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Researchers at Cornell University, in the US, have been looking at this process, and found that as the sodium ions move through the battery, the misorientation of crystal layers inside individual particles increases before the layers suddenly align just before the P2-O2 phase transition. “We’ve discovered a new and critical mechanism,” says Andrej Singer, assistant professor of materials science and engineering at Cornell. “During battery charging, the atoms suddenly realigned and facilitated that flawed phase transformation.” The team was able to observe the phenomenon after developing a new X-ray imaging technique using the Cornell High Energy Synchrotron Source, which allowed it to observe, in real time and at mass scale, the behaviour of single particles in the battery it was using. “The unexpected atomic alignment is invisible in conventional powder X-ray diffraction measurements, as it requires the ability to see inside individual cathode nanoparticles,” Singer says. “Our high-throughput data allowed us to reveal the subtle yet critical mechanism.” That led the team to propose new design options for the type of battery it was using, which it plans to investigate in future research projects. One solution is to modify the battery chemistry to introduce a strategic disorder into the particles just before the flawed transition phase, according to Jason Huang, a researcher on the project. “By changing the ratios of our transition metals, in this case nickel and manganese, we can introduce a bit of disorder and potentially reduce the ordering effect we observed,” he says. The new characterisation technique can be used to reveal complex phase behaviours in other nanoparticle systems such as sulphur batteries. “We’re pushing the frontiers of sodium-ion batteries and what we know about them,” Huang says. “We are also using this knowledge to design better batteries that will help unlock the technology for practical applications in the future.” Cathode materials Researchers from Skoltech and Lomonosov Moscow State University have developed a cathode material for NIBs that provides an energy density 15% higher than current cells and a discharge voltage of 4.0 V. It is a sodium-vanadium phosphate fluoride powder with a particular crystal structure that can be produced using a relatively low temperature process. “Our new material, and the one the industry has started using, are both called sodium vanadium phosphate fluoride – they’re made of atoms of the same elements,” Assistant Professor Stanislav Fedotov of Skoltech says. “What makes them different is how those atoms are arranged and in what ratio they are contained in the compound. “Our material also compares well with the class of layered materials for cathodes. It provides roughly the same battery capacity and greater stability, which translates into longer life and higher cost efficiency of the battery. Remarkably, even the theoretical predictions for the competing materials fall short of the practical performance of ours. That is far from trivial, because the theoretical potential is never fully realised. Recent research has uncovered a way to mitigate the poor durability of sodium cells caused by atomic shuttling in the battery’s operation (Courtesy of Cornell University) 58 Autumn 2022 | E-Mobility Engineering Deep insight | Sodium batteries