Innes McClelland on the EMU beamline with the sample cell. Credit: University of Sheffield
The development of high-capacity, efficient and long-lasting battery materials will open up new possibilities for energy storage in homes, vehicles and on the power grid. One promising cathode material for next-generation energy storage is LiNi0.8Mn0.1Co0.1O2, or NMC811, which has an excellent capacity, but often experiences an irreversible loss of capacity during the first charge-discharge cycle. A team led by University of Sheffield researchers has developed a new battery testing method to better understand the reasons for this pitfall, which could help engineers design improved NMC811 cathodes.
The new method is based on muon spectroscopy (µSR), in which electrically charged subatomic particles known as muons are implanted into the sample material. The muons originate from a proton beam in a particle accelerator; the protons generate pions upon collision with the nuclei of a material such as carbon, and these pions decay into muons and neutrinos. The implanted muons are spin-polarized, and their spin is affected by the local magnetic field in the sample material. Inside the sample, the muons rapidly decay into positrons, which are emitted from the sample at a trajectory that is dependent on the spin of the decaying muon. Using information from the emitted positrons, analysts can gain insight into the properties of the studied material, including information on the diffusion of ions through specific areas of the material. Muon spectroscopy has previously been used to study the properties of battery materials, but had not yet been applied to the study of battery cells in operation, i.e., during charge-discharge cycles.
The research team designed a novel battery cell, called the Battery Analysis by Muon (BAM) cell, which was optimized for operando muon spectroscopy. Using a BAM cell with a NMC811 cathode, the performed both µSR experiments and complementary analyses, including X-ray diffraction, electrochemical impedance spectroscopy (EIS) and the galvanostatic intermittent titration technique (GITT), during charge-discharge cycles. As expected, the researchers observed a slowing of lithium diffusion after a certain point during the first charge cycle, but were able to determine that this sluggishness was more prevalent on the surface of the cathode than in the bulk material. This localized examination of diffusion properties during battery operation revealed that focusing on stabilizing the surface of the NMC811 material could lead to the development of improved batteries. This research was published in the journal Chemistry of Materials.
“The exciting development of operando muon spectroscopy opens up a wide range of opportunities for researchers working on energy storage materials, allowing a unique perspective of ionic diffusion from inside the materials themselves whilst in operation,” said first author Innes McClelland. “I look forward to seeing future studies which can develop the field towards a variety of battery chemistries.”
The µSR experiments were conducted using the EMU instrument at the ISIS Neutron and Muon Source. The beamline is housed at theRutherford Appleton Laboratory of the Science and Technology Facilities Council.