Amorphous titanium dioxide nanotube anodes for sodium-ion batteries show ability to self-improve specific capacity
|Charge/discharge galvanostatic curves of amorphous TiO2NT in Na half cell (red for discharge and black for charge) cycled between 2.5 and 0.9 V versus Na/Na+ at 0.05A/g (C/3). Credit: ACS, Xiong et al.Click to enlarge.|
A team of researchers at the US Department of Energy’s Argonne National Laboratory has synthesized amorphous titanium dioxide nanotube (TiO2NT) electrodes directly grown on current collectors without binders and additives to use as an anode for sodium-ion batteries.
These electrodes can switch their phase as a battery is cycled, gradually boosting their operational capacity. Maximizing their capacity in operando, the electrodes reached reversible capacity of 150 mAh/g in 15 cycles in lab testing. The team also demonstrated for the the first time a full cell all-oxide Na ion battery exhibiting good rate capability at room temperature. A report on the material and the Na-ion cell is published in the ACS Journal of Physical Chemistry Letters.
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 Na1.0Li0.2Ni0.25Mn0.75Oδ 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.
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