"Incorporation of long-lived interactions into metal-ligand coordinating polymer electrolytes to improve bulk mechanical properties"

J. T. Bamford^a, S. D. Jones^a,b, N. S. Schauser^a, B. J. Pedretti^c,d, L. W. Gordon^a, N. A. Lynd^c, R. J. Clement^a, R. A. Segalman^a

^aUniversity of California, Santa Barbara, Santa Barbara, CA
^bCalifornia Polytechnic State University, San Luis Obispo, CA
^cUniversity of Texas at Austin, Austin, TX
^dMassachusetts Institute of Technology, Boston, MA

Nonflammable and electrochemically stable solid polymer electrolytes (SPEs) may enable the next generation of safe and energy-dense Li⁺ batteries. However, designing SPEs that simultaneously exhibit robust mechanical properties and high Li⁺ conductivity is fundamentally challenging due to their liquid-like ion transport mechanism. Recently, bulk mechanics and total conductivity have been partially decoupled in dynamic network SPEs, including “mixed salt” electrolytes that use metal-ligand interactions to form transient crosslinks. However, the impact of network formation on Li⁺ transport is not well understood. Herein, a polymer electrolyte with one type of ligand functionality is simultaneously blended with two metal-cation salts, one of which forms long lasting dynamic crosslinks with the ligand to improve bulk mechanics and the other comprises electrochemically-relevant Li⁺. The polymer backbone is based on polyethylene oxide (PEO), the ligand comprises an imidazole (Im) side chain, and the salts are nickel bis(trifluoromethanesulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide. Oscillatory shear rheology reveals a mechanically-reinforcing rubbery plateau in systems with Ni²⁺-Im bonding that is not present in systems with only Li⁺-Im bonding. Consequently, adding Ni²⁺ improves the shear storage modulus from 0.014 to 1.907 MPa. Meanwhile, adding Ni²⁺ only marginally reduces Li⁺ conductivity from 9.8 to 3.7 *10^-6 S/cm at 90 ˚C. Transient Ni-Im crosslinks primarily affects bulk mechanics and the diffusion of polymer chains at larger length scales, whereas the local dynamics that govern conductivity are relatively unaffected. As a result, blending metal-coordinating polymer electrolytes with multiple metal cations offers a straightforward route to independently tuning mechanical properties and Li⁺ transport.