Finding a viable electrolyte for next-generation 5 V-class lithium-ion batteries is of primary importance. A long-standing obstacle has been metal-ion dissolution at high voltages. The LiPF6 salt in conventional electrolytes is chemically unstable, which accelerates transition metal dissolution of the electrode material, yet beneficially suppresses oxidative dissolution of the aluminium current collector; replacing LiPF6 with more stable lithium salts may diminish transition metal dissolution but unfortunately encounters severe aluminium oxidation. Here we report an electrolyte design that can solve this dilemma. By mixing a stable lithium salt LiN(SO2F)2 with dimethyl carbonate solvent at extremely high concentrations, we obtain an unusual liquid showing a three-dimensional network of anions and solvent molecules that coordinate strongly to Li(+) ions. This simple formulation of superconcentrated LiN(SO2F)2/dimethyl carbonate electrolyte inhibits the dissolution of both aluminium and transition metal at around 5 V, and realizes a high-voltage LiNi0.5Mn1.5O4/graphite battery that exhibits excellent cycling durability, high rate capability and enhanced safety.

Superconcentrated Electrolytes for a High-Voltage Lithium-Ion Battery

In the progress of implementing solid polymer electrolytes (SPEs) into batteries, fundamental understanding of the processes occurring within and in the vicinity of the SPE are required. An important but so far relatively unexplored parameter influencing the ion transport properties is the ion coordination strength. Our understanding of the coordination chemistry and its role for the ion transport is partly hampered by the scarcity of suitable methods to measure this phenomenon. Herein, two qualitative methods and one quantitative method to assess the ion coordination strength are presented, contrasted and discussed for TFSI-based salts of Li+, Na+ and Mg2+ in polyethylene oxide (PEO), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC). For the qualitative methods, the coordination strength is probed by studying the equilibrium between cation coordination to polymer ligands or solvent molecules, whereas the quantitative method studies the ion dissociation equilibrium of salts in solvent-free polymers. All methods are in agreement that regardless of cation, the strongest coordination strength is observed for PEO, while PTMC exhibits the weakest coordination strength. Considering the cations, the weakest coordination is observed for Mg2+ in all polymers, indicative of the strong ion-ion interactions in Mg(TFSI)2, whilst the coordination strength for Li+ and Na+ seems to be more influenced by the interplay between the cation charge/radius and the polymer structure. The trends observed are in excellent agreement with previously observed transference numbers, confirming the importance and its connection to the ion transport in SPEs.

/pdf/quantifying-the-ion-coordination-strength-in-polymer-1zy9sr0m.pdf

Quantifying the ion coordination strength in polymer electrolytes.

Concentrated ionic solutions present a potential improvement for liquid electrolytes. However, their conductivity is limited by high viscosities, which can be attenuated via cosolvation. This study employs a series of experiments and molecular dynamics simulations to investigate how different cosolvents influence the local structure and charge transport in concentrated lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI)/acetonitrile solutions. Regardless of whether the cosolvent's dielectric constant is low (for toluene and dichloromethane), moderate (acetone), or high (methanol and water), they preserve the structural and dynamical features of the cosolvent-free precursor. However, the dissimilar effects of each case must be individually interpreted. Toluene and dichloromethane reduce the conductivity by narrowing the distribution of Li+-TFSI- interactions and increasing the activation energies for ionic motions. Methanol and water broaden the distributions of Li+-TFSI- interactions, replace acetonitrile in the Li+ solvation, and favor short-range Li+-Li+ interactions. Still, these cosolvents strongly interact with TFSI-, leading to conductivities lower than that predicted by the Nernst-Einstein relation. Finally, acetone preserves the ion-ion interactions from the cosolvent-free solution but forms large solvation complexes by joining acetonitrile in the Li+ solvation. We demonstrate that cosolvation affects conductivity beyond simply changing viscosity and provide fairly unexplored molecular-scale perspectives regarding structure/transport phenomena relation in concentrated ionic solutions.

Structure-Dynamics Interrelation Governing Charge Transport in Cosolvated Acetonitrile/LiTFSI Solutions.

Acetonitrile (AN) electrolyte solutions display uniquely high ionic conductivities, of which the rationale remains a long-standing puzzle. This research delves into the solution species and ion conduction behavior of 0.1 and 3.0 M LiTFSI AN and propylene carbonate (PC) solutions via Raman and dielectric relaxation spectroscopies. Notably, LiTFSI-AN contains a higher fraction of free solvent uncoordinated to Li ions than LiTFSI-PC, resulting in a lower viscosity of LiTFSI-AN and facilitating a higher level of ion conduction. The abundant free solvent in LiTFSI-AN is attributed to the lower Li-solvation power of AN, but despite this lower Li-solvation power, LiTFSI-AN exhibits a level of salt dissociation comparable to that of LiTFSI-PC, which is found to be enabled by TFSI anions loosely bound to Li ions. This work challenges the conventional notion that high solvating power is a prerequisite for high-conductivity solvents, suggesting an avenue to explore optimal solvents for high-power energy storage devices.

Low Solvating Power of Acetonitrile Facilitates Ion Conduction: A Solvation-Conductivity Riddle.

A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temperature range. Four new crystalline solvate structures-(PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, respectively-have been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI(-)···Li(+) cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI(-) anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI(-)···Li(+) cation interactions possible and the overlap of the corresponding spectroscopic data signatures.

Solvate Structures and Spectroscopic Characterization of LiClO4 Electrolytes

Neutron diffraction measurements on 6Li/7Li isotopically substituted 10 and 33 mol % *LiTFSA (lithium bis(trifluoromethylsulfonyl)amide)-AN-d3 (acetonitrile-d3) and 10 and 33 mol % *LiTFSA-DMF-d7(N,N-dimethylformamide-d7) solutions have been carried out in order to obtain structural insights on the first solvation shell of Li+ in highly concentrated organic solutions. Structural parameters concerning the local structure around Li+ have been determined from the least squares fitting analysis of the first-order difference function derived from the difference between carefully normalized scattering cross sections observed for 6Li-enriched and natural abundance solutions. In 10 mol % LiTFSA-AN-d3 solution, 3.25 ± 0.04 AN molecules are coordinated to Li+ with a intermolecular Li+···N(AN) distance of 2.051 ± 0.007 A. It has been revealed that 1.67 ± 0.07 AN molecules and 2.00 ± 0.01 TFSA- are involved in the first solvation shell of Li+ in the 33 mol % LiTFSA-AN solution. The nearest neighbor Li+···NAN and Li+···OTFSA- distances are obtained to be r(Li+···N) = 2.09 ± 0.01 A and r(Li+···O) = 1.88 ± 0.01 A, respectively. The first solvation shell of Li+ in the 10 mol % LiTFSA-DMF-d7 solutions contains 3.4 ± 0.1 DMF molecules with an intermolecular Li+···ODMF distance of 1.95 ± 0.02 A. In highly concentrated 33 mol % LiTFSA-DMF-d7 solutions, there are 1.3 ± 0.2 DMF molecules and 3.2 ± 0.2 TFSA- in the first solvation shell of Li+ with intermolecular distances of r(Li+···ODMF) = 1.90 ± 0.02 A and r(Li+···OTFSA-) = 2.01 ± 0.01 A, respectively. The Li+···TFSA- contact ion pair stably exists in highly concentrated 33 mol % LiTFSA-AN and -DMF solutions.

Solvation Structure of Li+ in Concentrated Acetonitrile and N,N-Dimethylformamide Solutions Studied by Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods.

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Solvation Structure of Li+ in Concentrated Acetonitrile and N,N-Dimethylformamide Solutions Studied by Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods.