NMR Determination of the Relative Binding Affinity of Crown Ethers for Manganese Cations in Aprotic Nonaqueous Lithium Electrolyte Solutions

Polymeric chelating agents placed in the interelectrode space of a Li-ion battery (LIB) have been suggested as a means of sequestering Mn cations dissolved from positive electrodes of LIBs to prevent their migration to, and deposition onto, negative electrodes and thus mitigate the associated degradation of LIB performance and life. In order to select the most effective chelating agent and optimize its polymeric form, it is desirable to determine the binding affinity of various chelating agents for manganese cations. The present study evaluates the relative binding affinity of crown ethers for manganese cations in a lithium-containing environment through the detection of the 7Li nucleus chemical shift. Results are presented for the relative binding affinities of 15-crown-5 and 1-aza-15-crown-5 ethers for Mn2+ and Mn3+. Significant differences in relative binding affinity were discovered with particular crown ether–manganese oxidation state combinations. Additionally, a substantial decrease in binding affinity was observed for the polymeric crown ether relative to its molecular form. These results indicate that the NMR titration technique is a useful screening tool, which will inform and assist the development of more effective manganese cations trapping materials. Quantification of the Mn trapping efficiency will permit the screening of trapping groups at the molecular level (before attachment to a polymer), as well optimization of their polymeric forms (type of backbone, morphology, length of linker, amount of cross-linking, etc.). The methodology developed in the present work will therefore accelerate the development of the Mn trapping technology.