05-06-2020 | | By Liam Critchey
Metal-ion batteries are a staple energy storage technology in modern-day electronics, with the high stability and relatively good efficiency of lithium-ion (Li-ion) batteries dominating the market. While lithium is main type of metal-ion battery, there are many more out there, but for one reason or another―often related to issues regarding stability or efficiency―they have not become a commercial success, yet. However, following on from the success of Li-ion batteries (best showcased by winning the Nobel Prize in Chemistry in 2019) a lot of research is going into making other types of metal-ion battery a commercially viable option―especially when lithium deposits are not going to last forever.
Out of all the metal-ion batteries besides Li-ion, sodium-ion (Na-ion) batteries show the most promise. However, their progress has been held back over the years as there has not been an anode material which is suitable for commercial and consumer use. The structure of a Na-ion battery is very similar to Li-ion batteries, however, the use of graphite of the anode is not suitable in Na-ion batteries like it is in Li-ion. This has been a sticking point as graphite has become the current gold standard in commercial batteries. Graphite is not suitable for sodium-ion batteries because the intercalation of the sodium ions is not thermodynamically favourable because of the larger ionic radius that sodium has compared to lithium.
One material that shows a lot of promise is red phosphorus, however, there are currently some challenges surrounding its use in Na-ion batteries. Red phosphorus is an amorphous (non-crystalline) allotrope of phosphorus that is considered to be a derivative of the P4 molecule, where the phosphorus atoms are arranged into molecular tetrahedrons which are chemically linked together. Its use in batteries is thought to be more suitable than other allotropes of phosphorus as it is more stable and cheaper than other allotropes, and it is found in abundance naturally (so it can be obtained in large quantities for commercial applications).
While the molecular arrangement is suitable for intercalating smaller ions, it undergoes similar volumetric expansion issues that many silicon anodes have in Li-ion batteries (i.e. expansion of the electrode). Volumetric expansion in anodes causes the anode to fall away, resulting in a loss of the active material in the anode. Over time, this causes a loss in efficiency and can cause the lifetime of the battery to be severely reduced. So, these are key issues that need to be overcome if Na-ion batteries are to have any look in on the Li-ion dominated market.
Despite the challenges associated with using pure red phosphorus as the anode in Na-ion batteries, researchers believe that it has a lot of potential to be used in Na-ion batteries, so they are looking at different ways of integrating it into the anode―namely by mixing it with other materials to create hybrid anode materials.
The tactic to hybridize an anode material with a conductive scaffold, most commonly a carbon-based material, has already been trialled with silicon anodes in Li-ion batteries, and with success. So, given that similar challenges are presented with red phosphorus in Na-ion batteries, it is a promising option for potentially tackling the current limitations. An international research team has tested the suitability of this idea by impregnating red phosphorus into a carbon nanofibre composite, to see if similar effects are seen to when silicon was hybridized for Li-ion anodes.
The carbon nanofibers were specifically chosen for this piece of research as their one-dimensional structure enables the sodiation process and the capacity decay mechanism of the red phosphorus to be easily observed. Moreover, the carbon nanofiber network could act as an electron pathway to improve the charge transfer kinetics of the red phosphorus. The aim was that if this initial study into hybridizing red phosphorus with a carbon-based nanomaterial showed promise from a mechanistic perspective, then it could open the doors to creating efficient red phosphorus-based anodes in the future.
A combination of physical and computational analysis methods allowed the researchers to deduce that the red phosphorus softens during the sodiation process. This ‘softening’ causes the red phosphorus molecules to adopt liquid-like mechanical properties which reduce the degree of volumetric expansion. So, the volumetric expansion via these routes is much less than silicon anodes in Li-ion batteries, and it was deduced that the instabilities of pure red phosphorus electrodes arises from unfavourable side reactions that occur when the sodium enters the phosphorous anode―leading to sodiated phosphorus compounds forming, degrading the anode in the process.
The researchers found that these side reactions were suppressed when the red phosphorous molecules were implanted within the carbon nanomaterial scaffold. This led to an anode being created that was much more stable than a pure red phosphorous anode and the team reported that the battery possessed long-term cycling stability and performance. The discovery, that hybridizing the red phosphorus with carbon nanomaterials produces a stable anode, opens the door to creating new anodes that are inherently more stable and effective than what we have seen for Na-ion batteries in the past; and this may help to make them a more commercially viable option in the near future.
It has become apparent that the development of different battery technologies cannot rely on the successes found within Li-ion batteries, even if the batteries are similar chemically and structurally. This has been found with graphite not being suitable for Na-ion batteries despite its commercial success in Li-ion batteries because of sodium having a bigger ionic radius. So, each battery technology is going to need to find its own “graphite” solution. For Na-ion batteries, this could be red phosphorus, but there are few challenges lying ahead before it becomes a commercially viable option, but hybridizing it with other materials appears to be a promising direction for Na-ion batteries to be heading in.