Cathode material of layered dichalcogenide

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 K_x(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. Li_xTaS₂ 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 Na_xTiS₂ or Na_xTaS₂ 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 Ti_1+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 Li_xVSe₂ (x=0.25), then Li_xVSe₂ 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 Li_xCoO₂, 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, Li₂Mn_0.5Ni_0.5O₂ 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.