2:45 PM - EN08.07.03
Atomic-scale Computational Study of the Solid Electrolyte Interphase in Na-Ion Batteries with a Nanoporous Hard Carbon Anode
Emilia Olsson1,Qiong Cai1
University of Surrey1
The solid electrolyte interphase (SEI) is known to have a direct impact on sodium ion battery performance. The composition of the SEI has been shown to have a compositional dependence upon electrolyte, cycling time, anode material, and sodium salt. For hard carbon anodes with organic solvent and NaPF6 there are a number of common features. These involve the fluoridation of the hard carbon surface suggested to result from the salt dissociation, in addition the breakdown of the electrolyte molecules can result in carboxyl, carbonyl and carbonate motifs being observed, together with both inorganic and organic SEI constituents.1,2 This make the atomic scale understanding of the anode electrolyte interphase, and its influence on both charge transfer and electrochemical performance challenging.
In this study, we combine atomic scale computational modelling with experimental characterisation of anode electrolyte interphases to investigate the structure, charge transfer, and electronic processes at the hard carbon anode electrolyte interphase in Na-ion batteries.3,4 Hard carbons are complex disordered carbon structures with randomly oriented graphene sheets, closed and open pore systems, and turbostratically stacked graphitic stacks. Adding to their complexity, these anode materials are also prone to defects and oxygen functionalities. This complex anode material structure leads a plethora of different anode electrolyte interphase structures, which all can have different influence on the electrochemical performance.4–7 Here, we construct, based on experimental guidance, a number of different interphase models to capture the effect of anode surface termination, roughness, defects, functional groups, pore size, and combinations of these on the electrochemical performance and charge transfer across the electrolyte anode interphase. Through a combination of ab initio molecular dynamics, and density functional theory simulations, the electronic structure, ionic diffusion, and electrolyte dissociation products can be investigated. These simulations showed, using an organic solvent based electrolyte that the local structure of the anode surface directly influences the solvent molecule breakdown and interphase immobile solvent molecule layer. Introducing carbon vacancy defects, and oxygen functionalities, together with different pore systems were further shown to affect the organic solvent molecule breakdown and lead to irreversible capacity loss. Similarly, the sodium ion transfer from the electrolyte bulk to the anode (both through intercalation and surface adsorption) was probed at different interphases to explore the effect of morphology, the liquid phase composition, and electronic structure on charge transfer.
The financial support from EPSRC (Engineering and Physical Sciences Council) under the grant number EP/M027066/1, and EP/R021554/2, is acknowledged.
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2 C. Bommier and X. Ji, Small 14, 1703576 (2018).
3 H. Au, H. Alptekin, A.C.S. Jensen, E. Olsson, C.A. O’keefe, T. Smith, M. Crespo-Ribadeneyra, T.F. Headen, C.P. Grey, Q. Cai, A.J. Drew, and M.M.-M. Titirici, Energy Environ. Sci. (2020).
4 H. Alptekin, H. Au, A.C. Jensen, E. Olsson, M. Goktas, T.F. Headen, P. Adelhelm, Q. Cai, A.J. Drew, M.-M. Titirici, and A.C. Jensen, ACS Appl. Energy Mater. acsaem.0c01614 (2020).
5 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. 30, 1908209 (2020).
6 E. Olsson, G. Chai, M. Dove, and Q. Cai, Nanoscale 11, 5274 (2019).
7 A.C.S. Jensen, E. Olsson, H. Au, H. Alptekin, Z. Yang, S. Cottrell, K. Yokoyama, Q. Cai, M.-M. Titirici, and A.J. Drew, J. Mater. Chem. A 8, 743 (2020).