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Construction of sandwich-type Co9S8-C anchored on carbonized melamine foam toward lithium-ion battery
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Construction of sandwich-type Co9 S8 -C anchored on carbonized melamine foam toward lithium-ion battery Pengcheng Zhang , Yi Feng , Mengjue Cao , Jianfeng Yao PII: DOI: Reference:
S0013-4686(20)31613-3 https://doi.org/10.1016/j.electacta.2020.137220 EA 137220
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Electrochimica Acta
Received date: Revised date: Accepted date:
24 July 2020 18 September 2020 2 October 2020
Please cite this article as: Pengcheng Zhang , Yi Feng , Mengjue Cao , Jianfeng Yao , Construction of sandwich-type Co9 S8 -C anchored on carbonized melamine foam toward lithium-ion battery, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.137220
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Construction of sandwich-type Co9S8-C anchored on carbonized melamine foam toward lithium-ion battery Pengcheng Zhanga, Yi Fenga, Mengjue Caoa, Jianfeng Yaoa,b,* a
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China b
Jiangsu Province Key Laboratory of Green Biomass-based Fuels and Chemicals, Nanjing
210037, China *Corresponding author. E-mail: [email protected]
Abstract Co9S8 nanoparticles (NPs) anchored on carbonized melamine foam with polydopamine derived carbon coating (CMF-Co9S8-C) was successfully constructed for advanced lithium-ion battery anode. The sandwich-type structure consists of massive Co9S8 NPs and dual N-doped carbon layers that not only accommodate the volume expansion effect but also shortens the transfer distance of Li+/e-. Moreover, the nanocomposite inherits 3D carbon network with hierarchical porous structure, which is beneficial for providing sufficient transport channels and contacting with electrolyte. With the prominent synergistic effects from structure and chemical properties, CMF-Co9S8-C hybrid compound exhibited high specific capacity of 645 mAh g-1 at 0.5 A g-1 after 200 cycles, and it delivered excellent cycling stability (520 mAh g-1) under a high current (2 A g-1) after 1000 cycles.
Keywords Sandwich-type; Co9S8; Melamine foam; Anode; Lithium-ion battery
1. Introduction As lithium-ion batteries (LIBs) have vast peculiar merits, including high specific energy, long cycle life, safe and environmental benignity, they have been used in a variety of electronic devices and vehicles [1]. Until now, graphite is still used as 1
anode materials for commercial LIBs. However, its low theoretical capacity (372 mAh g-1) restricts the development of next-generation LIBs with a higher rate capacity [2]. Previous reports have demonstrated that transition metal sulfide (TMS) was a promising alternative to graphite due to its abundant reserves, multi-form of sulfuration states as well as special crystal structures [3, 4]. Compared with other TMSs [5-8], cobalt sulfide has received numerous attentions owing to its inherent high theoretical capacity and low cost. Especially, lithiation mechanisms featured with conversion reaction can transfer more electrons, delivering higher specific capacity [9]. However, during the large current charge-discharge processes, the reaction rapidly leads to the pulverization and collapse of active materials, giving rise to the large capacity loss and poor rate performance. Furthermore, the practical application of cobalt sulfide is also limited by its low conductivity [10]. To shorten the transfer distance of electron/Li+, fabricating the cobalt sulfide with diversified morphologies and nano-sizes is an effective and rational way [11]. For instance, Co1-xS hollow nanospheres grown on reduced graphene oxide layers were synthesized by Lu et al. When used for LIBs, it delivers capacity of 527 mAh g-1 at 2.5 A g-1 after 107 cycles [12]. To coat or embed cobalt sulfide with/into porous carbonaceous matrixes is another effective method, which can improve the conductivity of materials and accommodate the volume expansion to stabilize the structural integrity. In addition, hybrid compound possesses double lithiation mechanisms including “insertion” mechanism and “conversion reaction” that would provide high specific capacity and excellent energy density. Therefore, such properties make cobalt sulfide a potential novel material to replace graphite. Zhao et al. built Co9S8/C/CNT by embedding Co9S8 nanoparticles (NPs) into porous C/CNT micro/nano spheres via spray-drying, showing 546 mAh g-1 at 0.2 A g-1 after 100 cycles [13]. Recently, polydopamine (PDA), obtained by dopamine self-polymerizing in an alkaline solution, is an intriguing biomimetic polymer [14]. PDA also has been regarded as a fantastic carbon source considering that it inherited the abundant nitrogen-doped graphite-like carbon after carbonization. Qu et al. synthesized 2
Co9S8@C by carbonizing PDA coated metal coordination polymers and in-situ sulfurization, which exhibits a specific capacity of 606 mAh g-1 at 1 C after 300 cycles [15]. To the best of our knowledge, introducing nitrogen-doped carbon in hybrid plays a crucial role in enhancing conductivity. Melamine foam (MF) is a multifunctional, cheap, nitrogen-rich and low-density carbon support [16, 17]. After calcining MF under Ar, the carbonized MF (CMF) inherits three dimension (3D) hierarchical porous structure and abundant nitrogen contents [18]. In terms of above advantages, CMF not only facilitates mass transfer but also provides sufficient active sites for reacting with electrolyte. In the present work, a rational design route was adopted to synthesize Co9S8 NPs anchored on 3D N-doped CMF, and they were further protected by N-doped carbon layer derived from PDA. In the hybrid composites, the unique structure of CMF provides the sufficient active sites and promotes the contact of electrolyte. Meanwhile, the outer carbon layer from PDA not only further accelerates the transmission of electron/ion but also accommodates volume expansion effect. As a consequence, the sandwich-type CMF-Co9S8-C exhibits an excellent reversible capacity (645 mAh g-1 at 0.5 A g-1 after 200 cycles) and stable long-term cycling performances (520 mAh g-1 at 2 A g-1 after 1000 cycles).
2. Experimental 2.1 Material Synthesis All reagents were used directly without further purification. Typically, the carbonized carbon foam (CMF) was obtained by calcining melamine foam (MF, ~6×3×2 cm3, SINOYQX, Chengdu,China) at 800 °C for 2 h [19]. 0.761 g of CoCl2·6H2O (≥98.5%, Sinopharm Chemical Reagent) and 0.487 g of CH4N2S (≥98.5%, Sinopharm Chemical Reagent) were dissolved in 20 ml of ethyl alcohol respectively and stirred continuously for 30 min. 0.1 g of CMF was soaked into the Co2+ solution, followed by pouring CMF/Co2+ solution and thiourea solution into 100 ml stainless steel autoclave and keeping at 180 oC for 12 h. After the reaction, the product (CMF-Co1-xS) was obtained after washing three times with deionized water 3
and drying at 80 oC overnight. 0.1 g of dopamine hydrochloride (99%, Shanghai Aladdin) was dissolved into a buffer solution with pH ≈ 8.5, which was obtained by dissolving 0.121g tris (hydroxymethyl) aminomethane (≥99.8%, Shanghai Aladdin) in 100 ml of deionized water. CMF-Co1-xS was soaked in above PDA solution for 12 h at room temperature. After that, the samples were collected and washed three times with deionized water and dried at 80 oC, followed by annealing at 800 oC for 3 h with a heat ramping of 5 o
C min-1 in a tube furnace under Ar atmosphere and naturally cooling to room
temperature. The resultant products were nominated as CMF-Co9S8-C. 2.2 Characterizations The crystalline structures of samples were measured by XRD (Rigaku Smartlab with Cu-Kα radiation (λ=0.1542 nm) at 40 KV. Scanning electron microscopy (SEM) images of samples were obtained by JSM-7600F (JEOL Ltd., Japan). Transmission electron microscopy (TEM) images were collected by JEOL-2100 instrument (JEOL Ltd., Japan). Raman spectra were performed by using Thermo with 780 nm laser excitation. The pore size distribution and nitrogen adsorption-desorption isotherm were analyzed by ASAP 2020 (Micromeritics, USA) at 77 K. X-ray photoelectron spectroscopy (XPS) measurement and analysis were used on Thermo ESCALAB 250 spectrometer
(Thermo
Ltd,
UK)
equipped
with
Al-Kα
X-ray
source.
Thermogravimetric analysis (TGA) was detected by STA 2500 Regulus (NETZSCH, Germany) with a heat ramping of 10 oC/min in air atmosphere. 2.3 Electrochemical measurements Electrochemical performances of the samples, as anode electrode for LIBs, were tested in CR2016 type coin half cells. The working electrode was obtained by coating slurry (~0.9 mg) which was mixed by 70 wt.% samples, 20 wt.% acetylene black carbon and 10 wt.% polyvinylidene difluoride soultion (6 wt.% in N-Methyl pyrrolidone) on the copper foil and dried overnight in the vacuum oven. The lithium foil was used as a counter electrode, the Celgard 2500 was used as a separator, and LiPF6 was dissolved in a 1:1 (w:w) mixture of ethylene carbonate and dimethyl carbonate as electrolyte. The assembly process takes place in Ar-filled glove box, 4
where both the water and oxygen value were blow 0.1 ppm. Cylic voltammetry (CV) and electrochemical impedance spectra (EIS) were evaluated by Chenhua CHI760e (shanghai, China). Galvanostatic tests were measured from Land CT2001A.
3. Results and discussion The crystal structures of CMF-Co1-xS and CMF-Co9S8-C were measured by XRD (Fig. 1a). Co1-xS particles were directly grown on the CMF by solvothermal method, and the diffraction peaks are corresponded to the pattern of Co1-xS (PDF#42-0826). The typical peaks that can be ascribed to the (100), (101), (102) and (110) planes are extremely weak [20]. Similarly, for CMF-Co9S8-C, the typical peaks are well indexed to the standard Co9S8 (PDF#02-1459) and assigned to (111), (311), (222), (511) and (440) planes [21]. The intensity of Co9S8 peaks becomes strong, which manifests the reinforcement of crystallinity degree. The phase and size transformation of cobalt sulfide were resulted from high-temperature and carbon-coating [13]. In addition, after annealing, a broad peak located at about 25 o is due to the formation of amorphous carbon with the carbonization of PDA layers [22].
Figure 1. XRD patterns (a) and Raman spectra (b) of CMF-Co1-xS and CMF-Co9S8-C, N2 adsorption-desorption isotherm (c) of CMF-Co9S8-C, the inset is the pore size distribution, and TGA curves (d) of CMF-Co1-xS and CMF-Co9S8-C.
5
The Raman spectra of CMF-Co1-xS and CMF-Co9S8-C were detected in Fig. 1b. Two apparent peaks at about 1348 and 1587 cm-1 represent defective carbon (D band) and graphitic carbon (G band) [23]. The ID/IG ratio of CMF-Co9S8-C (2.1) is much lower than that of CMF-Co1-xS (3.2), indicating that there are mass of graphitic carbon in CMF-Co9S8-C and it can significantly enhance the electrical conductivity [24]. Compared with CMF-Co1-xS, the other two Raman peaks at 475 and 678 cm-1 are ascribed to the existence of Co9S8 [25]. The Raman peaks at 473, 517 and 683 cm-1 for CMF-Co1-xS are ascribed to Co1-xS and they are relatively weak [12]. The N2 adsorption-desorption isotherm was used to analyze the Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the CMF-Co9S8-C, as shown in Fig. 1c. The BET surface area is 65 m2 g-1, which is much larger than that of CMF-Co1-xS (4 m2 g-1) and pristine CMF (2.5 m2 g-1) [18] (Details in Fig. S1). The pore size distribution implies that the hybrid compound provides hierarchical structure with micro and mesoporous, which is beneficial for facilitating the intimate contact of electrolyte and electrode. To evaluate the content of cobalt sulfide NPs in CMF-Co9S8-C and CMF-Co1-xS, TGA was implemented in air (Fig. 1d). Initially, the weight loss before 150 oC (~8%) is mainly due to the evaporation of adsorbed water. The weight loss from 150 to 750 o
C can be attributed to different phase changes of cobalt sulfide, the decomposition of
carbon and the transformation of cobalt sulfide to cobalt oxide [25]. The weight loses of CMF-Co9S8-C and CMF-Co1-xS are 54% and 68%, respectively, confirming that the PDA derived carbon coating accounts for ~14% of the CMF-Co9S8-C weight. Moreover, the weight content of Co9S8 in CMF-Co9S8-C can be calculated to ~28% and that of Co1-xS in CMF-Co1-xS is ~40%. [11] (The calculation details in supporting information, Fig. S2). The morphological and structure of CMF, CMF-Co1-xS and CMF-Co9S8-C were investigated by SEM. The surface of CMF is smooth and the width is about 5-10 μm, as shown in Fig. 2a. As for CMF-Co1-xS, a mass of Co1-xS particles with sizes about 1-2 μm were densely grown on the carbon skeleton (Fig. 2b). Fig. 2c, d shows the structure details of CMF-Co9S8-C, it is clearly observed that the Co9S8 NPs are 6
embedded into carbon layers after the annealing process. TEM image of CMF-Co9S8-C (Fig. 2e) suggests smaller Co9S8 NPs (10-20 nm) have anchored on the CMF carbon skeleton with a PDA derived carbon coating. Moreover, HRTEM image (Fig. 2f) reveals that amorphous carbon protects Co9S8 NPs. The lattice spacing is determined to be 0.29 nm, which corresponds to (311) planes of Co9S8 [26]. PDA derived carbon coating on Co9S8 plays a crucial role in slowing down volume expansion effect. Moreover, the hybrid composites inherit the intact 3D CMF structure and hierarchical porous, which is beneficial for contacting with electrolyte.
Figure 2. SEM images of CMF (a), CMF-Co1-xS (b) and CMF-Co9S8-C (c, d), TEM (e) and HRTEM (f) images of CMF-Co9S8-C.
X-ray photoelectron spectroscopy (XPS) was proposed to analyze the surface composition of CMF-Co9S8-C and CMF-Co1-xS. Fig. 3a shows the full survey spectra of CMF-Co9S8-C and CMF-Co1-xS. The XPS spectrum of N 1s is shown in Fig. 3b. There are three peaks at 398.5, 399.8 and 400.8 eV, corresponding to pyridinic N, pyrrolic N and graphitic N. It is worth noting that, compared with CMF-Co1-xS, the weight ratios of pyridinic N and graphitic N rise from 21% to 39% and 33% to 76% respectively. The pyridinic N provides quantities of active sites for lithium-ion storage and the graphitic N can greatly improve the conductivity [9]. The Co 2p spectrum was analyzed in Fig. 3c, where two apparent peaks located at 797.8 and 781.8 eV with two satellite peaks at 803.2 and 786.5 eV, corresponding to Co 2p1/2 and Co 2p3/2 [24]. As for S 2p (Fig. 3d), the spectrum was fitted into four peaks, where two peaks at 161.4 7
and 162.3 eV are ascribed to S 2p3/2 and S 2p1/2. Meanwhile, another two peaks at 164.1 and 169.3 eV are consistent with C-S and SO42-, which manifests that excess sulfur species have doped into carbon lattice and partially oxidized during the solvothermal reaction [27]. According to the XPS results, the atom contents of N and S in the CMF-Co9S8-C are 14.02 at.% and 13.39 at.%, respectively. As a control experiment, the Co 2p and S 2p of CMF-Co1-xS were also fitted, confirming the presence of Co1-xS NPs in CMF-Co1-xS [28] (Fig. S3).
Figure 3. XPS spectra of CMF-Co9S8-C and CMF-Co1-xS: survey spectra (a), N 1s (b); Co 2p (c) and S 2p (d).
To evaluate the electrochemical properties, CMF-Co9S8-C and CMF-Co1-xS were assembled as CR2016 type coin half cells and tested in the potential range of 0.01-3.0 V (vs. Li+/Li). The CV curves of CMF-Co9S8-C at a scan rate of 0.1 mV s-1 are shown in Fig. 4a. At the first cycle, an obvious cathodic peak appears at 0.6 V, which demonstrates the formation of solid electrolyte interphase (SEI) [29]. Another peak at 1.3 V is related to the reduction of Co9S8 to Co and LiS. Subsequently, two anode peaks at 2.1 and 2.3 V are attributed to the oxidation of Co to Co9S8 [26]. In the second and third cycles, the almost overlapping CV curves suggest that the CMF-Co9S8-C electrode has a prominent reversibility during the lithiation/delithiation 8
processes. Correspondingly, the first three galvanostatic cycling profiles at 0.1 A g-1 are shown in Fig. 4b. The discharge and charge plateaus are consistent with the cathodic and anodic CV curves respectively. At the first cycle, CMF-Co9S8-C electrode exhibits discharge and charge capacities of 1257 and 893 mAh g-1, which leads to the initial coulombic efficiency (CE) of 71%. The high initial CE mainly results from the Co9S8 NPs embedding into the dual N-doped carbon. For comparison, the CV measurements and charge-discharge profiles of CMF-Co1-xS were also tested (Fig. S4).
Figure 4. CV curves at 0.1 mV s-1 (a) and the charge-discharge curves (b) at 0.1 A g-1 for CMF-Co9S8-C electrode, the rate performances at different current (c), the cycling test at 0.5 A g-1 (d) for CMF-Co9S8-C and CMF-Co1-xS, and long-term cycling test of CMF-Co9S8-C at 2 A g-1 (e).
The rate performances of CMF-Co1-xS-C and CMF-Co9S8-C were measured at 9
current of 0.1, 0.2, 0.5, 1 and 2 A g-1 (Fig. 4c). CMF-Co9S8-C electrode delivers the capacities of 850, 743, 584, 485 and 406 mAh g-1, and the capacity can be recovered to 884 mAh g-1 when the current switched to 0.1 A g-1, indicating the excellent rate performance durability. However, CMF-Co1-xS, as a comparison, only delivers the capacities 630, 164, 118, 92 and 68 mAh g-1 under the same conditions. The cycling performances were also estimated at current of 0.5 A g-1 for 200 cycles in the Fig. 4d. Initially, the CMF-Co9S8-C shows capacity of 914 mAh g-1 with the CE of 71%. Although the capacity decreases slightly due to the slow activation of electrodes and the formation of SEI [30], the capacity rises dramatically to 645 mAh g-1 after the activation process and the CE tends to stabilize at 98%. As for the fluctuation phenomenon of capacities and CE during cycling test, it is mainly due to the gradual activation of Co9S8 NPs coated by PDA-derived carbon layer [31]. The outstanding electrochemical performances mainly originate from the sufficient contacts between the electrolyte and unique hierarchical pores of electrode. On the contrary, the capacity of CMF-Co1-xS decreases rapidly to the 136 mAh g-1, leading to the initial CE of 58%. For the CMF-Co9S8-C, the ultralong cycling performance was investigated under 2 A g-1 for 1000 cycles (Fig. 4e). The specific capacity can reach 520 mAh g-1 with the CE of 99% after the transient activation process, which further indicates the superior stability and high specific capacity. Moreover, to investigate the stability of structure, the SEM images of CMF-Co1-xS and CMF-Co9S8-C electrodes after charge-discharge 100 cycles test at 0.5 A g-1 have been compared (Fig. S5). As for CMF-Co9S8-C, the sandwich-type structure can be still maintained except the surface becomes thicker due to the SEI film coating [32]. Apparently, it is a feasible method to improve the long-term cycling performance of electrode when Co9S8 nanoparticles are coated by N-doped carbon layer. Furthermore, the electrochemical impedance spectra (EIS) of CMF-Co9S8-C and CMF-Co1-xS were investigated (Fig. 5a). It is clearly observed that the semicircle of CMF-Co9S8-C is smaller than that of CMF-Co1-xS in the high frequency region from the Nyquist plots, confirming carbon coatings from PDA on the Co9S8 NPs surface effectively enhance the charge transfer ability at the interface [15]. In addition, the 10
specific capacities of CMF-Co9S8-C were compared with other similar Co9S8 compounds (Fig. 5b) [9, 13, 15, 33-35], which reveals that CMF-Co9S8-C delivers excellent lithium-ion storage ability even at high current. According to the schematic illustration of CMF-Co9S8-C hybrid nanocomposite for LIBs (Fig. 5c), the superior electrochemical performance is ascribed to the unique sandwich-type structure, shortening the transfer distance of Li+/e- and improving the conductivity. In particular, Co9S8 NPs were embedded between the carbonized melamine foam skeleton and N-doped carbon layer from PDA. Such structure can effectively accommodate volume expansion effect during the large current charge/discharge processes, and further enhance the stability of the electrode.
(Ref. (Ref. (Ref. (Ref. (Ref. (Ref.
33) 15) 13) 9) 34) 35)
Figure 5. EIS profiles of CMF-Co9S8-C and CMF-Co1-xS electrode (a), comparison of the specific capacities of the CMF-Co9S8-C with similar electrodes (b), and schematic illustration of the sandwich-type CMF-Co9S8-C hybrid nanocomposite for lithium-ion batteries (c).
In order to further explicate the storage mechanism and confirm the specific capacity contribution, the electrochemical kinetics and pseudocapacitive contribution of the CMF-Co9S8-C were analyzed with CV measurements. As displayed in Fig. 6a, the CV profiles show the similar shapes at different scan rates from 0.2 to 1.5 V, suggesting a stable pseudocapacitive behavior. In general, the relationship of peak 11
current (i) and scan rate (v) can be obtained by the equation (1) and simply transformed to the equation (2). The b is an adjustable parameter, which represents the slope of log i-log v plots. Generally speaking, when b is 1.0, it means a capacitive-controlled process; when b is 0.5, it means a diffusion-dominated process [36]. Fig. 6b shows the slope b of log i-log v plots, where the b value for cathodic and anodic current are 0.74 and 0.73, respectively. The lithiation and delithiation processes
are
constituted
by
both
the
diffusion-controlled
and
pseudocapacitive-controlled. So as to investigate the specific contribution of capacitive effect and diffusion-controlled process, equation (3) was introduced [37]. The i comprised of pseudocapacitive effects (k1v) and diffusion controlled reactions (k2v0.5) represents the current at particular potential. k1, as a slope of i/v0.5-v0.5 plots, can be obtained by precise calculations (details in supporting information). The results are shown in Fig. 6c, d. The shaded part in red signifies the pseudocapacitive effects contribution of CMF-Co9S8-C at the scan rate of 0.6 mV s-1, and the value is about 67.1%. Notably, the percentage of pseudocapacitive effects increases gradually with the scan rate increasing from 0.2 to 1.5 mV s-1, as shown in Fig. 6d. The results confirm that the high contribution of pseudocapacitive effects improve the lithium storage capacity and rate performances. The hierarchical porous structure of CMF-Co9S8-C electrode facilitates the contacting with electrolyte; meanwhile, the design of N-doped carbon coating improves the conductivity and alleviates the volume expansion. 𝑖 = 𝑎𝑣 𝑏
(1)
log 𝑖 = 𝑏log 𝑣 + log 𝑎 𝑖 = 𝑘1 𝑣 + 𝑘2 𝑣 0.5
12
(2) (3)
Figure 6. Electrochemical kinetics of CMF-Co9S8-C electrode toward Li+: CV curves (a) at scan rate from 0.2 to 1.5 mV s-1, log i-log v plots (b), contribution of the diffusion and pseudocapacitive effects to the total capacity at 0.6 mV s -1 (c) and contribution ratio at different scan rate (d).
4. Conclusions In summary, Co9S8 nanoparticles (10-20 nm) anchored on the carbonized melamine foam with PDA derived carbon coating (CMF-Co9S8-C) was successfully synthesized via typical solvothermal sulfurization and high-temperature carbonization. The sandwich-type architecture, Co9S8 nanoparticles between CMF and PDA derived carbon coating, can effectively relieve volume expansion, and massive pyridinic and graphite carbon significantly enhance conductivity. In addition, the nanocomposite inherits the 3D hierarchical porous structure of melamine foam, promoting the intimate contact with electrolyte and providing sufficient active sites. On account of the above advantages, CMF-Co9S8-C electrode indeed gives superior electrochemical performance as lithium-ion battery electrode (645 mAh g-1 at 0.5 A g-1 after 200 cycles and 520 mAh g-1 at 2 A g-1 after 1000 cycles). Considering the unique structure and simply synthesis method, our work may provide a rational solution for diverse metal sulfides, selenides and phosphides for energy storage. 13
Credit Author Statement
Pengcheng Zhang: Conceptualization, Data curation, Formal analysis, Writing - original draft Yi Feng: Conceptualization, Formal analysis Mengjue Cao: Data curation, Formal analysis Jianfeng Yao: Conceptualization, Formal analysis, Supervision, Writing review & editing
Notes The authors declare no competing financial interest. Acknowledgements The authors are grateful for the financial support of the Youth Fund of Natural Science Foundation of Jiangsu Province (BK20170919) and the National Natural Science Foundation of China (21808112).
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Construction of sandwich-type Co9 S8 -C anchored on carbonized melamine foam toward lithium-ion battery Pengcheng Zhang , Yi Feng , Mengjue Cao , Jianfeng Yao PII: DOI: Reference:
S0013-4686(20)31613-3 https://doi.org/10.1016/j.electacta.2020.137220 EA 137220
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
24 July 2020 18 September 2020 2 October 2020
Please cite this article as: Pengcheng Zhang , Yi Feng , Mengjue Cao , Jianfeng Yao , Construction of sandwich-type Co9 S8 -C anchored on carbonized melamine foam toward lithium-ion battery, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.137220
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Construction of sandwich-type Co9S8-C anchored on carbonized melamine foam toward lithium-ion battery Pengcheng Zhanga, Yi Fenga, Mengjue Caoa, Jianfeng Yaoa,b,* a
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China b
Jiangsu Province Key Laboratory of Green Biomass-based Fuels and Chemicals, Nanjing
210037, China *Corresponding author. E-mail: [email protected]
Abstract Co9S8 nanoparticles (NPs) anchored on carbonized melamine foam with polydopamine derived carbon coating (CMF-Co9S8-C) was successfully constructed for advanced lithium-ion battery anode. The sandwich-type structure consists of massive Co9S8 NPs and dual N-doped carbon layers that not only accommodate the volume expansion effect but also shortens the transfer distance of Li+/e-. Moreover, the nanocomposite inherits 3D carbon network with hierarchical porous structure, which is beneficial for providing sufficient transport channels and contacting with electrolyte. With the prominent synergistic effects from structure and chemical properties, CMF-Co9S8-C hybrid compound exhibited high specific capacity of 645 mAh g-1 at 0.5 A g-1 after 200 cycles, and it delivered excellent cycling stability (520 mAh g-1) under a high current (2 A g-1) after 1000 cycles.
Keywords Sandwich-type; Co9S8; Melamine foam; Anode; Lithium-ion battery
1. Introduction As lithium-ion batteries (LIBs) have vast peculiar merits, including high specific energy, long cycle life, safe and environmental benignity, they have been used in a variety of electronic devices and vehicles [1]. Until now, graphite is still used as 1
anode materials for commercial LIBs. However, its low theoretical capacity (372 mAh g-1) restricts the development of next-generation LIBs with a higher rate capacity [2]. Previous reports have demonstrated that transition metal sulfide (TMS) was a promising alternative to graphite due to its abundant reserves, multi-form of sulfuration states as well as special crystal structures [3, 4]. Compared with other TMSs [5-8], cobalt sulfide has received numerous attentions owing to its inherent high theoretical capacity and low cost. Especially, lithiation mechanisms featured with conversion reaction can transfer more electrons, delivering higher specific capacity [9]. However, during the large current charge-discharge processes, the reaction rapidly leads to the pulverization and collapse of active materials, giving rise to the large capacity loss and poor rate performance. Furthermore, the practical application of cobalt sulfide is also limited by its low conductivity [10]. To shorten the transfer distance of electron/Li+, fabricating the cobalt sulfide with diversified morphologies and nano-sizes is an effective and rational way [11]. For instance, Co1-xS hollow nanospheres grown on reduced graphene oxide layers were synthesized by Lu et al. When used for LIBs, it delivers capacity of 527 mAh g-1 at 2.5 A g-1 after 107 cycles [12]. To coat or embed cobalt sulfide with/into porous carbonaceous matrixes is another effective method, which can improve the conductivity of materials and accommodate the volume expansion to stabilize the structural integrity. In addition, hybrid compound possesses double lithiation mechanisms including “insertion” mechanism and “conversion reaction” that would provide high specific capacity and excellent energy density. Therefore, such properties make cobalt sulfide a potential novel material to replace graphite. Zhao et al. built Co9S8/C/CNT by embedding Co9S8 nanoparticles (NPs) into porous C/CNT micro/nano spheres via spray-drying, showing 546 mAh g-1 at 0.2 A g-1 after 100 cycles [13]. Recently, polydopamine (PDA), obtained by dopamine self-polymerizing in an alkaline solution, is an intriguing biomimetic polymer [14]. PDA also has been regarded as a fantastic carbon source considering that it inherited the abundant nitrogen-doped graphite-like carbon after carbonization. Qu et al. synthesized 2
Co9S8@C by carbonizing PDA coated metal coordination polymers and in-situ sulfurization, which exhibits a specific capacity of 606 mAh g-1 at 1 C after 300 cycles [15]. To the best of our knowledge, introducing nitrogen-doped carbon in hybrid plays a crucial role in enhancing conductivity. Melamine foam (MF) is a multifunctional, cheap, nitrogen-rich and low-density carbon support [16, 17]. After calcining MF under Ar, the carbonized MF (CMF) inherits three dimension (3D) hierarchical porous structure and abundant nitrogen contents [18]. In terms of above advantages, CMF not only facilitates mass transfer but also provides sufficient active sites for reacting with electrolyte. In the present work, a rational design route was adopted to synthesize Co9S8 NPs anchored on 3D N-doped CMF, and they were further protected by N-doped carbon layer derived from PDA. In the hybrid composites, the unique structure of CMF provides the sufficient active sites and promotes the contact of electrolyte. Meanwhile, the outer carbon layer from PDA not only further accelerates the transmission of electron/ion but also accommodates volume expansion effect. As a consequence, the sandwich-type CMF-Co9S8-C exhibits an excellent reversible capacity (645 mAh g-1 at 0.5 A g-1 after 200 cycles) and stable long-term cycling performances (520 mAh g-1 at 2 A g-1 after 1000 cycles).
2. Experimental 2.1 Material Synthesis All reagents were used directly without further purification. Typically, the carbonized carbon foam (CMF) was obtained by calcining melamine foam (MF, ~6×3×2 cm3, SINOYQX, Chengdu,China) at 800 °C for 2 h [19]. 0.761 g of CoCl2·6H2O (≥98.5%, Sinopharm Chemical Reagent) and 0.487 g of CH4N2S (≥98.5%, Sinopharm Chemical Reagent) were dissolved in 20 ml of ethyl alcohol respectively and stirred continuously for 30 min. 0.1 g of CMF was soaked into the Co2+ solution, followed by pouring CMF/Co2+ solution and thiourea solution into 100 ml stainless steel autoclave and keeping at 180 oC for 12 h. After the reaction, the product (CMF-Co1-xS) was obtained after washing three times with deionized water 3
and drying at 80 oC overnight. 0.1 g of dopamine hydrochloride (99%, Shanghai Aladdin) was dissolved into a buffer solution with pH ≈ 8.5, which was obtained by dissolving 0.121g tris (hydroxymethyl) aminomethane (≥99.8%, Shanghai Aladdin) in 100 ml of deionized water. CMF-Co1-xS was soaked in above PDA solution for 12 h at room temperature. After that, the samples were collected and washed three times with deionized water and dried at 80 oC, followed by annealing at 800 oC for 3 h with a heat ramping of 5 o
C min-1 in a tube furnace under Ar atmosphere and naturally cooling to room
temperature. The resultant products were nominated as CMF-Co9S8-C. 2.2 Characterizations The crystalline structures of samples were measured by XRD (Rigaku Smartlab with Cu-Kα radiation (λ=0.1542 nm) at 40 KV. Scanning electron microscopy (SEM) images of samples were obtained by JSM-7600F (JEOL Ltd., Japan). Transmission electron microscopy (TEM) images were collected by JEOL-2100 instrument (JEOL Ltd., Japan). Raman spectra were performed by using Thermo with 780 nm laser excitation. The pore size distribution and nitrogen adsorption-desorption isotherm were analyzed by ASAP 2020 (Micromeritics, USA) at 77 K. X-ray photoelectron spectroscopy (XPS) measurement and analysis were used on Thermo ESCALAB 250 spectrometer
(Thermo
Ltd,
UK)
equipped
with
Al-Kα
X-ray
source.
Thermogravimetric analysis (TGA) was detected by STA 2500 Regulus (NETZSCH, Germany) with a heat ramping of 10 oC/min in air atmosphere. 2.3 Electrochemical measurements Electrochemical performances of the samples, as anode electrode for LIBs, were tested in CR2016 type coin half cells. The working electrode was obtained by coating slurry (~0.9 mg) which was mixed by 70 wt.% samples, 20 wt.% acetylene black carbon and 10 wt.% polyvinylidene difluoride soultion (6 wt.% in N-Methyl pyrrolidone) on the copper foil and dried overnight in the vacuum oven. The lithium foil was used as a counter electrode, the Celgard 2500 was used as a separator, and LiPF6 was dissolved in a 1:1 (w:w) mixture of ethylene carbonate and dimethyl carbonate as electrolyte. The assembly process takes place in Ar-filled glove box, 4
where both the water and oxygen value were blow 0.1 ppm. Cylic voltammetry (CV) and electrochemical impedance spectra (EIS) were evaluated by Chenhua CHI760e (shanghai, China). Galvanostatic tests were measured from Land CT2001A.
3. Results and discussion The crystal structures of CMF-Co1-xS and CMF-Co9S8-C were measured by XRD (Fig. 1a). Co1-xS particles were directly grown on the CMF by solvothermal method, and the diffraction peaks are corresponded to the pattern of Co1-xS (PDF#42-0826). The typical peaks that can be ascribed to the (100), (101), (102) and (110) planes are extremely weak [20]. Similarly, for CMF-Co9S8-C, the typical peaks are well indexed to the standard Co9S8 (PDF#02-1459) and assigned to (111), (311), (222), (511) and (440) planes [21]. The intensity of Co9S8 peaks becomes strong, which manifests the reinforcement of crystallinity degree. The phase and size transformation of cobalt sulfide were resulted from high-temperature and carbon-coating [13]. In addition, after annealing, a broad peak located at about 25 o is due to the formation of amorphous carbon with the carbonization of PDA layers [22].
Figure 1. XRD patterns (a) and Raman spectra (b) of CMF-Co1-xS and CMF-Co9S8-C, N2 adsorption-desorption isotherm (c) of CMF-Co9S8-C, the inset is the pore size distribution, and TGA curves (d) of CMF-Co1-xS and CMF-Co9S8-C.
5
The Raman spectra of CMF-Co1-xS and CMF-Co9S8-C were detected in Fig. 1b. Two apparent peaks at about 1348 and 1587 cm-1 represent defective carbon (D band) and graphitic carbon (G band) [23]. The ID/IG ratio of CMF-Co9S8-C (2.1) is much lower than that of CMF-Co1-xS (3.2), indicating that there are mass of graphitic carbon in CMF-Co9S8-C and it can significantly enhance the electrical conductivity [24]. Compared with CMF-Co1-xS, the other two Raman peaks at 475 and 678 cm-1 are ascribed to the existence of Co9S8 [25]. The Raman peaks at 473, 517 and 683 cm-1 for CMF-Co1-xS are ascribed to Co1-xS and they are relatively weak [12]. The N2 adsorption-desorption isotherm was used to analyze the Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the CMF-Co9S8-C, as shown in Fig. 1c. The BET surface area is 65 m2 g-1, which is much larger than that of CMF-Co1-xS (4 m2 g-1) and pristine CMF (2.5 m2 g-1) [18] (Details in Fig. S1). The pore size distribution implies that the hybrid compound provides hierarchical structure with micro and mesoporous, which is beneficial for facilitating the intimate contact of electrolyte and electrode. To evaluate the content of cobalt sulfide NPs in CMF-Co9S8-C and CMF-Co1-xS, TGA was implemented in air (Fig. 1d). Initially, the weight loss before 150 oC (~8%) is mainly due to the evaporation of adsorbed water. The weight loss from 150 to 750 o
C can be attributed to different phase changes of cobalt sulfide, the decomposition of
carbon and the transformation of cobalt sulfide to cobalt oxide [25]. The weight loses of CMF-Co9S8-C and CMF-Co1-xS are 54% and 68%, respectively, confirming that the PDA derived carbon coating accounts for ~14% of the CMF-Co9S8-C weight. Moreover, the weight content of Co9S8 in CMF-Co9S8-C can be calculated to ~28% and that of Co1-xS in CMF-Co1-xS is ~40%. [11] (The calculation details in supporting information, Fig. S2). The morphological and structure of CMF, CMF-Co1-xS and CMF-Co9S8-C were investigated by SEM. The surface of CMF is smooth and the width is about 5-10 μm, as shown in Fig. 2a. As for CMF-Co1-xS, a mass of Co1-xS particles with sizes about 1-2 μm were densely grown on the carbon skeleton (Fig. 2b). Fig. 2c, d shows the structure details of CMF-Co9S8-C, it is clearly observed that the Co9S8 NPs are 6
embedded into carbon layers after the annealing process. TEM image of CMF-Co9S8-C (Fig. 2e) suggests smaller Co9S8 NPs (10-20 nm) have anchored on the CMF carbon skeleton with a PDA derived carbon coating. Moreover, HRTEM image (Fig. 2f) reveals that amorphous carbon protects Co9S8 NPs. The lattice spacing is determined to be 0.29 nm, which corresponds to (311) planes of Co9S8 [26]. PDA derived carbon coating on Co9S8 plays a crucial role in slowing down volume expansion effect. Moreover, the hybrid composites inherit the intact 3D CMF structure and hierarchical porous, which is beneficial for contacting with electrolyte.
Figure 2. SEM images of CMF (a), CMF-Co1-xS (b) and CMF-Co9S8-C (c, d), TEM (e) and HRTEM (f) images of CMF-Co9S8-C.
X-ray photoelectron spectroscopy (XPS) was proposed to analyze the surface composition of CMF-Co9S8-C and CMF-Co1-xS. Fig. 3a shows the full survey spectra of CMF-Co9S8-C and CMF-Co1-xS. The XPS spectrum of N 1s is shown in Fig. 3b. There are three peaks at 398.5, 399.8 and 400.8 eV, corresponding to pyridinic N, pyrrolic N and graphitic N. It is worth noting that, compared with CMF-Co1-xS, the weight ratios of pyridinic N and graphitic N rise from 21% to 39% and 33% to 76% respectively. The pyridinic N provides quantities of active sites for lithium-ion storage and the graphitic N can greatly improve the conductivity [9]. The Co 2p spectrum was analyzed in Fig. 3c, where two apparent peaks located at 797.8 and 781.8 eV with two satellite peaks at 803.2 and 786.5 eV, corresponding to Co 2p1/2 and Co 2p3/2 [24]. As for S 2p (Fig. 3d), the spectrum was fitted into four peaks, where two peaks at 161.4 7
and 162.3 eV are ascribed to S 2p3/2 and S 2p1/2. Meanwhile, another two peaks at 164.1 and 169.3 eV are consistent with C-S and SO42-, which manifests that excess sulfur species have doped into carbon lattice and partially oxidized during the solvothermal reaction [27]. According to the XPS results, the atom contents of N and S in the CMF-Co9S8-C are 14.02 at.% and 13.39 at.%, respectively. As a control experiment, the Co 2p and S 2p of CMF-Co1-xS were also fitted, confirming the presence of Co1-xS NPs in CMF-Co1-xS [28] (Fig. S3).
Figure 3. XPS spectra of CMF-Co9S8-C and CMF-Co1-xS: survey spectra (a), N 1s (b); Co 2p (c) and S 2p (d).
To evaluate the electrochemical properties, CMF-Co9S8-C and CMF-Co1-xS were assembled as CR2016 type coin half cells and tested in the potential range of 0.01-3.0 V (vs. Li+/Li). The CV curves of CMF-Co9S8-C at a scan rate of 0.1 mV s-1 are shown in Fig. 4a. At the first cycle, an obvious cathodic peak appears at 0.6 V, which demonstrates the formation of solid electrolyte interphase (SEI) [29]. Another peak at 1.3 V is related to the reduction of Co9S8 to Co and LiS. Subsequently, two anode peaks at 2.1 and 2.3 V are attributed to the oxidation of Co to Co9S8 [26]. In the second and third cycles, the almost overlapping CV curves suggest that the CMF-Co9S8-C electrode has a prominent reversibility during the lithiation/delithiation 8
processes. Correspondingly, the first three galvanostatic cycling profiles at 0.1 A g-1 are shown in Fig. 4b. The discharge and charge plateaus are consistent with the cathodic and anodic CV curves respectively. At the first cycle, CMF-Co9S8-C electrode exhibits discharge and charge capacities of 1257 and 893 mAh g-1, which leads to the initial coulombic efficiency (CE) of 71%. The high initial CE mainly results from the Co9S8 NPs embedding into the dual N-doped carbon. For comparison, the CV measurements and charge-discharge profiles of CMF-Co1-xS were also tested (Fig. S4).
Figure 4. CV curves at 0.1 mV s-1 (a) and the charge-discharge curves (b) at 0.1 A g-1 for CMF-Co9S8-C electrode, the rate performances at different current (c), the cycling test at 0.5 A g-1 (d) for CMF-Co9S8-C and CMF-Co1-xS, and long-term cycling test of CMF-Co9S8-C at 2 A g-1 (e).
The rate performances of CMF-Co1-xS-C and CMF-Co9S8-C were measured at 9
current of 0.1, 0.2, 0.5, 1 and 2 A g-1 (Fig. 4c). CMF-Co9S8-C electrode delivers the capacities of 850, 743, 584, 485 and 406 mAh g-1, and the capacity can be recovered to 884 mAh g-1 when the current switched to 0.1 A g-1, indicating the excellent rate performance durability. However, CMF-Co1-xS, as a comparison, only delivers the capacities 630, 164, 118, 92 and 68 mAh g-1 under the same conditions. The cycling performances were also estimated at current of 0.5 A g-1 for 200 cycles in the Fig. 4d. Initially, the CMF-Co9S8-C shows capacity of 914 mAh g-1 with the CE of 71%. Although the capacity decreases slightly due to the slow activation of electrodes and the formation of SEI [30], the capacity rises dramatically to 645 mAh g-1 after the activation process and the CE tends to stabilize at 98%. As for the fluctuation phenomenon of capacities and CE during cycling test, it is mainly due to the gradual activation of Co9S8 NPs coated by PDA-derived carbon layer [31]. The outstanding electrochemical performances mainly originate from the sufficient contacts between the electrolyte and unique hierarchical pores of electrode. On the contrary, the capacity of CMF-Co1-xS decreases rapidly to the 136 mAh g-1, leading to the initial CE of 58%. For the CMF-Co9S8-C, the ultralong cycling performance was investigated under 2 A g-1 for 1000 cycles (Fig. 4e). The specific capacity can reach 520 mAh g-1 with the CE of 99% after the transient activation process, which further indicates the superior stability and high specific capacity. Moreover, to investigate the stability of structure, the SEM images of CMF-Co1-xS and CMF-Co9S8-C electrodes after charge-discharge 100 cycles test at 0.5 A g-1 have been compared (Fig. S5). As for CMF-Co9S8-C, the sandwich-type structure can be still maintained except the surface becomes thicker due to the SEI film coating [32]. Apparently, it is a feasible method to improve the long-term cycling performance of electrode when Co9S8 nanoparticles are coated by N-doped carbon layer. Furthermore, the electrochemical impedance spectra (EIS) of CMF-Co9S8-C and CMF-Co1-xS were investigated (Fig. 5a). It is clearly observed that the semicircle of CMF-Co9S8-C is smaller than that of CMF-Co1-xS in the high frequency region from the Nyquist plots, confirming carbon coatings from PDA on the Co9S8 NPs surface effectively enhance the charge transfer ability at the interface [15]. In addition, the 10
specific capacities of CMF-Co9S8-C were compared with other similar Co9S8 compounds (Fig. 5b) [9, 13, 15, 33-35], which reveals that CMF-Co9S8-C delivers excellent lithium-ion storage ability even at high current. According to the schematic illustration of CMF-Co9S8-C hybrid nanocomposite for LIBs (Fig. 5c), the superior electrochemical performance is ascribed to the unique sandwich-type structure, shortening the transfer distance of Li+/e- and improving the conductivity. In particular, Co9S8 NPs were embedded between the carbonized melamine foam skeleton and N-doped carbon layer from PDA. Such structure can effectively accommodate volume expansion effect during the large current charge/discharge processes, and further enhance the stability of the electrode.
(Ref. (Ref. (Ref. (Ref. (Ref. (Ref.
33) 15) 13) 9) 34) 35)
Figure 5. EIS profiles of CMF-Co9S8-C and CMF-Co1-xS electrode (a), comparison of the specific capacities of the CMF-Co9S8-C with similar electrodes (b), and schematic illustration of the sandwich-type CMF-Co9S8-C hybrid nanocomposite for lithium-ion batteries (c).
In order to further explicate the storage mechanism and confirm the specific capacity contribution, the electrochemical kinetics and pseudocapacitive contribution of the CMF-Co9S8-C were analyzed with CV measurements. As displayed in Fig. 6a, the CV profiles show the similar shapes at different scan rates from 0.2 to 1.5 V, suggesting a stable pseudocapacitive behavior. In general, the relationship of peak 11
current (i) and scan rate (v) can be obtained by the equation (1) and simply transformed to the equation (2). The b is an adjustable parameter, which represents the slope of log i-log v plots. Generally speaking, when b is 1.0, it means a capacitive-controlled process; when b is 0.5, it means a diffusion-dominated process [36]. Fig. 6b shows the slope b of log i-log v plots, where the b value for cathodic and anodic current are 0.74 and 0.73, respectively. The lithiation and delithiation processes
are
constituted
by
both
the
diffusion-controlled
and
pseudocapacitive-controlled. So as to investigate the specific contribution of capacitive effect and diffusion-controlled process, equation (3) was introduced [37]. The i comprised of pseudocapacitive effects (k1v) and diffusion controlled reactions (k2v0.5) represents the current at particular potential. k1, as a slope of i/v0.5-v0.5 plots, can be obtained by precise calculations (details in supporting information). The results are shown in Fig. 6c, d. The shaded part in red signifies the pseudocapacitive effects contribution of CMF-Co9S8-C at the scan rate of 0.6 mV s-1, and the value is about 67.1%. Notably, the percentage of pseudocapacitive effects increases gradually with the scan rate increasing from 0.2 to 1.5 mV s-1, as shown in Fig. 6d. The results confirm that the high contribution of pseudocapacitive effects improve the lithium storage capacity and rate performances. The hierarchical porous structure of CMF-Co9S8-C electrode facilitates the contacting with electrolyte; meanwhile, the design of N-doped carbon coating improves the conductivity and alleviates the volume expansion. 𝑖 = 𝑎𝑣 𝑏
(1)
log 𝑖 = 𝑏log 𝑣 + log 𝑎 𝑖 = 𝑘1 𝑣 + 𝑘2 𝑣 0.5
12
(2) (3)
Figure 6. Electrochemical kinetics of CMF-Co9S8-C electrode toward Li+: CV curves (a) at scan rate from 0.2 to 1.5 mV s-1, log i-log v plots (b), contribution of the diffusion and pseudocapacitive effects to the total capacity at 0.6 mV s -1 (c) and contribution ratio at different scan rate (d).
4. Conclusions In summary, Co9S8 nanoparticles (10-20 nm) anchored on the carbonized melamine foam with PDA derived carbon coating (CMF-Co9S8-C) was successfully synthesized via typical solvothermal sulfurization and high-temperature carbonization. The sandwich-type architecture, Co9S8 nanoparticles between CMF and PDA derived carbon coating, can effectively relieve volume expansion, and massive pyridinic and graphite carbon significantly enhance conductivity. In addition, the nanocomposite inherits the 3D hierarchical porous structure of melamine foam, promoting the intimate contact with electrolyte and providing sufficient active sites. On account of the above advantages, CMF-Co9S8-C electrode indeed gives superior electrochemical performance as lithium-ion battery electrode (645 mAh g-1 at 0.5 A g-1 after 200 cycles and 520 mAh g-1 at 2 A g-1 after 1000 cycles). Considering the unique structure and simply synthesis method, our work may provide a rational solution for diverse metal sulfides, selenides and phosphides for energy storage. 13
Credit Author Statement
Pengcheng Zhang: Conceptualization, Data curation, Formal analysis, Writing - original draft Yi Feng: Conceptualization, Formal analysis Mengjue Cao: Data curation, Formal analysis Jianfeng Yao: Conceptualization, Formal analysis, Supervision, Writing review & editing
Notes The authors declare no competing financial interest. Acknowledgements The authors are grateful for the financial support of the Youth Fund of Natural Science Foundation of Jiangsu Province (BK20170919) and the National Natural Science Foundation of China (21808112).
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