Electrochemical Properties of Nb-Based Oxides As Active Materials for the Negative Electrode of Lithium Secondary Batteries

During the past twenty years, much effort has been undertaken to obtain lithium secondary batteries with higher energy densities, and a great progress has been made. However, there is still an increasing demand for further lightweight and small-size rechargeable batteries for portable electronic devices. In addition, recent environmental concerns about global warming and the need to reduce CO 2 emissions provide us a strong driving force for developing large-scale lithium secondary batteries with high energy densities for use in electric vehicles. One of the important requirements of negative electrode material to get a lithium secondary battery having a large energy density, the redox potential at the negative electrode should be as low as possible. In the present work, we report Nb-based oxides as active materials for the negative electrode of lithium secondary batteries. Composite electrodes were used for charge/discharge tests and cyclic voltammetry (CV). The test electrode was prepared by coating a mixture of the Nb-based oxide powder (80 wt.%) and a carbon black (10 wt.%) and a poly(vinylidene difluoride) binder (10 wt.%) on copper foil. The charge and discharge tests were carried using conventional three-electrode cells at a constant current of 18.1 mA g –1 . The electrolyte solution was 1 mol dm –3 (M) LiClO 4 dissolved in 1:1 (by volume) mixtures of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC + DEC) (Kishida Chemical Co., Lithium Battery Grade). The counter and reference electrodes were lithium foil. CV was performed between 3.0 and 30.0 V at a sweep rate of 0.5 mV s –1 . All electrochemical measurements were carried out in an argon-filled glovebox with a dew point < –60 °C. All potentials are referred to as volts vs . Li/Li + . The electrochemical properties of Li 1.1 Nb 0.9 O 2-x material was theoretically predicted by first principal method and experimentally tested by electrochemical method. The potential of Li 1.1 Nb 0.9 O 2-x for lithium ion insertion into and extraction out of the material, as predicted through theoretical calculations, was quite low below 0.3 V vs . Li/Li + , giving high cell voltage as a negative electrode material for lithium secondary batteries. Li 1.1 Nb 0.9 O 2-x was successfully synthesized by a solid state reaction and shown to have good cycle performance, although there is a large irreversible capacity in the first cycle. The main reversible reaction of the material is occurred at almost 0 V vs . Li/Li + . The low potential is originated from the electronic structure of the Li 1.1 Nb 0.9 O 2-x ; it acts as a favorable factor for an increase in energy density even though the possibility of an increase in electrolyte decomposition is present in initial charging process. The study on the electrochemical properties of Li 1.1 M 0.9 O 2-x (M=Ti, Cr, Zr, and Mo) might be meaningful because those compounds present low potentials for lithium ion insertion as predicted by the first principal method and show high cell voltage as a negative electrode material for lithium secondary batteries. The NbO 2 electrode showed a large irreversible capacity and small discharge capacity. The results of X-ray photoelectron spectroscopy indicate that the poor electrode performance of NbO 2 may be caused by niobium pentoxide (Nb 2 O 5 ) formed on the surface of active material. The Nb 2 O 5 could be removed by chemical etching to some extent, thus improving the electrode performance. In addition, The electrochemical properties of niobium monoxide, NbO, were investigated as a negative electrode material for lithium-ion batteries. Lithium ions were inserted into and extracted from NbO material at potentials < 1.0 V versus Li/Li + . The process involved the formation of a solid electrolyte interface (SEI) on the NbO surface in the first cycle. The reversible capacity was ~67 mAh g –1 with a capacity retention of ~109% after 50 cycles. The magnitude of the charge transfer resistance was greatly decreased by ball-milling the pristine NbO, but the ball-milling had no effect on the SEI resistance. References 1) K.T. Jacob, C. Shekhar, M. Vinay, Y. Waseda, J. Chem. Eng. Data, 55 ,4854 (2010). 2) L.-F. Cui, Y. Yang, C.-M. Hsu, Y. Cui, Nano Lett., 9 , 3370 (2009).