VCU team proposes “super-LRAP” solid electrolyte with performance comparable to liquid electrolytes for solid-state Li-ion batteries
Virginia Commonwealth University researchers have devised a new lithium superionic conductor with Li+ conductivity comparable with that of the organic liquid electrolytes. In a paper published in Proceedings of the National Academy of Sciences (PNAS), they report that cluster-based lithium superionic conductors can have very high conductivities of 10−2 to more than 10−1 S/cm at room temperature; low activation energy under 0.210 eV; a giant band gap of 8.5 eV; and desired mechanical properties that entail great flexibility and can inhibit the growth of lithium dendrites.
In a lithium-ion battery, positive lithium ions flow between electrodes via electrolytes. Solid electrolytes offer greater safety, higher power, and higher energy densities; however, while lithium ions can flow freely through liquid-state electrolytes, they are less mobile in a solid-state electrolyte, which adversely affects conductivity.
… development of the all-solid-state batteries is limited by the relatively low conductivity of the solid electrolyte materials. Most families of the superionic conductors have an activation energy in the range of 0.3–0.6 eV and exhibit ionic conductivities of the order of 10−4–10−3 S/cm at room temperature (RT). However, a typical organic liquid electrolyte or a gel electrolyte in practical batteries has an RT conductivity around 10−2 S/cm. Attaining an Li+ conductivity over 10−3 S/cm in the solid state is particularly challenging, and it is highly desirable to develop superionic conductors that exhibit 3D RT Li+ conductivities over 10−2 S/cm and activation energies smaller than 0.25 eV. Very few lithium solid electrolytes can reach an RT conductivity of 10−2 S/cm.
… Here, by exploring a set of lithium-rich antiperovskites (LRAPs) composed of cluster ions (Li3O+/Li3S+ and BH4−/AlH4−/BF4−), we report crystalline materials that have estimated RT conductivity of 10−2 S/cm and activation energies around 0.2 eV. We coin the term “super-LRAP” for these materials, because they are composed of the cluster cations and cluster anions, which are known as superalkalis and superhalogens, respectively.—Fang and Jena (2017)
To improve the conductivity in solid-state electrolytes, the researchers produced a computational model in which a single negative ion is removed. Negative cluster ions—groups of atoms with more electrons than protons—replace the absent ion.
The VCU team of Hong Fang and Puru Jena conceptualized a twist on a specific solid-state electrolyte previously tested by other researchers. Originally, the electrolyte, which belongs to a family of crystals called antiperovskites, contained positive ions made of three lithium atoms and one oxygen atom. The positive ions were joined with a single chlorine atom that was a negative ion.
In a computational model, they replaced the chlorine atom with a negative cluster ion created by one boron atom and four fluorine atoms joined to the existing positive ions. Other combinations of negative cluster ions were identified to potentially enhance conductivity.
Broadly, they found that the lithium superionic conductors Li3SBF4 and Li3S(BF4)0.5Cl0.5 have the potential to be ideal solid electrolytes.
Li3SBF4 exhibits a band gap of 8.5 eV, an RT conductivity of 10−2 S/cm, an activation energy of 0.210 eV, a relatively small formation energy, and desired mechanical properties.
Its mixed phase with halogen, Li3S(BF4)0.5Cl0.5, exhibits an RT conductivity of more than 10−1 S/cm and an activation energy of 0.176 eV.
Among their conclusions:
Cluster ions, called superhalogens, having higher VDE than that of chlorine, can produce larger band gaps of the super-LRAP than Li3OCl.
With proper ionic radius and proper internal charge distribution, a cluster ion can stabilize the antiperovskite structure with large channel size, which provides more space for Li+ to migrate.
A large channel size produces a set of low-energy phonon modes called q-RUMs, which correspond to the translational and rotational motions of the superhalogens acting more like rigid bodies. These motions generate a constantly shifting and varying potential surface throughout the material, which then facilitates the fast ion migration of Li+ ions from one site to another.
Partial replacement of the large superhalogen with halogen inside the antiperovskite structure creates large redundant space around the halogen sites. This enables an unusually large channel size of the material and further improves the ionic conductivity of a super-LRAP.
Replacing the chlorine ion with cluster ions improves conductivity because these ions are larger and allow the lithium ions to move quickly, as if they were in a liquid.—Hong Fang
Jena and Fang are now in search of collaborators to test their computational model in a laboratory setting for eventual lithium-ion battery applications.
Hong Fang and Puru Jena (2017) “Li-rich antiperovskite superionic conductors based on cluster ions” PNAS 114 (42) 11046-11051 doi: 10.1073/pnas.1704086114