Transport of External Lithium Along Phase Boundary in LiF-Ti Nanocomposite Thin Films.

Hao Zheng,Hong Li,Rui Wang,Jian Gao,Jieyun Zheng

doi:10.17344/acsi.2016.2346

Abstract

The electronic and ionic conductivity of the LiF-Ti nanocomposite films prepared by the co-sputtering have been investigated by the method of impedance spectrum (IS), current-voltage curves (IV) and isothermal transient ionic current (ITIC) measurements. It is found that the ionic conductivity of the obtained LiF-Ti nanocomposite film is very low. After electrochemical and chemical lithiation, ionic conductivities of the lithiated composite films are increased to be 10-3 and 10-4 S/cm separately. This phenomenon indicates that the phase boundary between LiF and Ti could be the ionic conducting channels for external lithium in the LiF-Ti nanocomposite. Our results suggest a new strategy to design ionic or mixed ionic conductor.

Highlights

Transport behaviours of ions in solid can be classified into four types:1–7 1) motion of ions through vacancy or interstitial sites in a defected host lattice; 2) diffusion of ions in the free space through the segment wriggle of a polymer; 3) diffusion of ions on the surface of a solid; 4) diffusion of the ions along or across the interfaces between grains or phase
The electrochemical lithiation may lead to the coverage of the insulating solid electrolyte interphase (SEI) film
All results from IV, impedance spectrum (IS) and isothermal transient ionic current (ITIC) confirms that electrochemically lithiated LiF: Ti to a Ti-poor composite (LiF)-Ti nanocomposite thin film shows higher electronic and ionic conductivity compared than pristine LiF-Ti thin film

Summary

Introduction

Transport behaviours of ions in solid can be classified into four types:1–7 1) motion of ions through vacancy or interstitial sites in a defected host lattice; 2) diffusion of ions in the free space through the segment wriggle of a polymer; 3) diffusion of ions on the surface of a solid; 4) diffusion of the ions along or across the interfaces between grains or phase. It is noticed that extra lithium can be stored reversibly at the low voltage range after the formation of LiF-Ti nanocomposite.[14] Jamnik and Maier explained such heterogeneous storage phenomenon as the interfacial charging.[15,16]. The transport properties of the lithium ions and electrons in the phase boundaries of the LiXM nanocomposite and the SEI film are difficult to be measured separately. One valuable in situ TEM investigations on the reaction of Li with FeF2 in a solid cell showed that FeF2 can be fully lithiated and converted into the LiF-Fe nanocomposite.[17] Wang et al suggested that the massive interface formed between nanoscaled solid phases provides pathways for ionic transport during the conversion process. The ionic and electronic conductivity of the LiF-Ti nanocomposites with and without external lithium can be measured and compared

Experimental

Results and Discussion

Summary

Reference