Carbon-Based Anode Materials for Alkali Metal Ion Batteries – Atomic Scale Studies of Defects, Functional Groups, and Electrolytes and Their Impact on Metal Storage and Diffusion
Authors: Olsson Emilia , Cai Qiong 
Published: February 2020

Sodium ion batteries (SIBs) and potassium ion batteries (PIBs) are currently being investigated as credible post lithium ion batteries (LIBs) technologies.1–3 Due to their low cost, natural abundance, suitability for LIBs, SIBs, and PIBs, carbon-based anode materials, especially hard carbons, have gained scientific interest.3–6 These materials need to be optimized for high performance batteries. However, 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. Hence, we in this work aim to shed light on these mechanisms through systematic computational modelling of LIB, SIB, and PIB carbon-based anode materials.

Experimental studies have postulated that surface defects, and functional groups (especially oxygen-containing such), can have a marked effect on metal storage.4,7–1
1 To investigate this, we have used dispersion corrected density functional theory (DFT) to study defects and functional groups present with the carbon fragments. Our calculations show that oxygen-, and nitrogen-containing defects are energetically favorable to form on one single layer carbon fragment (i.e. graphene) surfaces, and that the adsorption of oxygen functional groups on the graphene basal plane is favored as well. The effect of these defects and functional groups on metal storage was found to be highly beneficial, with Li, Na, and K adsorption energies greatly improved as compared to the pristine graphene surface. Hence, it was deduced that the presence of surface defects could indeed lead to a higher degree of metal storage, as has been postulated experimentally. The effect of these defects on metal migration, which would influence the anode materials cycling properties, was investigated through nudged elastic band calculations. Comparing the metal energy migration barriers on the defective surface to the pristine, it was concluded that, even though these defects are highly beneficial for metal storage, they are detrimental to metal migration. 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 for 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.12

We have also made efforts to model a more complex, disordered hard carbon structure. Metal intercalation energy as a function of carbon interlayer distance, cell voltage, and charge transfer of Na, Li, and K were calculated. It was seen that the defect formation energies calculated for graphene were further replicated in the carbon layers. Metal migration in the disordered structure showed metal migration energy barriers close to those observed by our experimental collaborators for sodiated hard carbons. This paper will also report our most recent study on the interactions of different electrolytes with the carbon surface, to understand the effects of electrolytes on the solid electrolyte interface.

Acknowledgments
The financial support from EPSRC (Engineering and Physical Sciences Council) under the grant number (EP/M027066/1, EP/P003354/1, EP/R021554/1) is acknowledged.

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