Sodium-ion battery cathode has highest energy density to date

September 12, 2013 by Lisa Zyga feature
This Ragone plot for the new cathode material (red circles) and other cathode materials for Na-ion batteries shows that the new cathode has the highest energy density for a wide range of charge and discharge rates. Credit: Park, et al. ©2013 American Chemical Society

( —Although sodium-ion (Na-ion) batteries don't perform as well as lithium-ion (Li-ion) batteries, they have the potential to be a cheaper alternative. In a new study, scientists have designed a new cathode for Na-ion batteries that provides an energy density of 600 Wh kg-1, which is the highest reported so far for Na-ion batteries and even rivals the energy densities of some Li-ion batteries. The new cathode material also has a greatly improved cycle life, bringing Na-ion batteries a step closer to realization as part of a large-scale energy storage system.

The researchers, Young-Uk Park, et al., from Seoul National University and KAIST, both in South Korea, have published their paper on the new high-energy cathode in a recent issue of the Journal of the American Chemical Society.

As the researchers explain, Na-ion batteries have the potential to meet and even exceed the performance of today's Li-ion batteries.

"The fascinating thing is that Na chemistry is much richer and has more variety than that of Li," coauthor Kisuk Kang, Professor of Materials Science and Engineering at Seoul National University, told "This makes us believe that there will be unexplored Na electrodes out there that can far excel the current Li batteries."

Both Na-ion and Li-ion batteries are candidates for being part of a large-scale system that stores energy generated by new technologies, such as solar, wind, and geothermal technology, where energy is produced intermittently. Although Li-ion batteries' high energy densities enable them to store a large amount of energy in a small space, the downsides of these batteries are their high cost and low stability. Since sodium is abundant in the earth, it is much cheaper than lithium, even though Na-ion batteries face their own challenges.

One of these challenges is a low . Until now, the highest energy density of a Na-ion battery has been about 520 WH kg-1. The root of this problem can be traced to the inherent characteristics of sodium (in particular, a less negative redox potential compared to lithium), which reduces the operating voltage and leads to the lower energy density.

Another major challenge for Na-ion batteries is that, even moreso than Li-ion batteries, they suffer from poor long-term stability. This problem also stems from an inherent characteristic of sodium, which is that sodium ions (1.02 Å) are nearly twice as large as lithium ions (0.59 Å). The large size causes a greater change in the host structure upon insertion and removal, which results in a decrease in capacity after repeated cycles.

The new improves in both areas. The researchers attribute the 600 Wh kg-1 energy density to the cathode material's open crystal framework with vanadium redox couples, which leads to a high voltage that in turn increases the energy density.

The new cathode material also allows the Na-ion battery to retain 95% of its capacity over 100 cycles and 84% for 500 cycles. This outstanding cycle life arises from the fact that the cathode material has the smallest volume change among Na cathodes so far, which is due to the rigid framework that is less sensitive to Na ion insertion and extraction compared to other frameworks.

In the future, the researchers plan to further improve the electrochemical properties of this Na-ion battery cathode with the goal of designing next-generation Na-ion batteries for new applications.

"Further improvements in the cycle stability need to be accomplished in order to be considered for large-scale systems because it requires exceptionally long cycle life (>10 years), unlike small electronic devices," Kang said. "In this respect, not only the electrode stability, but also other aspects such as the compatibility between electrode and the electrolyte, and the electrolyte stability itself, have to be simultaneously considered."

The researchers plan to address these challenges when designing future .

"We are currently searching for more new electrode materials that can outperform the material that was reported this time," Kang said. "We have both computational and experimental tools to search for them, which will accelerate the identification of this new material."

Explore further: Sodium-air battery offers rechargeable advantages compared to Li-air batteries

More information: Young-Uk Park, et al. "A New High-Energy Cathode for a Na-Ion Battery with Ultrahigh Stability." Journal of the American Chemical Society. DOI: 10.1021/ja406016

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5 / 5 (4) Sep 12, 2013
Corrections ... new sodium ion battery cathode has highest energy density to date as compared to other sodium ion batteries.

devilish little detail
5 / 5 (3) Sep 12, 2013
" - highest energy to date" Not a mistake. They lied to get your attention.
2.2 / 5 (6) Sep 12, 2013
84% at 500 cycles is good enough for EVs. A 200 mile range EV would get 100,000 miles from a set of batteries. If the price is significantly cheaper than lithium ion batteries then car owners could opt for a new set of batteries in order to return to full range for another 100k or delegate/sell to use as a lower range vehicle.

Lots of people would love to purchase a used EV that had a >100 mile range and cost very little per mile to drive.

600 Wh/kg is more than 4x the Nissan LEAF batteries. That sort of capacity could take the LEAF to 200 miles with batteries about half the size/weight/material use of present batteries.
not rated yet Sep 13, 2013
84% at 500 cycles is good enough for EVs. A 200 mile range EV would get 100,000 miles from a set of batteries.

Anyone have the numbers for the charge/discharge current supported by these relative to Li-ion batteries? I do seem to remember that it is very low (i.e. cars would need to charge for very long) but I can't find a source.
Because if the charge/discharge speed isn't at least as good as with Li-ion batteries then it doesn't matter how much energy it stores. It's not aceptable to have to charge your car 5 days to be able to drive it for one (unless we go for that whole battery-swap thing).
1.4 / 5 (11) Nov 05, 2013
600 Wh/kg is more than 4x the Nissan LEAF batteries. That sort of capacity could take the LEAF to 200 miles with batteries about half the size/weight/material use of present batteries.

Don't forget that nearly 1/3 of actual battery pack weight in an electric car comes from the support structure and shielding and temperature control systems, so decreasing cell weight alone will not let you put in 4x the capacity for the same weight, but more like 2x.

With the amount of auxillary equipment and stuff required to make the batteries work safely, like fire and water and puncture proof compartments, the best you can hope for is to increase the range by about 3x per weight. Maybe less if it turns out modern battery systems aren't safe and sturdy enough.

1 / 5 (5) Nov 05, 2013
I was talking about battery weight.

You are talking about pack weight.

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