Researchers from IIT Madras and Karlsruhe Institute of Technology (KIT), Germany, have developed a new class of 'high entropy materials' for use as cathodes in long-lasting and better performing lithium ion batteries.
While lithium ion batteries have a wide-range use, their limited lifetime remains a challenge. Poor lifetime of lithium ion batteries arises from the poor cycling stability of the cathodes - the performance of the cathode reduces with multiple charge-discharge cycles because of side reactions between the active material and the electrolyte.
Improvements in cycling stability and better performance have therefore hinged on developing better cathode materials, such as those developed by the IIT Madras and KIT researchers.
The team's research was recently published in the prestigious peer-reviewed RSC journal Energy and Environmental Science. Their research reports the synthesis of non-toxic and relatively inexpensive and more earth-abundant cathode materials with enhanced lithium ion storage properties and considerably improved cycling performance over currently used cathode materials.
Prof. S.S. Bhattacharya from the Department of Metallurgical and Materials Engineering, IIT Madras led the research along with Prof. Dr.-ing. Horst Hahn, Executive Director, Institute of Nanotechnology, KIT led the research. Prof. Hahn is also a Distinguished Honorary Professor of IIT Madras.
Dr. Bhattacharya described the cathode materials the team developed as complex compounds that contain five or more metal ions in equal amounts.
"The uniqueness of our cathode materials is that despite their high chemical complexity, they have phase purity," added Prof. Bhattacharya.
The phase purity is believed to result from high configurational entropy that results from the five cat-ions occupying random sites in the crystal lattice, hence the name 'High Entropy Oxides' or HEOs. Dr. Bhattacharya and his co-scientists developed HEOs containing the ions of five metals, manganese, cobalt, nickel, copper and zinc. While only oxygen ions occupy the anionic site in traditional HEOs, they replaced some of the oxides with fluoride ions to adjust the electrochemical properties.
They then compared the performance of these cathodes to conventional nickel-based cathode materials and found that not only was the specific capacity higher for the HEO, but the performance degradation during cycling was also much smaller than conventional cathodes. Better lithium ion storage and cycling performance were due to the entropy stabilization in these compounds.
Speaking about the real-life implications of their discovery, Prof Bhattacharya said, "In practical terms, the retention of Coulombic efficiency with cycling points to longer life of batteries. Better lithium ion storage and cycling stability are not the only advantage of the HEOs developed in this collaboration. We were able to vary the anions with the use of these HEOs."
Prof. Hahn added, "This possibility of using multiple anions opens up pathways for developing not only better performing lithium ion batteries but also futuristic sodium ion batteries that are expected to be even cheaper than the former." As an alternative, it is possible to synthesize oxy-chlorides for use in sodium ion batteries.
The icing on the cake is that the use of HEOs as cathode materials in lithium ion batteries would eliminate the use of toxic and costly cathodic materials such as those used today, with no loss in energy density.
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