Reaction Dynamics of Magnesium Ions Insertion/Extraction of Olivine-Type FePO4
Authors: Yoshinari Takahiro , Yamamoto Kentaro , Orikasa Yuki , Masese Titus , Mori Takuya , Huang Zhen-Dong , Minato Taketoshi , Uchimoto Yoshiharu 
Published: February 2020

Magnesium battery is one of the candidates for the next generation battery system due to the high energy density and the environmentally friendly nature 1,2. Developing suitable cathode materials is a clue to the practical application. Divalent Mg2+ insertion/extraction reactions in host compounds is difficult, due to the stronger ionic interaction and harder charge redistribution of magnesium compared to lithium ions 3. Therefore suitable cathode materials are limited at this moment. In order to design the host materials for magnesium battery cathodes, it is important to understand the difference between the reaction dynamics of magnesium and that of lithium.
Olivine-type lithium iron phosphate (LiFePO4) is a well-known cathode material in lithium ion batteries 4. By using delithiated FePO4, the electrochemical insertion/extraction of magnesium ions in FePO4 was reported by Le Paul et al. 5 Hence, FePO4 is suitable as a model compound to clarify the reaction dynamics of magnesium battery. In this study, we investigated the crystal structure change and the reaction dynamics during electrochemical magnesium insertion/extraction reactions of olivine-type FePO4.
LiFePO4 was prepared by using a solid state reaction using Li2CO3, FeC2O4-2H2O and (NH4)2HPO4 as raw materials. After carbon-coated with acetylene black 10 wt% to LiFePO4, the mixture was pelletized and calcined. FePO4 was prepared by chemical oxidation of the obtained carbon coated LiFePO4 using nitronium tetrafluoroborate (NO2BF4) as an oxidizing agent. The particle size of the completed carbon coated FePO4 was about 65 nm determined by SEM observation. Electrochemical measurements were carried out using a three-electrode cell with an Ag+/Ag double junction reference electrode. The cathode material consists of 80 wt% FePO4, 15 wt% acetylene black and 5 wt% polytetrafluoroethylene. The counter electrode was a magnesium rod and the electrolyte was 0.5 M magnesium trifuluoromethanesulfonyl-imide (Mg(TFSI)2) in acetonitrile (AN). Galvanostatic discharge/charge measurements were conducted with C/30 rate at 55°C. X-ray absorption spectroscopy (XAS) measurements were carried out in a transmission mode at BL02B2 at SPring-8, Japan. Synchrotron X-ray diffraction (XRD) measurements were also carried out at BL01B1 and BL14B2 at SPring-8, Japan.
Figure 1 shows discharge/charge profile of FePO4 in 0.5 M Mg(TFSI)2/AN. The obtained discharge capacity indicates 0.4 magnesium ions insertion into FePO4, comparable to 0.8 lithium ions insertion. In the charge process, the obtained capacity corresponds to the extraction of 0.3 magnesium ions. This result shows that magnesium insertion /extraction reaction of FePO4is irreversible and that the extraction is more difficult than the insertion process. In addition, discharge/charge curves are considerably dissimilar, suggesting that the reaction mechanism could be quite different between insertion and extraction processes.
In order to characterize discharged and charged MgxFePO4, XAS and XRD measurements were performed. Fe K-edge XANES spectra shows lower and higher energy shifts with the discharge/charge reactions, respectively. This indicates the reduction and oxidation of Fe in FePO4 by magnesium insertion/extraction. XRD patterns show lower and higher peak shifts with the discharge/charge reactions, respectively. The peaks attributed to the two-phase reaction did not appear as shown in the charge/discharge reactions of LiFePO4. This result reveals that magnesium ions insertion/extraction reactions of FePO4proceeds via not a two-phase but a single-phase mechanism.
Furthermore, we will present the reaction dynamics of magnesium ions insertion/extraction by using FePO4thin film as a model electrode.

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.

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