Titanium dioxide: New anode material for batteries

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2012

Researchers discover an alternative anode material to carbon for use in rechargeable batteries.

KEY CONCEPTS
• Amorphous titanium dioxide has been discovered to be an alternative anode material to carbon in lithium-ion batteries.
• When in use as the anode, the amorphous titanium dioxide converts to a crystalline material after several charge and discharge cycles.
• The amorphous titanium dioxide anode was also used in preparing an all-oxide sodium battery for the first time.

MUCH OF THE INTEREST IN THE DEVELOPMENT OF BATTERY TECHNOLOGIES HAS FOCUSED on working with lithium ions because they exhibit the highest energy density. But lithium-ion batteries also have operational problems.

Chris Johnson, project leader, Lithium Battery Materials Group, Electrochemical Energy Storage Technologies at Argonne National Laboratory in Argonne, Ill., says, “Lithium-ion batteries have been plagued by safety problems where there is the possibility of them catching fire during extreme conditions. There is also a limited availability of lithium, with most of the sourcing being in South America.”

In an effort to improve performance, nanostructure materials have been evaluated for use in battery anodes. A previous TLT article discusses the use of an anode containing a gradient of silicon, aluminum and carbon in the shape of a scoop of ice cream (1). This anode material was developed to better handle the strain occurring during the uptake and discharge of lithium ions. Power densities from this nanoscoop electrode were one order of magnitude better than a conventional battery anode.

An attractive alternative to lithium is the use of sodium ions in rechargeable batteries. Tijana Rajh, group leader, Nanobio Interfaces Group at the Center for Nanoscale Materials at Argonne National Laboratory, says, “In contrast to lithium, sodium is available at a low cost and can be obtained in large quantities from such sources as seawater. Sodium is also not subject to the cost and speculation that occurs with lithium.”

This aspect is important because raw materials represent one-third of the cost of a battery. Use of sodium ions in a battery has been hindered because no metal-oxide anode has been developed to operate at room temperature. Johnson says, “The problem has to do with the size of the sodium ion as compared to the lithium ion. A sodium ion has a much higher radius (1.02 angstroms versus 0.76 angstroms) and twice the volume of lithium ions, which makes it very difficult to insert a sodium ion into a metal-oxide anode lattice.”

The traditional anode material used has been carbon, but the high voltages render the battery prone to a dangerous overcharge condition. It is desirable to develop a high-performing battery that can be overcharged without incident.

Progress has now been made toward the development of working with a brand new anode material and using it in a sodium-ion battery.

TIO₂NT
Johnson and Rajh, in collaboration with their associates at Argonne, have developed a new anode material based on titanium dioxide known as amorphous titanium dioxide nanotubes (TiO₂NT). This material is prepared through an electrochemical anodization process that starts with pure titanium thin foil.

Titanium dioxide was selected because of its availability, wide use in industrial applications, relatively low cost and, most important, stability. The interesting aspect of this research is the use of an amorphous version of titanium dioxide, as shown in Figure 2. Johnson says, “For batteries, the mantra is to use crystalline materials because conductivity must be available to facilitate the intercalation of the lithium ions into an electrode material such as graphite.”


Figure 2. Amorphous titanium dioxide has the potential to be a more suitable material for use as an anode in lithium-ion batteries due to better availability, relatively low cost and better stability than carbon. (Courtesy of Argonne National Laboratory)

The researchers also worked with a nanoscale material because of the expectation that diffusion of transporting ions is enhanced. Rajh says, “Nanostructured materials exhibit superior power and energy densities compared to macroscale materials because of enhanced kinetics and the larger surface area, which enables better access for the transporting ions.”

In initial work using TiO₂NT as the anode in a lithium-ion battery, the researchers observed that the amorphous titanium dioxide converted to a crystalline material as the battery went through several charge and discharge cycles. Rajh says, “From a simulation that we did, we noted that lithium ions diffuse slowly into the amorphous titanium dioxide. When we flooded the TiO₂NT with lithium ions, they acted as a lubricant in high concentrations enabling the flexible amorphous material to convert to a more crystalline structure which can accommodate more ions.”

This movement from an amorphous to a crystalline state starts with the first charge, discharge cycle. Johnson terms this unexpected phenomenon a terrific story as the titanium dioxide forms a face-centered cubic structure. He adds, “Other battery materials usually convert from crystalline to amorphous structures in contrast to titanium dioxide.”

The information obtained from the lithium-ion battery was used in an all-oxide sodium battery for the first time. A TiO₂NT electrode was used in conjunction with a sodium metallic-oxide cathode in a battery that reached a reversible capacity of 150 milli-ampere hour per gram in 15 cycles at room temperature.

A key factor in the success of the battery is the size of the nanotubes used in the amorphous titanium dioxide. Rajh says, “We found no evidence for intercalation of sodium ions if the diameter of the titanium dioxide nanotubes is less than 120 nanometers. The reason is that the sodium ions are just too big, which makes it less likely for them to adsorb on the surface of the titanium dioxide anode, so one needs more electrolyte volume within the titanium dioxide nanostructure and a larger tube diameter enables just that.”

Johnson notes that the titanium dioxide does not reach a crystalline phase in the sodium-ion battery. He says, “A very flexible, elastic structure is formed in the anode that accommodates the intercalation of sodium ions.”

One of the other benefits of using titanium dioxide is safety. Johnson says, “The carbonaceous material present in graphite electrodes has a much greater chance of catching fire than titanium dioxide.”

Future work will involve development of sodium-ion batteries with double the capacity and the use of sodium chloride as an electrolyte. Additional information will be available in a paper recently submitted for publication (2) or by contacting Jared Sagoff of Argonne at jsagoff@anl.gov.

REFERENCES
1. Canter, N. (2011), “New Lithium-Ion Battery Technology,” TLT, 67 (3), pp. 12-13.
2. Xiong, H., Slater, M., Balasubramanian, M., Johnson, C. and Rajh, T., “Amorphous TiO₂ Nanotube Anode for Rechargeable Sodium Ion Batteries,” Journal Physical Chemistry C, submitted for publication.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.