Ag Nanoparticle-Decorated MoS2 Nanosheets for Enhancing Electrochemical Performance in Lithium Storage

Metallic phase 1T MoS2 is a well-known potential anode for enhancing the electrochemical performance of lithium-ion batteries owing to its mechanical/chemical stability and high conductivity. However, during the lithiation/delithiation process, MoS2 nanosheets (NSs) tend to restack to form bulky structures that deteriorate the cycling performance of bare MoS2 anodes. In this study, we prepared Ag nanoparticle (NP)-decorated 1T MoS2 NSs via a liquid exfoliation method with lithium intercalation and simple reduction of AgNO3 in NaBH4. Ag NPs were uniformly distributed on the MoS2 surface with the assistance of 3-mercapto propionic acid. Ag NPs with the size of a few nanometers enhanced the conductivity of the MoS2 NS and improved the electrochemical performance of the MoS2 anode. Specifically, the anode designated as Ag3@MoS2 (prepared with AgNO3 and MoS2 in a weight ratio of 1:10) exhibited the best cycling performance and delivered a reversible specific capacity of 510 mAh·g−1 (approximately 73% of the initial capacity) after 100 cycles. Moreover, the rate performance of this sample had a remarkable recovery capacity of ~100% when the current decreased from 1 to 0.1 A·g−1. The results indicate that the Ag nanoparticle-decorated 1T MoS2 can be employed as a high-rate capacity anode in lithium-ion storage applications.


Introduction
Recently, there has been increasing interest among researchers in transition metal chalcogenides (TMCs), which are graphene-like two-dimensional (2D) materials consisting of a transition metal atom layer sandwiched between two chalcogenide atom layers. Each monolayer of TMC is formed as a 2D structured layer, and these layers are bonded to each other by van der Waals forces in the bulk structure. Therefore, the TMCs can be easily exfoliated into a single layer or a few layers. Moreover, various properties of these 2D materials have been investigated, and they have been found to be superior to bulk materials in strength, physical and chemical stability, and conductivity [1][2][3][4]. Therefore, TMCs have been employed in several electronic, optical, and energy conversion/storage applications, for example, in energy applications such as solar cells, light emitting diodes, hydrogen evolution reactions, and metal-ion batteries [5][6][7][8][9][10][11][12]. Among TMC materials, MoS 2 shows great potential for easy processing and high stability. Therefore, numerous studies have reported on its characteristics and applications. In particular, with a direct band-gap structure, the 1T phase of MoS 2 nanosheets (NSs) is attractive owing to its high mechanical/chemical stability and high conductivity [13][14][15][16]. Recently, MoS 2 NSs have been used as potential candidates for anodes in lithium-ion batteries (LIBs) [17][18][19][20][21][22]. With a nanosheet structure, MoS 2 has a large surface area and flexibility for the lithiation and delithiation processes and enhances the electrochemical performance of LIBs. Furthermore, the 1T phase of MoS 2 , with metallic properties, can afford high conductivity, which facilitates the processes of lithiation and delithiation. Various methods have been used to enhance the electrochemical performance of MoS 2 in LIBs by using metals doping, metal particles or metal oxides. Zhu et al. used TiO 2 nanoparticles (NPs) decorating on 2H-MoS 2 NS via hydrothermal method to achieve the reversible capacity of 604 mAh·g −1 after 100 cycles [23]. Pan et al. developed Ag methanesulfonic-acid capped NPs with 2H-MoS 2 NSs by sonication method to get high reversible capacity of~920 mAh·g −1 after 50 cycles [24]. In addition, Li et al. synthesized lithium molten salt of MoS 2 as a precursor at 1050 • C for liquid exfoliation of 1T MoS 2 [25]. This report showed that the superior properties of 1T MoS 2 to 2H MoS 2 due to the existence of abundant monolayer structures, providing diffusion path for lithium ion insertion/desertion. Wang et al. reported vertically aligned MoS 2 NSs patterned on graphene for LIBs, which exhibited high-rate energy storage [26]. This structure also enables sodium-ion storage capability. Tang et al. developed hollow 1T MoS 2 grown on carbon cloth and demonstrated high rate performance, high capacity, and good stability in sodium-ion batteries [27]. Li et al. combined 1T MoS 2 with MnO in lithium molten salts assisted with a ball milling method to develop a high-stability LIB anode. This MoS 2 /MnO composite anode retained a high capacity of~589 mAh·g −1 after 2000 cycles [28]. Bai et al. fabricated a 1T MoS 2 /C hybrid anode material through a hydrothermal method [20]. These carbon-covered MoS 2 NS materials also exhibited a high rate performance in LIBs. Therefore, 1T phase MoS 2 could be a potential anode for high capacity and high rate performance in LIBs. However, the commercialization of MoS 2 anode materials requires an easy fabrication process and further improvement in stability and rate performance.
In this study, we successfully investigated Ag NP-decorated 1T MoS 2 nanosheets as a potential anode for high-rate performance and stable LIBs. MoS 2 was prepared by a liquid chemical exfoliation method with lithium intercalation. By adding 3-mercapto propionic acid (MPA) as a functional group, Ag NPs were uniformly decorated on the MoS 2 surface. The presence of Ag NPs not only improves the specific capacity but also significantly enhances the rate performance and stability of the anode material in lithium storage. Notably, the Ag3@MoS 2 anode can restore~100% capacity after high-rate cycling.

Synthesis MoS 2 NSs
MoS 2 NSs were prepared as described in our previous report [17]. The loading of butyllithium was conducted in Ar-filled glovebox to prevent the reactions between butyllithium and oxygen/moister. First, 1.0 g of MoS 2 powder was added to 3 mL of 2.5 M butyllithium in hexane. Then, the solution was sealed with parafilm, taken out to be sonicated for 1 h, and kept for two days to obtain lithium-intercalated MoS 2 (Li x MoS 2 ) in glove box. The excess lithium was removed by washing with hexane. The obtained Li x MoS 2 was placed in 100 mL of deionized (DI) water. The interlayer lithium reacted with DI water to break the layer structure of the bulk MoS 2 and form MoS 2 NSs. The solution was further sonicated for 1 h and stirred for 1 h to obtain a complete dispersion of the MoS 2 NS. Finally, the dispersion was centrifuged and washed four times to remove excess lithium.

Ag-Decorated MoS 2 NS
The surface of the NSs was modified by MPA in order to obtain Ag-decorated MoS 2 NSs. The prepared LixMoS 2 , after being washed with hexane, was added to 100 mL of 0.045 M MPA solution. This process was similar to the synthesis of MoS 2 NSs. After sonication and washing with DI water, MPA-modified MoS 2 NSs were redispersed in DI water via sonication. Then, amounts of AgNO 3 with different weight ratios (1:50, 1: 20, and 1:10) to MoS 2 were added to the solution during stirring, and the samples were denoted as Ag1@MoS 2 , Ag2@MoS 2 , and Ag3@MoS 2 , respectively. Naturally, MPA contains both thiol and carboxyl groups. The thiol group is able to exchange with the S atom on MoS 2 with the appearance of Li ions [29,30]. Meanwhile, the carboxyl group induces a partial negative charge, which attracts Ag + ions in the solution. Then, a solution of 0.5 M NaBH 4 was added to the aforementioned solution to reduce Ag + to Ag nanoparticles. The solution was further washed via centrifugation with DI water three times to remove NaBO 2 . The final product was obtained after drying at 70 • C for 12 h.

Characterization
Scanning electron microscopy (SEM; Hitachi S4700, Tokyo, Japan) and transmission electron microscopy (TEM; TECNAI G2F30, FEI Corp., Hillsboro, OR, USA) were used to analyze the morphologies and sizes of the as-prepared materials. Samples were coated a few-nanometers Pt layer via magnetron sputtering system for high quality SEM images. A high-resolution X-ray diffractometer (XRD; SmartLab, Rigaku, Tokyo, Japan) was used to investigate material structures. XRD patterns were recorded over the 2θ range 10-70 • .

Electrochemical Measurements
Anode materials were assembled in a half-cell LIB using coin-type cells (CR 2032, Rotech Inc., Gwangju, Korea). Typically, the anode electrode was prepared using a doctor blade on a Cu foil using a slurry of 70% active material, 15% PVDF, and 15% Super P in NVP. Then, the electrodes were dried in a vacuum oven at 70 • C for 24 h before use. The anodes were punched into 12 mm diameter circular disks. The loading of active materials was 0.7-1.0 mg cm −2 . Then, battery half-cell structures were assembled in an Ar-filled glovebox with positive pressure (>1.0 atm). Lithium foil and polyethylene were used as the reference electrode and separator, respectively. A solution of 1 M LiPF 6 in ethylene carbonate-diethylene carbonate (1:1 by volume) was employed as the electrolyte. Galvanostatic electrochemical discharge/charge analysis of the different cells was performed using a battery cycle tester (WBCS3000, WonAtech Co., Ltd., Seocho-gu, Seoul, Korea) over the voltage range 0.01-3.00 V versus Li/Li+. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests were performed using a ZIVE MP1 apparatus (WonAtech Co., Ltd., Seocho-gu, Seoul, Korea). EIS measurements were recorded at 3.0 V over the frequency range between 100 kHz and 0.1 Hz. CV tests were performed over the voltage range 0.01-3.0 V at a scan rate of 0.1 mV·s −1 .

Results and Discussion
The MoS 2 NSs were fabricated using a liquid exfoliation method [17]. MoS 2 bulk powder was mixed with butyllithium in hexane to form intercalated lithium ions in MoS 2 as interlayer structures. The lithium ions easily filled the MoS 2 , forming inter-layers between two MoS 2 layers by the following process: According to Dines [31], the x value is in the range 1.1-1.5. This implies that the lithium ions easily fill the bulk MoS 2 . Then, reactions between the intercalated lithium and water create hydroxide ions and hydrogen gas. This reaction and the generated gas exfoliate the MoS 2 layers. Finally, the bulk MoS 2 is cleaved into MoS 2 NSs. Figure 1a,b are SEM images of exfoliated MoS 2 NSs at different scale bars of 5 µm and 500 nm, respectively. The lateral size of the MoS 2 NSs was between 100 nm and 3 µm. This wide range of MoS 2 sizes is because of the strong reaction with Li(1) and the random shape of the bulk MoS 2 powder. According to previous reports, the butyllithium intercalation process converts the 2H MoS 2 to 1T due to the effect of lithium ion [10,32]. In this phase, the material could have high conductivity (10-100 S cm −1 ), which is about 10 5 times that of 2H-MoS 2 [25,33]. Therefore, it is thought that highly conductive 1T MoS 2 phase would generate different electrochemical performance (discussed later). Besides, AgNO 3 was selected as the Ag source in order to decorate Ag nanoparticles on the MoS 2 . MoS 2 NSs were functionalized by MPA to prevent random decoration and achieve uniform decoration. Using this functional group, metal ions are easily attracted to the partial negative charge of -COO − to uniformly distribute the Ag + ions. Then, H 2 generated by the hydrolysis of NaBH 4 reduces Ag + ions into Ag NPs, as illustrated in Figure 1c. Three samples with different AgNO 3 :MoS 2 weight ratios-1:50, 1:20, and 1:10-were prepared, and, as aforementioned, they are designated herein as Ag1@MoS 2 , Ag2@MoS 2 , and Ag3@MoS 2 , respectively. Figure 1d, showing the Ag1@MoS 2 sample and an inset image of its magnified surface, indicates that the surface of MoS 2 has only a few Ag NPs owing to the small amount of Ag + used. When the amount of AgNO 3 was increased in Ag2@MoS 2 , the Ag NPs were more uniformly decorated with higher density. Figure 1e and its inset image show uniform Ag NPs on the MoS 2 surface; however, they do not fully cover the surface. The 1:10 ratio of the Ag3@MoS 2 sample reveals the highest coverage of Ag nanoparticles with a size of <10 nm on the MoS 2 nanosheets, as shown in Figure 1f. Figure 1a,b are SEM images of exfoliated MoS2 NSs at different scale bars of 5 μm and 500 nm, respectively. The lateral size of the MoS2 NSs was between 100 nm and 3 μm. This wide range of MoS2 sizes is because of the strong reaction with Li(1) and the random shape of the bulk MoS2 powder. According to previous reports, the butyllithium intercalation process converts the 2H MoS2 to 1T due to the effect of lithium ion [10,32]. In this phase, the material could have high conductivity (10-100 S cm −1 ), which is about 10 5 times that of 2H-MoS2 [25,33]. Therefore, it is thought that highly conductive 1T MoS2 phase would generate different electrochemical performance (discussed later). Besides, AgNO3 was selected as the Ag source in order to decorate Ag nanoparticles on the MoS2. MoS2 NSs were functionalized by MPA to prevent random decoration and achieve uniform decoration. Using this functional group, metal ions are easily attracted to the partial negative charge of -COO − to uniformly distribute the Ag + ions. Then, H2 generated by the hydrolysis of NaBH4 reduces Ag + ions into Ag NPs, as illustrated in Figure 1c. Three samples with different AgNO3:MoS2 weight ratios-1:50, 1:20, and 1:10-were prepared, and, as aforementioned, they are designated herein as Ag1@MoS2, Ag2@MoS2, and Ag3@MoS2, respectively. Figure 1d, showing the Ag1@MoS2 sample and an inset image of its magnified surface, indicates that the surface of MoS2 has only a few Ag NPs owing to the small amount of Ag + used. When the amount of AgNO3 was increased in Ag2@MoS2, the Ag NPs were more uniformly decorated with higher density. Figure 1e and its inset image show uniform Ag NPs on the MoS2 surface; however, they do not fully cover the surface. The 1:10 ratio of the Ag3@MoS2 sample reveals the highest coverage of Ag nanoparticles with a size of <10 nm on the MoS2 nanosheets, as shown in Figure 1f. The structure of MoS2 and Ag-decorated MoS2 were analyzed by XRD. Figure 2 shows the XRD patterns of bulk MoS2, LixMoS2, and Ag-decorated MoS2 NSs. It is noted that the bulk MoS2 powder contains many peaks contributed by various planes in the lattice. However, after lithium ions are intercalated in the MoS2 layers, the XRD pattern of LixMoS2 has a main peak for the (002) plane of MoS2 at ~14.36° and a peak at 15.1°, which is related to the interlayer plane of Li between the MoS2 layers. The other peaks were much reduced and negligible due to the high intensity of (002) plane. After exfoliation and Ag The structure of MoS 2 and Ag-decorated MoS 2 were analyzed by XRD. Figure 2 shows the XRD patterns of bulk MoS 2 , Li x MoS 2 , and Ag-decorated MoS 2 NSs. It is noted that the bulk MoS 2 powder contains many peaks contributed by various planes in the lattice. However, after lithium ions are intercalated in the MoS 2 layers, the XRD pattern of Li x MoS 2 has a main peak for the (002) plane of MoS 2 at~14.36 • and a peak at 15.1 • , which is related to the interlayer plane of Li between the MoS 2 layers. The other peaks were much reduced and negligible due to the high intensity of (002) plane. After exfoliation and Ag decoration, the sample shows some main peaks for MoS 2 at~14.4 • ,~29.0 • , and 32.7 • , which correspond to the (002), (004), and (100) planes according to the #JCPDS card number 00-037-1492. The XRD peaks of MoS 2 are broadened in comparison to those of the bulk material. According to the Scherrer equation, the average size of the crystals can be calculated by D = 0.9λ/βcosθ, where D is the average size, λ is the X-ray wavelength, β is the full width at half maximum of the peak, and θ is the diffraction angle. Thus, the broadening of these peaks indicates small crystal sizes in those planes, which implies that the bulk MoS 2 material was exfoliated into nanosheets. These results are similar to other synthesized MoS 2 nanosheets by bottom-up methods such as hydrothermal method and hot injection method [34][35][36][37]. The appearance of Ag peaks for the (111) and (200) planes confirms the successful reduction of AgNO 3 into Ag nanoparticles. decoration, the sample shows some main peaks for MoS2 at ~14.4°, ~29.0°, and 32.7°, which correspond to the (002), (004), and (100) planes according to the #JCPDS card number 00-037-1492. The XRD peaks of MoS2 are broadened in comparison to those of the bulk material. According to the Scherrer equation, the average size of the crystals can be calculated by D = 0.9λ/βcosθ, where D is the average size, λ is the X-ray wavelength, β is the full width at half maximum of the peak, and θ is the diffraction angle. Thus, the broadening of these peaks indicates small crystal sizes in those planes, which implies that the bulk MoS2 material was exfoliated into nanosheets. These results are similar to other synthesized MoS2 nanosheets by bottom-up methods such as hydrothermal method and hot injection method [34][35][36][37]. The appearance of Ag peaks for the (111) and (200) planes confirms the successful reduction of AgNO3 into Ag nanoparticles. The TEM image has many dark points representing Ag particles decorated on MoS2. The HRTEM image was also analyzed to reveal the lattice spacing of 0.24 nm resulting from the Ag lattice structure. The size of the Ag NPs is a few nanometers. The SAED pattern of the Ag-decorated sample is changed in comparison to that of the pure MoS2 sample. The pattern shows that the reflective planes reveal wide, blurred points, which could be because of the overlap of Ag NPs on the MoS2 NSs. The high uniformity of Ag decoration on MoS2 NSs is, thus, confirmed. To investigate the effect of Ag-decorated MoS2 NSs on the electrochemical properties in lithium batteries, CV tests were performed at a scan rate of 0.1 mV⋅s −1 over the range 0.01-3.00 V (vs. Li/Li + ). Figure 4a-d shows the initial three CV curves for MoS2 NSs and Ag1/2/3@MoS2 anodes. The electrochemical processes in the anode can be expressed by the following reactions: In the cathodic process, the MoS2 NSs anode shows a peak at 1.3 V (vs. Li/Li + ), which is the intercalation process of lithium ions into MoS2 NSs to form LixMoS2, corresponding to reaction (2) [26]. The strong peak at 0.5-1.0 V (vs. Li/Li + ) is related to the strong formation of a solid electrolyte interface (SEI) layer and the transformation of LixMoS2 to metallic Mo nanoparticles and the Li2S conversion reaction, as shown in Equations (3) and (4) [17,19]. In the 2 nd and 3 rd cycles, the SEI layer is stable; therefore, the peak at 0.5-1.0 V is significantly reduced. The peak at 1.3 V is strong and stable, indicating stable lithium intercalation process. In the anodic process, the peaks at 1.8 and 2.3 V are associated with the oxidation of Mo to Mo +4 and the delithiation of Li2S to sulfur, respectively. In the case of the Ag1@MoS2 anode, the CV curves indicate some different electrochemical processes. The intercalation peak in the first cycle is shifted to ~1.0 V (vs. Li/Li + ). The CV profiles mainly have two pairs of redox peaks at 1.1/1.7 V and 1.75/2.3 V, which are related to the multiple steps of lithiation and delithiation. When the amount of Ag is increased, the reduction peaks shift to higher potentials. In the Ag2@MoS2 anode, there are two pairs of redox peaks at 1.3/1.75 and 1.75/2.4 V. The peak at ~0.3 V is the formation of Ag-Li alloys [38]. The dealloying peak of Ag-Li is not shown as a clear peak; however, the hump at To investigate the effect of Ag-decorated MoS 2 NSs on the electrochemical properties in lithium batteries, CV tests were performed at a scan rate of 0.1 mV·s −1 over the range 0.01-3.00 V (vs. Li/Li + ). Figure 4a-d shows the initial three CV curves for MoS 2 NSs and Ag1/2/3@MoS 2 anodes. The electrochemical processes in the anode can be expressed by the following reactions: Li + + e − + electrolyte → SEI layer (4) In the cathodic process, the MoS 2 NSs anode shows a peak at 1.3 V (vs. Li/Li + ), which is the intercalation process of lithium ions into MoS 2 NSs to form Li x MoS 2 , corresponding to reaction (2) [26]. The strong peak at 0.5-1.0 V (vs. Li/Li + ) is related to the strong formation of a solid electrolyte interface (SEI) layer and the transformation of Li x MoS 2 to metallic Mo nanoparticles and the Li 2 S conversion reaction, as shown in Equations (3) and (4) [17,19]. In the 2nd and 3rd cycles, the SEI layer is stable; therefore, the peak at 0.5-1.0 V is significantly reduced. The peak at 1.3 V is strong and stable, indicating stable lithium intercalation process. In the anodic process, the peaks at 1.8 and 2.3 V are associated with the oxidation of Mo to Mo +4 and the delithiation of Li 2 S to sulfur, respectively. In the case of the Ag1@MoS 2 anode, the CV curves indicate some different electrochemical processes. The intercalation peak in the first cycle is shifted to~1.0 V (vs. Li/Li + ). The CV profiles mainly have two pairs of redox peaks at 1.1/1.7 V and 1.75/2.3 V, which are related to the multiple steps of lithiation and delithiation. When the amount of Ag is increased, the reduction peaks shift to higher potentials. In the Ag2@MoS 2 anode, there are two pairs of redox peaks at 1.3/1.75 and 1.75/2.4 V. The peak at~0.3 V is the formation of Ag-Li alloys [38]. The dealloying peak of Ag-Li is not shown as a clear peak; however, the hump at 0.2-0.5 V may indicate the multiple Ag-Li phases of the de-alloying process [38]. Notably, when the Ag content is increased, the oxidation peak position shifts to a higher potential, and the shape is broadened. This might be because of the enhanced multiple steps of the oxidation process for Li 2 S [39]. Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 12 0.2-0.5 V may indicate the multiple Ag-Li phases of the de-alloying process [38]. Notably, when the Ag content is increased, the oxidation peak position shifts to a higher potential, and the shape is broadened. This might be because of the enhanced multiple steps of the oxidation process for Li2S [39]. The initial voltage profiles of the as-prepared anodes are shown in Figure 5. It can be observed that with Ag decoration, the discharge/charge capacity of the MoS2 anodes is improved from ~500 to ~900 mAh⋅g −1 , while the MoS2 NS reveals a low initial discharge/charge capacity of only ~490/466 mAh⋅g −1 . Notably, the capacity does not decrease significantly during the initial cycles. The Ag1@MoS2 demonstrates a high initial discharge/charge capacity of 866/780 mAh⋅g −1 and ~800/773 mAh⋅g −1 at the 2 nd and 3 rd cycles, which is >92% of that of the first cycle. The Ag2@MoS2 anode exhibits a high discharge/charge capacity of 937/720 mAh⋅g −1 at the 1 st cycle and ~773/739 mAh⋅g −1 at the 2 nd and 3 rd cycles. The enhancement of the lithium storage capacity could be owing to the Ag decoration improving the conductivity of the materials, thus, facilitating the lithiation/delithiation process. For the Ag3@MoS2 anode, the discharge/charge capacity is slightly reduced to ~840/660 at the 1 st cycle and 694/679 mAh⋅g −1 at the 2 nd and 3 rd cycles, which can be attributed to the higher amount of Ag NPs, leading to stable cyclic performance. Meanwhile, the initial voltage profile of the MoS2 anode shows a sloping plateau at ~1.8 V, which is ascribed to the insertion of Li ions into MoS2, according to Equation (2) [26]. A sloping plateau at ~1.2 V corresponds to the reaction of lithium with sulfur in Equation (4). At below 0.5 V, the sloping plateau is related to the deep conversion reaction of lithium with MoS2 and formation of the SEI layer. The MoS2 NSs, Ag1@MoS2, and Ag2@MoS2 have similar reaction potentials. In contrast, the Ag3@MoS2 electrode has a higher plateau voltage at 2 V. The charging process of the Ag3@MoS2 anode also shows a higher plateau at ~2.4 V, higher than the plateau at ~2.2 V for the other electrodes. This process is because of a shift in the redox potential, which was indicated by the CV profiles of these anodes. The initial voltage profiles of the as-prepared anodes are shown in Figure 5. It can be observed that with Ag decoration, the discharge/charge capacity of the MoS 2 anodes is improved from~500 to~900 mAh·g −1 , while the MoS 2 NS reveals a low initial discharge/charge capacity of only~490/466 mAh·g −1 . Notably, the capacity does not decrease significantly during the initial cycles. The Ag1@MoS 2 demonstrates a high initial discharge/charge capacity of 866/780 mAh·g −1 and~800/773 mAh·g −1 at the 2nd and 3rd cycles, which is >92% of that of the first cycle. The Ag2@MoS 2 anode exhibits a high discharge/charge capacity of 937/720 mAh·g −1 at the 1 st cycle and~773/739 mAh·g −1 at the 2nd and 3rd cycles. The enhancement of the lithium storage capacity could be owing to the Ag decoration improving the conductivity of the materials, thus, facilitating the lithiation/delithiation process. For the Ag3@MoS 2 anode, the discharge/charge capacity is slightly reduced to~840/660 at the 1 st cycle and 694/679 mAh·g −1 at the 2nd and 3rd cycles, which can be attributed to the higher amount of Ag NPs, leading to stable cyclic performance. Meanwhile, the initial voltage profile of the MoS 2 anode shows a sloping plateau at~1.8 V, which is ascribed to the insertion of Li ions into MoS 2 , according to Equation (2) [26]. A sloping plateau at~1.2 V corresponds to the reaction of lithium with sulfur in Equation (4). At below 0.5 V, the sloping plateau is related to the deep conversion reaction of lithium with MoS 2 and formation of the SEI layer. The MoS 2 NSs, Ag1@MoS 2 , and Ag2@MoS 2 have similar reaction potentials. In contrast, the Ag3@MoS 2 electrode has a higher plateau voltage at 2 V. The charging process of the Ag3@MoS 2 anode also shows a higher plateau at~2.4 V, higher than the plateau at~2.2 V for the other electrodes. This process is because of a shift in the redox potential, which was indicated by the CV profiles of these anodes. To further investigate the long-term cyclability, the performance of these anodes was analyzed in half-cells for 100 charge-discharge cycles at a current rate of 100 mA⋅g −1 , as shown in Figure 6a-d. The MoS2 NS anode exhibits a stable cycling performance during the initial 15 cycles; however, the capacity gradually decays thereafter and remarkably fades from the 30th cycle to the 100th cycle to a capacity of ~100 mAh⋅g −1 . This can be attributed to the formation of a broken MoS2 structure after the cycling process and a restacking of the MoS2 NS layers. By addition of a small amount of Ag nanoparticles, the Ag1@MoS2 anode improves the electrochemical performance for the initial 30 cycles; however, a dramatic capacity decay still occurs after that. A small amount of Ag enhances the conductivity of MoS2; however, the applied amount in the Ag1@MoS2 anode does not appear to be enough to protect the entire MoS2 structure. Thus, after 40 cycles, the MoS2 NS structure collapses, and the lithium storage capability worsens. A further increase in the amount of Ag diminishes the collapse of the MoS2 structure, leading to an enhancement in the cycling performance of the anode. The Ag2@MoS2 anode retains a capacity of 330 mAh⋅g −1 up to 100 cycles. The Ag3@MoS2 anode exhibits the best cycling performance; it retains a specific capacity of ~510 mAh⋅g −1 after 100 cycles, corresponding to a capacity retention of ~73%. To further investigate the long-term cyclability, the performance of these anodes was analyzed in half-cells for 100 charge-discharge cycles at a current rate of 100 mA·g −1 , as shown in Figure 6a-d. The MoS 2 NS anode exhibits a stable cycling performance during the initial 15 cycles; however, the capacity gradually decays thereafter and remarkably fades from the 30th cycle to the 100th cycle to a capacity of~100 mAh·g −1 . This can be attributed to the formation of a broken MoS 2 structure after the cycling process and a restacking of the MoS 2 NS layers. By addition of a small amount of Ag nanoparticles, the Ag1@MoS 2 anode improves the electrochemical performance for the initial 30 cycles; however, a dramatic capacity decay still occurs after that. A small amount of Ag enhances the conductivity of MoS 2 ; however, the applied amount in the Ag1@MoS 2 anode does not appear to be enough to protect the entire MoS 2 structure. Thus, after 40 cycles, the MoS 2 NS structure collapses, and the lithium storage capability worsens. A further increase in the amount of Ag diminishes the collapse of the MoS 2 structure, leading to an enhancement in the cycling performance of the anode. The Ag2@MoS 2 anode retains a capacity of 330 mAh·g −1 up to 100 cycles. The Ag3@MoS 2 anode exhibits the best cycling performance; it retains a specific capacity of~510 mAh·g −1 after 100 cycles, corresponding to a capacity retention of~73%. The discharge/charge rate performance of MoS2 NS without and with Ag nanoparticle decoration is shown in Figure 7a-c. These cycles were recorded at 0.1, 0.2, 0.5, and 1.0 A⋅g −1 . As observed, bare MoS2 NSs show an inferior rate performance with a dramatic decrease in capacity from ~600 to 300, 180, and ~100 mAh⋅g −1 , which corresponds to capacity retentions of 50, 30, and 17%, respectively. The restored capacity when going back from 1 to 0.1 A⋅g −1 is ~430 mAh⋅g −1 , which is approximately 72%. When Ag nanoparticles are introduced, however, the rate performance of the anodes significantly improves. The Ag1@MoS2 anode shows capacity reductions from ~730 to 690, 650, and 550 mAh⋅g −1 , respectively, which correspond to capacity retentions of 94, 89, and 75%, respectively. The restored capacity reaches ~676 mAh⋅g −1 (~92% of the initial capacity). Moreover, the Ag3@MoS2 electrode demonstrates the best rate performance, exhibiting capacity retentions of 98%, 96%, and 92%, which correspond to capacity values from 700 to 690, 677, and 646 mAh⋅g −1 , respectively. The capacity after the high-rate test at 1 A⋅g −1 was 100% recovered when returning to 0.1 A⋅g −1 . To further investigate the effect of Ag in MoS2, EIS measurements were performed to evaluate the change in the charge-transfer resistance, as illustrated in Figure 7d. The equivalent circuit using the modified Randles model contains a series resistance, SEI resistance, charge-transfer resistance, and a Warburg impedance element, and this was used to simulate the Nyquist plot [40]. The extracted charge-transfer resistances of the MoS2 NS and Ag1/2/3@MoS2 anodes are 210.5, 152.3, 99.1, and 95.8 Ω, respectively. The presence of Ag NPs clearly leads to an improvement in the anode conductivity [41,42]. Between the Ag2@MoS2 and the Ag3@MoS2 anode, the charge-transfer resistance is not significantly reduced, which indicates that the amount of Ag NPs is sufficient to decorate and enhance the electronic properties of the MoS2 NSs. Thus, the 1:10 weight ratio of AgNO3:MoS2 can contribute to the best performance in lithium-ion storage of MoS2 anode materials. The discharge/charge rate performance of MoS 2 NS without and with Ag nanoparticle decoration is shown in Figure 7a-c. These cycles were recorded at 0.1, 0.2, 0.5, and 1.0 A·g −1 . As observed, bare MoS 2 NSs show an inferior rate performance with a dramatic decrease in capacity from~600 to 300, 180, and~100 mAh·g −1 , which corresponds to capacity retentions of 50, 30, and 17%, respectively. The restored capacity when going back from 1 to 0.1 A·g −1 is~430 mAh·g −1 , which is approximately 72%. When Ag nanoparticles are introduced, however, the rate performance of the anodes significantly improves. The Ag1@MoS 2 anode shows capacity reductions from~730 to 690, 650, and 550 mAh·g −1 , respectively, which correspond to capacity retentions of 94, 89, and 75%, respectively. The restored capacity reaches~676 mAh·g −1 (~92% of the initial capacity). Moreover, the Ag3@MoS 2 electrode demonstrates the best rate performance, exhibiting capacity retentions of 98%, 96%, and 92%, which correspond to capacity values from 700 to 690, 677, and 646 mAh·g −1 , respectively. The capacity after the high-rate test at 1 A·g −1 was 100% recovered when returning to 0.1 A·g −1 . To further investigate the effect of Ag in MoS 2 , EIS measurements were performed to evaluate the change in the charge-transfer resistance, as illustrated in Figure 7d. The equivalent circuit using the modified Randles model contains a series resistance, SEI resistance, charge-transfer resistance, and a Warburg impedance element, and this was used to simulate the Nyquist plot [40]. The extracted charge-transfer resistances of the MoS 2 NS and Ag1/2/3@MoS 2 anodes are 210.5, 152.3, 99.1, and 95.8 Ω, respectively. The presence of Ag NPs clearly leads to an improvement in the anode conductivity [41,42]. Between the Ag2@MoS 2 and the Ag3@MoS 2 anode, the charge-transfer resistance is not significantly reduced, which indicates that the amount of Ag NPs is sufficient to decorate and enhance the electronic properties of the MoS 2 NSs. Thus, the 1:10 weight ratio of AgNO 3 :MoS 2 can contribute to the best performance in lithium-ion storage of MoS 2 anode materials. aterials 2021, 11, x FOR PEER REVIEW 10 of 12 Recent works are summarized in Table 1. The reversible capacity of modified MoS2 NSs can deliver up to ~1000 mAh⋅g −1 . From our method, the MoS2 bulk was exfoliated into few layer MoS2, with decoration of Ag on MoS2 NSs. The Ag decorated MoS2 NSs exhibited stable cyclability and high-rate performance. Moreover, the butyllithium assisted technique and uniform decorating technique for metal-particles can be easily scaled up to industrial purpose. This work can be further improved by optimizing and modifying the synthesis of MoS2 to develop uniform MoS2 single layer with the insertion of butyllithium by applying the pressure or temperature.  Recent works are summarized in Table 1. The reversible capacity of modified MoS 2 NSs can deliver up to~1000 mAh·g −1 . From our method, the MoS 2 bulk was exfoliated into few layer MoS 2 , with decoration of Ag on MoS 2 NSs. The Ag decorated MoS 2 NSs exhibited stable cyclability and high-rate performance. Moreover, the butyllithium assisted technique and uniform decorating technique for metal-particles can be easily scaled up to industrial purpose. This work can be further improved by optimizing and modifying the synthesis of MoS 2 to develop uniform MoS 2 single layer with the insertion of butyllithium by applying the pressure or temperature.

Conclusions
In this study, we successfully prepared MoS 2 NS with Ag NP decoration, using the assistance of MPA functionalization. The structure and morphology of the Ag NPs on the MoS 2 NSs were confirmed by SEM, XRD, and TEM measurements. The size of the MoS 2 NSs was from 100 nm to~3 µm. Ag NPs with a size of a few nm were decorated on the surface of the MoS 2 NSs. The MoS 2 NS shows inferior cycling performance of lithium storage capacity (~500 mAh·g −1 ) and rate performance. By incorporating Ag NPs, the storage capacity and rate performance of anodes were significantly improved. Among the three anodes prepared, the Ag3@MoS 2 anode demonstrated the best cycling performance retention capacity of 73% compared to that in the first cycle after 100 cycles. Moreover, this anode could restore~100% of the capacity after high rate performance. These results suggest that Ag-decorated MoS 2 can be a potential anode for a high-rate and high-stability anode in lithium storage applications in the future.