing carbon nanofibers (TRGO/CNFs) are used as scaffolds to house Sn/SnO₂ particles, enhance anode capacity beyond that of graphite, and prolong cycle life of Sn-based electrodes. This study attempts to elucidate structure–composition relationships of tin-TRGO/CNF electrodes that lead to increased capacity retention. The composition and morphology of tin-TRGO/CNFs are assessed as a function of heat-treatment temperature and Sn loading as a means to understand and correlate electrochemical performance with physical features. We find: (1) the oxidation state of tin in TRGO/CNFs is in part determined by temperature-dependent, thermal-decomposition products of polyacrylonitrile-derived CNFs, and (2) precursor Sn(IV) loadings ≤10 wt% in the tin-TRGO/CNFs lead to Sn(0) or SnO₂ particles embedded within the fiber + TRGO matrix. Electrodes with precursor Sn(IV) loading ≤10 wt% have smaller tin particles than electrodes with Sn(IV) loadings >10 wt%, and have longer cycle-lives; reversible capacities of ∼600 mA h g⁻¹ are observed at 0.2C rates, while capacities of ∼400 mA h g⁻¹ are observed after hundreds of cycles at 2C rates. The durable graphene-containing nanofiber matrix, coupled with the high-capacity of tin, provides a promising anode material for Li-ion cells.