Optimization of Cation Composition on Charge-Discharge Behavior in Rock-Salt Type Lithium Titanium Oxynitride Anode Material

Abstract

The lithium-ion batteries have been widely used in portable electronic devices such as laptop computers, mobile phones and so on. Despite lithium batteries possess obvious advantages over nickel–cadmium and lead–acid cells with regards to high energy density, there are still some safety concerns with the carbon anode and a safer alternative which can offer the same performance would be preferable. Bonding in metal oxynitrides show lower ionicity and higher covalency than that in the metal oxides, since nitrogen possesses lower electronegativity than oxygen and the difference in electronegativity between metal and anions (oxygen and nitrogen) should be smaller in oxynitrides. Covalent bond can be featured by a directed bond as a results of forming an electron pair between the bond whereas ionic bond can be regarded as a non-directional bond due to electrostatic attraction between metal cations and anions. When temperature rises, durability of bond against heat would be stronger in the more covalent oxynitrides than in the more ionic oxides. In this study, we focused on rock-salt type oxynitride Li0.30Ti0.60O0.50N0.50. Although there is a synthetic report on this material[1], there is no evaluation report as an electrode material. Li rich oxynitrides such as Li7.9MnN3.2O1.6 have already been reported as superior anode materials[2]. We referred to this idea and tried to obtain a better electrochemical performance in Li0.30Ti0.60O0.50N0.50 as an anode material by enriching Li against Ti. We investigated the effects of different Ti/Li ratio in LTON on the electrochemical performance.As the result, we successfully synthesized the rock-salt type LTON (lithium titanium oxynitride, with Ti/Li=1.25, 1.5, 1.75, 2.0) without impurity phase by solid state reaction technique as shown in Fig.1. XPS spectra revealed that the valence state of Ti increased as Ti/Li ratio decreased. Electrochemical charge–discharge cycle tests were performed at various current density of 20–200 mAg−1 with the cutoff voltage of 0.01–3.2 V. At relatively low current density (20 mA g-1), the corresponding initial discharge capacity of LTON with Ti/Li=1.25 reached about 140 mAh g-1, which was the highest capacity of all the LTON electrodes, and smaller charge-discharge hysteresis was observed. Moreover, the LTON with Ti/Li=1.25 showed excellent performance at all of current density compared with other LTON electrodes tested, as shown in Fig.2. For further understanding of the improved rate capability by changing the Ti/Li ratio, EIS measurements for LTON electrodes (Ti/Li=1.25, 2.0) were carried out, and it was found that the lithium ion diffusion coefficient increased as Ti/Li ratio decreased. In addition, In-situ XRD result suggested that the discharge and charge reactions should be reversible. XPS analysis for the pristine and discharged LTONs demonstrated that the charge compensation due to Li insertion would be performed by a reduction of Ti. The decrease of Ti/Li ratio in the oxynitrides would affect on the existence of the more number of high valent Ti such as Ti4+, which would possess a larger allowance to compensate the reduction during the discharging to show the better electrochemical performance.[1] T. Katsumata et al., Solid State Commun. 132, 583-587 (2004).[2] J. Cabana et al., Electrochem. Commun. 12, 315-318 (2010). Figure 1