Full Length ArticleAmorphous MoSx embedded within edges of modified graphite as fast-charging anode material for rechargeable batteries
Graphical abstract
Amorphous MoSx was embedded within edges of modified graphite through mechanical and chemical procedures for fast-charging anode material.
Introduction
Rechargeable lithium-ion batteries (LIBs) have been extensively used in portable electronics owing to their flexibility and lack of memory effects [1], [2], [3], [4]. Graphite has several advantages as an anode material, including a flat potential profile (0.3 V vs Li/Li+), relatively low cost, superior electrical conductivity, and structural stability during lithiation/de-lithiation [2], [5], [6], [7], [8]. However, as demands for eco-friendly vehicles have risen, several researchers have attempted to replace graphite—which has drawbacks such as a low theoretical capacity (372 mAh g−1, LiC6) and slow lithium ion diffusivity—with nanomaterials having high capacity and fast charging/discharging characteristics [9], [10], [11]. Among the alternative anode materials, molybdenum sulfide (MoSx) has been considered because its conversion reaction leads to a higher theoretical specific capacity (~670 mAh g−1) compared with commercial graphite [12], [13], [14]. Nevertheless, MoSx exhibits some disadvantages, including structural deterioration during repeated charging and discharging and semiconducting property that lead to low stability [12], [15], [16], [17].
To address these critical issues, several strategies to form composites of MoSx and nanocarbon have been proposed [18], [19], [20], [21], [22]. It is well known that nanocarbon materials provide large electrochemically active surface areas and electron transfer pathways owing to their porous structure with exposed edges and sp2 hybridization, respectively [23], [24], [25], [26], [9], [27]. In addition, nanocarbon-based electrodes exhibit good contact with the electrolyte and a short pathway for both electron and Li-ion transports, leading to fast charging/discharging rates [28], [29]. Li et al. reported that MoS2 directly grown on the surface of carbon nanotubes (CNTs) showed excellent electrochemical performance owing to the strong interaction between the semiconducting MoS2 and the conductive CNTs [30]. Similarly, Shi et al. synthesized hierarchical MoSx/CNT nanocomposites via a solvent thermal method. This hierarchical structure maximized the surface area and increased the interlayer spacing, resulting in a lower strain and a smaller intercalation barrier of Li ions, which yielded a large lithium storage capability, stable cycling performance, and excellent charge/discharge rates [31]. Nevertheless, these approaches have the serious drawback of forming an excessively thick solid electrolyte interphase (SEI) layer at the interface between the electrode and the electrolyte, owing to the high specific surface area [32], [33], [34]. As a result, the electrodes show a low first coulombic efficiency (CE). Furthermore, owing to their low tap density, nanocarbon-based electrodes are not suitable for commercial use, such as in electric vehicles.
In this study, we introduce a facile synthesis method for preparing anode materials based on commercial graphite composites. The graphite was modified through ball-milling and mild oxidation as mechanical and chemical procedures, respectively, to increase the number of exposed edges, which provides larger sites for MoSx loading. The MoSx grown in the opened interlayer gaps at the edges enhances the Li storage capacity of the material. The resulting MoSx embedded within ball-milled reduced graphite oxide (MoSx@B-rGtO) electrode showed a high specific capacity of 1239 mAh g−1 at a current density of 0.13 A g−1 and a retention capacity of 95% after 100 cycles (at 0.26 A g−1). The rate-capability of the composite was ~3.3 times that of pristine graphite at a high current density. Accordingly, we suggest that our graphite-based composite can be utilized as an anode material in LIBs.
Section snippets
Materials
Graphite flakes (natural, 99.8%) were purchased from Alfa Aesar. They were mildly oxidized in sulfuric acid (H2SO4, 95–98%) with potassium permanganate (KMnO4, ACS reagent grade, 99%) as the oxidizing agent. To remove the residual oxidizing agent, hydrogen peroxide (H2O2, 30%) was added to the solution. Ammonium tetrathiomolybdate ((NH4)2MoS4, 99.97%), as the MoSx precursor, and hydrochloric acid (HCl, ACS reagent grade, 36%) were used to synthesize the MoSx according to reaction formula (1)
Structural analysis of modified graphite
The structural changes in the B-Gt, GtO, and B-GtO were investigated using XRD and Raman spectroscopy. As shown in Fig. 2a, the structure of the mechanically modified B-Gt was similar to that of graphite, despite its larger specific surface area (458.4 cm2 g−1) than pristine graphite (Gt, 2.5 cm2 g−1), as shown in Fig. S1. However, the chemically modified GtO and mechanically/chemically modified B-GtO showed a (0 0 1) plane at 12–13° because the chemical oxidation resulted in an increase in the
Conclusions
A MoSx@B-rGtO anode with outstanding electrochemical performance was easily synthesized from graphite modified using ball-milling and a mild oxidation process. This approach significantly increased the number of edge sites on the graphite in which MoSx can effectively be embedded. In addition, various oxygen functional groups on the modified graphite led to a strong interaction between the MoSx and modified graphite. Electrochemical tests showed that the MoSx@B-rGtO electrode, which had high
CRediT authorship contribution statement
Youn-Ki Lee: Conceptualization, Validation, Investigation, Resources, Writing - Original Draft, Formal analysis, Visualization. Mihwa Lee: Conceptualization, Validation, Investigation, Resources, Writing - Original Draft, Formal analysis, Visualization. Gwan Won Lee: Investigation, Visualization. KwangSup Eom: Formal analysis, Writing - Review and Editing. Myong-Hoon Lee: Formal analysis, Writing - Review and Editing. Sungho Lee: Conceptualization, Writing - Review and Editing, Supervision,
Acknowledgements
This work was supported by a grant from the National Research Foundation of Korea (NRF) (NRF-2018M1A2A2061989 and NRF-2019M3E6A1064735), Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry and Energy, (No. 20194010201790 and No. 20193091010110) and the Korea Institute of Science and Technology (KIST) Institutional program, Carbon Cluster Development Program (No.
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These authors contributed equally to this work.