Oxide and sulfide solid electrolytes in all solid-state battery electrolyte materials
Oxide solid electrolyte
According to the material structure, the solid oxide electrolyte can be divided into two types: crystalline state and glass state (amorphous state). Among them, crystalline electrolytes include perovskite type, NASICON type, LISICON type and garnet type, etc. The research hotspot of glass oxide electrolyte is the LiPON type electrolyte used in thin film batteries.
The oxide electrolyte has high air and thermal stability, low raw material cost, and is easier to achieve large-scale preparation. Among the oxide electrolytes, the room temperature conductivity of the amorphous (glass) oxide electrolyte is low, and it is more sensitive to water vapor in the air. The preparation often requires high temperature quenching, which is difficult to apply to actual batteries. In oxides, lithium ions conduct in the gaps of the skeleton structure composed of O2- which is much larger in size, weakening the Li-O interaction, realizing the three-dimensional transmission of lithium ions, and optimizing the ratio of lithium ion to vacancy concentration in the transmission channel are all conducive to improving the conductivity of lithium ions. In 1992, the Oak Ridge National Laboratory (ORNL) in the United States used a radio frequency magnetron sputtering device to sputter a high-purity Li3PO4 target in a high-purity nitrogen atmosphere to prepare a lithium phosphorous oxygen nitrogen (LiPON) electrolyte film. The material has excellent comprehensive properties, the room temperature ionic conductivity is 2.3×10-6S/cm, the electrochemical window is 5.5V (vs.Li/Li+), and the thermal stability is good, and it has good compatibility with positive electrodes such as LiCoO2, LiMn2O4, and negative electrodes such as lithium metal and lithium alloy.
The ionic conductivity of the LiPON film depends on the amorphous structure and the N content in the film material. The increase in the N content can improve the ionic conductivity. It is generally believed that LiPON is the standard electrolyte material for all solid-state thin-film batteries and has been commercialized. The RF magnetron sputtering method can prepare a large area and uniform surface film, but at the same time, it has the disadvantages that it is difficult to control the film composition and the deposition rate is low. Therefore, researchers try to use other methods to prepare LiPON films, such as pulsed laser deposition, electron beam evaporation, and ion beam assisted vacuum thermal evaporation. In addition to the changes in the preparation method, the method of element substitution and partial substitution has also been used by researchers to prepare a variety of LiPON-type amorphous electrolytes with better performance. Based on these concepts, some oxide lithium ion conductor materials with complex structures have appeared one after another. Representative ones include garnet structure system, perovskite structure system, and sodium fast ion conductor structure system. However, among these materials, only the garnet-type structural system is stable to metallic lithium, and the materials with higher electrical conductivity in the other two structural systems contain elements such as Ti and Ge that can be reduced by metallic lithium. In addition, the garnet-type structural system material has good stability to air, low raw material cost, and high mechanical strength of the sintered body, so it has the potential to be widely used as an ideal solid-state electrolyte in all-solid-state lithium batteries. The current methods to improve conductivity are mainly element replacement and heterovalent element doping. In addition, the compatibility with the electrode is also an important issue that restricts its application.
Sulfide solid electrolyte
According to the material structure, the sulfide solid electrolyte can be divided into crystalline and glass and glass ceramic electrolytes. The most typical sulfide crystalline solid electrolyte is thio-LISICON, which was first discovered in the Li2S-GeS2-P2S system by Professor KANNO of Tokyo Institute of Technology. Its chemical composition is Li4-xGe1-xPxS4, the room temperature ionic conductivity is up to 2.2×10-3S/cm (where x=0.75), and the electronic conductivity is negligible. The general chemical formula of thio-LISICON is Li4-xGe1-xPxS4 (A=Ge, Si, etc., B=P, Al, Zn, etc.). Glassy electrolyte is usually composed of P2S5、SiS2、B2S3 and other network forming bodies and network modifier Li2S. The system mainly includes Li2S-P2S5, Li2S-SiS2, Li2S-B2S3, etc. The advantages of sulfide solid electrolytes are: compared with O2-, S2- has a larger radius and strong polarization. Sulfur replaces oxygen in the oxide crystalline electrolyte, which can increase the cell volume and expand the Li+ meter transmission channel; compared with oxides, sulfides weaken the attraction and binding of Li+ by the crystal structure and increase the concentration of mobile carriers Li+; High ionic conductivity, up to 10-4~10-2S/cm; in addition, it also has the characteristics of high thermal stability, good safety performance, and wide electrochemical stability window (up to 5V); it has outstanding advantages in high-power and high-low-temperature solid-state batteries, and is a promising electrolyte material for solid-state batteries. But it has the disadvantage of being too sensitive to air. Professor TATSUMISAGO of Osaka Prefecture University in Japan’s research on Li2S-P2S5 electrolyte is at the forefront of the world. They first discovered that Li2S-P2S5 glass was partially crystallized to form glass ceramics by high-temperature treatment, and the crystal phase deposited in the glass matrix greatly improved the conductivity of the electrolyte.