RESUMO
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.
RESUMO
Correction for 'Quantifying the ion coordination strength in polymer electrolytes' by Rassmus Andersson et al., Phys. Chem. Chem. Phys., 2022, 24, 16343-16352, https://doi.org/10.1039/D2CP01904C.
RESUMO
Correction for 'Quantifying the ion coordination strength in polymer electrolytes' by Rassmus Andersson et al., Phys. Chem. Chem. Phys., 2022, https://doi.org/10.1039/d2cp01904c.
RESUMO
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.
