Electrochemical Performance of Spinel-Type LiNi0.5Mn1.5O4 Thin-Film in Aqueous Electrolytes

Abstract

Introduction Due to the increase of the demand for use of sustainable energy, lithium-ion batteries attract more attention as one of the leading candidates of application in large-scale energy facilities. While the safety of lithium-ion batteries are gradually improved, the flammable organic solvents used in the electrolytes have fatal drawbacks. In addition, the cost of preparing and handling ultra-dry organic solvents is also a serious concern, and therefore alternative electrolytes have been studied. Aqueous electrolytes could resolve these concerns, but their narrow electrochemical window (1.23 V) raises another problem. The electrodes reactions in aqueous electrolytes are accompanied by the decomposition of H2O on the surface of electrodes. Therefore the voltages of aqueous lithium-ion batteries are limited (less than 1.5 V), and it diminishes the advantages that plentiful active materials can be used for lithium-ion batteries [1]. Considerable effort has been made to overcome the limitation of the electrochemical window. The most common method is the modification of the electrode surface. The coated electrodes retard the access of water molecules [2]. Another method to obtain aqueous electrolytes with wide electrochemical windows is to increase the concentration of salts in aqueous electrolyte since the activity of water can be decreased. In this work, we enhanced the resistance to oxidative decomposition of aqueous electrolytes by adding organosulfate to aqueous electrolytes and show the electrochemical properties of spinel-type LiNi0.5Mn1.5O4(LNMO) electrode in the electrolytes. Experimental The electrochemical potential windows of aqueous electrolytes were measured by using a rotating-disk electrode (RDE) of Pt with a rotation speed of 400 rpm. Linear sweep voltammetry was carried out in aqueous electrolytes of 0.5 M LiNO3 and 0.25 M Li-PO4 buffer with saturated PDSS. Buffer solution consisted of LiOH and H3PO4with a molar rate of 3:2 (pH 7). LNMO thin-films were prepared by sol-gel method. Prepared LNMO thin-films were characterized by X-ray diffraction (XRD) measurement.A three-electrode cell was used for electrochemical measurements. LNMO thin-film was used as a working electrode and Pt mesh as a counter electrode and Ag/AgCl electrode was used as a reference electrode. Cyclic voltammetry was carried out between 0.6 and 1.6 V at a scan rate of 1.0 mV s−1. Results and discussion XRD patterns of synthesized LiNi­0.5Mn1.5O4 thin-films had good agreements with ICDD diffraction patterns of spinel-type LNMO. Fig. 1 shows linear sweep voltammograms of Pt disk electrode. While the oxidation current was observed at 1.0 V (vs. Ag/AgCl) in the electrolyte of 0.5 M LiNO3, the onset potential of water oxidation shifted positively to 1.6 V (vs. Ag/AgCl) in the PDSS buffer solution [3]. The electrochemical behaviors of the LNMO thin films in 0.25 M Li-PO4 buffer with saturated PDSS are shown in Fig. 2. When we conducted cyclic voltammetry in solutions of 0.5 M LiNO3, oxidation currents began to increase at a potential of 1.3 V (vs. Ag/AgCl), but there were no clear redox pairs that corresponded to Li+ insertion/extraction of LNMO. On the other hand, in the PDSS buffer solution the oxidation current for oxygen evolution was suppressed, and two redox pairs appeared at around 1.5 V (vs. Ag/AgCl) which appeared to Li+ insertion/extraction of LNMO. This implies that the Li+insertion/extraction at LNMO thin film occurred even in aqueous solution by suppressing oxygen evolution and relieving the decrease in local pH corresponding oxidation of water. Reference W. Li et al., Science, 264, 1115-1118 (1994).I. B. Stojkovic et al., Electrochem. Commun., 12, 371-373 (2010).K. Miyazaki et al., Chem. Commun., 52, 4979-4982 (2016). Figure 1