Figure 1 shows the XRD patterns of Li10.35Ge1.35P1.65S12 sintered at different temperatures in comparison to the computed result. The main phase of LGPS was observed in all these materials. Although the diffraction peaks of the 550 °C sintered sample completely coincide with the computed result, the peaks intensity is very weak, indicating poor crystallinity. As the sintering temperature is sequentially raised up to 580 °C, the diffraction peak intensity under exactly the same measurement conditions becomes gradually stronger and thus the 580 °C sample shows the best crystallinity. However, with a further increase in the sintering temperature to 590 and 600 °C, the crystallinity starts to degrade. Furthermore, other phases are noticeable in the XRD patterns and are distinctly different from those obtained in the samples sintered below 580 °C, which are confirmed to match well with γ-Li3PS4 and GeS2 via the JADE program and are marked as hollow triangle (∇) and hollow box (□) in Fig. 1, respectively. It can be seen from the figure that the peak intensity of these impurity phases becomes stronger when the sintering temperature increases from 590 to 600 °C, revealing that heat treatment of Li10.35Ge1.35P1.65S12 at high temperatures above 580 °C would lead to the formation of impurity phases like γ-Li3PS4 and GeS2. Therefore, it is determined that the most suitable sintering temperature is 580 °C for the synthesis of Li10.35Ge1.35P1.65S12. A lower sintering temperature would result in lower crystallinity, while a higher sintering temperature would lead to the formation of impurity phases.
The structural profile parameters of Li10.35Ge1.35P1.65S12 sintered at 580 °C were refined by Rietveld analysis with the refinement program FullProf. Figure 2 and Table 1 provide the Rietveld refinement pattern and results. The space group P42/nmc is verified and the unit cell parameters obtained in our present work (a = 8.7002 Å, c = 12.6274 Å) are similar to those reported in the literature.15−18
Table 1
Rietveld refinement results for Li10.35Ge1.35P1.65S12 sintered at 580 °C.
Atom
|
Site
|
x
|
y
|
z
|
Li1
|
14 h
|
0.24593
|
0.27197
|
0.19500
|
Li2
|
4d
|
0
|
1/2
|
0.94434
|
Li3
|
8f
|
0.23543
|
= x(Li3)
|
0
|
Li4
|
4c
|
0
|
0
|
0.26841
|
Ge1
|
4d
|
0
|
1/2
|
0.69126
|
P1
|
4d
|
0
|
1/2
|
0.69126
|
P2
|
2b
|
0
|
0
|
1/2
|
S1
|
8 g
|
0
|
0.18862
|
0.40801
|
S2
|
8 g
|
0
|
0.29416
|
0.09556
|
S3
|
8 g
|
0
|
0.70168
|
0.79410
|
Note: Space group P42/nmc (137), a = 8.7002(3) Å, c = 12.6274(5) Å, V = 955.8119 Å3, Rp=14.2, Rwp=14.9, Rexp=11.5, RB=2.9, RF=2.6.
|
AFM analysis was used here for characterization of Li10.35Ge1.35P1.65S12’s microstructure and grain size. Figure 3 shows the two-dimensional AFM micrographs of Li10.35Ge1.35P1.65S12 sintered at 550, 580 and 600 °C, respectively. For each AFM image, the area in view represents a 10 µm × 10 µm square. The roughly estimated grain size of 550 °C sintered sample is about 1–2 µm. As shown in the figure, with an increase in sintering temperature, the grain size increases gradually, and thus, a positive correlation between grain size and sintering temperature is obtained in Li10.35Ge1.35P1.65S12, like what was generally observed in many other inorganic materials. The most uniform and dense microstructure is, however, achieved in the 580 °C sintered sample.
Figure 4 presents the room-temperature impedance spectra for the samples sintered at different temperatures and the high frequency parts are magnified in the inset. Similar to the widely reported results for sulfide electrolytes, these plots only exhibit an oblique line in the frequency range of 1 MHz to 1 Hz, and similarly, the horizontal intercept of oblique lines presented in Fig. 4 can be identified as the total resistance R of samples. Accordingly, the ionic conductivity is calculated as σ = L/(R × A), where L and A are the thickness and area of the pellets, respectively.30
Figure 5 illustrates the calculated room-temperature conductivities as a function of the sintering temperature. As shown in this figure, the conductivity increases first and then decreases with the increase in sintering temperature. When the sintering temperature is 550 °C, the obtained conductivity is 13.8 mS cm− 1, and is close to the previously reported result.29 Very interestingly, the highest ionic conductivity, obtained in the 580 °C sample, reaches as high as 19 mS cm−1. This is the highest lithium-ion conductivity obtained experimentally at room temperature for Li10+δGe1+δP2−δS12, to the best of our knowledge. However, when the sintering temperature exceeds 580 °C, the ionic conductivity decreases sharply. For example, the conductivity of the sintered sample at 600 °C (10.2 mS cm−1) is just 53.7% of the value of the 580 °C sintered one. Taking the XRD patterns into account, the crystallinity of samples peaks as the sintering temperature rises up to 580 °C. Moreover, the grain size of these samples increases with increasing the sintering temperature, as previously presented in Fig. 3. Accordingly, the total superficial area of grains does change in the same way as grain size and the Li+ migration distance among Li10.35Ge1.35P1.65S12 grains changes as well, and therefore, the ionic conductivity is enhanced. Besides, the γ-Li3PS4 phase, with a low ionic conductivity on the order of 10− 4 mS cm− 1,31 forms in the high temperature region, which attributes to the sharp decrease of the overall conductivity. With the highest ionic conductivity as well as the best crystallinity and microstructure, it is concluded that 580 °C is the best sintering temperature for Li10.35Ge1.35P1.65S12.