Why sodium-ion batteries don’t last as long

The unintended presence of hydrogen, which leads to degradation of battery electrodes, is behind the short lifetimes of sodium-ion batteries, according to new research.

Batteries power our lives: we rely on them to keep our cell phones and laptops buzzing and our hybrid and electric cars on the road. But ever-increasing adoption of the most commonly used lithium-ion batteries may actually lead to increased cost and potential shortages of lithium—which is why researchers are focused intensely on sodium-ion batteries as a possible replacement. They perform well, and sodium, an alkali metal closely related to lithium, is cheap and abundant.

The challenge? Sodium-ion batteries don’t last as long as their lithium-based siblings.

“Hydrogen is commonly present during the fabrication of the cathode material, or it can be incorporated from the environment or from the electrolyte,” says coauthor Zhen Zhu, who is now 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 study this, the researchers used computational techniques that are capable of predicting the structural and chemical effects that arise from the presence of impurities.

Coauthor Hartwin Peelaers, now at the University of Kansas, describes the key findings: “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.”

“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,” says coauthor Chris Van de Walle, a computational materials scientist at the University of California, Santa Barbara.

“Our new results show that the loss of manganese can occur anywhere in the material, if hydrogen is present. 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.”

In fact, the researchers suspect that even the ubiquitous lithium-ion batteries may suffer from the ill effects of unintended hydrogen incorporation. Whether this causes fewer problems because fabrication methods are further advanced in this mature materials system, or because there is a fundamental reason for the lithium batteries to be more resistant to hydrogen is not clear at present, and will be an area of future research.

The findings appear in the journal Chemistry of Materials. Funding for the work came from the Office of Science of the US Department of Energy. The National Energy Research Scientific Computing Center, with support from the Department of Energy, provided computing resources.

Source: UC Santa Barbara

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