Due to the high cost of lithium-ion batteries and potential limited accessibility of lithium reserves, sodium-ion batteries have emerged as an alternate battery chemistry [1]. Sodium-ion batteries trade lithium ions for sodium ions while retaining the rocking chair like mechanism for electrochemical transport. The advantage of using sodium instead of lithium is its low cost, which is largely due to its widespread availability. Titanates have been investigated for use as anode materials in Na and Li cells. Recently and new family of titanates, NaxMyTi1-yO2 (M = Li, Ni, Co, or Cr), have been reported. These materials show remarkable low voltage cycling stability [2-5]. Herein another member of this family, namely the NaxVxTi1-xO2 (0.6 ≤ x ≤ 1) series, was studied for use as an anode material in sodium-ion batteries.
Experimental
Samples were synthesized via high energy ball milling of Na2CO3, V2O3, and TiO2 then heating pellets to 950 °C for 6 hours in an argon/H2:95/5 atmosphere. A 10 % Na excess was added due to the volatility of Na at these elevated temperatures. Samples for X-ray diffraction measurement were loaded under argon into an air-tight sample holder with an aluminized Mylar window. Electrodes comprising NaxVxTi1-xO2, carbon black and PVDF binder 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 NaPF6 in PC electrolyte.
Results and Discussion
Figure 1 shows the XRD patterns of the as synthesized NaxVxTi1-xO2 (0.6 ≤ x ≤ 1) materials. They all have layered type structures. Electrochemical measurements showed that these materials can intercalate Na in bulk particles at low voltages and with very low hysteresis. Figure 2 shows the voltage profile of the x = 0.66 material for the first cycle and a half. These materials have substantial low voltage capacity and remarkably low hysteresis, as shown in Figure 2. As such they are promising candidates for Na-ion battery negative electrodes. Structural changes and stability during cycling, for different cut-off voltages, will be discussed.
References:
[1] D. Kundu et al., Angew. Chem. Int. Ed. Engl., 54 (2015) 3431-3448.
[2] R. Fielden, M.N. Obrovac, J. Electrochem. Soc., 161 (2014) A1158-A1163.
[3] Y. Wang et al., Nature Comm., 6 (2015) 6954-6962.
[4] H. Yu et al., Angew. Chem. Int. Ed. Engl., 53 (2014) 8963-8969.
[5] Y. Wang et al., Nature Comm., 4 (2013) 1-7.