Unveiling a simple way to boost Na-ion batteries’ capacity
During the first charge/discharge cycles of a battery, a reaction occurs at the surface of the negative electrode. A thin layer is formed. Well-known by industrialists and researchers, the solid electrolyte interface (SEI) is of importance to the final properties of the battery (protecting its structural integrity, blocking the reduction reactions of the electrolyte, electronic passivation...).
Schematic Na3P synthesis (yellow) by ball milling of Sodium (blue) and Phosphorus (red). Credit: researchers
Beyond these positives aspects, some of the SEI’s components are unfortunately lithium ions originating from the positive electrode that are trapped at the surface of the negative electrode. Furthermore each lithium ion represents a tiny part of the capacity of the battery. The more captured ions, the less capacity will be retained by the consumer. This well-known problem for Li-ion batteries, has been resolved by a better understanding of the mechanisms governing the formation of the SEI and the addition of excess Li (lithium) to compensate for the loss.
The revival of sodium batteries and our latest advances made towards its commercialization (see: a 18650 Na-ion battery prototype by CNRS and CEA) have raised awareness within RS2E (French research network on electrochemical energy storage) an in particular within Jean-Marie Tarascon’s group (Collège de France) on the urgency of addressing the problem for sodium batteries as well. Indeed, the sodium ion trapping mechanism in the SEI is causing a capacity loss of up to 15%.
Ball milling cell used in the experiment. Credit: RS2E/Clement Colin
The solution found by the researchers is to add an excess of sodium (also called sacrificial sodium) in the positive electrode to compensate for the inevitable loss. This excess of sodium is supplied by Na3P or metallic sodium (Na) which serve as reducing agents once added to the positive electrode material. Thus obtaining positive electrode materials enriched in Na, some of which (such as Na4V2(PO4)2F3) were not known up until there. The study, which led to the filing of two patent applications, is published in the journal Nature Communications. For this study the group included two postdoctoral Zhang Biao and Romain Dugas (Collège de France), Gwenaelle Rousse (crystallographer), Patrick Rozier (a researcher from CIRIMAT who produced one of the compounds studied) and a Russian researcher.
The Na3P compound, already known but never used in this context, was the subject of a simplified method of production using a 2-hours ball milling (mechanical grinding) synthesis route. A production method far simpler and more economical than previous ones (electrochemical sodiation, alloying by heating in solvents or annealing at 450 ° C in special silica ampoules).
Results in full cells. Left: on top, the initial compound, bottom, the sodium-enriched compound (we see the bonus in capacity which is alsmost of 200 mAh/g). Right: capacity of batteries using enriched compounds (blue and red) over 20 cycles vs. the capacity of the original battery (green). Credit: researchers
Following the success of this ball milling method, researchers also used it to incorporate the Na3P or metallic Na to both positive electrodes compositions used. The results are clear with increases in the overall capacity of the battery of 10 to 30%! The approach is successful: two compounds carrying sacrificial Na have been identified as well as a simpler, more efficient, method of synthesis/mixing (mechanical grinding).
This approach would be ideal if these compounds were not sensitive to moisture. Used as such they require the use of a "dry room" for their production. This is why the group of researchers is now developing barrier layers that act as a buffer between moisture and Na compounds.
Insertion compounds and composites made by ball milling for advanced sodium-ion batteries
Biao Zhang, Romain Dugas, Gwenaelle Rousse, Patrick Rozier, Artem M. Abakumov & Jean-Marie Tarascon
Nature communications, 01/19/2016, DOI : 10.1038/ncomms10308