Alkali Ion Storage and Migration in Carbonaceous Anode Materials – an Atomic Scale Study
Authors: Olsson Emilia , Cai Qiong 
Published: December 2020

Lithium-ion and post-lithium ion battery technologies are receiving increasing attention due to the need for a transition to a more sustainable energy economy. Apart from lithium ion batteries (LIBs), sodium ion batteries (NIBs) and potassium ion batteries (KIBs) are currently being investigated. Due to their low cost, natural abundance, and suitability for LIBs, NIBs, and KIBs, carbon-based anode materials, especially hard carbons, have gained scientific interest. The molecular structure of hard carbons is complex, and the atomistic understanding of metal ion storage and intercalation/de-intercalation during charge and discharge is limited. In this work we aim to shed light on these mechanisms through systematic computational modelling of LIB, NIB, and KIB carbon-based anode materials. Experimental studies have postulated that surface defects, planar, and curved pores can have a marked effect on metal storage in hard carbon.1–4

Density functional theory simulations of Li, Na, and K on different carbon motifs (defective graphene basal planes, planar graphitic layers, and curved pores) found in hard carbon anode materials were conducted to investigate their effect on the metal incorporation mechanism. Our calculations show that oxygen-, and nitrogen-containing defects are energetically favorable to form on single layer carbon fragment surfaces,5 and on curved morphologies. These defects were found to be highly beneficial for the initial metal storage, with Li, Na, and K adsorption energies greatly improved as compared to the non-defective carbon surface. Hence, it was deduced, as also suggested by experiment, that the presence of surface defects could indeed lead to increased metal storage. The effect of these defects on metal migration, which would influence the anode materials cycling properties, was investigated through nudged elastic band calculations, and showed that metal adsorption on surface defects can lead to capacity loss and irreversible metal storage. From defect formation energy calculations, it was seen that the defect expected to be present in the highest concentration on the carbon surface under equilibrium conditions is the 2OCNC defect, where three adjacent carbon have been substituted by two oxygen, and one nitrogen, respectively. This defect further has metal migration energy barriers many times higher than the corresponding ones at pristine surfaces, and would be highly detrimental for metal diffusion.5,6

We have also made efforts to model a more complex, disordered hard carbon structure. Metal intercalation energy as a function of carbon interlayer distance, and charge transfer of Na, Li, and K were calculated, based on nanoporous structures (expanded planar graphitic pores and cylindrical pores) identified by experimental analysis.1,2 Our simulations showed that metal incorporation and diffusion will be directly influenced by the pore shape and size, regardless of metal type. Planar graphitic pores were shown to play an important role in metal storage, with a direct impact observed on pore expansion, whereas curved morphologies contribute to rapid metal diffusion.2 In summary, the experimentally proposed metal charge/discharge mechanism of initial simultaneous metal storage on defects and intercalation in expanded pores, followed by pore filling was confirmed from atomic scale studies, showing the direct impact of carbon structure on the intercalation and storage properties of these metal ion battery anode materials.
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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