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A new type of Li-Al-O solid electrolyte to protect lithium a

Time:2020/01/06 丨 source:未知 丨 visit count:

A new type of Li-Al-O solid electrolyte to protect lithium anode


[ Research Background ]
The lithium metal has a very high specific mass capacity (3860mAh g-1) and a minimum electrochemical potential (- 3.04V) . But the long-cycle stability of the anode material is influenced by the organic electrolyte and Lithium Dendrite. Lithium can be stabilized by adding electrolyte additives, increasing the molar ratio of lithium salts to solvents, building SEI , solid electrolyte and coating. Because of its electrochemical inertness and high mechanical strength, Al2O3 is often used to build protective coatings, such as Atomic layer deposition(ALD). However, Al2O3 is not an ionic conductor and the improvement of the lithium ion conductivity of aluminate group has yet to be solved. Secondly, the problems of poor conductivity at room temperature, poor electrode-electrolyte interface contact and lithium dendrites still exist in the developed solid electrolytes.
 

 
[Introduction]
In view of this, a new type of Li-Al-O based solid electrolyte (SSE) has been reported by the research team of Professor Yun-hui Huang from HUST. The SSE has a High Lithium Ion conductivity and good lithium anode protection capability. At Room temperature, the total ionic conductivity was as high as 1.42 ~ 10 -4 S cm-1. The SSE thin film is in situ formed on the surface of lithium metal, which can effectively protect the lithium anode from the erosion of H2O, O2 and organic solvent, and thus inhibit the growth of lithium dendrite.
 

 
Figure 1. a) Schematic diagram of Li-Al-OSSE formed on the surface of lithium foil in Situ; b) SEM Of Li-Al-O SSE surface; c,d ) SEM and local magnification of cross section of Li-Al-O SSE; e) The element distribution of Li- Al-O SSE.
 
From SEM, it can be seen that the interface between the lithium metal and the coating is compact, and no voids are observed. The thickness of Li-Al-O SSE can be adjusted by changing the soaking time. EDX showed that the elements were uniformly distributed, and the C and F elements might come from the residual LiTFSI/DME electrolyte.
 

 
Figure 2. a, b) Li | Li-Al-O | Li battery and its Equivalent Circuit Diagram; c) Li | Li-Al-O | Li Electrochemical impedance spectroscopy at room temperatured; d) Li–Al–OSSE Ionic conductivity curve; e) Li | Li-Al-O | Li Electrochemical impedance spectroscopy at room temperature (with liquid electrolyte wetting)
 
The ionic conductivity of Li-Al-O SSE can be measured by Eis of Li |Li-Al-O | Li battery. The Li-Al-O SSE is formed in Situ on the Li foil on one side and manually pressed on the other side, with an impedance of 4.27 x 104 Ω. The impedance can be reduced to 7.5Ω when the manually pressed side is wetted with 1 MLiTFSI/TEGDME electrolyte.
 

Figure 3. XPS, FTIR, HRTEM and electron diffraction patterns of Li-Al-O SSE
 
In order to understand the cause of high ionic conductivity, the composition of Li-Al-O SSE was investigated by XPS. Li-Al-O SSE contains O, F, C and Al elements, which is consistent with EDS results. XPS analysis showed that LiAlO2 like compounds were probably present, which was also confirmed by FTIR results. HRTEM shows that Li-Al-O SSE is polycrystalline, and the different interplanar spacing corresponds to the different crystal planes of Al2O3, LiAlO2 and Li3AlO3. According to the above results, Li-Al-O SSE is a polycrystalline film composed of LiAlO2, Li3AlO3, Al2O3, Li2CO3, LiF and organic compounds. Compared with pure LiAlO2, this structure has more internal defects, which may be the reason for the increase of the mobility of lithium ions.
 

Figure 4. Schematic Diagram, cycling performance and XRD of Li | Li–Al–O | Li and Li | liquid electrolyte | Li symmetric cells
 
The method of constructing Li | Li–Al–O | Li symmetric cell is very ingenious. As shown in Fig. 4a, the middle of the cell is wetted with 1 M LiTFSI / TEGDME electrolyte. Compared with the symmetrical cell with liquid electrolyte, the polarization voltage is stable and the cycle time is over 1400h. XRD Patterns show that the reason for the poor contrast is the serious passivation at the interface, and the passivation is the result of the accumulation of the by-product LiOH. Under the protection of Li-Al-O SSE, this kind of side reaction can be basically eliminated.
 

Figure 5. SEM and Cross section SEM of Li | Li–Al–O and unmodified Li surface after different cycles
 
The SEM images of different cycles show that the protective effect of Li-Al-O is very obvious. During the whole cycle, the surface of Li-Al-O remains flat. After 70 cycles, SSE can still maintain a thickness of about 40 µm. The Metal Li in Li | liquid electrolyte | Li symmetric cell is heavily pulverized and the thickness of SEI is greatly increased, which is consistent with XRD.
 

Figure 6. Photos of Li | Li–Al–O and unvarnished Li after being exposed to air for 15 days in oxygen-saturated TEGDME solution
 
Li Foil with Li–Al–O protection and Li foil without modification were exposed to air, the Li foil with Li–Al–O protection remained bright after 5 h, while the control became dark after 1 h. At the same time soaked in oxygen-saturated TEGDME solution. After 15 days, the surface of Li foil with protective layer remained bright, while the control surface became dark. These results show that Li–Al–O SSE formed on the surface of Li can effectively protect Li from H2O, O2 and organic solvents.
 

Figure 7.  a,d) Charge-discharge curves of Li | O battery with Li-Al-O protection and control samples;
b, e) The SEM image after 30 cycles;  c, f) EIS after different cycles; g) Long cycle performance
 
Li | O2 battery was prepared by the author, which was inspired by the protective action of Li-Al-O SSE. The charge-discharge curves show that Li-Al-O protected lithium-oxygen batteries can be cycled about 180 times, while the lithium-oxygen batteries without any modification have failed after 31 cycles. After 30 cycles of the same cycle, the Li-Al-O protected lithium-oxygen battery's lithium anode kept a smooth surface, whereas the unprotected lithium anode was severely crushed. For AC impedance testing, the impedance of Li-Al-O protected lithium-oxygen batteries is much more stable.
 
[ Conclusion ]
A simple and effective method has been used to produce a solid electrolyte of Li–Al–O by in situ reaction of LiOH with Triethylaluminium on Li foil. The electrolyte exhibits a conductivity of 1.42x10-4 S cm-1 at room temperature, it has good protection to lithium anode from water, oxygen and organic solvent. The reversibility of Li | O2 battery can be improved by inhibiting the unnecessary side reaction of the negative electrode. The preparation technology of Li-Al-O SSE is simple and the interface is stable. Li-Al-O SSE has potential application value in high energy density lithium metal battery.
 
 
Meilan Xie, Xing Lin, Zhimei Huang, Yuyu Li, Yun Zhong, Zexiao Cheng, Lixia Yuan, Yue Shen, Xing Lu, Tianyou Zhai, Yunhui Huang, A Li–Al–O Solid‐State Electrolyte with High Ionic Conductivity and Good Capability to Protect Li Anode, Adv. Funct. Mater., 2019, DOI:10.1002/adfm.201905949


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