Structural Study on Magnesium Ion Solvation in Diglyme-Based Electrolytes: IR Spectroscopy and DFT Calculations

Ultralong-life and high-capacity magnesium/sodium hybrid-ion battery using a ternary CoSe/NiSe2/CuSe2 cathode and dual-ion electrolyte

An ultrahigh-performance magnesium/sodium hybrid-ion battery (MNHB) is developed using ternary CoSe/NiSe2/CuSe2 (CNCS) "micro-flowers" as cathode materials, working with a coordinative [Mg2Cl2][AlCl4]2 and bis(trifluoroethylsulfonyl)imide anionic sodium salt in triglyme electrolyte. After 2000 cycles at 2.0 A g-1, the MNHB shows a stable capacity of 115.5 mA h g-1 and a high Coulombic efficiency exceeding 99.8%. The battery shows very rapid charging, and good stability in extreme environments, providing new opportunities to develop other hybrid-ion systems.

Seeing the Unseen: Mg2+, Na+, and K+ Transference Numbers in Post-Li Battery Electrolytes by Electrophoretic Nuclear Magnetic Resonance

The growing demand for energy storage devices worldwide combined with limited resources for lithium attracts interest in other alkali or alkaline earth metals. In addition to conductivity, the cation transference number T+ is a decisive parameter to rank the electrolyte performance. However, the existing experimental methods for its determination suffer from various intrinsic problems. We demonstrate here a novel approach for T+ determination based on determining the total conductivity with impedance spectroscopy (IS) and the partial conductivity of the anion species, with the latter being obtained from the anion mobility by electrophoretic NMR. First, this eNMR/IS approach is validated by comparing T+ values from different methods in a Li-based solvate ionic liquid electrolyte. Then, it is applied to obtain T+ of cations with nuclei not detectable in NMR transport measurements, employing bis(trifluoromethanesulfonyl)imide (TFSI)-based metal salts. Solvate ionic liquids consisting of triethylene glycol dimethyl ether (G3) and Mg(TFSI)2 or NaTFSI yield values of TNa and TMg on the order of 0.4, similar to TLi. Furthermore, we apply the method to polymer electrolytes, again testing the concept with LiTFSI, and finally investigating NaTFSI, KTFSI, and Mg(TFSI)2 in poly(ethylene oxide). Values of TNa and TK are in the range of 0.14-0.2, similar to those of TLi, while Mg2+ shows a higher transference number (TMg = 0.3). The method is very versatile as it allows quantification of T+ for any type of cation, and moreover, it is applicable to highly concentrated electrolytes without suffering from assumptions about dissociation or from unknown interfacial resistances which impede electrochemical methods.

Organic Electrolyte Design for Rechargeable Batteries: From Lithium to Magnesium

Rechargeable magnesium batteries (RMBs) have been considered as one of the most viable battery chemistries amongst the "post" lithium-ion battery (LIB) technologies owing to their high volumetric capacity and the natural abundance of their key elements. The fundamental properties of Mg-ion conducting electrolytes are of essence to regulate the overall performance of RMBs. In this Review, the basic electrochemistry of Mg-ion conducting electrolytes batteries is discussed and compared to that of the Li-ion conducting electrolytes, and a comprehensive overview of the development of different Mg-ion conducting electrolytes is provided. In addition, the remaining challenges and possible solutions for future research are intensively discussed. The present work is expected to give an impetus to inspire the discovery of key electrolytes and thereby improve the electrochemical performances of RMBs and other related emerging battery technologies.

A structural and electrochemical study of lithium-ion battery electrolytes using an ethylene sulfite solvent: from dilute to concentrated solutions

Coordination structures of Li-ion complexes in ethylene sulfite (ES)-based electrolytes exhibiting the fast charge–discharge behavior.

Investigation of alkali and alkaline earth solvation structures in tetraglyme solvent

This study compares molecular calculations performed with molecular and periodic codes through an investigation of the solvation structures of alkali and alkaline earth metal ions in tetraglyme solution. The two codes are able to produce equivalent structural and energetic information at the same level of theory, and in the presence of the implicit solvation model or not. This comparison reveals that molecular optimisations can be performed with periodic codes and used directly as input models for interface or electrochemistry calculations in order to preserve the solvent-solute interaction and the cavitation energy. By a rigorous comparison, we have demonstrated that equivalent energetic values can be obtained with the conventional PBE-D3 and the newly developed SCAN-rVV10 functionals. Nevertheless, as far as the vibrational features are concerned and when the molecule possesses a highly conjugated system, the SCAN-rVV10 functional is required to describe the vibrational modes properly. The computed IR/Raman spectra can thus be used as essential information to determine the first solvation shell of metal ions in glyme-based solutions. In tetraglyme solution, the alkali and alkaline earth metal ions exhibit a diverse solvation structure. Small ions like Li⁺ and Mg²⁺ tend to adopt a coordination number of five or six, while larger ions, Na⁺, K⁺, and Ca²⁺, prefer an eight-coordinated environment, and the metal-ligand interaction increases in the order K⁺-O < Na⁺-O < Li⁺-O < Ca²⁺-O < Mg²⁺-O. The solvation spheres play a significant role in the stability and the reactivity of the solvated ions, and can thus be used as input models to construct the solvation structure in more sophisticated electrolytes, such as polyethylene oxide, or perform electrochemical calculations.

Quantum-Chemical and Molecular Dynamics Investigations of Magnesium Chloride Complexes in Dimethoxyethane Solutions

Quantum-chemical calculations and classical and ab initio molecular dynamics simulations have been performed to study the Mg²⁺-conducting electrolytes based on Mg(TFSI)₂/MgCl₂ solutions in dimethoxyethane. It has been shown that depending on the TFSI/Cl^- ratio, the Mg₂Cl₂ ²⁺ or Mg₃Cl₄ ²⁺ complexes are preferred as stable ion aggregates. In the initial stages of the ion association process, MgCl⁺, MgCl₂, and Mg₂Cl₃ ⁺ are formed as intermediate species. Calculations of harmonic frequencies and simulations of the IR spectrum of the electrolyte from the ab initio MD trajectories have been used to identify the spectral range of vibrations of ion aggregates found in the modeled electrolyte. The results have been discussed in the context of experimental data.

Fluorinated alkyl-phosphate-based electrolytes with controlled lithium-ion coordination structure

Herein, we propose Li-ion solvation-controlled electrolytes based on non-flammable organic solvent TFEP and an LiFSA salt [TFEP: tris(2,2,2-trifluoroethyl)phosphate, LiFSA: lithium bis(fluorosulfonyl)amide] to allow Li-ion insertion into a graphite electrode for Li-ion batteries. Comprehensive structural study based on (1) infrared (IR)/Raman spectroscopy, (2) high-energy X-ray total scattering (HEXTS), and (3) molecular dynamics (MD) simulation revealed the solvation (or coordination) structures of Li ions in TFEP-based electrolytes at the molecular level. In binary LiFSA/TFEP with a Li salt concentration (c_Li) < 1.0 mol dm^-3, Li ions are coordinated with both TFEP and FSA components; in detail, two TFEP molecules coordinate in an O-donating monodentate manner and one FSA in an O-donating bidentate manner to form [Li(TFEP)₂(bi-FSA)] as the major species. We demonstrated that adding acetonitrile (AN) to the LiFSA/TFEP electrolytes caused structural changes in the Li-ion complexes. The bi-FSA bound to the Li ion changed its coordination mode to mono-FSA, which was induced by solvating AN molecules to Li ions. The redox reaction corresponding to insertion/deinsertion of Li ions into/from the graphite electrode successfully occurred in 1.0 mol dm^-3 LiFSA/TFEP with an AN electrolyte system, while there was no or reduced Li-ion insertion in the electrolyte without AN. We discussed the relationship between the structure and electrode reaction of the Li-ion complexes based on the FSA-coordination characteristics; i.e., in LiFSA/TFEP with the AN system, the mono-FSA bound to the Li ion is easier to decoordinate due to weaker Li⁺mono-FSA^- interactions rather than the Li⁺bi-FSA^- interactions, which mainly contribute to charge-transfer at the electrode/electrolyte interface to allow Li-ion insertion/deinsertion in the graphite anode.