X-ray analysis reveals the charging mechanism of a promising electrode material

November 30, 2018, Agency for Science, Technology and Research (A*STAR), Singapore
A titanium dioxide electrode takes in lithium ions as the battery discharges. Credit: Ref. 1. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

An experimental technique developed by A*STAR researchers has been used to track the chemical and structural changes in an electrode as a battery discharges. The X-ray-based technique should help to improve the performance of materials in next-generation batteries.

Lithium-ion batteries are widely used in our daily lives, for example in mobile devices and electric vehicles. They store and release energy by shuttling lithium ions between two electrodes (see image). But the amount of energy that these electrodes can store, and the speed at which the batteries charge or discharge, is still relatively limited. Moreover, repeated use can cause the electrodes to expand and contract, degrading their performance over time.

Electrodes containing titanium dioxide nanotubes organized in a form known as the bronze phase could help to overcome these restrictions because the material has a high theoretical charge capacity and its volume changes little during operation. However, its charging mechanism is not fully understood, due to the limitations of analytical tools that can directly probe the surface charging process.

Yonghua Du of the A*STAR Institute of Chemical and Engineering Sciences, and Xiaodong Chen's group at Nanyang Technological University have now tackled this problem by using the Singapore Synchrotron Light Source to perform X-ray absorption spectroscopy measurements on the titanium dioxide electrodes during operation.

They discovered that the average charge of the material's titanium atoms, known as their , dropped steadily from roughly four to three as the material accumulated lithium ions during discharge. The experiments also revealed how the material's crystal structure expanded as lithium ions accumulated in the . Since titanium atoms in a low-valence state are slightly larger than ones in a higher valence state, this further distorted the crystal structure. "A phase transition occurs during charging and discharging," explains Du.

Different mechanisms for charge storage can occur at the electrode's surface, which the experiments quantified for the first time. They showed that most of the battery's storage capacity depends on the change in titanium's valence state. Further tests demonstrated that hollow could store more charge than nanowires of the same material.

As the discharge rate increased, a greater proportion of lithium ions were stored at the electrode's surface, rather than deep within its structure. This reduced the change in the average valence state of , which ultimately lowered the electrode's energy capacity.

This analysis of how the work will help to guide researchers as they design electrode nanostructures to improve lithium-ion storage and mobility. Du notes that their X-ray absorption spectroscopy technique could be also applied to other electrode materials.

Explore further: Making it crystal clear: Crystallinity reduces resistance in all-solid-state batteries

More information: Yuxin Tang et al. Identifying the Origin and Contribution of Surface Storage in TiO2 (B) Nanotube Electrode by In Situ Dynamic Valence State Monitoring, Advanced Materials (2018). DOI: 10.1002/adma.201802200

Related Stories

A novel approach of improving battery performance

September 18, 2018

New technological developments by UNIST researchers promise to significantly boost the performance of lithium metal batteries in promising research for the next-generation of rechargeable batteries. The study also validates ...

Recommended for you

Field-responsive mechanical metamaterials (FRMMs)

December 11, 2018

In a recent study published in Science Advances, materials scientists Julie A. Jackson and colleagues presented a new class of materials architecture called field-responsive mechanical metamaterials (FRMM). The FRMMs exhibit ...

Researchers develop smartphone-based ovulation test

December 11, 2018

Investigators from Brigham and Women's Hospital are developing an automated, low-cost tool to predict a woman's ovulation and aid in family planning. Capitalizing on advancements in several areas, including microfluidics, ...

CRISPR method for conditional gene regulation

December 11, 2018

A team of engineers at the University of Delaware has developed a method to use CRISPR/Cas9 technology to set off a cascade of activities in cells, a phenomenon known as conditional gene regulation. Their method, described ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.