Oxygen Generation from 18oxygen-Doped Li2CuO2 Caused By Electrochemical Oxidation

Abstract

INTORDUCTION Li-ion batteries with high energy density have been required for power sources of electric vehicles. In particularly, it is crucial that the positive electrodes exhibit a large specific capacity with high cell voltage. The practical candidates for the electrode are limited such as Li2MnO3-based solid solutions. Based on our previous investigations on LiNi0.5Mn0.5O2, we have focused on Li2CuO2 and Li2CuO2-Li2NiO2 solid solutions [1, 2, 3]. They have some unique advantages which are different from other popular positive electrodes. Firstly, copper has abundant resources and less toxicity compared to cobalt or nickel. Secondary, they expect to exhibit a high-capacity due to a large amount of Li like as Li2MnO3. Furthermore, they adopt mutually similar structure which has the square planner CuO4 with the copper atom in the center and the oxygen atoms at the corners. Thus, these structures lead to characteristics of electronic structure that Cu 3d and O2p orbitals overlap dominantly over a wide range of energy level. Hence, both copper and oxygen electrons in the Cu-containing oxides may participate in the electrochemical oxidizing and reducing reactions. Our ab-initio electronic calculation strongly suppports that electrons by electrochemical oxidation may be removed considerably from both copper and then oxygen atoms [1]. In addition, XRD patterns using synchrotron radiation showed the formation of CuO after the first charging [1, 2]. By combining our experimental and calculated results, oxygen could participate in charging process of Li2CuO2 and Li2CuO2-Li2NiO2solid solutions. The objective of this paper is to detect O2 release directly from 18O isotope-doped Li2CuO2 by use of GC-MS analysis, and then to discuss the role of the oxygen on the electrochemical mechanism of Li2CuO2 and Li2CuO2-Li2NiO2solid solutions. Experimental The samples for x = 0-0.4 in Li2Cu1-x Ni x O2 were prepared by a combination of co-precipitation and solid state reaction. The co-precipitates were obtained from Cu(CH3COO)2∙H2O and Ni(CH3COO)2∙4H2O by dropping 1 M LiOH solution. After heating dried co-precipitates at 873 K for 6 h in air, Cu and Ni oxides were obtained. They were mixed with chemical grade Li2O and sintered at 1073 K for 24 h under N2 flow. A sample for 18O isotope - doped was prepared by filling and sealing with 18O2 gas. Electrochemical testing was carried out by coin-type cells with Li/1M LiPF6 in EC:DMC(1:1)/samples at 298 K. The positive electrode was composed of the following materials; specimen: acetylene black: PTFE = 30:15:1.5 (wt.%). The rate of constant current was 1/20 C in the range of 2.0-4.3 V. The in-situelectrochemical testing cell was connected with GC-MS (Gas Chromatography Mass Spectrometry) analysis equipment and the measurement was performed with on-line to detect oxygen generation directly during charging process. RESULTS AND DISCUSSIONS All the obtained samples for 18O isotope-doped Li2CuO2 and x = 0-0.4 in Li2Cu1-x Ni x O2 showed an orthorhombic single phase and formed a Li2CuO2-Li2NiO2 solid solution. Fig.1(a) shows a charge curve of 18O-doped Li2CuO2 and GC-MS spectra collected at m/z = 35.90-36.40 is shown in Fig.1(b). The results of electrochemical testing were good agreement with those of our reported earlier [3]. A remarkable increasing of the detected intensity was observed from 4.1 to 4.3 V and then after the completion of the electrochemical testing the intensity decreased. Whereas, the intensity corresponding to the mass number of 36 was not detected for Li2CuO2 prepared by conventional oxygen gas. These results shows direct O2 gas generation clearly from Li2CuO2 by its electrochemical oxidation. On the other hands, the charge and discharge specific capacities increased with the amount of Ni contents. A reversible capacity of about 300 mAhg-1 for x = 0.3 in Li2Cu1-x Ni x O2 was obtained and the capacity was maintained after 10 cycles. Based on these results, we will discuss the role of the oxygen on the electrochemical mechanism of Li2CuO2 and Li2CuO2-Li2NiO2solid solutions. REFERENCES 1. Y. Arachi et al., J. Power Sources, 196, (2011) 6939. 2. Y. Arachi et al., Solid State Ionics, 225, (2012) 611. 3. Y. Arachi et al., ECS Trans, 50(2013)143. Figure 1