Contents lists available at ScienceDirect Nano Energy journal homepage: www.elsevier.com/locate/nanoen Full paper Origin of excellent rate and cycle performance of Na + -solvent cointercalated graphite vs. poor performance of Li + -solvent case Sung Chul Jung, Yong-Ju Kang, Young-Kyu Han ⁎ Department of Energy and Materials Engineering and Advanced Energy and Electronic Materials Research Center, Dongguk University-Seoul, Seoul 100- 715, Republic of Korea ARTICLE INFO Keywords: Na ion battery Li ion battery Solvent cointercalation Ion conductivity Graphite ABSTRACT Despite its high reversibility for Li + intercalation, graphite is known to be electrochemically inactive for Na + intercalation. On the contrary, recent studies have demonstrated that graphite is active and shows excellent rate and cycle performance for Na + -solvent cointercalation but it exhibits poor performance for Li + -solvent cointercalation. Herein, we elucidate the mechanism of Li + - and Na + -solvent cointercalation into graphite and the origin of the strikingly different electrochemical performance of Li + - and Na + -solvent cointercalation cells. Na + intercalation into graphite is thermodynamically unfavorable, but Na + -diglyme cointercalation is very favorable. The diglyme–graphene van der Waals interaction reinforces the interlayer coupling strength and thereby improves the resistance of graphite to exfoliation. The transport of solvated Na ions is so fast that the diffusivity of Na + -diglyme complexes is markedly faster (by five orders of magnitude) than that of Li + -diglyme complexes. The very fast Na + -diglyme conductivity is attributed to facile sliding of flat diglyme molecules, which completely solvate Na ions in the interlayer space of graphite. The slow Li + -diglyme conductivity is ascribed to steric hindrance to codiffusion caused by bent diglyme molecules that incompletely solvate Li ions. The bent and flat diglyme molecules surrounding Li and Na ions, respectively, are highly associated with the strong Li + – graphene and weak Na + –graphene interactions, respectively. 1. Introduction Graphite intercalation compounds (GICs) are layered materials with periodically stacked intercalant and graphene layers, and they are formed by inserting guest species, such as atoms, molecules, and ions, into the interlayer space of the host graphite. The control of the type and amount of guest species can lead to the formation of GICs with peculiar features such as superconducting behavior [1] and high transparency [2]. The GICs have a variety of applications as chemical reagents, electrochemical electrodes, highly conductive materials, catalysts, and so on [3]. Since the discovery of monolayer graphene in 2004 [4], GICs have been widely used as starting materials to produce large-area graphene sheets via exfoliation [5]. Many hundreds of GICs have been examined by utilizing various intercalant species such as alkali metals, metal oxides, metal chlorides, bromides, fluorides, oxyhalides, acidic oxides, and strong acids [3]. Particularly, one of the most extensively studied GICs is Li x C 6 (0 < x≤1), namely Li-GICs, used as the standard anode material in lithium-ion batteries (LIBs) [6–11]. LIBs are currently the most commonly used power sources for portable electronic devices, but are facing a potential challenge in price due to the low abundance of Li resources in the Earth's crust [12]. Sodium-ion batteries (SIBs) have attracted much attention as a promising alternative to LIBs. Unfortunately, however, Na + intercala- tion into graphite is electrochemically difficult. The maximum sodia- tion capacity of graphite is < 35 mAh g –1 for NaC 64 , which is much lower than that for lithiation (372 mAh g –1 for LiC 6 ). The poor Na + storage capability of graphite has been thought to be due to the small interlayer spacing of graphite, which is not sufficient to accommodate Na ions [13–17]. However, this prevailing view has been contested in experiments showing that K ions, which are larger than Na ions, can electrochemically intercalate into graphite [18,19]. The low activity of graphite for sodiation can be ascribed to a weak Na + –graphene cation–π interaction rather than to any mismatch between the graphite interlayer spacing and ion size, considering that, among alkali metals, Na has the weakest binding to graphite [20]. Until recent years, graphite has been considered inappropriate for applications in SIBs unless it is modified by chemical methods such as oxidation, reduction [15,21], and heteroatom doping [22]. Intriguingly, Jache et al. [23] and Kim et al. [24,25] recently reported that pristine graphite can be successfully used as the anode http://dx.doi.org/10.1016/j.nanoen.2017.03.015 Received 29 December 2016; Received in revised form 10 February 2017; Accepted 6 March 2017 ⁎ Corresponding author. E-mail address: ykenergy@dongguk.edu (Y.-K. Han). Nano Energy 34 (2017) 456–462 Available online 07 March 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved. MARK