Liquid-Phase Exfoliation of Arsenic Trisulfide (As2S3) Nanosheets and Their Use as Anodes in Potassium-Ion Batteries

Here, we demonstrate the production of 2D nanosheets of arsenic disulfide (As2S3) via liquid-phase exfoliation of the naturally occurring mineral, orpiment. The resultant nanosheets had mean lateral dimensions and thicknesses of 400 and 10 nm, and had structures indistinguishable from the bulk. The nanosheets were solution mixed with carbon nanotubes and cast into nanocomposite films for use as anodes in potassium-ion batteries. These anodes exhibited outstanding electrochemical performance, demonstrating an impressive discharge capacity of 619 mAh/g at a current density of 50 mA/g. Even after 1000 cycles at 500 mA/g, the anodes retained an impressive 94% of their capacity. Quantitative analysis of the rate performance yielded a capacity at a very low rate of 838 mAh/g, about two-thirds of the theoretical capacity of As2S3 (1305 mAh/g). However, this analysis also implied As2S3 to have a very small solid-state diffusion coefficient (∼10–17 m2/s), somewhat limiting its potential for high-rate applications.


A. SAED pattern, EDX and
Step-height analysis of exfoliated nanosheets of As 2 S 3 .
a. Selected area-electron diffraction (SAED) pattern from a few-layer thick As 2 S 3 nanosheet.
The selected area diffraction pattern (SAED) obtained from a few-layered thick nanosheet provides valuable insights into the crystallography and structural characteristics of the material.It reveals the nanosheet of As 2 S 3 is oriented in the [010] direction and show the periodicity of the crystal lattice (Crystal System: Monoclinic, SPGR: P21/n).Exfoliated As 2 S 3 nanosheets were analyzed using EDX spectra, and the results are shown in Figure S2.The analysis confirmed the existence of As and S elements with an average atomic ratio of <S/As> : 1.5 c.
Step-height analysis of exfoliated nanosheet.
In this context, step-height analysis pertains to the precise measurement and evaluation of the nanosheet's thickness at the atomic scale.Atomic force microscopy (AFM) is employed to accurately determine the thickness of these nanosheets.Within the AFM images, nanosheets are identified at locations exhibiting distinct steps or edges, exemplified in Figure S3.A line profile is generated across these steps using AFM software Gwyddion, and the height difference referred to as step height, is recorded.To enhance the accuracy of our analysis, measurements are conducted at multiple positions on various nanosheets, and statistical analysis was performed to find the minimum step-height.Subsequently, the actual thickness (   ) of the nanosheets is derived based on these measurements, utilizing the following formula: The values for t apparent are compiled from the AFM height measurements conducted on over 100 nanosheets, while the minimum step height is determined to be 2 nm.Notably, the monolayer thickness is calculated as half of the unit cell thickness along the b-axis, which is determined to be 0.48 nm.This rigorous approach ensures accurate assessment of the nanosheet thickness.

a. Free standing As 2 S 3 @CNT electrode
The electrode is composed of 70% by weight of As 2 S 3 nanosheets and 30% by weight of SWCNT.The mass loading of the electrode is measured at 0.44 mg/cm 2 .To prepare the electrode materials, the dispersions of As 2 S 3 and SWCNT in solvent 2-proponal were mixed at the required weight ratio (7:3) to create the final dispersion containing 1D SWCNT and 2D-As 2 S 3 nanosheets.This mixture is then bath sonicated for 30 minutes at room temperature to ensure uniform mixing, followed by filtering the final dispersion onto a Celgard membrane.
Once the entire solution was filtered, the film on the membrane was allowed to dry under a vacuum pump, and subsequently stored in a glove box overnight.The resulting dried film was easily peeled off from the membrane, yielding a free standing As 2 S 3 @CNT electrode, as depicted in Figure S4.

b. Specific capacity of SWCNTs alone K-ion battery anode
To determine the capacity contribution of CNTs, we conducted a cycling test on SWCNTs electrodes alone at 50 mA/g for 10 cycles and 200 mA/g for 40 cycles, as shown in Figure S5.
The results indicate that the specific capacity of SWCNTs for K-ion storage is approximately 32 mAh/g at 200 mA/g.As we used 30 wt% SWNTs, the maximum contribution of SWCNTs in our electrodes is 9.6 mAh/g, which is relatively small compared to the overall electrode capacity of over 450 mAh/g.

Figure S1 :
Figure S1: (A) SAED from an As 2 S 3 nanosheet.(B) Assignment of the diffraction spot pattern to the monoclinic crystal system, and space group P21/n.

Figure S3 :
Figure S3: AFM image of an As 2 S 3 nanosheet with clear steps.A line profile (black line) generated across the nanosheet display clear steps as shown, and the step height analysis over various nanosheets shows a minimum step height of 2 nm.

Figure S4 :
Figure S4: (A) The photograph of a free-standing As 2 S 3 @CNT electrode.The electrode is held with forceps and rotated to capture the top, side, and bottom views of the electrode.(B) SEM cross-section image of the composite film with the corresponding elemental composition maps, for (C) As; (D) S. (E) EDX spectra of the composite electrode, indicate the presence of As and S elements in the composite electrode.Elemental maps reveal a uniform distribution of As and S elements with an expected stoichiometry of As 2 S 3.1 .

Figure S5 :
Figure S5: Cycling performance of SWCNTs film alone for K-ion storage at a current density of

Figure S7 :
Figure S7: SEM image and X-ray diffraction pattern of the electrode after 200 charge/discharge cycles.

Figure S8 .
Figure S8.(A) SEM cross-section image of As 2 S 3 /CNT composite electrode after 200 chargedischarge cycles.The elemental mapping on the post-cycled electrode surface, for (B) S; (C) As; (D) P (E) K and (F) fluorine from the electrolyte.(G) O on the electrode surface (from SEI and oxidation after washing the electrodes).(G) EDX spectra on the post-cycled electrode represent the presence of As and S elements.Elemental maps of the electrode revealed a uniform distribution of As, S, K, P, and F elements.

Table S1 .
The literature comparison on the specific capacities, and capacity retention other 2D materials based KIB anodes of this work with other state-of-the-art latest 2D materials based KIB anodes (except for carbon/graphite).Rate performance and cycling comparison of As 2 S 3 (this work) with reported Sb 2 S 3 , and Bi 2 S 3 based KIB anodes.

Table S2 .
A comparative analysis of the rate and cycling performance of As 2 S 3 (this work) in comparison with Sb 2 S 3 and Bi 2 S 3 -based KIB anodes in published studies.