Na Insertion and Extraction Reaction of Li2-XMnO3 for the Use As a Positive Electrode Material of the Room Temperature Na Ion Battery
For the positive electrodes of the LIBs, Li2MnO3 is one of the promising materials because of its high reversible capacity of more than 200 mAhg-1(3). Were “Na2MnO3” present, it would also work as a good electrode for the SIB. But no one has yet succeeded in synthesizing “Na2MnO3”. In the present study, we investigated the Na insertion/extration behavior of Li2-xMnO3 which was prepared by extracting Li from Li2MnO3through the electrochemical charging process. Also investigated were the detailed crystal structure of the samples and structural change during the charge and discharge process.
The initial sample, Li2MnO3, was obtained by heating a mixture consisting of Li2CO3, Mn2O3 in the temperature range between 400 and 1000 oC for 10 h in air. The electrode was prepared by attaching a mixture of the prepared Li2MnO3 (84 wt.%), acetylene black (AB, 8 wt.%) and polytetrafluoroethylene (PTFE, 8 wt.%) to aluminum mesh. The electrochemical test of the electrodes was conducted in CR2032 type coin cell, consisting of the electrode and lithium or sodium metal foils as the counter electrodes and 1 M APF6 /EC-DEC (A = Li or Na, 50 : 50 vol.%) as the electrolytes. At first, the electrodes were charged up to 4.95 V (vs. Li/Li+) at the current density of 10 mAg-1 to extract Li from Li2MnO3. The positive electrode after the lithium extraction was taken out of the cell and washed with dimethyl-carbonate to remove residual salt such as LiPF6. The present phase of the samples and the in-situstructural change during the charge/discharge process were examined using XRD with Cu-Kα and Mo-Kα radiation, respectively.
Figure 1 shows the initial charge curve (vs. Li/Li+) and subsequent discharge curves (vs. Li/Li+or vs. Na/Na+) of Li2MnO3 prepared at 800 oC. The first charge capacity was 460 mAhg-1, which is translated to 2.0 Li per Li2MnO3. The following discharge capacities were 244 mAh(g-Li2MnO3)-1 (1.06 Li per Li2MnO3 under the cut-off potential of 2.5 V vs. Li/Li+) and 237 mAh(g-Li2MnO3)-1 (1.03 Na per Li2MnO3 under the cut-off potential of 1.5 V vs. Na/Na+), respectively. Although the cut-off potentials differ for Li and Na, almost the same amount of alkaline cations were inserted into Li2-xMnO3. Figure 2 shows the cycling performances of the electrodes tested versus Li and Na, respectively. More Li ion is reversibly inserted into the electrode than Na ion during the initial 10 cycles. However, the cycling performance in the Na system eventually exceeded that in the Li system..
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