Structural analysis

3. Mg 2+ co-intercalation into Natural graphite

3.2 Results and discussion

3.2.3 Structural analysis

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Figure 23 Ex-situ XRD (a) spots and (b) patterns of natural graphite ranging 20 - 30 °

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Figure 24 Ex-situ XRD patterns of natural graphite ranging 10 – 40 ° (ordering is the same with figure 22)

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Figure 25 TEM images of (a) pristine (b), (c) Mg intercalated natural graphite

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Figure 26 EDS mapping of natural graphite after magnesium intercalation(c of Figure 25) by TEM

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3.3.3 Co-intercalation of magnesium with solvent

The co-intercalation of magnesium into natural graphite is supported by ex situ Fourier transform infrared (FT-IR) analysis. After magnesium intercalation process, cell was disassembled and cycled electrode was washed with THF to remove residual salt and electrolyte on the electrode surface.

Compard with pristine electrode, vibrations of bondings that is related with electrolyte was observed in the case of cycled electode. It indicates that solvated magnesium ions were co-intercalated with solvents into natural graphite during discharge process. Ether bases electrolytes could solvate magnesium ion in non-polar like form, which makes it easier easier to intercalate in natural graphite that has hydrocpopic characteristic.¹⁹

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Figure 24 FT-IR analysis of electrolyte and natural graphite at pristine and discharged state

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4. Conclusion

In this research, magnesium ion intercalation materials were introduced for electrode materials for magnesium based battery system. One of the Prussian blue analogues, Na0.69Fe2(CN)6 is introduced as cathode materials that can store magnesium ions. It showed approximately 70 mA h g-1 of capacity with stable cycle performance over 35 cycles in 2.0 ~ 3.6 V (vs. Mg/Mg²⁺, calibrated). The reversible structural change was observed with ex situ XRD during intercalation and de-intercalation of magnesium. Due to the phase transition of alkali-rich state of Na0.69+ Fe2(CN)6 (0.31 < ), it showed larger structural change during discharge. However, the maximum volume expansion was only 103 % than pristine state. This negligible volume expansion leads stable cycle performance. The reversible intercalation and de-intercalation were observed with TEM and EDS mapping. 0.24 mol of magnesium was detected after discharge, which matches with first discharge capacity, and 0.15 mol of magnesium was detected after following charge. This indicates that 0.09 mol of magnesium was reversibly de- intercalated with existing sodium in Na0.69Fe2(CN)6. The possibility of sodium de-intercalation can be confirmed with electrochemical test of Na0.69Fe2(CN)6 starting with charge. Because of bivalency of magnesium ions, coulombic attraction between Mg-(CN) bonding is larger than Na-(CN) bonding. This affects de-intercalation kinetics of magnesium leading more de-intercalation of sodium than magnesium.

By comparing Fe2(CN)6 and Na0.69Fe2(CN)6, it seems that existence of sodium make magnesium ions easier to be intercalated into Prussian blue analogues.

The co-intercalation of magnesium with ether solvent into natural graphite was introduced. It showed reversible cycling with constant capacity of 180 mA h g^-1. The reversible structural change was observed by ex situ XRD during intercalation and de-intercalation. (002) peak of natural graphite was disappeared with intercalation and recovered with following de-intercalation. Detailed structural change was investigated with TEM and EDS mapping. There were spots that (002) layers of natural graphite are collapsed and magnesium ions were mainly detected in that region. To confirm participant of magnesium in the reaction, SEM analysis of magnesium metal surface was performed. There were hundreds of micrometer sized pores and deposits on the surface of magnesium. It indicates that the magnesium actually participated in reaction. To confirm co-intercalation of solvents into natural graphite, ex situ FT-IR was conducted with magnesium intercalated electrode. It showed there were peaks of electrolytes in the electrode which means co-intercalation of solvents into natural graphite.

Na0.69Fe2(CN)6 and natural graphite were introduced as magnesium ion intercalstion electrode materials that can be used as cathode and anode respectively. Intercalation of magnesium into Prussian blue compounds with sodium and co-intercalation of magnesium into natural graphite with ether solvents could suggest new method for development of magnesium based battery systems.

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