O-doped g-C3N4 Nanospheres for

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In-Situ Growing Mesoporous CuO/O-doped g-C3N4 Nanospheres for Highly Enhanced Lithium Storage Hemdan S. H. Mohamed, Liang Wu, Chao-Fan Li, Zhi-Yi Hu, Jing Liu, Zhao Deng, Lihua Chen, Yu Li, and Bao-Lian Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10171 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

In-Situ Growing Mesoporous CuO/O-doped g-C3N4 Nanospheres for Highly Enhanced Lithium Storage Hemdan S. H. Mohamed,†,‡ Liang Wu,† Chao-Fan Li,†,§ Zhi-Yi Hu,†,§ Jing Liu,† Zhao Deng,† LiHua Chen,† Yu Li,*,†,§ and Bao-Lian Su†,& †State

Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China. ‡

Physics Department, Faculty of Science, Fayoum University, El Gomhoria Street, 63514

Fayoum, Egypt. §Nanostructure

Research Centre (NRC), Wuhan University of Technology, 122 Luoshi Road,

430070 Wuhan, Hubei, China. &Laboratory

of Inorganic Materials Chemistry (CMI), University of Namur, 61 rue de Bruxelles,

B-5000 Namur, Belgium.

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ABSTRACT: Development of lithium-ion batteries (LIBs) using transition metal oxides (TMOs) becomes more attractive these days, due to their higher specific capacities, better rate capability and high energy densities. Herein, the in-situ growth of an advanced mesoporous CuO/O-doped g-C3N4 nanospheres is carried out in two steps hydrothermal at 180 C and annealing in air at 300 C. When used as anode material, the CuO/O-doped g-C3N4 nanospheres achieve a high reversible discharge specific capacity of 738 mAhg-1 and a capacity retention 75.3% after 100 cycles at current density 100 mAg-1 compared with the pure CuO (412 mAhg-1, 47%) and O-doped g-C3N4 (66 mAhg-1, 53%). Even at high current density 1 Ag-1, they exhibit a reversible discharge specific capacity of 503 mAhg-1 and capacity retention

80% over 500 cycles. The excellent

electrochemical performance of the CuO/O-doped g-C3N4 nanocomposite is attributed to the following factors: (I) the in-situ growing CuO/O-doped g-C3N4 avoids CuO nanoparticles aggregation, leading to the improved lithium ions transfer and electrolyte penetration inside the CuO/ O-doped g-C3N4 anode, thus promoting the utilization of CuO; (II) the porous structure provides efficient space for Li+ transfer during insertion/extraction process to avoid large volume change; (III) the O-doping g-C3N4 decreases its band gap, ensuring the increased electrical conductivity of CuO/O-doped g-C3N4; (IV) the strong interaction between CuO and O-doped gC3N4 ensures the stable structure during cycling. KEYWORDS: CuO; g-C3N4; in-situ growth; lithium-ion batteries; reaction kinetics

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INTRODUCTION Nowadays, the explosive growth of electronics and rapid advancements in technologies such as electric vehicles, audio players, cell phones, laptops and numerous electronic products have received much attention all over the world. To confront this growth, developments of simple and low cost batteries become more urgent for next generation of these electronic devices. Lithium ion batteries (LIBs) is one of the most attractive choice for these developed batteries, owing to their advantageous like cost-effective, high energy density, durability and good rate capability 1-4. Currently, graphite used for commercial LIBs as an anode material owing to various features such as chemical stability, low cost, and abundant resource5. Due to its low theoretical specific capacity (372 mAhg−1), graphite can`t achieve the future demands for LIBs6. Therefore, the searching for alternative anode materials with superior specific capacity, low cost and chemical stability becomes urgent for developing LIBs 7-8. TMOs could be the most probable candidates for achieving the future demands of LIBs because they own superior specific capacities (600~1300 mAhg−1) and high rate capability compared to other commercial anode materials

9-12.

In the last

few years, CuO is investigated as one of most favorable TMOs anode materials due to its high theoretical specific capacity (~670 mAhg−1), facile synthesis, compatible for environmental requirements, low cost, and high safety 13-15. Nevertheless, its low electrical conductivity leads to anode pulverization, sluggish kinetics and poor cyclability 16. In addition, the large volume change of CuO (~174%) tends to make it expholiated from the collector during the cycling process 17-18. To achieve the high electrical conductivity, the long term cycling stability and the satisfactory capacity retention for CuO electrode material, different effective strategies



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conducted for addressing these issues. The most widely employed strategy is to integrate carbon materials such as carbon nanoflakes 19, carbon microspheres 20, graphene 21, carbon nanotubes 2, 22,

mesoporous carbon

23

to enhance the electrical conductivity, limit the aggregation and

pulverization of CuO elctrode materials. However, it is still necessary to further address these issues and achieve better electrochemical properties of CuO-based anode materials. Another important stratgy to adress the defects of TMOs is to use graphatic carbon nitride (g-C3N4) 24-25. The stability and cycle performance of TMOs could be improved using g-C3N4 due to large surface area, cost-effective availability 26 and tunable porous structure27, layer structure 28, suitability of physical-chemical proparties 29. In particular, the porous g-C3N4 structure can offer void space to buffer the volume changes, resulting in shortened diffusion distance for Li+ ions and improved charges transportation, especially when the pores are highly interconnected

30.

For

example, Shi et al. 31 formed sandwich-type nanosheets of iron oxide nanoparticles and porous gC3N4/graphene, achieving an improvement of reversible capacity (1023 mAhg-1) and high columbic efficiency (97.6%). Hou et al.

32

designed porous MoS2/g-C3N4 composite, the results

confirm the enhancement of specific capacity after 800 mAhg-1. Although, there are a notable progress for improving cycle performace of TMOs using modified g-C3N4, these results are still far from the expected good performance. We propose that, this comes from the weak interaction between TMOs and g-C3N4. For that, using oxygen-doped g-C3N4 to modified TMOs could be a feasible way to alleviate their disadvantages. On the one hand, the intrinsic band structure of gC3N4 could be decreased via oxygen doping leads to improve its conductivity33. On the other hand, the interaction between g-C3N4 and TMOs could be strengthened due to the existence of carbon– oxygen-carbon bond and nitrogen-carbon-oxygen bonds. Also, the O-doped g-C3N4 should be


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helpful to avoid the TMOs aggregation. Therefore, combing CuO nanostructures with O-doped gC3N4 is quite promising to largely improve the electrochemical properties of CuO for LIBs. Herein, we report the construction of anode electrode using mesoporous CuO/O-doped gC3N4 nanospheres for advanced rechargeable LIBs via hydrothermal method. Suggesting that, the modification of CuO nanostructures using O-doped g-C3N4 for LIBs is considered as the first important report in this field. The obtained results of designed CuO/O-doped g-C3N4 nanocomposite reveal a high specific capacity (738 mAhg-1), high capacity retenation (75%) after 100 cycles at current density 100 mAg-1. Even after 500 cycles, the achieved specific capcity is 503 mAhg-1 at 1 Ag-1. Thus, the in situ growth process can disperse CuO nanoparticles in O-doped g-C3N4 nanospheres to prevent its agllomeration, promoting the utilization of the active CuO nanoparticles. In addition, the porous structure of O-doped g-C3N4 nanospheres can allow the diffusion of lithium ions, facilitate the electrolyte penetration and alleviate the volume change of CuO leads to achieve a superior performance of LIBs. EXPERIMENTAL SECTION Materials Dicyandiamide (DCNA), (Cu(NO3)2·3H2O), acetonitrile, N,N-Dimethylmethanamide (DMF), anhydrous ethanol, polyvinylidene fluoride (PVDF), acetylene black, lithium hexafluorophosphate (LiPF6) with volume ratio of 1:1. DCN, DMF, LiPF6, Cu(NO3)2·3H2O bought from Shanghai Aladdin. PVDF, acetylene black and anhydrous ethanol were obtained at Sinopharm Chemical. All obtained chemicals were employed without purification. Synthesis of O-doped g-C3N4 nanospheres Nanospheres of O-doped g-C3N4 were carried out in two steps. First, 50 mL mixed solution of accetonitrile and DMF (40: 10) was used to dissolve1.5 g of DCNA, stirring for 2 h at room



temperature. Then, sealed 100 mL Teflon-lined autoclave was used to heat the obtained mixture at 180 °C for 12 h. After that, the obtained samples were washed three times with water and three times with ethanol followed by drying for 24 h at 60 C. Second, the obtained solid powder was annealed at 300 C in air for 2 h at heating rate 2 C min-1 then cooled down to 25 C. Finally, the given yellow product was O-doped g-C3N4 nanospheres. Synthesis of pure CuO nanoparticles Pure CuO was also synthesized via solvothermal method. Typically, 2.155 g of Cu (NO3) was dissolved in 50 mL mixed solution of accetonitrile and DMF (40: 10), and stirring for 2 h at room temperature. The following steps for preparing CuO are the same of O-doped g-C3N4 nanospheres. The resultant black powder was donated as pure CuO nanoparticles. Synthesis of CuO/O-doped g-C3N4 nanospheres Nanospheres of CuO/O-doped g-C3N4 were carried out in two steps. First, 50 mL mixed solution of accetonitrile and DMF (40: 10) was used to dissolve 2.155 g of Cu(NO3), the mixture was vigorously stirred for 1h. Then, 1.5 g of dicyandiamide was mixed with the above product, and then stirred for another 1h. The following steps are the same of O-doped g-C3N4 nanospheres. The final resultant was donated as CuO/O-doped g-C3N4 nanospheres. Characterizations Advanced XRD machine Bruker-D8 with Cu-Kα radiation was adjusted at 40 kV, 40 mA and wavelength=1.541 Å) to investigate the crystallographic phases of all samples. The morphologies and elemental composition of these samples were performed on (FESEM, Hitachi S-4800) at 5 kV and equipped TEM machine with energy dispersive X-ray spectroscopy (EDS) (TEM, Thermo Fisher Scientific Talos F200S) Super-X system at 200 kV. Then, the samples were prepared before


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and after cycling for testing X-ray photoelectron spectroscopy (XPS) spectra on a customized Xray photoelectron spectrometer (VG Multilab 2000-X equipped with a monochromatic Al Kα source). Surface area and porosity for the prepared samples were also carried out using (Micromeritics Tristar II 3020) Nitrogen adsorption-desorption isotherms (NADI). Also, Pore size distribution and average pore diameters of the samples were carried out using (BET) and (BJH) method, respectively. To investigate the weight content of each compound of CuO/O-doped g-C3N4, thermogravimetric analysis (TGA)-model-Evo S60/58458) was used at 800 °C and rate 5 °C/min in air. Electrochemical measurements An argon-filled glove box was optimized for preparing coin cells (type CR2025). The batteries were prepared as following. Different ratio 10 wt%, 20 wt% and 70 wt% of PVDF as binder, acetylene black and active material, respectively were dissolved in n-methyl pyrrolidinone (NMP) followed by milling for 30 min. The obtained paste was covered the cleaned copper foil and then left for drying in the air for 20 minutes. After that, the films were dried in drying oven for 6 h at 60 °C. Finally, to remove the solvents from the prepared films, drying vacuum oven was used for 12 h at 120°C. To assemble full coin cell batteries, each film was cut into a small coin cell as anode material; weight of loaded active materials on the anode was about1.44 mg cm-2. A commercial lithium metal used as counter anode, the used electrolyte composed from ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume) was 1 mol L−1 of LiPF6. A separator Celgard 2400 membrane fabricated from porous polypropylene. The electrochemical measurements such as Cyclic Voltammetry (CV), electrochemical performance, and electrochemical impedance spectroscopy (EIS) were investigated. These measurements have been done on the following 7 ACS Paragon Plus Environment


instruments: electrochemical workstation CHI 660D, galvanostatic charge-discharge battery test system (CT2001A, LAND) and electrochemical workstation (PGSTAT 302N, Metrohm Autolab) in frequency ranging from 100 KHz to 0.01 Hz, respectively. The used voltage ranged between 0.01 and 3.0 V (vs. Li/Li+) and various current densities (100m Ag-1-1 Ag-1). RESULTS AND DISCUSSION In-situ growth of CuO/O-doped g-C3N4 nanospheres is carried out in two steps as illustrated in Fig. 1a. First, the copolymerization process of porous CuO/O-doped g-C3N4 precursor is achieved using Cu(NO3) and dicyandiamide mixed together in acetonitrile/DMF solution through hydrothermal method at 180 C, in consistent with our previous work33. This allows the formation of O-doped g-C3N4, which is helpful for CuO nanoparticles homogeneously dispersing inside gC3N4 matrix

34.

Fig S1a-c display the SEM images at different magnifications for the prepared

porous CuO/O-doped g-C3N4 precursor, clearly show spherical porous structure. Moreover, XRD patterns were carried out to investigate the prepared precursor as shown (Fig. S1d). The pattern indicates the formation of two peaks for O-doped g-C3N4 at 10.8 and 27.4, consistent with patterns obtained for the O-doped g-C3N4 (Fig. 1b). Pattern at 10.8 is the (001) crystal plane refer to formation of intra-planar tri-s-triazine. Other one at 27.4 belongs to (002) crystal plane reflecting to the aromatic structure 35, 36. Note that, the diffraction peak of (001) shifts toward the lower diffraction angle 10.8 compared with the prepared g-C3N4 by thermal method 35. Suggesting that, this shift is due to the replacement of some O atoms with N atoms, reveals the enlarged of tri-s-traiazine packing

35, 33, 37.

The other small peaks for the CuO/O-doped g-C3N4 precursor

should correspond to the formed Cu complex.


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Fig.1 (a) the schematic description for the in-situ growth CuO/O-doped g-C3N4 nanospheres, (b) XRD patterns for three products and (c-d) SEM images for in-situ growth CuO/O-doped g-C3N4 nanospheres at different magnification.

Second, the mesoporous CuO/O-doped g-C3N4 nanospheres are obtained using heat treatment for the above precursor at 300 C. Fig. 1b presents XRD patterns for three products, respectively. For the pure CuO nanoparticles, the results confirm formation of a monoclinic CuO consistent with pervious report (JCPDS No. 005-00661) 38. No other peaks are obtained, revealing the high crystallinity of prepared sample and high oxidation CuO precursor to CuO at 300 C. Fig. S2 displays the SEM images of pure CuO nanoparticles, clearly show aggregation of small CuO nanoparticles together to form big spheres. For CuO/O-doped g-C3N4, the diffraction peaks can be perfectly indexed as CuO with additional two peaks of O-doped g-C3N4 at 10.8 and 27.4. A small series peaks correspond to the crystalline dicyandiamide C2H4N4 phase due to the annealing at 300C 39. 9 ACS Paragon Plus Environment


Fig. 1c presents the SEM image of the CuO/O-doped g-C3N4 nanospheres at low magnification, clearly demonstrate formation spherical structure with diameters varied from 200 nm to 500 nm. By increasing the magnification of SEM observation, one can obviously see many small CuO nanopartiles highly dispersed in the surface of O-doped g-C3N4 (Fig 1d). This indicates that these small CuO nanoparticles are in situ formed after annealing at 300 C, leading to highly disperse CuO nanoparticles inside the formed structure. Fig. S3 displays the obtained images of O-doped g-C3N4 nanospheres at different magnifications; they show formation of big spheres with diameter varied from 5 µm to 7µm. This SEM observation obviously shows the porous structure of O-doped g-C3N4 nanospheres, consistent with our previous result 33. Fig. 2a presents the typical HAADF-STEM image of the as-prepared CuO/O-doped g-C3N4 nanospheres, showing the homogeneously dispersion of the CuO nanoparticles in O-g-C3N4 nanospheres, consistent with the SEM results. Fig. 2b presents the selected area of HRTEM image for CuO/O-doped g-C3N4 composite, clearly illustrating in situ growing of the CuO nanoparticles on O-doped g-C3N4. Fig. 2c presents selected area electron diffraction (SAED) for O-doped gC3N4 (0002) diffraction ring and CuO (110), (111), (111) diffraction rings respectively. Fig. 2dg show the corresponding elemental mapping analysis of CuO/g-C3N4 nanocomposite, clearly indicate the constructing elements Cu, C, N, displayed a well defined compositional profile of CuO/O-doped g-C3N4 nanospheres. In addition, the mapping confirms the distributions of all elements in the surface are homogeneous.


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Fig. 2. (a) HAADF-STEM image of CuO/O-doped g-C3N4, (b) HRTEM and corresponding FFT pattern (inset) for selected red box, (c) SAED pattern of selected green box and (d-g) corresponding elemental mapping analysis of CuO/O-doped g-C3N4.

N2 adsorption–desorption isotherms of the three products are presented in (Fig. 3a). The results show that the specific surface area for CuO nanoparticles is 18.4 m²g-1, O-doped-g-C3N4 is 17.6 m²g-1 and its value about 18.9 m²g-1 for the composite sample. Fig. 3b presents corresponding pore size distribution for the three samples, corresponding to Barrett–Joyner–Halenda distribution plots. The pore diameter centres at 11 nm for pure CuO due to the small nanoparticles aggregation.



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For O-doped g-C3N4, the pore size distribution ranges from 13 to 28 nm, and the pores becomes narrow and certres at 15 nm for CuO/O-doped g-C3N4 nanospheres.

Fig. 3. (a) N2 adsorption-desorption isotherms for the three products and (b) corresponding pore size distributions curves.

The surface and chemical configuration of the three products were investigated via XPS pattern. Fig. 4a shows high resolution Cu 2p spectra of CuO and CuO/O-doped g-C3N4 nanospheres. For pure CuO nanoparticles, the pattern clearly demonstrates the observation of two main components Cu 2p1/2 and Cu 2p3/2 at 933.6 and 953.4 eV. While, For CuO/O-doped g-C3N4, they observed at 933.6 and 952.8 eV, respectively. Spectra of Cu 2p3/2 can be decomposed into two peaks at 953.4 and 955.0 eV. Cu 2p strong satellites peaks can be observed at 941.7, 944.1 and 962.1 eV reveal the existence of Cu2+ in the obtained structures 40. Fig. 4b shows the O 1s spectra of the three samples. For O-doped g-C3N4, O 1s spectrum shows one single peak deconvoluted into three peaks at binding energies 531.7, 532.5 and 534.1 eV, confirm the existence of C-O-C, N-C-O bonds and adsorbed O2

33, 41.

Accordingly, the

electronic states of O-doped g-C3N4 are changed to those obtained thermally33. While, O 1s spectrum of pure CuO validates two peaks at 529.9 eV and 531.1 eV assigned to Cu-O bond and O-H group absorbed on the surface of CuO42. For CuO/O-doped g-C3N4, the O 1s spectrum can


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be divided to four peaks. One peak corresponds to Cu-O bond as obtained for CuO. The other three peaks at higher binding energies 531.7, 531.9 and 532.5 eV belong to N-C-O bond, adsorbed O-H group and C-O-C bond, respectively. Most possibly, the peaks at 531.7 and 532.5 eV are originated from the in-situ growing CuO/O-doped g-C3N4 nanospheres. The small shift of the OH peak indicates the strong interaction between CuO and g-C3N4.

Fig. 4. XPS spectra of three prepared samples (a) Cu 2p (b) O 1s (c) C 1s, and (d) N1s.

Fig. 4c describes the C 1s spectra; the core level C 1s spectrum of O-doped g-C3N4 has two main peaks, divided into four bonds. One peak observed at 284.6 eV corresponds to sp2 C-C bond originated from the adventitious carbon in species of sample. Second peak, observed at 286.6 eV ascribes to C-O-C bond confirming the O-doping g-C3N4

43.


Third peak, obtained at 288.2 eV


belong to sp3 (N=C-N) bond corresponds to carbon bond of the s-triazine rings and carboncontaining contaminations 37. Final peak observed at 289.2 eV ascribes to C-O bond, suggesting that, this peak formed due to copolymerization process33. Also, the core level spectrum of C 1s for CuO/O-doped g-C3N4 can validate the same four peaks44. It is note that , the peaks corresponding to C-C, C-O-C are observed at higher intensities than those observed for O-doped g-C3N4, confirming strong interaction between CuO and O-doped g-C3N4 45. Fig. 4d displays N 1s spectra; the core level N 1s spectrum of the O-doped g-C3N4 has one peak divided into three dominant peaks. First peak observed at 398.7 eV corresponds to C-N=C bond in triazine rings of g-C3N433. The other two peaks observed at higher binding energies400.4 and 401.2 eV ascribed to C-N-H and N-(C)3 bonds33. Also, the core level spectrum of N 1s for CuO/O-doped g-C3N4 can validate the same three peaks. There is a little shift in the peak corresponds to C-N=C bond, this shift confirm the strong interaction between CuO and O-doped g-C3N4. TGA spectra for the three products were investigated at temperature up to 800 °C in air at heating rate 5 °C min-1 as shown in Fig.S4. For pure CuO, it has observed, the initial weight loss about 4.7% at 200°C could be owing to the chemisorbed water 46. In the temperature range 282398 °C, the second weight loss is about 22%, which might be due to consequent evaporation process of CuO47. For O-doped g-C3N4, the observed weight loss is very high about 70% in the temperature rang 368-684 °C. This means most of O-doped g-C3N4 might be decomposed at 684 °C. For the CuO/O-doped g-C3N4 nanocomposite, it is similar to pure CuO sample with two weight loss. The contrary is that, the second weight loss of the CuO/O-doped g-C3N4 sample is (39%) which is higher than that obtained for CuO sample. These results indicate that the CuO/O-doped


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g-C3N4 nanocomposite contains about 17% of g-C3N4, consistent with those obtained from EDX elemental maps about19% (Fig. S5). To evaluate the lithium storage performance of the three samples, the electrochemical measurements such as Cyclic Voltammetry (CV) and electrochemical performance were investigated at voltage ranged between 0.01 and 3.0 V (vs. Li/Li+). The overall reversible electrochemical process is confirmed for all samples as follows 48: 𝐶𝑢𝑂 + 2𝐿𝑖 + +2𝑒 ― ↔𝐶𝑢0 + 𝐿𝑖2𝑂

(1).

Fig. 5a shows the CV curves for CuO/O-doped g-C3N4 nanocomposite. During the first lithiation process, the abroad and weak reduction peak observed at ~2.0 V ascribed to the initial construction of LixCuO 49. The other two peaks observed at 1.14 and 0.56 V belong to the phase transformation from LixCuO to Cu2O and to Cu0 respectively. Peak observed at voltage below 0.3V correspond to solid electrolyte interphase (SEI). While, the observed peaks at 2.4 and 2,73 V during the first de-lithiation process refer to the obverse conversion reaction of Cu0 to Cu2O then to CuO50. The observed peak at 1.27 V belongs to SEI decomposition. The following cycles for the CuO/O-doped g-C3N4 nanocomposite have the same behavior as in the first cycle confirm the improving of lithium storage performance. It clearly noted that, the CV curves area of CuO/Odoped g-C3N4 is much larger than that of the pure CuO (Fig. 5b), suggesting that the capacity of the as-synthesized composite has been improved 51. Fig. S6a displays the CV curves of O-doped g-C3N4. Two main peaks are observed during the first lithiation process at 1.33 and 0.76 V respectively, while one broad peak is obtained during the de-lithiation anodic process at 0.7-1.0 V, suggesting that, these peaks of cathodic sweep play important role in improving the discharge capacitance of the CuO/O-doped g-C3N4 anode. These CV results confirm the highly improved reaction kinetics of CuO/O-doped g-C3N4 after the in situ growth of CuO in O-doped g-C3N4.



Fig. 5. (a) CV curves for CuO/O-doped g-C3N4, (b) CV curves for pure CuO, (c) charge-discharge voltage profiles of CuO/O-doped g-C3N4 nanocomposite (d) the cycling performance for the three samples.

The discharge/charge profiles of the three samples were investigated at 0.01-3.0 V to show the advantage of the CuO/O-doped g-C3N4 nanocomposite for lithium storage. Fig. 5c describes the profiles of CuO/O-doped g-C3N4 with voltage versus Li/Li+ for different chosen cycles. These curves display constant slope with multiple small voltage plateaus, signifying the phase transition of the CuO/O-doped g-C3N4 nanocomposite during the conversion reaction. For the first cycle, the first discharge specific capacity is 980 mAhg-1and the charge specific capacity is 928 mAhg-1, achieve initial columbic efficiency 94.7%.and irreversible capacity loss up to 5.3%. This initial irreversible loss is likely due to the irreversible processes as the creation of SEI layer inside CuO/O-doped g-C3N4 nanocomposite anode, interfacial lithium storage and incomplete 16 ACS Paragon Plus Environment

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decomposition of Li2O phase consistent with most of TMOs,23, 52-55. Also, the three discharge voltage plateaus can be occurred approximately at 2.36, 1.16-1.36 and 0.8 V; in addition, the three charge voltages can be observed at 1.28, 2.4 and 2.73 V respectively. For the second cycle, specific capacities are abruptly decreased to 931 and 913 mAhg-1 for discharge and charge profile. Although the specific capacities are decreased, the columbic efficiency is increased up to 98.1% and the shape of slope is constant with multiple voltage plateaus. The discharge voltage plateaus of this cycle occur at approximately 2.36, 1.26 and 0.81 V and charge voltage plateaus are observed as those obtained in the first cycle, revealing the obtained results for CV curves. For the next cycles at 20th, 50th and 100th, shapes of the curves not changed and the columbic efficiency values are increased to 98.2 %, 98.4% and 98.8%, respectively. Fig. 5d shows the cycle performance of the three prepared anodes at current density 100 mAg1

over 100 cycles. It shows that, the profiles of pure CuO anode achieve the first discharge specific

capacity 869 mAhg-1 and charge specific capacities 564 mAhg-1, displaying a low initial coulombic efficiency 64.9 %. For O-doped g-C3N4 anode, they achieve 123 and 79 mAhg-1, corresponding to the initial coulombic efficiency 64.6 %. Thus, the initial obtained results for both prepared anodes are lower than those obtained for CuO/O-doped g-C3N4 anode. After 100 cycles, the obtained discharge reversible specific capacities are 738, 412 and 66 mAhg-1 achieve capacity retention 75.3 %, 47.4% and 53.7%, for the three prepared anodes, respectively. Table 1 compares the performance of our CuO/O-doped g-C3N4 anode and other works, clearly showing the superior electrochemical performance.



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Table 1. Comparison of electrochemical performance of CuO/O-doped g-C3N4 anode with pervious work Materials CuO/MWCNTs CuO/C microspheres CuO NWs@Cu foam CuO nanowires/CNTs MOF-CuO/graphene CuO hollow octahedral CuO/grapheme CuO/O-doped gC3N4

Current density (mAg-1) 100 100 100 100 100 100 65 100

Cycle numbers 50 50 100 50 50 100 100 100

Specific capacity (mAhg-1) 540 470 462 460 405 470 600 738

Refs 2 20 42 56 57 58 59 This work

XPS was further conducted on the reacted CuO/O-doped g-C3N4 anode after lithiation to reveal the capacity decay. Fig. S6b presents the survey spectra of the CuO/O-doped g-C3N4 anode before and after lithiation. The results show that spectra of all elements Cu, C, N, O before and after lithiation are clearly constructed and well-defined compositional profile. Fig. 6a displays high resolution Cu 2p spectrum of the CuO/O-doped g-C3N4 anode after lithiation. The obtained pattern clearly demonstrates the observation of two main components Cu 2p1/2 and Cu 2p3/2. Cu 2p1/2 spectrum can be decomposed into three peaks at 932.4, 933.6 and 934.7 eV ascribed to Cu0, Cu2O and CuO, respectively18, 60. While, Cu 2p3/2 spectrum can be deconvoluted into two peaks at 952.6 and 954.1 eV corresponds to Cu0 and CuO, respectively. These results confirm that after lithiation, Cu and Cu2O are existed. According to our previous work, the formed Cu and Cu2O could not be fully converted to CuO again in the reverse reaction 52. These results could interpret of capacity decay at the initial cycles. On the other side, formation of irreversible Cu nanoparticles can avoid the electrical conductivity decrease during the reaction between CuO/O-doped g-C3N4 anode and Li+, leading to improve its cycle stability. To confirm these results, XRD was carried out on the reacted CuO/O-doped g-C3N4 anode after lithiation (Fig S7). The diffraction peaks can be perfectly reveal the existence of two phases Cu0 cubic structure with (111), (200) and (220) crystal planes and CuO monoclinic with (110), (-111), and (220) crystal planes17, 18 ACS Paragon Plus Environment

38.

Suggesting that, high

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crystallinity peaks of Cu0 reveal reversible electrochemical process and the obverse conversion reaction. Fig 6b shows the O 1s spectrum after lithiation, which is obviously different from O 1s before lithiation (Fig 4b). There is only one peak decomposed into three species situated at 530, 531 and 532.15 eV, ascribed to Cu-O, N-C-O and C-O-C bonds. OH group disappears after lithiation, most probably due to the existence of Li2O after lithiation process.

Fig.6. XPS spectrum for CuO/O-doped g-C3N4 after lithiation: (a) Cu 2p, (b) O1s, (c) C1s and (d) N1s.

Fig 6c presents high resolution C 1s spectrum after lithiation. The results confirm the formation of four bonds obtained before lithiation (Fig 4c). These bonds shift a little to lower binding energies and the intensities are also incompatible with those before lithiation. The C-O-C 19 ACS Paragon Plus Environment


bond becomes strong and C=N bond appears weak after lithiation. Sugesting that, the intensity CO bond decreases due to the formation of more Li2O layers. Fig 6d presents the core level N1s spectrum after lithiation. The peak comes from three N species, similar to those obtained before lithiation (Fig 4d). It is noted that, the structure still maintains C-N-H bond after lithiation, confirming the strong interaction between CuO and Odoped g-C3N4. This ensures the highly improved electrochemical performance of CuO. Fig. 7a presents the rate capability performance of the three prepared anodes at 0.01-3.0 V with different current densities from 100-2000 mAg-1. It clearly show that the rate performance and specific capacity of two anodes are lower than those obtained for CuO/O-doped g-C3N4 anode. At lower current density 100 mAg-1, discharge specific capacity of CuO/O-doped g-C3N4 anode is still higher than 800 mAhg-1 after 10 cycle. For the other current densities (200-2000 mAg-1), discharge specific capacities slowly reduced to 696, 617, 549, 497 and 458 mAhg-1, respectively. Then, the reversible discharge capacity still maintains at 736 mAhg-1 when returned back to lower current density 100 mAg-1. To prove the improving of rate capability performance, the voltage plots of the CuO/O-doped g-C3N4 anode at different current densities were investigated as shown in Fig. 7b. They clealy show the similarity of overpotential for all the current densities. Fig. 7c presents long cycling performance of the three prepared anodes at the current density 1 Ag-1 over 500 cycles. The initial discharge/charge capacities of CuO/O-doped g-C3N4 anode are 628 and 605 mAhg-1, which are respectively maintained at 503 and 490 mAhg-1 after 500 cycles, demonstrating excellent reversibility and outstanding stability. For the pure CuO anode, they have 172 and 170 mAhg-1 at the current density 1 Ag-1 over 500 cycles, achieving coulombic efficiency and capacity retention about 98.8% and 39.4%, respectively. Moreover, After 500 cycles and at the same current density (1 Ag-1), they have 59 and 57 mAhg-1, respectively for the O-doped g-


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C3N4 anode, achieving coulombic efficiency (96.6%) and capacity retention(96.6%) . However, The enhanced rate capability performance and the long cycling performance for the O-doped gC3N4 anode, the specific capacity still very low due the semiconductor nature and high surface area of g-C3N4.

Fig. 7. (a) Rate performances the three prepared anodes, (b) the corresponding voltage plots of CuO/O-doped gC3N4 anode and (c) the cycling performance at 1 Ag-1 for 500 cycles and the corresponding Coulumbic efficiency of CuO/O-doped g-C3N4.

The enhanced rate capability performance and the long cycling performance of CuO/O-doped g-C3N4 anode compared with other two anodes should come from two main factors. First, the porous of O-doped g-C3N4 structure makes the CuO nanoparticles fully contact with electrolyte, prevent its agglomeration, facilitate the Li+ insertion/extraction, and accelerate charges transfer 19,



50.

Second, the oxygen doping can increases the electrical conductivity of g-C3N4, resulting in in

improving the electrical conductivity of the CuO/O-doped g-C3N4 anode33. Fig. 8 shows EIS of fresh samples and after 100 cycles. The results reveal that the charge transfer resistance (CTR) of prepared CuO/O-doped g-C3N4 anode is smaller than those of two other anodes. Also, after 100 cycles, clearly observe no increase in its resistance unlike the resistance CuO anode. Thus, the improving of EIS of CuO/O-doped g-C3N4 anode is attributed to the oxygen doping that could increase the electrical conductivity of CuO/O-doped g-C3N433. Moreovere, the irreversible formation Cu0 nanoparticles in reacted CuO/O-doped g-C3N4 anode could also be helpful for the electrical conductivity, according to XRD analysis after cycling (Fig. S7).

Fig. 8. EIS curves of (a) fresh anodes and (b) anodes after 100 cycles for three products.

The mechansim structure of CuO/O-doped g-C3N4 anode for lithium storage, SEM and TEM analyses after lithiation are conducted (Fig. 9). During the lithiation process, CuO is reduced to Cu2O and further to Cu as a result of reaction between lithium ions and CuO nanoparticles as shown in Fig. 9a. The formed Cu nanograins facilitate the electron transfer through the pathway in O-doped g-C3N4. During the de-lithiation process, part of these Cu nanoparticles can not been


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converted to Cu2O or CuO. This allows the faster electrons transfer from active anode material to counter anode consitent with XRD results after cycling Fig. S7. Fig. 9b displays the SEM image of the cycled CuO/O-doped g-C3N4 anode after lithiation. It clearly shows the spherical and porous stucture, similar to the structure before cycling.

Fig. 9. (a) The schematic description explain reaction mechanism of CuO/O-doped g-C3N4 anode during charge discharge process, (b) SEM image of CuO/O-doped g-C3N4 anode after lithiation, (c) TEM of prepared composite CuO/O-doped g-C3N4 anode after lithiation (d) HAADF-STEM image, (e-h) EDX mapping Cu (green), C (red), N (blue) and O (yellow).

TEM, STEM and STEM-EDS elematal mappings were used to reveal the structure and elemental distribution of CuO/O-doped g-C3N4 after lithiation (Fig. 9c-h). The results show the 23 ACS Paragon Plus Environment


well-kept structral form of the CuO/O-doped g-C3N4 nanocomposite (Fig. 9c and d) and the homogenerous distribution of all construction elements (Fig. 9e-h). It is clearly observed Cu elemental mapping is more dispersive than those observed O. On the one hand, copper grid supported carbon film is used as support to load the TEM specimen. On the other hand, the irreversible Cu nanoparticles can be deposited on the carbon film during the preparation of the TEM specimen. Thus, the prepared porous CuO/O-doped g-C3N4 anode can bring excellent rate capability , superior cycling stability and enhanced capacity due to the following factors: (I) in-situ growing of CuO/O-doped g-C3N4 avoids the CuO nanoparticles aggregation, leading to the improved Li+ transfer and electrolyte penetration into the CuO/g-C3N4 anode, finally beneficial for the utilization of CuO; (II) the porous CuO/O-doped g-C3N4 nanospheres provide efficient space to decrees volume change; (III) the O-doping and the irreversible Cu nanoparticles lead to the reduce CTR of CuO/O-doped g-C3N4 anode; (IV) the O-doping strengthens the interaction between CuO and O-doped g-C3N4, ensuring the stable structure during the cycling processes due to the existence of C-O-C and C-O bonds. CONCLUSIONS We demonstrate the preparation of CuO/ O-doped g-C3N4 nanospheres with superior properties by in-situ hydrothermal growth and subsequent annealing process. The spherically and mesoporousity of CuO/O-doped g-C3N4 can facilitate Li+ and electrons transfer, allow electrolyte penetration and relieve the volume expansion for the active CuO nanoparticles. The O-doped g-C3N4 has significant features for improving the electrical conductivity , leading to the enhanced stability and cycling performance of the prepared compsite. It is believed that, the O-doped g-C3N4 could be utilized to modify other transition metal oxides to alleviate their disadvantages for LIBs. In


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addition, the multiple functions of mesoporous CuO/O-doped g-C3N4 composite could be applied for sodium-ion batteries, supercapacitors, etc. ASSOCIATED CONTENT Supporting Information The Supporting Information contains SEM images, XRD pattern, CV curves, XPS survey spectra for CuO/O-doped g-C3N4 precursor, pure CuO and O-doped g-C3N4. AUTHOR INFORMATION * Corresponding

author. Email: [email protected] and [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. This work is supported by National Key R&D Program of China (2016YFA0202602), Academy of Scientific Research& Technology (ASRT, Egypt), Science & Technology Development Fund (STDF, Egypt), National Natural Science Foundation of China (U1663225, 21671155, 21805220), Natural Science Foundation of Hubei Province (2018CFB242, 2018CFA054), Major Technology Innovation of Hubei Province (2018AAA012) and Program for Changjiang Scholars Innovative Research Team in University (IRT_15R52). REFERENCES



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Batteries

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57. Ji, D.; Zhou, H.; Tong, Y.; Wang, J.; Zhu, M.; Chen, T.; Yuan, A. Facile Fabrication of MOFderived Octahedral CuO Wrapped 3D Graphene Network as Binder-free Anode for High Performance Lithium-ion Batteries Chem. Eng. J. 2016, 313. 1623-1632. 58. Wu, R.; Qian, X.; Yu, F.; Liu, H.; Zhou, K.; Wei, J.; Huang, Y. MOF-templated Formation of Porous CuO Hollow Octahedra for Lithium-ion Battery Anode Materials J. Mater.Chem. A 2013, 1, 11126-11129. 59. Qi, W.; Zhao, J.; Shan, W.; Xia, X.; Xing, L.; Xue, X. CuO Nanorods/Graphene Nanocomposites for High-performance Lithium-ion Battery Anodes J. Alloys Compd. 2014, 590, 424-427. 60. Wang, G.; Sui, Y.; Zhang, M.; Xu, M.; Zeng, Q.; Liu, C.; Liu, X.; Du, F.; Zou, B. One-pot Synthesis of Uniform Cu2O-CuO-TiO2 Hollow Nanocages with Highly Stable Lithium Storage Properties J. Mater. Chem. A 2017, 5, 18577-18584.



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O-doped g-C3N4 Nanospheres for

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