In Situ Alloying Strategy for Exceptional Potassium Ion Batteries - ACS

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In-Situ Alloying Strategy for Exceptional Potassium Ion Batteries Jue Wang, Ling Fan, Zhaomeng Liu, Suhua Chen, Qingfeng Zhang, Longlu Wang, Hongguan Yang, Xinzhi Yu, and Bingan Lu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00634 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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In-Situ Alloying Strategy for Exceptional Potassium Ion Batteries Jue Wang,† Ling Fan,† Zhaomeng Liu,† Suhua Chen,† Qingfeng Zhang,† Longlu Wang,† Hongguan Yang,† Xinzhi Yu,† and Bingan Lu*,†,‡ †

School of Physics and Electronics, State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, P. R. China ‡

Fujian Strait Research Institute of Industrial Graphene Technologies, Quanzhou 362000, P. R. China *Corresponding email: [email protected]

ABSTRACT: We report an in-situ alloying strategy for obtaining homogeneous (Bi,Sb) alloy nanoparticles from (Bi,Sb)2S3 nanotubes for the exceptional anode of potassium ion batteries (KIBs). The operando X-ray diffraction results, along with transmission electron microscopy and energy-dispersive X-ray spectroscopy mappings, successfully reveal the phase evolution of this material, which is (Bi,Sb)2S3 → (Bi,Sb) → K(Bi,Sb) → K3(Bi,Sb) during the initial discharge and K3(Bi,Sb) → K(Bi,Sb) → (Bi,Sb) in the charging process. The in-situ alloying strategy produces a synergistic effect and brings an outstanding electrochemical performance. It achieves ultrahigh discharge capacities of 611 mAh g-1 at 100 mA g-1 (0.135C) and 300 mAh g-1 at 1000 mA g-1 (1.35C), and remains a capacity as high as 353 mAh g-1 after 1000 cycles at 500 mA g-1 (0.675C) with a Coulombic efficiency close to 100%. In addition, the KIBs full cell, which is composed of this anode and a perylenetetracarboxylic dianhydride cathode, reaches an initial discharge capacity as high as 276 mAh g-1 at 500 mA g-1 and retains a discharge capacity of 207 mAh g-1 after 100 cycles.

KEYWORDS: potassium ion battery, anode, in-situ alloying strategy, synergistic effect, operando X-ray diffraction

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Considering the uneven distribution and shortage of lithium minerals on the earth (0.0017 wt%), the fast increase of the electric vehicle and renewable energy markets stimulates the pursuit of energy storage devices with the cost, stability, energy density, and power density competitive with lithium ion batteries.1, 2 In view of the natural abundance of potassium and its similar position to lithium in the periodic table, potassium ion batteries (KIBs) are one of the most promising candidates to meet these requirements.3-6 Besides, the standard electrode potential of potassium is -2.93 V (vs. standard hydrogen electrode), close to that of lithium ( -3.05 V vs. standard hydrogen electrode).7 However, the development of suitable electrodes to accommodate the large-sized K ions (1.38 Å), in comparison to Li ions (0.76 Å), is highly challenging. The large size of K ions introduces a large volume change during potassiation/depotassiation, which severely undermines the stability of the electrode.8 The previous study on the anode of KIBs has been focusing on carbonaceous materials, in regard to their inexpensiveness and high electric conductivity.9-15 But the formation of C8K for the storage of K ions and the difficulty to reversibly accommodate the large-sized K ions in carbonaceous materials give rise to a low capacity and unsatisfied rate capability, which hinders the commercialization of KIBs. Therefore, searching for high performance anode material is highly demanded. Generally speaking, anodes associating with conversion reactions and/or alloy reactions, such as SnO2,16 Co3O4-Fe2O3,17 metal-organic framework (MOF),18 P,19, 20 Bi,21-25 Sb,26 Sn,27-29 and CoP,30 could offer a high capacity for KIBs.31 However, the cycling stability of these anodes is a severe concern due to the large volume expansion. Metal selenides, including VSe28 and MoSe2,32, 33 have been reported for the anodes of KIBs, displaying a relatively high capacity and good stability. Sulfur, which is more in common with selenium since they are in the same group in the periodic table, is less heavy than selenium. It suggests that the metal sulfides


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are very promising anodes because they are supposed to provide a specific capacity even higher than the corresponding metal selenides while maintain the similar chemical properties. Metal sulfides with the high theoretical capacity and superior redox reversibility are attractive materials for the anode of lithium/sodium-ion batteries, which have been intensively investigated.34-41 However, their electrochemical performance in KIBs is restricted to CoS,42 FeS2,43-45 ReS2,46 MoS2,47-49 SnS2,50 and Sb2S3.51, 52 These sulfides generally present discontented cycling stability and relatively high charge/discharge plateaus, which is not able to effectively address the issues of the current anode materials for KIBs. Bi2S3 and Sb2S3 have been intensively explored in lithium/sodium ion batteries, but the size of K ions is a severe problem. Sb2S3 has been demonstrated to be an anode for KIBs with a high capacity.51, 52 However, the capacity, charge/discharge plateaus, and stability definitely should be considerably upgraded. Bi2S3 has not been reported for KIBs yet, but it is found to be struggling with the stability and capacity in our pilot experiments. Recently, a synergistic effect has been achieved within binary and ternary alloy systems for the improved performance of the anode of lithium ion batteries and sodium ion batteries, in comparison to the pure elemental counterparts.53-55 Inspired by those work, a solid solution of Bi2S3 and Sb2S3 is adopted in this study for the anode of KIBs, which may yield a much better electrochemical performance than the pure counterpart. Importantly, an in-situ alloying strategy is discovered within this material, which realizes an exceptional battery performance for the anode of KIBs. Due to the solid state, the alloying of Bi and Sb using this strategy will be limited to the local area and avoid aggregation, thus developing a more homogeneous distribution of (Bi,Sb) alloy. Meanwhile, this is an atomic-level alloying process, unlike the alloy obtained from the traditional ball milling of bulky precursors. More importantly, the potassiation/depotassiation mechanism and phase


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evolution of this material are unravelled through the operando X-ray diffraction (XRD) to understand the in-situ alloying process in depth.

RESULTS AND DISCUSSION (Bi,Sb)2S3 is generally prepared at a temperature higher than 180oC,56, 57 which is not favorable for mass production. Meanwhile, a tubular nanostructure of (Bi,Sb)2S3 for accommodating the large sized K ions should be designed, which is not available in the literature. Consequently, a low temperature synthesis for obtaining (Bi,Sb)2S3 nanotubes is developed in this study. Bi2S3 and Sb2S3 are isostructural to each other (Figure 1a), belonging to the same space group Pnma (No. 62) with tightly-bonded infinite (M4S6)n rods extending along [010]. This isostructural characteristic and the similar chemical properties between Bi and Sb should allow the formation of (Bi,Sb)2S3 through a co-precipitation method, and the chemical composition may be controlled by tuning the ratio of Bi:Sb in the precursor. To validate this idea, a facile co-precipitation method displayed in Figure 1b was used to synthesize (Bi,Sb)2S3 nanotubes. First, Bi- and Sb-based salts were completely dissolved in an organic solvent to form a transparent and homogeneous solution, generating a uniform distribution of Bi and Sb elements. Second, the sulfur source, thioacetamide, was dissolved in another organic solution. Finally, (Bi,Sb)2S3 solid-solution nanotube was acquired after adding the solution with Bi- and Sb-based salts into the thioacetamide solution dropwise and let it stay at 60oC for 24 hours. As displayed in the high-magnification scanning electron microscope (SEM) image (Figure 1c), a nanotube with a width around 150 nm is observed. A large amount of nanotubes with a relatively good uniformity are shown in the low-magnification SEM image in Figure S1. Meanwhile, the low magnification transmission electron microscope (TEM) image in Figure 1d,


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together with high magnification TEM image in the inset, also illustrates the fairly good uniformity of the well-defined (Bi,Sb)2S3 nanotubes and the existence of graphene nanosheets. Graphene nanosheets were introduced in the sulfur precursor during synthesis, which serves two functions. One is to prevent the aggregation of nanotubes. The other is to enhance the conductivity of the electrodes. To explore the elemental distribution in the nanotubes, high-angle annular dark-field imaging (HAADF)-scanning transmission electron microscope (STEM) and energy dispersive X-ray spectra (EDS) elemental mappings were employed and the outcomes are presented in Figure 1e. They reveal a tubular structure with Bi, Sb, and S dispersing homogeneously throughout the whole nanotube, as well as a graphene layer beneath the nanotube. High-resolution transmission electron microscope (HRTEM) in Figure 2a offers the lattice information of these nanotubes. The facets with a lattice distance of 0.565 nm are spotted along the length while a lattice distance about 0.396 nm is detected along the width, corresponding to (200) and (010) facets, respectively. According to the literature, XRD pattern of these nanotubes in Figure 2b can be indexed as the orthorhombic phase structure with lattice constants of a = 1.132 nm, b = 0.395 nm, and c = 1.116 nm,56 consistent with the HRTEM results. The chemical composition of (Bi,Sb)2S3 nanotubes is verified to be Bi, Sb, and S through X-ray photoelectron spectroscopy (XPS) in Figure 2c. The peaks at 164.00 and 158.70 eV are indexed to Bi3+; the peaks located at 529.90 and 539.20 eV correspond to Sb3+; the peak appearing around 161.5 eV is referred to S2-. EDS quantitative analysis from TEM images produces a formula Bi1.11Sb0.89S3 for (Bi,Sb)2S3 nanotubes synthesized using a precursor with Bi:Sb equivalent to 1:1. The ratio of Bi:Sb in (Bi,Sb)2S3 nanotubes can be easily adjusted through tuning the ratio of Bi to Sb in the precursor. With the ratios of Bi:Sb equivalent to 1:3 and 3:1 in the precursor, nanotubes with the


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formulas of Bi0.26Sb1.74S3 and Bi1.57Sb0.43S3 were acquired, respectively, which are demonstrated in Figure S2 and Figure S3. SEM and TEM images as well as elemental mappings of Bi2S3 and Sb2S3 synthesized using the same procedures are shown in Figure S4 and Figure S5, respectively. In the SEM image of Bi2S3 (Figure S4a), a large number of Bi2S3 nanotubes are observed. The tubular structure, along with the presence of graphene nanosheets, is verified by the TEM image in Figure S4b. The shape and chemical composition of these nanotubes are revealed in HAADFSTEM image (Figure S4c) and EDS elemental mappings (Figure S4d and S4e). The crystal structure of Bi2S3 is shown by the XRD pattern in Figure S6a, which matches the standard card of Bi2S3 (JCPDS-74-9437).58 In Figure S5, the spherical morphology and chemical constituents of Sb and S are illustrated for Sb2S3. The diameter of Sb2S3 spheres is around 70 nm. From the XRD pattern in Figure S6b, those Sb2S3 nanospheres are amorphous, which could be due to the low synthesizing temperature since it typically requires a temperature at least 180°C to obtain Sb2S3 with a good crystallinity.56, 57 The mechanism of forming a crystalline (Bi,Sb)2S3 nanotube, which is similar to the Bi2S3 nanotube, is possibly due to a self-crimping process.59, 60 Driven by the anisotropic growth, the lamellar structures are generated initially. Then they will roll up from the edges when a sufficient energy is offered to overcome the barrier of strain energy. A generation of the Sb2S3 spherical equilibrium shape in an amorphous phase is to minimize the surface energy. The relationship between the value of x in (BixSb1-x)2S3 and the atomic ratio of Bi/(Sb+Bi) in the precursor is plotted in Figure S7. There is a good linear relationship, with an R squared value of 0.98, between the ratio of Bi/(Sb+Bi) in the precursor and that in the products of (Bi,Sb)2S3. This indicates that Bi-to-Sb ratio in (Bi,Sb)2S3 could be easily tuned through adjusting the value of Bi/(Sb+Bi) in the precursor.To facilitate the study, all (Bi,Sb)2S3 nanotubes in the following


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discussions were fabricated by using the precursor with a 1:1 Bi-to-Sb ratio, corresponding to a chemical formula of Bi1.11Sb0.89S3. The electrochemical performance of Bi1.11Sb0.89S3 nanotube and its counterparts, Bi2S3 nanotube and Sb2S3 nanosphere, is evaluated with the selected charge/discharge profiles at 200 mA g-1 (0.27C based on the theoretical capacity of 737 mAh g-1 for Bi1.11Sb0.89S3) in the half cell, which is illustrated in Figure 3a–c. Their initial curves are shown in Figure S8. Duirng the tests, the investigated sample is the working electrode, a potassium metal foil serves as the counter electrode and reference electrode, and 3 M potassium bis(fluorosulfonyl)imide (KFSI) in dimethyl ether (DME) is the electrolyte. The electrolyte of 3 M KFSI in DME is selected based on our previous study on the potassium-ion based energy storage devices, where a more stable SEI film and a superior performance can be obtained using the electrolyte of 3 M KFSI in DME in comparison with 1.5 M KFSI in DME or the traditional KPF6 in a carbonate solvent.61, 62 In Figure S8a, Bi2S3 nanotube electrode shows a discharge capacity of 851.3 mAh g-1 in the initial cycle, along with a Coulombic efficiency of 52.6%. The extremely high discharge capacity of the first cycle is due to some irreversible processes, such as the formation of SEI film stemming from the reactions between electrodes and KFSI salt, which is identified for all three electrodes. The discharge capacity of Bi2S3 nanotube is clearly not stable after 50 cycles, presenting a degradation of 32.9% from 50th cycle to 100th cycle. Charge/discharge profiles of Sb2S3 nanosphere are revealed in Figure 3b and Figure S8b. It achieves a high discharge capacity of 1072.1 mAh g-1 with a Coulombic efficiency of 51.9% in the beginning cycle. A discharge capacity of 527.5 mAh g-1 remains for the 50th cycle. Nevertheless, it severely deteriorates to 403.8 mAh g-1 in the 100th cycle, generating a rapid capacity decay of 23.5% over 50 cycles. In contrast to the unstable performance of Bi2S3 and Sb2S3, a dramatically improved performance of


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Bi1.11Sb0.89S3 is described in Figure 3c. Besides an initial Coulombic efficiency of 59.8%, Bi1.11Sb0.89S3 nanotube electrode becomes relatively stable after 50 cycles, where the charge/discharge curves of the 50th cycle and the 100th cycle almost overlap and the capacity only loses by 2.99% from 445.3 mAh g-1 at the 50th cycle to 432.0 mAh g-1 at the 100th cycle, equivalent to a decay of 0.0597% per cycle. In addition to the superior stability, Bi1.11Sb0.89S3 nanotube electrode exhibits charge/discharge plateaus around 0.40 V, lower and broader than that of Bi2S3 and Sb2S3 electrodes, which are not only able to avoid the formation of detrimental metal dendrites but also provide a large cell voltage in the full cell. From the above discussion, Bi1.11Sb0.89S3 nanotube possesses a higher initial Coulombic efficiency, better stability, and more appropriate discharge plateaus than Bi2S3 nanotube and Sb2S3 nanosphere. To explore the potassiation/depotassiation process, cyclic voltammetry (CV) scans of Bi1.11Sb0.89S3 nanotubes, as well as Bi2S3 nanotubes and Sb2S3 nanospheres, were performed. The initial four CV scans of Bi1.11Sb0.89S3 nanotubes are depicted in Figure 3d. A strong peak appearing at 1.07 V in the first cathodic scan is mainly referred to the formation of SEI films, while the other peak showing up at 0.50 V is likely associated with the alloy reactions between K and (Bi,Sb) alloy, which is skewed a little by SEI formation. During the subsequent scans, the cathodic peaks shift to 1.55, 0.77, and 0.59 V and then sustain the peak position and intensity, expressing the realization of a stable SEI film after the first scan. The peak at 1.55 V should stem from the conversion reactions (which is sheltered by the intense peak from SEI formation in the first scan), and the other two peaks belong to the alloy reactions between K and (Bi,Sb) alloy. In the anodic scans, four peaks emerge. The peak at 2.11 V derives from the oxidation of (Bi,Sb) alloy, and peaks at 0.62, 0.86, and 1.16 V correspond to the depotassiation reactions. CV profiles of Bi2S3 nanotubes and Sb2S3 nanospheres are shown in Figure S9a and Figure S9b, respectively.


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Their peak positions are close to Bi1.11Sb0.89S3, but the intensity of cathodic and anodic peaks of Bi1.11Sb0.89S3 is much stronger than that of Sb2S3 nanospheres after the initial scan. This implies that Bi1.11Sb0.89S3 nanotubes are bound to have charge/discharge plateaus wider than that of Sb2S3 nanospheres, which is validated in the following discussions. The discharge capacities of Bi1.11Sb0.89S3 nanotube, Bi2S3 nanotube, and Sb2S3 nanosphere at different current densities, together with the corresponding Coulombic efficiency of Bi1.11Sb0.89S3 nanotube, are revealed in Figure 3e. Bi1.11Sb0.89S3 nanotube easily records a discharge capacity as high as 611 mAh g-1 at 100 mA g-1, 7% higher than that of Sb2S3 nanosphere electrode and 74% more than that of Bi2S3 nanotube. Meanwhile, charge/discharge curves of a Bi1.11Sb0.89S3 nanotube electrode with a heavy coating of ~2.9 mg cm-2 (equal to an active material loading of ~2 mg cm-2) are depicted in Figure S10 to explore the influence of coating. It delivers a stable discharge capacity close to 600 mAh g-1 over 10 cycles at 100 mA g-1 (0.135C) at such a heavy coating, indicating an excellent conductivity of this material. It is worth noting that the cycling stability and the electrode stability of the heavy-coating electrode is not as good as the normal-coating electrode, considering a larger volume change and more severe exfoliation of the active material from the current collector in the heavy-coating anode. With a high Coulombic efficiency, the discharge capacity of Bi1.11Sb0.89S3 nanotube electrode moderately decreases to 547, 477, 418, and 302 mAh g-1, respectively, as the current density increases to 200, 400, 600, and 800 mA g-1, corresponding to 90%, 78%, 68%, and 49% of the discharge capacity at the current density of 100 mA g-1. Even at a high current density of 1000 mA g-1 (1.35C), Bi1.11Sb0.89S3 nanotube electrode is still able to achieve a high capacity of 300 mAh g-1. More importantly, after experiencing the high current density, Bi1.11Sb0.89S3 nanotube maintains a stable discharge capacity about 500 mAh g-1 and a high Coulombic efficiency near


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98% when the current density is back to 100 mA g-1, expressing an excellent rate capability. On the contrary, Bi2S3 nanotube electrode reveals a very poor rate performance, only delivering a discharge capacity of 45 mAh g-1 at 1000 mA g-1, 13% of that at the current density of 100 mA g1

. Sb2S3 nanosphere electrode obtains a discharge capacity of 226 mAh g-1 as the current density

reaches 1000 mA g-1, which is significantly less than that of Bi1.11Sb0.89S3 nanotube. With the current density reducing to 100 mA g-1, Sb2S3 nanosphere electrode can regain a capacity over 500 mAh g-1, but it deteriorates fast in the following cycles, suggesting a low stability over a long cycling test. Overall speaking, Bi1.11Sb0.89S3 nanotube electrode exhibits a considerably improved rate performance. It tells that Bi1.11Sb0.89S3 nanotube owns a very resilient nanostructure which is able to accommodate various amplitudes of strain caused by the volume change at different current densities. To explore the stability, the discharge capacities of Bi1.11Sb0.89S3, Bi2S3 nanotube, and Sb2S3 nanosphere over long cycling tests at 500 mA g-1 (0.675C) are illustrated in Figure 3f (after activation). It is evident that the Bi1.11Sb0.89S3 nanotube electrode possesses a considerably better cycling stability than Bi2S3 nanotube and Sb2S3 nanosphere. Even after 1000 cycles, the Bi1.11Sb0.89S3 nanotube still delivers a capacity as high as 353 mAh g-1 at 500 mA g-1, achieving a capacity retention of 78% using the 6th cycle as the reference. More encouragingly, the Coulombic efficiency is close to 100% when it is approaching the 1000th cycle, expressing an outstanding stability of this electrode. Even at a small current density of 50 mA g-1, the cycling performance of Bi1.11Sb0.89S3 is stable for 200 cycles (tested for about 5 months), which is displayed in Figure S11. On the contrary, the capacities of Bi2S3 nanotube and Sb2S3 nanosphere are lower than that of Bi1.11Sb0.89S3 nanotube and deteriorate sharply with cycles going on. The electrochemical impedance spectra (EIS) of Bi1.11Sb0.89S3 are adopted to further investigate the


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electrochemical process, which is illustrated by the Nyquist plots in Figure S12. It is clear that the charge-transfer resistance (Rct) derived from the diameter of the semicircle in the middle frequency decreases dramatically after 10 cycles at 500 mA g-1, indicating a formation of SEI film in the initial cycles. After 50 cycles, Rct only increases slightly, illustrating that a stable SEI film has been formed, which is beneficial to the cycling performance of the battery. The specific capacity, discharge plateau, and cycling performance are three critical electrochemical parameters of an electrode. Therefore, comparison of the these three factors between Bi1.11Sb0.89S3 nanotube and the state-of-the-art anodes reported for KIBs with at least 500 cycles is illustrated in Figure 3g and Table S1.4, 8, 11, 32, 45, 49, 52, 63, 64 It is apparent that the Bi1.11Sb0.89S3 nanotube electrode displays the leading capacity, a highly appropriate discharge plateau, and an outstanding cycling performance, not mentioning the easy fabrication process at a low temperature of 60oC, which is a significant advancement of the anode for KIBs. As illustrated in Figure 4a, the charge/discharge curves from the 400th cycle to the 1000th cycle almost overlap, displaying an admirable stability with a promising discharge plateau around 0.3 V. CV scans of the Bi1.11Sb0.89S3 nanotube electrode after being cycled at 500 mA g-1 for 910 cycles are depicted in Figure 4b. Three CV profiles of the cycled electrode exhibit the constant position and intensity, proving the superior stability of the electrodes. For the cathodic scans, two tiny peaks appear at 0.56 and 0.43 V, and an intense peak is observed at around 0.28 V, agreeing with the wide discharge plateau around 0.3 V. The anodic peaks are located at 0.55, 0.80, and 1.19 V. All of these peaks point to the alloy reactions between K+ and (Bi,Sb) alloy nanoparticles. It is clear that the redox couple peaks of 1.55/2.11 V, which associate with the conversion reactions, disappear after 910 cycles, suggesting the reversibility of the conversion reactions is limited. Meanwhile, the cathodic peaks obviously shift negatively in comparison to


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the initial few CV cycles in Figure 3d, which is due to the complete formation of (Bi,Sb) alloy over long cycles since the conversion reaction and alloying process are not finished in the first few CV cycles. Besides, TEM image, HAADF-STEM image, and EDS mappings of the Bi1.11Sb0.89S3 nanotube at 0.01 V after 670 cycles are displayed in Figure 4c and 4d, further illustrating that the integrity of Bi1.11Sb0.89S3 nanotube is well reserved after 670 cycles and explaining the outstanding stability of the electrode. It is worth mentioning that a small current density of 10 mA g-1 was adopted in our experiments to reach the desired potential to obtain an equilibrium status. Meanwhile, TEM images in Figure S13a and S13b portrait the morphology of Bi1.11Sb0.89S3 nanotubes being discharged to 1.2 V (after the conversion reaction) and 0.01 V (fully potassiated), respectively, while Figure S13c displays the morphology being charged to 2.2 V (depotassiated). These results clearly demonstrate that the integrity of Bi1.11Sb0.89S3 nanotube is retained during potassiation/depotassiation. To interpret the outstanding electrochemical performance of Bi1.11Sb0.89S3 nanotube, a contour plot of the operando XRD results of Bi1.11Sb0.89S3 nanotube during discharging/charging process for the initial three cycles and the phase evolution are revealed in Figure 5. With the electrode being discharged, the peak intensity of Bi1.11Sb0.89S3 becomes weaker and weaker, indicating the intercalation of K+ into Bi1.11Sb0.89S3 which weakens the crystallinity. When the potential drops to 1.2 V where the conversion reactions happen, peaks from Bi1.11Sb0.89S3 are difficult to be detected while the peaks around 27.50o emerges, indexed to (012) facet of (Bi,Sb) alloy.55 It tells that Bi3+ and Sb3+ in Bi1.11Sb0.89S3 are reduced to Bi and Sb which form (Bi,Sb) alloy thereafter. The existence of (Bi,Sb) alloy is further verified by the HRTEM image of an Bi1.11Sb0.89S3 nanotube electrode being discharged to 0.8 V in Figure 6a, where nanoparticles with a size around 10 nm and a lattice space of 0.319 nm are detected. The lattice spacing


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corresponds to (012) facet of (Bi,Sb) alloy, consistent with XRD results.65 The ratio of Bi:Sb in the alloy is quantified to be 1.03:0.97 by EDS analysis, which is close to that in the Bi1.11Sb0.89S3 nanotube. Because of the solid state, the alloying of Bi and Sb is confined to the local area, thus establishing homogeneous distribution of (Bi,Sb) alloy through the entire nanotube, which is exhibited in the HAADF-STEM image and the related EDS mappings of the electrode being discharged to 0.8 V in Figure S14. The peaks at 25.4 o and 28.5o are identified to (013) and (0-43) facet of K2S4 (PDF#30-0992), respectively. As the electrode is further discharged 0.6 V, the alloying reactions between (Bi,Sb) alloy and K ions are going to occur and K(Bi,Sb) appears, which is confirmed by the peak around 31.3 o in Figure 5, corresponding to (121) facet of K(Bi,Sb) (PDF#42-0791). With the discharging process moving on to 0.2 V, a peak corresponding to (220) of K3(Bi,Sb) is distinguished at 29.7o (PDF#19-0935). In the charging process, the K(Bi,Sb) peak shows up again around 0.6 V, corresponding to depotassiation of K3(Bi,Sb) to yield K(Bi,Sb). The depotassiation of K(Bi,Sb) continues with the charging process going on, generating (Bi,Sb) alloy around 1.2 V. Peaks associating with Bi1.11Sb0.89S3 are not recovered at all during the charging process, suggesting that the formation of (Bi,Sb) alloy from destruction of the lattice structure of Bi1.11Sb0.89S3 is irreversible. The potassiation/depotassiation process of (Bi,Sb) alloy is highly reversible in the following two cycles, which is clearly demonstrated in Figure 5. Based on the above results, the detailed potassiation/depotassiation process related to various potentials in the first cycle is summarized in Figure 6b. During the discharging process, the K+ intercalation proceeds above 1.5 V; then reduction of Bi3+ and Sb3+ and formation of (Bi,Sb) alloy occur from 1.5 V and 0.8 V, majorly around 1.2 V; below 0.8 V, alloying reactions between K and (Bi,Sb) alloy take place to generate K(Bi,Sb), primarily around 0.6 V; at about


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0.2 V, alloying reactions proceed to produce K3(Bi,Sb). In the charging process, the depotassiation of K3(Bi,Sb) to form K(Bi,Sb) happens about 0.6 V; thereafter, the depotassiation of K(Bi,Sb) to produce (Bi,Sb) alloy follows around 1.2 V. Meanwhile, the potassiation/depotassiation process of (Bi,Sb)2S3 nanotube can be separated into six stages vividly illustrated in Figure 6c, where the reversibility of stage I and II is limited while the stage III , IV, V, and VI are highly reversible. (Bi,Sb)2S3 + K+ + e → Bi + Sb + K2S4

(around 1.2 V)

(I)

Bi + Sb → (Bi,Sb)

(around 1.2 V)

(II)

(Bi,Sb) + K+ + e → K(Bi,Sb)

(around 0.6 V)

(III)

K(Bi,Sb) + K+ + e→ K3(Bi,Sb)

(around 0.2 V)

(IV)

K3(Bi,Sb) - K+ - e → K(Bi,Sb)

(around 0.6 V)

(V)

K(Bi,Sb) - K+ - e → (Bi,Sb)

(around 1.2 V)

(VI)

During the discharge process, K+ intercalates into the (Bi,Sb)2S3 above 1.5 V. The conversion reactions start below 1.5 V in stage I, where Bi3+ and Sb3+ are reduced to Bi atom and Sb atom (majorly around 1.2 V), respectively, while S2- forms K2S4 with K+. Then in-situ alloying takes place between Bi atom and Sb atom to create homogeneous (Bi,Sb) alloy nanoparticles dispersed evenly through the whole nanotube, which is stage II. During the stage III, (Bi,Sb) alloy reacts with K+ and produces K(Bi,Sb), which further forms K3(Bi,Sb) in stage IV. In the charging process, depotassiation of K3(Bi,Sb) to generate K(Bi,Sb) occurs in the stage V, and it is further depotassiated to return to (Bi,Sb) in stage VI. The synergistic effect between Bi atom and Sb atom in the alloy generates a buffering effect which successfully reduces the strain caused by the volume change during potassiation/depotassiation, herein maintaining the structure integrity, improving the cycling stability, and retaining the high capacity from the alloying reactions


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beween K+ and (Bi,Sb) alloy. It is worth mentioning that although the tubular structure is beneficial to maintain the nanostructure by providing a buffering room, the capacity of Bi2S3 nanotube still decays very fast, verifying that the synergistic effect from (Bi,Sb) alloy is the key in realizing the outstanding performance. To study the feasibility of this material, Bi1.11Sb0.89S3 nanotube anode was coupled with a reported organic cathode, perylenetetracarboxylic dianhydride (PTCDA),62, 66 to build KIBs full cell, which is illustrated in Figure 7a. The typical charge/discharge curves of PTCDA cathode at 100 mA g-1 from 1.5 to 3.5 V is presented in Figure S15. Based on the charge/discharge profiles of the anode and cathode, the voltage range of the full cell is set to be 0.8–3.4 V, and the corresponding charge/discharge curves at 500 mA g-1 are exposed in Figure 7b. The full cell shows a discharge plateau residing at 2.3 V, and two charge plateaus around 2.5 and 3.1 V. Inspiringly, it achieves a discharge capacity as high as 260 mAh g-1 (calculated based on the mass of the active material of the anode) in the first 10 cycles at a large current density of 500 mA g-1. The cycling performance of the full cell is illustrated in Figure 7c. It delivers an initial discharge capacity as high as 276 mAh g-1, and then it decays moderately to 222 mAh g-1 at the 50th cycle. In the following cycles, it is prone to be stable and a capacity of 207 mAh g-1 is reserved at the 100th cycle, corresponding to decay rate of ~0.1% per cycle. The Coulombic efficiency is always close to 98%. In comparison with the half cell, a lower capacity of Bi1.11Sb0.89S3 in the full cell is presented. This is due to the lower charge voltage in the full cell (around 1.2 V vs. K+/K), while the electrode is charged to 2.2 V (vs. K+/K) in the half cell, in addition to the rough match of capacities between the anode and cathode. The full cell performance can be further improved by optimizing the cathode material and the cell balance. CONCLUSIONS


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In summary, (Bi,Sb)2S3 nanotubes with various Bi:Sb ratios are successfully synthesized using a scalable one-step method at a low temperature of 60oC. An in-situ alloying strategy is revealed to develop homogeneous (Bi,Sb) alloy nanoparticles distributed evenly within the nanotubes. During the initial discharge, the phase transformation is (Bi,Sb)2S3 → (Bi,Sb) → K(Bi,Sb) → K3(Bi,Sb). During the charge, the phase evolution of K3(Bi,Sb) → K(Bi,Sb) → (Bi,Sb) emerges. The in-situ alloying strategy brings a synergistic effect and yields a leading anode material for KIBs, achieving an ultrahigh specific capacity (611 mAh g-1 at a current density of 100 mA g-1, 0.135C), an excellent rate capability (300 mAh g-1 at a current density of 1000 mA g-1, 1.35C), and a super-long cycle life (over 1000 cycles with 353 mAh g-1 remained at 500 mA g-1, 0.675C). Considering the outstanding electrochemical performance, relatively low cost, easy fabrication, and environmental friendliness, this anode material is highly promising and a significant advancement of KIBs.

EXPERIMENTAL SECTION Material Synthesis. (Bi,Sb)2S3 nanotubes with a 1:1 ratio of Bi:Sb in the precursor were synthesized by a facile co-precipitation method. Frist, 0.375 mmol SbCl3 (≥ 99.0%, Sinopharm Chemical Reagent Co., Ltd) and 0.375 mmol Bi(NO3)3·5H2O (≥ 99.0%, Sinopharm Chemical Reagent Co., Ltd) were dissolved in 15 mL ethylene glycol in turn to form a homogeneous solution, noted as solution A. Second, graphene nanosheets (Nanjing/Jiangsu XFNANO Materials Tech Co., Ltd) were uniformly dispersed in 15 mL ethylene glycol by sonication to obtain a concentration of 1 mg mL-1, and then the sulfur source, 1.125 mmol thioacetamide (Shanghai Macklin Biochemical Co., Ltd), was added into this solution and let it stir for half an hour to be completely dissolved. This solution was designated as solution B. Third, solution A


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with Bi(NO3)3 and SbCl3 was added into solution B dropwise. The mixtures were magnetically stirred for half an hour and then maintained at 60°C for 24 hours. The dark products were washed with ethanol and deionized water for several times, and thereafter separated by centrifugation. The final products were obtained after freeze drying. Same experimental procedures were employed for the synthesis of (Bi,Sb)2S3 nanotubes with various ratios of Bi:Sb in the precursor as well as Bi2S3 nanotubes and Sb2S3 nanospheres. It is worth mentioning that the total amount of Bi3+ and Sb3+ is always 0.75 mmol in the precursor. Meanwhile, only 0.75 mmol Bi(NO3)3 was used for preparing Bi2S3 nanotubes while merely 0.75 mmol SbCl3 was added for synthesizing Sb2S3 nanospheres. Material Characterizations. Powder XRD data were obtained using RIGAKU RINT-2000 (Cu Kα). Operando XRD results were acquired through Bruker D8 ADVANCE (Cu Kα) at a current density of 100 mA g-1, with the investigated sample as the working electrode, potassium metal foil as the counter electrode and reference electrode, and 3 M KFSI in DME as the electrolyte. XPS analysis was carried out by ESCALAB 250Xi for obtaining the chemical composition. The morphology was characterized by using field emission scanning electron microscope (FESEM, Hitachi S-4800, 20 kV). A Titan G2 60-300 TEM with selected area electron diffraction (SAED) was used for obtaining detailed structural information. Electrochemical Measurements. Bi1.11Sb0.89S3 nanotube electrode, Bi2S3 nanotube electrode, and Sb2S3 electrode were prepared by mixing active materials, carbon black, and carboxymethyl cellulose (CMC) with a weight ratio of 7:2:1 in a solution of H2O and C2H5OH, followed by casting the slurry onto a copper foil. The average loading of active material on the anode is ~1.0 mg cm-2. The loading of the active material has been doubled to ~2.0 mg cm-2 in the tests exploring the influence of the heavy coating on the electrochemical performance of Bi1.11Sb0.89S3


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nanotube electrodes. The cathode was fabricated by mixing the thermally annealed PTCDA, carbon black, and CMC at a weight ratio of 8:1:1, which was spread on an aluminum foil. Half cells were assembled using 2032 coin cells under Ar with the investigated sample as the working electrode, a potassium metal foil as the counter electrode and reference electrode, and 3 M KFSI in DME as the electrolyte. The amount of electrolyte for each coin cell is around 100 l. The cells were cycled at 100 and 200 mA g-1 between 0.01 and 2.2 V for a few times to obtain densely packed SEI films before the stability tests at 500 mA g-1. Full cells were obtained with 2032 coin cells by utilizing Bi1.11Sb0.89S3 nanotube anode, PTCDA cathode, the electrolyte of 3 M KFSI in DME, and Whatman glass fibers serving as the separator. Bi1.11Sb0.89S3 nanotube anode was cycled in half cells and then discharged to 0.01 V at a current density of 500 mA g-1 before being integrated into the full cell. The charge/discharge behaviors were assessed by the testing system Neware BTS-53. The voltage ranges are 0.01–2.2 V for half cells and 0.8–3.4 V for full cells. CV scans were performed through an electrochemical workstation. To study the cycled electrodes, the coin cells were carefully disassembled in a glovebox, followed by washing the electrodes in DME to get rid of the residuals of potassium salt.

Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Low-magnification SEM image of Bi1.11Sb0.89S3; TEM image, HAADF-STEM image, and EDS mappings of Bi0.26Sb1.74S3; TEM image, HAADF-STEM image, and EDS mappings of Bi1.57Sb0.43S3; SEM image, TEM image, HAADF-STEM image, and EDS mappings of Bi2S3; XRD patterns of Bi2S3 and Sb2S3; the relationship between the value of x in (BixSb1-x)2S3 and the atomic ratio of Bi/(Sb+Bi) in the precursor; initial discharge/charge curves of Bi2S3, Sb2S3 and Bi1.11Sb0.89S3; CV profiles of Bi2S3 and Sb2S3; charge/dischare curves of Bi1.11Sb0.89S3 with a heavy coating of ~2.9 mg cm-2; cycling performance of Bi1.11Sb0.89S3 at 50 mA g-1; Nyquist plots of Bi1.11Sb0.89S3 and the equivalent-circuit model; TEM images of Bi1.11Sb0.89S3 being discharged and charged; HAADF-STEM image and EDS mappings of Bi1.11Sb0.89S3 being discharged to 0.8 V; the typical charge/discharge curves of the thermally annealed PTCDA cathode; the table of 18 ACS Paragon Plus Environment

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electrochemical-performance parameters of the reported anode materials for KIBs in comparison with this work.

ASSOCIATED CONTENT The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by Natural Science Foundation of China (Nos. 51672078, 21473052, 61474041), Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Independent Research Project (No. 71675004), Hunan Youth Talents (2016RS3025), and Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-903).

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(44) Yu, Q.; Hu, J.; Gao, Y.; Gao, J.; Suo, G.; Zuo, P.; Wang, W.; Yin, G. Iron Sulfide/Carbon Hybrid Cluster as an Anode for Potassium-Ion Storage. J. Alloys Compd. 2018, 766, 1086– 1091. (45) Zhao, Y.; Zhu, J.; Ong, S. J. H.; Yao, Q.; Shi, X.; Hou, K.; Xu, Z. J.; Guan, L. High-Rate and Ultralong Cycle-Life Potassium Ion Batteries Enabled by In Situ Engineering of YolkShell FeS2@C Structure on Graphene Matrix. Adv. Energy Mater. 2018, 8, 1802565. (46) Mao, M.; Cui, C.; Wu, M.; Zhang, M.; Gao, T.; Fan, X.; Chen, J.; Wang, T.; Ma, J.; Wang, C. Flexible ReS2 Nanosheets/N-Doped Carbon Nanofibers-Based Paper as a Universal Anode for Alkali (Li, Na, K) Ion Battery. Nano Energy 2018, 45, 346–352. (47) Ren, X.; Zhao, Q.; McCulloch, W. D.; Wu, Y. MoS2 as a Long-Life Host Material for Potassium Ion Intercalation. Nano Res. 2017, 10, 1313–1321. (48) Xie, K.; Yuan, K.; Li, X.; Lu, W.; Shen, C.; Liang, C.; Vajtai, R.; Ajayan, P.; Wei, B. Superior Potassium Ion Storage Via Vertical MoS2 “Nano-Rose” with Expanded Interlayers on Graphene. Small 2017, 13, 1701471. (49) Jia, B.; Yu, Q.; Zhao, Y.; Qin, M.; Wang, W.; Liu, Z.; Lao, C.; Liu, Y.; Wu, H.; Zhang, Z.; Qu, X. Bamboo-Like Hollow Tubes with MoS2/N-Doped-C Interfaces Boost Potassium-Ion Storage. Adv. Funct. Mater. 2018, 28, 1803409. (50) Lakshmi, V.; Chen, Y.; Mikhaylov, A. A.; Medvedev, A. G.; Sultana, I.; Rahman, M. M.; Lev, O.; Prikhodchenko, P. V.; Glushenkov, A. M. Nanocrystalline SnS2 Coated Onto Reduced Graphene Oxide: Demonstrating the Feasibility of a Non-Graphitic Anode with Sulfide Chemistry for Potassium-Ion Batteries. Chem. Commun. 2017, 53, 8272–8275. (51) Lu, Y.; Chen, J. Robust Self-Supported Anode by Integrating Sb2S3 Nanoparticles with S,NCodoped Graphene to Enhance K-Storage Performance. Sci. China: Chem. 2017, 60, 1533– 1539. (52) Liu, Y.; Tai, Z.; Zhang, J.; Pang, W. K.; Zhang, Q.; Feng, H.; Konstantinov, K.; Guo, Z.; Liu, H. K. Boosting Potassium-Ion Batteries by Few-Layered Composite Anodes Prepared Via Solution-Triggered One-Step Shear Exfoliation. Nat. Commun. 2018, 9, 3645. (53) Ji, L.; Gu, M.; Shao, Y.; Li, X.; Engelhard, M. H.; Arey, B. W.; Wang, W.; Nie, Z.; Xiao, J.; Wang, C.; Zhang, J.; Liu, J. Controlling SEI Formation on SnSb-Porous Carbon Nanofibers for Improved Na Ion Storage. Adv. Mater. 2014, 26, 2901–2908. (54) Farbod, B.; Cui, K.; Kalisvaart, W. P.; Kupsta, M.; Zahiri, B.; Kohandehghan, A.; Lotfabad, E. M.; Li, Z.; Luber, E. J.; Mitlin, D. Anodes for Sodium Ion Batteries Based on TinGermanium-Antimony Alloys. ACS Nano 2014, 8, 4415–4429. (55) Zhao, Y.; Manthiram, A. High-Capacity, High-Rate Bi–Sb Alloy Anodes for Lithium-Ion and Sodium-Ion Batteries. Chem. Mater. 2015, 27, 3096–3101. 23 ACS Paragon Plus Environment

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(56) Zhao, Y.; Manthiram, A. Bi0.94Sb1.06S3 Nanorod Cluster Anodes for Sodium-Ion Batteries: Enhanced Reversibility by the Synergistic Effect of the Bi2S3-Sb2S3 Solid Solution. Chem. Mater. 2015, 27, 6139–6145. (57) Wang, J.; Yu, H.; Wang, T.; Qiao, Y.; Feng, Y.; Chen, K. Composition-Dependent Aspect Ratio and Photoconductivity of Ternary (BixSb1-x)2S3 Nanorods. ACS Appl. Mater. Interfaces 2018, 10, 7334–7343. (58) Dutková, E.; Sayagués, M. J.; Zorkovská, A.; Real, C.; Baláž, P.; Šatka, A.; Kováč, J. Properties of Mechanochemically Synthesized Nanocrystalline Bi2S3 Particles. Mater. Sci. Semicond. Process. 2014, 27, 267–272. (59) Zhu G.; Liu P.; Zhou J.; Bian X.; Wang X; Li J. Hydrothermal Synthesis of Bismuth Sulfide Nanotubes and Its Formation Mechanism. Chem. Res. Chin. Univ. 2008, 29, 240–243. (60) Wang, D.; Hao, C.; Zheng, W.; Ma, X.; Chu, D.; Peng, Q.; Li, Y. Bi2S3 Nanotubes: Facile Synthesis and Growth Mechanism. Nano Res. 2009, 2, 130–134. (61) Fan, L.; Lin, K.; Wang, J.; Ma, R.; Lu, B. A Nonaqueous Potassium-Based Battery– Supercapacitor Hybrid Device. Adv. Mater. 2018, 30, 1800804. (62) Fan, L.; Ma, R.; Wang, J.; Yang, H.; Lu, B. An Ultrafast and Highly Stable Potassium– Organic Battery. Adv. Mater. 2018, 30, 1805486. (63) Liu, L.; Chen, Y.; Xie, Y.; Tao, P.; Li, Q.; Yan, C. Understanding of the Ultrastable K-Ion Storage of Carbonaceous Anode. Adv. Funct. Mater. 2018, 28, 1801989. (64) Yu, Q.; Jiang, B.; Hu, J.; Lao, C.; Gao, Y.; Li, P.; Liu, Z.; Suo, G.; He, D.; Wang, W. (.; Yin, G. Metallic Octahedral CoSe2 Threaded by N-Doped Carbon Nanotubes: A Flexible Framework for High-Performance Potassium-Ion Batteries. Adv. Sci. 2018, 5, 1800782. (65) Gao, H.; Niu, J.; Zhang, C.; Peng, Z.; Zhang, Z. A Dealloying Synthetic Strategy for Nanoporous Bismuth–Antimony Anodes for Sodium Ion Batteries. ACS Nano 2018, 12, 3568–3577. (66) Chen, Y.; Luo, W.; Carter, M.; Zhou, L.; Dai, J.; Fu, K.; Lacey, S.; Li, T.; Wan, J.; Han, X.; Bao, Y.; Hu, L. Organic Electrode for Non-Aqueous Potassium-Ion Batteries. Nano Energy 2015, 18, 205–211.


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Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

a

b

c

d

e

Figure 1. a) Illustration of the crystal structure of Bi2S3 and Sb2S3 viewed along [010] direction; b) The synthesis procedures of (Bi,Sb)2S3 nanotubes; c) SEM image, d) TEM images, and e) HAADF-STEM image and EDS mappings of as synthesized Bi1.11Sb0.89S3 nanotubes.


ACS Nano

(015) (214)

(113) 013) (402) (312) (410) (411)

(212)

Bi1.11Sb0.89S3

(210)

(103) (202) (011)

(200) (201)

15

(211)

b

a

25 35 2 (Degree)

45

c Bi4f7/2

Sb3d3/2

Bi4f5/2 Sb3d5/2

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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S2p3/2

174 171 168 165 162 159 156 153 Binding Energy (eV)

O1s

546 543 540 537 534 531 528 525 Binding Energy (eV)

Figure 2. a) HRTEM image, b) XRD pattern, and c) XPS of Bi1.11Sb0.89S3 nanotubes.


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1.0 0.5

0.62 0.2

Bi1.11Sb0.89S3

1.16 1.21

0.86

0.1

2.11

0.0 -0.1 0.77 0.59 1.07 0.50

-0.2 0.0

1st 2nd 3rd 4th

1.55

0.5 1.0 1.5 2.0 + Potential (V vs. K /K)

1.5

50th 100th 200 mA g-1

1.0 0.5

0

e 900 600 300

0.135C

10

Sb2S3

Bi1.11Sb0.89S3

100

600 800

1000

1.08C

1.35C

30

40

Cycle Number

80

Bi1.11Sb0.89S3

60 40

300 20

0 600

Coulombic Efficiency (%)

Sb2S3

g

60

0.135C 40

50

0

Our Work Bi1.11Sb0.89S3, 500 mA g-1

350 300 FeS2, 300 mA g-1

250

K2TP, 1000 mA g

-1 200 CoSe2, 2000 mA g

-1

MoSe2, 1000 mA g-1 N-carbon NS, 500 mA g-1

VSe2, 2000 mA g-1 N-carbon NF, 500 mA g-1 Sb2S3, 1000 mA g-1 + /K) -1 MoS2, 500 mA g 0.0 vs. K 500 V 0.5 au ( 600 te Cy cle 700 800 Pla 1.0 e Nu g r a mb 900 h c er 1000 Dis

150

0 1000

800

80

20 0.81C

20

Coulombic Efficiency of Bi1.11Sb0.89S3

400

100 200 300 400 500 Specific Capacity (mAh g-1)

Coulombic Efficiency of (Bi,Sb)2S3

0.54C

0

353

200

0

0.27C

600

0

Bi2S3

400

2.5

Bi2S3

0.675C

0.5

100

0.01–2.2 V -1 Current Density: mA g 100 200

-1

0.01–2.2 V

1.0

1200

100

900 Current Density: 500 mA g-1

50th 100th 200 mA g-1 0.27C

1.5

0.0

100 200 300 400 500 Specific Capacity (mAh g-1)

f 1200

Bi1.11Sb0.89S3

2.0

-1 ) city (m Ah g Sp ec ific Ca pa

d

Sb2S3

0.0

100 200 300 400 500 Specific Capacity (mAh g-1) -1

0

2.0


1.5

50th 100th 200 mA g-1

Potential (V vs. K+/K)

Bi2S3


2.0

0.0

Current (mA)

c

b

Specific Capacity (mAh g )


a

Specific Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Cycle Number

Figure 3. Charge/discharge curves of a) Bi2S3, b) Sb2S3, and c) Bi1.11Sb0.89S3 at 200 mA g-1 (0.27C based on a theoretical capacity of 737 mAh g-1 for Bi1.11Sb0.89S3) from 0.01 to 2.2 V; d) CV profiles of Bi1.11Sb0.89S3 nanotubes with a scan rate of 0.1 mV/s; e) Rate performance; f) Cycling stability of Bi2S3, Sb2S3, and Bi1.11Sb0.89S3; g) Comparison of the specific capacity, discharge plateau, and cycling performance between the Bi1.11Sb0.89S3 nanotube electrode and other reported electrodes for KIBs with at least 500 cycles (K2TP means dipotassium terephthalate, NS stands for nanosheet, and NF is nanofiber). [4, 8, 11, 32, 45, 49, 52, 63, 64]


ACS Nano

a 2.0

b

500 mA g-1 Bi1.11Sb0.89S3 400th 600th 800th 1000th

1.5

500th 700th 900th

1.0

Bi1.11Sb0.89S3 after 910 cycles 1st 0.55 2nd 0.80 1.19 3rd

0.2

Current (mA)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.1

-0.1

0.56 0.42 0.28

-0.2 0

100 200 300 Specific Capacity (mAh g-1)

c

0.0

0.5 0.0

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0.0

0.5 1.0 1.5 2.0 Potential (V vs. K+/K)

HAAD F

2.5

d

K

Sb

Bi

S

Figure 4. a) Charge/discharge curves of Bi1.11Sb0.89S3 nanotube from the 400th to the 1000th cycle; b) CV profiles of Bi1.11Sb0.89S3 after being cycled at 500 mA g-1 for 910 cycles; c) TEM image of Bi1.11Sb0.89S3 at 0.01 V after being cycled at 500 mA g-1 for 670 cycles; d) HAADFSTEM image and EDS mappings of Bi1.11Sb0.89S3 nanotube at 0.01 V after being cycled at 500 mA g-1 for 670 cycles.


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(Bi,Sb) K2S4 K3(Bi,Sb) K(Bi,Sb) Bi:Sb=1.03:0.97 (121) (012) (0-43) (220)

70 60

0.6 V

0.2 V

0.6 V

40

30

1.2 V

0.6 V

10

5 16

20

0.2 V

0.6 V

10

20

1st cycle

10

0

0.6 V

1.2 V

30

40

15

Index

50

20

0.6 V

0.2 V

2nd cycle

60

25

1.2 V

50

70

80

30

80

3rd cycle

90

90

K2S4 (013)

Index Time (hour)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

17

18

19

20

21

22

23

1.2 V 24 25 25

26

2627

28

29 27

30

31 28 32

33

34 29

35

3630

37

38

31

2 1 0 222Theta (Coupled 25 TwoTheta/Theta) 26 WL=1.54060 27 2Theta 28 (Coupled29 30 WL=1.54060 31 TwoTheta/Theta) Potential (V vs. K+/K) (202) (103) (211)

2θ (Degree) Figure 5. Contour plot of the operando XRD results of Bi1.11Sb0.89S3 during discharging/charging process for the initial three cycles.


32

33

34

32 (212) Bi1.11Sb0.89S3

ACS Nano

0 300

900

e arg

600

K+ intercalation Reduction of Bi3+ and Sb3+and formation of (Bi,Sb) alloy Formation of K(Bi,Sb) Formation of K3(Bi,Sb)

Ch ar ge

b Specific Capacity (mAh g-1)

a

ch Dis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1200

Depotassiation of K3(Bi,Sb) to form K(Bi,Sb) Depotassiation of K(Bi,Sb) to form (Bi,Sb) alloy

3 2 1 0 Potential (V vs. K+/K)

c

Figure 6. a) HRTEM image of Bi1.11Sb0.89S3 after being discharged to 0.8 V during the first cycle; b) The detailed potassiation/depotassiation process in the first cycle; c) Schematic illustration of the potassiation/depotassiation process in (Bi,Sb)2S3.


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a

b Cell Voltage (V)

3.0

Bi1.11Sb0.89S3-PTCDA Full Cell

2.5

2nd 5th 10th 500 mA g-1

2.0 1.5 1.0

c -1

100 200 300 Specific Capacity (mAh g-1) 100

300

80

200

-1

Bi1.11Sb0.89S3-PTCDA Full Cell Current Density: 500 mA g Charge 100 Discharge 0 Coulombic Efficiency 0 20 40 60 80 Cycle Number

60

0.8–3.4 V

40 20

0 100


0 Specific Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 7. a) Demonstration of KIBs full cell with a Bi1.11Sb0.89S3 anode and a PTCDA cathode; b) Charge/discharge curves of the full cell at 500 mA g-1 with a cell voltage range of 0.8–3.4 V; c) Cycling test of the full cell at 500 mA g-1 with a cell voltage range of 0.8–3.4 V.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic (For Table of Contents Only)


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In Situ Alloying Strategy for Exceptional Potassium Ion Batteries - ACS

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