Batteries, batteries, batteries

Recently we have had a flurry of activity on battery materials, battery metamaterials and battery properties. I thought this was a good opportunity to briefly summarise our recent work.

We have worked with Andrew Hector and coworkers and Deregallera on Sn3N4 battery anodes for Na batteries. This experimentally realised material (made by Andrew and co) shows remarkable capacity and after the initial cycle shows a large capacity >200 mAhr/g over repeat cycling. It represents a much more realisable anode material for Na than other materials considered.

We have also done substantial work on examining the intercalation materials, the Transition Metal Dichalogenides (TMDC). Conor Price led an excellent work on characterising the entire set theoretically, using a clever stability metric to assess their intercalation capacity. A highlight of this work was ScS2, which shows considerable potential as a cathode, potentially better than the industry standard, NMC. This system still has significant room to grow as the alloying concepts that followed from LiCoO2 could easily be applied here.

Another output of this same work is a series of anodes which would be highly suitable for Mg batteries, and the ranges which they could be cycled in. Given the double valency of these systems, it shows a significant potential compared to Li ion.

A significant effort has been to explore the role of metamaterial concepts in battery materials. Two areas are of particular interest, the first is the classical layering, creating nanolaminates or superlattices and the second is macroscale patterning which has seen considerable early success in the battery community.

In the former of these, we have explored TMDC/graphene and TMDC/TMDC heterostructures (which has the voltages for MS2 shown above). As you can see, the voltages tend to sit in the average between the two constituent materials, and this we find that many of these properties can be well approximated through consideration of the equivalent property for the component layers. An additional example, is if the superlattice volumetric expansion were to be estimated by calculating the mean of the volumetric expansion arising in the component TMDCs, we could expect the result to deviate by up to a 2% error from what is observed in the actual superlattice, and the voltage profiles of the component materials provide bounds to the voltage profile exhibited by the constructed superlattice. Further, the unoccupied states of the host material are progressively filled with the addition of an intercalant, which follows the behaviour observed with the individual TMDCs. Most interestingly, the construction of superlattices allows for many improvements to component materials: formation of a superlattice can result in a reduction of the electronic band gap, hence improving electronic conductivity; conversion-resistant materials can be used to increase the stability of conversion-susceptible materials, extending their cyclability and lifetime; and materials can be chosen such that the overall voltage can be tuned towards specific values.

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