Fugen Sun*,
Yahui Li‡
,
Zilong Wu‡,
Yu Liu,
Hao Tang,
Xiaomin Li,
Zhihao Yue and
Lang Zhou*
Institute of Photovoltaics, Nanchang University, 999 Qianhu Road, Nanchang 330031, China. E-mail: sunfugen@ncu.edu.cn; lzhou@ncu.edu.cn
First published on 21st September 2018
A facile methodology to fabricate a metallic and selenophilic Ag2Se coating on a Se/nitrogen-doped mesoporous carbon composite, has been successfully developed based on the in situ redox reaction between Se and AgNO3 under ambient conditions. The in situ reactive growth of Ag2Se on Se ensures the complete encapsulation of Se by the Ag2Se coating, which endows the Ag2Se coating with the dual effects of physical entrapment and chemical binding to effectively confine polyselenide intermediates within the cathodes. With the further assistance of mesopore confinement of the nitrogen-doped carbons, the Ag2Se-coated Se/nitrogen-doped mesoporous carbon composites present much improved electrochemical performances with a high initial discharge capacity of 652 mA h g−1, a high coulombic efficiency of 95.4% and a high reversible capacity of 382 mA h g−1 after 100 cycles. These encouraging results suggest that the in situ reactive construction of metallic and chalcogenophilic coating layers on the chalcogen (e.g. S, Se and Te)-based electrode materials should be a promising and easy to scale-up method for practical applications of lithium batteries in light of the very simple in situ reaction processes involved.
Analogous to Li–S batteries, Li–Se batteries also face the dissolution issue of high-order polyselenide intermediates, resulting in poor cycling performance and low coulombic efficiency.11,12 Motivated by the significant works on using carbon materials to encapsulate sulfur, improvements of the Li–Se batteries have been achieved by coupling the Se phases with various carbon materials, such as porous carbons,13 carbon nanotubes,14 and graphene15 (as listed in Table S2†). These carbon hosts not only act as electronic conduits to the Se encapsulated within, but also function as electrochemical reaction microchambers to physically confine the polyselenide intermediates, which results in enhanced utilization of Se.16,17 However, the weak interactions between non-polar carbon materials and polar polyselenides reduce the ability to bind and entrap these soluble polyselenides.18,19 As a consequence, the only use of non-polar carbon hosts can benefit the cell kinetics/reversibility and alleviate the capacity fading, but still fail to resolve the short cyclic problem of the Li–Se batteries.
From this aspect, using hybrid hosts of polar metal compounds with conductive carbons, has been proved to be a very promising approach for addressing the polyselenide dissolution issue in Li–Se batteries. Polar metal compounds including CoSe2,20 NbS2,21 Ni(OH)2 (ref. 22) and metal Co23 have been coupled with porous carbons for improving the performances of selenium cathodes. The relatively strong chemical binding between these polar compounds and polyselenides, combining with the physical adsorption of porous carbons, synergistically restrain the polyselenide shuttling. However, very few have concentrated on the rational integration of polar compounds with conductive carbons. Undoubtedly, there is still an urgent need to explore stronger encapsulating materials and to develop more facile synthetic procedures that yield optimized hybrid host structures for boosting the electrochemical performances of Se cathodes.
Herein, we demonstrate a simple methodology to fabricate an effective and multifunctional Ag2Se coating on the Se/nitrogen-doped mesoporous carbon particles, which is realized by an in situ redox reaction between Se and AgNO3 under ambient conditions. Unlike most of the insulated metal compounds used as polar hosts in the literature, the obtained orthorhombic Ag2Se coating possess bulk metallic conductivity as high as 2 × 103 S cm−1 at 298 K,24,25 which could interconnect with the conductive carbon frameworks for the fast electron transportation. Moreover, the in situ reactive growth of Ag2Se on Se ensures the complete encapsulation of Se by the Ag2Se coating with the dual effects of physical and chemical adsorption. The physical adsorption is created by the Ag2Se coating architectures, and chemical adsorption is affected by the selenophilic Ag–Se–Ag units, which lead to effective confinement of polyselenide intermediates within the cathodes. As a result, the Ag2Se coated Se/nitrogen-doped mesoporous carbon nanocomposites exhibit much improved cycling stability and rate performance. These encouraging results suggest that the in situ reactive coating of the metallic and selenophilic layers on the Se/C composite cathode materials would be a promising strategy to elevate the electrochemical performances of the advanced Li–Se batteries.
Fig. 1 Schematic illustration for the synthesis and cycling process of Se/NMC (a) and Ag2Se@Se/NMC (b). |
The Ag2Se coating on the Se/NMC composites could be observed from the SEM and TEM images, as shown in Fig. 2. The NMCs, which were obtained via a colloidal silica nanocasting process, has a well-developed spherical mesoporous structures (Fig. 2a) with a large specific surface area of 731 m2 g−1 and pore volume of 2.6 cm3 g−1 (more detailed characterizations on the porous structures and nitrogen-doped surface chemistry of the NMCs are shown in Fig. S1–S3†). After Se loading, the mesopores of NMCs were filled with Se, undoubtedly accompanying with a Se layer coating on the external surface of NMCs (Fig. 2b). The further reaction of AgNO3 with Se outside of NMCs results in a dense surface morphology which consists of interconnected Ag2Se nanoparticles (Fig. 2c). The scanning transmission electronic microscopy (STEM) elemental mapping images in Fig. S4† further indicate the position of Ag2Se coating on the surfaces of Se/NMC composites. Furthermore, the HRTEM image of the Ag2Se@Se/NMC composites in Fig. 2d shows the distinct lattice fringes with fringe spacings of 0.353 and 0.260 nm which are respectively corresponding to the (020) and (121) planes of orthorhombic Ag2Se phase, suggesting the interconnected orthorhombic Ag2Se nanocrystallites coating on the Se/NMC composites.
The crystal phases of the Ag2Se coating on the Se/NMC composites were further revealed by XRD analysis, as shown in Fig. 3a. After Ag2Se coating, the Ag2Se@Se/NMC composites contain typical P212121 orthorhombic Ag2Se diffraction peaks with strong peak intensity, further confirming the orthorhombic Ag2Se coating with high crystallinity. The in situ reactive crystallization of interconnected Ag2Se nanoparticles onto the Se ensures the complete enclosure of the Se by the Ag2Se coating. TGA analysis in Fig. 3b shows that the Se/NMC and Ag2Se@Se/NMC composites respectively exhibit the weight loss of approximately 73 and 67 wt% between 250 and 650 °C in a nitrogen flow, corresponding to the evaporation of Se in the composites during the heating process. Moreover, compared with the Se/NMC composites, the Se in the Ag2Se@Se/NMC composites evaporates at a significantly elevated temperature, indicating the strong encapsulation effect of selenophillic Ag2Se coating on the Se/NMC composites.
Fig. 3 (a) X-ray diffraction patterns of NMCs, Se/NMC and Ag2Se@Se/NMC. (b) Thermogravimetric curves of Se/NMC and Ag2Se@Se/NMC. |
XPS survey scan of the Ag2Se@Se/NMC composites (Fig. 4a) reveals that the basic elemental composition consists of C, N, O, Se and Ag, without any other impurities. High-resolution Ag 3d XPS spectra of the Ag2Se@Se/NMC composites (Fig. 4b) displays two well-defined peaks at 373.60 and 367.55 eV which are corresponding to the Ag 3d3/2 and 3d1/2 of the Ag2Se coating, respectively. Due to the different chemical state of Se in the elemental Se and Ag2Se, the Se 3d XPS spectra of the Ag2Se@Se/NMC composites could be deconvoluted and curve-fitted into two doublet peaks with the binding energies of 55.88/55.03 and 54.28/53.43 eV, which are respectively attributed to the Se–Se bonds of the elemental Se and the Se–Ag bonds of the Ag2Se. Moreover, compared with the Se/NMC composites, the doublet Se 3d peaks of elemental Se in the Ag2Se@Se/NMC composites locate at the slightly reduced binding energy which shift toward that of selenides (Fig. 4c). This shift of binding energy indicates the change of electronic structures in elemental Se after Ag2Se coating, further suggesting the strong binding of the selenophillic Ag2Se with selenium. Furthermore, the digital image in Fig. S5† shows a significant decoloration of Li2Se6 solution after the adsorption of NMCs coated with Ag2Se, confirming the enhanced adsorption capability toward polyselenides after Ag2Se coating.
Fig. 4 (a) Survey XPS spectra of Ag2Se@Se/NMC. (b) High-resolution Ag 3d XPS spectra of Ag2Se@Se/NMC. (c) High-resolution Se 3d XPS spectra of Se/NMC and Ag2Se@Se/NMC. |
The electrochemical performances of the Ag2Se@Se/NMC and Se/NMC composites were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge testing, and electrochemical impedance spectroscopy (EIS). As shown in the CV curves of the Ag2Se@Se/NMC and Se/NMC composites (Fig. 5a), two well-defined reduction peaks for both the composites exist distinctly, which are centered at 2.13 and 2.02 V corresponding to the two-step conversion of Se to soluble lithium polyselenides (Li2Sex, 4 ≤ x ≤ 8) and soluble lithium polyselenides to solid-state Li2Se2/Li2Se, respectively. Only one broad oxidation peak is observed, suggesting the transformation of Li2Se2/Li2Se to the polyselenides and then to Se with an overlapped delithiation process. Except for the intensity difference of these peaks, there are no other obvious differences observed. The higher peak intensity of the Ag2Se@Se/NMC composites than the Se/NMC composites indicates that the Se exhibit higher electrochemical activity after the metallic and selenophilic Ag2Se coating.
Fig. 5 Cyclic voltammograms (a) and the initial charge–discharge curve (b) of Se/NMC and Ag2Se@Se/NMC. |
The discharge and charge properties of the Ag2Se@Se/NMC and Se/NMC composites at a current density of 0.2C were further investigated, as shown in Fig. 5b. Two plateaus, corresponding to the formation of soluble polyselenides (Li2Sex, 4 ≤ x ≤ 8) at 2.1 V and solid-state Li2Se2 and Li2Se at 2.0 V, are observed for both the composites in the discharge process, which agree well with two apparent reduction peaks in the CV curves. The initial discharge and charge capacities of the Se/NMC composites are 620 and 691 mA h g−1, respectively, corresponding to an overcharge capacity of 71 mA h g−1 which is a typical feature of the polyselenide shuttling phenomenon.26 This implies that only confinement of Se in the nitrogen-doped mesoporous carbons cannot entirely prevent the shuttling phenomenon. Interestingly, after Ag2Se coating, the obtained Ag2Se@Se/NMC composites deliver a slightly higher discharge capacity of 652 mA h g−1 (approaching the theoretical capacity of 675 mA h g−1) than the Se/NMC composites, in accompany with a negligible overcharge capacity. Therefore, the multifunctional Ag2Se coating on the Se/NMC composites should provide physical confinement and chemical interaction, thus showing excellent efficiency for trapping the polyselenides.
The cycling performance of the Ag2Se@Se/NMC and Se/NMC composites at 0.2C were compared in Fig. 6. The Se/NMC sample delivers a considerable discharge capacity of 290 mA h g−1 after 100 cycles, which is comparable to the other carbon-selenium composites due to the mesopore confinement of nitrogen-doped carbons. On this basis, the Ag2Se coating further suppress the shuttle effect and facilitate the redox reaction kinetics, resulting in much improved cycling performances. As a result, The Ag2Se@Se/NMC sample exhibits better electrochemical performances with a higher reversible capacity of 382 mA h g−1 and a higher coulombic efficiency of 95.4% after 100 cycles than the Se/NMC counterpart (Fig. 6a). Moreover, the Ag2Se@Se/NMC sample presents better cycling responses to continuously varying current densities, as shown in Fig. 6b. At the maximum discharging rate of 5C (3.4 A g−1), the Ag2Se@Se/NMC delivers a reversible capacity of 294 mA h g−1, much higher than that of Se/NMC. After extended cycling for 400 cycles at 0.5 and 1C, the relatively high reversible capacities of 206 and 193 mA h g−1 have been retained for the Ag2Se@Se/NMC sample (Fig. 6c). The electrochemical performances of the Ag2Se@Se/NMC sample are among the best series of the carbon-selenium cathode materials (Table S2†). Such excellent kinetic behavior of the Ag2Se@Se/NMC composite is further supported by their much lower charge-transfer resistances and diffusion impedances than those of the Se/NMC composite from the EIS spectra (Fig. S6†) and powder electrical conductivity measurement results (Table S1†). There results further demonstrate the effective confinement of Li2Se and Li2Sex species via the combination of chemical and physical entrapment provided by the metallic and selenophilic Ag2Se coating nanostructure.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra06484a |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2018 |