Young investigator group „Anion and cation conducting electrolytes for solid state batteries“
Helmholtz-Institut Münster (HI MS), IEK-12, Forschungszentrum Jülich
The increasing utilization of renewable energy sources and the advance of electromobility demand the provision of suitable energy storage technologies. The Helmholtz-Institute Münster (HI MS) brings together scientists from Münster University, Forschungszentrum Jülich and RWTH Aachen University to develop and improve batteries for these applications.
Focus on electrolytes
The young investigator group „Anion and cation conducting electrolytes for solid state batteries“ in Aachen focuses on the investigation and development of solid state electrolytes using computational and experimental methods.
Solid state electrolytes are key element of all solid state lithium-ion batteries as well as high temperature batteries like the Rechargeable Oxide Battery. We are especially interested in relations between composition, structure and properties as well as mechanisms of ionic transport and the prediction of ionic conductivity.
Rechargeable Oxide Battery
One focus is on battery concepts utilizing oxygen ion conductors like the Rechargeable Oxide Battery (ROB). This battery concept features high safety, low material cost and a scalable capacity.
The energy is stored in a metal/metal-oxide redox mass with an H2/H2O-mixture acting as shuttle gas to connect the redox mass with an electrochemical cell. During the charging process, hydrogen is produced from steam at the electrochemical cell. The produced hydrogen then reduces the metal oxide of the redox mass to metal.
During discharge the process is reversed and the metal is oxidized by steam to produce hydrogen that is again converted at the electrochemical cell.
From atomic to macroscopic scale
Density functional theory (DFT) allows the ab-initio prediction of energy barriers and interaction energies for ionic defects in solid state materials. In materials with high defect concentrations the energy barriers depend on the position of the moving defect due to the interactions with the environment. Kinetic Monte Carlo (KMC) methods allow the simulation of the ionic conductivity based on ab-initio energies by performing a sequence of random jumps for which the probability depends on the local environment. The simulations thereby connect the atomistic scale with the macroscopic conductivity for varying temperatures and compositions. We develop and apply KMC code to predict ionic conductivities and investigate the influence of different interactions.
Doped ceria: An oxygen ion conductor
Doped ceria is an oxygen ion conductor where doping with rare-earth oxides like yttrium oxide generates oxygen vacancies in the lattice. The vacancies provide high ionic conductivity at elevated temperatures (around 500 °C). The ionic conductivity depends not only on the number of oxygen vacancies but also on the interaction of the vacancies with each other and the interaction with the dopant ions. We can divide these interactions into „blocking“ (increase of the barrier when passing a dopant) and „trapping“ (vacancies associate with the immobile dopants). The strength of these effects determines the shape of the conductivity curve with respect to the dopant fraction.
Interstitial transport in apatites
Lanthanum-apatites of the general composition
La8+x B 2-ySi6O26+3/2x -y (with B being divalent cations) show high oxygen ion conductivities comparable to those of doped ceria. In contrast to ceria the conductivity is not enabled by oxygen vacancies but by oxygen interstitials instead. Lanthanum cations form a channel for oxygen ions and oxygen interstitials can easily migrate within this channel. The transport of the ions proceeds by a cooperative interstitialcy mechanism where one ion pushes the next ion from its regular position.
Li conduction in NASICON
NASICON (Na+ Super Ionic Conductor) structured materials are known to possess high ionic conductivities for Na+ and Li+ ions. Therefore NASICONs are possible electrolytes for all-solid-state batteries. We are interested in the relation between composition, structure and conductivity of these materials. Computational and experimental methods are used to investigate migration barriers and conductivities depending on the composition.
S. Grieshammer: “Influence of the lattice constant on defects in cerium oxide”, Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/C8CP03677B
A. Rossbach, F. Tietz, S. Grieshammer: “Structural and Transport Properties of Lithium-Conducting NASICON Materials”, J. Power Sources, 2018, 391, 1 9, DOI: 10.1016/j.jpowsour.2018.04.059
J. Koettgen, S. Grieshammer, P. Hein, B. Grope, M. Nakayama, M. Martin: “Understanding the Ionic Conductivity Maximum in Doped Ceria: Trapping and Blocking”, Phys. Chem. Chem. Phys., 2018, 20, 14291 - 14321, DOI: 10.1039/c7cp08535d
S. Grieshammer, M. Martin: “Entropies of defect association in ceria from first principles”, Phys. Chem. Chem. Phys., 2017, 19, 29625 – 29628, DOI: 10.1039/c7cp03817h
S. Grieshammer, “Defect Interactions in the CeO2−ZrO2−Y2O3 Solid Solution”, J. Phys. Chem. C, 2017, 121, 15078-15084, DOI: 10.1021/acs.jpcc.7b03507