Probing the Dynamics of Li⁺ Ions on the Crystal Surface: A Solid-State NMR Study

Polyethylene oxide-based solid polymer electrolytes (SPEs) are of research interest because of their potential applications in all-solid-state Li⁺ batteries. However, despite their advantages in terms of compatibility with the electrodes and easy processing, polyethylene oxide (PEO)/Li⁺ complexes often suffer from low conductivity at room temperature. Understanding the conduction mechanism and, in turn, developing strategies to improve the conductivity have long been the main objectives underlying research into PEO/Li⁺ complex electrolytes. Here, we prepared several special PEO/Li⁺ complex samples where the PEO/Li⁺ complex structures were located on the surfaces of PEO crystals and consisted of high content chain ends. We found two different Li⁺ species in the PEO/Li⁺ complex structures via solid-state nuclear magnetic resonance (NMR). The 2D ⁷Li exchange NMR showed the exchange process between the different Li⁺ species. The exchange dynamics of the Li⁺ ions provide a molecular mechanism of the Li⁺ transportation in the surface of PEO crystal lamella, which is further correlated with the ionic conduction mechanism of the PEO/Li⁺ complex structure.