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The latest development of practical lithium ion solid electrolyte

- Jan 02, 2019 -

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.

On October 2, 2017,PNAS (Proceedings of the National Academy of Sciences of the United States of America) published online a research paper entitled li-rich antiperovskite superior perovskite conductors based on cluster ions(Direct Submission) the author is working in the United States at the university of Virginia commonwealth and Dr Fang Hong Puru Jena professor researchers have found that the use of cluster ions as primitives, can be constructed out of the performance of lithium ion solid electrolyte to reveal the characteristics of the different from basic clusters ions such as ultra high electron affinity, large size and have an internal charge division to block changes of material properties, and found that the vibration mode of cluster ions and size effect to promote the mechanism of lithium ion migration in solid

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Basic properties of anti-perovskite lithium ion superconductor based on cluster ions

A. Crystal structure of anti-perovskite materials based on cluster ions. In the ground state, a triple rotational axis of symmetry (C3) of the cluster ions (such as BF4-, tetrahedron in the figure) overlaps with the body diagonal of the cube monocyte (as shown by the green arrow line).

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.

These new lithium ion solid electrolytes consist of Li3O+ and Li3S+, called superalkali metal ions, and BH4-, AlH4- and BF4-, called superhalogen ions. Superbasic clusters are named because their ionization potentials are less than those of the periodic table alkali metal Li, and superhalogens are named because they have greater electron affinity than any halogen in the periodic table, such as Cl. The new lithium ion solid electrolyte formed by them has the crystal structure of antiperovskite.

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.

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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.