Investigation on Electrochemical Properties of TiNb6O17 As the New Anode Materials for High Power Lithium Ion Batteries

Abstract

Among the commercial anode materials, graphite isn`t suitable for high-power applications such as electric vehicles and large-scale energy storage due to formation of the solid-electrolyte interphase (SEI) leading to lithium irreversibly at about 0.8V and inactive high rate performance. Although, Li4Ti5O12 (LTO) are being pursued as alternatives to graphite electrodes due to high lithium storage voltages (1.0~2.0V) which are higher than the formation of SEI voltage and high rate capability, it has low theoretical capacity (~175mAhg-1). Recently titanium-niobium oxides (TNO) such as TiNb2O7, Ti2Nb10O29 with high redox potential range have been considered as interesting candidates for anode material of LIB. Because they have higher theoretical capacity of 380~396mAhg-1 due to multiple redox couples such as Ti4+/Ti3+, Nb5+/Nb4+, Nb4+/Nb3+ by compared with LTO having only Ti4+/Ti3+ redox couple. In addition, they have good stability, high rate capability. However, they are show limited reversible capacities (240~270mAhg-1) owing to low electronic conductivity and lithium diffusion coefficient. Chunfu Lin et al. presented that TiNb6O17 showed higher capacity, lithium diffusion coefficient and high rate performance than Ti2Nb10O29. Because it is consist of larger amount of Nb5+ ions with larger size (0.64A) than that of Ti4+ ions (0.605A). As a result, it has more open lithium insertion structure and lithium diffusion coefficient than Ti2Nb10O29. But this study not exactly proposed chemical properties such as oxidation state, lithium insertion behavior of TiNb6O17. Based on above study, we have compared physical properties and electrochemical performance of new compound TiNb6O17 and TiNb2O7 which were fabricated using a solid-state reaction method. In a typical synthesis, TiO2 and Nb2O5 in the molar ratio (1:2, 1:6) were mixed with ethanol in a ball mill at 300rpm for 4h. After drying, calcination treatment is carried at 1100°C for 10h in air atmosphere. The crystal structures were determined by X-ray diffraction (XRD), rietveld refinement in order to compare lattice parameters and unit cell volume. X-ray absorption spectroscopy (XAS) was measured to confirm variation of oxidation state and lithium insertion/extraction behavior after charge and discharge. The morphologies were observed by scanning electron microscope (SEM). Finally, we investigated electrochemical performance by measuring cyclic voltammetry, galvanostatic charge-discharge curve and C-rate. Also, electrochemical impedance spectroscopy (EIS) and slow scan rate cyclic voltammetry (SSCV) were measured to compare lithium diffusion coefficients of TiNb2O7 and TiNb6O17. As the result of, TiNb6O17 exhibited better electrochemical performance with lithium diffusion coefficient, charge-discharge capacities and high rate performance than TiNb2O7.