English

• 1. INTRODUCTION

  Rechargeable lithium-ion batteries (LIBs) currently dominate the battery market for portable energy storage devices including laptops, smartphones, cameras, and electric vehicles^[1]. However, the scarcity and uneven global distribution of lithium resources raise a concern of long-term sustainability for LIBs, which motivates researchers to explore low-cost alternative battery systems. In this context, sodium-ion batteries (SIBs)^[2] and potassium-ion batteries (PIBs) are attracting rapidly growing attention due to the earth-abundance and cost-effectiveness of sodium and potassium resources. In comparison to SIBs, PIBs possess unique advantages: (1) The standard redox potential of K/K⁺ (–2.93 V vs. standard hydrogen electrode, SHE) is higher than that of Na/Na⁺ (–2.71 V vs. SHE)^[3], implying a higher working voltage and thus a possibly higher energy density^[4]. (2) In comparison to Li and Na-ions, K-ions have weaker Lewis acidity and thus smaller Stoke's radius of solvated ions, which enables larger transport number and higher mobility of K-ions in the electrolyte as well as lower desolvation energy for potentially fast ion transport at the electrolyte/electrode interface^[4]. (3) K-ions readily intercalate into commercial graphite anode whereas Na-ion intercalation is impracticable unless accompanying with solvent co-intercalation. That is, the well-established graphite industry in LIBs could be adopted in PIB system^[5]. However, K-ion has a much larger ionic radius (1.38 Å) compare to that of Li-ion (0.76 Å)^[6], and the insertion of large-size K-ions induces not only inferior structural stability and short lifespan but also sluggish ion transport to the electrode materials, representing a key challenge for the development of the practical PIBs.

  Two-dimensional layered transition metal dichalcongenides (TMD) have been regarded as a promising electrode category for electrochemical energy storage systems such as Li-ion^[7-9] and Na-ion batteries^{[10, 11]}. Taking inspiration from these previous reports, TMD structures have been recently reported as promising both anode^[12-14] and cathode^[15] materials for PIBs. Among them, WS₂ with large layer spacing allows high reversible K-ion intercalation into its layered structure with a small volume change of 37.8%, and exhibits ultralong cycle life up to 1000 cycles as well as excellent rate capability. All these merits make it a very promising K-ion host with high structural stability. However, bulk WS₂ delivers a relatively low capacity of ~67 mAh/g. Recently, Choi has reported that the Li-ion storage capacity of MoS₂ can be largely elevated in the liquid-phase exfoliated monolayer MoS₂ structure due to its larger surface area and more Li-ion storage sites^[16].

  Inspired by the previous work, we here report a WS₂-graphene composite structure via the filtration of liquid-phase exfoliated WS₂ (E-WS₂) and graphene nanosheets as an anode material for PIBs. The graphene in the composite not only provides extra reversible K-ion storage capacity, but also maintains high structural stability and electric conductivity through the entire binder-free composite anode^{[17, 18]}. The exfoliated-WS₂-graphene (E-WS₂-G) delivers a significantly higher reversible capacity of 137 mAh·g^-1 at a current density of 10 mA·g^-1, which is twice that of the bulk WS₂ (B-WS₂). Equally important, the E-WS₂-G anode also exhibits excellent rate capability and long-term cyclability over 500 cycles. These results suggest that the E-WS₂-G anode could be a material choice of anode for potassium-ion batteries with high stability and long lifespan, and will trigger further studies into this type of electrode materials.

  2. EXPERIMENTAL

  2.1 Synthesis of exfoliated WS₂

  Bulk WS₂ powders (analytical pure, 99.9%) purchased from Adamas-Beta were hand-ground for one hour and then sieved through a 200-mesh screen. 2 g sieved fine powder was added into 1000 mL N-methyl pyrrolidone (NMP), then the suspension was ultrasonicated for 5 hours with circulating water to prevent overheating. After resting for 24 hours, the supernatant was poured into a beaker to isolate the exfoliated WS₂ from the sediment (unexfoliated or less-exfoliated WS₂). 50 mL of such supernatant was vacuum-filtered on a membrane with a pore size of 0.45 μm (Nylon6) and washed several times with ethyl alcohol to remove residual NMP solvent. The obtained powder was dried under vacuum at 80 ºC for 12 hours and then weighed to calculate the concentration of E-WS2 in the supernatant.

  2.2 Synthesis of graphene

  The graphene was also obtained by ultrasonication. 50 mg graphite was added into 1000 mL NMP, then the solution was ultrasonicated for 30 hours. After resting for 24 hours, the solution was centrifuged for 3 minutes at 3000 rpm and the supernatant was extracted out with a pipette. The concentration of graphene in the supernatant was determined using the same method as for E-WS₂.

  2.3 Synthesis of WS_2-graphene composite

  The exfoliated WS₂ and graphene NMP solutions were mixed with a WS₂: graphene mass ratio of 8:2, and subsequently filtered on a membrane (Nylon6, pore size: 0.45 μm) by vacuum filtration. The obtained E-WS₂-G was washed for several times with ethyl alcohol to remove residual NMP solvent and dried under vacuum at 80 ºC for 12 hours prior to the fabrication of batteries.

  2.4 Materials characterization

  The X-ray powder diffraction (XRD) patterns were collected using a powder X-ray diffractometer (Rigaku Miniflex 600) with CuKα radiation at a voltage of 40 kV and a current of 15 mA. The morphology of both bulk and exfoliated WS₂ was characterized on a field-emission scanning electron microscope (Hitachi SU-8010).

  2.5 Electrochemical measurements

  The E-WS₂-G composite films were used directly as binder-free electrodes, whereas the electrode of B-WS₂ was fabricated based on a slurry-coating approach with a mixture of B-WS₂, super P carbon black and carboxymethyl cellulose (CMC) (80 wt%: 10 wt%: 10 wt%) in deionized water. The slurry was subsequently cast on an Al foil and dried under vacuum at 80 ºC for 12 hours. The mass loading of E-WS₂-G and B-WS₂ is ~1.2 and ~1.3 mg·cm^-2, respectively. CR2032-type coin cells were assembled in an argon-filled glovebox (Vigor) with both oxygen and water content lower than 0.1 ppm. Potassium metal foils were applied as counter and reference electrodes, and glass fibers (Grade GF/F, Whatman) were used as separators. 5 M potassiumbis (trifluoromethylsulfonyl)imide (KTFSI) in diethylene glycoldimethyl ether (DEGDME)) was adopted as an electrolyte. The specific capacities of B-WS₂ and E-WS₂-G were calculated based on the mass of WS₂ and the total mass of E-WS₂-G composite, respectively. Galvanostatic charge/discharge cycling tests were conducted in the voltage range between 0.1~1.5 V on a LANHE CT-2001A at a constant temperature of 28 ºC. Cyclic Voltammetry (CV) measurements were carried out on a Bio-Logic SP-300 electrochemical workstation at a scanning rate of 0.1 mV·s^-1 in the voltage range of 0~3 V.

  3. RESULTS AND DISCUSSION

  The X-ray diffraction (XRD) patterns of B-WS₂ and E-WS₂ are shown in Fig. 1a. The dominant Bragg peaks of B-WS₂ at 14.32°, 28.88°, 49.30° and 59.77° can be assigned to (002), (004), (103) and (008) reflections of WS₂ (JCPDS no.35-0651) and confirms the P63/mmc structure. After exfoliation, those reflections remain at the same position with a well-preserved P63/mmc crystal structure of WS₂. However, the (002) reflection becomes broader as a consequence of the decrease in the lateral thickness of WS₂. In addition, a new peak appears at 13.75° around (002) plane reflection, which is indicative of the partially expanded interlayer spacing of the WS₂ upon liquid-phase exfoliation^[16].

  Figure 1

  

  Figure 1.  (a) XRD profiles of B-WS₂ and E-WS₂; (b-e) SEM images of B-WS₂ (b, c) and E-WS₂ (d, e); (f) SEM EDS mapping of the binder-free E-WS₂-G composite anode

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  The morphology and microstructure of B-WS₂ and E-WS₂ were examined by scanning electron microscopy (SEM). As shown in Figs. 1b-1e, B-WS₂ exhibits nanosheet morphology with a thickness of about 100~150 nm and lateral size of about 1~3 μm. After liquid-phase exfoliation, the E-WS₂ flakes display obviously reduced thickness (~50 nm) and lateral size (~0.66 μm) in comparison to the original B-WS₂. Through a simple filtration process with a mixture solution of the liquid-phase exfoliation E-WS₂ and graphene, the binder-free E-WS₂-G composite anode is obtained. The energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. 1f) reveals a homogeneous distribution of the C, W and S elements, which indicates a uniform dispersion of E-WS₂ and graphene in the E-WS₂-G composite anode.

  The electrochemical K-ion storage behaviors were studied by CV and galvanostatic charge-discharge tests. The comparative CV curves of B-WS₂ and E-WS₂-G in the first cycle are shown in Fig. 2a. For the B-WS₂, peaks around 0.67 and 0.47 V in the cathodic sweep can be attributed to the intercalation of K-ions. The peak below 0.1 V represents the reduction degradation and the formation of W metal and K_xS as well as solid electrolyte interphase^[19]. Note that, to avoid the reduction degradation, the cutoff voltage for battery cycling was set to 0.1 V. In the reverse anodic scan, three peaks at 0.66, 1.06 and 1.33 V are ascribed to the deintercalation of K-ions from the WS₂ layered structure^[19]. The E-WS₂-G composite exhibits similar CV curves but accompanied with peak shifting and a new anodic peak at 0.33 V. Two cathodic peaks for E-WS₂-G shift to higher voltage (0.67 → 0.74 V, 0.47 → 0.51 V), while three anodic peaks shift to lower voltage (0.66 → 0.61 V, 1.06 → 1.01 V, 1.33 → 1.28 V). These observations indicate lower polarization and faster reaction kinetics are achieved on the E-WS₂-G composite anode. The new cathodic peak at 0.33 V can be attributed to the extraction of K-ions from graphene^{[5, 20]}. After one activation cycle, the subsequent CV curves are well overlapped (Figs. 2b, 2c), suggesting high reversibility of K-ion insertion/extraction into/from the composite anode.

  Figure 2

  

  Figure 2.  (a-c) Cyclic voltammetry curves of B-WS₂ and E-WS₂-G at a scan rate of 0.1 mV·s⁻¹; (d, e) Voltage profiles of B-WS₂ and E-WS₂-G at a current density of 10 mA·g^-1; (f) Electrochemical cyclabiltiy and Coulombic efficiencies of B-WS₂ and E-WS₂-G at a current of 20 mA·g^-1

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  Fig. 2d presents the first cycle voltage profiles of B-WS₂ and E-WS₂-G at a current of 10 mA·g^-1. B-WS₂ achieves discharge/charge capacities of 117 and 59 mAh·g⁻¹ with an initial Coulombic efficiency of 50.43%. In sharp contrast, the E-WS₂-G composite anode delivers significantly improved discharge/charge capacities of 532 and 136 mAh·g⁻¹ but a relatively lower initial Coulombic efficiency of 25.71%. This charge capacity is about twice that of B-WS₂ and indicates elevated active sites for K-ions insertion in E-WS₂-G. It is also observed that flat voltage plateaus disappear in the composite anode, indicating that the K-ion storage capacity is predominantly contributed by capacitance rather than intercalation capacity in B-WS₂. Based on these results, it is concluded that the largely improved K-ion storage capacity in E-WS₂-G arises from the larger amount of active sites created by the liquid-exfoliation process. The cycling performances of B-WS₂ and E-WS₂-G at a current of 20 mA·g^-1 are shown in Fig. 2f. The B-WS₂ and E-WS₂-G can maintain capacities of 43 and 74 mAh·g ^-1 after 50 repeated cycles, demonstrating good cycling stability of both B-WS₂ and E-WS₂-G composite anodes.

  The rate capability and long-term cyclability of B-WS₂ and E-WS₂-G are also evaluated. As shown in Fig. 3a, B-WS₂ delivers discharge capacities of 50, 44, 41, 36, 28 and 17 mAh·g⁻¹ at current density of 10, 20, 30, 50, 100 and 200 mA·g^-1, respectively. Remarkably, the corresponding discharge capacities of the E-WS₂-G composite anode reach 120, 88, 79, 65, 41 and 32 mAh·g⁻¹, all of which are about twice those of B-WS₂. What's more, the discharge capacity recovers from 32 to 106 mAh·g⁻¹ when switching the current density back to 10 mA·g^-1, evidently confirming the good rate capability of the composite anode. It is equally important that the E-WS₂-G composite anode also exhibits good long-term cycling stability. As shown in Fig. 3b, E-WS₂-G still delivers a reversible capacity of about 50% of the initial value over 500 cycles, corresponding to a capacity decay rate of as low as 0.1% per cycle. This excellent electrochemical cyclability indicates high structural stability of the composite anode even upon repeated long-term cycling.

  Figure 3

  

  Figure 3.  (a) Rate capabilities of B-WS₂ and E-WS₂-G; (b) Long-term cyclability of E-WS₂-G composite anode at a current density of 50 mA·g^-1

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  4. CONCLUSION

  In summary, we have demonstrated the fabrication of a liquid-phase exfoliated WS₂ and graphene composite structure as well as the feasibility of reversible K-ion insertion into this composite. The composite anode achieves ~132% more capacity (137 mAh·g^-1) than that of the original bulk WS₂ (59 mAh·g^-1), good rate capability (32 mAh·g⁻¹ at a current density of 200 mA·g⁻¹), and a long lifespan (~50% capacity retention over 500 cycles). These results highlight the WS₂-graphene composite as a promising anode for high-stability and long-lifespan K-ion batteries. Moreover, the "liquid-exfoliation to filtration" method may directly extend to other two-dimensional electrode materials to further explore advanced electrode materials for K-ion batteries.

  
  
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  Figure 1  (a) XRD profiles of B-WS₂ and E-WS₂; (b-e) SEM images of B-WS₂ (b, c) and E-WS₂ (d, e); (f) SEM EDS mapping of the binder-free E-WS₂-G composite anode

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  Figure 2  (a-c) Cyclic voltammetry curves of B-WS₂ and E-WS₂-G at a scan rate of 0.1 mV·s⁻¹; (d, e) Voltage profiles of B-WS₂ and E-WS₂-G at a current density of 10 mA·g^-1; (f) Electrochemical cyclabiltiy and Coulombic efficiencies of B-WS₂ and E-WS₂-G at a current of 20 mA·g^-1

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  Figure 3  (a) Rate capabilities of B-WS₂ and E-WS₂-G; (b) Long-term cyclability of E-WS₂-G composite anode at a current density of 50 mA·g^-1

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