Electrochemistry Characteristics of NaNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> As the Cathode Material for Sodium Ion Batteries

Introduction                                Lithium ion batteries have been widely used in portable electronic devices and deploying in electric vehicles and power grid energy storing. After the topic of the limited lithium source was mentioned, sodium ion batteries resurged in recent years. Many kinds of cathode materials for sodium ion batteries have been studied, but more research is needed to realize the commercial use of sodium ion batteries, especially competing with current lithium ion batteries.                 Because of large ion size, Na+ cannot stay in tetragon position in close-packed oxygen structure, so spinel structure of sodium oxide isn’t thermal stable phase, which can only synthesized at high pressure[1]. Naoaki Yabuuchi et al. [2] reported that product of electrochemical exchanged LiMn2O4 isn’t NaxMn2O4, but NaMnO2. The NaMnO2 prepared by electrochemical exchange show a very high capacity of 190 mAh/g and better capacity retention in the 2.3-4.3V range than the directly prepared NaMnO2.                 Here we report the structure change and electrochemical performance of NaNi0.5Mn1.5O4 prepared by electrochemical exchange method.                                    Experimental                                Spinel LiNi0.5Mn1.5O4 was prepared by solid state method. Stoichiometry Li2CO3, Ni(OH)2 and Mn2O3 were ball-milled and then sintered at 800℃ for 24h in air. The obtained LiNi0.5Mn1.5O4 was chartered by XRD, SEM and charge-discharge testing. For preparation of NaNi0.5Mn1.5O4 via electrochemical exchange, LiNi0.5Mn1.5O4 was first charged to 4.9V in a cell with Li electrolyte and Li anode to de-insert all of the lithium ions. Then the cell was disassembled and the cathode electrode was took out and washed by DMC for three time followed by assembling it with Na electrolyte (NaClO4 in PC+FEC) and Na anode. Charge and discharge the sodium ion cell at 10 mA/g current between 2 and 4.5V.                                    Results and discussion                                LiNi0.5Mn1.5O4 shows a long charge plateau at 4.7V in lithium cell, which represents de-intercalation of Li+ and oxidation of Ni2+.  At the end of charge process, Ni0.5Mn1.5O4 with very few lithium ions was gotten whose character of Na+ intercalation and de-intercalation was investigated in following cycles in sodium cell. It could be found that a discharge plateau at about 3.6V corresponding about a capacity of 80 mAh/g (Fig.1a). The electrochemical behavior is similar as the NaMn2O4 whose initial discharge plateau locates at about 3.2V with a capacity of 80 mAh/g in sodium cell [2]. However, in NaNi0.5Mn1.5O4 system, the capacity between 2 and 3V is little, so the total capacity is only about 110mAh/g for the first discharge process and decrease quickly during the following cycles.                 The structure of cycled NaNi0.5Mn1.5O4 was investigated by XRD, shown in Fig.1b. The spinel structure is kept when lithium ion is replaced by sodium ion, though some should peaks appears, which caused by strain because of a size gap between Li+ and Na+. After cycling, though no peaks corresponding to layered NaMnO2 can be found, the capacity decreases obviously, so new capacity decrease mechanism should be considered.                                    Reference                                [1] Xizheng Liu,  Xi Wang,  Akira Iyo,  et al., J. Mater. Chem. A, 2014, 2, 14822-14826                 [2] Naoaki Yabuuchi, Masaya Yano, Satoru Kuze, et al., Electrochim. Acta, 2012,82, 296–301                                                                                                                              Figure 1