ã€introduction】
Since the reserves of lithium resources on the earth are not abundant, researching the next generation of metal ion batteries and preserving their related technologies in advance will have very important scientific significance for preventing the possible energy storage crisis in the future. Magnesium metal is rich in resources (storage is 1000 times that of lithium), the market price is low (only 7% of lithium metal), and the small ionic radius (0.66?). In addition, since magnesium ions carry two positive charges, magnesium has an extremely high specific capacity (2205 mA hg-1) and a volume specific capacity (3833 mA h cm-1), while the chemical stability of metallic magnesium is Safety is superior to lithium and sodium, so magnesium batteries are widely concerned. However, since magnesium ions have a bivalent positive charge, they have a strong electrostatic interaction with non-metal elements in the electrode material. For example, related research proves that magnesium ions are difficult to intercalate in the oxygen environment. Due to the strong interaction between magnesium ions and oxygen atoms in the electrode, large volume changes and complex phase transitions of the electrodes are caused, resulting in magnesium ions. The transport in the positive electrode is difficult, and the overall cycle characteristics of the battery are poor. Therefore, how to realize the rapid transport of magnesium ions in the positive electrode material is considered to be one of the key problems that magnesium ion batteries urgently need to solve.
[Introduction]
The research team of Professor Shao Guosheng of Zhengzhou University published an article entitled "High-capacity cathodes for magnesium lithium chlorine tri-ion batteries through chloride intercalation in layered MoS2: a computational study" in Journal of Materials Chemistry A. In this paper, the material genomic method was used to systematically study the synergistic action of magnesium, lithium and chloride ions in chloride intercalation MoS2, so as to realize the rapid transport of magnesium ions. The basic design ideas are as follows: (1) Select layered sulfide as carrier: electrostatic interaction between sulfur and magnesium ions is moderate; (2) use of repulsion between metal ions: transport of magnesium ions by lithium ions that are easily transported (3) Chloride ion expansion technology: widening the interlayer distance of layered sulfides to further weaken the interaction between magnesium ions and sulfur; (4) chloride ion transport: under the action of electric field, realize magnesium, lithium and chlorine Three-particle coordinated transport, in which chloride ions are transported, provides additional energy density to the battery. The theoretical energy density of this positive electrode material is 277.4 mA hg-1, and the positive volume change during the multi-ion intercalation process is small, and the ion transport performance is estimated, which provides feasibility for subsequent experimental research. Program.
The material genomic methods used in this paper include: (1) USPEX genetic algorithm: search for MxMoS2Cl0.5 global energy minimum structure; (2) formation energy calculation: energy stability evaluation for related compounds; (3) phonon band Calculation: structural stability evaluation for related compounds; (4) molecular dynamics AIMD calculation: evaluation of mass transfer ability for related compounds; (5) HSE06 electronic energy band structure calculation: for related compounds, Make an assessment of the conductivity; (6) make an estimate of the voltage platform, energy density, and volume change.
The work was selected as the cover for the current period.
[Graphic introduction]
Figure 1: Using the USPEX genetic algorithm, the global domain searches for the most stable structure of the MxMoS2Cl0.5 component.
The global domain search by USPEX obtained the most energy-stable structure of the corresponding components of MxMoS2Cl0.5. (a) Mg0.25MoS2Cl0.5, (b) Li0.25Mg0.25MoS2Cl0.5, (c) Mg0.5MoS2Cl0.5, (d) Li0.5Mg0.25MoS2Cl0.5, (e) LiMoS2Cl0.5, and Li0. 5Mg0.5MoS2Cl0.5.
Figure 2: Formation energy, volume change and voltage of MxMoS2Cl0.5
(a) The formation energy of MxMoS2Cl0.5 (M=Li, Mg or Li/Mg mixture) is always negative, indicating that the corresponding product energy is stable;
(b) Due to the intercalation of Cl ions, the volume change of the positive electrode can be effectively overcome, for example, the variation of the interlayer spacing of MxMoS2Cl0.5 is only 2%. Small volume changes help to improve the cycle stability of the electrode material;
(c) Due to the interaction between Mg2+ and Cl- ions, it helps to further increase the working voltage of the electrode by 2.4 V v. s. Mg, much higher than the highest voltage platform of the known sulfide layered magnesium system 1.1 V v. s. Mg.
Figure 3: The phonon band of MxMoS2Cl0.5
Calculated by phonon energy band structure, intercalating related compounds of chloride ion (a) Mg0.25MoS2Cl0.5, (b) Li0.25Mg0.25MoS2Cl0.5, (c)Mg0.5MoS2Cl0.5, (d)Li0.5Mg0 .25MoS2Cl0.5, (e) LiMoS2Cl0.5, (f) Structural stability evaluation of Li0.5Mg0.5MoS2Cl0.5, where the black dashed line represents 0 THz and the blue dashed line represents an acceptable imaginary frequency range -0.3 THz . Chloride ion intercalation related products all show good structural stability. In this paper, by comparison with the phonon band without Cl-intercalation related compounds, it is found that the phonon band moves to the low frequency region after chloride ion intercalation (such as The arrow position is shown). Therefore, Cl-intercalation has a significant effect on lattice softening and may contribute to the rapid transport of charged ions.
Figure 4: Diffusion coefficient (AIMD) calculation
The diffusion coefficients of the individual particles in the different components were calculated by AIMD molecular dynamics. The diffusion activation energy of Cl- (red line) in Mg0.5MoS2Cl0.5 is 0.6 eV, while the diffusion activation energy of Mg2+ ion (blue line) is 0.86 eV, which is obviously better than Mg2+ ion transport in Mg0.5MoS2. Situation (Ea = 1.8 eV). In addition, under the driving of Li, Mg2+ and Cl- ions can achieve fast transport, for example, Li+ (black line), Mg2+ (blue line) and Cl- (red line) diffusion activation energy in Li0.25Mg0.25MoS2Cl0.5 Equally, about 0.2 eV. Therefore, it is promising to form a three-ion transport magnesium battery as shown in FIG.
Figure 5: Establishment of a Mg-Li-Cl three-particle battery
ã€summary】
In this paper, the possibility of chloride intercalation MoS2 as the positive electrode material of magnesium ion battery and its key performance were evaluated by material genomics method and system theory. The calculation results show that in the sulfide layered structure, the coordinated transport of magnesium, lithium and chlorine ions can be realized. With the help of interlayer intercalation Cl-, the energy and structure of the related system are stable, the volume change is small, and the diffusion activation energy of charged ions is about 0.2 eV. Therefore, the magnesium electropositive material is expected to exhibit good cycle characteristics, providing a new perspective for the study of magnesium metal batteries and other metal batteries.
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