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New Titanate Negative Electrode Materials for Na-Ion Batteries
Lithium-ion batteries are widely used because they are lightweight, have a high energy density, and can be cycled many times. Sodium-ion has shown promise as a potentially lower cost, more sustainable battery chemistry [1]. Titanates have been investigated for use as anode materials in Na and Li cells [2]. However they are non-conductive, requiring the use of large amounts of carbon black to cycle [3]. We have found new titanate phases that can reversibly intercalate Na as bulk micron-sized particles without the need for large amounts of carbon black.
Experimental
Samples were synthesized via heating precursors to 900 °C for 10 hours in argon. A 10 % Na excess was added due to the volatility of Na at these elevated temperatures. Products were found to be air or moisture sensitive. Samples for X-ray diffraction measurement were loaded under argon into an air-tight sample holder with an aluminized Mylar window. The sample holder was purged with He gas during XRD measurement. Electrodes comprising active materials, carbon black and PVDF binder in a 80:10:10 weight ratio were cast using NMP as a solvent and dried at 120°C. Electrodes were cycled in 2325 coin type cells at a rate of C/10 (assuming a 100 mAh/g capacity) using Na metal counter/reference electrodes and 1M NaPF6in PC electrolyte.
Results and Discussion
Figure 1 shows a voltage curve of a new titanate negative electrode material. In-situ XRD shows that reversible Na-insertion occurs via an intercalation mechanism in which the titanate's volume only expands by 1.6% after full sodiation. The volumetric capacity of the titanate is 400 Ah/L, which is comperable to that of hard carbon.
To our knowledge, this is the first demonstration of intercalation of sodium into bulk oxides at such low voltages without the use of large amounts of carbon black. The use of these new titanate phases presents a new strategy towards making negative electrodes for Na-ion cells, from which high energy density bulk intercalation materials may be developed.
References:
[1] M.D. Slater et al., Adv. Funct. Mater., 23(2012) 947-958.
[2] L. Zhao, Journal of Power Sources, 242(2013) 597-603.
[3] P. Senguttuvan et al., Chem. Mat., 23(2011) 4109.
Figure 1 Voltage curve of a new titanate negative electrode vs. sodium metal.