To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA)2)/triglyme, magnesium borohydride (Mg­(BH4)2)/tetrahydrofuran (THF), and Mg­(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg­(TFSA)2/triglyme and Mg­(BH4)2/THF, while the reactions do not occur in Mg­(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]− in the Mg­(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]− in Mg­(TFSA)2/2-MeTHF and [BH4]− in Mg­(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]−, strongly bound to the magnesium ion in the Mg­(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg­(BH4)2/THF electrolyte, because the [BH4]− coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]−. Conversely, because [TFSA]− is weakly bound to the magnesium ion in Mg­(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

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To clarify the effects of anion species and solvents on the Coulombic efficiency and polarization of magnesium deposition/dissolution reactions, the anode/electrolyte interfacial behavior of magnesium tetrakis­(hexafluoroisopropyloxy) borate (Mg­[B­(HFIP)4]2) and magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA)2) was investigated and compared in triglyme and 2-methlytetrahydrofuran (2-MeTHF). When using triglyme, which has strong interaction with magnesium ions, decomposition of [B­(HFIP)4]− in Mg­[B­(HFIP)4]2/triglyme was hard to occur because of the high reduction stability of the uncoordinated [B­(HFIP)4]− anion, resulting in significantly higher Coulombic efficiency and smaller polarization than Mg­(TFSA)2/triglyme. When 2-MeTHF was used as the solvent, magnesium deposition/dissolution reactions occurred in the Mg­[B­(HFIP)4]2/2-MeTHF electrolyte but not in the Mg­[TFSA]2/2-MeTHF electrolyte. This is because the coordinated [B­(HFIP)4]− anion in Mg­[B­(HFIP)4]2/2-MeTHF is stable at the magnesium deposition potential. However, the reductive stability of the coordinated [B­(HFIP)4]− anion is inferior to that of the uncoordinated [B­(HFIP)4]− anion, resulting in the Mg­[B­(HFIP)4]2/2-MeTHF Coulombic efficiency being lower than that of Mg­[B­(HFIP)4]2/triglyme. Our results indicate that solvents that could not be used with Mg­(TFSA)2 are suitable in weakly coordinating anion electrolytes, such as Mg­[B­(HFIP)4]2. Controlling the interaction between magnesium ions and anions by selecting suitable anions and solvents is essential for designing new electrolytes for magnesium rechargeable batteries.

The 3d transition metal oxides with a spinel structure are among the most promising cathode materials for magnesium batteries. In this study, we investigated the reaction mechanism of magnesium ion insertion for magnesium spinel oxides, MgMn2O4, by electrochemical measurements, X-ray absorption spectroscopy (XAS), and synchrotron X-ray diffraction (XRD) with Rietveld analysis. Open-circuit-potential and XAS measurements showed that Mg2+ insertion into MgMn2O4 does not proceed via a simple two-phase coexistence reaction between the spinel and rock-salt phases. Synchrotron XRD measurements showed that Mg2+ insertion into MgMn2O4 involves crystal structural changes in three stages. In the early stage of the Mg2+ insertion process (0 < x < 0.2), Mg2+ is inserted into the spinel (MgMn2O4) phase and rock-salt (Mg1.2Mn2O4) phases, which are included in the pristine samples, without significant volume changes. In the middle stage of the Mg2+ insertion process (0.2 < x < 0.4), Mg2+ is inserted into the Mg1+αMn2O4 spinel phase and the Mg2−βMn2O4 rock-salt phases with a large volume change. In the last stage of Mg2+ insertion process (0.4 < x < 0.56), Mg2+ insertion proceeds via a two-phase coexistence reaction between Mg1.4Mn2O4 spinel and Mg1.6Mn2O4 rock-salt phases without Mg content changes in either phase. The phase transition from the Mg1+αMn2O4 spinel phase to the Mg2−βMn2O4 rock-salt phase with a large volume change resulted in significant polarization during the Mg2+ insertion process. Suppressing the phase transition, accompanied by a large volume change, is important in designing a spinel oxide cathode with a high rate performance.

Pyrazolyl magnesium halide/tetrahydrofuran (THF) solutions were obtained by the simple reaction of pyrazole compounds with Grignard reagents in THF. Their electrochemical performances as rechargeable magnesium battery electrolytes are reported. The pyrazolyl magnesium halide/THF solutions were characterized in term of anodic stability and reversibility of magnesium deposition-dissolution using cyclic voltammetry and galvanostatic charge-discharge techniques. The composition and morphology of the deposit were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is concluded that the substituents on the pyrazole compound and the molar ratio of the pyrazole to the Grignard both affect the electrochemical performance. An electrolyte consisting of 1 mol•L-1 1-methylpyrazole-PhMgCl (1:1 molar ratio)/THF has an anodic oxidation decomposition potential of 2.4 V (vs Mg/Mg 2 +) on stainless steel (SS), a low potential for magnesium deposition-dissolution, and a high cycling reversibility, and can be prepared easily, making it a promising candidate for rechargeable battery electrolytes.