PNNL

Abstract

Surface reactivity and ion transfer processes of anatase TiO2 nanocrystals were studied using Lithium bis(trifluoromethylsulfone)imide (LITFSI) as a probing molecule. The HRTEM analysis of synthesized anatase TiO2 reveals aggregated nanoparticles (average size ~8nm) with significant defects (holes and cracks). With the introduction of LiTFSI salt, the Li propensity towards defective surface along the anatase (100) plane is evident in XRD and HRTEM analysis. Ab initio molecular dynamics (AIMD) analysis corroborates the site-preferential interaction of Li+ cations with oxygen vacancies and the thermodynamically favourable transport through the (100) step edge plane. Using 7Li NMR chemical shift and relaxometry measurements, the Li+ cation near the TiO2 interface and the bulk LiTFSI phase were speciated, and subsequent diffusion properties were analyzed. The lower activation energy derived from NMR analysis reveals enhanced mobility of Li+ cations along the surface, in good agreement with AIMD calculations. On the other hand, the TFSI anion interaction with defect sites leads to CF3 bond dissociation and subsequent generation of carbonyl fluoride-type species. The multimodal spectroscopic analysis including NMR, EPR and XPS analysis confirms the decomposition of TFSI anions near the anatase surface. The reaction mechanism and electronic structure of interfacial constituents were simulated using AIMD calculations. Overall, this work demonstrates the role of defects at the anatase nanoparticle surface on charge transfer and interfacial reaction processes.