Piezoelectric COFs Function as Dynamic "Ion Pumps" to Facilitate Li⁺ Transport in Solid-State Batteries

The development of solid-state electrolytes is restricted by sluggish ion transport and unstable electrode-electrolyte interfaces. To address this issue, we introduce a paradigm-shifting approach that actively converts cycling-induced mechanical stress into an electrochemical driving force for ion migration. Through strategically structural engineering of a covalent organic framework (COF), we create a piezoelectric COF (CityU-57) with a broken structural symmetry, enabling a built-in electric field under mechanical stress (piezoelectric field). This structural modification not only decreases the HOMO energy level to improve oxidative stability but also enhances Li⁺ affinity and reduces migration barriers, especially under a piezoelectric field. When implemented as a solid electrolyte, CityU-57 achieves exceptional performance, including a high Li⁺ transference number (0.539), low interfacial resistance, and unprecedented cycling stability exceeding 5000 h in symmetric cells. Comprehensive characterization through piezo-response force microscopy, electrochemical analysis, and theoretical calculations, we verify a "mechano-electric coupling" mechanism where mechanically induced piezoelectric fields function as a dynamic "ion pump" to facilitate Li⁺ transport and homogenize the deposition.