Jul 18 2019
Batteries power the lives of people: All are dependent on them to keep their cell phones and laptops active and their hybrid and electric cars on the road.
An illustration depicts hydrogen-induced degradation of a sodium-ion battery: (1) When hydrogen is present (circled in black), (2) an Mn atom (purple) can move from the MnO2 layer to the Na layer (yellow); (3) Mn can then move within the Na layer, and will be lost. (Image credit: Hartwin Peelaers)
However, rising adoption of the most widely used lithium-ion batteries may indeed result in increased cost and possible deficiency of lithium—for this reason, sodium-ion batteries are being studied very much as a potential alternative. They operate well, and sodium, an alkali metal closely associated with lithium, is plentiful and cheap.
However, the problem with sodium-ion batteries is that they have shorter lifetimes when compared to their lithium-based counterparts.
Currently, UC Santa Barbara computational materials researcher Chris Van de Walle and teammates have found out a reason for this loss of capacity in sodium batteries: the accidental presence of hydrogen, which results in degradation of the battery electrode. Van de Walle and co-authors Zhen Zhu and Hartwin Peelaers reported the study outcomes in the journal Chemistry of Materials.
“Hydrogen is commonly present during the fabrication of the cathode material, or it can be incorporated from the environment or from the electrolyte,” stated Zhu, who is currently working at Google. “Hydrogen is known to strongly affect the properties of electronic materials, so we were curious about its effect on NaMnO2 (sodium manganese dioxide), a common cathode material for sodium-ion batteries.”
To explore this, the scientists employed computational methods that can predict the structural and chemical impacts that occur due to the presence of impurities.
Professor Peelaers, now at the University of Kansas, explained the important study outcomes: “We quickly realized that hydrogen can very easily penetrate the material and that its presence enables the manganese atoms to break loose from the manganese-oxide backbone that holds the material together. This removal of manganese is irreversible and leads to a decrease in capacity and, ultimately, degradation of the battery.”
The studies were carried out in Van De Walle’s Computational Materials Group at UC Santa Barbara.
Earlier research had shown that loss of manganese could take place at the interface with the electrolyte or could be associated with a phase transition, but it did not really identify a trigger. Our new results show that the loss of manganese can occur anywhere in the material if hydrogen is present.
Chris Van de Walle, Computational Materials Researcher, UC Santa Barbara
Van de Walle continued, “Because hydrogen atoms are so small and reactive, hydrogen is a common contaminant in materials. Now that its detrimental impact has been flagged, measures can be taken during fabrication and encapsulation of the batteries to suppress incorporation of hydrogen, which should lead to better performance.”
Actually, the scientists doubt that even the omnipresent lithium-ion batteries may experience the adverse effects of accidental hydrogen incorporation. Presently, it is not clear if this poses fewer challenges since fabrication techniques are more advanced in this mature materials system, or as there is a basic reason for the lithium batteries to be more resistant to hydrogen, and it will continue to be a domain of future research.
This research was funded by the Office of Science of the U. S. Department of Energy (DOE). Computing resources were offered by the National Energy Research Scientific Computing Center, supported by DOE.