Breaking the strength-dendrite paradox in polymer electrolytes: spherical lithium deposition via redox-active Fe-O/Cl...

Polyethylene oxide (PEO)-based electrolytes are essential for solid-state batteries due to their intrinsic flexibility and affordability for large-scale manufacturing. However, they suffer from severe Li dendrite growth. Although traditional strategies focus on improving the mechanical modulus to resist dendrites, they lead to compromised interfacial compatibility. Herein, we propose a cation-anion synergy strategy to regulate Li plating behavior and achieve spherical Li deposition in PEO-based electrolytes. Specifically, Fe-O/Cl centers are formed by simply incorporating ferric chloride (FeCl₃) into a PEO matrix, which serve as nucleation sites for Li plating. The electronegative Cl^- anions act as a "Li⁺ pump" to concentrate Li⁺ around nucleation centers, whereas redox-active Fe ⁿ⁺ cations function as an "electron reservoir" that traps interfacial electrons and enables preferential Li deposition. Meanwhile, Fe ⁿ⁺ cations catalyze TFSI^- decomposition, constructing a robust LiF/Li₂O-rich interfacial layer. This collaborative cation-anion synergy enables highly uniform spherical Li plating. Consequently, the as-prepared FeCl₃-modified electrolyte with a lower modulus (3.1 MPa) exhibits superior ability against dendrite growth, breaking the conventional strength-dendrite paradox. The obtained electrolyte enables a high critical current density of 2.8 mA cm^-2. The symmetric cell sustains stable cycling over 3000 h, while the full cell maintains a high capacity of 129.4 mAh g^-1 after 1000 cycles. This work provides a facile and effective route to regulate Li deposition for high-performance polymer electrolytes.