Phase transition and ionic conductivity of pyrrolidinium-based ionic plastic crystals with magnesium salts

Abstract

Despite being safe for use in secondary Mg batteries, solid electrolytes exhibit lower ionic conductivities than those of traditional liquid electrolytes. Organic ionic plastic crystals—soft crystals with excellent thermal and electrochemical stabilities and ionic conductivities—are promising solid electrolytes. Herein, we investigated the effects of various anion species and Mg salt concentrations on the properties of pyrrolidinium-based organic ionic plastic crystals (N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide [[C₂epyr][FSA]] and N,N-diethylpyrrolidinium bis(trifluoromethylsulfonyl)amide [[C₂epyr][TFSA]]) upon Mg(TFSA)₂ addition. The Mg-ion transference number (t_Mg2+) was measured using the Vincent–Bruce method; ionic conductivity via impedance measurements; and phase transition via differential scanning calorimetry. The phase transition behavior, dissociation state of the Mg salt, and electrochemical properties varied with the organic ionic plastic crystal anionic structure. The FSA system became liquid when the Mg salt concentration exceeded 15 mol%. The ionic conductivity of the pyrrolidinium-based organic ionic plastic crystals increased substantially with the Mg salt concentration. In the solid state, [C₂epyr][FSA]/Mg(TFSA)₂ (5 mol%) (FT5) showed the highest ionic conductivity (2.9 × 10⁻⁴ S cm⁻¹ at 25 °C). The t_Mg2+ of FT5 at 60 °C was 0.29. Mg exhibited redox behavior in FT5 but not in [C₂epyr][TFSA]/Mg(TFSA)₂ (5 mol%). The FSA⁻ structure is suitable for Mg electrochemistry and will aid in developing high-performance secondary Mg batteries.

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1. Introduction

The depletion of lithium resources and associated rise in costs limit the development and application of lithium-ion secondary batteries (LIBs). Therefore, high-performance, low-cost, and safe secondary battery energy storage systems must be developed.^1–6 Mg-ion secondary batteries (MIBs) are considered promising alternatives to LIBs owing to several advantages. As next-generation secondary batteries, MIBs possess a relatively low redox potential (−2.375 V vs. SHE), making them materials with high specific energy capacity per unit mass and volume when used as battery anodes.^7,8

In contrast to Li, Mg does not form dendrites on the electrode surface, preventing short circuits, and has a lower reactivity when exposed to air, making it nontoxic and safer.^9,10 However, Mg tends to passivate, hindering redox reactions in highly reducing electrolytes. Most MIB research uses nonaqueous liquid electrolytes composed of complex salts and organic solvents to prevent anode passivation.¹¹ However, organic Mg-ion electrolytes exhibit low conductivities, limiting diffusion rates and necessitating high temperatures to maintain battery performance.^12–16 Various approaches have been investigated to address these issues.

Solid electrolytes have been developed to make MIBs safer. Solid electrolytes are more stable thermally and electrochemically than organic solvents, with no leakage concerns; thus, they are safer. They also eliminate the need for separators between cathodes and anodes, enabling the design of more flexible and compact batteries.¹⁷ However, transporting ions in solids is more challenging than in liquids, typically resulting in higher cell resistance in solid electrolytes. Therefore, developing solid electrolytes with excellent ionic conductivities (>10⁻³ S cm⁻¹) and low-resistance electrode–electrolyte interfaces is essential.

Organic ionic plastic crystals (OIPCs) exhibit a plastic crystal (PC) phase, which is a mesophase between the solid and liquid phases. In the PC phase, component ions have 3D positional order but rotate freely in place, resulting in the orientational disorder.^18–22 The ionic conductivity, plasticity, and safety of OIPCs make them attractive as solid electrolytes that can be used in flexible devices with low-resistance electrode interfaces. OIPCs enable diverse molecular designs via combinations of various cations and anions. Combining a cation with 2 ethyl groups on a pyrrolidinium ring with the bis(fluorosulfonyl)amide (FSA) anion N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C₂epyr][FSA]) results in high ionic conductivity.²³ Adding Li or Na salts to OIPCs improves ionic conductivity by 1 to 2 orders of magnitude, with several OIPC/inorganic salt composites exhibiting an ionic conductivity of approximately 10⁻⁴ S cm⁻¹ at room temperature of ∼25 °C.^24–27 Zhou and Matsumoto successfully fabricated a Li-ion conductor by doping 5 mol% Li[CF₃BF₃] into N,N-diethyl-N-methyl-N-(n-propyl)ammonium trifluoro(trifluoromethyl)borate ([N₁₂₂₃][CF₃BF₃]). This material exhibits high solid-state ionic conductivities of 10⁻⁴ to 10⁻³ S cm⁻¹ at room temperature, and the deposition/stripping of Li has been observed at 25 °C.²⁸ Wang et al. prepared composite electrolytes by combining poly(vinylidene difluoride) and pyrrolidinium-salt-based OIPC N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide ([C₂mpyr][FSA]). Lithium metal/OIPC/LiFePO₄ cells exhibit high specific capacity and excellent cycling stability (99.8% coulombic efficiency after 1,200 cycles at 2 C, room temperature), demonstrating long-term cycling performance at high rates with OIPC-based electrolytes.²⁹

OIPCs are expected to have a broad application scope in all-solid-state energy storage devices after chemical structural elucidation, analysis of physicochemical properties, and combination with composite materials.

In this study, we aimed to develop novel solid electrolytes composed of OIPCs doped with Mg salts for next-generation secondary batteries. To the best of our knowledge, this is the first report on the doping of Mg salts into OIPCs. By adding Mg(TFSA)₂ to pyrrolidinium-based OIPCs with various anions, ([C₂epyr][FSA] and N,N-diethylpyrrolidinium bis(trifluoromethylsulfonyl)amide [C₂epyr][TFSA]; Fig. 1), OIPC/Mg-salt composites with various Mg salt concentrations were prepared, and their thermal and electrochemical properties were evaluated.

2. Experimental

2.1 Materials

N-Ethylpyrrolidine (98%) was purchased from Tokyo Chemical Industry Co., Ltd., and iodoethane (>98.0%) and acetonitrile (99.8%) were purchased from FUJIFILM Wako Pure Chemical Corporation. All reagents were purified via distillation before use. Lithium bis(fluorosulfonyl)amide (LiFSA, 99%), magnesium(II) bis(trifluoromethylsulfonyl)amide (Mg(TFSA)₂ 99.9%), and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA, 99.9%) were purchased from Kishida Chemical Co., Ltd. All the solvents were purchased from Kanto Chemical Co. Inc. or FUJIFILM Wako Pure Chemical Corporation and purified as necessary.

2.2 Synthesis of OIPCs

[C₂epyr][FSA] and [C₂epyr][TFSA] were synthesized according to previously reported methods.^30,31 The chemical structure was confirmed through ¹H NMR spectroscopy (Bruker AVANCE III HD NanoBay 400 MHz), fast atom bombardment mass spectrometry (JMS-T100LC), and elemental analysis (Perkin Elmer PE2400-II).

2.3 Preparation of OIPC/Mg(TFSA)₂ composites

The OIPCs and Mg(TFSA)₂ were dissolved in dichloromethane in a glove box in an argon atmosphere, and the solutions were stirred at room temperature for 24 h. The mixtures were then dried in vacuo at 45 °C for 24 h to obtain OIPC/Mg-salt composites. [C₂epyr][FSA]/Mg(TFSA)₂ and [C₂epyr][TFSA]/Mg(TFSA)₂ are denoted as FTx and TTx, respectively, where x represents the molar percentage of the Mg salt.

2.4 Methods

2.4.1 Phase transition behavior

The phase transition behavior was investigated via differential scanning calorimetry (DSC) using a DSC7020 system (Hitachi High-Tech) at a scanning rate of 10 °C min⁻¹. The samples were sealed in aluminum pans in an argon-filled glove box. The reported data were obtained from the second heating scan. The measurement temperature ranged from −100 to 150 °C.

2.5 X-ray diffraction

X-ray diffraction (XRD) profiles were obtained over the 2θ range of 1.5° to 40° with a SmartLab X-ray diffractometer (Rigaku) operating at 45 kV and 200 mA using a Ni-filtered copper target at a temperature of −40 to 60 °C.

2.6 Raman spectroscopy

The samples were placed on a glass slide and covered with a cover glass in a glove box in an argon atmosphere. Raman spectroscopy was performed using a Jasco NRS-3100 (JASCO Corporation) instrument with 1 min exposure time, 100 mW laser intensity, and 64 accumulations.

2.7 Ionic conductivity

Impedance measurements were performed using an impedance analyzer, VSP-300 (BioLogic) over the frequency range of 100 mHz to 1 MHz (applied voltage: 10 mV) to measure the ionic conductivity of the samples. A donut-shaped spacer made from Kapton tape No. 650S-P (Kenis Co., Ltd.) with an outer diameter of 24 mm, an inner diameter of 8 mm, and a thickness of 50 μm was attached to a Pt electrode. The samples were placed in the donut hole of the Kapton tape in a TYS-00DM01 cell (Toyo System Co., Ltd.) in a glove box in an argon atmosphere. The temperatures of the samples were controlled by placing the measurement cell in a thermostatic chamber (SU-261, ESPEC Corp.). The resistance values were determined using the Nyquist plots of the measured data.

2.8 Mg-ion transference number

The Mg-ion transference number (t_Mg2+) was measured electrochemically according to the Vincent–Bruce method.³² Mg plates (AZ31, 0.5 mm thickness, Fuji Sangyo) were used after surface polishing with a razor in a glove box under an argon atmosphere. After stabilizing the measurement cells at 60 °C for 24 h, DC polarization was performed at 60 °C using a VSP-300 (BioLogic) to investigate the t_Mg2+ in FT5 and FT15. A voltage of 500 mV was applied for the DC polarization measurements. Temperature was controlled using a TB-1 thermostatic chamber (BAS Inc.). The t_Mg2+ was calculated using Equation (1):

where I_s and I₀ represent the stationary and initial currents, respectively.

2.9 Cyclic voltammetry

Cyclic voltammetry (CV) was performed to investigate the redox reactions of Mg in the electrolyte. A donut-shaped spacer made from Kapton tape No. 650S-P (Kenis Co., Ltd.) with an outer diameter of 24 mm, inner diameter of 8 mm, and a thickness of 50 μm was attached to a Pt electrode. The samples were placed in the donut hole of the Kapton tape in a TYS-00DM01 cell (Toyo System Co., Ltd.) in a glove box in an argon atmosphere. CV was performed at 60 °C using a VSP-300 (BioLogic) in a thermostatic chamber, TB-1 (BAS Inc.). The voltage range of the measurements was −0.50 to 2.0 V, and the scan rate was 10 mV s⁻¹. A Pt electrode was used as the working electrode. Mg electrodes were used as the reference and counter electrodes.

3. Results and discussion

3.1 Thermal behavior

Figure 2 shows the phase diagrams of the OIPC/Mg-salt composites. DSC profiles of FTx and TTx are shown in Supplementary Fig. S1. FT0 ([C₂epyr][FSA]) shows a solid–solid phase transition temperature (T_s–s) of −34.4 °C and a melting point (T_m) of 130 °C. TT0 ([C₂epyr][TFSA]) shows multiple T_s–s (−48.9, −33.6, 3.7, and 31.3 °C) and a T_m of 94.6 °C, consistent with the phase transition temperatures reported by Yunis et al.³¹ Generally, the temperature range from T_m to the first T_s–s is called Phase I, followed by Phase II, Phase III, and so on. The two OIPCs investigated herein exhibited more than one T_s–s before melting, with the FSA system in Phase I and TFSA system in Phase II at room temperature. PCs exhibit high entropy owing to the orientational disorder of the component molecules or ions. Therefore, the entropy of fusion (ΔS_f) of PCs is low. Timmermans reported that the ΔS_f of molecular PCs is <20 J K⁻¹ mol⁻¹.³³ The ΔS_f of [C₂epyr][FSA] was 8.2 J K⁻¹ mol⁻¹, consistent with Timmermans's criterion. OIPCs with FSA⁻ tend to have low ΔS_f values³⁴ owing to the high mobility of ions in Phase I. The ΔS_f of [C₂epyr][TFSA] was 23 J K⁻¹ mol⁻¹. Timmermans's criterion is based on molecular PCs, and some ionic compounds exhibit ΔS_f values higher than 20 J K⁻¹ mol⁻¹ owing to ionic interactions.^35,36

The T_m of FT5 could not be detected in the DSC measurement due to a low phase transition enthalpy. The solid–solid phase transition enthalpy of FT5 was lower than that of FT0. FT10 exhibited a eutectic point at −26 °C. When the Mg salt concentration exceeded 15 mol%, the samples were in the liquid state at room temperature, and only glass transition was observed as a phase transition event. The glass transition temperature (T_g) values of FT15, 20, 30, 40, and 50 were −71.7, −78.1, −71.2, −55.9, and −44.7 °C, respectively, and increased with Mg salt concentration, likely because of the increase in viscosity due to the strong interactions between Mg²⁺ and the anions.

In TTx, unlike in the FSA system, all samples exhibited T_m and multiple solid–solid phase transitions, with the phase transition enthalpies decreasing as the Mg salt concentration increased. T_m decreased with increasing Mg salt concentration. A eutectic point was observed at 65 °C for x = 15 to 40. A new solid–solid phase transition was observed at ∼50 °C for x = 40 and 50, suggesting that a new solid phase was formed (Supplementary Fig. S2). High concentrations of inorganic salts added to OIPCs form crystal structures.³⁷ The FSA system showed a lower T_m and fewer solid-phase components with increasing Mg salt concentration, making low concentrations desirable for solid electrolytes. The TFSA system remained solid at approximately room temperature regardless of the Mg salt concentration.

3.2 Crystal structure

Figure 3 shows the XRD pattern of FTx at 30 °C. Owing to the random orientation of the constituent ions, the [C₂epyr] cation can be considered spherical. Similarly symmetric OIPCs, such as N,N-dimethylpyrrolidinium bis(fluorosulfonyl)amide ([C₁mpyr][FSA]), have a CsCl-type crystal structure with [C₁mpyr] at the center and FSA⁻ at the corners of the cubic unit cell.³⁸ FT0 and FT5 show 2 diffraction peaks at 12° and 17° and similar diffraction patterns, suggesting that they have a CsCl-type structure and that the crystal structure did not change with the addition of the Mg salt. For site symmetry in ions with low symmetry, spherical or, in some cases, nonspherical free rotation is assumed. Molecules in the crystal have symmetry, and whether the anion conformation is syn or anti, the anion and cation can form a CsCl lattice as freely rotating bodies without spatial stress.³⁸ FT5 shows decreased peak intensity at 17°, suggesting that the crystallinity of FT0 was reduced. In FT0, a broad diffraction peak, possibly due to diffuse scattering, is observed above 15°, consistent with the characteristics of a PC. In contrast, FT5 shows broad diffraction peaks in a range similar to that of FT0. In the case of FT5, the decrease in the phase transition enthalpy and melting point suggests that solid–liquid coexistence occurred, and this broad scattering may be attributed to the amorphous component and not just diffuse scattering.

3.3 Dissociation state of Mg salt

Figure 4 shows the Raman spectra of Mg(TFSA)₂ and the OIPC/Mg-salt composites. Mg(TFSA)₂ tends to form associated states in electrolytes, making dissociation difficult.³⁹ TFSA⁻ and FSA⁻ exhibit characteristic peaks at ∼742 and 728 cm⁻¹, respectively, originating from the symmetric stretching vibration band of N–S bond vibrations (ν_s(N−S)).^40,41 Coordination with Mg²⁺ causes these specific peaks to shift to higher wavenumbers.³⁹ The peaks in the Raman spectra ideally follow Lorentz functions but may mimic Gaussian functions for amorphous materials. In this experiment, the peaks were fitted using Voigt functions, which are composites of the Lorentz and Gaussian functions, to calculate the peak components.

The Raman spectrum of FT0 shows peaks corresponding to the ν_s(N−S) vibrations of dissociated FSA⁻ (free FSA) at 725 cm⁻¹. FT5 can be separated into free FSA and bound anions (TFSA and FSA). For FT5, the spectrum shows a peak top at 737 cm⁻¹, which is considered to originate from both the FSA⁻ component coordinated with Mg²⁺ and from TFSA⁻ (free TFSA). This peak shift suggests the dissociation of Mg salt owing to electrostatic interactions between Mg²⁺ and FSA⁻. The TT0 spectrum shows peaks ascribed to free TFSA⁻ at 744 cm⁻¹. The spectra of TT20 and TT50 show the peak ascribed to bound TFSA⁻ at 748 and 751 cm⁻¹, respectively. The peak of the Raman shift of bound TFSA⁻ was observed at 754 cm cm⁻¹ for Mg(TFSA)₂. The spectra of TT5, TT20, and TT50 indicate that as the Mg salt concentration increased, the amount of coordinated components increased and the degree of dissociation decreased. FT5 exhibited a stronger interaction with Mg ions than did TT5, suggesting a more stable Mg-TFSA coordination. These results are consistent with the DSC findings, which indicated that the ΔS_f of the TFSA system tends to be higher than that of the FSA system. Furthermore, calculations employing the hybrid density functional theory method (B3LYP) and triple zeta quality (6-311+G*) have revealed that the dissociation energy with the Li cation is higher for TFSA⁻ than for FSA⁻.⁴² These results indicate that the dissociation of the Mg salt is more favorable in the FSA system.

3.4 Ionic conductivity

Figure 5 shows the Arrhenius plots of the ionic conductivities of the OIPCs and OIPC/Mg-salt composites. OIPCs exhibit sudden changes in ionic conductivity owing to changes in the crystal structure and molecular mobility/diffusivity associated with solid–solid phase transitions.⁴³ The OIPCs used in this study, [C₂epyr][FSA] and [C₂epyr][TFSA], show discontinuities in ionic conductivity before and after solid–solid phase transitions, correlating with the phase transition behavior. The ionic conductivities of [C₂epyr][FSA] and [C₂epyr][TFSA] at 25 °C are 2.21 × 10⁻⁶ and 5.32 × 10⁻¹¹ S cm⁻¹, respectively.

The ionic conductivities of FTx and TTx increased by more than 2 orders of magnitude at higher Mg salt concentrations. This is attributed to the increased ion mobility owing to the higher entropy. The Arrhenius plot of the ionic conductivity of FT5 is linear below the eutectic point at −30 °C but becomes convex upward at higher temperatures, characteristic of the Vogel–Fulcher–Tammann (VFT) behavior. This behavior is related to ionic conduction in amorphous regions. FT5 exhibited a liquid phase at the eutectic point, suggesting preferential ion diffusion in the liquid phase. FT10 and FT15 exhibit VFT-type behavior in the measured temperature range, with FT10 exhibiting a higher ionic conductivity. Higher Mg salt concentrations likely increased crystallinity and viscosity.⁴⁴

TTx exhibited the highest ionic conductivity at TT15. The slope of the Arrhenius plot at 25 °C increases with Mg salt concentration, indicating a higher activation energy for ion conduction, corresponding to a higher energy barrier. Raman spectroscopy suggests the formation of cluster ions with Mg²⁺ and TFSA⁻ in TTx, which increases ion size and activation energy. Above the eutectic point at 65 °C, the slope of the Arrhenius plot decreases, indicating lower energy barriers for ion conduction in the amorphous region than in the crystalline region.

Among solid samples, FT5 shows the highest ionic conductivity of 2.9 × 10⁻⁴ S cm⁻¹ at 25 °C. The ionic conductivity of the [C₂epyr][FSA]/LiFSA (5 mol%) composite at 25 °C is 1.1 × 10⁻⁴ S cm⁻¹. The ionic conductivity of Mg-salt-doped systems is considerably higher than that of Li-salt-doped systems,²⁶ possibly because the Mg system has a larger number of anions, which makes the system disordered and liquid like. In addition, the strong coordination between Mg ions and anions is considered a factor increasing the disorder in the system.

3.5 Mg-ion transference number

Figure 6 shows the DC polarization curves of the Mg/FT5/Mg cells at 60 °C. Mg ions have a high solvation energy and interface resistance owing to their high charge density. Thus, t_Mg2+ was calculated using the ratio of the initial to steady-state current (I_s/I₀). The t_Mg2+ values of FT5 and FT15 were 0.29 and 0.019, respectively. At 60 °C, the ionic conductivity of FT5 was lower than that of FT15, whereas the t_Mg2+ of FT5 was much higher than that of FT15. FT5 showed a solid-solid phase transition (see Fig. 1a), suggesting that the PC phase is superior to the liquid phase for Mg-ion transport. The Li-ion transference number of the [C₂epyr][FSA]/LiFSA (5 mol%) composite at 60 °C is 0.27.²⁵ The transference number of the Mg-salt-added system is comparable with that of the Li-salt-added system. Additionally, the t_Mg2+ of a polyethylene oxide system containing the Mg(CF₃SO₃)₂ salt and 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid is 0.22.⁴⁵ These organic electrolyte systems show similar t_Mg2+ values. Higashi et al. reported that the t_Mg2+ of a system using an inorganic molecular material, Mg(BH₄)(NH₂), is 0.46.⁴⁶ For all systems mentioned here, I_s/I₀ was used to calculate the t_Mg2+ values. Generally, inorganic solid electrolytes exhibit higher transference numbers than organic solid electrolytes. Solid polymer electrolytes exhibit high ionic conductivities and transference numbers in high-concentration regions.^47,48 If a similar trend is observed for the transference number of the Mg system, this property of Mg-based organic electrolytes may be further improved. However, such a trend was not conclusively observed in the present study. This is a study limitation that must be acknowledged and addressed in future studies.

3.6 Redox behavior

Figure 7 shows the CV results for FT5 and TT5. In FT5, a reduction current based on Mg-ion reduction is observed at a potential below 0 V vs. Mg/Mg²⁺, and an oxidation current based on Mg oxidation is observed at ∼1.5 V vs. Mg/Mg²⁺. These redox reactions suggest that Mg was reversibly plated and stripped onto the surface of the Pt electrode. FSA⁻ forms a solid electrolyte interface (SEI) film via reductive decomposition on the electrode surface.⁴⁹ The SEI film suppresses excessive decomposition of the electrolyte and stabilizes reversible redox reactions. This suggests that SEI films were formed from the 1st to 10th cycle and that the Mg redox reactions were supported from the 20th cycle onward.

In TT5, a reduction in the current owing to Mg-ion reduction can be observed at potentials below 0 V vs. Mg/Mg²⁺ in the 1st cycle, but no current response is observed from the 2nd cycle onward. This suggests the formation of an inert film on the electrode surface. Controlling the degree of dissociation of the Mg salts is essential for the use of the OIPC as a Mg-ion conductor. Using a similar organic electrolyte system, Ab Aziz and Tominaga confirmed the deposition and stripping behavior of Mg in a sample with 40 mol% Mg(TFSA)₂ added to poly(ethylene carbonate). Mg deposition occurs at voltages exceeding 2 V.⁵⁰ Higashi et al. demonstrated that Mg²⁺ conduction is mediated by inorganic molecular material Mg(BH₄)(NH₂). Mg(BH₄)(NH₂) was predicted to be a bandgap insulator with ionic bonding characteristics, and CV confirmed the reversible deposition and stripping behavior of Mg in this material.⁴⁶ In this study, FT5 used FSA⁻ as the anion, and it is the first report confirming the reversible deposition and stripping behavior of Mg with FSA-based anions.

4. Conclusion

This study was aimed at developing solid electrolytes for secondary Mg batteries using Mg-doped OIPCs and investigating the effects of adding Mg(TFSA)₂ to pyrrolidinium-based OIPCs with various anion species. The thermal and electrochemical stabilities of the OIPC/Mg-salt composites are applicable to solid electrolytes. The OIPC with FSA became amorphous, and the liquid-phase content increased with Mg salt concentration. In the TFSA system, the eutectic and melting points decreased with increasing Mg salt concentration. The Mg salt was more dissociated, and the anion was coordinated to Mg in both the FSA and TFSA systems. In both systems, the addition of the Mg salt increased the ionic conductivity by more than 2 orders of magnitude. In the solid state, FT5 showed the highest ionic conductivity of 2.9 × 10⁻⁴ S cm⁻¹ at 25 °C. The Mg-ion transference number at 60 °C for FT5 was 0.29. There results clearly suggested that OIPCs worked as Mg-ion conductors. Mg exhibited redox behavior in FT5, and the use of the FSA anion is key to the reversible redox reaction of Mg. The use of FSA as the anion for both the OIPC and Mg salts is expected to enhance the redox properties of Mg, thereby facilitating the development of solid electrolytes with higher ionic conductivities for next-generation secondary Mg batteries.

Acknowledgments

We would like to thank Editage (www.editage.jp) for English language editing.

Supplementary data

Supplementary material is available at Bulletin of the Chemical Society of Japan online.

Funding

This study was supported by JSPS KAKENHI (22K19072), a research grant from the Murata Science and Education Foundation and a Sophia University Special Grant for Academic Research.

Data availability

All data are incorporated into the article and its online Supplementary material.

References

Masahiro Yoshizawa-Fujita

Masahiro Yoshizawa-Fujita received his PhD degree (2002) from Tokyo University of Agriculture and Technology. During his PhD studies, he received a Research Fellowship for Young Scientists at the Japan Society for the Promotion of Science (JSPS). He spent 2 years as a postdoctoral research fellow (Discovery-Project) at Monash University. He moved to Sophia University as an assistant professor in 2006. He was promoted to professor in 2019. His recent research activities are concerned with the design of organic salts (ionic liquids, ionic plastic crystals, zwitterions, etc.), especially for battery research and biomass processing.

Author notes