Na Insertion and Extraction Reaction of Li2-XMnO3 for the Use As a Positive Electrode Material of the Room Temperature Na Ion Battery

Sodium ion battery (SIB) is attracting much attention as alternative to the lithium ion battery (LIB) due to the wide and uniform distribution of sodium all over the world which can consequently lead to the lower cost than lithium. Up to now, various types of positive electrode materials for SIB  have been reported⁽¹⁾⁽²⁾many of which their structures are similar to those of the LIBs.  

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 Li_2-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 Li_2-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.. 

References

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