Controlling the Phase Separation of Dimethyl Carbonate Solvents Using a Dual-Cation System: Applications in High-Power Lithium Ion-Based Hybrid Capacitors

Abstract

To achieve lithium ion-based energy storage devices having high power densities and stabilities, the use of a low-viscosity and low-dielectric-constant solvent [such as dimethyl carbonate (DMC)] as well as a chemically stable BF4 –-based salt (such as LiBF4 or quaternary ammonium salts [spiro-(1,1)-bipyrrolidinium tetrafluoroborate (SBPBF4))] is promising for application in next-generation electrolytes. However, these combinations are impractical for several reasons, including the low ionic conductivity of LiBF4/DMC and the phase separation of SBPBF4/DMC systems. Thus, we developed a DMC-based dual-cation system (1 M LiBF4 + 1 M SBPBF4/DMC) possessing a higher ionic conductivity (5.7 mS cm–1) than that of single-cation systems (1 M LiBF4/DMC, 0.5 mS cm–1) and realizing a stable single-phase solution. Raman measurements suggest that the dual-cation system constitutes one DMC and two or three BF4 – complexes (not neutral-charged states), which should result in high ionic conductivity. Furthermore, the DMC-based dual-cation system exhibited a higher power performance in a Li4Ti5O12//activated carbon hybrid capacitor than the single-cation system (88 and 13% capacity retention at 50 mA cm–2, respectively) and demonstrated high Li+ conductivity (dual-cation: 1.6 mS cm–1, single-cation: 0.2 mS cm–1). Therefore, the dual-cation strategy could aid the development of diverse electrolyte combinations involving salts and solvents that have been considered impracticable.

Strategy for Ultrafast Cathode Reaction in Magnesium-Ion Batteries Using BF4 Anion Based Dual-Salt Electrolyte Systems: A Case Study of FePO4

“…The Raman spectra of the neat DMC solvent (880–1000 cm –1 ) in the single-salt system (Figure b) showed a major peak at 918 cm –1 , which was assigned to the cis–cis conformer of the DMC molecule (O–C–O symmetric stretching vibration) and termed as free DMC . With increasing Mg-salt concentration, the intensity of the DMC peak decreased, and a new peak appeared at ∼943 cm –1 corresponding to the DMC bound to Mg 2+ ions, termed as bound DMC . In addition, an isosbestic point was observed at 935 cm –1 between free DMC (918 cm –1 ) and bound DMC (943 cm –1 ), suggesting that they coexist in the electrolyte.…”

“… In the single-salt system, the intensities of all TFSA – anion peaks increased with increasing Mg­(TFSA) 2 salt concentration, whereas peak shape was maintained, indicating that the solvation structure around Mg 2+ with TFSA – is maintained in this salt concentration range. The Raman spectra of the neat DMC solvent (880–1000 cm –1 ) in the single-salt system (Figure b) showed a major peak at 918 cm –1 , which was assigned to the cis–cis conformer of the DMC molecule (O–C–O symmetric stretching vibration) and termed as free DMC . With increasing Mg-salt concentration, the intensity of the DMC peak decreased, and a new peak appeared at ∼943 cm –1 corresponding to the DMC bound to Mg 2+ ions, termed as bound DMC .…”

“…The ionic conductivities of the prepared electrolytes were measured at T = 298 K using a conductivity meter (Mettler Toledo, S230) equipped with a four-electrode Pt conductivity sensor (Mettler Toledo, InLab 710). Raman measurements of the electrolytes were conducted according to a previously reported protocol using a Horiba Jobin Yvon LabRAM HR800 spectrometer (0.5 cm –1 resolution) equipped with a 785 nm semiconductor laser. , The obtained Raman spectra were deconvoluted into single bands for free and bound solvent or anion species using least-squares curve fitting with a pseudo-Voigt function, as per a previously reported method. , Equations and represent the integrated intensities of the free ( I f ) and bound ( I b ) molecules of the DMC solvent ( I f_DMC and I b_DMC ) or TFSA – anion ( I f_TFSA and I b_TFSA ). I normalf = J normalf c normalf I normalb = J normalb c normalb where J f and c f are the Raman scattering coefficients ( J f_DMC or J f_TFSA ) and concentrations ( c f_DMC or c f_TFSA ) of free DMC and TFSA – species, and J b and c b are the Raman scattering coefficients ( J b_DMC and J b_TFSA ) and concentrations ( c b_DMC and c b_TFSA ) of the bound DMC and TFSA – species, respectively.…”

“…34 With increasing Mgsalt concentration, the intensity of the DMC peak decreased, and a new peak appeared at ∼943 cm −1 corresponding to the DMC bound to Mg 2+ ions, termed as bound DMC. 34 In addition, an isosbestic point was observed at 935 cm −1 between free DMC (918 cm −1 ) and bound DMC (943 cm −1 ), suggesting that they coexist in the electrolyte. We calculated the solvation numbers of the Mg 2+ cation coordinated to DMC (n DMC ) and TFSA − (n TFSA ) in the single-salt system using the least-squares curve-fitting analysis from our previously reported method (representative curve-fitting results are shown in Figures S12−S13).…”

“…Raman measurements of the electrolytes were conducted according to a previously reported protocol using a Horiba Jobin Yvon LabRAM HR800 spectrometer (0.5 cm −1 resolution) equipped with a 785 nm semiconductor laser. 29,34 The obtained Raman spectra were deconvoluted into single bands for free and bound solvent or anion species using least-squares curve fitting with a pseudo-Voigt function, as per a previously reported method. 35,36 Equations 1 and 2 represent the integrated intensities of the free (I f ) and bound (I b ) molecules of the DMC solvent (I f_DMC and I b_DMC ) or TFSA − anion (I f_TFSA and I b_TFSA ).…”