Sodium-ion batteries: How doping works

The schematic illustration shows a sodium ion battery: The positive electrode or cathode (left) consists of layered transition metal oxides which form a host structure for sodium ions. The transition metal nickel can be replaced either by magnesium or scandium ions.

The schematic illustration shows a sodium ion battery: The positive electrode or cathode (left) consists of layered transition metal oxides which form a host structure for sodium ions. The transition metal nickel can be replaced either by magnesium or scandium ions. © HZB

Sodium-ion batteries still have a number of weaknesses that could be remedied by optimising the battery materials. One possibility is to dope the cathode material with foreign elements. A team from HZB and Humboldt-Universität zu Berlin has now investigated the effects of doping with Scandium and Magnesium. The scientists collected data at the X-ray sources BESSY II, PETRA III, and SOLARIS to get a complete picture and uncovered two competing mechanisms that determine the stability of the cathodes.

Lithium-ion batteries (LIB) have the highest possible energy density per kilogramme, but lithium resources are limited. Sodium, on the other hand, has a virtually unlimited supply and is the second-best option in terms of energy density. Sodium-ion batteries (SIBs) would therefore be a good alternative, especially if the weight of the batteries is not a major concern, for example in stationary energy storage systems.

However, experts are convinced that the capacity of these batteries could be significantly increased by a targeted material design of the cathodes. Cathode materials made of layered transition metal oxides with the elements nickel and manganese (NMO cathodes) are particularly promising. They form host structures in which the sodium ions are stored during discharge and released again during charging. However, there is a risk of chemical reactions which may initially improve the capacity, but ultimately degrade the cathode material through local structural changes. This has the consequence of reducing the lifetime of the sodium-ion batteries.

"But we need high capacity with high stability," says Dr Katherine Mazzio, who is a member of the joint research group Operando Battery Analysis at HZB and the Humboldt-Universität zu Berlin, headed by Prof Philipp Adelhelm. Spearheaded by PhD student Yongchun Li, they have now investigated how doping with foreign elements affects the NMO cathodes. Different elements were selected as dopants that have similar ionic radii to nickel (Ni 2+), but different valence states: magnesium (Mg 2+) ions or scandium ions (Sc 3+). 

Three years of experiments at BESSY II, PETRA III, and SOLARIS

To decipher the influence of the two elements, they had to carry out experiments at three different X-ray sources. At BESSY II, they analysed the samples using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) in the soft and hard X-ray ranges, at PETRA III they evaluated structural changes with X-ray diffraction (XRD) and pair distribution function analysis (PDF) with hard X-rays, and for more detailed insights on the element magnesium, they carried out additional soft XAS investigations at the PIRX beamline at SOLARIS.

Scandium does not improve stability

"The results surprised us," explains Mazzio. Although doping with scandium leads to fewer structural changes during the electrochemical cycle than doping with magnesium, it does not improve stability. "Until now, it was thought that supressing phase transitions (and thus volume changes) would also improve the cathode material cycling performance over many cycles. But that's not enough."

It all depends on the ratio

Magnesium doping suppresses the oxygen redox reaction in NMO even more. This was also unexpected, as magnesium is known to trigger an oxygen redox reaction in layered manganese oxides. "We analysed different Mg/Ni ratios in NMO and found that the oxygen redox reaction reaches a minimum at a ratio close to 1," explains Mazzio. "Only through a combination of advanced X-ray techniques could we show that it is more than just suppression phase transitions that is important for improving the long-term cycling behavior, but also the interplay between Ni and O redox activity dictate performance," says Mazzio.

arö

  • Copy link

You might also be interested in

  • BESSY II: New procedure for better thermoplastics
    Science Highlight
    04.11.2024
    BESSY II: New procedure for better thermoplastics
    Bio-based thermoplastics are produced from renewable organic materials and can be recycled after use. Their resilience can be improved by blending bio-based thermoplastics with other thermoplastics. However, the interface between the materials in these blends sometimes requires enhancement to achieve optimal properties. A team from the Eindhoven University of Technology in the Netherlands has now investigated at BESSY II how a new process enables thermoplastic blends with a high interfacial strength to be made from two base materials: Images taken at the new nano station of the IRIS beamline showed that nanocrystalline layers form during the process, which increase material performance.
  • Hydrogen: Breakthrough in alkaline membrane electrolysers
    Science Highlight
    28.10.2024
    Hydrogen: Breakthrough in alkaline membrane electrolysers
    A team from the Technical University of Berlin, HZB, IMTEK (University of Freiburg) and Siemens Energy has developed a highly efficient alkaline membrane electrolyser that approaches the performance of established PEM electrolysers. What makes this achievement remarkable is the use of inexpensive nickel compounds for the anode catalyst, replacing costly and rare iridium. At BESSY II, the team was able to elucidate the catalytic processes in detail using operando measurements, and a theory team (USA, Singapore) provided a consistent molecular description. In Freiburg, prototype cells were built using a new coating process and tested in operation. The results have been published in the prestigious journal Nature Catalysis.
  • Rutger Schlatmann re-elected as ETIP PV Chair
    News
    24.10.2024
    Rutger Schlatmann re-elected as ETIP PV Chair
    The European Technology and Innovation Platform for Photovoltaics (ETIP PV) was created by the European Commission in order to promote photovoltaic technologies and industries in Europe. Now, the ETIP PV Steering Committee elected a new Chair, as well as two Vice-Chairs for the term 2024 – 2026. Rutger Schlatmann, head of the division Solar Energy at the HZB, and professor at HTW Berlin, was re-elected as the ETIP PV Chair.