Sodium-ion batteries (earlier post) are considered a potential attractive alternative to lithium-ion batteries. A battery that uses sodium ions instead of lithium ions could potentially be much less expensive and safer, and it would be more environmentally benign. However, Na-ion batteries have exhibited weak charge-discharge behavior except at high temperature—indicative of sluggish kinetics in standard carbon anodes. (Earlier post.)

Despite several studies of cathode materials for sodium ion batteries involving layered oxide materials, the Argonne team notes in their paper, there are few low-voltage metal oxide anodes capable of operating sodium-ion batteries reversibly at room temperature.

They attributed this as being likely due to the prohibitively large ionic radius of the sodium ion (1.02 Å) as compared to the Li ion (0.76 Å); insertion of Na ion therefore requires large distortion of the metal oxide lattice, which would require unacceptably elevated temperatures not realistic for the operation of batteries.

In this study, we utilized amorphous electrochemically synthesized 1D titanium dioxide nanotube (TiO₂NT) as an anode for Na-ion batteries and demonstrated reversible self-improving specific capacity of ~150 mAh/g. In addition, we demonstrated for the first time all-oxide reversible Na-ion batteries using TiO₂NT anode and a lithium-substituted sodium-layered transition-metal oxide cathode operating at room temperature.

...TiO₂ has garnered significant attention in energy-storage applications, in particular, in lithium-ion batteries. TiO₂ is one of a few transition metal oxide materials that intercalates Li ions at reasonably low voltage (~1.5 V vs Li/Li⁺) with comparable capacities to the dominant graphite anodes. Although a variety of metal oxide materials have been identified as sodium-ion cathode materials, there are very few materials reported to be suitable for anode materials. With the success of TiO₂ as anode materials for Li-ion batteries, it is interesting to investigate its utilization for Na-ion batteries.

—Xiong et al.

In previous unpublished work, the team had found that amorphous TiO₂NT electrodes underwent irreversible phase transition by self-organization upon electrochemical cycling with Li⁺. The newly formed cubic phase exhibits self-improving specific capacity as high as 310 mAh/g in a Li full cell.

When they used amorphous TiO₂NTs as electrodes in a sodium-ion half-cell, they found that cycling of narrow NTs of amorphous TiO₂ (<45 nm ID [internal diameter] and 10 nm wall thickness) does not lead to noteworthy intercalation of Na ions. However, when they gradually increased the size of the nanotube to >80 nm ID (wall thickness >15 nm), they observed cycling with relatively low specific capacity initially that self-improves as cycling proceeds.

Starting at a 75 mAh/g in the first cycle, the specific capacity almost doubled to reach 150 mAh/g after only 15 cycles. This self-improving of the specific capacity upon cycling occurs at a slow rate of 0.05 A/g compared with observed self-improving of the specific capacity for a Li/TiO₂NT system that occurs only at fast cycling of 3 A/g.

This is highly unusual material behavior. We’re seeing some nanoscale phase transitions that are very interesting from a scientific standpoint, and it is the deeper understanding of these materials’ behaviors that will unlock mysteries of materials that are used in electrical energy storage systems.

—Jeff Chamberlain, Argonne chemist who leads the laboratory’s energy storage major initiative

Charge/discharge voltage profile of the full Na-ion battery at ambient temperature cycled between 2.6 and 1 V at various rates. Credit: ACS, Xiong et al. Click to enlarge.

To show the practical application of the material as an anode for Na-ion batteries operating at room temperature, they built a Na-ion cell with a TiO₂NT anode and a Na_1.0Li_0.2Ni_0.25Mn_0.75O_δ cathode with the cell capacity limited by the mass of the anode. The cell shows an operation voltage of ~1.8 V and a discharge capacity of ~80 mAh/g. (The obtained voltage is smaller compared with a Li-ion battery, but still higher than 1.2 V, the operation voltage of the NiMH battery used in most hybrids now, the authors noted.)

This cell showed excellent rate capability with ~70% low-rate capacity retained at 11C (0.56 A/g).

This Na-ion oxide battery therefore indeed holds promise for the further development of new ambient temperature Na-ion battery systems that combine novel electrodes to form energy storage devices that are inexpensive with good performance.

—Xiong et al.

Resources

• Hui Xiong, Michael D. Slater, Mahalingam Balasubramanian, Christopher S. Johnson, and Tijana Rajh (2011) Amorphous TiO₂ Nanotube Anode for Rechargeable Sodium Ion Batteries. The Journal of Physical Chemistry Letters 2 (20), 2560-2565 DOI: 10.1021/jz2012066