# Constructing Enhanced Composite Solid-State Electrolytes with Sb/Nb Co-Doped LLZO and PVDF-HFP
## 3 Results and Discussion
It is found that most Li jumps in cubic LLZO do not contribute to the diffusion rate because of the local reciprocating motion of its jump. Therefore, the low vacancy concentration is the main obstacle to Li\({}^{+}\) diffusion in LLZO. As observed in the experiment, it is an effective strategy to increase the vacancy concentration by substituting higher valence cations. The relative density and ion conductivity of LLZSNO are calculated using Formulas (1) and (2), as shown in Table 1. It was observed that the relative densities of the co-doped LLZSNO all increased compared with LLZSO, and LLZSNO with x = 0.05 has the highest densities of 88.7%. Li\({}_{6.65}\)La\({}_{3}\)Zr\({}_{1.65}\)Sb\({}_{0.3}\)Nb\({}_{0.05}\)O\({}_{12}\) also exhibits the highest ionic conductivity of 2.08 \(\times\) 10\({}^{-4}\) S/cm, which is a significant enhancement compared to LLZSO and is related to its high densification. Moreover, with the increase in Nb content in co-doped garnets, the ionic conductivity of the studied garnets decreases as a result of the garnet lattice constant decreasing, which narrows the Li-ion transport paths. The cross-sectional microstructure of Li\({}_{6.65}\)La\({}_{3}\)Zr\({}_{1.65}\)Sb\({}_{0.3}\)Nb\({}_{0.05}\)O\({}_{12}\) ceramics is shown in Figure 4. Figure 4a shows the cross-section of the green pellet before sintering, where the ball-milled powder particles were uniformly pressed. The ceramic after sintering is observed overall at low magnification, as shown in Figure 4b; it can be seen that the ceramic is composed of large particles surrounded by some small particles. Figure 4c,d demonstrate that large particles with clear grain boundaries are observed at high magnification, and the filling of the voids between the large particles by small particles may increase the density of the ceramic. In addition, spherical pores can also be seen in the grain boundary, and these  
Figure 3: Nyquist plots of LLZSO and LLZSNO at 25 \({}^{\circ}\)C.  
voids are usually accompanied by a certain liquid-phase sintering process [38]. Figure 4e shows the elemental distribution of LLZSNO using EDS; the results indicate that in the prepared LLZSNO, the elements of La, Zr, O, Sb, and Nb all have a uniform distribution. The SEM image and EDS mapping of LLZSNO powder are shown in Figure S1. The particle morphology of LLZSNO powder after crushing in a high-speed ball mill, with a particle size of submicron, can be observed in the SEM images; the individual particles are very fine and present irregular polyhedrons, and some of the particles are clustered together. Since the interface problem is still a big obstacle for the application of ceramic electrolytes [39], Li\({}_{6.65}\)La\({}_{3}\)Zr\({}_{1.65}\)Sb\({}_{0.3}\)Nb\({}_{0.05}\)O\({}_{12}\) was used as the inorganic filler to prepare CSE membranes for displaying their electrochemical performance. The thermal stability \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Nb Content** & **Impedance (f)** & **Relative Density (\%)** & **Ionic Conductivity (S/cm)** \\ \hline
0 & 1750 & 82.4 & \(8.53\times 10^{-5}\) \\ \hline
0.05 & 720 & 88.7 & \(2.08\times 10^{-4}\) \\ \hline
0.10 & 930 & 86.5 & \(1.61\times 10^{-4}\) \\ \hline
0.15 & 1080 & 87.2 & \(1.38\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Impedance, Relative Density, and Ionic Conductivity of LLZSNO. Figure 4: (**a–d**) SEM image of LLZSNO, and (**e**) EDS mapping of La, Zr, O, Sb, and Nb in LLZSNO. of the solid polymer electrolyte PVDF-HFP and the composite solid electrolyte PVDF-HFP/LLZSNO (PL-x) was conducted by TG-DSC; the results are shown in Figure S2. It can be seen that the thermal stability of the CSE membrane has not been lost compared to the PVDF-HFP membrane. In a way, the CSE membrane's thermodynamic property might be able to satisfy the application requirements of most LIBs. Figure S3 shows the microscopic morphology of the PVDF-HFP/LLZSNO.