Designing Zwitterionic Bottlebrush Polymers to Enable Long-Cycling Quasi-Solid-State Lithium Metal Batteries

Abstract

Ionogel polymer electrolyte (IPE), incorporating ionic liquid (IL) within a polymer matrix, presents a promising avenue for safe quasi-solid-state lithium metal batteries. However, sluggish Li⁺ kinetics, resulting from the formation of [Li(anion)<inf>n</inf>]⁻⁽ⁿ⁻¹⁾ clusters and the occupation of Li⁺ transport sites by organic cations, limit their practical applications. In this study, we have developed zwitterionic bottlebrush polymers-based IPE with promoted Li⁺ conduction by employing poly(sulfobetaine methacrylate)-grafted poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVC-g-PSBMA) bottlebrushes as matrices of IL. The grafted zwitterionic side chains greatly facilitate the dissociation of [Li(anion)<inf>n</inf>]⁻⁽ⁿ⁻¹⁾ clusters to produce more movable Li⁺. Moreover, the positively charged −NR<inf>4</inf>⁺ groups in zwitterionic side chains effectively restrain anions migration, while the negatively charged −SO<inf>3</inf>⁻ groups immobilize IL cations, preventing them from occupying Li⁺ hopping sites and reducing the energy barrier for Li⁺ migration. These synergistic effects contribute to a notable ionic conductivity (7.5×10⁻⁴ S cm⁻¹) and Li⁺ transference number (0.62) of PVC-g-PSBMA IPE at 25 °C. As a result, PVC-g-PSBMA IPE enables ultralong-term (over 6500 h) reversible and stable Li plating/stripping in Li||Li symmetric cells. Remarkably, the assembled Li||LiFePO<inf>4</inf> full batteries demonstrate unprecedented cycling stability of more than 2000 cycles with a superior capacity retention of 93.7 %.