Computational insights into the effect of carbon structures at the atomic level for non-aqueous sodium-oxygen batteries H.R. Jiang, M.C. Wu, X.L. Zhou, X.H. Yan, T.S. Zhao * Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China highlights  The effect of atomic carbon structures is investigated for NaeO 2 batteries.  SV defect has the largest adsorption energy for NaO 2 among samples studied.  The dangling atoms and the O-attachment are the origin of large adsorption energy.  Increasing the number of SV defect leads to large capacity and good cyclability. article info Article history: Received 22 March 2016 Received in revised form 21 May 2016 Accepted 30 May 2016 Keywords: First-principles study Non-aqueous sodium-oxygen batteries Atomic carbon structures Adsorption energy Solution mechanism abstract Carbon materials have been widely used to form air cathodes for non-aqueous sodium-oxygen (NaeO 2 ) batteries due to their large specific surface area, high conductivity and low cost. However, the effect of carbon structures at the atomic level remains poorly understood. In this work, a first-principles study is conducted to investigate how representative carbon structures, including graphite (0001) surface, point defects and fractured edge, influence the discharge and charge processes of non-aqueous NaeO 2 bat- teries. It is found that the single vacancy (SV) defect has the largest adsorption energy (5.81 eV) to NaO 2 molecule among the structures studied, even larger than that of the NaO 2 molecule on NaO 2 crystal (2.81 eV). Such high adsorption energy is attributed to two factors: the dangling atoms in SV defects decrease the distance from NaO 2 molecules, and the attachment through oxygen atoms increases the electrons transfer. The findings suggest that SV defects can act as the nucleation sites for NaO 2 in the discharge process, and increasing the number of SV defects can facilitate the uniform formation of small- sized particles. The uniformly distributed discharge products lower the possibility for pore clogging, leading to an increased discharge capacity and improved cyclability for non-aqueous NaeO 2 batteries. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Rechargeable metal-oxygen batteries are considered to be the potential energy storage systems for future electric vehicles (EVs) due to their high theoretical energy densities, which are achieved by the facts that the anode materials are metals and the cathode reactant O 2 is retrieved from ambient air without occupying the cathode volume [1]. Especially, non-aqueous lithium-oxygen (LieO 2 ) batteries have been widely investigated in the past few years [2e6], but varieties of critical issues (e.g., poor electrolyte stability, low energy efficiency, short cycle life and poor power capacity [7,8]) limit their further commercial exploitation, most of which are related to the high charge overpotential during oxygen evolution reaction (OER) process. One widely applied strategy to decrease the high charge overpotential is developing catalysts, such as carbon-based materials [9e11], noble metals [12,13], metal ox- ides [14,15] and metal alloys [16,17]. However, some investigated electrocatalysts undesirably promote the decomposition of elec- trolytes [18,19]. Even worse, the discharge product Li 2 O 2 itself in non-aqueous LieO 2 batteries was supposed to be the origin of high charge overpotential [20e22]. By contrast, non-aqueous sodium-oxygen (NaeO 2 ) batteries exhibit a much lower charge overpotential (<300 mV) than non- aqueous LieO 2 batteries (typically >1V) do, thus attracting great attention recently [23e26]. In 2010, the rechargeable NaeO 2 bat- teries were firstly investigated and demonstrated to run for several cycles at 105  C by Peled et al. [27]. After that, Sun et al. reported the * Corresponding author. E-mail address: metzhao@ust.hk (T.S. Zhao). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2016.05.132 0378-7753/© 2016 Elsevier B.V. All rights reserved. Journal of Power Sources 325 (2016) 91e97