Speaker
Dr
Martin Mansson
(Laboratory for Quantum Magnetism (LQM), EPF Lausanne)
Description
One of the most important scientific problems to solve for our modern society is how to convert and store clean energy. In order to accomplish a paradigm shift in this field we need to understand the fundamental dynamical processes that govern the transfer of energy on an atomic scale. For future energy devices like solid-state batteries (SSB) as well as solid-oxide fuel cells (SOFC) this means understanding and controlling the complex mechanisms of ion diffusion in solid matter. Only recently, developments of state-of-the-art large scale experimental facilities e.g. neutron/muon spallation sources as well as free electron lasers, have opened new possibilities for studying such intrinsic material properties in a straightforward manner.
Layered transition metal oxides (TMOs) have been extensively studied both for their correlated electronic properties (frustrated magnetism and superconductivity) as well as for energy applications e.g. Li-ion batteries or thermoelectrics. Recently these two fields have been unified under the framework of the layered NaxCoO2 family where Na-ion vacancy order as well as dynamics has been shown to tailor low-temperature magnetic and thermoelectric properties. In addition, room-temperature sodium batteries are currently receiving considerable attention since the available lithium reserves of our planet are very limited. In many ways the NaxCoO2 compound is the Na-analog of the most common Li-ion battery electrode LixCoO2. Hence, understanding Na-ion diffusion mechanisms of NaxCoO2 would seem a logical first step. Consequently, we have conducted systematic studies of this compound using neutron powder diffraction (NPD), quasi-elastic neutron scattering (QENS) and muon spin relaxation/rotation ($\mu$SR) as a function of temperature as well as Na-content (x) and pressure.
Our high-resolution T-dependent NPD data display a "melting" of the ordered Na-ion planes in two steps, involving an intriguing crossover from 1D to 2D Na diffusion [1]. It is evident that the onset and evolution of ion-diffusion is intrinsically linked to a series of subtle structural transitions that unlocks the diffusion pathways. The diffraction data is readily supported by our preliminary QENS experiments [2] that show a strong increase of inelastic intensity at two different temperatures, indicating the activation of two unique diffusion mechanisms.
Further, from our recently developed $\mu$SR techniques [3] it is possible to follow the Na-ion hopping-rate ($\nu$) on a local scale. $\nu$(T) is found to increase exponentially around room-temperature, in line with the onset of “melting” in the NPD/QENS data. The temperature dependence is well fitted by an Arrhenius type equation, indicative of a diffusive process and the activation energy as well as diffusion constant (DNa) can be extracted. Additional $\mu$SR measurements as a function of Na-content (x) [4] display an interesting, yet counterintuitive increase of Na-diffusion with decreasing c-axis length (increasing x). Such behavior motivated us to perform also pressure dependent NPD measurements, which indicated the same type of increase in Na-dynamics with elevated pressure (i.e. smaller c-axis) [5].
In summary, our current research has established a novel and detailed insight into the ion diffusion mechanisms in this group of compounds. This allows us to contemplate and actively consider future possibilities for tuning fundamental physical properties as well as solid state engineering of energy related materials with improved functional properties.
REFERENCES
[1] M. Medarde, M. Månsson et al., arXiv:1302.0708
[2] F. Juranyi et al., publication in progress
[3] J. Sugiyama, M. Månsson et al., PRL., 103, 147601 (2009)
[4] M. Månsson et al., Submitted for publication
[5] Y. Sassa, M. Månsson et al., publication in progress
Primary author
Dr
Martin Mansson
(Laboratory for Quantum Magnetism (LQM), EPF Lausanne)
Co-authors
Prof.
Bertram Batlogg
(Lab. for Solid State Physics, ETH Zurich, Switzerland)
Dr
Denis Sheptyakov
(LNS, PSI, Switzerland)
Dr
Ekaterina Pomjakushina
(LDM, PSI, Switzerland)
Dr
Fanni Juranyi
(LNS, PSI, Switzerland)
Prof.
Hand Rudolf Ott
(Lab. for Solid State Physics, ETH Zurich, Switzerland)
Dr
James Lord
(ISIS, RAL, UK)
Dr
Jorge Gavilano
(LNS, PSI, Switzerland)
Dr
Jun Sugiyama
(Toyota Central Research and Development Laboratories, Inc., Japan)
Dr
Kazuya Kamazawa
(Neutron R&D, CROSS, Japan)
Dr
Marisa Medarde
(LDM, PSI, Switzerland)
Mr
Mattia Mena
(LNS, PSI, Switzerland)
Prof.
Tsunehiro Takeuchi
(Nagoya University, Japan)
Dr
Vladimir Pomjakushin
(LNS, PSI, Switzerland)
Dr
Yasmine Sassa
(Lab. for Solid State Physics, ETH Zurich, Switzerland)
