Byung-Ho
Kang‡
a,
Seulgi
Shin‡
b,
Kunwoo
Nam
a,
Joonwon
Bae
c,
Jong-Min
Oh
b,
Sang-Mo
Koo
b,
Hiesang
Sohn
d,
Sung-Hoon
Park
*a and
Weon Ho
Shin
*b
aDepartment of Mechanical Engineering, Soongsil University, 369 Sangdo-ro, Dongjakgu, Seoul 06978, Republic of Korea. E-mail: leopark@ssu.ac.kr
bDepartment of Electronic Materials Engineering, Kwangwoon University, Seoul 01897, Republic of Korea. E-mail: weonho@kw.ac.kr
cDepartment of Applied Chemistry, Dongduk Women's University, Seoul 02748, Republic of Korea
dDepartment of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea
First published on 29th August 2023
Transition metal dichalcogenides (TMDCs), as next-generation two-dimensional materials, have gained considerable attention in energy storage applications through the incorporation of functional materials. In this work, we investigated NbSe2 TMDCs possessing metallic properties through the exfoliation of mono/few layered nanosheets and their subsequent incorporation with polypyrrole, a conducting polymer. The charge–charge interaction between the positively charged pyrrole and negatively charged NbSe2 nanosheets was facilitated through a chemical incorporation method, resulting in NbSe2@polypyrrole hybrid nanocomposites where the pyrrole molecules polymerized along the surface of NbSe2 nanosheets. The synergistic effect observed in NbSe2@polypyrrole hybrid nanocomposites demonstrated high capacity for lithium storage (955 mA h g−1) with excellent cycling stability (>100 cycles) and rate performance (4 A g−1), surpassing the performance of pristine NbSe2 or polypyrrole electrodes. In particular, the NbSe2@polypyrrole hybrid nanocomposite exhibited 361 mA h g−1 discharge capacity retention within a charge/discharge time of less than 6 minutes, which is comparable with the capacity of conventional graphite anodes. Our hybrid approach utilizing TMDCs and carbon structures could provide significant insight for the utilization of novel TMDC materials in energy storage applications.
Among various TMDCs, NbSe2 has been gaining attention due to its metallic properties, although limited research has been conducted compared to that on MoSe2 and WSe2. Despite its potential as a superconducting material, NbSe2 has not been thoroughly studied. Wang et al. synthesized monolayer NbSe2via chemical vapor deposition (CVD) and demonstrated its superconducting properties according to the number of layers and temperature.13 Additionally, charge density waves and superconducting phases were observed in NbSe2 down to a thickness of one monolayer, with the transition temperature increasing from 33 K in the bulk to 145 K in the monolayer by Xi et al.14 Moreover, NbSe2-based composite materials have also been studied for energy storage devices, both theoretically and experimentally. For example, Khan et al. reported visible light active photocatalytic NbSe2@TiO2 nanocomposites using the increased surface area and visible light absorption capability of NbSe2.10 Furthermore, Nguyen et al. synthesized NbSe2@graphene composites via wet ball milling as an anode material for lithium-ion batteries (LIBs).4 NbSe2 nanosheets embedded in carbon nanofibers co-doped with N and Se were used in a potassium-ion hybrid capacitor by Chen et al.15 and Liu et al. reported a WS2/NbSe2 van der Waals heterostructure as an ultrafast charging and discharging anode material for LIBs.16 Despite these efforts, there is still a need to develop NbSe2-based materials with controlled properties for next-generation energy and electronic applications.
Utilization of TMDC materials often requires a monolayer of the material. Chemical vapor deposition (CVD) is typically used to prepare monolayer TMDCs due to its potential to synthesize high-quality and large-sized TMDC layers.13,17 However, the high cost of the high vacuum system and the limited amount of synthetic materials hinder versatile applications. An alternative method for large-scale synthesis of monolayer TMDCs is the Li intercalation/exfoliation process, where Li atoms intercalate into weak van der Waals interlayers.18–20 In this work, we successfully synthesized monolayer NbSe2 through the Li intercalation/exfoliation process. The prepared NbSe2 nanosheets were combined with a conducting polymer to enhance their electrical transport properties. Polypyrrole (PPy) is a promising conducting polymer with a nitrogen-containing conjugated structure that can provide higher electronic conductivity than carbon alone.21–23
Numerous studies have reported PPy's cost-effectiveness, environmental stability, and simple fabrication.3,24–26 Moreover, PPy is a positively charged conducting polymer that can readily realize a composite structure with negatively charged NbSe2 monolayer nanosheets. Therefore, we fabricated an NbSe2@PPy hybrid nanocomposite structure as an anode material for a lithium-ion battery for the first time. The prepared NbSe2@PPy structures exhibited (i) a high gravimetric capacity of 955 mA h g−1, (ii) stable operation for more than 300 cycles without any capacity loss, and (iii) comparable capacity (361 mA h g−1) with conventional graphite anodes within 6 min of discharge time. Overall, the hybrid TMDC structure with NbSe2 and the PPy composite showed outstanding power and cycling performance that could be used to realize sustainable energy storage systems.
2Li + 2H2O → H2 (gas) + 2LiOH (aq.) | (1) |
The resultant H2 gas is the driving force of NbSe2 exfoliation. During the intercalation reaction, the NbSe2 layers reduce to have a partially negative charge, and exfoliated NbSe2 nanosheets are also charged negatively to stably disperse in polar solvent (water). The reddish solution (shown in Fig. 1) is obtained after centrifugation, confirming that NbSe2 is successfully exfoliated by the Li intercalation process. After exfoliation, we coated PPy on both sides of NbSe2 monolayer nanosheets, with the charge–charge interaction between negatively charged NbSe2 nanosheets27,28 and the positively charged pyrrole monomer.3 Due to the tendency of the resultant suspension of Li ion intercalation to restack, it is preferred to synthesize with PPy immediately after exfoliation. The high binding between NbSe2 and PPy originated from coulombic forces and the precipitates could be obtained after pyrrole polymerization. The conducting polymer facilitates the electronic transport to the NbSe2 nanosheet which could lead to the promotion of the electrochemical reactions.
The morphology of the NbSe2 nanosheets after exfoliation is depicted in Fig. 2. As shown in Fig. 2a, bulk NbSe2 powders are composed of a layered structure held together by weak van der Waals interactions, making it easy for foreign species to intercalate between the layers. In this work, we used butyllithium for Li intercalation with the reduction of NbSe2 layers. The SEM image of exfoliated NbSe2 monolayer nanosheets after spin coating on a Si wafer is presented in Fig. 2b. The large lateral sizes of NbSe2 nanosheets range from 200 nm to 5 μm, with no significant size reduction. The UV-vis spectrum of exfoliated NbSe2 solution is shown in Fig. 2c. Since NbSe2 is metallic, a broad spectrum can be detected within the visible light range (Fig. 2c).7,29–31 The AFM images in Fig. 2d show the thickness of exfoliated NbSe2 nanosheets, and the apparent thickness of NbSe2 nanosheets is ∼1 nm.32,33 This implies that the exfoliated NbSe2 nanosheets mostly consist of monolayers. Raman analysis was performed to compare the chemical bonding nature of bulk and exfoliated NbSe2 (shown in Fig. 2e). Bulk NbSe2 displays main characteristic peaks of ∼240 cm−1 and ∼230 cm−1, representing the in-plane vibration mode and out-of-plane vibration mode, respectively, denoted as E12g and A1g. The mechanism of in-plane and out-of-plane vibration modes is displayed in Fig. 2f. After exfoliation, two major features are observed. The peak of E12g vibration mode in exfoliated NbSe2 nanosheets red-shifts compared to that in bulk NbSe2 powder (from 236.57 cm−1 to 242.30 cm−1), while the A1g vibration mode is blue-shifted (from 232.74 cm−1 to 230.83 cm−1). This is consistent with previous reports.14,34,35 Since these characterization peaks depend on the number of layers, the interlayer interaction between the layers is weakened, and the intralayer interaction within the layers becomes stronger for properly exfoliated NbSe2 nanosheets. In summary, we successfully fabricated NbSe2 nanosheets through the Li intercalation and exfoliation process.
The morphology of the NbSe2@PPy composite is presented in Fig. 3, displaying its 2D sheet structure, similar to that of NbSe2 alone (as depicted in Fig. 2b), while PPy has a sphere-shaped structure (see Fig. S1a and b†). It is well-known that randomly distributed PPy in a solvent tends to form spherical structures to reduce surface energy. When NbSe2 nanosheets are introduced during the polymerization of pyrrole monomers, PPy deposits on the NbSe2 nanosheets and follows their morphology. The coulombic interaction between positively charged pyrrole and negatively charged NbSe2 nanosheets leads to strong binding between them. Therefore, the adsorbed pyrrole monomers on NbSe2 nanosheets are polymerized when an initiator is introduced. The EDS mapping images presented in Fig. S1c and d† clearly show the homogeneous carbon structure coated on NbSe2 nanosheets without any agglomeration. Moreover, we have prepared another composition with a lower amount of PPy, and there is no detectable difference in morphology, as shown in Fig. 3. Hereafter, we refer to the samples with a high amount of PPy and a low amount of PPy as NbSe2@PPy-1 and NbSe2@PPy-2, respectively.
The TGA analysis is performed to estimate the ratio of NbSe2 and PPy in NbSe2@PPy composites as shown in Fig. 4a. Pristine NbSe2 stays without any noticeable change up to 900 °C, while the NbSe2@PPy composites show a significant mass drop near 400 °C, which originated from the dissociation of PPy.36 Therefore, the mass ratio of NbSe2 in NbSe2@PPy composites is calculated at 28% for NbSe2@PPy-1 and 64% for NbSe2@PPy-2. The Raman spectra of NbSe2@PPy composites are presented in Fig. 4b to investigate the crystallinity of the carbon structure (PPy). Two distinguishable peaks are detected: the D-band (1344 cm−1), attributed to defects or disorder of the carbon structure and the G-band (1570 cm−1), commonly attributed to the graphite carbon structure.37,38 The crystallinity of carbon-based materials can be compared by calculating the intensity ratio of the D-band and G-band. The ID/IG ratios of all samples in this study are comparable (ID/IG = ∼0.89), indicating that the coated carbon structure does not significantly change with the introduction of NbSe2 nanosheets during the polymerization process. Therefore, we can directly compare the effect of NbSe2 nanosheets on the following lithium storage properties.
The XPS spectra are shown in Fig. 4c–f to determine the electronic states of the elements and composition of NbSe2@PPy composites. From the survey scan in Fig. 4c, the Nb 3d and Se 3d peaks are easily detected on the NbSe2@PPy hybrid. The N 1s detailed spectrum in Fig. 4d has two different peaks. The higher binding energy peak (400.6 eV) represents the quaternary nitrogen where the nitrogen atoms replace the carbon structure, and the lower binding energy peak (398 eV) is attributed to the pyridinic nitrogen. The nitrogen configuration is a typical feature of annealed PPy as reported elsewhere.39 A similar N 1s spectrum is found for PPy and the NbSe2@PPy hybrid, indicating that the presence of NbSe2 nanosheets does not alter the PPy structure. The detailed spectra of Nb and Se for NbSe2@PPy are shown in Fig. 4e and f. The 3d2/3 (209 eV) and 3d2/5 (206.2 eV) peaks for Nb 3d and 3d2/3 (56.2 eV) and 3d2/5 (55.4 eV) peaks for Se 3d are detected, consistent with other reports.7,40 This indicates that the chemical structure of NbSe2 nanosheets is maintained during synthesis of NbSe2@PPy. Therefore, we can conclude that the NbSe2@PPy hybrid nanocomposites are successfully synthesized.
The prepared NbSe2@PPy materials were utilized as LIB anode materials, and their electrochemical properties were compared to those of NbSe2 and PPy alone. Fig. 5a displays the CV graphs of the NbSe2@PPy sample obtained at a 0.5 mV s−1 scan rate for the first four cycles, illustrating redox reactions during the lithiation/delithiation process. The low potential peaks observed below 1 V originate from the formation of a solid–electrolyte interface (SEI) layer on the active materials through the decomposition of the liquid electrolyte.4 In the second cycle, distinct oxidation/reduction peaks were observed at ∼1.4 V for the cathodic peak and ∼1.7 V for the anodic peak. This phenomenon can be attributed to the lithiation/delithiation of NbSe2 nanosheets, which follows the electrochemical reaction mechanism (2).
NbSe2 + 4Li+ + 4e− ↔ 2Li2Se + Nb | (2) |
The theoretical capacity of NbSe2 conversion based on the above reaction is approximately 1538 mA h g−1. According to other literature studies, the lithiation process in the NbSe2 system starts with the intercalation of Li+ between the NbSe2 layers before the above-mentioned conversion mechanism occurs.41 To investigate this process, we performed cyclic voltammetry (CV) measurements on the pristine NbSe2 electrode, as shown in Fig. S2.† The results displayed the intercalation peak at approximately 2 V. However, the NbSe2@PPy composite electrodes (Fig. 5a) are composed of mono- or few-layers of NbSe2, and therefore, the Li+ intercalation process hardly occurs. After one cycle, the CV graphs were almost identical to those in the second cycle, indicating excellent stability and reversibility of the NbSe2@PPy electrode system. In contrast, the CV graph of the PPy electrode did not exhibit noticeable peaks, which is typical of amorphous carbon structures. Only the peaks of SEI formation (below 1 V) were detected in the first cycle.
Fig. 5b presents the galvanostatic charge–discharge (GCD) profiles of the NbSe2@PPy-1 electrode system at a current density of 0.1 A g−1 for the first five cycles. The NbSe2@PPy-1 electrode system displays a high capacity of approximately 1562 mA h g−1 with a coulombic efficiency of 50.5% during the first discharge. The major electrochemical reaction in NbSe2@PPy-1 occurs from around 1.5 V due to the electrochemical conversion reaction of NbSe2 nanosheets in PPy. The potential of NbSe2@PPy-1 decreases without a distinct plateau, indicating that the solid solution reaction occurs on the active materials during the lithiation/delithiation process. The low coulombic efficiency during the first cycle is due to the formation of the solid–electrolyte interface (SEI) layer, which is consistent with the aforementioned cyclic voltammetry (CV) results. From the second cycle, the discharge capacity remains stable at approximately 955 mA h g−1 with a high coulombic efficiency of 92%. In contrast, the NbSe2 electrode alone has a first discharge capacity of only 837 mA h g−1 at a current density of 0.1 A g−1 (Fig. S3†), which is less than the theoretical capacity of the reaction (1). The discharge capacity of the NbSe2 electrode gradually decreases to 175 mA h g−1 after five cycles at a current density of 0.1 A g−1, which is attributed to the low intrinsic electrical conductivity of NbSe2. NbSe2 with a stacked structure has low electrochemical activity, which may be present near the surface. In contrast, the well-coated NbSe2@PPy-1 structure exhibits great reversibility because the highly conductive PPy readily transfers electrons from the current collector to the active NbSe2, and the porous structure of PPy could deliver Li+ ions to NbSe2.
Fig. 5c displays the cycling performances of NbSe2, PPy, NbSe2@PPy-1, and NbSe2@PPy-2 at a current density of 0.5 A g−1. The first five cycles are performed at a current density of 0.1 A g−1 for the stabilization of the electrodes. The discharge capacity of the NbSe2@PPy-1 electrode in the 210th cycle reaches approximately 670 mA h g−1, while the NbSe2 electrode is significantly degraded to 17 mA h g−1 in the 210th cycle. The coulombic efficiency of every cycle is around 99% for the NbSe2@PPy-1 electrode, indicating highly reversible electrochemical reactions. The PPy and NbSe2@PPy-2 electrodes have discharge capacities of 416 mA h g−1 and 531 mA h g−1, respectively, in the 100th cycle, with high cycling stability. Notably, the NbSe2@PPy-2 electrode has a higher NbSe2 content than the NbSe2@PPy-1 electrode. Since the PPy on the NbSe2@PPy-2 electrode is not homogeneously covered on the NbSe2 nanosheets, the NbSe2@PPy-2 electrode may have lower electrochemical activity.
In order to investigate the electrochemical mechanism of the NbSe2@PPy-1 electrode, we conducted EIS measurements. As shown in Fig. 5d, the semicircle in the high frequency region corresponds to the charge transfer resistance (Rct), while the straight line in the low frequency region corresponds to the Warburg impedance related to the mass transfer. The Rct value of NbSe2@PPy-1 for the as-prepared cell is 126 Ω, which is significantly lower than that of the NbSe2 electrode (589 Ω). This improvement in Rct is attributed to the enhanced electron mobility resulting from the well-coated PPy on the NbSe2@PPy-1 electrode. Furthermore, after 100 cycles of lithiation/delithiation, the Rct value of NbSe2@PPy-1 is 287 Ω (Fig. 5e), which is 128% higher than that in the first cycle, while the Rct of the NbSe2 electrode increases significantly (1322 Ω). These results indicate that our NbSe2@PPy-1 electrode exhibits significant improvements in capacity and stability simultaneously.
The rate performances of the NbSe2, PPy, NbSe2@PPy-1, and NbSe2@PPy-2 electrodes were evaluated by varying the current density from 0.1 A g−1 to 4 A g−1, as shown in Fig. 6a. The PPy, NbSe2@PPy-1, and NbSe2@PPy-2 electrodes exhibit reversible lithiation/delithiation reactions even at high rates of 4 A g−1, while the NbSe2 electrode degrades after a few cycles. The discharge capacity of the NbSe2@PPy-1 electrode is 361 mA h g−1 at a current density of 4 A g−1, which is 58.9% lower than that at 0.1 A g−1. The discharge capacities of the PPy and NbSe2@PPy-2 electrodes are 178 mA h g−1 and 286 mA h g−1, respectively, at a current density of 4 A g−1. After 5 cycles at 4 A g−1, the current density was changed back to the initial state of 0.1 A g−1, and the discharge capacity of the NbSe2@PPy-1 electrode returned to the initial value of 819 mA h g−1, indicating excellent electrochemical stability at high current densities. In contrast, the NbSe2 nanosheet electrode exhibits significant degradation at high current densities and poor rate performance. The GCD profiles of NbSe2@PPy-1, NbSe2, PPy, and NbSe2@PPy-2 electrodes for different current densities from 0.1 A g−1 to 4 A g−1 are displayed in Fig. 6b and S5a–c.† Therefore, the NbSe2@PPy hybrid nanocomposites show high performance anode properties in terms of high capacity, fast kinetics, and excellent stability, which makes it possible to apply them to next generation energy storage devices.
Fig. 6 (a) Rate performance of NbSe2@PPy-1, NbSe2@PPy-2, NbSe2, and PPy, (b) GDC graphs of NbSe2@PPy-1 by increasing the current density from 0.1 A g−1 to 4 A g−1. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta01335a |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |