Development of Novel Magnesium Rechargeable Battery Using Polyanion Compounds

Magnesium rechargeable battery is one of candidates for next generation battery system.¹ The advantage of magnesium battery is the high theoretical capacity, low cost and stability of magnesium metal anode. One of challenges for the practical application is development of cathode materials. Compared to lithium ions, insertion/extraction reaction of magnesium ions into the host compounds has more difficulty due to stronger ionic interaction between the inserted multivalent cations and host materials.² Therefore, at this moment, the compounds which can serve as magnesium hosts are limited. In this study we apply polyanion compounds to magnesium battery cathodes. We investigate electrochemical magnesium insertion / extraction of polyanion compounds. And then the mechanism of magnesium insertion / extraction is analyzed by using X-ray absorption spectroscopy and X-ray diffraction.

Electrodes were prepared from iron-based polyanion compounds to which carbon black (acetylene black) was added and ball-milled. Polytetrafluoroethylene binder was thereafter added. Three electrodes were used. Mg rod and double junction-type Ag⁺ / Ag were used as the counter and reference electrode respectively. 0.5 M Mg [N(CF₃SO₂)₂]₂ / acetonitrile was used as electrolyte. Measurements were performed at 55^oC. XAS spectra were measured in a transmission mode at the beam line BL01B1 at SPring-8 (Japan). XRD measurements were carried out at the beam line BL02B2 at SPring-8 (Japan).

Figure 1 shows charge profile of the polyanion compound under lithium salt electrolyte system and discharge profile under magnesium salt system. For the charge reaction, two-lithium extraction can be achieved. For the discharge reaction in magnesium system, the theoretical capacity is achieved. Subsequent electrochemical cycling tests show that Mg²⁺ can be reversibly inserted / extracted at 55^oC. As shown in Fig. 2, the absorption edge from XAS is shifted towards lower / higher energy with discharging / charging, respectively. This result indicates that the formal valence of iron is decreased / increased with magnesium insertion/extraction process. We further demonstrate the feasibility of reversible magnesium deposition and dissolution using Mg[N(CF₃SO₂)₂]₂ / triglyme electrolyte that exhibits a high-voltage window. Combination of a triglyme–based electrolyte system with polyanion compounds not only presents a practical high-energy-density magnesium rechargeable battery, but also is free of toxic and explosive chemicals.

Acknowledgments:

  This work was supported in part by Core Research for Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (JST) in Japan.

References:

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2. E. Levi, Y. Gofer and D. Aurbach, Chem. Mater., 22, 860 (2010).