Surfactant-Mediated Synthesis of Graphene-TiO2 Nanocomposites for Lithium-Ion Batteries

Abstract

Metal oxides are attractive electrode materials for lithium-ion batteries due to inherent safety from electroplating, and mechanical and chemical robustness. However, poor electronic conductivity limits bulk oxides to slow charging rates. In addition to nano-sized oxide particles, conductive additives are often used to improve high-rate performance. Functionalized graphene sheets (FGSs) are an ideal additive as they have high electrical conductivities and specific surface areas. Hybrid nanocomposites of titanium dioxide (TiO2) and FGSs were synthesized via an aqueous surfactant-mediated approach using sodium dodecyl sulfate (SDS). The Li-storage capacity of FGS-TiO2 was enhanced compared to TiO2 nanoparticles. Nevertheless, neither the synthesis nor the enhanced performance is well understood.

In the synthesis of FGS-TiO2, SDS acts to disperse FGSs and promote TiO2 growth on FGSs. The key process for both is SDS adsorption onto FGSs, which is investigated using conductometric surfactant titration. We propose a four-stage adsorption model: (i) adsorption of isolated monomers at very low concentrations; (ii) formation of an adsorbed monolayer at ~12 μM SDS; (iii) formation of hemi-cylindrical surface micelles at ~1.5 mM; (iv) micelle formation in bulk solution at ~8 mM.

SDS adsorption was then related to the colloidal stability of FGSs. Using optical microscopy, we observed branched aggregates in the absence of SDS, a transition to compact aggregates as the SDS concentration increased to 10 μM, and dispersed FGSs above 20 μM. We quantified FGS settling with UV-Vis absorbance: the largest absorbance decreases were measured in compact aggregate-containing suspensions. Persistent settling below 40 μM indicated that compact aggregates formed either through branched-aggregate restructuring or aggregation of initially-dispersed FGSs. Interaction energy calculations supported these results; we determined that stably-dispersed FGSs are obtained above ~40 μM SDS.

Finally, the influence of reaction parameters on the properties of various FGS-TiO2 samples was investigated. Clear differences in high-rate Li-storage performance were observed, particularly at high mass loadings. This is attributed to differences in power loss during cycling. Characterization of the electrodes showed that electron transport might not be the main source of performance differences; instead, Li<super>+</super> transport in the electrode and electrolyte likely has a greater impact on electrochemical performance.