Salt cathode could lead to cheaper energy storage

Published on: October 12, 2017 10:11 amBy: Jade Beevor

 

The latest research on a sodium ion battery by a Stanford University team has resulted in a design for a sodium cathode that claims to cost 80% less than of a lithium-ion battery with the same storage capacity.

By binding sodium ions to a compound known as myo-inositol the scientist have calculated a sodium cell would cost $35/kWh (based on $10/kg for its sodium salt Na6C6O6) compared to $48/kWh for NMC ($30/kg for NMC) in lithium cells.

In a half cell (vs Na metal anode), the team reported that the specific energy density of their cell was 726Wh/kg, and the maximum specific power is 3.151 W/kg.

The findings by chemical engineer Zhenan Bao and her faculty collaborators, materials scientists Yi Cui and William Chueh, were published in the October 9 edition of Nature Energy journal.

Having already optimized the cathode and charging cycle, the researchers plan to focus next on the anode of their sodium-ion battery.

In September a Stanford paper authored by postdoctoral scholar Min Ah Lee and Bao presented a high performance sodium-organic battery by using a biomass-derived ionic crystal, disodium rhodizonate (Na2C6O6).

The paper found that to achieve a viable Na-ion technology for grid storage applications, the battery would require sodium hosts with material sustainability, sufficient energy density and cycle stability.

The paper stated: “Given the high theoretical specific capacity (501 mAh/g) and abundance of disodium rhodizonate, it is one of the most promising cathode materials for SIB, which can be obtained from biomass through green chemistry.

“The material disodium rhodizonate, is readily available, low cost and easy to make the desirable morphology as a cathode for SIB. We consider that the feasibility of sodium-organic battery now depends on the development of successful anode material to be coupled with our cathode material.”

The scientists aren’t the first researchers to look into sodium-ion technology; UK-based Faradion received £3.2 million ($4.2 million) in funding in January to move to large-scale prototype production. In March Aquion Energy working in the same field went into Chapter 11 bankruptcy — although it has since emerged with a new owner, with reports the company is set to move to China from its US headquarters.

Postdoctoral scholar Min Ah Lee answers key questions from ESJB about the technology:

ESJ: How did you come up with the costing for the sodium battery (you say its 80% cheaper than a lithium-ion), and what is the cost per kWh?

Postdoctoral scholar Min Ah Lee at Standford University

Min Ah: To be precise, we are comparing the cathode costs to build a battery. Our calculation is based on the full-cell energy density for graphite-NMC lithium cell and our phosphorous-Na6C6O6 sodium cell. Based on material costs— $30/kg for NMC and $10/kg for our sodium salt — the cost per kWh for NMC in lithium cell is about $48/kWh, and that for our sodium salt (Na6C6O6) in sodium cell is $35/kWh.

With further development of a better anode having lower operating potential in the future, the cost should be decreased by $20/kWh with an increase in full cell energy density. We are expecting the wholesale price or cost for mass production of our cathode material to be even cheaper than $10/kg as it is originated from biomass (e.g. corn liquor).

ESJ: What is the specific power of your technology?

Min Ah: In a half cell (vs Na metal anode), the specific energy density is 726Wh/kg and the maximum specific power is 3151 W/kg. Note that this value is normal to “only” cathode mass. In a full cell (vs phosphorous), the energy density normal to total anode and cathode mass was 281Wh/kg and we did not conducted any rate tests of full cells for getting a high power density number as it would be limited not by our cathode but by the anode that we used as one of possible candidates.

It should also be remembered when considering the total mass for battery casing, it should be roughly one third, and this should be carefully optimized in industry level.

ESJ: At what stage is your research, and how long before your technology would be available for mass production?

Min Ah The mass production of our cathode materials and the resource myo-inositol is already available. Myo-inositol is naturally present in the human body being a cell membranes component and is found in many foods, particularly in corns, nuts, fruits.

Inositol is used as a nutritive supplement in infant formula and is available as an over-the-counter 50 nutritional supplement. The synthesis of sodium rhodizonate from myo-inositol is through oxidation in aqueous solution at room temperature which should be scalable.

But the entire shift from lithium to sodium for a battery is still limited by the performance of anode. We are currently working on developing better sodium anodes.

ESJ: Can you explain how the new technology works?

Min Ah: In this study, we mainly focused on making a sustainable and energy-dense cathode and used the phosphorous as an example to complete a full cell. We first carefully monitored the evolution of structure and morphology during sodium storage in the cathode, and revealed that the redox reaction is not fully reversible, which is in turn deteriorating redox capability in the following cycles.

So then we further conducted a systematic study on how to facilitate fast and reversible four sodium storage in this electrode, and identified the size of active particle and solvating power of electrolytes holds the key to realize this mechanism by reducing the kinetic barrier. So smaller size, and higher solvation of sodium ion allow us to have better performance.

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