Ionic and Electronic Conduction in TiNb₂O₇.

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Titanium niobium oxide (TiNb2O7) is a Wadsley-Roth phase with potential for high-rate lithium-ion energy storage.
• Fundamental understanding of lithium insertion mechanisms and ion conduction in TiNb2O7 is limited.

Purpose of the Study:

• To elucidate the inherent properties of bulk TiNb2O7 using combined experimental and computational approaches.
• To understand the lithium insertion mechanism and ion conduction pathways in TiNb2O7.

Main Methods:

• Experimental techniques (e.g., NMR spectroscopy) were employed to study electronic and ionic conductivity.
• Density Functional Theory (DFT) calculations were used to model lithium diffusion pathways and energy barriers.
• Combined analysis of experimental and computational data provided insights into material properties.

Main Results:

• Electronic conductivity increased by seven orders of magnitude upon lithiation, with electrons exhibiting both localized and delocalized character.
• Lithium diffusion is rapid with low activation barriers in the single-redox region (Li<=3TiNb2O7), with D_Li = 10^-11 m^2 s^-1 at 525-650 K.
• Ionic diffusion is anisotropic, with significantly lower barriers along tunnels compared to across blocks; mobility is hindered in the multiredox region (Li>3TiNb2O7).

Conclusions:

• Lithium insertion leads to n-type self-doping and high-rate conduction in TiNb2O7, but ionic motion is eventually hindered at high lithiation.
• The TiNb2O7 structure is specifically suited for Li+ mobility compared to other alkali and alkaline-earth metal ions.
• Understanding these properties is crucial for optimizing TiNb2O7 as a high-performance electrode material for lithium-ion batteries.