Relaxation Behavior of Magnetization for Electrochemically Lithium Inserted Gamma-Fe2O3
From the gradual potential change of γ-Fe2O3 after the termination of electrochemical lithium insertion, we have found that structural change occurs in the electrode material from the kinetically favorable to thermodynamically stable states. X-ray diffraction experiment coupled with the Rietveld structure refinement revealed not only the structure relaxation process after the termination of lithium insertion but also electrochemical lithium insertion mechanism. We named this technique as “Relaxation analysis” and applied on various electrode materials, such as γ-Fe2O3 [1,2], LiMn2O4 , LiFePO4 , LiCoO2 , or graphite .
γ-Fe2O3 possesses spinel-type structure with the space group , where iron ions essentially occupy 8a and 16d sites with a small amount of vacancy and the 16c site is vacant. In the previous study [1,2], it has been found that occupation factor of 8a site decreases and that of 16c site increases at the lithium insertion process. In addition, after the insertion termination, 8a site occupancy increases while 16c site declines during relaxation time as shown in Fig. 1. Therefore, we concluded that lithium ions at first insert into preferred 8a site pushing iron ions out to 16c site, thereafter iron ions return to 8a site after the termination of lithium insertion. According to the above model, magnetic moment should vary during the relaxation process, since the site movement of iron is involved in the model. In the present study, we measured the magnetization of lithium inserted γ-Fe2O3 during relaxation process to consider from the viewpoint of magnetic properties.
Electrochemical lithium insertion into γ-Fe2O3 has been made by using an Ar-sealed glass beaker cell. The mixture of γ-Fe2O3 powder (AlfaAesar, >99%), Acetylene Black (AB) and PTFE powder with the weight ratio of 70:30:5 was employed for cathode material, which was spread onto a Ni mesh as a current collector. Lithium foil was used as the reference and counter electrodes, and 1 mol∙dm-3 LiPF6 in EC/DMC (2:1 v/v%, Kishida chemical Co., Ltd) was used as the electrolyte. Lithium ion has been electrochemically inserted into the sample to obtain LixFe2O3 (x = 1.5) at a constant current of 0.01 Ag-1. After the termination of lithium insertion, sample was scraped immediately out of the Ni mesh and set into a quartz holder for SQUID.
Magnetization has been measured by using an MPMS SQUID magnetometer under an external field of 103 Oe at 300K. Magnetization of the sample was monitored for 70 hours.
Results and discussion
Fig. 2 shows the measured magnetization of Li-inserted γ-Fe2O3 (Li1.5Fe2O3). It was found that the magnetization gradually increases with relaxation time after the termination of lithium insertion. This is the first observation of the relaxation in the magnetization. The magnetization of γ-Fe2O3 comes from the superexchange interaction of iron via oxygen and it is well known that the change of crystal site of iron changes the superexchange interaction. Then it is considered that the change of the magnetization comes from the change of the superexchange interaction due to the change of occupation site of iron at the relaxation time.
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