European scientists have made a breakthrough in developing a solid-state electrolyte they claim has the same conductivity levels as liquid.
The team from Graz University of Technology, the Technical University of Munich and Belgian university UCLouvain, have developed a crystalline ionic conductor, which exhibits high lithium-ion mobility.
Their research was published in the journal Chem.
It is thought solid-state batteries will replace traditional lithium-ion technology in energy storage and electric vehicle applications because they are safer and display higher energy density characteristics.
The researchers claim the new material exhibits one of the fastest lithium mobility processes ever measured in a lithium-ion conductor. The team measured the degree of mobility using nuclear magnetic resonance (NMR) spectroscopy techniques.
The team used a lithium-titanium thiophosphate (LTPS), which has an unusual crystalline structure characterised by its “geometric frustration”.
As LTPS offers no energetically favourable sites for the ions to occupy, they are put in a state of ‘frustration’ that results in exceptionally high lithium mobility, unlike other ionic conductors, say the researchers.
“The lithium-ions seek out suitable sites in a rather frantic way, meaning that they move through LTPS’s crystallographic structure extremely rapidly,” said Martin Wilkening from TU Graz’s Institute for Chemistry and Technology of Materials and director of the university’s Christian Doppler Laboratory for Lithium Batteries.
“This high ionic mobility is exactly what we’re after for use in solid-state electrolytes for solid-state batteries,” Wilkening said.
“We found clear evidence of two jump processes that entirely corroborate the results of our calculations. In the structure of LTPS, the lithium-ions can jump via ring-shaped paths back and forth, and from one ring to the next. The latter of these processes, the inter-ring process, enables the long-range ionic transport.”
Researchers found the ions were still mobile at 20 kelvin (-253oC), a rare behaviour because ions are normally sapped of thermal energy at low temperatures, Wilkening said.
The team will now search for compounds with characteristics that facilitate similar conduction mechanisms.
The study was conducted in collaboration with Toyota. UCLouvain has filed a patent for the discovery of LTPS.
Expert opinion from BEST’s technical editor Dr Mike McDonagh
I would not like to rely on the values listed here as it would need more research to verify them, however they do provide some idea of the relative speed of lithium ions in these electrolytes.
D is the diffusion coefficient, which is a measure of the diffusion speed, taken from Fick’s diffusion equation. The power numbers are the most significant as they are all minus, the higher the negative value the lower the value of D i.e the slower the diffusion.
LiPF6 (Organic solvent) D = 1.1×10 -6 cm 2 s-1
Li7P3S11 (solid electrolyte) D= 1.7×10-14cm2s-1
LiTi2(PS4)3 (LTPS Frustration) D = 4.62 × 10−11 cm2 s−1
These values are important for speed of ion transport and the LTPS (geometric frustration structure) is the fastest solid electrolyte diffusion but is far slower than the organic solvent.
However, the main issue for the solid electrolyte lithium-ion battery is the poor contact between the electrodes and the electrolyte, which causes low cycle life and high resistance (see our safety feature in BESTmagNo.65, summer 2019). Click here if you are not a subscriber.
I would like to see the real diffusion coefficients over a range of temperatures. Geometric frustrated structures do not have minimum energy positions for atoms in crystal lattice.
This means there is no natural place for particles to reside in the structure so they just keep rolling along. It is a complex area and is where metallurgy meets quantum mechanics, so is at present rather esoteric. The main benefit is probably the low temperature operation.
