Atomic Scale Insight into Metal Storage and Migration in Novel Anode Materials for Next Generation Battery Technologies
Authors: Olsson Emilia , Cai Qiong 
Published: December 2020

The need for sustainable energy storage and green technology is currently driving the demand for lithium ion batteries (LIBs) to never before seen levels. However, due to the limited Li supply, novel battery technologies are urgently needed. To this end, Li has sought to be replaced by other monovalent metals, such as Na and K, and divalent metals such as Mg, Ca, and Zn. If these battery technologies can be successfully developed, they could make a significant contribution towards meeting the future energy demands. Furthermore, the development of novel LIB electrode materials could potentially increase the current LIB performance. In this work, we have focused on nanoporous carbons for divalent metal ion batteries, and will also compare these to the performance of monovalent metal ions in the same materials. Furthermore, we have investigated bismuth-based anode materials for novel monovalent ion battery materials.1,2

Carbonaceous anode materials have shown promising performance for post-lithium ion batteries such as sodium ion batteries (NIBs), and potassium ion batteries (KIBs), but are also starting to be explored for divalent ion battery technologies.3–5 These carbonaceous anode materials commonly consist of sp2-hybridized carbon sheets with a plethora of defects. Hence, we in this study investigate the fundamental influence of defects in graphene on the energy storage mechanism, and their implication on the battery performance of Mg, Ca, and Zn ion batteries.5 Through density functional theory calculations, it was found that pristine graphene does not provide suitable metal storage sites for either Mg or Zn, whereas the calculated metal adsorption energies suggest that Na, K, and Ca may be readily adsorbed and stored on pristine graphene, even though adsorption energies similar to those calculated for Li on graphene were not obtained for any of these metals. The inclusion of defects on the graphene sheet, especially O- and N-heteroatom defects, lead to an increase in metal binding energies, suggesting that these defects could be used to boost the metal storage potential, showing a direct link between electrode structure with electrochemical performance. One caveat with metal storage on defect sites is the risk of capacity loss between the initial and subsequent charge/discharge cycles due to strong binding between metals and defect sites. Calculations of metal migration energies showed that the weak interaction between zinc and the carbon systems lead to rapid diffusion, but would not necessarily make these systems suitable as zinc ion battery electrodes. The only defect that would lead to irreversible capacity for the Ca and Mg ion batteries is the NC2OC defect, with calculated metal migration barriers of >0.5 eV. This defect was also found to be detrimental to Na and K diffusion. In summary, heteroatom defects were shown to be beneficial for anode performance, but the exact structure of the defect is important as not to trap metal ions irreversibly.
Finally, we also investigated bismuth-based electrode materials, both alloying with Li, Na, and K, and novel LIB intercalation-type anodes (Bi2S3 and Bi2MoO6). These materials were shown to have energetically favourable Li intercalation, and the potential to form different Li, Na, and K intermetallic alloys depending on metal loading during charge/discharge, making them promising next generation battery technologies.
Acknowledgment
The financial support from EPSRC (Engineering and Physical Sciences Council) under the grant number EP/M027066/1, and EP/R021554/2, is acknowledged.
References

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2 T. Zhang, E. Olsson, M. Choolaei, V. Stolojan, C. Feng, H. Wu, S. Wang, and Q. Cai, Materials (Basel). 13, 1132 (2020).

3 E. Olsson, J. Cottom, H. Au, Z. Guo, A.C.S. Jensen, H. Alptekin, A.J. Drew, M.-M. Titirici, and Q. Cai, Adv. Funct. Mater. 1908209 (2020).

4 E. Olsson, G. Chai, M. Dove, and Q. Cai, Nanoscale 11, 5274 (2019).

5 E. Olsson, T. Hussain, A. Karton, and Q. Cai, Carbon N. Y. 163, 276 (2020).