Edge-enriched MoS2 for kinetics-enhanced potassium storage

Potassium-ion batteries (PIBs) hold great promise as alternatives to lithium ion batteries in post-lithium age, while face challenges of slow reaction kinetics induced by the inherent characteristics of large-size K+. We herein show that creating sufficient exposed edges in MoS2 via constructing ordered mesoporous architecture greatly favors for improved kinetics as well as increased reactive sites for K storage. The engineered MoS2 with edge-enriched planes (EE-MoS2) is featured by three-dimensional bicontinuous frameworks with ordered mesopores of ~ 5.0 nm surrounded by thin wall of ~9.0 nm. Importantly, EE-MoS2 permits exposure of enormous edge planes at pore walls, renders its intrinsic layer spacing more accessible for K+ and accelerates conversion kinetics, thus realizing enhanced capacity and high rate capability. Impressively, EE-MoS2 displays a high reversible charge capacity of 506 mAh·g−1 at 0.05 A·g−1, superior cycling capacities of 321 mAh·g−1 at 1.0 A·g−1 after 200 cycles and a capacity of 250 mAh·g−1 at 2.0 A·g−1, outperforming edge-deficient MoS2 with nonporous bulk structure. This work enlightens the nanoarchitecture design with abundant edges for improving electrochemical properties and provides a paradigm for exploring high-performance PIBs.


Introduction
As one of the most promising substitutes for commercial lithium-ion batteries (LIBs), potassium-ion batteries (PIBs) have captured increasing attention owning to natural abundance of potassium in contrast to lithium (17,000 vs. 20 ppm in the earth crust) [1]. Additionally, an intrinsic merit lies in that the redox potential of K + /K (−2.93 V vs. standard hydrogen electrode, E o ) is quite close to that of Li + /Li (−3.04 V vs. E o ), endowing PIBs with a high working voltage and energy density analogue to LIBs [2][3][4][5]. Nonetheless, the large radius of K + (1.38 Å), approximately twice as that of Li + (0.76 Å), undoubtedly retards "rocking chair" chemistry kinetics and induces acute volume expansion of electrode [6][7][8][9], which renders the promising PIBs into a dilemma. Hence, resolving the above-mentioned problems is crucial but challenging for the further development of PIBs.
Recent years have witnessed the enormous endeavors devoted to exploring high-performance anodes for PIBs. Given the low cost and easy availability, carbon materials have been investigated, however, their theoretical specific capacity (278 mAh·g -1 ) remains unsatisfactory according to the formation of one-stage KC8 in graphite structure via the intercalation chemistry [10][11][12]. In contrast, materials involved in the conversion or alloying chemistry for K + storage are anticipated to provide much higher capacity, such as transition metal dichalcogenides (TMDs) [13,14] and phosphides (TMPs) [15,16]. TMDs have been receiving extensive attention owing to their potential advantages of layered structures with large interlayer spacing, which is favorable for ions diffusion and conversion kinetics [17]. However, a vast number of layers with compact stacking by van der Waals interactions undoubtedly pose limitations to the efficient utilization of TMDs in micrometer size, especially at high rate operations for electrochemical energy storage. To make full utilization of interlayer space in TMDs, exposing more sites/edges is a promising way, including (1) constructing TMDs nanostructures like hollow tubes [18], microspheres [19] and yolk-shell structure [12] and (2) further expanding the interlayer spacing via intercalation technology, such as increasing the interspace of MoS2 from 0.64 to 0.92 and 1.085 nm [20,21]. However, large gaps still exist between the achieved results and their theoretical capacity (e.g., 670 mAh·g −1 for MoS2). For instance, MoS2@CT with enlarged layer spacing of 1.085 nm exhibited a capacity of only 232 mAh·g −1 at 1.0 A·g −1 [20]. Even with addition of conductive carbon, the performance is still limited to a capacity of 131 mAh·g −1 at 2.0 A·g −1 for MoS2/ N-doped-C [18]. Evidently, the above structure designs are still incapable of accomplishing the maximum utilization of laminar MoS2 on account of inferior ion diffusion and limited active sites. Recently, the concept of edge engineering has been demonstrated to play a great role in boosting electrochemical activities, such as constraining Li dendrites [22], improving performance of aluminum batteries [23], Na/Li batteries [24] and Li-S batteries [25]. It could thus be envisioned while not yet explored that designing structures with highly exposed edges could resolve the sluggish kinetics and limited capacity caused by large-size K + , considering that plenty of edge planes could offer more easy access of K + to promote ions diffusion and increase reactive sites in MoS2. Therefore, elaborate engineering of MoS2 with more exposed edge planes is highly promising to further facilitate the utilization of two-dimensional layered structure for boosting K storage capability.
Herein, we propose a strategy of constructing three-dimensional (3D) bicontinuous MoS2 skeletons at mesoscale to expose enriched edges (EE-MoS2) by template-assisted approach, which was found helpful to address the challenge of sluggish kinetics and limited active sites. Mesoscale MoS2 skeletons with thickness about 9.0 nm, corresponding to ~ 15 layers theoretically, enable more edge planes such as (002) plane to expose as compared with edge-deficient MoS2 (ED-MoS2). Combined with regular mesopores of ~ 5 nm surrounded by mesoscale skeleton, these exposed edges planes at mesopore walls would provide great potential in enhancing kinetics and K + storage capacity. As expected, the electrochemical performances reveal that the as-prepared EE-MoS2 maintains a high stable capacity of 321 mAh·g −1 that is twice as high as 145 mAh·g −1 of ED-MoS2 at 1.0 A·g −1 after 200 cycles. The prominent capacity of 245 mAh·g −1 (the 25 th cycle for rate test) at higher rate of 2.0 A·g −1 has also been achieved for EE-MoS2, representing one of the best performances for TMDs-based materials for PIBs anode. Our finding suggests that fabricating edge-enriched planes may open a new avenue to obtain advanced electrochemical energy storage materials.

Results and discussion
Conceptually, by confining the MoS2 skeleton at mesoscale, the number of stacked layers within skeleton will decrease considerably exposing more edge planes at the mesopore walls, thus enhancing kinetics by promoted K + diffusion and offering more accessible active sites for conversion reaction, as illustrated in Fig. 1. In contrast, bulk and nonporous ED-MoS2 in micrometers exhibit large extended sheets without mesoscale skeleton, indicative of limited edge planes/sites for facilitating K storage. According to this concept, we constructed EE-MoS2 by hard template-assisted approach. The starting Mo compound, phosphomolybdic acid hydrate (H3PO4·12MoO3, PMA), was exerted a smooth conversion into EE-MoS2 via an initial impregnation into KIT-6, followed by calcination, sulfidation and the final template removal (Fig. S1 in the Electronic Supplementary Material (ESM)). As schematically depicted in Fig. 2(a), EE-MoS2 has a 3D bicontinuous interconnected porous structure and the mesoscale skeleton is composed of the inherent laminar structure stacked by van der Waals interaction, thus exposing more edge planes at the mesopore walls, as shown in side view and top view. Scanning electron microscopy (SEM) image reveals the porous structure of EE-MoS2, while ED-MoS2 exists in the bulk particles ( Fig. S2 in the ESM). The regular interlinked skeletons and periodic mesopores can be confirmed by transmission electron microscopy (TEM) image, in which stripe-like structures with space groups P6mm and Ia3d are present [26] (Fig. 2(b)), illustrating the porous ordered nanoarchitecture of EE-MoS2. In a magnified TEM image ( Fig. 2(c)), the thickness of interconnected skeletons (mainly composed of (002) plane) was measured to be about 9.0 nm, corresponding  to only ~ 15 layers. The reduction of layers cuts down ions migration distance and exposes more edges, facilitating conversion kinetics and increasing reactive sites. And the size of mesopores surrounded by these skeletons was measured to be ~ 5.0 nm. In a high-resolution TEM (HRTEM) image ( Fig. 2(d)), the crystalline lattices with spacings of 2.61 and 6.20 Å are indexed as (100) and (002) planes of MoS2, respectively [27,28]. As is observed from high-angle annular dark-field (HAADF) image and energy dispersive spectra (EDS) mappings, only Mo and S are uniformly distributed, demonstrating the phase purity with complete removal of the template (Figs. 2(e)-2(g)).
The phase characteristics was furthermore evaluated by X-ray diffraction (XRD) pattern, which shows the main Bragg peaks at 14.4°, 32.7°, 39.5° and 58.3° ( Fig. 2(h)), attributed to respective (002), (100), (103) and (110) planes of the hexagonal MoS2 with the space group P63/mmc (194) (JPCDS No. 37-1492) [27,29]. For the pore properties of EE-MoS2, the sample bears a H1-type hysteresis loop ranging from 0.45 to 0.90 (P/P0 value) assigned to the type IV isotherms via N2 adsorption/desorption test ( Fig. 2(i)), further corroborating the mesoporous characteristics of EE-MoS2 [30]. The mesoporous feature endows EE-MoS2 with a large Brunauer-Emmett-Teller (BET) surface area of 124 m 2 ·g −1 and average pore size is centered at ~ 4.8 nm (Table S1 in the ESM and Fig. 2(j)), consistent with the TEM result. To our best knowledge, EE-MoS2 owns the highest BET surface area among the reported pure MoS2. To exhibit the overwhelming superiority of EE-MoS2 in K storage, ED-MoS2 was also synthesized via the direct decomposition and subsequent sulfidation of PMA without template (Fig. S3 in the ESM), which exists in bulk form with nonporous structure and extended sheets in micrometer size range.
Cyclic voltammetry (CV) test was conducted to unravel K storage behaviors (Fig. S4 in the ESM). Two observable peaks at ~ 0.78 and ~ 0.45 V, in the first potassiation process, can be attributed to the irreversible reaction of solid electrolyte interphase (SEI) layer and the conversion from MoS2 to Mo and K2S [31]. During the depotassiation scan, the oxidation peak located at ~ 1.74 V corresponds to the transformation of reduced products back into MoS2 [32]. The well overlapped CV profiles of 2 nd and 3 rd cycles verify the prominent electrochemical reversibility. Figure 3(a) describes the galvanostatic charge/discharge profiles of 3 rd cycle at 0.1 A·g −1 for MoS2. EE-MoS2 possesses a lower polarization voltage (e.g., 1.04 V) than ED-MoS2 (e.g., 1.65 V), indicating that the plenteous edge planes decrease the polarization. Rate performance is an important index for evaluating reaction kinetics of electrode materials. As to EE-MoS2, the average capacities of 484, 409, 393 and 342 mAh·g −1 at stepwise current densities from 0.1 to 1.0 A·g −1 were obtained, respectively ( Fig. 3(b) and Fig. S5(a) in the ESM). Strikingly, the average capacity of 250 mAh·g −1 is still achieved at a higher current density of 2.0 A·g −1 . And the capacity recovers completely when the current density switches back to 0.1 A·g −1 , even higher than its original level (e.g., 620 mAh·g −1 ). In contrast, ED-MoS2 displays lower capacities than EE-MoS2 at various rates ( Fig. 3(b) and Fig. S5(b) in the ESM). Figure 3(c) depicts the superior rate performance of EE-MoS2 over previously reported TMDs and the other anode materials ( Table S2 in the ESM). The rate performance of EE-MoS2 exceeds the majority of TMDs, including MoS2 [18,20,21,31,32], MoSe2 [17,33,34], VSe2 [35], NiS2 [36], and even those composited with conductive carbons. Considering that the rate capability is governed by both electronic and ionic conductivity, the superior rate capability of EE-MoS2 without compositing with carbon further highlights the importance of edge engineering for boosting K + transport/diffusion. As for the cycling stability, the variations of discharge capacity of EE-MoS2 and ED-MoS2 are described in Fig. 3(d). Different from ED-MoS2 with solid bulk particles, EE-MoS2 undergoes a capacity increase in the first 100 cycles probably due to the activation process with electrolyte penetration into the interior of porous EE-MoS2. The firstcycle discharge and charge capacities of 713 and 506 mAh·g −1 were obtained at 0.05 A·g −1 , respectively, corresponding to an initial Coulombic efficiency (ICE) of 71.0% for EE-MoS2 (Fig. S6(a) in the ESM). However, under the same conditions, ED-MoS2 reveals a discharge capacity of 603 mAh·g −1 with lower ICE of 55.3% (Fig. S6(b) in the ESM). Generally, increasing the surface area will give rise to the irreversible electrolyte decomposition and thus a reduced ICE [37]. In our case, although EE-MoS2 exhibits 31-time higher surface area than ED-MoS2, its ICE is still much higher than that of edge-deficient one, manifesting the edges actually could mitigate the undesirable side reactions with more K + participating in reversible storage. After the initial two cycles at 0.05 A·g −1 , the cells were further cycled at 1.0 A·g −1 , and a high and stable capacity of 321 mAh·g −1 is retained after 200 cycles with Coulombic efficiency over 99% during the whole cycling, demonstrating the stability of such edge-enriched structures for reversible K storage. The capacity is 2.2 times as high as that of ED-MoS2 with 145 mAh·g −1 under the same conditions (Fig. S6(c) in the ESM). The prominent cycling stability and high capacity are also confirmed by welloverlapped charge/discharge curves of EE-MoS2 at different cycles at 1.0 A·g −1 (Fig. 3(e)). Such an enhancement on electrochemical performance could be originated from the prominent conversion activity promoted by the increased edge planes. As is depicted in Fig. 3(f) and Fig. S7 in the ESM, all the electrochemical impedance spectroscopy (EIS) plots are comprised of a deformed semicircle in high-medium frequency domain and an oblique line in low frequency region for both anodes [17]. The equivalent circuit models for EIS spectra are described in the inset. In fresh batteries, EE-MoS2 possesses charge transfer resistance (Rct) of ~ 6,700 Ohm, clearly lower than that (e.g., ~ 9,000 Ohm) of ED-MoS2 (Fig. S7 in the ESM). More importantly, Rct decreases to ~ 2,533 Ohm after 40 th cycles for the edge-enriched one, whereas Rct of the edge-deficient counterpart decreases to only ~ 4,093 Ohm (Table S3 in the ESM). And the Rct value of EE-MoS2 is also lower than that of some reported works ( Table S4 in   ED-MoS2 within the equilibrium voltage hysteresis, indicative of a higher energy conversion efficiency and a lower polarization for EE-MoS2 [38]. Moreover, the edge-enriched sample has lower reaction internal resistances than the edge-deficient one in both charge/discharge processes (Fig. S9 in the ESM), suggesting the faster reaction kinetics over that of ED-MoS2 [39]. In addition, K + diffusion coefficients (~ 10 −11 cm 2 ·s −1 ) of EE-MoS2 are higher than that (~ 10 −13 cm 2 ·s −1 ) of the ED-MoS2 counterpart (Fig. 4(c)). It is inferred that this remarkable promotion in diffusion coefficient stems from edge-enriched planes with easy access for K + . To further verify this point, ratescan CV was carried out to calculate the diffusion-dominated contribution to capacity (Fig. 4(d) and Fig. S10(a) in the ESM).
The b values of peaks A and C are 0.78 and 0.94 (calculated according to Eq. S(1) in the ESM) for EE-MoS2 (Fig. 4(e)), respectively, suggesting the simultaneous presence of diffusiondominated and capacitive contributions to capacity [40][41][42]. Whereas the b values of peaks A and C are 0.91 (diffusiondominated and capacitive contributions) and 1.13 (only capacitive contribution) for ED-MoS2 (Fig. S10(b) in the ESM), respectively. For EE-MoS2, the diffusion-dominated fraction occupies 35.0% of its total capacity at 0.3 mV·s −1 , while the corresponding ratio for ED-MoS2 is 25.7% (Figs. 4(f) and 4(g), calculated according to Eq. S(2) in the ESM). Further diffusion-dominated proportions at various rates were revealed in Fig. 4(h), and Figs. S11 and S12 in the ESM. Although they both display decreasing diffusion fractions with incremental scan rates, EE-MoS2 exhibits higher diffusion-dominated portions in contrast to ED-MoS2. This disparity is especially obvious at 0.9 mV·s -1 and ED-MoS2 lags by 19.3% behind EE-MoS2. The promoted diffusion-dominated contribution is attributed to the facile electrolyte diffusion and through ordered mesostructure and easy access for K + diffusion generated by thin skeletons with exposed edges. It is undeniable that the enormous edges with ordered mesostructure and high surface area improve diffusiondominated contribution.
To further gain an insightful understanding, we conducted the theoretical calculations based on density functional theory (DFT) approach. As mentioned above, mesostructures endow EE-MoS2 with sufficient exposed edges and large interior surface area in comparison with ED-MoS2. Hence, we simplify EE-MoS2 and ED-MoS2 as monolayer and double-layer MoS2 models, respectively ( Fig. 5(a)). The adsorbed sites for K in MoS2 are usually classified into two types: octahedral (Oh) and tetrahedral (Th) sites [36,43]. For both ED-MoS2 and EE-MoS2, Th adsorption energies of −1.72 and −2.31 eV are higher than those of Oh sites (e.g., −1.94 and −2.54 eV), respectively ( Fig. 5(b)), meaning that Th sites are unstable configuration and finally would transform into Oh types. Meanwhile, the adsorption energies of Oh and Th sites in EE-MoS2 are both lower than those of ED-MoS2, implying the more favorable conversion reaction in EE-MoS2. And the ions diffusion capability was further evaluated via diffusion energy barriers calculation. The realization of K + migration demands the continuous Oh-Th-Oh transformation. As for ED-MoS2, the energy barrier is 0.29 eV, while EE-MoS2 needs a lower activation energy of 0.19 eV to complete the transformation (Fig. 5(c)), which implies a faster K + transfer in EE-MoS2 consistent with GITT and rate-scan CV results. Such rapid K + transfer and smooth conversion reaction are closely associated with much more edges in EE-MoS2.
Ex-situ HRTEM was applied to investigate the possible (de)potassiation mechanism of MoS2. Potassiated MoS2 still exists in form of laminated structure with the expanding lattice fringe of (002) plane from 6.20 to 6.65 and 6.85 Å upon discharging to 0.78 V, indicative of the K + insertion in MoS2 ( Fig.  6(a)). At the final discharge state (0.01 V), the layered structure disappears and Mo 0 and K2S emerge as evidenced by d(110) = 1.98 Å and d(220) = 2.81 Å, respectively, representing the occurrence of conversion reaction ( Fig. 6(b)). Then when charged to 1.74 V, the laminar structure reappears with lattice spacing of 7.28 Å belonging to (002) plane, suggesting the incomplete extraction of K + (Fig. 6(d)). At fully charged state (3.00 V), MoS2 recovers its laminar crystalline structure with lattice fringe of 6.76 Å, illustrating the electrochemical reversibility of (de)potassiation (Fig. 6(e)). In ex-situ X-ray photoelectron spectroscopy (XPS) (Figs. 6(c) and 6(f)), despite the presence of Mo 5+ and Mo 4+ in the sample, Mo 0 species is present at 0.01 V, consistent with ex-situ HRTEM results. At fully charged state (3.00 V), most of Mo species exist in Mo 6+ , Mo 5+ and Mo 4+ with little of Mo 0 , meaning the approximately complete removal of K + . In in-situ Raman spectra, peaks E 1 2g and A1g vibrations located at ~ 380 and 410 cm −1 are attributed to the in-plane and out-of-plane Mo-S phonon modes in MoS2, respectively ( Fig. 6(g)) [21,31,44]. With continuous discharging to 0.01 V, peaks E 1 2g and A1g gradually fade and they reemerge in charge process, further corroborating the (de)potassiation mechanism of K storage in MoS2.

Conclusions
In summary, we demonstrated the rational design of EE-MoS2 for high capacity and fast K storage. The bicontinuous mesoscale skeletons with mesopores enable sufficient edge planes exposed at pore walls, thus rendering its intrinsic layer spacing readily accessed by K + with enhanced diffusion-dominated process and accelerated conversion reaction, as verified in detail by GITT, rate-scan CV investigations and theoretical calculations. Consequently, EE-MoS2 exhibits high capacity, remarkable rate performance and supeorior cycle stability, far exceeding its counterpart edge-deficient bulk MoS2 and the majority of reported TMDs. The engineering and utilization of edgeenriched active sites in compact stacked laminar-structured materials could be valuable for various TMDs families in electrochemical energy storage in future.

Materials synthesis
Typically, 2.0 g SiO2 template (KIT-6, synthesized according to Ref. [30]) was added into ethanolic solution of phosphomolybdic acid (H3PO4·12MoO3, PMA; 1.48 g PMA dissolved into 3 mL ethanol) and stirred for 1 h at room temperature. Subsequently, the obtained mixture was dried at room temperature overnight. Afterwards, the resulted sample was calcined at 450 °C for 5 h to crystallize MoO3 and then sulfurized in a center of tubular furnace at 600 °C for 3 h with thiourea as a sulfur source laid at upstream in N2 flow. Finally, EE-MoS2 was achieved by etching the SiO2 template with 4% HF aqueous solution overnight. For comparison, ED-MoS2 was prepared via the direct decomposition of PMA and subsequent sulfidation under the same conditions as EE-MoS2.

Characterization
The sample phase composition was determined by powder XRD on a Bruker D8 ADVANCE diffractometer (Cu Kα radiation, 0.15406 nm). N2 adsorption/desorption measurements were conducted on a Micromeritics ASAP 2020. The specific surface area and pore size distribution curve were obtained according to the BET theory and the Barret-Joyner-Halenda (BJH) method using the absorption data. And the average pore size was calculated by BJH adsorption (4V/A) approach. TEM investigation was carried on a JEOL JEM-2100F. XPS data were collected on a Kratos (Shimadzu, Axis Suprato) spectrometer to characterize the surface chemistry state. In-situ Raman spectra data were collected on a Renishaw Invia RM200 (London, UK) combined with CV test at a scan rate of 1 mV·s −1 on a CHI 660E electrochemical workstation (Shanghai Chenhua, China).

Electrochemical measurements
The half-cell electrochemical tests were conducted in 2032-type coin cells with metal K plate as the counter electrode and with 1 M potassium bis(fluorosulfonyl)amide (KFSI) in ethyl methyl carbonate (EMC) as electrolyte. The active material, Super P, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binder in a weight ratio of 7:2:1(CMC:SBR = 8:2) in deionized water was mixed into slurry and coated on copper foil. The coated foil was dried in a vacuum oven at 80 °C for 8 h and then cut into round electrode in diameter of 10 mm. The total loading amount of each electrode is ~ 1.2 mg·cm −2 . The cells were assembled in an Ar-filled glovebox with oxygen and moisture concentrations kept below 0.1 ppm. Land CT 2001A battery testing system (Land, China) was employed to conduct galvanostatic discharge/charge tests ranging from 0.01 to 3.00 V versus K/K + at room temperature. A CHI 660E electrochemical workstation (Shanghai Chenhua, China) was used to carry out CV and EIS measurements within a frequency range of 100 kHz-0.01 Hz. A current pulse of 50 mA·g −1 for 1 h followed by relaxation for 4 h was exerted to complete GITT analysis.

Computational calculation methods
The theoretical calculations were employed to depict K + migration in MoS2 based on DFT. Vienna ab-initio simulation package (VASP) was employed to conduct all calculations with given pseudopotential according to the projector-augmented wave (PAW) and the Perdew-Burke-Ernzerhof (PBE) method. Moreover, the cut-off energy was set to 400 eV and the Brillouin zone was described with a 4×4×2 Γ-centered k-mesh for 3×3×1 supercell. The optimizations of all structures were terminated until the self-consistent force less than 10 −3 eV·Å −1 and the energy between two consecutive steps less than 10 −6 eV. And the barrier energy of K + migration was calculated utilizing the climbing nudged elastic band (CI-NEB) method. In our calculation, the adsorption energy was calculated from the equation: Eads = Etotal − EMoS 2 − EK, where Etotal is the total energy of MoS2 with K atom absorbed structure, EMoS 2 and EK is the total energy of MoS2 structure and K + , respectively.
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