Carboxymethyl Cellulose Binder Greatly Stabilizes Porous Hollow

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Carboxymethyl Cellulose Binder Greatly Stabilizes Porous Hollow Carbon Sub-Microspheres in Capacitive K-Ion Storage Jinliang Li, Ning Zhuang, Junpeng Xie, Yongqian Zhu, Haojie Lai, Wei Qin, Muhammad Sufyan Javed, Weiguang Xie, and Wenjie Mai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02060 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Carboxymethyl Cellulose Binder Greatly Stabilizes Porous Hollow Carbon Sub-Microspheres in Capacitive K-Ion Storage Jinliang Li,† Ning Zhuang,‡ Junpeng Xie,† Yongqian Zhu,† Haojie Lai,† Wei Qin,§ Muhammad Sufyan Javed, †∥ Weiguang Xie, † Wenjie Mai†* † Siyuan

Laboratory, Guangdong Provincial Engineering Technology Research Center

of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou 510632, People's Republic of China. ‡

Department of Materials Science and Engineering, Jinan University, Guangzhou

510632, People's Republic of China §

College of Materials Science and Engineering, Changsha University of Science and

Technology, Changsha, 410114, People's Republic of China ∥ Department

of Physics, COMSATS University Islamabad, Lahore Campus, Lahore,

54000, Pakistan Corresponding

author. E-mail: [email protected]

Keywords: carboxymethyl cellulose binder; porous hollow carbon sub-microsphere; K-ion storage; anode material. 1

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Abstract

On account of the large radius of K-ions, the electrodes can suffer huge deformation during K-ion insertion and extraction processes. In our work, we unveil the impact of using carboxymethyl cellulose (CMC) instead of poly(vinylidene fluoride) (PVDF) as binders for K-ion storage. Our porous hollow carbon sub-microsphere anodes using CMC binder exhibit a reversible capacity of 208 mAh g-1 after 50 cycles at 50 mA g-1, and even at a high current density of 1 A g-1, they achieve the reversible capacity of 111 mAh g-1 over 3000 cycles with almost no decay, demonstrating remarkably improved reversibility and cycling stability than those using PVDF (18 mAh g-1 after 3000 cycles at 1 A g-1). It is showed that CMC binder can result in higher adhesion force and better mechanical performance than PVDF binder, which can restrain the crack during potassiation-depotassiation process. According to the test of adhesion force, the hollow carbon sub-microspheres using CMC binder show above 3 times of average adhesion force than that using PVDF binder.

Furthermore,

based

on

the

rational

design,

our

hollow

carbon

sub-microspheres also exhibit 62.3% specific capacity contribution below 0.5 V vs K/K+ region, which is helpful to design the full cell with high energy density. We believe that our work will highlight the binder effect to improve K-ion storage performance.

2


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1. Introduction In the past decade, Li-ion batteries (LIBs) obtain dominant position in portable-type energy devices.1-4 In order to break the energy density limitation, some new-type batteries including Li-S batteries, Li-O2 batteries, Zn-Air Batteries and flow batteries have been proposed.5-10 Nevertheless, for many large-scale energy storage systems, especially for the grid, energy densities are no longer the highest priority considered, instead of the cost.11, 12 It is well known that Li element certainly suffers high cost due to its poor abundance.13,

14

To decrease the acquisition cost, some

low-cost alternatives such as Na-ion batteries (NIBs) are proposed, which is due to its abundant natural resource availability.15-19 However, due to relatively high potential, NIBs also suffered some restrictions like low energy densities compared with LIBs. Recently, another promising energy storage system, K-ion batteries (KIBs) were also proposed, since K element exhibits similar earth-abundant like Na element and low redox potential compared with Na (Na+/Na = −2.714 V; K+/K = −2.936 V vs SHE).20, 21

However, there is a challenge for KIBs due to its larger radius of K ions, which

often suffer a large strain during potassiation-depotassiation process and consequently undergo irreversibility.22,

23

Therefore, moving forward KIBs’ technologies to

improve their electrochemical performance became a top priority. In recent years, scientists have attempted many anode materials including alloy, metal sulphides and carbon for KIBs.24-27 Considering the economy and sustainability, 3



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carbon materials were deemed to one of the most promising alternative anodes for KIBs.28, 29 Currently, many results about carbon-based materials anode for KIB are affirmative, which encourages the KIBs’ development. Xiulei Ji’s group synthesized hard-soft composite carbon as anode for KIB and found that the carbon composite exhibited about 200 mAh g-1 at 279 mA g-1 with a capacity retention of 93% after 200 cycles.11 M. P. Paranthaman’s group prepared tire-derived carbon anode for KIBs and it exhibited a capacity of 155 mAh g-1 after 200 cycles at 139.5 mA g-1.30 Shaojun Guo’s group designed mesoporous carbon for KIBs and a storage capacity of 146.5 mA h g-1 obtained at 1 A g-1 with a capacity retention of about 70% after 1000 cycles.31 In spite of these excellent works, many issues of carbon-based material for KIBs’ application still exist, particularly in their cycle life, because of the large expansion during potassiation-depotassiation process. Previously, some researches found that choosing suitable binder to replace traditional binder of poly(vinylidene fluoride) (PVDF) could improve the cycle life effectively in LIBs and NIBs. Jang Wook Choi’s group found that incorporation of 5 w% polyrotaxane in polyacrylic acid as binder could restrain disintegration of silicon particles in LIBs.32 Shinichi Komaba’s group found that NIBs using carboxymethyl cellulose (CMC) binder exhibited much longer cycle life than those using traditional PVDF binder.33 Previously, a literature has been also reported about the binder for KIBs, but this work only simply described this phenomenon.34 Up to date, no relevant 4


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researches have been conducted to deep analyse the binder for KIBs yet. In our work, we study the effects of binders on the electrochemical performance of porous hollow carbon sub-microspheres for K-ion storage. Two different binders of CMC and PVDF were selected in our investigation. We found that our porous hollow carbon sub-microspheres electrode using CMC binder exhibited excellent reversible capacity and stable cycle life compared to that using PVDF binder. We believe that our work will offer a new route to design the high-performance K-ion storage system. 2. Experimental 2.1. Synthesis Silica sub-microspheres were synthesized by modified Stober method.35 Typically, 9 mL ammonium hydroxide (28 wt%) and 16.25 mL ethanol were added in 24.75 mL water under stirring to obtain solution A. 4.5 mL tetraethoxysilane was dissolved in 45.5 mL ethanol with stirring for 30 min to obtain solution B. Subsequently, the solution B was dropped into solution A rapidly with vigorous stirring at room temperature. After reaction of 2 h, the product was corrected, washed and dried at 80 °C for 24 h. Finally, the silica sub-microspheres were obtained. Porous hollow carbon sub-microspheres were obtained by hydrothermal process, which used silica sub-microspheres as templates, as shown in Figure S1. Typically, 8.107 g glucose and 4 g silica sub-microspheres were added in 60 mL deionized water under stirring and ultrasonic treatment. Subsequently, the mixture solution was loaded 5



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into a 100 mL hydrothermal reactor and heated to 180 °C for 12 h. Then the precipitate was collected, washed and dried at 80 °C for 24 h. The obtained products were thermally treated at different temperature in nitrogen for 2 h. After that, the products were corroded for 12 h under stirring in hydrofluoric acid (10 wt%) to remove silica sub-microspheres. The porous hollow carbon spheres thermally treated at 400, 600 and 800 °C were named as HCS-400, HCS-600 and HCS-800, respectively. Carbon sub-microspheres were prepared at 600 °C by a similar process without silica sub-microspheres, which are named as CS-600. For comparison, sucrose derived porous hollow carbon sub-microspheres were also prepared by similar method except 8.107 g glucose instead of 7.702 g sucrose, which was named as S-HCS-600. 2.2. Materials Characterizations The products’ morphologies were obtained by scanning electron microscopy (SEM, Zeiss-Ultra-55), transmission electron microscopy (TEM, JEOL-2100). The crystallographic phase of the product was characterized by X-ray diffraction (XRD, Panalytical PRO PW3040/60). The disorder degrees of products were tested by Raman spectra (Horiba T64000) with a wavelength of 532 nm. The surface property was measured by X-ray photoelectron spectroscopies (XPS, Thermo Fisher Scientific, K-Alpha) and Fourier-transform infrared spectroscopy (FTIR, Nicolet, NEXUS 670). Thermal gravimetric analysis (TGA) was tested by a thermal gravimetric analyzer 6


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(STA449F3, NETZSCH) with a temperature range of 30~1000 °C in air. Brunauer-Emmett-Teller (BET) specific surface areas were evaluated by nitrogen adsorption

isotherms

using

a

nitrogen

adsorption

apparatus

(Micrometitics-Norcross-GA). The roughness parameters were performed by atomic force microscopy (AFM, Bruker dimension icon). The adhesion force was test by a universal tension gauge with a speed of 50 mm min-1. The adhesive tape was attached to the electrodes with the size of 15 mm × 160 mm, and the stripping angle between the active material layer and the Cu foil is 90°. The corresponding schematic of adhesion force measurement was showed as Figure 6a 2.3. Electrochemical measurement Typically ， 70 wt% products, 10 wt% Super-P carbon , and 20 wt% CMC in water or PVDF in N-Methyl pyrrolidone were mixed to form a uniform sizing. Then the sizing was coated on Cu foil, dried overnight and subsequently cut into a diameter of 14 mm as an electrode. The mass loading of active materials of all the samples are about 0.93 mg cm-2. The obtained electrode was packaged into R2032 coin-type cell in Etelux glove box (Lab2000). In the coin-type cell, the metal K foil served as counter electrode and the Whatman glass fiber acted as separator. 1 M KPF6 solution in propylene carbonate and ethylene carbonate (1:1, v/v) was used as electrolyte. The CMC binders were distinguished by their viscosities. Among the CMC binders, the viscosities were 300-600 mpa, 600-1000 mpa and 1000-1500 mpa, which were named 7



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as CMC-1, CMC-2 and CMC, respectively. Galvanostatic charge-discharge curves were performed by battery test system (Neware BTS-4000). Cyclic voltammetry (CV) was

tested

by

a

Princeton

electrochemical

workstation

(VersaSTAT3).

Electrochemical impedance spectroscopy (EIS) was recorded by the identical electrochemical workstation, which tested in a frequency range of 0.1 Hz-10 kHz. 3. Results and discussion Hollow carbon sub-microspheres were synthesized by hydrothermal process utilizing glucose precursor and silica sub-microspheres template. In order to evaluate the etching degree of SiO2 templates, TGA curve of HCS-600 was provided, as shown in Figure S2. It can be seen that the weight of sample completely exhausts after 650 °C, indicating that SiO2 templates have been completely removed after corrosion. Figure 1a shows the SEM image of silica sub-microspheres template, which is observed a homogeneous sub-microspheres structure with an average diameter of ~350 nm. Figure 1b displays the SEM images of CS-600, which shows a diameter of ~380 nm with smooth surface. After using silica sub-microspheres as template, the SEM images of HCS-400, HCS-600 and HCS-800 show in Figure 1c-e, respectively. All of them exhibit similar morphologies with hollow structure. Different from the surface of CS-600, the hollow carbon sub-microspheres exhibit rougher surface, which was shown in the enlarged image of HCS-600 in Figure 1f. In order to further investigate the hollow carbon sub-microspheres, TEM images of HCS-600 are also 8


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provided in Figure 2. The hollow structure of carbon sub-microspheres is further demonstrated. From the enlarged TEM image of HCS-600, it is found that the thickness of shell is ~35 nm and the rough surface was constituted by amorphous carbon. Such carbon layer covered on the surface of hollow carbon sub-microspheres can provide more sites for K-ion insertion, which is helpful to improve the K-ion storage performance. In high resolution TEM, disorder lattice can be detected, proving the amorphous structure of our hollow carbon sub-microspheres.

Figure 1 FESEM images of (a) SiO2 sphere, (b) CS-600, (c) HCS-400, (d) HCS-600, (e) HCS-800 and (f) enlarged image of HCS-600.

9



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Figure 2 (a) TEM image, (b) enlarged TEM image and (c) high-resolution TEM image of HCS-600. According to XRD patterns of samples (Figure S3), all of them exhibit two broad peaks at 26° and 44.5°, which is the typical hard carbon structure.36 From the XRD patterns, a slight right-shift in (002) facet can be seen, possibly due to the decrease of interlayer spacings with the temperature increasing.37, 38 In order to further investigate the structure, Raman spectra of the samples were obtained, as shown in Figure 4a. Two obviously characteristic peaks located at 1352 and 1626 cm-1, which are appointed to D-band and G-band, respectively. As well known that the D-band is related to the disordered carbon, and the G-band is related to the graphitic carbon.39, 40 Therefore, the peak intensity ratio (ID/IG) of G and D band should be considered a parameter of the disorder degree of the carbon materials. Based on the calculation, the values of ID/IG are 0.89, 0.81, 0.92 and 1.05 for CS-600, HCS-400, HCS-600 and HCS-800, respectively. These results suggest that hollow structure and high temperature thermal treatment can lead to more disordered structure in hard carbon material. Usually, such disorder structure is helpful to improve the insertion reaction 10


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of K-ion.41, 42 Figure 3b shows the FTIR spectra of CS-600, HCS-400, HCS-600 and HCS-800. All of samples contain O-H (3446 cm-1), C=O (1639 cm-1), C-H (1358 cm-1), C=C (1169 cm-1) and C-O (883 cm-1) bands, stemming from the glucose precursor. It is worth noting that HCS-400 exhibits more oxygen-containing group than others, which is attributed to the incomplete carbonization. To further estimate the structure of hollow carbon sub-microspheres, XPS measurement of HCS-600 was carried out in Figure 3c. According to the fitting of C 1s XPS spectra of HCS-600, it is found that the C-C/C=C bond occupies dominant position, indicating that our hollow carbon sub-microspheres have been carbonized.43 Figures 3d and Figure S4 show the nitrogen adsorption-desorption isotherms and pore size distribution of CS-600, HCS-400, HCS-600 and HCS-800, respectively. The specific surface areas, external surface area, micropore area and pore volume were determined by the BET method and listed in Table 1. Compared with CS-600, hollow carbon sub-microspheres exhibit larger external surface areas. Such larger external surface areas rather than micropore areas are probably in favor of the capacitive contribution due to the abundant adsorption behavior.44 More micropore areas may restrain the depotassiation behavior, which is harmful to enhancement of its electrochemical performance. It's worth noting that HCS-400 exhibits much low specific surface area compared with other samples, which is due to the incomplete pyrolysis of precursor, resulting in that the pores cannot be generated effectively at the low temperature.45 11



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Figure 3 (a) Raman spectra and (b) FTIR spectra of CS-600, HCS-400, HCS-600 and HCS-800; (c) C 1s XPS spectra of HCS-600; (d) nitrogen adsorption and desorption isotherms of CS-600, HCS-400, HCS-600 and HCS-800. The cycling performance at 50 mA g-1 of CS-600, HCS-400, HCS-600 and HCS-800 using CMC binder was also provided in Figure 4a. The initial reversible specific capacity of CS-600 is 188 mAh g-1 and maintains 139 mAh g-1 after 50 cycles, with a retention rate of only 74%. Such capacity decay was speculated that the solid structure leads to the break of interior structure during potassiation-depotassiation process. As a contrast, HCS-600 exhibits an initial reversible specific capacity rises to 194 mAh g-1 and the capacity of HCS-600 still maintains 211 mAh g-1 after 50 cycles, higher than that of CS-600. Such improvement of electrochemical performance should be ascribed to the rough surface and larger external surface areas, which have 12


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been proved in our previous characterizations. Furthermore, different thermal treatment temperatures also affect the electrochemical performance of hollow carbon sub-microspheres. HCS-400 only exhibits ultra-low initial reversible specific capacity of 6.7 mAh g-1, with a specific capacity of 73 mAh g-1 after 50 cycles. Some inordinate specific capacity during the cycles is due to the incomplete carbonization of HCS-400. However, HCS-800 also exhibits a slight lower reversible specific capacity of 181 mAh g-1, and 169 mAh g-1 after 50 cycles, which is possibly due to the smaller interlayer spacings.37, 38 It is noted that the specific capacity of HCS-600 after 50 cycles is still larger than the initial, which is due to the activation step, as like in the report of Liu’ group.46 The corresponding Coulombic efficiencies of CS-600, HCS-400, HCS-600 and HCS-800 using CMC binder showed as Figure 4(b). The untidy Coulombic efficiencies of HCS-400 should be attributed to the incomplete carbonization. We also find that the initial Coulombic efficiencies increase with the annealing temperature raised, which possibly cause by the reduction of oxygen-containing groups. The rate performance of HCS-600 was also shown in Figure 4c. As seen, the HCS-600 exhibits reversible specific capacities of 252, 217, 183, 154 mAh g-1 at 50, 100, 250, and 500 mA g-1, respectively. The specific capacity still maintains 130 mAh g-1 even at a high current density of 1 A g-1. With the current density reversing, the capacity can almost fully recover, illustrating that the excellent reversibility of our sample. The charge-discharge curves of rate performance were 13



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shown in Figure 4d. All of curves show a similar shape, illustrating that the electrochemical process at different current densities scarcely changes. Table 1 Specific surface areas, external surface area, micropore area and pore volume of CS-600, HCS-400, HCS-600 and HCS-800. External Specific surface Sample

Micropore

Pore volume

area (m2 g-1)

(cm3 g-1)

surface area area

(m2

g-1) (m2 g-1)

CS-600

501.7

62.1

439.6

0.249

HCS-400

74.2

69.9

4.3

0.204

HCS-600

478.2

84.2

394.0

0.313

HCS-800

369.1

97.7

271.4

0.287

14


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Figure 4. (a) Cycling performance and (b) the corresponding Coulombic efficiency of CS-600, HCS-400, HCS-600 and HCS-800 using CMC binder; (c) rate performance of HCS-600 and (d) the corresponding charge-discharge profiles (10 cycles for each current density). The electrochemical reactions of HCS-600 using PVDF and CMC binders were evaluated by CV and galvanostatic charge-discharge measurement. Figures 5a and b show the CV curves of HCS-600 using PVDF and CMC binder for the initial four cycles between 0.01 and 3 V (vs K/K+) at a scan rate of 0.2 mV s-1, respectively. In the case of using PVDF binder, during the initial discharge, a peak at ~0.64 V appeared, which is due to the formation of SEI layer.24,

47

Furthermore, large

irreversible slope in lower potential region can be observed, stemming from the trapping mechanism of potassiation behavior in micropores.48, 49 In the following CV 15



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cycles, there are some slight fade using PVDF binder, which is attributed to the maturation process of SEI layer. Due to the existence of some hydrophilic group (proved in FTIR and XPS spectra), using CMC as binder can pre-form SEI layer in HCS-600, which probably help to reduce the SEI layer and restrain the irreversible potassiation behavior.34 It is obviously observed that using CMC binder exhibits less irreversible discharge in the initial CV cycle. From all of the CV cycles at high voltage, a weak redox peak of the CV curves in a wide voltage range can be seen, implying that K-ion storage exhibits a capacitive behavior. It's worth noting that the initial cathodic peak from the case of using CMC binder exhibits lower voltage than that using PVDF binder. This phenomenon illustrates that the polarization of HCS-600 using CMC binder is weaker than that using PVDF binder, which is helpful to increase the activity of the electrochemical reaction.50 Furthermore, CMC binder can form a uniform passivation layer as pre-SEI layer on the surface of active materials, which has been proved by the XPS spectra (Figure S12).51 Thanks to the passivation layer, only a handful of K-ions are irreversibly trapped, resulting in the improvement of its initial Coulomb efficiency (Figure 5g).51,

52

Figures 5c and d

illustrate the initial four galvanostatic charge-discharge curves of HCS-600 using PVDF and CMC binder at 50 mA g-1, respectively. From the initial charge-discharge curves, HCS-600 using CMC binder exhibits more reversible specific capacity than that using PVDF binder, in agreement with the CV measurement. We also find an 16


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interesting phenomenon that our sample exhibits a large discharge specific capacity contribution in low voltage region. As shown in Figure 5e, the second discharge specific capacity contribution below 0.5 V achieve 62.3%. Compared with other hard carbon in previous reports, our porous hollow carbon sub-microspheres exhibit better electrochemical performance for KIBs, as shown in Table S1.53-62 Unfortunately, due to we failed to grasp cathode technical, we are not able to provide the electrochemical performance of full battery to further support our statement. Figures 5f and g show the cycling performance and Coulombic efficiencies of HCS-600 using PVDF and CMC binder at 50 mA g-1, respectively. We find that the initial discharge specific capacity of the HCS-600 with PVDF binder is 617 mAh g-1, while the reversible specific capacity only exhibits 162 mAh g-1, with a low initial Coulombic efficiency of 26%. When the binder was changed to CMC, the initial discharge specific capacity drops to 370 mAh g-1, and the corresponding reversible specific capacity rises to 194 mAh g-1, with an initial Coulombic efficiency of 52%. After 50 cycles, the capacities of HCS-600 using PVDF and CMC binder are 89 and 211 mAh g-1, respectively, indicating the better electrochemical reversibility and stability of CMC binder in our sample. In order to further compare the electrochemical performance, the rate performance of HCS-600 electrode using PVDF binder was also provided, as shown in Figure S5. The electrode using PVDF binder exhibits 248, 198, 166, 124, 90 mAh g-1 at 50, 100, 250, 500 and 1000 mA g-1, respectively. This result is worse than that 17



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electrode using CMC binder (Figure 4c). In order to better demonstrate this phenomenon, long-term cycling performance of HCS-600 using PVDF and CMC binder were provided (Figure 5h), and the corresponding charge-discharge profiles are shown in Figure S6. At a large current density of 1000 mA g-1, HCS-600 using CMC binder still exhibits 111 mAh g-1 after 3000 cycles with almost no fade, which is significantly better than HCS-600 using PVDF binder (18 mAh g-1 after 3000 cycles). To testify the universality of our view, the long-term cycle performance of hollow carbon spheres derived by sugar was also provided in Figure S7. S-HCS-600 using CMC binder exhibits a stable specific capacity of 115 mAh g-1 at 1 A g-1 after 1000 cycles, which is much higher than that using PVDF binder (a specific capacity of 47 mAh g-1 at 1 A g-1 after 1000 cycles). This result further proven that CMC binder exhibits better performance than PVDF binder. We also investigate the cycling performance of electrodes using different viscosities of CMC, as shown in Figure S8. After 500 cycles, the electrode using CMC-2 as binder exhibits similar capacity retention compared that using CMC (a specific capacity of 113 mAh g-1 at 1 A g-1 after 500 cycles). However, when the viscosity further decreases, the electrode using CMC-1 exhibits a slight capacity fade (a specific capacity of 78 mAh g-1 at 1 A g-1 after 500 cycles), indicating that viscosity of CMC for KIB’s binder is important and the high viscosity of CMC is helpful to improve the cycle life of electrode for KIBs.

18


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Figure 5 CV curves of HCS-600 using (a) PVDF and (b) CMC binder; galvanostatic charge-discharge profiles of HCS-600 using (c) PVDF binder and (d) CMC binder; (e) the second discharge specific capacity contribution of HCS-600 using CMC binder below 0.5 V vs K/K+. (f) cycling performance and (g) Coulombic efficiency of HCS-600 using PVDF and CMC binder tested at 50 mA g-1. (h) long cycling performance tested at 1000 mA g-1. In order to measure the adhesion of PVDF and CMC binders, the 90° adhesion forces were preferred for evaluation, and the corresponding test schematic is shown in Figure 6a. Figure 6b show the adhesion curves of HCS-600 using CMC and PVDF binders. As seem, HCS-600 using PVDF binder only exhibits an adhesion forces of 35 N m-1. Instead of PVDF, CMC binder exhibits an adhesion forces of 114 N m-1, 19



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much higher than that of PVDF binder, demonstrating that CMC binder can provide stronger adhesion than PVDF binder. We also studied the morphology change of electrodes after cycles. Figures 6c-e shows the SEM images of HCS-600 using PVDF and CMC binder after 50 cycles at 50 mA g-1. In Figure 6c, obviously cracks can be observed after cycling, demonstrating that the electrode using PVDF binder suffers great damage during potassiation-depotassiation process. However, when the binder was changed to CMC (Figures 6d and e), no large crack can be seen, implying that CMC binder exhibits higher adhesion force between hollow carbon spheres and copper foil. Figure S9 shows the photograph of peeled off separator using PVDF and CMC binder after 50 cycles at 50 mA g-1. We can observe that the separator using PVDF binder attach some material, while the separator using CMC binder is clean, further indicating that CMC binder possesses stronger adhesion force than PVDF binder. This stronger adhesion force of CMC binder can restrain the pulverization of electrode, resulting in the realization of superior cycle performance.63 To study the roughness state, AFM images of HCS-600 using PVDF and CMC binder before and after 50 cycles were provided in Figure S10. According to the AFM characterization, the average roughness can be calculated. Before cycle, the electrode using CMC binder exhibits an average roughness of 214 nm, which is higher than that of electrode using PVDF binder (average roughness of 131 nm). After 50 cycles, the average roughness of electrode using PVDF binder is 455 nm, with an increment rate of 247%. 20


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The significantly increase of the average roughness using PVDF as binder is stemmed the inflation and crack during potassiation-depotassiation process. However, the average roughness of electrode using CMC binder increase to 373 nm, with an increment rate of 74%, which exhibits a smaller change. This phenomenon is ascribed to the strong adhesion force of CMC binder, which can restrain the cracks in electrode during potassiation-depotassiation process. In order to better illustrate the roughness of samples, 3D AFM images were also provided in Figure S11.

Figure 6 (a) The schematic of the 90° adhesion forces test, (b) Adhesion forces curves of HCS-600 using PVDF and CMC binders. SEM images of HCS-600 using (c) PVDF and (d) CMC binders after 50 cycles at 50 mA g-1, (e) enlarged SEM image of HCS-600 using CMC binder. To investigate the source of strong adhesion force of CMC binder, we also 21



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measure the electrodes using different binders by XPS, as shown in Figure S12. It can be seen that the C-C bond peak strength of the electrode using CMC binder weaken significantly compared with that using PVDF. It is well known that the signal from XPS are mainly comes from the surface of materials, indicating that CMC binder will form a passivation layer, which was coated on the surface of hollow carbon spheres uniformly. Furthermore, a strong peak of C-O-H bond was detected, indicating that the electrode using CMC binder exhibits lots of hydroxyl groups. Based on abundant hydroxyl groups in the CMC macromolecular chain, hydrogen bond will exhibit strong adhesive force, which is helpful to restrain the exfliation of active material. Furthermore, the EIS spectra before and after cycles of HCS-600 using PVDF and CMC binders were also studies (Figures 7a and b, respectively). The Rp value of electrode using PVDF and CMC binder are 1.85 and 1.42 kΩ, with only a little difference, which is may be cause by the subtle difference of assembly process. After 50 cycles, the electrode using PVDF exhibits the Rp value of 5.41 kΩ, significantly larger than that before cycles, which is ascribed to the crack of electrode after cycle. While the electrode using CMC exhibits the Rp value of 2.42 kΩ, further proving that the electrode using CMC binder maintains better adhesion after cycles.

22


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Figure 7 EIS spectra of HCS-600 using PVDF and CMC binder (a) before and (b) after 50 cycles; (c) CV curves of HCS-600 using CMC binder at the scan rates of 0.4~1.2 mV s-1 after 4 CV cycles; (d) the relation between log (i) and log (v); (e) the capacitive contribution area at 1.2 mV s-1 for HCS-600 using CMC binder and (f) contribution ratio of the capacitive and diffusion controlled contribution in different scan rates. The CV curves of HCS-600 using CMC binder at the scan rates of 0.4~1.2 mV s-1 23



Page 24 of 39

were shown in Figure 7c. At different scan rates, we can observe that all of CV curves exhibit similar shapes. According to the equation 1 of the scan rate (v) and current (i) in the previous work:64, 65 i = avb

(1)

which can transform to equation 2: log i = log a + b log v

(2)

where the current i follows with the scan rate v. Both of them, b is decided by the slope of the plot of log i vs log v. Typically, the b value of 0.5 illustrates that the current exhibits a diffusion-controlled behavior. While the b value is 1, indicating that the current predominantly exhibits capacitive behavior. In Figure d, according to the linear relationship of log i vs log v, b values of HCS-600 for an anodic peak (peak 1) and a cathodic peak (peak 2) are calculated to be 0.95 and 0.91, respectively, illustrating that the specific capacity of HCS-600 is predominantly controlled by the capacitive contribution. To further evaluate the contribution of capacitive behavior in HCS-600, equation 3 was utilized for investigation:59, 66 i(V) = k1v + k2v1/2

(3)

For analytical purposes, we rearrange equation 3 to i(V)/v1/2 = k1v1/2 + k2

(4)

In equation 3, k1v and k2v1/2 correspond to the contributions of capacitive behavior and diffusion-controlled behavior, respectively. Therefore, we can quantify each of 24


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these contributions by determining k1 and k2. According to the equation 4, we are able to calculate the capacitive contribution of 80% at a scan rate of 1.2 mV s-1, as shown in Figure 7e. Figure 7f shows the contributions of the capacitive behaviors at different scan rates, which contribution ratios are 69%, 72%, 74%, 77% and 80% at the scan rates of 0.4, 0.6, 0.8, 1.0 and 1.2 mV s-1, respectively. It is found that the capacitive contribution is enhanced with the scan rate increasing, indicating that capacitive contribution plays a dominant role in the total capacity in our sample. Furthermore, Due to the abundant oxygen functional group in precursor, our porous hollow carbon spheres still exit slight surface functional groups after thermal treatment. The oxygen functional group will result the Faradaic reactions for K-ion storage, which have been provided in the CV curves (Figure 5b).67 The contributions of the capacitive behaviors of electrodes in different scan rates also indicate that Faradaic reactions of functional groups will be sluggish at higher scan rate. 4. Conclusions In our work, we first unveiled that CMC binder can improve the K-ion storage performance in hollow carbon spheres effectively. Furthermore, our result also demonstrates that hollow carbon sphere anodes with CMC binder show better reversibility and cycling stability than that of PVDF. Even at a high current density of 1000 mA g-1, the reversible capacity of hollow carbon sphere using CMC binder achieved 111 mAh g-1 over 3000 cycles, with almost no decay, much better than that 25



Page 26 of 39

using PVDF binder. We also employ SEM, AFM to investigate the changes of electrode during potassiation-depotassiation process, testifying CMC binder can provide better adhesive force than PVDF, which is owing to CMC’s enormous hydroxyl groups. We believe that our work will offer a new route to improve K-ion storage performance for prospective energy storage applications. Supporting Information Supporting Information including schematic of the synthesis process, TGA curve, XRD patterns, pore size distributions, charge-discharge curves, long-term cycling performance, photograph of separator, AFM images and XPS spectra of samples. The comparison of K-ion storage performance of carbon in previous literatures were also provided. Notes The authors declare no competing financial interest. Acknowledgements We thank the financial supports from the National Natural Science Foundation of China (51702056, 51772135, 51702371), the Natural Science Foundation of Guangdong Province, China (2014A030306010), the Ministry of Education of China (6141A02022516) and the Project funded by China Postdoctoral Science Foundation (2017M622902, 2018M633280). References 26


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Table of Contents (ToC) Graphic

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Carboxymethyl Cellulose Binder Greatly Stabilizes Porous Hollow

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