All-solid-state batteries with the advantages of high safety, high power and high energy density will play a key role in the next generation of energy storage materials. However, the development of all-solid-state batteries is constrained by the search for excellent solid-state electrolytes. Practical solid electrolyte materials should have ionic conductivity (10-2 S/cm) comparable to that of liquid electrolytes at room temperature. Through theoretical research, the researchers found that by substituting cluster ions for basic ions, new lithium ion superconductors Li3SBF4 and Li3S(BF4)0.5Cl0.5 could be obtained. These new materials not only have very high ionic conductivity (> 10-2 S/cm) and very low ionic excitation energy (< 0.25 eV) at room temperature, but also have the advantages of large band gap and high melting point. Further research shows that these excellent material properties are derived from the low-energy vibration modes of cluster ions in the material (called quasi-rigid body mode, quasirigid unit modes) and the size effect caused by cluster ions.
B. Comparison of lattice dynamic properties of crystal Li3OBH4, Li3SAlH4 and Li3SBF4 shows that red vibration modes in phonon spectrum are all quasirigid unit modes;
C. Band gap comparison of crystal Li3OBH4, Li3SAlH4 and Li3SBF4;
D. Correlation functions of atomic pairs at 400 K (black line) and 600K (red line) obtained by molecular dynamics simulation.
Compared with Li3OBH4 and Li3SAlH4, Li3SBF4 has the largest band gap (~ 8.5ev) and a high melting point (> 600K). The energy required for the preparation of Li3SBF4 based on LiBF4 +Li2S to Li3SBF4 is also relatively low (~ 39.4 meV/atom). These excellent material properties are due to the appropriate physicochemical properties of cluster ions Li3S+ and BF4-. In particular, superhalide ion BF4- proper ion size, internal charge fraction, and its high electron affinity.
The researchers found that in the cubic cell of the crystal, the superhalide ion BF4- with tetrahedral structure has a specific symmetry orientation, that is, one of its three rotational axes of symmetry coincides with the diagonal of the cubic cell body, which makes the energy the lowest.
It is used to study the vibration of cluster ions and the physical model of lithium ion migration in solids
The blue circles represent lithium ions, each of which has four clusters of ions (yellow highlighted tetrahedrons) as neighbors. The translational and rotational modes of these clusters of ions constantly change the potential plane felt by the lithium ions, which promotes the migration of lithium ions in the solid (e.g., from A1 position to A2 position).
In these crystals, each lithium ion has four clusters of neighboring ions. When these cluster ions vibrate due to thermal excitation, their coulomb effect on lithium ions will continue to change, forming a small potential barrier, which is transferred to the kinetic energy of lithium ions and promotes the migration of lithium ions in the lattice. At room temperature, only the low-energy cluster ion vibrational modes can be excited. In these vibrations, the cluster ions basically do not deform (quasi-rigid body), so these vibration modes are called quasi-rigid body modes. It is the translational and rotational vibrations of the cluster ions as quasi-rigid bodies that enhance the conductivity of lithium ions in these new materials.