Incorporation of Long-Lived Interactions into Metal-Ligand Coordinating Polymer Electrolytes to Improves Bulk Mechanical Properties

Next-generation Li batteries require solid state electrolytes that decouple Li⁺ transport from bulk mechanics. We demonstrate that these properties can be separated by blending a metal-ligand coordinating polymer (MCP) with two types of metal cation salts, such that each salt independently tunes mechanics and ion transport via their interactions with the polymer ligand. Specifically, a polyether-based MCP with one type of ligand, imidazole (Im), can effectively decouple Li⁺ conductivity from bulk mechanics by mixing with Li⁺ and Ni²⁺ salts. Li⁺ has weaker ML interactions and diffuses rapidly. Ni²⁺ salt has stronger ML interactions and forms a dynamic network. A unique rheological plateau appears in the mixed-salt MCP that increases the storage modulus by over two orders of magnitude, from 0.014 MPa to 1.907 MPa. Dynamic Ni-Im crosslinks inhibit chain diffusion at larger length scales, while local Li⁺ hopping dynamics are only moderately stiffened. Thus, while introduction of Ni²⁺ salts to PEO-PIGE reduces Li⁺ conductivity by a factor of 2.6, from 9.8 to 3.7 *10^-6 S/cm at 90 °C, bulk mechanics are significantly improved by a factor of 135.