The search for clean and green energy in the 21st century requires a better and more efficient battery technology. The key to attaining that goal may lie in designing and building batteries not from the top down, but from the bottom up — beginning at the nanoscale. A team of researchers from Argonne National Laboratory and the University of Chicago has taken such an approach by developing titanium dioxide (TiO₂) electrodes that can actually improve their own electrochemical performance as they are used.

The experimenters synthesized TiO₂ nanotubes and assembled them into Li-ion coin cells, then cycled them galvanostatically between 0.8 V and 2.0 V. Electrode samples from the cells were then examined using x-ray diffraction (XRD) at the GeoSoilEnvirioCARS 13-ID-D insertion device beamline and x-ray absorption spectroscopy (XAS) at the X-ray Science Division 20-BM bending magnet beamline, both at the U.S. Department of Energy’s Advanced Photon Source at Argonne.

In addition to the synthesis of the TiO₂ nanotubes, scanning electron microscopy imaging and molecular dynamics simulations also were performed at the Argonne Center for Nanoscale Materials. All these techniques provided a window into the inclusion and removal of ions (intercalation/deintercalation process) occurring within the TiO₂ nanotubes.

Using the amorphous nanoscale TiO₂ nanotubes as an anode in lithium half-cells, the researchers noted a consistently linear decreasing voltage during the first discharge, followed by a “hump” at ~1.1 V vs Li/Li⁺. This indicated an irreversible phase transition in the nanotube material.

On subsequent cycles, Li⁺ ions reversibly intercalated/deintercalated into the TiO₂ nanotubes with capacities far beyond those observed in other TiO₂ varieties such as anatase.

The team concluded that this is due to a different structure or intercalation mechanism occurring as a result of the phase transition. Compared to anatase, the phase-transformed TiO₂ nanotube anode displayed greatly improved Li-ion diffusion, especially at high cycling rates. The TiO₂ nanotube anode demonstrated both much higher energy and higher power compared to its structural TiO₂cousins, which displayed a decrease in capacity in similar experiments using fast cycling.

The XRD and XAS studies, along with computational simulations, displayed how the anode structure changes upon cycling. Above ~1.1 V, no changes were observed with cycling, but below 1.1 V, a highly-symmetric, closely-packed cubic oxygen crystalline structure formed, with Ti and Li randomly distributed among octahedral sites.

Interestingly, the type of short-range order that would be expected in such a fully-ordered octahedral system apparently does not develop in this case. However, this does not affect thermodynamic stability, and the cubic structure remained both highly stable and reversible following the phase transition.

It appears that the intercalation/deintercalation of Li⁺ ions initiates a new structure that allows even better intercalation of Li⁺ ions. Because all layers of the new structure retain metal atoms even in the charged state, the cubic phase of the material is preserved. Molecular dynamics simulations of Li-ion diffusion in other types of TiO₂ structures showed that the most efficient diffusion and the lowest activation barrier (0.257 eV) occurs in the amorphous cubic Li₂Ti₂O₄ form, compared to other TiO₂ varieties such as, again, anatase.

The amorphous-to-cubic TiO₂ nanotube anode was tested in a full cell configuration with a 5-V spinel cathode (LiNi_0.5Mn_1.5O₄). On repeated cycling, the cell displayed an average voltage of 2.8 V and improving capacity.

Another distinct advantage of the TiO₂ nanotube anode is that because it does not suffer capacity degradation, it avoids Li plating at the graphite anode and electrode over-potentials that create possible safety hazards in other types of Li-ion batteries.

By creating a nanoscale electrode material that can actually order itself into a more efficient and powerful electrochemical structure as it is subjected to repeated discharging and charging, the research team forged a new pathway for the design and development of higher capacity, higher power, safer batteries. In our world of smart phones technology and electric cars, the importance of such an advance can hardly be overestimated. — Mark Wolverton

See: Hui Xiong¹, Handan Yildirim¹, Elena V. Shevchenko¹, Vitali B. Prakapenka², Bonil Koo¹, Michael D. Slater¹, Mahalingam Balasubramanian¹, Subramanian K. R. S. Sankaranarayanan¹, Jeffrey P. Greeley¹, Sanja Tepavcevic¹, Nada M. Dimitrijevic¹, Paul Podsiadlo¹, Christopher S. Johnson¹*, and Tijana Rajh¹**, “Self-Improving Anode for Lithium-Ion Batteries Based on Amorphous to Cubic Phase Transition in TiO₂ Nanotubes,” J. Phys. Chem. C 116, 3181 (2012). DOI:10.1021/jp210793u

Author affiliations: ¹Argonne National Laboratory, ²The University of Chicago

Correspondence: *cjohnson@anl.gov; **rajh@anl.gov

This work, use of the Argonne Center for Nanoscale Materials, and use of the Argonne Advanced Photon Source were supported by the U.S. Department of Energy Office of Science (DOE-SC) under Contract DE-AC02-06CH11357. Use of beamline 20-BM was supported by the DOE-SC and by the Natural Sciences and Engineering Research Council of Canada. GeoSoilEnviroCARS is supported by the National Science Foundation Earth Sciences (EAR-1128799) and DOE Geosciences (DE-FG02-94ER14466).

The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science to carry out applied and basic research to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels, provide the foundations for new energy technologies, and support DOE missions in energy, environment, and national security. To learn more about the Office of Science x-ray user facilities, visit http://science.energy.gov/user-facilities/basic-energy-sciences/.

The Center for Nanoscale Materials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://science.energy.gov/bes/suf/user-facilities/nanoscale-science-research-centers/.

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