Single‐Layered MoS2 Fabricated by Charge‐Driven Interlayer Expansion for Superior Lithium/Sodium/Potassium‐Ion‐Battery Anodes

Abstract Single‐layered MoS2 is a promising anode material for lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), and potassium‐ion batteries (PIBs) due to its high capacity and isotropic ion transport paths. However, the low intrinsic conductivity and easy‐agglomerated feature hamper its applications. Here, a charge‐driven interlayer expansion strategy that Co2+ replaces Mo4+ in the doping form to endow MoS2 layers with negative charges, thus inducing electrostatic repulsion, together with the insertion of gaseous groups, to drive interlayer expansion which once breaks the confinement of interlayer van der Waals force, single‐layered MoS2 is obtained and uniformly dispersed into carbon matrix arising from the transformation of carbonaceous gaseous groups under high vapor pressure, is proposed. Co atom doping helps enhance the intrinsic conductivity of single‐layered MoS2. Carbon matrix effectively prevents agglomeration of single‐layered MoS2. The doped Co atoms can be fully transformed into ultrasmall Co nanoparticles during conversion reaction, which enables strong spin‐polarized surface capacitance and thus significantly boosts ion transport and storage. Consequently, the prepared material delivers superb Li/Na/K‐ion storage performances, which are best in the reported MoS2‐based anodes. The proposed charge‐driven interlayer expansion strategy provides a novel perspective for preparing single‐layered MoS2, which shows huge potential for energy storage.


Introduction
Rechargeable lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs) have been extensively studied and partially commercialized. [1,2,3] As the most popular anode material, graphite has shown huge success for LIBs over the past decade. However, graphite suffers from low specific capacity (372 mAh g −1 ) and unsatisfactory rate capability mainly caused by anisotropic lithium-ion storage paths and small interlayer distance (0.334 nm). [4] Graphite is becoming increasingly difficult to cope with the growing demand for high energy density and fast charging capability in LIBs. Meanwhile, graphite is not suitable as anode material for SIBs owing to the thermodynamic instability of the binary Na-intercalated graphite compounds, [5,6] and also exhibits low capacity of 279 mAh g −1 and poor rate performance for PIBs. [7] In this context, developing a universal high-performance anode material for LIBs, SIBs, and PIBs is of practical importance. MoS 2 is a potential anode material for LIBs, SIBs, and PIBs because of its high theoretical capacity (670 mAh g −1 ) and unique layered structure with a relatively large interlayer space of 0.62 nm. [8] However, there are still some obstacles to be addressed in developing high-performance MoS 2 electrodes. The main challenges arise from its unsatisfactory intrinsic electrical conductivity (EC), large ion diffusion barrier, and large volume change upon cycling. These issues result in poor rate capability and cycle stability and thus severely restrict its application in LIBs, SIBs, and PIBs. [9][10][11] Many different strategies including enlarging interlayer distance of MoS 2 , [12][13][14] constructing MoS 2 /graphene interoverlapped structure, [15,11] doping heteroatom, [16,17] fabricating fewlayered or even single-layered MoS 2 , [18,19] and compositing with carbon materials [20] have been implemented to tackle with the above-mentioned tricky problems. Although few-layered MoS 2 have larger interlayer spaces and are cheap in production, the adjacent layers form barriers to ion transport, resulting in much longer and more tortuous ion transport paths. On the contrary, the single-layered MoS 2 is completely exposed and easily accessible to the ions transported from all directions, providing short and easy ion transport paths. [21,22] In addition, the two surfaces of the single-layered MoS 2 are capable of storing ions, increasing the ion storage capacity. Previous reports also showed that the electrochemical properties of single-layered MoS 2 are obviously better than few-layered MoS 2 , [11,23] suggesting the great potential of single-layered MoS 2 on LIB/SIB/PIB applications. Many methods have been reported to prepare the single-layered MoS 2 , e.g., one-pot colloidal wet-chemical approach, [24] mechanical/chemical exfoliation, [25][26][27] and chemical vapor deposition. [28] Especially, the methods of using minerals as precursors based on top-down exfoliation process show great potential in the mass production of single-layered MoS 2 and other 2D materials. [29][30][31] Notwithstanding the outcomes in preparing single-layered MoS 2 , its poor intrinsic EC and serious restacking/agglomeration problems severely inhibit its application in batteries. To circumvent these difficulties, various techniques including solvothermal synthesis, [32] hydrothermal method, [33] and CTAB-assisted synthesis [34] have been involved to construct the sandwich structure of single-layered MoS 2 /graphene, which effectively improves EC of the electrode and avoids agglomeration of the single-layered MoS 2 and thereby enhanced electrochemical performances. However, such a sandwich structure reintroduces the interlayer ion diffusion barrier. Although the interlayer space of the adjacent MoS 2 layers in the sandwich structure is enlarged (≈0.96 to ≈1.15 nm) compared to the theoretical value (0.62 nm), the presence of graphene between adjacent MoS 2 layers makes the actual interlayer distance (from MoS 2 to graphene) available for ion transport to be only ≈0.53 nm, which is even smaller than that of bulk MoS 2 . This inevitably sets up barriers for the transport of Li + in the interlayer, let alone Na + and K + with a larger radius. Given this situation, further efforts such as electrospinning, [23] dual-template method, [35] and emulsiontemplated solvothermal method, [36] have been carried out to synthesize composites with uniformly dispersed single-layered MoS 2 in carbon matrix. This strategy eliminates the interlayer ion diffusion energy barrier while suppressing the agglomeration of single-layered MoS 2 . The corresponding electrochemical performances are also greatly improved based on the advantages of uniformly dispersed single-layered MoS 2 . Unfortunately, the intrinsic electrical conductivity of MoS 2 is not increased, far from being sufficient to perform fast charge storage, thus causing the insufficient release of the huge potential of single-layered MoS 2 in the capacity and rate performances. Hence, it is of very importance to develop a novel route to synthesize single-layered MoS 2 with enhanced intrinsic electrical conductivity and uniformly dispersed in carbon matrix to achieve superior Li/Na/K-ion storage performances, which will certainly boost the application pace of MoS 2 materials in LIBs, SIBs, and PIBs.
Heteroatom doping can enlarge the interlayer distance of layered transition metal sulfides due to the electrostatic repulsion between the adjacent layers caused by atomic substitution during the doping process, showing high potential in MoS 2 -based electrode material. [17] Recently, Li et al. demonstrated the presence of strong spin-polarized surface capacitance on Co nanoparticles in LIBs system. Specifically, the CoO electrode can generate ultrasmall Co nanoparticles during conversion reactions, and numerous spin-polarized electrons are injected into Co nanoparticles while lithium ions are stored in Li 2 O matrix, causing a spin capacitance. [37] It is reasonable to believe that a strategy that introduces Co atom into MoS 2 lattices in the form of doping cannot only induce the charge-driven interlayer expansion effect to obtain the single-layered MoS 2 , but also bring surface capacitance effect during conversion reactions, thus boosting ion transport and storage.
Herein, we propose a novel charge-driven interlayer expansion strategy to prepare single-layered MoS 2 with enhanced intrinsic EC in carbon matrix based on Co doping. Co 2+ replaces Mo 4+ of MoS 2 lattice to complete the doping process, which results in MoS 2 layers with negative charges to induce the interlayer electrostatic repulsion, together with the insertion of gaseous groups, to drive interlayer expansion. When the electrostatic repulsion is large enough to break the limitation of interlayer van der Waals forces, the Co-doped single-layered MoS 2 is obtained and uniformly dispersed in N, O co-doped carbon matrix (Co-SLMoS 2 /NOC). Utilizing Co-SLMoS 2 /NOC as anode material for LIBs, SIBs, and PIBs holds the following advantages: 1) the fully exposed single-layered MoS 2 basal planes can perform fast charge transport due to the disappearance of interlayer ion transport barrier; 2) Co doping hugely enhances the intrinsic EC of single-layered MoS 2 , thereby significantly accelerating electrons transfer; 3) Co-SLMoS 2 /NOC has a maximization of contact area between single-layered MoS 2 and carbon matrix to fully improve the electrical conductivity of single-layered MoS 2 , thus accelerating the transfer rate of charges; 4) the doped Co atoms in Co-SLMoS 2 /NOC can transform into ultrasmall Co nanoparticles (≈2 nm) during conversion reaction, which can generate strong spin-polarized surface capacitance to enhance ion transport and storage. As a result, the Co-SLMoS 2 /NOC delivers the best comprehensive electrochemical performances in terms of capacity, cyclability, and rate performances compared with the recently reported MoS 2 -based anode materials for LIBs, SIBs, and PIBs.

Fabrication of Samples
The samples are prepared by pyrolysis of the precursor solution containing distinct amounts of cobalt naphthenate, (NH 4 ) 2 MoS 4 , and N, N-dimethylformamide (DMF) through a facile one-step pressure-induced vapor synthetic route (Figure 1, detailed experimental content in Supporting Information). The corresponding microstructure characterizations of the prepared samples are shown in Figure 2. As shown in Figure 1a, (NH 4 ) 2 MoS 4 thermally decomposes to form multilayered MoS 2 (MLMoS 2 , ∼10 layers) with a typical interlayer space of 0.62 nm, which is confirmed by high-resolution transmission electron microscopy (HRTEM) observation (Figure 2b,c). After introducing DMF (Figure 1b), vast gaseous groups (OHCN, OHC, ·CH 3 , etc.) caused by the pyrolysis of DMF insert the MoS 2 interlayer, which induces the reduction of MoS 2 layers (3-5 layers, Figure 2d,e) and the enlargement of interlayer distance (0.96 nm, Figure 2f). Meanwhile, the pyrolysis products surrounding the few-layered MoS 2 can be transformed into N, O co-doped carbon matrix under large vapor pressure and annealing to obtain FLMoS 2 /NOC sample. Cobalt naphthenate, an organic ionic compound ( [(C 5 H 9 )(CH 2 ) n COO] 2 Co containing one positively charged Co 2+ and two negatively charged (C 5 H 9 )(CH 2 ) n COO − ), is further introduced into the system as the dopant to achieve Co doping for MoS 2 (Figure 1c). It is worth noting that Co 2+ replaces Mo 4+ of the MoS 2 lattice to make MoS 2 www.advancedsciencenews.com www.advancedscience.com layers with negative charges and leave negatively charged gaseous groups (carboxylic anions) in the interlayer, thus inducing interlayer electrostatic repulsion to achieve the charge-driven interlayer expansion for MoS 2 . As can be seen in Figures 2g-i, a further increased interlayer distance of 1.28 nm for few-layered MoS 2 can be obtained when introducing a small amount of cobalt naphthenate (named as Co-FLMoS 2 /NOC). Continue to increase the amount of cobalt naphthenate can increase the number of the negative charges to bring stronger interlayer electrostatic repulsion (Figure 1d), which is evidenced by the disappeared fewlayered MoS 2 and the formed numerous single-layered MoS 2 (Figures 2j,k), thus obtaining Co-SLMoS 2 /NOC sample. This means that the increased interlayer electrostatic repulsion far surpasses the interlayer van der Waals interaction of MoS 2 . It is clearly indicated that the present material form with the MoS 2 layers embedded in the carbon matrix avoids agglomeration of the MoS 2 layers (Figure 2k). Meanwhile, the single-layered MoS 2 is extremely small (a length size of ≈6.5 nm) and thin (a thickness size of ≈0.4 nm, in agreement with the standard distance data of single-layered MoS 2 (JCPDS 37-1492); Figure 2l). The lateral dimension of ≈6.5 nm enables extremely short lateral transport length of charges and small local electroactive mass. The latter point is crucial for the spin-polarized surface capacitance (discussed below). Note that the lateral size decreased from ≈12 nm of Co-FLMoS 2 /NOC to ≈6.5 nm of Co-SLMoS 2 /NOC. This may be because 1) the short time for MoS 2 growth is not enough to obtain the large-sized MoS 2 layers in the pressure-induced vapor synthetic system, and 2) the repulsion of Co-SLMoS 2 /NOC system is much larger than that of Co-FLMoS 2 /NOC system during the synthesis process (discussed below), which may inhibit the growth of MoS 2 , thus leading to a smaller size of MoS 2 in Co-SLMoS 2 /NOC compared to Co-FLMoS 2 /NOC. In addition, the structure of MoS 2 remains intact after Co doping, demonstrating the successful fabrication of Co-doped single-layered MoS 2 in Co-SLMoS 2 /NOC. The morphology of the synthesized samples is exhibited in Figure S1 (Supporting Information), irregular micronscale bulks are obtained for (NH 4 ) 2 MoS 4 and MLMoS 2 . However, FLMoS 2 /NOC, Co-FLMoS 2 /NOC, and Co-SLMoS 2 /NOC display a nano-sized morphology, which can effectively shorten the ion transport length and relieve the stress caused by the volume expansion of the electrode. Besides, the high-angle annular darkfield (HAADF) images and the energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirm the uniform distribution of all expected elements in FLMoS 2 /NOC ( Figure S2, Density functional theory (DFT) calculations are carried out to further evidence the reliability of the charge-driven interlayer expansion (the detailed computational method exhibited in Supporting Information). Specifically, we investigate the equilibrium interlayer spacing of adjacent MoS 2 with different additional charge concentrations caused by the Co doping process. For example, when two Co 2+ replace two Mo 4+ of adjacent MoS 2 layers to complete the doping, the two adjacent MoS 2 layers each carry two negative charges. Meanwhile, the donor (cobalt naphthenate) of these two Co 2+ provides four negative charges in the interlayer, together with the four negative charges on MoS 2 layers to leave eight negative charges for the MoS 2 model. The accumulated negative charges can induce interlayer electrostatic repulsion, leading to the expansion of the interlayer distance of MoS 2 . Note that the simulation of charge amounts by regulating the number of electrons in the model. In Figure 2m, MoS 2 maintains its typical interlayer distance of 0.62 nm under a vacuum layer thickness of 30 Å. For the model with enlarged interlayer distance (corresponding to Co-FLMoS 2 /NOC), a Co/Mo atomic ratio of 1:5 is constructed (from XPS analysis results in Table S2, Supporting Information), and the corresponding negative charges are also introduced to the MoS 2 model (Figure 2n). Note that the interlayer distance of MoS 2 calculated from the model should be lower than that in Co-FLMoS 2 /NOC. This is because the interlayer distance of MoS 2 in Co-FLMoS 2 /NOC is caused by the combined impetus of the insertion of gaseous groups and the charge-driven interlayer repulsion rather than the single interlayer repulsion set for the model. However, the calculated interlayer distance (1.44 nm) is slightly larger than in Co-FLMoS 2 /NOC (1.28 nm) under a similar Co doping amount (based on XPS results below). This may be because of the volume shrinkage arising from the conversion from gas to the solid phase under high vapor phase pressure and the incomplete insertion of negatively charged gaseous groups in the interlayer. Further increasing the number of Co atoms and negative charges in the model (≈1/2, atomic ratio of Co/Mo, Table S2, Supporting Information), a hugely increased interlayer distance of 2.36 nm is obtained (Figure 2o), which signals the formation of single-layered MoS 2 . [38] Importantly, the lattice constants of MoS 2 in models are well preserved after interlayer expansion and separation. Although there are some differences in the interlayer distance between the calculation in the model and the experimental samples, the variation trend of the interlayer distance keeps consistent under charge driving. This result demonstrates the effectiveness of the charge-driven interlayer expansion strategy. This expansion method is promising in the mass production of single-layered MoS 2 , which is dependent on the successful construction of the production equipment. It is expected that this method is of high potential to produce other 2D materials following a similar principle. Figure 2p presents the X-ray diffraction (XRD) pattern of the synthesized samples. All samples display the (100) and (110) diffraction peaks of 2H-MoS 2 (JCPDS: 37-1492), signaling the successful preparation of MoS 2 in these samples. The (002) diffraction peak of MLMoS 2 (14.3°), FLMoS 2 /NOC (9.2°), and Co-FLMoS 2 /NOC (7.0°) samples can be used to analyze their interlayer distance, and the corresponding values are 0.62, 0.96, and 1.26 nm, respectively (according to the Bragg's Law). No (002) diffraction peak can be detected in Co-SLMoS 2 /NOC, demonstrating the presence of single-layered MoS 2 . [23,36] These results strongly support that the single-layered MoS 2 can be prepared by the charge-driven interlayer expansion strategy and are consistent with the HRTEM results (Figure 2k,l). In addition, no graphitic peak at ≈25°can be detected, indicating the carbon phase in these samples is amorphous. Figure 2q exhibits the Raman spectra, in which two characteristic peaks at ≈380 and 405 cm −1 correspond to the in-plane vibration (E 1 2g ) and outof-plane vibration (A 1g ) of MoS 2 , respectively. [39] Note that the difference in peak frequency of E 1 2g and A 1g can offer layer number information for MoS 2 . [40] The frequency differences for FLMoS 2 /NOC and Co-FLMoS 2 /NOC are 22.4 and 21.9 cm −1 (≈3 layers), respectively, as compared with 26.2 cm −1 (>5 layers) for MLMoS 2 . The reduction of frequency difference indicates that FLMoS 2 /NOC and Co-FLMoS 2 /NOC possess a thinner layer structure than MLMoS 2 . [40] Especially, the frequency difference of Co-SLMoS 2 /NOC (20.5 cm −1 ) falls within the range of singlelayered MoS 2 (20.2-21.2 cm −1 ), [40] which further evidence that single-layered MoS 2 exists in Co-SLMoS 2 /NOC and is consistent with HRTEM ( Figure 2l) and XRD (Figure 2p) results. In addition, no carbon peaks can be detected in MLMoS 2 , whereas FLMoS 2 /NOC, Co-FLMoS 2 /NOC, and Co-SLMoS 2 /NOC show the characteristic peaks of D-band (≈1339.5 cm −1 ) and G-band (≈1588.3 cm −1 ) of carbon-based materials ( Figure S5, Supporting Information) with a relatively high I D /I G intensity ratio of ≈0.93 (Table S1, Supporting Information). This indicates rich structural defects in the carbon matrix because of N, O co-doping, which is conducive to the transport of electrons and ions. [41] Moreover, the chemical states of these samples are analyzed by X-ray photoelectron spectrometer (XPS). From Figure S6 and Table S2 Table S3 (Supporting Information) suggests that Co doping amounts in Co-FLMoS 2 /NOC and Co-SLMoS 2 /NOC are 5.42 and 11.30 at.%, respectively, which represents an increased tendency of Co doping amounts with the rise of cobalt naphthenate. In Figure S7a,b (Supporting Information), the peaks of Mo 3d (Mo 3d 5/2 , ≈229 eV; Mo 3d 3/2 , ≈232 eV) and S 2p (S 2p 3/2 , ≈162 eV; S 2p 1/2 , ≈163 eV) are attributed to Mo 4+ and S 2− , respectively, again indicating the formation of MoS 2 . [42] Two distinct peaks at 778.7 and 793.7 eV correspond to the Co 2p 3/2 and Co 2p 1/2 orbitals of CoMoS (Figure 2r), respectively, [43] denoting that the Co atoms are doped into MoS 2 lattices to obtain the CoMoS phase in Co-FLMoS 2 /NOC and Co-SLMoS 2 /NOC. In the high-resolution C 1s, N 1s, and O 1s spectra, peaks of 284.5 eV (C-C), 285.8 eV (C-N/C-O), 288.8 eV (C=O) ( Figures  S7c and S8, Supporting Information), 397.7-400.6 eV (pyridinic N, pyrrolic N, and graphitic N, Figures S7d and S9, Supporting Information), and 531.8 eV (C=O; Figure S7e, Supporting Information) confirm the successful doping of N and O into the carbon lattice. [44,45] The specific N and O doping amounts in the carbon matrix are shown in Table S4 Figure  S7f, Supporting Information). Furthermore, we also explore the effect of excessive Co doping on sample structure. As can be seen in Figure S10 (Supporting Information), the sample with excessive Co doping (≈3/4, Co/Mo atomic ratio, XPS result in Figure  S10c, Supporting Information) shows a bulk morphology (Figure S10a, Supporting Information). In Figure S10b Figure S10d). [46] Therefore, it is believed that the optimal value of Co/Mo atomic ratio for the preparation of single-layered MoS 2 in this system is ≈1:2.

Electrochemical Characterizations for LIBs
The electrochemical data including the first charge/discharge curves at 0.1 A g −1 (Figure 3a), cycling curves at 0.1 A g −1 (Figure 3b), and rate capability (Figures 3c-g) of these samples are tested. As revealed, MLMoS 2 shows quite poor first charge capacity (533.6 mAh g −1 ), cycle stability (28.4% capacity retention after 100 cycles), and rate capability (37.2 mAh g −1 at 20 A g −1 ) because of its micron-sized bulk feature and the lack of carbon matrix, consistent with the previous report. [33] After compounding few-layered MoS 2 with carbon matrix, FLMoS 2 /NOC exhibits obvious enhancement in first charge capacity (865.2 mAh g −1 ), cycle stability (112.0% capacity retention after 100 cycles), and rate capability (241.5 mAh g −1 at 20 A g −1 ), which are much higher than that of MLMoS 2 . The increased capacity during cycling may be ascribed to the formation of gel-like polymeric layer, and the increase of interfaces with increasing the cycle number, which can act as active storage sites of lithium ion. [16,47] The increase of active interfaces may arise from 1) the appearance of more exposed surfaces as the activation of FLMoS 2 /NOC electrode upon cycling, 2) the redistribution of sulfur species, 3) the formation of ultrasmall Mo nanoparticles during conversion reaction (discussed below). Further introducing Co doping, a better improvement in initial charge capacity (1165.6 mAh g −1 ) and rate capability (536.2 mAh g −1 at 20 A g −1 ) are achieved for Co-FLMoS 2 /NOC due in large part to the strong spin-polarized surface capacitance effect (confirmed in Figure 4) caused by Co doping and large interlayer distance. Remarkably, Co-SLMoS 2 /NOC exhibits unprecedented electrochemical performances, far superior to Co-FLMoS 2 /NOC, FLMoS 2 /NOC, and MLMoS 2 . Specifically, Co-SLMoS 2 /NOC shows an ultrahigh first charge capacity of 1520.1 mAh g −1 with an initial Coulombic efficiency (CE) of 76.3% (Figure 3a), and a high reversible capacity of 1596.2 mAh g −1 after 100 cycles (Figure 3b). When gradually increasing the current density from 0.1 to 20 A g −1 , the charge/discharge profiles at each current density remain stable and deliver a superior rate capacity of 1068.2 mAh g −1 at 20 A g −1 (≈70.3% capacity retention relative to the capacity obtained at 0.1 A g −1 ). Such superior electrochemical performances of Co-SLMoS 2 /NOC can be attributed to 1) its vanishing interlayer ion diffusion barrier, more efficient open ion diffusion mode, lower charge transfer resistance (R ct ; Figure 3h,i), higher EC (Table S5, Supporting Information), and bigger specific surface area ( Figure S11, Supporting Information) facilitate the ion transport on the MoS 2 plane, and 2) stronger spin-polarized surface capacitance is induced due to the formation of smaller Co nanoparticles (discussed below) during the conversion reaction, which hugely boosts ion transport and storage. We point out in passing that the first charge capacity of Co-doped single-layered MoS 2 can reach 1775.0 mAh g −1 www.advancedsciencenews.com www.advancedscience.com when removing the contribution of the N, O co-doped carbon matrix in Co-SLMoS 2 /NOC (detailed discussion in Figures S12 and S13, Supporting Information). So high capacity of Co-doped single-layered MoS 2 is unprecedented, fully demonstrating the sufficient release of the huge potential of single-layered MoS 2 in the capacity by using the charge-driven interlayer expansion strategy. Remarkably, the Co-SLMoS 2 /NOC has an ultralow volume swelling of 10.3% in electrode thickness, while the MLMoS 2 exhibits a thickness change of 82.5% after 100 cycles ( Figure S14, Supporting Information), confirming that the structural superiority of Co-SLMoS 2 /NOC as LIB anode.

Study on the Kinetics and Behavior of Li-Ion Storage in Co-SLMoS 2 /NOC
To explain the reason of the excellent rate capability and high capacity of Co-SLMoS 2 /NOC, electrochemical kinetic studies and Li-ion storage behavior in Co-SLMoS 2 /NOC are investigated.  (1): [48 ] i = av b (1) where i is the current density, v is the scan rate, and a and b are empirical constants. When the b-value is close to 1, the cell system is mainly controlled by the capacitance, and when the b-value approaches 0.5, ion diffusion-controlled process dominates. In Figure 4b, the b-values of peaks 1-5 in both cathodic and anodic processes for Co-SLMoS 2 /NOC are 0.90, 0.85, 0.97, 0.98, and 0.95, respectively. This suggests that Li-ion storage dynamics are mainly controlled by the capacitive process. Moreover, the capacitive contribution can be further quantified according to Equation (2). [49] i where i(V), k 1 v, and k 2 v 1/2 represent the total current at a fixed potential, the pseudocapacitance behavior, and the diffusioncontrolled process, respectively. With the increase of the sweep rate from 0.1 to 1.0 mV s −1 , the capacitive contribution enlarges from 56.3% to 85.3% (Figure 4c), indicating that capacitive behavior is essential to the high-rate capability. In addition, Co-SLMoS 2 /NOC also shows huge advantages in capacitive contribution when compared with MLMoS 2 (4.3% to 14.1%, Figure  S15, Supporting Information), FLMoS 2 /NOC (24.2% to 46.4%, Figure S16, Supporting Information), and Co-FLMoS 2 /NOC (41.3% to 67.7%, Figure S17, Supporting Information), further evidence that capacitive behavior boosts Li-ion transport and storage in Co-SLMoS 2 /NOC. The Li-ion diffusion coefficient (D Li + ) is calculated to confirm the fast Li-ion transport dynamics according to Equation (3). [50,51] i p = 2.69 × 10 5 n 3∕2 AD 1∕2 where i p , v, A, n, and C Li+ represent the peak current, the sweep rate, the contact area between the electrolyte and active materials, the number of electrons in the electrochemical reaction, and the Li + bulk concentration (calculated in Supporting information), respectively. According to the linear relationship of i p versus v 1/2 of the redox peaks ( Figure S18, Supporting Information), the values of D Li+ for Peaks 1-5 range from 2.65 × 10 −10 to 1.97 × 10 −9 cm 2 s −1 . So high D Li+ strongly supports the fact of the high-rate capability of Co-SLMoS 2 /NOC. Notably, Co-SLMoS 2 /NOC also shows huge advantages when comparing the obtained values of D Li + with recently reported MoS 2 -based anode materials (Table  S7, Supporting Information).
Investigating the conversion reaction of Co-SLMoS 2 /NOC in LIBs system is of great interest. As exhibited in the ex situ TEM observation (Figure 4d,e, and Figure S19, Supporting Information), after discharging to 0.01 V, well-dispersed nanoparticles with an average size of ≈2 nm can be observed. The diffraction rings of (310) and (321) Figure 4f) indicates that the nanoparticles produced in conversion reaction contain Mo and Co. Note that Co and Mo nanoparticles produced from Co-SLMoS 2 /NOC can induce the formation of the space charge zone to store extra Li + . [37,52] Specifically, the d-orbit electrons of Co and Mo are not full, spin-polarized electrons can be injected into the d orbits of the metallic Co and Mo nanoparticles with a depth of the Thomas-Fermi screening length in an electric field. Meanwhile, lithium ions are stored at the surfaces of the Co and Mo nanoparticles to construct the space charge zone, thus leading to the spin-polarized surface capacitance. [37] An interesting feature of space-charge storage is that it possesses the potential to be very fast, which is because ions and electrons in the space charge zone travel along separate pathways rather than in bulk storage where ions and electrons must travel in the same phase. [52] To evidence the formation of the space charge zone, ex situ XPS spectra of Co-SLMoS 2 /NOC electrode at various discharge/charge states are performed and exhibited in Figure 4g,h. In the pristine state, the binding energy of Mo 3d and Co 2p contains 229.0 eV (Mo 3d 5/2 ), 232.0 eV (Mo 3d 3/2 ), 778.7 eV (Co 2p 3/2 ), and 793.7 eV (Co 2p 1/2 ). When the electrode is fully discharged to 0.01 V, these characteristic peaks shift obviously to lower binding energy (228.3 eV, Mo 0 3d 5/2 ; 231.3 eV, Mo 0 3d 3/2 ; 777.8 eV, Co 0 2p 3/2 ; 793.3 eV, Co 0 2p 1/2 ), confirming the production of metallic Mo and Co due to the conversion reaction. [37,53,54] In the fully charged state, these peaks show a further negative shift (227.9 eV, Mo 0 3d 5/2 ; 230.8 eV, Mo 0 3d 3/2 ; 777.0 eV, Co 0 2p 3/2 ; and 792.0 eV, Co 0 2p 1/2 ). This may be because numerous positively charged lithium ions stored on the surface of Mo and Co nanoparticles increase the binding energy. Therefore, the binding energy shows negative shift after the delithiation from the Mo and Co particle surface. These results demonstrate the formation of the space charge zone. [37] Because the magnetism is sensitive to the change in the phase, structure, and electronic state of transition metal-based materials, [55] the Li-ion storage behavior of the Co-SLMoS 2 /NOC electrode is further studied by magnetic characterization. As shown in the ex situ magnetic hysteresis loops in Figure 4i, a pronounced increase in magnetization for Co-SLMoS 2 /NOC can be detected from ≈0 emu g −1 (before cycling) to 12.6 emu g −1 (after discharging to 0.01 V), signaling the formation of Mo and Co nanoparticles due to the reduction of Mo 4+ and Co 2+ . [37] After charging to 3 V, the magnetization presents a further increase www.advancedsciencenews.com www.advancedscience.com to 17.7 emu g −1 . Specifically, many extra electrons stored in Mo and Co nanoparticles partially cancel out the spin majority bands of the 4d (Mo) and 3d (Co) energy levels, reducing the magnetization. Therefore, the magnetization shows a further increase after the delithiation from the surface of Mo and Co nanoparticles, which further confirms the formation of the space charge zone. [37] To reveal the main responsible for the strong surface capacitance effect of the Co-SLMoS 2 /NOC electrode, MLMoS 2 , FLMoS 2 /NOC, and Co-FLMoS 2 /NOC electrodes are also investigated by ex situ TEM and XPS spectra. After the full discharge, the size of Mo nanoparticles generated from MLMoS 2 is ≈10 nm (Figure S20, Supporting Information), while that from FLMoS 2 /NOC is ≈3.0 nm ( Figure S21, Supporting Information), suggesting the nanoscale few-layered structure of MoS 2 and the presence of carbon matrix effectively inhibit the growth and aggregation of Mo nanoparticles during the conversion reaction. The smaller Mo nanoparticle with larger specific surface area can provide more interfaces for extra Li + storage and result in a stronger surface-capacitance effect, [56] which is confirmed by XPS spectra and magnetic hysteresis loops (detailed discussion in Figures S22 and 23, Supporting Information). After introducing a small amount of Co doping, Co and Mo nanoparticles generated from Co-FLMoS 2 /NOC show an average size of ≈3.0 nm ( Figure S24, Supporting Information), which is similar to the size of Mo nanoparticles produced from FLMoS 2 /NOC ( Figure  S21, Supporting Information). However, the magnetization of Co-FLMoS 2 /NOC is much higher than that of FLMoS 2 /NOC after discharge (0.56 emu g −1 , FLMoS 2 /NOC; 6.6 emu g −1 , Co-FLMoS 2 /NOC) and charge process (1.31 emu g −1 , FLMoS 2 /NOC; 11.1 emu g −1 , Co-FLMoS 2 /NOC), as shown in Figures S23  and S25 (Supporting Information). Theoretically, higher electrode magnetization enables more spin-polarized electrons to inject into d orbits of transition metal,resulting in a larger space charge zone, thereby enhancing the spin-polarized surface capacitance. [37] This means that the strong spin-polarized surface capacitance is mainly caused by the small amounts of Co nanoparticles. Notably, compared to Co-FLMoS 2 /NOC electrode, Co-SLMoS 2 /NOC electrode with a higher Co doping amount (Table S3, Supporting Information) produces more Co nanoparticles with a smaller average size of ≈2 nm after fully discharging (Figure 4e). The smaller nanoparticles generated from Co-SLMoS 2 /NOC electrode are due largely to the smaller MoS 2 microcell (lateral dimension×thickness: ≈6.5 × 0.4 nm, in Co-SLMoS 2 /NOC, Figure 2l; ≈12×5 nm, in Co-FLMoS 2 /NOC, Figure 2h) and the confinement of carbon matrix. With the synergistic effect of the smaller size and larger amount of Co nanoparticles, the Co-SLMoS 2 /NOC electrode exhibits higher magnetization than the Co-FLMoS 2 /NOC electrode after discharge (6.6 emu g −1 , Co-FLMoS 2 /NOC; 12.6 emu g −1 , Co-SLMoS 2 /NOC) and charge process (11.1 emu g −1 , Co-FLMoS 2 /NOC; 17.7 emu g −1 , Co-SLMoS 2 /NOC), thereby causing stronger spin-polarized surface capacitance.

Practical Exploration of Co-SLMoS2/NOC in LIBs
We further explore the potential of Co-SLMoS 2 /NOC for practical applications in view of its superior electrochemical perfor-mances and advanced Li-ion storage mechanism. As can be seen, Co-SLMoS 2 /NOC delivers a reversible capacity of 1372.2 mAh g −1 at 1 A g −1 over 1000 cycles with a high capacity retention of 98.9% (Figure 5a). Even at 5 A g −1 , Co-SLMoS 2 /NOC still delivers a high capacity of 1084.3 mAh g −1 after 3000 charge-discharge cycles (90.1%, capacity retention, Figure 5b), denoting excellent stability during repeated rapid charge and discharge process. When comparing the performances of Co-SLMoS 2 /NOC with recently reported MoS 2 -based anode materials for LIBs ( Figure 5c and Table S8, Supporting Information), Co-SLMoS 2 /NOC shows huge advantages in terms of capacity, cyclability, and rate capability. In practice, the areal capacity is a crucial criterion to evaluate the application potential of the LIBs electrode materials. The areal capacity strongly depends on the areal mass loading of the electrode materials. Consequently, the cycling stability of the Co-SLMoS 2 /NOC with active mass loading ranging from 2.2 to 4.6 mg cm −2 is tested (Figure 5d). Obviously, Co-SLMoS 2 /NOC exhibits good stability with high mass loading, and especially a high areal capacity of 5.26 mAh cm −2 still remains for loading 4.6 mg cm −2 after 100 cycles at 0.1 A g −1 (Figure 5d). Even after long-term cycling (500 cycles) at 5 A g −1 , the electrode (4.6 mg cm −2 ) still maintains a capacity of 706.6 mAh g −1 with a capacity retention of 83.8% (Figure 5e). In addition, Figure 5f shows that the electrode with high loading (4.6 mg cm −2 ) also exhibits excellent rate capability (675.9 mAh g −1 at 20 A g −1 ), further confirming the potential value of Co-SLMoS 2 /NOC in LIBs application. Afterward, the full cells are assembled with commercial LiFePO 4 cathode to further assess the application potential of the Co-SLMoS 2 /NOC anode, which successfully powers the light-emitting diode array (Figure 5g). During the first and 100th cycles, the full cell presents almost overlapped charge/discharge curves, which indicates stable electrode structure upon cycling (Figure 5g). A high reversible capacity of 160.0 mAh g −1 can be obtained with a capacity retention of 97.2% after 100 cycles at 0.1 C (Figure 5h). As being tested at 1 C, the capacity can still remain at 141.1 mAh g −1 after 200 cycles ( Figure S26, Supporting Information), signaling the outstanding cycle stability of the full cell. The full cell also exhibits excellent rate performance. Upon gradually elevating the current density, the galvanostatic discharge-charge voltage profiles of the full cell remain stable ( Figure S27, Supporting Information), and a high capacity of 135.8 mAh g −1 is obtained at 3 C (Figure 5i). After reducing the current density back to 0.1 C, the capacity of the full cell rapidly increases to 163.0 mAh g −1 , implying a fine capacity recovery. After charging for 15.3 min at 3 C, the full cell shows a high energy density of 125.5 Wh kg −1 (the calculation of the energy density presented in Supporting information), which is 76.5% of the energy density at 0.1 C (164.0 Wh kg −1 ; Figure 5j). Moreover, a high reversible capacity of 109.6 mAh g −1 can still be maintained after 500 cycles at 3 C (81.9%, capacity retention, Figure 5k). The full-cell performances are superior to previous reports on full cells when MoS 2 -based materials as the anode (Figure 5l and Table S9, Supporting Information), fully demonstrating the excellent high-rate stability and high practical potential of Co-SLMoS 2 /NOC.
We further investigate the behavior of Li-ion diffusion on MoS 2 plane of Co-SLMoS 2 /NOC. The DFT calculations are carried out to provide Li-ion diffusion energy barriers for undoped and Codoped single-layered MoS 2 , in which the doping amount of Co   obeys the XPS results (Table S2, Supporting Information). Figure 6a-c shows the relative energy curves along the diffusion pathways, as can be seen, the Li-ion diffusion barrier of Co-doped single-layered MoS 2 at each site is much lower than that of pure single-layered MoS 2 , implying Co doping can hugely improve the Li-ion diffusion kinetics, which results in an excellent Liion transport capability. In addition, the doped Co atoms can obviously reduce the bandgap of single-layered MoS 2 to enable high carrier transportation, as presented in the calculated density of states (DOS) in Figure 6d. The pure single-layered MoS 2 possesses a semiconductor feature (≈1.40 eV, bandgap), while the metallic property is achieved after introducing Co atoms in the MoS 2 lattices (0 eV, bandgap). Co-doping can therefore improve carrier transportation in single-layered MoS 2 . This is mainly due to Co doping can interfere with the surrounding atoms and induce lattice distortion of MoS 2 , thus tuning the electronic structure. [57,58] Figure 6e shows the Li-ion diffusion mechanism in Co-SLMoS 2 /NOC. The amorphous carbon possesses a large number of defects, through which the ions are transported to the MoS 2 layers and stored at every active spot on the surface of both sides. The presence of the Co atoms in the MoS 2 layers accelerates this process as the Co atoms present electronegativity due to the presence of the excess electrons. Meanwhile, Co doping generates a strong spin-polarized surface capacitance effect on Co nanoparticles (Figure 6f). The Li storage performance is effectively enhanced by the above factors.

Conclusions
In summary, a novel charge-driven interlayer expansion strategy is proposed for fabricating single-layered MoS 2 based on Co doping, which induces strong electrostatic repulsion to break the confinement of interlayer van der Waals forces, and is www.advancedsciencenews.com www.advancedscience.com demonstrated by DFT calculation. Co-doped single-layered MoS 2 with fully exposed MoS 2 basal planes, increased intrinsic EC, low lithium ion diffusion energy barrier, and vanishing interlayer ion diffusion barrier hugely boosts ion transport and storage capability. The Co-doped single-layered MoS 2 uniformly dispersed into N, O co-doped carbon matrix is highly stable during cycling, which shows an ultralow volume expansion of 10.3% after 100 cycles. The doped magnetic Co atom in single-layered MoS 2 lattices can transform into ultrasmall Co nanoparticles (≈2 nm) during conversion reaction, which induces strong spin-polarized surface capacitance to further enhance ion transport and storage. As a consequence, the Co-SLMoS 2 /NOC electrode delivers superior electrochemical performances than all the previous reports on MoS 2 -based anode for Li/Na/K-ion batteries. The proposed single-layered MoS 2 synthesis strategy may be generalized to other 2D layered materials stabilized by weak van der Waals attraction. This work may provide new insight into the design of single-layered transition metal dichalcogenides for rechargeable ion batteries.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.