Realizing the full potential of lithium batteries requires an electrolyte medium that has both high lithium transference (i.e. high ionic conductivity) and favorable mechanical properties to prevent the formation of lithium metal dendrites that cause battery failure. The current industry standard in lithium battery technology uses a liquid electrolyte that is incompatible with pure lithium metal electrodes which limits the lifetime and energy density of the battery. This research presents, for the first time, the synthesis and characterization of hyperbranched phosphonium ionenes for use as solid polymer electrolytes in lithium metal batteries. The synthesis of hyperbranched phosphonium ionenes was accomplished by the step-growth polymerization of a terminal diphosphinoalkane with a branching molecule, 1,3,5-tris(bromomethyl) benzene. Using different diphosphinoalkanes with varying alkyl chain length to prepare these hyperbranched polymers, we hypothesize that the lithium conductivity will be increased in hyperbranched polymers that were synthesized with shorter diphosphinoalkane monomers. Gel permeation chromatography shows that the molecular weight of the polymer does not depend on the alkyl chain length in the diphosphinoalkane and that the hyperbranched phosphonium ionenes can be synthesized to molecular weights up to 25 kDa. Rheological characterization of these unique polymers shows that hyperbranched phosphonium ionenes have relatively high modulus (≈10^6 MPa) compared to the current state-of-the-art solid polymer electrolytes. Also, thermal analysis of the hyperbranched phosphonium ionenes using thermogravimetric analysis reveals their thermal stability at temperatures exceeding 300 °C. Further, initial attempts to prepare melt processed films that incorporate lithium-based salts indicate that solvent-free and flexible homogeneous films containing lithium bis(trifluoromethanesulfonyl)imide can be produced. In conclusion, the high modulus and thermal stability of these unique polymers indicate that they are excellent candidates for preventing the formation of lithium metal dendrites during battery operation. Future work for this project will directly address the hypothesis for this project by including a thorough electrochemical analysis to determine the ionic conductivity, lithium transference number, and charge/discharge properties of coin cell batteries that incorporate these solid polymer electrolyte membranes.

Hyperbranched phosphonium-based polyelectrolytes for application in lithium batteries

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.