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Understanding Electrolytes with NMR

Requirements for energy storage grow almost daily, from the widespread use of portable electronics to the rapidly growing market for electric vehicles. Ensuring that this demand can be met requires continued development of new battery technologies and optimisation of the manufacturing of current technologies. Benchtop NMR has an important role to play in many areas of battery technology.

Currently, all commercial batteries and those technologies expected to come into production in the next 5 years are based on liquid state electrolytes. This makes NMR a perfect analytical tool for supporting their development. The role of the electrolyte is essentially to provide a free deintercalation environment for ions and to achieve current conduction between the positive and negative electrodes of the battery.

Due to the wide variety of salts, organic solvents and additives, electrolytes with different compositions and ratios may have significant differences in thermal resistance, chemical stability, ionic conductivity, and electrode compatibility. This can greatly affect the performance of the battery, Life, safety, and scope of application. Therefore, accurately, and comprehensively characterizing the electrolyte as well as understanding and controlling its method of action are indispensable in development and quality control of battery technology.

Electrolyte development

Current electrolytes are based on either Li+ or Na+ salts dissolved in a mixture of organic solvents, typically ethyl carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) or propylene carbonate (PC). One of the key parameters to understand the performance of these electrolytes is the ion mobility. Using our X-Pulse broadband benchtop spectrometer, the diffusion coefficient and therefore ion mobility of all of the different species in the electrolyte can be measured quickly and in the same lab in which the developmental chemistry takes place. The wide range of NMR active nuclei available in a single X-Pulse instrument means that your cation (7Li, 23Na), anion (31P, 11B, 19F) and solvent (1H,13C) components can all be measured independently.

PFGSE experiment results for 3 nuclei collected on a single system, 1H spectra showing the solvent, 19F showing the anion, and 7Li showing the Li cation

It is well known that current generation electrolytes can be unstable. Through numerous charge and discharge cycles a wide variety of breakdown products can form. With a single 19F spectrum it is possible to identify most of these. Common compounds such as LiF or hydrofluoric acid identify very separate regions of the spectrum, even at 60MHz. The flexibility of benchtop NMR opens the possibility of performing rapid checks of electrolyte after each discharge cycle to understand the processes that underpin degradation.

7Li: Quantify Li concentration and quickly identify variations in chemical environment

19F: Identify major fluorinated components as well as breakdown products and contaminants

Quality control

Ensuring the quality of the highest performance batteries requires the highest quality materials. Benchtop NMR spectroscopy allows rapid measurement of raw materials and final product to ensure consistency and quality. The broadband capability of X-Pulse allows the measurement of a wide range of nuclei, 1H, 19F, 7Li, 31P, 11B etc. This means that purity of raw materials and the consistency of product can be checked quickly and easily using a single instrument. For example, the purity of LiF6P or LiF4B will be very obvious from a single 31P or 11B spectrum. The presence of H2O in either raw materials or final product can be quickly identified from a single 1H spectrum, and as previously discussed the majority of breakdown products and contaminants can be identified using a single 19F spectrum.

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