A group of electron-donating molecules and ionic compounds are embedded in layered dichalcogenides (especially TaS2). The intercalation reaction of these host-guest materials changes their original physical properties. In particular, it has been discovered that it can transform superconductivity. The temperature increased from 0.8K to above 3K. When TaS2 is embedded in alkali metal hydroxides, it exhibits the highest superconducting transition temperature. Research on this formation has led to the discovery that these alkali metal ions have very high free energy to react with layered substances. Therefore, the stability of Kx(H2O)-TaS2 may be explained as their salt-like properties, which is contrary to the metal-like properties of the corresponding graphite compounds.
Among all the layered dichalcogenides, TaS2 is the most attractive for energy storage electrodes. TaS2 is a semiconductor, so there is no need for a conductive diluent in the positive electrode structure. LixTaS2 forms a single phase with lithium in the entire range of 0≤x≤1. When the phase change cannot make all the lithium reversibly extracted, it will not be accompanied by the energy loss of the nucleation of the new phase, and it will not be possible due to the change in the lithium content. Energy loss caused by slow response when the body is rearranged. In the LiCoO2 system, the phase change causes only about 0.5 lithium to be easily extracted and intercalated from the compound during cycling. It is quite attractive to use disulfide as the cathode material of sodium battery. When the sodium content in NaxTiS2 or NaxTaS2 changes, it is more conducive to the formation of triangular bipyramidal coordination when the value of x is 0.5, and when x is close to 1. The octahedral coordination is adopted, so the phase change of this type of system is very complicated.
Sulfur is a hexagonal close-packed lattice, and titanium ions are located in octahedral positions between alternating sulfur layers. Sulfide ions are deposited as ABAB, and the TiS2 layer is directly deposited on top of another TiS2 layer. For non-stoichiometric sulfides Ti1+yS2 or TiS2, some titanium was found to be present in the empty van der Waals layer. These irregular titanium ions prevent the intercalation of macromolecular ions and at the same time prevent the intercalation of small ions like lithium, thus reducing the diffusion coefficient of lithium ions. Therefore, lithium high-performance reactive materials should have a regular structure, which requires preparation at less than 600°C.
The electrostatic cycle of lithium deintercalation in TiS2 is shown in Figure 1, and the set current is 10mA/cm². It can be seen that TiS2 to LiTiS2 is a typical single-phase behavior from beginning to end, so no energy is consumed in the nucleation of the new phase. However, a more precise detection of the intercalation potential with the first compatibilization method shows that there is a local adjustment of lithium ions. The electrolyte used is a 2.5mol/L LiClO4/dioxolane system, this solvent will not co-intercalate with lithium into the sulfide. The difference is that when propylene carbonate is used as a solvent, a small amount of moisture will cause the co-intercalation of propylene carbonate, which is accompanied by a tendency to increase and expand the crystal lattice. Before a suitable non-intercalation solvent was discovered, the co-intercalation of the solvent prevented graphite from being used as a lithium negative electrode. This non-intercalating solvent was initially dioxolane and then a mixture of carbonates. Dioxolane has also been found to be an effective electrolyte for NbSe3 batteries. However, the electrolyte LiClO4 in dioxolane is inherently unsafe. This clean electrolyte system allows these intercalation reactions to be easily tracked by in-situ radiation and optical microscopes. These methods can reveal the microscopic details of the embedding process.
Charge and discharge curve of Li/TiS2 at 10mA/cm²
Most other disulfide compounds are also electrochemically active, and they also exhibit similar single-phase behavior when intercalating with lithium. VSe2 is an exception, which exhibits two-phase behavior as shown in Figure 2. Initially, VSe2 is in equilibrium with LixVSe2 (x=0.25), then LixVSe2 is in equilibrium with LiVSe2, and finally LiVSe2 is in equilibrium with Li2VSe2. The initial two-phase behavior may be related to the c/a ratio, which may be the result of the general easy coordination of the fifth subgroup elements with sulfur or selenium triangular bipyramid. But in VSe2, vanadium is a regular octahedral coordination. When lithium is inserted, there is a standard c/a ratio, and the structure becomes a typical octahedron. The rapid reversibility of lithium in VSe2 indicates that single-phase properties are not the most critical for effective use as a battery positive electrode. However, there is only a slight octahedral deformation difference in the phase formation in VSe2, instead of being transformed into an oxyanion layer during delithiation like LixCoO2, which makes it move in the entire range.
Figure 2 Electrochemical performance of VSe2 with two-phase behavior
VSe2 appears to be able to insert a lithium into its crystal lattice. The LiVSe2/Li2VSe2 system is two-phase, because the lithium in LiVSe2 is octahedral coordination, while the lithium in Li2VSe2 moves to tetrahedral coordination or occupies two positions at the same time. With butyl lithium, the intercalation of two lithiums can be achieved by chemical or electrochemical methods, and the remaining dilithium layered materials like Li2NiO2 can also be formed electrochemically or chemically with lithium benzophenone in tetrahydrofuran. Its structure changes from 3R-LiNiO2 phase to the equivalent structure of Li2TiS2 and Li2VSe2, that is, lithium atoms are located in all tetrahedral positions between NiO2 layers, forming a 1T structure. In a similar way, Li2Mn0.5Ni0.5O2 can also be synthesized electrochemically, and when part of the manganese is replaced by titanium atoms, two lithium atoms can still be cyclically deintercalated.
Group VI layered disulfides, such as MoS2, which appears in nature as molybdenite, if its coordination mode can be changed from a triangular bipyramid to an octahedron, the formed MoS2 can also be effectively used as a positive electrode material. This conversion is achieved by inserting a lithium into its crystal lattice for each MoS2, and then allowing it to transform into a new phase.
Although Li/TiS2 batteries are usually constructed with pure lithium or LiAl anodes when charging, they can also be discharged with LiTiS2 anodes like all LiCoO2 batteries currently in use. In this concept, the battery must first be charged by the extraction of lithium ions. Although both LiVS2 and LiCrS2 are well known in the literature, their lithium-free compounds have not been successfully synthesized because VS2 and CrS2 are thermodynamically unstable at the usual synthesis temperature. These compounds can be formed by the deintercalation of lithium at room temperature. This work has led to the emergence of a new route for the synthesis of metastable compounds-the deintercalation of the stable phase.
The metastable spinel TiS2 compound in which the sulfide ions are closely packed in cubic form can be similarly extracted from CuTi2S4with copper ions. This cubic structure can also reversibly extract lithium, although the diffusion coefficient is not as high as the layered structure. For example, when considering the use of TiS2, the laboratory block titanium is used to synthesize electronic grade TiS2, and the sponge-like titanium is used to provide battery research grade TiS2. Its specific surface area is 5m²/g, allowing current density to reach 10mA/cm² . However, when titanium reacts with sulfur, it takes only a few hours for the sponge metal, and several days for the bulk metal. Observation of the commercial production process of sponge titanium disulfide shows that the precursor is TiCl4 which is liquid at room temperature. Titanium tetrachloride is available in tonnage quantities because it is used in paint pigments containing TiO2. The designed processing process is to obtain the stoichiometric TiS2 through the vapor reaction deposition of TiCl4 and H2S. The sulfide obtained in this way exhibits excellent electrochemical properties, and its morphology is obtained from a single central point in three-dimensional growth. Many faces.
The measurement and regularity of titanium are very critical to the electrochemical performance of TiS2. If the temperature is kept below 600°C, stoichiometric and regular TiS2 exists and has metal conductivity. The disorder of titanium can be easily determined by trying to insert weakly bonded substances like NH3 or pyridine. In fact, a slight excess (≤1%) of titanium is beneficial. It can reduce the corrosiveness of sulfur, but it will not significantly affect the battery voltage or the diffusion coefficient of lithium atoms. Of course, it is best to additionally add metallic titanium to the initial reaction medium.