Conducting scientific experiments usually involves several carefully planned steps which consist of selecting a topic of research, determining the parameters of the experiment and finally carrying it out. All careful planning notwithstanding, experiments sometimes do not only yield the desired results; however, they may lead to completely unexpected insights and findings.
Such was the case in an experiment recently conducted by scientists at Argonne National Laboratory (2021). Originally, they were looking for a new superconductor with unconventional behaviour and analysing a material that was four atoms thick. The target material was composed of silver, potassium and selenium (α-KAg3Se2) in a four-layered structure. This so-called 2D material enabled them to study the motion of charged particles in two dimensions. However, the material turned out not to possess any of the characteristics of a superconductor. Instead, to their great surprise it proved to be a very potent superionic conductor.
In superionic conductors the charged ions in a solid material can move about as unhindered as in the liquid electrolytes used for batteries, they therefore have very high ionic conductivity and can conduct electricity extremely well. Also, they exhibit low thermal conductivity, so that heat does not pass through easily. These properties make superionic conductors excellent media for energy storage and conversion devices.
The scientists first noticed that they may have discovered a material with special properties when they heated it up to between 232 and 315 degrees Celsius, at which point it suddenly transformed into a more symmetrical layered structure. The team also found that this transformation could be reversed when the temperature was lowered.
The analysis results revealed that before the transition the silver ions were immobile the two dimensions of the material, but after the transition they started to move around. The team measured how the ions diffused in the solid and found it to equal that of a heavily-salted water electrolyte. Quasi-elastic neutron scattering and ab initio molecular dynamics simulations proved that the superionic Ag+ ions were confined to sub-nanometre sheets. The simulated local structure was validated by experimental X-ray powder pair-distribution-function analysis. Also, the experiments showed that the phase transition temperature could be controlled by chemical substitution of the alkali metal ions, of which the immobile charge-balancing layers consisted.
Superionic conductors for energy storage and conversion have been at the centre of scientific attention for several years. In 2011 scientists developed a lithium superionic conductor, Li10GeP2S12, with a new three-dimensional framework structure which showed very high lithium ionic conductivity at room temperature. The high conductivity which was achieved in the solid electrolyte exceeded even those of liquid organic electrolytes. Also, the new solid-state battery electrolyte had advantageous qualities in terms of device fabrication, stability, safety and top electrochemical properties.
In 2019 a new sulfide-based superionic conductor, a kesterite-structured Li2CuPS4 (LCPS) material, was designed based on density functional theory (DFT) calculations. Theoretical studies showed that the LCPS material was thermodynamically and dynamically stable and could be experimentally synthesized. LCPS was capable of forming electronically insulating and ionically conducting interphases at high lithium chemical potential, which could inhibit further reduction or oxidation, though making its surface immobile and improving its electrochemical stability due to the limited kinetics. LCPS proved to be a superionic conductor with a much higher ionic conductivity than the state-of-the-art Li10GeP2S12 material as a result of the weaker Li ion binding and the energy compensation for forming Li vacancies due to the variable valence state of the Cu+ cation.
The new type of superionic conductor has several advantages: it could immediately be used as a platform for designing other 2D materials with high ionic conductivity and low thermal conductivity. Also, further research into this superionic material could also lead to the development of new thermoelectrics able to convert heat to electricity in power plants, or being used for industrial processes and exhaust gas from automobile emissions. Moreover, the findings could advance the development of membranes for environmental cleanup and desalination of water.
While it is too early to tell if this particular superionic material will find a practical application, the unexpected discovery will certainly make a valuable contribution to enhancing the efficiency of batteries, fuel cells, and electricity conversion devices.