sodium storage anode materials

Journal Pre-proof Zeolitic imidazolate frameworks derived ZnS/Co3S4 composite nanoparticles doping on polyhedral carbon framework for efficient lithium/sodium storage anode materials Zheng Zhang, Ying Huang, Xudong Liu, Chen Chen, Zhipeng Xu, Panbo Liu PII:

S0008-6223(19)31067-X

DOI:

https://doi.org/10.1016/j.carbon.2019.10.052

Reference:

CARBON 14714

To appear in:

Carbon

Received Date: 4 September 2019 Revised Date:

18 October 2019

Accepted Date: 19 October 2019

Please cite this article as: Z. Zhang, Y. Huang, X. Liu, C. Chen, Z. Xu, P. Liu, Zeolitic imidazolate frameworks derived ZnS/Co3S4 composite nanoparticles doping on polyhedral carbon framework for efficient lithium/sodium storage anode materials, Carbon (2019), doi: https://doi.org/10.1016/ j.carbon.2019.10.052. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical abstract Schematic illustration of detailed synthesis of three composites material. Firstly, the ZIF polyhedral precursor was prepared by simple liquid phase diffusion method. Then the precursor was carbonized at 600 °C for 2 h under the protection of argon gas. Finally, the subsequent sulfidation process was completed under hydrothermal conditions with TAA, and finally three composites material was obtained.

Zeolitic imidazolate frameworks derived ZnS/Co3S4 composite nanoparticles doping on polyhedral carbon framework for efficient lithium/sodium storage anode materials Zheng Zhang, Ying Huang*, Xudong Liu, Chen chen Zhipeng Xu, Panbo Liu MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, School of Science, Northwestern Polytechnical University, Xi’an 710072, PR China Abstract: High-performance lithium-ion batteries (LIBs) and sodium-ion battery (SIBs) anode materials Co3S4-ZnS/NC are prepared by carbonization and subsequent sulfidation using Co, Zn-based zeolitic imidazolate frameworks (ZIF) as a precursor. ZnS and Co3S4 in-situ doping on polyhedral carbon framework, which enhances the conductivity and can buffer the volume expansion during the cycle and improve the kinetics. When Co3S4-ZnS/NC use as an anode material for LIBs, which can deliver reversible capacity 750 mA h g-1 after 200 cycles at current density of 1 A g-1. It also exhibits excellent sodium storage, with capacities remaining 316.5 mA h g-1, after 1000 cycles at current density of 1 A g-1. Compared with pure ZnS/NC and Co3S4/NC electrode, the Co3S4-ZnS/NC electrode also exhibits enhanced rate performance in both Li/Na storage. This excellent performance may be due to the following factors. (a) The polyhedral carbon skeleton makes the composites has superior conductivity and excellent structural stability. (b) The large specific surface area and porous structure can make the electrolyte fully infiltrate the electrode. (c) The high capacitance contribution gives the composite excellent Li/Na storage performance, especially at high current density. These results reveal the potential application prospects of Co3S4-ZnS/NC in high performance LIBs and SIBs. Keyword: Lithium-ion batteries, Sodium-ion battery, Anode, Zeolitic imidazolate frameworks

*

Corresponding author. Tel: +86 29 88431636. E-mail address: [email protected] (Ying Huang).

1. Introduction The huge consumption of global fossil fuels has caused the energy crisis of resource exhaustion, and directly or indirectly caused resulted in serious environmental problems [1-3]. Developing green, clean and renewable new secondary energy is considered as one of the most promising solutions for this dilemma. As two kinds of electrochemical devices in the development of sustainable energy system, lithium ion batteries (LIBs) and sodium ion batteries (SIBs) have attracted many researchers interest [4-6]. In the past few years, it has been proved that LIBs are the dominant power supply for portable electronic products and electric vehicles. However, LIBs face such problems as high cost and price, uneven distribution of lithium resources and limited reserves, which limit its further promotion in large-scale applications in the future [7-9]. In contrast, sodium resources are widely distributed all over the world, and the earth's crust is rich in reserves and cheap. More importantly, the redox point of sodium is -2.7 V (relative to standard hydrogen electrode), slightly higher than that of lithium metal (-3.04), making SIBs a strong competitor for LIBs [10-12]. However, the radius of Na+ (97 pm) is about 43% larger than that of Li+ (68 pm), which makes it more difficult for sodium ions to transfer inside the battery. In the process of sodium deintercalation, the volume changes greatly, which easily leads to the collapse of electrode material structure, resulting in poor cycle and rate performance [13-15]. Therefore, in order to meet the demand for high energy and power density of future batteries, it is urgent to develop an efficient anode material for storing lithium and sodium. Among many carbon anode materials, including soft carbon, hard carbon and graphite, exhibit excellent cycle stability in LIBs and SIBs [16, 17]. However, the carbon-based electrode has a low

specific capacity and it is difficult to meet the need of future high energy density and power density market. It has been previously reported that transition metal oxides (TMOs) or sulfides (TMSs) have abundant redox active sites and thus can provide much higher specific capacity than carbon-based materials, but due to the low conductivity of TMOs or TMSs, large volume change and electrode pulverization during insertion/extraction of Li+ and Na+ may result in poor cycle stability and rate performance [18-20]. So far, there have been many strategies to solve this problem. For example, reducing the particle size of the electrode material to the nano level can reduce the internal stress generated by the active material during the cycle, thereby suppressing the powdering [21]. In addition, another effective way is to combine some carbon materials, such as CNT, graphene, amorphous carbon, and etc into the electrode material to form a nanocomposite [22-24]. The carbon material has good electrical conductivity, which can significantly improve the electrical conductivity of the composite material, while also providing an effective space to alleviate volume expansion. Although there are many advantages, the rational design of the electrode structure and the controllable synthesis of the morphology still have a certain of challenges [25]. Metal-Organic Framework Materials (MOFs) is a kind of coordination polymer that has developed rapidly in the past decade [9, 25-27]. It has three-dimensional pore structure and various morphologies, which has been widely used in gas adsorption separation, catalysis, and electrochemical energy storage application [28, 29]. The preparation of electrode materials with different morphologies by using MOF as a precursors/templates is a recent research hotspot, such as nanocube s[29], nanoboxes [30], hollow structures [31], nanorods [32], nanosheets [33], and so on. It is worth noting that TMSs derived from MOFs can be uniformly embedded in a porous carbon

matrix. In addition, some organic ligands of MOFs usually contain N atoms, so that it is easy to introduce N elements into the carbon matrix when calcination. Previous studies have shown that doping N atoms can improve energy storage efficiency [13, 29, 34]. Therefore, in situ synthesis of TMSs/N-doped porous carbon composites by using MOFs as precursors/templates is a significant work. Herein, we have reported a simple way to successfully prepare metal sulfide/carbon matrix composites by using zeolitic imidazolate frameworks (ZIF) precursors and incorporating subsequent carbonization and sulfidation processes. The as-prepared ZnS and Co3S4 nanoparticles are in situ-doped on the polyhedral carbon frameworks, and the obtained composite material has a large specific surface area and pore volume. The porous carbon matrix provides efficient conductive channels for the transport of Li+/Na+, and to relieve volume expansion during the cycle. In addition, the two TMSs on the polyhedral carbon framework, namely, ZnS and Co3S4, when used as active electrode materials, exhibit enhanced lithium/sodium storage properties over a single component due to the synergistic effects of the different components. Inspired by these advantages, the as-prepared Co3S4-ZnS/NC materials exhibit excellent electrochemical properties when evaluated as LIBs and SIBs anode materials, which proves their potential value for next-generation high performance LIBs and SIBs.

2 Results and discussion Synthesis and characterization of the as-prepared materials

Scheme 1 describes the detailed synthesis of Co3S4-ZnS/NC composites material. Firstly, the

ZIF-CoZn polyhedral precursor was prepared by simple liquid phase diffusion method. Then the precursor was carbonized at 600 °C for 2 h under the protection of argon atmosphere. Afterwards, the subsequent sulfidation process was completed under hydrothermal conditions with TAA, and finally Co3S4-ZnS/NC composite material was obtained. It is worth noting that the 1-methylimidazole is not a synthesis of ZIF introduced as a ligand. One of its coordination sites is occupied by a methyl group and cannot form a ZIF material. When it is added to the system, it not only acts as a competitive ligand in the metal center, but also serves as a basis for protonation of the bridging ligand without generating a large number of crystal nucleation. Moreover, it also acts as a solvent to adjust the polarity of the solution system [35-37]. According to the literature, such a synthesis method can effectively control the size of the product and increase the yield of ZIF [38, 39]. The detailed morphology of the as-prepared materials is analyzed by FESEM. Figure 1a-c shows ZIF-8, ZIF-67, and ZIF-CoZn precursors respectively. According to the picture, three samples were obtained by covalent combination of Zn2+ and Co2+ with 1-methylimidazole and 2-methylimidazole. It exhibits a uniform rhombohedral dodecahedron with a smooth surface with a clear outline and an average particle size of approximately 300 nm, 1 µm and 500 nm, respectively. Figure S1 shows more details of the sample. We further characterized the synthesized ZIF precursor by XRD. All the samples showed sharp diffraction peaks, which confirmed its high crystallinity [39, 40] (Figure S1d). After carbonization and sulfidation, the polyhedral structure remains relatively intact. It can be clearly seen from the high magnification FESEM image that the in-situ ZnS and Co3S4 nanoparticles are uniformly distributed on the surface of the polyhedron.

Scheme 1. Schematic illustration for the detailed synthesis of Co3S4-ZnS/NC composites material.

Figure 1. FESEM image of a) ZIF-8, b) ZIF-67, c) ZIF-CoZn precursors, d) ZnS/NC, e) Co3S4/NC, f) Co3S4-ZnS/NC.

The TEM gives a more detailed internal structure for as-prepared samples. Figure 2a-c shows a typical TEM image of the synthesized ZIF-precursors, all the precursors showed a regular hexahedron shape with regular edge and uniform morphology, which is consistent with the results of SEM observations. After carbonization and sulfidation, all three samples maintain the ideal structure of the precursor. Figure 2d-f demonstrates that the in-situ ZnS and Co3S4 nanoparticles are uniformly distributed in the carbon matrix. According to the HRTEM images (Figure. S2), the particle diameter is about 13-18 nm, in addition it also shows that the nitrogen-doped carbon matrix is amorphous [41]. Figure 2g,f show the EDX spectra and elemental mappings of Co3S4-ZnS/NC composite, respectively. The results show that the five elements of Co (yellow), Zn (blue), S (cyan),

C (red), N (green) coexist and are evenly distributed. The height distribution structure of ZnS/NC and Co3S4/NC was proved by the uniform distribution of Zn, S, C, N (Figure S3a) and Co, S, C, N (Figure S3b) in the element mapping, respectively.

Figure 2. TEM image of a) ZIF-8, b) ZIF-67, c) ZIF-CoZn precursors, d) ZnS/NC, e) Co3S4/NC, f) Co3S4-ZnS/ NC, g) EDX of Co3S4-ZnS/NC, h) EDS mapping of Co, Zn, S, C, N.

Phase purity and crystal structure were detected by XRD，For ZnS/NC (Figure 3a), the diffraction peaks at 28.6, 33.3, 47.6, 56.3, 69.4 and 76.6°, which are characteristic of (111), (200), (220), (311), (400) and (331) planes of the cubic structure of ZnS (JCPDS: 77-2100). For Co3S4/NC (Figure 3a), the diffraction peaks at 16.2, 26.3, 31.5, 38.1, 50.4, 55.2° can be well-indexed to (111), (220), (311), (400), (511) and (440) planes of the cubic structure of Co3S4 (JCPDS: 73-1730). The diffraction peaks of Co3S4-ZnS/NC were sharp and intense, indicating it highly crystalline

nature. No impurity peaks were observed, confirming the high purity of the products. The HRTEM image in Figure S2 shows the lattice spacing of 0.31 nm and 0.28 nm attributed to the (111) and (311) plane of the ZnS/NC and Co3S4/NC, respectively. Since nitrogen-doped carbon is amorphous, XRD has no diffraction peaks. Moreover, the structure of ZnS/NC, Co3S4/NC and Co3S4-ZnS/NC was further characterized by Raman spectroscopy, and the results are shown in Figure 3b and Figure S4. For both Co3S4/NC and Co3S4-ZnS/NC, the Raman peak appears in 188, 462, 501, 604 and 664 cm-1, which are belong to the Ag, Eg, F2g, F2g and A1g modes, respectively, corresponding to the characteristic peak of Co3S4 [42-44]. There are two peaks near 1335 and 1575 cm-1, corresponding to D band and G band [45], respectively, which confirm the existence of nitrogen-doped carbon (NC). In addition, the peak intensity ratio of the D and G bands (ID/IG) can be used to measure the degree of disorder of carbon materials. The ID/IG of Co3S4-ZnS/NC is calculated as 0.91, which is significantly higher than ZnS/NC (ID/IG=0.74, Figure S4a, ESI†) and Co3S4/NC (ID/IG=0.87, Figure S4b, ESI†), the possible reason is that compared with a single component, when Co3S4 and ZnS nanoparticles are introduced into the system the carbon disorder and defects in the composites increased, resulting in the increase of ID/IG value [46-48], which contributes to the improvement of electrochemical performance, as reversible lithium/sodium storage locations can be increased at the defect [49, 50]. Surface element compositions of as-prepared materials were examined by XPS. The survey spectrum of the Co3S4-ZnS/NC composite indicates the presence of Zn, Co, S, C and N elements. High resolution XPS spectroscopy of Co 2p, Zn 2p, S 2p, C 1s and N 1s are show in Figure S5. The specific surface area, pore size distribution and pore volume of the three materials were

tested by nitrogen adsorption-desorption isotherm. The results are shown in Figure 3d and Figure S6. All curves show I and IV isotherms, and there is a significant hysteresis loop in the relative pressure range of 0.5-1.0, indicating the presence of mesopores in the material. According to the BET calculation method and the BJH pore size distribution, the specific surface area, pore volume and pore size distribution of the material were calculated. As can be seen from Table S1, the Co3S4-ZnS/NC composite material has the largest specific surface area (120.8 m2 g-1), pore volume is 0.172 (cm3 g−1), and the pore sizes of the three samples are concentrated around 3.7 nm. It is well known that the specific surface area and pore size distribution play a crucial role in the electrochemical performance of the electrode material. The large specific surface area and mesoporous characteristics can expand the contact area between the electrode material and the electrolyte and provide additional space to mitigate structural changes during Li+/Na+ insertion/desertion process. Moreover, large pore volumes facilitate Li+/e- dynamic transmission, which contribute to improved electrochemical performance [51, 52]. According to the results of structural analysis, we expected that Co3S4-ZnS/NC composite has excellent lithium storage and sodium storage properties.

Figure 3. a) XRD patterns of as-prepared materials, b) Raman spectrum of Co3S4-ZnS/NC, c) XPS spectrum of Co3S4-ZnS/NC, d) BET curve of Co3S4-ZnS/NC.

Lithium storage performance

The lithium storage performance of as-prepared materials was investigated by using CR2016 half cells. Figure 4a shows the CV curves of Co3S4-ZnS/NC composite, with a voltage range from 0.01 to 3 V and a sweep rate of 0.1 mV s-1. At the first scan, the redox peak at 0.65/0.54 V is ascribed to the alloying and de-alloying of Li-Zn, and the redox peak at 1.4/1.34 V corresponds to the conversion reaction between ZnS and Zn/Na2S. The redox peak at 2.47/1.58 V is attributed to the insertion and conversion reaction between Co3S4 and Co/Li2S. The relevant reactions according

to previous reports are as follows [31, 39, 53, 54]: + + 2−

+ +

8−

+ 3Co + 4

+

↔

+

(1)

↔

(2)

+ 2− +

↔

+

↔ + 8− ↔8

+

(3) (4)

↔ 3Co + 4

(5) (6)

In addition, it can be seen from the figure that the CV curves are highly overlapped, indicating that the electrode material has good stability and reversibility. Figure 4b and Figure S7a,b show a typical charge-discharge curves for as-prepared electrode material at a current density of 0.1 A g-1. The Co3S4-ZnS/NC composite has the highest discharge and charge specific capacity of 1600.1 mA h g-1 and 1205.8 mA h g-1, with the initial coulomb efficiency (CE) is 75.3%, which is obviously higher than Co3S4/NC (discharge and charge specific capacity of 1031.7 mA h g-1 and 809.7 mA h g-1 with CE of 78.5%, (Figure S7a) and ZnS/NC (discharge and charge specific capacity of 1421.3 mA h g-1 and 809.1 mA h g-1 with CE of 56.9%, (Figure S7b). Most of the capacity loss in the first cycle is attributed to the formation of the SEI film [54]. In the subsequent cycle, the curve remains intact, suggesting a good capacity retention of the electrode material, consistent with CV results. Based on the lithium storage mechanism of the Co3S4-ZnS/NC composite material and its excellent structure, we further investigated the long cycle performance of the three electrode materials at a current density of 0.1 A g-1, as shown in Figure 4c. The Co3S4-ZnS/NC electrode has the highest specific capacity and provides a reversible capacity of 1019.4 mA h g-1 after 110 cycles, a capacity retention

of 82% (compared to the second cycle), with CE approaching 100%. The Co3S4/NC electrode shows excellent cycle stability, the capacity is stable at 742.8 mA h g-1 after 110 cycles, and the capacity retention rate is 92% relative to the second cycle with CE approaching 100% (Figure S7c). For the ZnS/NC electrode capacity experienced rapid decay, it is interesting that the capacity begins to increase slowly after 25 cycles, and the capacity remains basically stable at 100 cycles, and the reversible capacity is 838.5 mA h g-1 after the 110 cycles with CE approaching 100% (Figure S7d). This phenomenon is common in metal oxides or sulfides, and similar situations have occurred in our previous reports [52]. The possible cause [39, 52, 55] is that (1) the polymer gel-like film generated from the electrolyte reversibly forms and decomposes resulting in increased capacity. (2) defects generated during the cycle can provide lithium storage sites. (3) the kinetic process is slow, the electrode has a gradual activation process. (4) additional effects may be provided certain capacity through the interface effect. Co3S4-ZnS/NC electrode not only exhibits excellent cyclic performance, but also enhances the rate performance. As shown from Figure 4d, the discharge capacity of Co3S4-ZnS/NC electrode is 1618.9, 1115.4, 1065.3, 961.2, 773.5 mA h g-1 at current densities of 0.1, 0.2, 0.4, 0.8, 1.6 A g-1 respectively, when the current density returns to 0.1 A g-1, deliver the reversible capacity is 1257.5 mA h g-1. which is significantly better than the Co3S4/NC and ZnS/NC electrodes. It can be found that the Co3S4-ZnS/NC electrode has excellent cycle performance at a low current density. In consideration of practical application, the battery charging time can be shortened under a large current condition. However, due to the increased internal polarization of the battery at high current densities, the capacity retention rate is not very satisfactory. Many electrode materials have a particularly severe capacity degradation at high

current densities. In order to meet the requirements of fast charging in the future market, we tested the charge and discharge performance of the Co3S4-ZnS/NC electrode at high current density. The initial discharge capacity was 1427.8 mA h g-1 and 1171.2 mA h g-1 (Figure 4e), and the reversible capacity was 1052.2 mA h g-1 and 750 mA h g-1 after 200 cycles at current density of 0.5 A g-1 and 1 A g-1, respectively. Compared with the fifth cycle, the capacity retention rates were 94.3% (1115.3 mA h g-1 at 0.5 A g-1) and 94.7% (791.8 mA h g-1 at 1 A g-1), respectively. Moreover, the long cycle shows outstanding stability, indicating that the as-prepared Co3S4-ZnS/NC electrode materials have great lithium storage properties.

Figure 4. Lithium storage performance of a) CV curves of Co3S4-ZnS/NC, b) typical charge-discharge curves of Co3S4-ZnS/NC, c) cycle performance at 0.1 A g-1 d) rate performance from 0.1 to 1.6 A g-1 e) Co3S4-ZnS/NC long cycle performance at 0.5 and 1 A g-1.

In order to understand the reaction kinetics of the three electrode materials mentioned above and the intrinsic reasons for the differences of their electrochemical properties, electrochemical

impedance spectroscopy (EIS) tests were carried out, the Nyquis plot results are shown in Figure 5a. A detailed explanation of the Nyquist plot and an analog equivalent circuit diagram of the EIS patterns are shown in Figure S8. It can be clearly seen from Table S2, the Co3S4-ZnS/NC electrode showed the smallest impedance value, indicating that the electrode stability was remarkable and the formed SEI film was relatively stable. In addition, in order to better explain the excellent electrochemical performance of the Co3S4-ZnS/NC electrode we studied the impedance after cycle (1 A g-1 after 100 cycles) the results are shown in Figure 5b. From Table S2 we can see that the impedance value after the cycle has increased compared to the first time. To further understand the lithium intercalation kinetics of the electrode material, we calculate the lithium-ion diffusion coefficient (D) according to the straight line in the low frequency region and the relevant formula (see the supplementary information for the detailed calculation process). σw is derived from the slope of the Zre ~ ω-1/2 fitting line in Figure 5c. The calculated D value of Co3S4-ZnS/NC electrode decreased from 6.73×10-14 cm2 s-1 to 2.805×10-14 cm2 s-1 after 100 cycles at a current density of 1 A g-1. On the one hand, this behavior may be related to the formation of the SEI film [56]. On the other hand, the particles may agglomerate during the cycle, and the volume expansion causes the charge transfer to be blocked, resulting in an increase in impedance and a decrease in the lithium ion diffusion coefficient [54, 57]. Subsequently, the surface morphology and structural features of the electrode material before and after the cycle were observed by SEM. It can be seen from Figure 5d,e that the morphology of the electrode material remains intact after the cycle, and no obvious pulverization is observed on the Co3S4-ZnS/NC surface, which is favorable for the stability of the cycle. It is well known that when

metal oxides and sulfides are used in the anode material for LIBs, the electrode material faces volume expansion during repeated lithiation and delithiation processes, which causes rapid decay of capacity. In order to study the thickness variation of the electrode piece, the cross section was observed by SEM, and the expansion ratio of the electrode was estimated by measuring the thickness variation before and after the cycle [58]. The results are shown in Figure 5f,g. After 100 cycles, the thickness of the electrode piece increased from 14.0 µm to 15.6 µm, and the volume expansion ratio was approximately 11.4%. In addition, it can be seen from the cross-sectional picture that the original dodecahedron morphology is maintained well. Although there is a certain volume expansion, it is still in close contact with the current collector, further indicating the structural stability of the electrode material. The TEM further characterizes the microscopic intrinsic structure of the electrode after the cycle. It can be seen from the TEM picture that the structure of Co3S4-ZnS/NC is completely preserved, which is consistent with the SEM observation. Moreover, it can be seen from the EDS mapping of Figure 5h,k that the elements are still highly dispersed after 100 cycles and the material does not agglomerate.

Figure 5. a) Nyquist plots of as-prepared materials, b) Nyquist plots of Co3S4-ZnS/NC before and after cycling, c) Zre ~ ω-1/2 fitting line in low frequency region of Co3S4-ZnS/NC, FESEM images of d) Co3S4-ZnS/NC electrode before cycling, e) Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, f) cross section image of Co3S4-ZnS/NC electrode before cycling, inset: partially enlarged view of the purple area, g) cross section image of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, h) cross section EDS mapping of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, Cu, Co, Zn, S, C, i) TEM image of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, j) partially enlarged view of the green area in Figure i, k) EDS mapping of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, Co, Zn, S, C, N.

Sodium storage performance

Based on lithium storage mechanism and excellent electrochemical performance in lithium half-cells, which stimulated our exploration of sodium storage performance. Since the radius of

sodium ions is larger than that of lithium ions and the reaction kinetics are poor, the anode materials generally suitable for LIBs are not necessarily suitable for SIBs [39, 54]. However, the materials we prepared simultaneously exhibited superior sodium ion storage. The first four CV curves are shown in Figure 6a, with a voltage range of 0.01-2.5 V and a scan rate of 0.1 mV s-1. The cathodic peak at 1.93 V corresponds to the Na+ insertion in the Co3S4 phase corresponding to the following equation [13, 39, 54, 57]: +

+

↔

(7)

The peak at 1.49 V corresponds to the following reaction: + 8−

+ 8−

↔4

+3

(8)

The peak at 0.33 V corresponds to the following reaction: 2

+2

+2

↔2

+

(9)

Peaks below 0.5 V may correspond to the formation of Na-Zn alloys as following: +

+

↔

(10)

For the anode peak, below 0.1 V due to the de-alloying of the Na-Zn alloy, the peak at 0.94 V corresponds to the conversion of the metal Zn to ZnS, and the oxidation peak at 2.15 V for the formation of Co3S4. In addition, the CV curves are highly consistent, indicating that the electrochemical process is reversible. Figure 6b and Figure S9a,b show the charge and discharge curves of the three electrode materials at different cycles. The first discharge capacity of the Co3S4-ZnS/NC electrode was 1022.8 mA h g-1, the charging capacity was 753.9 mA h g-1, and the initial CE was 73.7%. The initial discharge capacities of the Co3S4/NC and ZnS/NC electrodes were 869.1 mA h g-1, 639 mA h g-1, and the charging capacities were 552.4 mA h g-1 and 359.4 mA h g-1,

with the initial CE were 63.6% and 56.2%, respectively. The loss of the initial capacity and the low initial CE correspond to the irreversible phase transition and the SEI film formed by electrolyte decomposition on the electrode surface [40]. Figure 6c shows the cycling performance of the three electrode materials at a current density of 0.1 A g-1. It is apparent that the Co3S4-ZnS/NC electrode exhibits the best cycling performance, and the capacity maintained at 409.5 mA h g-1 after 200 cycles is significantly higher than that of the Co3S4/NC electrode 324.1 mA h g-1 and the ZnS/NC electrode 300.5 mA h g-1, along with the number of charge-discharge increases, the CE gradually approaches 100% (Figure S9c,d). Besides, Co3S4-ZnS/NC electrode also exhibits enhanced rate performance (Figure 6d), including specific capacity and cycle stability, as compared to the Co3S4/NC and ZnS/NC electrodes. The discharge capacity of Co3S4-ZnS/NC electrode is 1005.2, 619.4, 522.3, 462.4, 385.3 mA h g-1 at current densities of 0.1, 0.2, 0.4, 0.8, 1.6 A g-1 respectively, when the current density returns to 0.1 A g-1, deliver the reversible capacity is 564.4 mA h g-1. Figure 6e shows the Co3S4-ZnS/NC electrode long cycle at current densities of 0.5 and 1 A g-1, with capacities remaining at 377.3 and 316.5 mA h g-1, respectively, after 1000 cycles, which has a higher specific capacity among most of the reported MOF-derived metal sulfides.

Figure 6. Sodium storage performance of a) CV curves of Co3S4-ZnS/NC, b) typical charge-discharge curves of Co3S4-ZnS/NC, c) cycle performance at 0.1 A g-1 d) rate performance from 0.1 to 1.6 A g-1 e) Co3S4-ZnS/NC long cycle performance at 0.5 and 1 A g-1.

Adopt similar research ideas, we performed EIS tests on the above three electrode materials, and the results are shown in Figure 7a. It is easy to see from the figure that the Co3S4-ZnS/NC electrode has the smallest Rct, indicating good conductivity and structural stability. Table S3 lists the specific values of the impedance. Similar to lithium storage, when the Co3S4-ZnS/NC electrode still has the smallest impedance for the SIBs, we next analyzed the Nyquis plot after 100 cycles at current densities of 1 A g-1 (Figure 7b). The impedance value has increased compared to the initial (Table S3). The relationship between Zre and ω-1/2 is shown in Figure 7c. The D value of the Co3S4-ZnS/NC electrode can be calculated according to the formula (Eq. (S1) (S2), Supplementary Information), and it can be found that the reduction from 9.01×10-14 to 1.89×10-15 cm2 s-1 after 100 cycles at a current density of 1 A g-1. Figure 7d,e shows the SEM images of the electrodes before

and after cycling, it can be seen from the diagram that after 100 cycles, the polyhedron morphology still remains intact. From the cross-sectional SEM image of the electrode piece (Figure 7f,g), we roughly estimated the volume expansion rate to be 59.3%. It is more directly reflected from the TEM image (Figure 7i) that the structure of the Co3S4-ZnS/NC electrode remains intact after cycling, which indicating a peaceful transition in the repeated charge and discharge process. From the EDS mapping of the cross section (Figure 7h) and TME (Figure 7j), it can be seen that there is a certain amount of Na+ in the polyhedron, which is related to the formation of SEI and a small amount of sodium is not deintercalated from the electrode. According to previous reports, the remaining small amount of Na+ can increase the conductivity of the electrode material and contribute to fast pseudo-capacitance behavior [57].

Figure 7. a) Nyquist plots of as-prepared materials, b) Nyquist plots of Co3S4-ZnS/NC before and after cycling, c) Zre ~ ω-1/2 fitting line in low frequency region of Co3S4-ZnS/NC, FESEM images of d) Co3S4-ZnS/NC electrode

before cycling, e) Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, f) cross section image of Co3S4-ZnS/NC electrode before cycling, inset: partially enlarged view of the purple area, g) cross section image of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, h) cross section EDS mapping of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, Cu, Na, Co, Zn, S, C, i) TEM image of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, j) EDS mapping of Co3S4-ZnS/NC electrode after 100 cycles at 1 A g-1, Na, Co, Zn, S, C, N.

In order to further clarify the excellent cycle performance and enhanced rate performance of the Co3S4-ZnS/NC composite electrode material, the electrochemical kinetics of lithium/sodium storage was studied in detail by testing the CV at different scans from 0.1-1 mV s-1. As shown in Figure 8a,d the CV curves are similar in shape at different scans, and the faster the scanning speed, the greater the current intensity. The relationship between current and scan rate is as follows [39, 54, 59] : =a log

= #log

(11) + log

(12)

Here i represents the current intensity, ν represents the scan rate, a and b are constants. Usually, a b value of 0.5 indicates a diffusion process, and a b value of 1 indicates a capacitance process. We next calculated the b-values according to the formula are listed in Figure S10a,b. The b-values of all the peaks are between 0.7 and 1, indicating that the dynamic process of Co3S4-ZnS/NC electrode has both diffusion process and capacitance process, and is mainly controlled by the capacitance contribution. In addition, the capacitance contribution can be quantified by the following formula: % = &' + &

'/

or /

'/

= &'

'/

+&

(13)

Where k1ν and k2ν1/2 represent capacitance contribution and diffusion contribution, respectively.

The capacitance contribution is calculated as 44.4% and 71.0% for lithium and sodium storage, respectively, at a scan rate of 0.1 mV s-1 as shown in the shaded area of Figure 8b,e. As the scan rate increases, the proportion of capacitance contribution increases. For the lithium storage process, the capacitance contribution increased from 44.4% to 71.6% (Figure 8c). For the sodium storage process, the capacitance contribution increased from 71.0% to 88.6% (Figure 8f). High capacitance contribution is beneficial to the excellent cycle performance of electrode materials at high current density, and good rate performance [57, 60]. Figure 8g shows the lithium/sodium ion insertion/ desertion mechanism of the Co3S4-ZnS/NC composite electrode during charge and discharge. During the charging process, electrons pass from the external circuit to the anode, and lithium/sodium ions pass through the separator to react with Co3S4-ZnS/NC. The discharge process is the opposite. Figure 8h,i compares the rate performance of some metal compounds derived from MOF used for Li/Na storage, from which we can see that the Co3S4-ZnS/NC electrode material we prepared is superior to most of the reported anode electrode materials for LIBs and SIBs (ZnSe-NC@CoSe2-NC [54], N-ZnSe@rGO [61], CoSe@PCP [40], Co3O4/ZnO [33], ZnS/NPC [39], Co(L)MOF [62], Co3O4/C [63], CoO-NCNTs [62], CuO/Cu2O-GPC [64]). At the same time, the performance of NC in lithium/sodium batteries has been investigated in detail. The results listed in Figure. S11 and Table S4 further confirm the excellent properties of the prepared materials. Based on the above analysis of the structure and electrochemical properties of Co3S4-ZnS/NC, which demonstrate that the as-prepared anode materials exhibited excellent performance in LIBs and NIBs, including cycle stability, rate performance, and higher specific capacity. The possible causes are explained below. Firstly, the close combination of ZnS and Co3S4 nanoparticles are

tightly bound to the polyhedral carbon skeleton produced by calcination of ZIF precursors makes the composite electrode has superior conductivity and excellent structural stability, which facilitates the rapid transfer of Li+/Na+ and electrons and as well as to alleviating the volume expansion in the cycling process. Secondly the large specific surface area and porous structure can make the electrolyte fully infiltrate the electrode material. Finally, the high capacitance contribution gives the composite electrode excellent Li/Na storage performance, especially at high current density.

Figure 8. The electrochemical kinetics of LIBs/SIB a,d) CV curves at different scans from 0.1-1 mV s-1 b,c) CV curve with capacitance contribution shown in the shaded area at 0.1 mV s-1 c,f) capacitance contribution ration at different scans rates, g) lithium/sodium ion insertion/ desertion mechanism of the Co3S4-ZnS/NC composite electrode during charge and discharge. Comparison of rate performance between Co3S4-ZnS/NC electrode and

other reported MOF derived materials, h) for LIBs, i) for SIBs.

3 Conclusion In short, an in-situ doping of ZnS and Co3S4 nanoparticles onto a polyhedral carbon framework was prepared by carbonization and sulfidation of ZIF precursors. The framework can adapt to the volume expansion during the cycle and improve the conductivity of electrode materials. In addition, the Co3S4-ZnS/NC electrode exhibits excellent electrochemical performance. For lithium-ion battery, the reversible capacity of Co3S4-ZnS/NC composite electrode up to 1019.4 and 750 mA h g-1 after cycling 110 and 1000 cycles at current densities of 0.1 and 1 A g-1, respectively. For the sodium ion battery, the reversible capacity of Co3S4-ZnS/NC composite electrode up to 409.5 and 316.5 mA h g-1 after cycling 110 and 1000 cycles at current densities of 0.1 and 1 A g-1, respectively. More importantly, the Co3S4-ZnS/NC electrode not only exhibits a high specific capacity, but also exhibits excellent cycle stability and enhanced rate performance compared to the pure ZnS and Co3S4 electrodes. In addition, Li+/Na+ kinetic analysis indicates that the capacitive behavior is a major contributor to Li+/Na+ storage, which facilitates Li+/Na+ fast storage. This work demonstrates that it is of interest to prepare metal compounds and carbon-based composites from ZIF precursors, and that the method is also suitable for the preparation of other transition metal sulfur compounds, which can be used as anode materials for high performance LIBs and SIBs.

Acknowledgements The work was supported by the Joint Fund Project-Enterprise-Shaanxi Coal Joint Fund Project (No:2019JLM-32). The authors thank the Analysis and Testing Center of Northwestern Polytechnical University for Electron Microscopy for their technical assistance in SEM and TEM.

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There are no conflicts to declare.