Energy Focus: Rock-salt LiBH₄ phase solid electrolyte shows enhanced Li⁺ conduction

From electric cars to consumer electronics, the demand for weight-efficient and sustainable energy storage is growing rapidly. Commercial lithium-ion batteries use a flammable liquid or gel electrolyte that is unsatisfactory for use in cars and have a limited charge-discharge cycle life due to dendrite formation at the electrode-electrolyte surface. Both of these issues could be solved by use of a solid electrolyte Li⁺ ion conductor. Indeed, some solid electrolytes have achieved room-temperature conductivities comparable to those of liquid/gel electrolytes, such as the garnets (Li₅La₃M₂O₁₂, M = Nb, Ta), where the La and M sites have substituted with other metals to “stuff” the structure with Li ions for conduction. However, there are still many problems to overcome before such solid electrolytes can be integrated into batteries, including sinterability, stability in contact with the Li metal anode, and the ability to make a chemically stable and conductively continuous interface with the cathode.

Now, a research team at Tohoku University in Japan has developed a new method for achieving Li-ion conduction, as reported in the May issue of APL Materials (DOI: 10.1063/1.4876638). Their material is based on the normally high-pressure rock-salt phase of LiBH₄—a reducing agent familiar to organic chemists in its room-temperature orthorhombic phase—and which is stabilized by creating a solid solution between cubic KI and LiBH₄. This enables the rock-salt form of LiBH₄ to be stabilized at ambient pressures. LiBH₄ has good sinterability and stability in contact with Li metal, and its high-temperature tetragonal phase exhibits Li⁺ conductivities of 1 mS/cm, the minimum considered viable for consumer battery applications.

The solid solution has Li⁺ conductivities ranging from 5 × 10^–3 S/cm at 145°C to 10^–7 S/cm at 21°C with an activation energy for conduction slightly higher than seen in other rock-salt Li⁺ conductors, consistent with the mixed-cation effect. “At this moment, we are working to enhance the ionic conductivity by optimizing a host material and composition,” said H. Takamura. The microstructure of the material, which was produced by sintering KI with LiBH₄, is a mix of rock-salt solid solution 3KI • LiBH₄ and a secondary phase region which is K-deficient. The group reports that Li⁺ migrates in the rock-salt grains through a vacancy-mediated conduction mechanism. A transference number near unity suggests that the material is a pure Li⁺ conductor, such that K⁺ remains immobile in the lattice.

It is unusual to see pure Li⁺ conduction in a solid solution so low in Li (<25 mol%) and which has completely stationary K⁺. The researchers suggest that this occurs through a new phenomenon, which they term “parasitic conduction.” The light Li doping into the host lattice is sufficient for isotropic Li⁺ conduction. This may permit lithium electrolyte designers to propose systems previously considered inaccessible due to solubility limits.

“We believe that the ‘parasitic conduction’ of lithium ion is likely to happen in other systems such as oxides, sulfides, halides, and nitrides, if a small amount of lithium ion can be doped into the host materials. The concept opens the possibility of exploring new materials systems and/or crystal structures which have not been considered as a candidate for lithium ion conductors. We are working on that,” said Takamura.