Oxygen defect-mediated Li-ion transport for long-cycle solid-state lithium metal batteries

Solid-state lithium (Li) metal batteries have attracted significant attention due to their high energy density and improved safety performance. However, sluggish Li-ion transport and rapid anion migration in solid-state electrolytes often result in heterogeneous Li-ion flux distribution and thus Li dendrite growth. Herein, we developed a highly conductive composite solid electrolyte with an elevated Li-ion transference number through incorporating Gd-doped CeO₂ (GDC) nanofillers with abundant surface oxygen defects into poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrices. The defect concentrations were effectively controlled by regulating the Gd doping ratio in CeO₂. As a result, the highest oxygen concentration of 12.2% is achieved for the GDC with 10% Gd doping (GDC-10). The GDC-10 electrolyte demonstrated a high Li-ion transference number of 0.59 and an improved ionic conductivity of 0.40 mS/cm at room temperature, attributed to anion immobilization and enhanced Li-salt dissociation. This was due to the strong interactions between positively charged oxygen vacancies and anions, which effectively reduces surface concentration polarization and homogenizes Li-ion flux. Therefore, Li||Li symmetric cells exhibited exceptional cycling stability of 1500 h without noticeable Li dendrite growth at 1 mA/cm² and 1 mAh/cm². Furthermore, Li||LiFePO₄ full cell also stably cycles for 500 cycles with a capacity retention of 90.44% at 1 C. This work provides new insights into the design of composite solid electrolytes through the defect regulation of fillers.