Breaking the 10-nm grain size barrier in ultrahard metals
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F
rom electric cars to consumer electronics, the demand for weight-efficient and sustainable energy storage is growing rapidly. Commercial lithiumion batteries use a flammable liquid or gel electrolyte that is unsatisfactory for use in cars and have a limited chargedischarge 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
Breaking the 10-nm grain size barrier in ultrahard metals
H
igh-strength metals are important for many mechanical applications such as wind energy turbines. Niels Hansen from the Department of Wind Energy at the Technical University of Denmark in Roskilde studies the microscopic phenomena and the atomic mechanical processes that determine or limit possible further strengthening of metals. Refinement of the material microstructure is crucial for obtaining highstrength metals and is typically achieved by plastic deformation (e.g., cold-rolling). However, this is counteracted by a process of dynamic recovery. 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 rocksalt grains through a vacancy-mediated conduction mechanism. A transference number near unity suggests
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