UV Irradiation Treatment for Enhanced Sodium Storage Performance

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Energy, Environmental, and Catalysis Applications

UV Irradiation Treatment for Enhanced Sodium Storage Performance Based on Wide-Interlayer-Spacing Hollow C@MoS2@CN Nanospheres Jingying Duan, Guohui Qin, Luofu Min, Yuchen Yang, and Chengyang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13570 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Applied Materials & Interfaces

UV Irradiation Treatment for Enhanced Sodium Storage Performance Based on Wide-Interlayer-Spacing Hollow C@MoS2@CN Nanospheres Jingying Duan a,c, Guohui Qin b*, Luofu Min a, Yuchen Yang a, Chengyang Wang a,c* a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

b

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042,

Shandong, China c

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, PR

China *

Corresponding author.

E-mail address: [email protected], [email protected]

Keywords: UV irradiation, MoS2, double carbon layers, energy storage, sodium ion battery Abstract: The photochemistry and sodium storage process have been generally considered as two separated approaches without strong connection. Here, ultraviolet (UV) irradiation was applied to sodium-ion batteries to improve the electrochemical performance of MoS2 based composite. C@MoS2@CN nanospheres consist of double protective structures, including inner hollow carbon spheres with thin wall (C) and outer N-doping carbon nanosheets (CN) derived from polydopamine. The special nanostructure possesses the virtues such as wide-interlayer spacing, flexible feature with great structure integrity and rich active sites, which endow the fast electron transfer and shorten the ion diffusion pathways. Under the excitation of UV-light, intense electrons

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and holes are accumulated within MoS2 based composite. The excited electrons can promote the pre-insertion of Na+. And more importantly, dense electrons promote the electrolyte to decompose and hence form a stable solid electrolyte interphase (SEI) in advance. After UV-light irradiation treatment in electrolyte, the initial Coulombic efficiency of C@MoS2@CN electrodes increased from 48.2% to 79.6%. And benefiting from the fine nanostructure, the C@MoS2@CN electrode with UV irradiation treatment delivered a great rate performance of 116 mAh g-1 in 20 s and super cycling stability that 87.6% capacity was retained after 500 cycles at 500 mA g1

. When employed as anode for sodium-ion hybrid capacitors (SIHCs), it delivered a

maximum power density of 6.84 kW kg-1 (with 114.07 Wh kg-1 energy density) and a maximum energy density of 244.15 Wh kg-1 (with 152.59 W kg-1 power density). This work sheds new viewpoints into the applications of photochemistry in the development of energy storage devices. 1. Introduction: SIBs have attracted tremendous research attention due to the abundance and low cost of sodium.1,2 In order to satisfy the increased demand for power devices, it is promising to design SIBs possessing high rate capability and long durability.3-5 Since the rate capability and cycling life are predominately restrained by the sluggish kinetics of diffusion reaction as well as the large volume expansion,6-9 the reasonable structure design of active material is critically important.49-53 And the pseudocapacitive energy storage, which depends on surface faradaic reaction, can overcome the rate limitations effectively.11-14 In addition, the initial Coulombic efficiency is a vital indicator which


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reflects the irreversible losses during the forming process of SEI. Many investigations indicate that SEI is derived from decomposing of electrolyte and reactions between electrolyte and electrode. It is evident that intensive electrons and holes can be excited by UV irradiation in light responsive materials.10 When carried out in electrolyte, the holes will be trapped by anion in electrolyte rapidly and the electrons can induce the redox reaction between electrolyte and electrode material. Inspired by this idea, we tried to explore whether the UV-light treatment can enhance electrochemical stability of sodium-ion batteries. In support of this work, designing the light responsive battery material with pseudo-capacitance is significantly important. Among various candidates, MoS2 has been investigated as a functional material in diverse fields such as batteries,15 catalysis,16 lubrication,17 electronic transistors,18 and photovoltaics.19 MoS2 is a typical graphite-like layered structure composed of S-Mo-S motifs, displaying extraordinary photocatalytic behavior20 and sodium storage performance.21 But the commercial 2H-MoS2 exhibits a severe volume change during cycling which can cause pulverization of electrode material and poor conductivity, resulting in poor cycling stability, especially under high current density. In order to be more adjustable to Na+ (0.102 nm), and increase the conductivity of 2H-MoS2, various strategies, including optimizing nanostructures and morphologies,27,28 carbon doping layers,29 and introducing defects30-32 have been studied. The introduction of defects can not only change the electronic structure but also provide more open channels for the diffusion of Na+.25-26 To overcome such drawbacks of MoS2, wide-interlayer-spacing hollow C@MoS2@CN nanospheres were synthesized. Firstly, combining MoS2 with



flexible hollow thin carbon spheres can be well suited for alleviating the mechanical stress and also improving the conductivity of MoS2. Secondly, high temperature calcination in N2 atmosphere accompanied by the decomposing of polydopamine is an effective way to synthesize rich-defects material for enhanced kinetics of diffusion reaction. Besides, the CN sheets derived from polydopamine among MoS2 interlayer spacing maintain the wide interlayer spacing of MoS2 synthesized by hydrothermal method and increase the conductivity of MoS2. It is brilliant to design a composite in which MoS2 is encapsulated into double conductive carbon layers, which not only provide double insurance against the pulverization of MoS2 nanostructure, but also ensure preferable transport kinetics for both electron and sodium ion. The photochemistry and energy storage are generally regarded as two parallel processes without major overlap. Up to now, no report has been found about any mechanistic connections between the two approaches. Herein, we applied UV irradiation to the design of high performance electrode material. The result bridges the two processes and provides detailed information for the promotion of UV irradiation on energy storage performance. The MoS2 based composite combined with hollow carbon spheres and nitrogen-doping carbon sheets (C@MoS2@CN) was synthesized by the template-mediated method and annealing process. Besides the improvement on conductivity of MoS2, the thin hollow carbon spheres alleviate the volume change during charge and discharge process and shorten the distance of Na+ diffusion. The Ndoping carbon layers increase the interlayer lattice spacing, which accommodates the insertion and extraction of Na+. Therefore, C@MoS2@CN exhibits higher reversible


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capacity and better cyclic stability than C@MoS2 and commercial MoS2. With the assistance of photocatalytic technique, such C@MoS2@CN nanoparticles with an expanded interlayer spacing present high electrochemical stability and show extremely fast charging and discharging kinetics along with long cycling lifetimes. 2. Experimental section 2.1. Material SiO2 powders with a diameter of ~200 nm were purchased from Aladdin. All the regents including CTAB, resorcinol, NaOH, (NH4)6Mo7O24`4H2O, thiourea and polydopamine hydrochloride were analytical grade and supplied by Tianjin Chemical Corp. 2.2. Synthesis of SiO2@RF SiO2@RF spheres were prepared through a typical method.54 Firstly, 20 mg SiO2 spheres were dispersed in 28 mL H2O. 2 mL CTAB (1 mol/L) and 0.1 mL ammonia solution (25 wt%) were added to the suspension under vigorous stirring at room temperature for 30 minutes. After the addition of 0.05 g resorcinol, 0.07 mL formaldehyde (37 wt%) were dropped into the prepared suspension. The mixed solution was kept vigorous stirring at 50 ℃ for 2 hours. The products were collected by centrifugation. After washing with H2O and ethanol for a few times, the faint yellow solid powders were got through drying at 80 ℃. 2.3. Synthesis of the wide-interlayer-spacing hollow C@MoS2@CN The obtained 30 mg SiO2@RF spheres were dispersed in 35 mL H2O. 0.42 g (NH4)6Mo7O24`4H2O and 0.76 g thiourea were dissolved in the prepared suspension



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under ultrasonic dispersion for 30 minutes. Then it was transferred to 50 mL Teflon@lined autoclave and maintained at 180 ℃ for 12 hours to get the SiO2@RF@MoS2 spheres. Then, the products were dispersed into 150 mL Tris buffer (2mol/L) by ultrasonic. Under vigorous stirring, 75 mg polydopamine hydrochloride was added into the suspension and kept at 30 ℃ for 24 hours to get the SiO2@RF@MoS2@PDA

spheres.

After

separated

by

centrifugation,

the

SiO2@RF@MoS2 and SiO2@RF@MoS2@PDA spheres were calcined at 800 ℃ for 4 hours under N2 atmosphere to obtain SiO2@C@MoS2 and SiO2@C@MoS2@CN spheres. Finally, the hollow C@MoS2 and C@MoS2@CN nanospheres were obtained by removing SiO2 using sodium hydroxide (NaOH, 1 M) (Scheme 1). Scheme 1. Schematic diagram for the preparation of hollow C@MoS2@CN nanostructure process.

3. Results and discussion 3.1. Structure and morphology


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Figure 1. (a) The XRD patterns of commercial MoS2 and MoS2 based composites. (b) The Raman spectrum of commercial MoS2 and MoS2 based composites.

Figure 1a displays the X-ray diffraction (XRD) patterns of the commercial MoS2 and MoS2 based composites. The peaks of the three samples are basically consistent with the JCPDS card No.37-1492 which represents crystal planes of hexagonal phase MoS2. However, after combining with the CN sheets, the (002) peak was split into two peaks located at 9.6° and 18°, respectively. This phenomenon suggested the distance between two adjacent MoS2 layers expanded from 6.29 Å (C@MoS2) to 9.2 Å (C@MoS2@CN) calculated by the Bragg law. The peak at 18° reflects the layer distance (4.9 Å) between adjacent MoS2 layer and CN sheet, which is consistent with the HRTEM. To explore the reason for wide interlayer spacing, the XRD pattern of the product through hydrothermal process was investigated. As shown in Figure S1, the (002) peak of Hy-SiO2@RF@MoS2 appears at 9.5°, corresponding to a interlayer distance of 9.5 Å. It can be concluded that MoS2 synthesized by hydrothermal method had a large interlayer spacing, and the polydopamine (PDA) occupied the space after the doping of PDA, which could restrain the shrink of MoS2 interlayers during calcination process (Scheme 2).44.45 Obviously, the larger interlayer spacing of 7 ACS Paragon Plus Environment


C@MoS2@CN enables the rapid insertion and extraction of Na+. Meanwhile, combination with the CN sheets improves the conductivity of MoS2. Scheme 2. Mechanism for the forming of wide-interlayer spacing C@MoS2@CN.

Figure 1b shows the Raman spectra of commercial MoS2, C@MoS2 and C@MoS2@CN. The peaks at 1360 cm-1 and 1590 cm-1 refer to the D band and G band of carbon materials, respectively. The characteristic peaks of the three samples at ~381 cm-1 and ~408 cm-1 assign to the E2g1 and the A1g modes of MoS2 with typical hexagonal layered structure. The E2g1 characteristic peak represents the in-plane vibration of Mo and sulfur atoms, while A1g is corresponding to the out-of-plane vibration of sulfur atoms. Compared with the commercial MoS2, the A1g peak of C@MoS2@CN has an obvious shift (Table S1). The shift is derived from the expanded interlayer spacing of MoS2, which is in agreement with the XRD results.46 In addition, the E2g1 peak and A1g peak of C@MoS2@CN are close to the MoS2 with monolayer structure,35 which proves the CN sheets among MoS2 layers reduce van der Waals force between MoS2 monolayers, endowing excellent electrochemical performance.


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In order to evaluate the detailed carbon contents of MoS2 based composites, TG analysis of C@MoS2@CN and C@MoS2 was conducted in air and result was shown in Figure S2. The mass losses of C@MoS2@CN and C@MoS2 mainly occurred at 300 ℃~400 ℃, related to the oxidation of C and MoS2. The final weight percents of C@MoS2 and C@MoS2@CN are 90% and 73.54%. According to equations: C + O2 → CO2 and MoS2 + O2 → MoO3 + SO2, the contents of carbon in C@MoS2 and C@MoS2@CN can be calculated. There is 18.2 wt% carbon in C@MoS2@CN and scarcely any carbon in C@MoS2, indicating high contents of MoS2 in both samples. The composition and chemical states of C@MoS2@CN were further tested by XPS analysis. As shown in Figure 2, the existence of Mo, S, C, O, N elements in C@MoS2@CN can be confirmed (Figure 2a). The peaks at 163.61 eV and 162.41 eV correspond to the binding energy of S 2p1/2 and S 2p3/2 (Figure 2b), respectively, indicating the presence of S2-. The peaks at 232.81 eV and 229.61 eV assign to Mo 3d3/2 and Mo 3d5/2 of Mo4+ and the small peaks at 236.01 eV and 227.11 eV are attributed to Mo6+ and S 2s (Figure 2c). Besides, the peaks at 285.66 eV and 284.81 eV, which represent the binding energy of C=C/C-C bonds and C-O bonds (Figure 2d), the peaks located at 533.31 eV, 532.21 eV and 531.41 eV refer to C-O-H, C-O-Mo and C=O (Figure 2e), respectively. The weak peak at 392 eV~406 eV can be divided into three peaks at 401.06 eV (Pyrollic N), 398.51 eV (Pyridinic N) and 395.51 eV (Mo-N bonds) (Figure 2f), which reveals the successfully doping of N in C@MoS2@CN.



Figure 2. (a) XPS spectrum of CN@MoS2@C. (b) S, (c) Mo, (d) C, (e) N and (f) O narrow scan, respectively.

The detailed morphology of MoS2 based composites was characterized by SEM (Figure 3) and TEM (Figure 4). As shown in Figure 3a, flower-like MoS2 sheets grow on the surface of hollow carbon sphere, forming C@MoS2 nanospheres with diameter of ~300 nm. And it can be observed from Figure 3b that the sheets on the surface of hollow carbon spheres become thicker after the doping of CN. TEM was used to investigate the internal structure of C@MoS2@CN. MoS2@CN sheets tightly grow on the hollow carbon spheres with thickness of 12 nm (Figure 4a). The structure buffers the volume expansion of electrode material during charge/discharge process and shortens the distance of ion diffusion. More detailed information can be got from HRTEM images (Figure 4b). The thickness of CN sheets is ~10 nm and a large interlayer spacing of 0.913 nm is observed in Figure 4b. In addition, there are many defects exist in MoS2 interlayers which were formed during calcination. As temperature increasing, PDA among the MoS2 interlayers decomposed meanwhile a part of oxygen 10 ACS Paragon Plus Environment

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functional groups reacted with MoS2, resulting in many defects. The defects benefit the ion diffusion and forming of exciton under UV-light. Uniform Mo, C, S, and N elemental distributions throughout the hollow carbon layer are observed from the energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 4c).

Figure 3. (a) The SEM image of C@MoS2. (b) SEM image of C@MoS2@CN.

Figure 4. (a),(d) TEM images of C@MoS2@CN. (b) HRTEM image of C@MoS2@CN and profile of calibration (inset of b). (c) TEM image and the corresponding EDS elemental mapping images of C@MoS2@CN.



The pore structure of the C@MoS2 and C@MoS2@CN nanospheres were further explored by N2 adsorption/desorption test. As shown in Figure 5a, a typical IV behavior was observed with a significant hysteresis characteristic of the isotherms at a high relative pressure (0.5–1.0),17 indicating the C@MoS2 and C@MoS2@CN are both mesoporous materials. The C@MoS2@CN nanostructure possesses a specific surface area of 68.18 m2 g−1 and a specific pore volume of 15.6616 cm3 g-1, both of which are much larger than those of C@MoS2, a specific area of 42.17 m2 g-1 and a pore volume of 9.6876 cm3 g-1, calculated through the Brunauer-Emmett-Teller (BET) method. C@MoS2@CN has a narrow pore size distribution in the range of 2~3 nm while pore size of C@MoS2 is mainly distributed in the scope of 2.5~3 nm (Figure 5b). The higher large specific surface area and pore volume C@MoS2@CN are mainly contributed by the doping of CN sheets.

Figure 5. (a) N2 adsorption-desorption isotherm and (b) pore size distribution and magnification (inset) of C@MoS2@CN and C@MoS2.

3.2. Electrochemical performance The examination of electrochemical performance was explored via coin-type SIB cells (2430). The MoS2 based composite, carbon black, and polyvinylidene fluoride 12 ACS Paragon Plus Environment

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(PVDF) binder were mixed with a weight ratio of 8 : 1 : 1 (each electrode contains 1~1.5 mg cm-2 active material). The electrolyte (1 M NaClO4 in EC-DEC (1:1 v/v) + 5 wt% FEC) was used. As for the UV irradiation treatment, the UV-light with a wavelength of 365 nm was irradiated on C@MoS2@CN electrodes soaking in the electrolyte for 1 hour before the assembly of sodium ion batteries. To ensure the comparability, electrode without UV-light treatment was also soaked in electrolyte for 1 hour. The influence of UV-light illumination to the electrochemical performance of C@MoS2@CN was firstly researched via the cyclic voltammograms (CV). As shown in Figure 6a, there are three obvious reduction peaks (1.3, 0.6 and 0.001 V) and one obvious oxidation peak (2 V) in the first cycle for C@MoS2@CN whether with UVlight treatment or not. The peak at 1.3 V assigns to insertion of Na+ into C@MoS2@CN (MoS2 + xNa+ + xe- → NaxMoS2). The forming of SEI was occurred at 0.6 V. The peak at 0.001 V is derived from the conversion from NaxMoS2 to Na2S (NaxMoS2 + (4-x)Na+ + (4-x)e- → 2Na2S + Mo). The oxidation peak at 2V corresponds to the reaction: Mo + Na2S → Mo + S + 2Na+ + 2e-.37 Notably, the peaks at 1.3 V and 0.6 V of C@MoS2@CN with UV-light treatment are much smaller than those of C@MoS2@CN without UVlight treatment, confirming the significant contribution of UV light illumination to the enhance of initial Coulombic efficiency by promoting the pre-insertion of Na+ and redox reaction between electrolyte and electrode material before assembling. The two CV curves both are nearly overlapped during the subsequent two cycles, implying a highly reversible sodiation/desodiation process.



Figure 6. CV curves of (a) C@MoS2@CN with UV-light treatment and (b) C@MoS2@CN without UVlight treatment for the first three cycles at a scan rate of 0.1 mV s -1 between 0.001 V and 3 V.

Figure 7. (a) Nyquist plots of the two samples. (b) The initial charge/discharge profiles of the two samples at a current density of 50 mA g-1.

To elucidate the reason for shrunken peak of SEI in the first cycle in more details, Nyquist plots of new batteries assembled by the C@MoS2@CN electrode with UVlight treatment and electrode without UV-light treatment were measured and the results were showed in Figure 7a. Different from the C@MoS2@CN without UV-light treatment, there are two semi-cycles after treatment of UV-light illumination, one of which represents the typical resistance of SEI.42 It is convincing that UV-light treatment improves the electrochemical stability of electrodes by forming SEI on the surface of active materials in advance. Benefiting from the photo-induced electrons, active 14 ACS Paragon Plus Environment

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substance in the electrolyte was decomposed forming a stable SEI and such preformed SEI film can produce a repeated cyclic stability for sodium-ion batteries. Figure 7b shows the initial charge/discharge profiles of the two samples at 50 mA g-1 between 0.001 V and 3 V. The discharge specific capacity decreased apparently after treatment with UV-light, while no obvious change happened to charge specific capacity, resulting in Coulombic efficiency increasing from 48.2% to 79.6%. The low Coulombic efficiency was mainly caused by large specific surface area of C@MoS2@CN.

Figure 8. (a) Rate performance of commercial MoS2 and MoS2 based materials at current densities from 50 to 200 mA g-1. (b) Rate performance of C@MoS2@CN with UV-light treatment at current densities from 0.5~20 A g-1. (c) Cycle performance of C@MoS2@CN with UV-light treatment at a current density of 500 mA g-1.

The wide-interlayer spacing hollow C@MoS2@CN with rich active sites has advantages including alleviating volume expansion during charge/discharge process,



shortening the ion diffusion pathways and enhancing the conductivity of electrodes. Further electrochemical characterization of C@MoS2@CN with UV-light treatment was carried out. Figure 8a shows rate performance at current densities from 50 to 200 mA g-1 and cycle performance at 100 mA g-1 of commercial MoS2, C@MoS2 and C@MoS2@CN with UV-light treatment. The reversible capacities of C@MoS2@CN with UV-light treatment are 520, 477, 455, and 435 mAh g-1 at current densities of 50, 100, 150, and 200 mA g-1, which are much higher than those values of C@MoS2 and commercial MoS2. Besides, the reversible capacity of C@MoS2@CN with UV-light treatment is 454.3 mAh g-1 with 95% retention after 100 cycles. In contrast, C@MoS2 and commercial MoS2 showed an obvious degeneration as low as 150 mAh g-1, 50 mAh g-1 and did not reverse to their initial capacity. The rate performance of C@MoS2@CN with UV-light treatment at high current densities is shown in Figure 8b. It delivers high reversible capacities of 380, 340, 290, 275, 241, 210, 180, 150, and 126 mAh g-1 at current densities of 0.5, 1, 2, 3, 5, 8, 10, 15, and 20 A g-1. And C@MoS2@CN electrodes reveal excellent cyclic stability with a retention of 87.6% at 500 mA g-1 after 500 cycles (Figure 8c). These great electrochemical performances of C@MoS2@CN with UVlight treatment can be attributed to the rational nanostructure.

Figure 9. Nyquist plots and equivalent circuit model (inset).


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Figure 9 exhibits the Nyquist plots after 100 cycles at 100 mA g-1 for the three kinds of electrodes. All of the plots consist of a depressed semi-cycle at high-frequency region and a diffusion drift at low-frequency region. From the equivalent circuit model, three parameters, including RSEI (resistance of SEI), RCT (charge-transfer resistance at the interfaces) and ZW (Warburg impedance) can be got. RCT of the C@MoS2@CN electrodes with UV-light treatment (98.21 Ω) is much lower than those of C@MoS2 (188.6 Ω) and commercial MoS2 (3946 Ω), indicating an enhanced electron transfer kinetics of the C@MoS2@CN with UV-light treatment. The CN sheets among MoS2 interlayers not only expand interlayer spacing of MoS2, but also improve the conductivity of MoS2 layers. The difference between RCT of C@MoS2 and RCT of commercial MoS2 can also testify the improving effect of inner hollow carbon. To analyze the high rate performance of C@MoS 2@CN with the assistance of UVlight illumination, the CV scans at different scan rates from 0.1 to 2 mV s-1 were carried out, as shown in Figure 10a. The current (mA) and scan rate (mV s-1) obey the following equation:41 i=avb

(1)

where the a and b are adjustable values. The value of b=0.5 indicates a diffusioncontrolled battery reaction process, while b=1 indicating a surface capacitive response. The values of b can be calculated by fitting the log(i) versus log(v). It can be noted from Figure 10b, the values of b at reduction peaks (0.001V, 0.6V, 1.3V) and oxidation peak (2V) are 0.74246, 0.8898, 1.01964, 0.9538, respectively, which are close to 1,



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indicating high surface capacitive contribution. To investigate the surface capacitive contribution specifically, Equation (2) was used: i=k1v+k2v1/2

(2)

where k1 and k2 are appropriate values at fixed potential. In Eq. (2), k1v and k2v1/2 represent surface capacitive and diffusion contribution, respectively. The values of k1 can be obtained by rearranging Eq. (2) to i/v1/2=k1v1/2+k2

(3)

As shown in Figure 11, capacitive contribution of C@MoS2@CN is directly proportional to the scan rate, and the value changes from the initial 63.47% at a scan rate of 0.1 mV s-1 to 88.92% (2 mV s-1) (Figure 11 a to f), which is mainly resulted from the high interlayer spacing and hollow structure of the C@MoS2@CN. The lower ion diffusion barrier and shortened ion diffusion pathways are beneficial to the excellent rate performance.47,48

Figure 10. (a) CV curves at different scan rates. (b) Log(i) versus log(v) at different voltages.


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Figure 11. CV profiles of capacitive contribution at scan rates from 0.1 to 2 mV/s (a)~(e). (f) The specific capacities generated from battery contribution and capacitive contribution at different scan rates for C@MoS2@CN with UV-light treatment.

Considering the great contribution of pseudocapacitive capacity, it is expected that such

C@MoS2@CN

composite

with

UV-light

treatment

shows

excellent

electrochemical performance when evaluated as SIHCs. Based on such concern, the coin-type asymmetric supercapacitors were fabricated with C@MoS2@CN as anodes, commercial active carbon (AC) as cathodes. And 1 M NaClO4 in EC-DEC (1:1 v/v) with 5 wt% FEC was used as the electrolyte. The current were calculated based on the total weight of both electrodes active materials (≈ 4 mg). The electrochemical test was carried out within the voltage window of 1.0-4.3 V. Figure 12a presents galvanostatic charge/discharge curves at different current densities. A high Coulombic efficiency was achieved reading from the symmetric charge/discharge curves at different current densities. The specific capacitance was about 44.8 F g−1 (be equivalent to energy density of 244.5 Wh kg−1) (Table S2). The energy density and power density (Figure 12) of



such SIHCs surpass those of current SIBs and are comparable to the most supercapacitors.43

Figure 12. (a) Galvanostatic charge/discharge curves of C@MoS2@CN//AC at current densities from 25 mA g-1 to 1 A g-1. (b) Corresponding Ragone plots of SIHCs (based on the total weight of active materials) and most energy storage devices.

4. Conclusions In summary, this strategy displays creative coupling of the photochemistry and energy storage. The UV-light treatment within electrolyte can stabilize the surface of active material efficiently by promoting the decomposition of electrolyte to form SEI in advance and pre-insertion of Na+, causing a high Coulombic efficiency in the first discharge/charge cycle. The C@MoS2@CN nanospheres with expanded interlayer spacing (larger than 0.9 nm) were carefully designed to increase the sodium storage ability. The protective double carbon layers are conducive to decreasing the electron transfer and ion diffusion distance, improving conductivity of electrode meanwhile increasing active sites. When evaluated as anode materials, the C@MoS2@CN with UV-light treatment delivered a high initial Coulombic efficiency of 79.6% and a reversible specific capacity of 520 mA h g-1 at 0.05 A g-1. Even at 20 A g-1, a specific 20 ACS Paragon Plus Environment

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capacity of 126 mAh g-1 could be achieved. In addition, the electrode displayed superior cyclic stability with an outstanding capacity retention rate of 87.7% after 500 cycles at 0.5 A g-1. When applied to SIHCs, high power density and energy density can be achieved. The unification of the two developed processes with a consistent mechanism also supplies a new thought for developing the sustainable energy storage technology. Associated content Supporting Information XRD pattern of SiO2@RF@MoS2 synthesized by hydrothermal method; TG curves of C@MoS2 and C@MoS2@CN; Ultraviolet absorption spectrum of C@MoS2@CN in NMP; SEM of C@MoS2@CN with UV-light treatment after 500 cycles at 0.5 A g-1; FESEM-mapping images of C@MoS2@CN; Table of peaks in Raman spectrum for commercial MoS2 and MoS2 based composites; Table of correlated data of power density and energy density for SIHCs.



Acknowledgements The authors acknowledge the financial support of the doctor foundation of Shandong province (No.ZR2018BB033) and the National Natural Science Foundation of China (No. 21805152).

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UV Irradiation Treatment for Enhanced Sodium Storage Performance

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