# Constructing Enhanced Composite Solid-State Electrolytes with Sb/Nb Co-Doped LLZO and PVDF-HFP
## 3 Results and Discussion
From the overall view, it could be observed that LLZSNO particles distribute uniformly on the surface of CSE, and the film is smooth and dense. Moreover, the concrete image in Figure S4 shows that the SSEs have good flexibility, recoverability, and film-forming properties. Li I PVDF-HFP | LiFePO4 and Li I PVDF-HFP/LLZSNO | LiFePO4 batteries were assembled to study the electrochemical performance of SSEs. Figure 5a,b display the charging and discharging capacity of coin batteries at room temperature. The results show that the two batteries can cycle for more than 100 cycles at a rate of 1 C. The initial discharge-specific capacity of the pure polymer electrolyte is about 138.6 mAh/g, and the CSE PL-0.05 shows a higher initial specific capacity of 161.2 mAh/g with a specific capacity retention of 92.7% after 100 cycles, and during the 100 cycles, the CSE battery keeps a high Coulombic efficiency of 99.5-100%. Figure 5c,d exhibit that the voltage change of SSEs is small, which indicates that the solid-state lithium batteries have perfect cycling stability. Figure S5 shows the Arrhenius plots of the SSEs at different temperatures (25-80 \({}^{\circ}\)C). The results indicate that the impedance of the samples decreases along with the temperature increase throughout the reheating process, and their ionic conductivities are all  
Figure 5: (**a**,**b**) Cycle performance comparison graphs, and (**c**,**d**) charge and discharge voltage platform of PVDF-HFP and PL-0.05 at room temperature and 1 C.  
improved along with the temperature. As shown in Table S1, the composite solid electrolyte PL-0.05 has the highest ionic conductivity of 1.76 \(\times\) 10\({}^{-4}\) S/cm at room temperature and the lowest activation energy of 0.76 eV. Composite solid electrolytes have higher ionic conductivity than polymer solid electrolytes, which is a contribution made by inorganic ceramic fillers. Because the LLZO is a fast ionic conductor, it greatly improves the ionic conductivity of the CSE, and the garnet also reduces the crystallinity of the polymer, thus increasing the mobility of the polymer chains and improving the lithium-ion transport efficiency inside the composite solid electrolyte. As shown in Figure 6a, the important role of inorganic fillers is also emphasized by the results that the electrochemical window of pure polymer electrolytes is only 4.5 V, while the best-performing PL-0.05 composite solid-state electrolyte can reach 5.2 V. The improvement of the electrochemical window of the CSEs containing inorganic ceramic filler is attributed to the fact that LLZO possesses a high electrochemical window of more than 6 V. In addition, Lewis acid-base interactions on the surface of the LLZNO stabilize the TFSI\({}^{-}\) and inhibit their migration, which also contributes to the oxidative stability of the CSEs. The solid electrolyte with a higher electrochemical window could be more adaptable to match the high-voltage cathode, laying the foundation for improving the overall energy density of the battery. Figure S6 shows the AC impedance and DC polarization test results of the SSEs at 25 \({}^{\circ}\)C. The Li\({}^{+}\) transfer number of the PVDF-HFP solid electrolyte is about 0.24, while that of the PL-0.05 CSE is increased to 0.32. This enhancement could be explained by the following mechanism: the LLZO ceramic is an active conductor with a Li-ion transfer number near 1. The addition of LLZO ceramic filler can effectively enhance the Li transfer number of the polymer. Figure 6b shows the rate performance of PVDF-HFP and PL-x. The results exhibit that when the rate was gradually increased, the PVDF-HFP electrolyte did not perform as well as the composite electrolyte, and its specific capacity decreased very quickly, to only 96 mAh/g at a rate of 5 C. When the rate was returned to 2 C, the overall specific capacity recovered. These test results indicate that the solid-state batteries equipped with PVDF-HFP/LLZNO have a better rate performance and can adapt to more scenarios.