Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes-A Review

Abstract

Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes.

Keywords: Multivalent electrolytes; Renewable energy; Solvation structure.

Fig. 1

Relationships between de-solvation energies of Li ion and monovalent (Na: blue circle) ion and multivalent ion (Mg: red square) ions, as well as SbCl₅: green triangle). The dashed line corresponds to the energies of Li ion. Republished with permission of Journal of the Electrochemical Society from ref 41; permission conveyed through Copyright Clearance Center, Inc

Fig. 2

Solvation structure of Mg(TFSI)₂/diglyme solution from r-weighted form of the d-PDFs, rG(r), highlights the well-defined inner sphere features and broader solvation shells at longer distances (top). Mg-X radial distribution functions from MD simulations (O: red, N: blue, C: grey, S: yellow). The corresponding simulation showing Mg²⁺ in space-filling format (magenta), TFSI^_ and diglyme in stick format (bottom). Reproduced from Ref. 43 by permission of the PCCP Owner Societies

Fig. 3

Representative simulation snapshot of a 0.1 M; b 1.5 M Mg (TFSI)₂/diglyme at 298 K. Mg depicted in pink in space-filling format, TFSI in licorice, and diglyme in line format; c and d show the zoomed image of solvation structure around an Mg ion in 0.1 M and 1.5 M concentrations, respectively. Reproduced from Ref. 45 by permission of The Royal Society of Chemistry

Fig. 4

a Bond dissociation energy (BDE) of TFSI⁻ anion in different chemical environments corresponding to well-solvated, Mg¹⁺ ion paired and Mg²⁺ ion paired configurations from DFT calculations. Adapted with permission from Ref 30. Copyright 2015 American Chemical Society. b Energy-dispersive X-ray spectroscopy analysis of deposited Mg metal from a Mg(TFSI)₂/diglyme electrolyte. Inset shows zoomed image at low energy. Reproduced from Ref. 45 by permission of The Royal Society of Chemistry. c Galvanostatic cycling of Mg/Mg cells at a rate of C/20 after the first cycle at C/40. Adapted with permission from Ref. 20. Copyright 2014 American Chemical Society

Fig. 5

a Concentration-dependent ionic conductivity and b diffusion coefficient of Mg(TFSI)₂/diglyme electrolyte at a concentration range from 0.1 to 1.5 M. Reproduced from Ref. 45 by permission of The Royal Society of Chemistry. c Self-diffusion coefficients of Mg²⁺ and TFSI⁻ in seven different solvents (shown on x-axis) displayed with error bars. Adapted with permission from Ref 30. Copyright 2015 American Chemical Society

Fig. 6

Proposed mechanisms of structural diffusion in the Mg²⁺−IL electrolytes, where the dashed lines indicate interactions being formed (▶) and broken (●). The weakly coordinating monodentate and bridging TFSI⁻ anions tend to rapidly exchange between different ionic clusters, resulting in a faster process. On the other hand, strong coordination of bidentate TFSI⁻ anions results in a vehicular diffusion of the cluster followed by a slower process, which involves exchange of the bidentate anions via a bidentate to bridging mechanism. The Pyr1x⁺ cations are omitted for simplicity. Reprinted with permission from Ref 44. Copyright 2014 American Chemical Society

Fig. 7

a Refined SCXRD structure for MgTFSI₂ single crystal, recrystallized from MgTFSI₂/DME solution. b Raman spectra of pure DME (brown line), U-phase (orange line), L-phase (yellow line), 1.25 M MgTFSI₂ solution in DME, and powder pure MgTFSI₂: 750−900 cm⁻¹ spectral region. c Conductivity measurements of MgTFSI₂/DME solutions at various salt concentrations. Adapted with permission from Ref 54. Copyright 2016 American Chemical Society

Fig. 8

a Displacement of ellipsoid representation of [Mg₂Cl₃(THF)₆]⁺[TFSI]⁻. b ESI–MS spectrum of 0.5 M Mg(TFSI)₂-MgCl₂ at the ratio of 1:(0.5) in THF (experimental measurement in positive mode). Adapted with permission from Ref 56. Copyright 2016 American Chemical Society

Fig. 9

a Cyclic voltammograms (20 mVs^_1) of 0.01 M Mg(BH₄)₂ in glymes on a Pt electrode. b Radial distribution function of Mg–O(solvent) of Mg(BH₄)₂ in DME, diglyme, triglyme, and tetraglyme from MD simulations. Reprinted from Ref. 51 with permission from Elsevier

Fig. 10

a The exchange mechanisms and solvation structures in the combined electrolyte containing both Mg(BH₄)₂ and Mg(TFSI)₂ in diglyme (DGM), highlighting that Structure-A is changed to Structure-B via intermediate Structure-C. b Coordination number of Mg-BH₄, Mg-TFSI and Mg-DGM in mixed solutions of Mg(BH₄)₂ and Mg(TFSI)₂ (b) cyclic voltammetry of electrolyte solutions prepared in diglyme with different concentrations of Mg(BH⁠4) 2 and Mg(TFSI) 2 as labeled. The scan rate was 20 mV/s. Reprinted from Ref. 60 with permission from Nano Energy

Fig. 11

a ORTEP plot of [(μ-Cl)₃Mg₂(THF)₆]AlCl₄. b [(μ-Cl)₃Mg₂(THF)₆]AlPh₃Cl [(μ-Cl)₃Mg₂(THF)₆]AlCl₄ (Mg, blue; Cl, green; Al, pink; C, gray; O, red). Reproduced from Ref. 75 with permission from The Royal Society of Chemistry. c ORTEP plot (25% thermal probability ellipsoids) of (Mg₂(μ- Cl)₃6THF) (HMDSAlCl₃). Reproduced from Ref. 67 with permission from The Royal Society of Chemistry

Fig. 12

Complex structures of Mg electrolytes. a, b Tetrahedral dimer structure of neutral Mg complex in 0.25 M BuMgCl/THF: Mg₂Cl₂Bu₂THF₂. c, d Tetrahedral dimer structure of dicationic Mg complex in 0.25 M MgAlCl₂EtBu₂/THF: Mg₂Cl₂THF₄²⁺. e Tetrahedral monomer structures of Al complexes in 0.25 M MgAlCl₂EtBu₂/THF, and their equilibrium, where R = Et or Bu. Republished with permission of Journal of the Electrochemical Society from Ref 70; permission conveyed through Copyright Clearance Center, Inc. DFT-PBE optimized solvation structures for Mg₂Cl³⁺·4THF in approximate (f) C_3v and (g) D_2h symmetry and Mg₂Cl₄·5THF in approximate (h) C_3v and (i) D_2h symmetry. Reprinted with permission from Ref 76. Copyright 2014 American Chemical Society

Fig. 13

ORTEP plot (50% thermal probability ellipsoids) of [Mg₂(μ -Cl)₃THF₆][HMDSAlCl₃]THF. Hydrogen atoms are omitted for clarity. Reproduced from Ref. 86 with permission from The Royal Society of Chemistry

Fig. 14

a ¹¹B NMR spectra measured with boron-based electrolyte solutions of Mes₃B/THF (a), Mes₃B–PhMgCl/THF(b), Mes₃B–(PhMgCl)₂/THF (c), (b) ORTEP drawing of the molecular structure of the crystallized Mes₃B–(PhMgCl)₂ complex electrolyte. Hydrogen atoms have been omitted for clarity. Reproduced from Ref .88 with permission from The Royal Society of Chemistry

Fig. 15

Formation free energy (in eV) of magnesium-chloride complexes as a function of THF coordination for a MgCl ⁺ (monomer), b Mg₂Cl₃⁺(dimer), c Mg₃Cl₅⁺ (trimer), and d MgCl₂. Arrows indicate the most stable THF coordination environment for each complex. Snapshots of the most stable magnesium-chloride complexes are also depicted. Ref. 90 - Published by The Royal Society of Chemistry

Fig. 16

Snapshots from simulations of PEO/LiTFSI at 423 K (a, b, d) for EO:Li = 20:1 and c for EO:Li = 10:1. Hydrogen atoms are not shown for clarity. Only atoms within 4.0 Å of each Li + are shown. Reprinted with permission from Ref. 112. Copyright 2006 American Chemical Society

Fig. 17

a Spectrum of the symmetric SO₃ stretching region of an electrolyte of polyacrylonitrile (0.25 g), propylene carbonate (0.6 g), ethylene carbonate (0.6 g), and Zn(CF₃SO₃)₂ (1.25 M). The band at 1032 cm⁻¹ corresponds to free CF₃SO₃⁻, 1042 cm⁻¹ to ion pairs and 1055 cm⁻¹ to higher aggregates. b Percent composition of free ions, ion pairs, and higher aggregates as a function of salt concentration. ZnTr = Zn(CF₃SO₃)₂. Reprinted from Ref. 148, with permission from Elsevier

Fig. 18

Coordination numbers of Zn-anion and Zn-solvent for TFSI⁻ (a) and CF₃SO₃⁻ (b) at 0.1 M (filled bars) and 0.5 M (hollow bars) for four different solvents. Reprinted with permission from Ref 12. Copyright 2016 American Chemical Society