Halide Ion Intercalated Electrodeposition Synthesis of Co3O4

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Halide Ion Intercalated Electrodeposition Synthesis of Co3O4 Nanosheets with Tunable Pores on Graphene Foams as Free-Standing and Flexible Li-Ion Battery Anodes Yifan Yao, Yihua Zhu, Shunan Zhao, Jianhua Shen, Xiaoling Yang, and Chunzhong Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00351 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Energy Materials

Halide Ion Intercalated Electrodeposition Synthesis of Co3O4 Nanosheets with Tunable Pores on Graphene Foams as Free-Standing and Flexible LiIon Battery Anodes Yifan Yao, Yihua Zhu,* Shunan Zhao, Jianhua Shen, Xiaoling Yang, and Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China.

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ABSTRACT: Soft portable electronics has grabbed the attention of manufacturing industry. One of the issues is the flexible energy sources with high energy density, high operating voltage and long lifetime. Flexible lithium ion batteries could be the solution. However, the conventional electrode production process is paste casting which contains binder, conductive additive and substrates with no capacities and limits the flexibility of lithium ion batteries. Herein, we promote the electrode preparation by halide ion intercalated electrodeposition of Co(OH)2 nanosheet precursors on graphene foams. The Co3O4/graphene foam electrodes, the final product, exhibit good flexibility and mechanical strength. Meanwhile, by regulating the ratio of Br- and Cl-, specific surface areas, pore volumes, pore diameters and specific capacities can be modified with the highest specific capacity of 790 mA h g-1 after 100 cycles at a current density of 0.1 C in the voltage range of 0.2 - 3.0 V. Furthermore, because of the similar particle sizes of porous Co3O4 nanosheets, the influence of pore nature on the electrochemical performances is revealed. A full cell is assembled to testify the good electrochemical performance of porous Co3O4/graphene foam electrode. KEYWORDS: cobalt oxides; graphene foams; electrodeposition; ion intercalation; flexible lithium battery.


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1. INTRODUCTION The prosperous market of soft portable electronics is attracting more attention to the development of flexible lithium ion batteries. Thus, it is essential to integrate the energy sources into a flexible device without astricting their bending or folding.1-4 Moreover, as a major energy source, lithium ion batteries with higher energy density, higher operating voltage, and longer lifetime are recommended for application.1,

5, 6

However, the limited capacities and the

conventional fabrication process of electrodes impede the utilization of lithium ion batteries in flexible and wearable devices. To achieve higher capacities, transition metal oxides (MOx) are employed.3, 5, 7-9 Co3O4 is an encouraging MOx due to its high theoretical capacity of 890 mA h g-1 and extensive attentions have been attracted.7,

9, 10

Nevertheless, the large volume change during the electrochemical

process would result in pulverization and deterioration of the electrodes.11 Various structures and morphologies, such as hollow spheres, nanowires, nanofibers, nanosheets and porous structures, have been developed by hydrothermal reactions to buffer the volume changes. Hu et al. reported the porous Co3O4 nanofibers with enhanced cyclic stability of 900 mAh g-1 at 1 A g-1.12 Mesoporous bowknot-like Co3O4 anode with a reversible capacity of 1388.8 mA h g-1 at 0.2 C after 100 cycles is reported by Geng et al.13 Recently, Kim et al. reported the influence of pore nature and particle sizes on the electrochemical performances of Co3O4 nanosheets.14 However, hydrothermal process is used in which autoclaves are needed and is difficult to scale up to an industrial level.15 Meanwhile, the electrodes are prepared by paste coating method and copper foils are employed as substrates. Compared with the binder free electrode fabrication, binders are employed to bind the substrates and active materials which can increase the resistance of


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electrodes and decrease the integrity of electrode.10, 16 Compared with the free standing anodes, the weight of copper foils is huge and cannot supply extra capacities. To overcome the drawbacks, electrodeposition and carbon substrates can be applied to fabrication free standing anodes. Fe3O4 nanoparticles with carbon nanotubes (CNTs) and reduced graphene oxide (rGO) are electrodeposited on Cu foils as binder free anode by Zhao et al.5 Chen et al. reported the silicon nanowires on carbon cloth as anode for lithium ion batteries.17 Co(OH)2 nanosheets are electrodeposited on graphene foams as supercapacitors by Jun et al.18 However, the rGO and CNTs in solution has lower conductivity due to the introduction of acid treatment and surfactants.16 Compared with graphene foams, carbon fibers of the same thickness are heavier in per square meters. Nevertheless, to the best of our knowledge, electrodeposited porous Co3O4 on graphene foams has not been reported. Thus, it is essential to synthesize electrodeposited Co3O4 on graphene foams with controllable pore properties and particle sizes to achieve excellent lithium ion battery performances. Herein, we reported the electrodeposited Co(OH)2 nanosheets on graphene foams. Cl- and Brof different ratios was intercalated into the Co(OH)2 nanosheets during the formation of nanosheets to further control the crystallinity of Co(OH)2 nanosheets. Compared with the hydrothermal reaction, the electrodeposited synthesis of Co(OH)2 nanosheets was mild and cost effective. The graphene foams can be directly used as the electrodeposited electrode without any acid treatment which ensured the high integrity of graphene foams and the three dimensional high conductivity. The Co(OH)2 nanosheets with different crystallinity and different ratios of Cland Br- was calcined in argon to form porous Co3O4 nanosheets with a similar particle size. The pore formation process and the influences of pore sizes, pore volumes and specific surface areas on the electrochemical performances of porous Co3O4 nanosheets were discussed in detail.


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2. EXPERIMENTAL SECTION 2.1 Materials CoCl2•6H2O and KNO3 was purchased from the Sinopharm Chemical Reagent Company. NH4Cl was purchased from the Shanghai Lingfeng Chemical Reagent Company. NH4Br was purchased from Shanghai Macklin Biochemical Company. CoBr2 was purchased from the Shanghai Nine-Dinn Chemistry. All chemical reagents were used without further treatment. 2.2 Preparation of graphene foams. Graphene foams were prepared by chemical vapor deposition (CVD) method reported before with some modifications. First, nickel foams with a size of 7 × 12 cm were directly employed as the frame templates and catalyst and loaded into a quartz tube. Ar and H2 at a flow rate of 500 and 200 sccm were used before heated to 1000 °C. After treatment for 10 min, CH4 was aerated. The flow rate of Ar, H2 and CH4 were 800, 100 and 50 sccm, respectively. The CVD time was controlled to 7 min to obtain nickel supported graphene foams with sufficient mechanical strength. When the CVD was finished, the nickel foams with graphene were cooled to ambient temperature at a rate of ~100 °C min-1. Freestanding graphene foams were obtained by etching of nickel foams in a 3 M HCl solution at 85 °C. After etching, graphene foams were dried in vacuums at 80 °C. 2.3 Preparation of Co3O4 nanosheets on graphene foams. Graphene foams were used without any modification and all chemical reagents were used without any purification. Eletrodeposition of Co(OH)2 was carried out in a three-electrode system with a carbon cloth (2 × 3 cm) as the counter electrode, the graphene foams (1 × 1.5 cm) as the working electrode and the saturated Ag/AgCl electrode as the reference electrode. The electrolyte solution was obtained by dissolving CoCl2•6H2O, KNO3, NH4Cl, NH4Br and CoBr2 in deionized water (40 mL). The ratio of Co2+, NO3- and NH4+ was set as 1 : 2 : 10 and the concentration of Co2+ was 0.02 M. The sum


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concentration of Br- and Cl- was 0.24 M and the ratios of Br- and Cl- were set as 24 : 0, 12 : 12, 4 : 20, 0 : 24 which were named as BrPure, 8Br12Cl, 0Br20Cl and ClPure based on the concentration of NH4Br and NH4Cl, respectively. In a typical process, the graphene foams were immersed with the underwater length of 1 cm. A current density of -12.5 mA cm-2 was applied for 900 s at 70 °C to attain the Co(OH)2 nanosheets. The graphene foams were carefully washed with flowing deionized water to remove the possible residues. Before calcinations, the Co(OH)2 nanosheets were desiccated in vacuums at 60 °C. The Co3O4 nanosheets were obtained by calcining the Co(OH)2 nanosheets at 450 °C for 2 h in argon atmospheres with a heating rate of 3 °C min-1. The Co3O4 nanosheets on graphene foams were presently used as anodes. With different ratios of Br- and Cl-, Co3O4 nanosheets with different porosity were obtained. The mass proportions of Co3O4 nanosheets on CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure were 51.92 %, 58.52 %, 55.59 % and 51.80 %, respectively. 2.4 Material characterization The crystal structures of Co(OH)2 and Co3O4 naonsheets were characterized by X-ray diffractometer (RIGAKU, D/MAX 2550 VB/PC, λ = 1.5406 Å) from 10° - 80° under room temperature. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALab 250Xi Xray photoelectron spectrometer with a Mg Kα as source and the C1s peak at 284.8 eV as an internal standard. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained using a JEOL SM-6360LV microscope operated at 15 kV and a JEM-2100 microscope operated at 200 kV, respectively. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models were used to determine the specific surface areas, pore volumes, and pore sizes of the samples. 2.5 Electrochemical measurements


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Electrochemical studies were conducted in two-electrode coin-cell (CR 2032) assembled in an argon-filled glovebox. Graphene foams with Co3O4 nanosheets (1 × 1 cm) were directly used as working electrode. Metallic Li sheets were used both as the counter and reference electrodes. A polypropylene film (Celgard 2400) was used as a separator. The nonaqueous electrolyte in cells was 1.0 M LiPF6 in EC/DEC (1 : 1 w/w). Galvanostatical charge-discharge cycles were conducted on a LAND-CT2001A battery tester at various current densities in the voltage range of 0.2 - 3.0 V versus Li+/Li. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (Autolab PGSTAT30 potentiostat). The cyclic voltammograms were obtained over the potential range from 3.0 to 0.2 V at a scanning rate of 0.2 mV s-1. The impedance spectra were obtained by applying an AC voltage of 2 mV amplitude over the frequency range of 100 kHz to 0.01 Hz at delithiation states. 3. RESULTS AND DISCUSSION Scheme 1 shows the fabrication of Co3O4 nanosheets with tunable porosity on the graphene foams. Firstly, Co(OH)2 nanosheets (represented by CHO) were electrodeposited on graphene foams. The graphene foams were directly used in electrodeposition process without any modification which ensured the integrity of the graphene structure and better conductivity.5, 9, 10 During the formation of Co(OH)2 nanosheets,19,20 halogen ions were intercalated which simplified the intercalation procedure.21-26 The halogen ions would cause the mismatch of Co(OH)2 lattice. By tuning the ratio of Br and Cl, different lattice defects were developed and then affected the porosity of Co3O4 nanosheets (represented by CO). The electrodeposition treated graphene foams were then heated in argon to form porous Co3O4 nanosheets and remove the intercalated ions. With the increment of Cl-, higher surface area was evoked. The detailed properties of the porous Co3O4 nanosheets are discussed in the following parts. Based on the


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molar ratio of NH4Br and NH4Cl, BrPure, 8Br12Cl, 0Br20Cl and ClPure followed CHO and CO, representing the Co(OH)2 and Co3O4 derived from the solution containing 0.24 M NH4Br, 0.08 M NH4Br and 0.12 M NH4Cl, 0M NH4Br and 0.20 M NH4Cl, and 0.24 M NH4Cl. Besides the tunable porosity, the electrodeposition procedure can be applied to any conductive substrates, such as ITO, AAO, carbon cloth and metal substrates.10, 19, 20, 27 Scheme 1 Schematic illustration of electrodeposited fabrication of porous Co3O4 nanosheets on graphene foams.

The crystalline structure of electrodeposited Co(OH)2 nanosheets were affirmed by the XRD spectrum (Figure 1a). Four peaks at 26.6°, 54.8°, 44.7° and 60.0° correspond to the (004), (102), (008) and (106) planes of carbon (JCPDS No. 26-1080, carbon). Peaks at 11.1°, 22.2°, 33.2°, 33.9°, 38.1° and 58.8° can be indexed to the (003), (006), (101), (012), (015) and (110) planes of α-Co(OH)228-32 and JCPDS No. 46-0605. The similar patterns of Co(OH)2 verify that ratio of Br and Cl does not change the crystal structure of α-Co(OH)2. However, the peaks at 11.1° are different and the relevant peak of CHO-BrPure is broadened which demonstrates the smallest


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crystal size. The size reduction of CHO-BrPure may be due to the disorder of layers along c axis. Anion intercalation can jumble the c axis and the size reduction phenomena certify the intercalation of anions. Meanwhile, as marked by the red dashed line, the corresponding peaks of Co(OH)2 exhibits a slight shift to large angles with the increase of Cl- which also indicates the insertion of anions into the crystal lattice. The intercalation ratio of Br- and Cl- was further determined by the EDS analysis (Figure 1b). Except for the CHO-BrPure and CHO-ClPure, the ratio of Cl in Co(OH)2 are higher than the ratio in solution which means the preferable intercalation of Cl-. In polar protic solutions, Cl- should have a lower reactivity to positively charged Co(OH)2-x layers than Br-.22, 25 However, Cl- has a smaller anion diameter and is more likely to inert into the lattice which may be the critical factor to affect the ratio of intercalation anion. The ratio of Br- and Cl- in Co(OH)2 would further impact the crystal size and pore nature of Co3O4.

Figure 1 a) XRD patterns of Co(OH)2 nanosheets electrodeposited from solution with different ratio of Br- and Cl- on graphene foams; b) Normalized ratio of Br- and Cl- derived from the EDS data.


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The crystal nature of Co3O4 was verified by the XRD spectrum (Figure 2a). Peaks at 19.1°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.4° and 65.2° correspond to the (111), (220), (311), (222), (400), (422), (511) and (440) of Co3O4 (JCPDS No. 43-1003, Co3O4).7, 9, 33 Compared with the XRD spectra of Co(OH)2, no significant changes can be specified which indicates the less difference of particle sizes. Peaks of carbon still exist demonstrating the stability of graphene foams. To further manifest the crystal structures, particle sizes of Co(OH)2 and Co3O4 were exposed after refining the XRD spectra by Maud.34-36 As shown in Figure 2b, the particle size of CHO-BrPure is the smallest which is consist with the diffraction peak widening at 11.1° in Figure 1a. With the increase of Cl-, larger Co(OH)2 particles are developed. The increase particle size can be derived from the better crystallization. The diameter of Cl- is smaller than that of Brand the insertion of Cl- can decrease the stacking disorder of Co(OH)2 which increases the interaction of adjacent layers.22, 24, 26 , 37 Meanwhile, the crystalline structures are enhanced and particle sizes increase. The as-prepared Co3O4 displays a similar particle size increase trend. The formation of Co3O4 incorporated dehydration, expulsion of halogen ions and grain melting. Dehydration and halogen ion expulsion are particle-size-shrinkage procedures while grain melting is the particle-size-expansion process.14, 37 As for CHO-BrPure, the grain melting was the main process which enlarged the particle size of Co3O4. With the increment of Cl- and improvement of crystallinity, grain melting was depressed while dehydration and halogen ion expulsion were principal which shrank the crystal particles of Co3O4. However, the large particle sizes of Co(OH)2 determined that the particle sizes of Co3O4 were still hefty. The moderate size changes of Co3O4 particle are agreed with the slight changes of XRD patterns of Co3O4. As a consequence of the slight size changes of Co3O4 particles, the sizes of Co3O4 nanoparticles are assumed to be the same.


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Figure 2 a) XRD patterns of Co3O4 nanosheets on graphene foams stemming from the Co(OH)2 precursors; b) Particle sizes of Co(OH)2 and Co3O4 refined from the XRD data by Maud. The structure of graphene foams was characterized by FESEM and TEM (Figure S1). The graphene foams maintained the outer macropore structure after Ni foam removal. Meanwhile, the inner tunnel structure replaced the Ni foam framework which could benefit the lithium ion diffusion. The TEM image indicated the formation of multilayer graphene. The morphology of Co(OH)2 and Co3O4 were scrutinized by FESEM (Figure 3). The electrodeposited Co(OH)2 displayed nanosheet structures with similar thickness of ~50 nm and several micrometers in width (Figure 3a, d, g and j). The SEM images of Co(OH)2 validated that the concentration ratio of Br and Cl had no evident effect on the microstructures. Co3O4 was obtained after post heat treatment of Co(OH)2 nanosheets. As shown in Figure 3b, e, h and k, the nanosheet structures were well maintained. The Co3O4 nanosheets revealed no collapse which demonstrated the good mechanical strength of the nanosheet structures. The satisfactory mechanical strength would benefit the integrity of the electrodes and the electrochemical performance.7, 10, 11, 14, 33 Details of the Co3O4 nanosheets were verified in Figure 3c, f, i and l. The Co3O4 nanosheets exhibited porous structure. Interestingly, the SEM images revealed that the pore sizes increased with the


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increment of Cl and particle sizes showed no palpable variation. As mentioned in the XRD spectra refinement results, the particle sizes varied slightly and the SEM images supported the data. The porous structures originated from the dehydration, expulsion of halogen ions and grain melting.11, 14, 37 Large pores with diameters of ~50 nm were mainly derived from the dehydration and halogen ion expulsion while grain melting primarily engendered the collapse of micropores.7,

38, 39, 14

Porous structures could accumulate the volume expansion which would

promote the integrity of electrodes and consequently enhance the cycling stability.8,

9, 11, 33

Besides the accumulation of volume changes, the porous structure would benefit the wettability of graphene foam surface. The wet properties of graphene foams were estimated by contact angles in Figure S2. Compared with pure garphene foams, graphene foams with porous Co3O4 had the smaller contact angles indicating the improved wettability of graphene foams. The upgraded wet properties would assist the wetting of electrolytes which could thrive the electrochemical performances.7, 40


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Figure 3 FESEM images of a) CHO-BrPure; b,c) CO-BrPure; d) CHO-8Br12Cl; e,f) CO8Br12Cl; g) CHO-0Br20Cl; h,i) CO-0Br20Cl; j) CHO-ClPure; k,l) CO-ClPure. The pore sizes, Brunauer-Emmett-Teller (BET) surface areas and pore volumes of porous Co3O4 was acquired by the Nitrogen isotherm adsorption analysis (Figure 4). Based on the Barrett-Joyner-Halenda evaluation, the pore sizes of porous Co3O4 are calculated from the absorption curve. The pore sizes are mainly in the range of 1-20 nm, indicating the development of micropores and mesopores during the calcinations. These pores would accumulate the volume changes of Co3O4 on cycling. Moreover, pores with diameter below 10 nm are massive which


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would benefit the reversible reaction of organic layer formation on the Li2O matrix with Co nanoparticles and supply additional capacities and the electrodes may exhibit a capacity higher than the theoretical capacity.14 With the increment of Cl- concentration, the quantities of micropores are increased while the mesopores are decreased. The formation of micropores is probably due to dehydration, expulsion of halogen ions and the stack of the nanoparticles.7, 38, 39 Based on the hypothesis of particle size equation, dehydration and expulsion of halogen ions would affect the stack of the Co3O4 nanoparticles demonstrating the signification of halogen ions intercalation. The addition of Cl- further enlarges the BET surface area which can accelerate the electron/Li+ transfer and is able to achieve superior rate capabilities.

Figure 4 a) N2 adsorption-desorption isotherm curves of different porous Co3O4 nanosheets and the related pore size distribution; b) BET specific surface areas and pore volumes of different porous Co3O4. The structures of CHO-8Br12Cl and CO-8Br12Cl were further investigated by TEM. The microstructures were displayed in Figure 5a and c. As shown in Figure 5a and c, the particle sizes of CHO-8Br12Cl and CO-8Br12Cl are ~12 nm and ~30 nm, respectively, which are consistent with the results refined by Maud. The HRTEM images in Figure S3 also consist with


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the Maud results. The labelled lattice spacing of CHO-8Br12Cl is 0.24 nm corresponding to the (015) plane while the lattice spacing of CO-8Br12Cl is 0.29 nm corresponding to the (220) plane, which are in agreement with the XRD characteration. The porous morphology of CO-8Br12Cl was exhibited in Figure 5b. As expected, during calcination in Ar, the pore structures were induced by dehydration, halogen ions expulsion and grain melting. The crystallites are connected with each other forming the bicontinuous porous nanostructures. It is well known that the porous structures in nanosheets are influential in the electrolyte mass transport within the electrodes for fast redox reactions and double-layer performances.7,

14

The favorable crystallization of CO-

BrPure was also confirmed by the electron diffraction pattern. As shown in Figure 5d, the welldefined diffraction spots was analyzed by the CrysTBox41 suggesting the well crystalline structure of CO-BrPure.


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Figure 5 HRTEM images of a) CHO-8Br12Cl and b,c) CO-8Br12Cl; d) the SAED of CO8Br12Cl labelled by CrysTBox. The element composition and oxidation valence of CHO-8Br12Cl and CO-8Br12Cl was also exposed by X-ray photoelectron (XPS) measurement, as given in Figure 6. In the wide survey XPS spectrum of CHO-8Br12Cl, peaks of Co, O, Br, Cl and C are identified (Figure 6a). The existence of Br and Cl indicates the intercalation of halogen into the Co(OH)2. Co 2p spectrum of CHO-8Br12Cl exhibits two satellite peaks at 802.6 and 786.6 eV, corresponding to the shakeup satellite peaks of 2p1/2 and 2p3/2 (Figure 6b). The 2p1/2 peaks at 798.2 and 796.5 eV and the 2p3/2 peak at 782.1 eV can be assigned to Co2+ coordinated to oxygen atoms. The 2p3/2 peak at


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780.4 eV corresponds to the higher oxidation state of Co3+ which proves the formation of positively charged Co(OH)2 layers and would avail the intercalation of negatively charged halogen ions.42 The C and O 1s spectra of CHO-8Br12Cl were displayed in Figure S4. The C1s peak at 284.8 eV coincides with the C-C bond and a C=O bond is observed at 287.6 eV. The O1s peaks at 533.0 and 530.6 eV correspond to C-OH and C=O, respectively.43, 44 The existence of C=O and C-OH on the electrodeposited surface of graphene foams was also affirmed in the literature, demonstrating the partial oxidation of graphene foams.18, 45, 46 The oxygen-containing functional group on the surface of graphene foams would enhance the combination between Co(OH)2 nanosheets and graphene foams and Co(OH)2 are permitted to nucleate and crystallize on the surface of graphene foams. After calcinations, C=O group remains indicating the stable electrostatic interaction of Co3O4 nanosheets and graphene foams (Figure S4c, d). Except of the peaks of Br and Cl, the wide survey XPS spectrum of CO-8Br12Cl exhibit peaks similar to the peaks of CHO-8Br12Cl. The exclusion of halogen elements confirms the halogen ion expulsion process during the formation of Co3O4. Two satellite peaks at 804.4 and 789.5 eV correspond to the 2p1/2 and 2p3/2 of Co3O4.47, 48 Peaks centered at 796.1 and 794.8 eV can be assigned to Co 2p1/2 while peaks at 781.6 and 779.8 eV can be assigned to Co 2p3/2.49, 50 Both Co2+ and Co3+ exist in CO-8Br12Cl noting that the binding energy difference between Co 2p1/2 and Co 2p3/2 is 15.0 eV.50 The O 1s peaks at 531.1, 529.8 and 529.7 eV also indicate the phase of Co3O4.51-53


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Figure 6 Wide survey and high resolution XPS Co 2p spectra of a,b) CHO-8Br12Cl and c,d) CO-8Br12Cl. The electrochemical performances of porous Co3O4 were investigated as the coin-type cell anodes. Due to the possible extra capacity from graphene foams, it is essential to choose proper voltage range to exclude the interference from graphene foams. As shown in the Figure S5, a cyclic voltammetry (CV) test of CO-0Br20Cl was performed in the voltage range of 0.01 - 3.0 V at a scan rate of 0.2 mV s-1. The curves shows two cathodic peaks at ~0.8 and ~0.1 V in the first cycle and two peaks at ~1.2 and ~0.1 V in the following two cycles. The peak at ~0.8 V in the first cycle is attributed to the reduction of Co3+ to Co2+ and the formation of solid electrolyte


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interphases (SEI).54 In the following two cycles, the peak shifts to a higher voltage of 1.2 V demonstrating the stability of the SEI films and no other side reaction except of the reduction of Co3+ to Co2+.55 Furthermore, the reduction current intensity is stronger in the first cycle indicating the existence of irreversible reactions.54 The cathodic peak at ~0.1 V corresponds to the lithium ion insertion into the lattice of graphite.56 In the anodic process, two peaks at ~0.3 and ~2.2 V are related to the extraction of Li from graphite and the oxidation of cobalt metal to Co3O4, respectively.56 In the second and third cycles, the CV profiles are well maintained demonstrating the stable conversion reaction and the stable of the porous structures and graphite. Although graphene foams could supply stable structures in the CV test (the current intensity is stable in the second and third cycles), extra capacities would be added and this would hamper the deeper insight of porous Co3O4 somewhat. Hence, the electrochemical tests were carried out in the voltage range of 0.2 - 3.0 V and the reactions and structure stabilities are the same as shown in Figure 7a.


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Figure 7 a) CV curves of porous Co3O4 nanosheets on graphene foams within 0.2 - 3.0 V at a scan rate of 0.2 mV s-1; b-e) Discharge-charge curves of CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure at the current density of 0.1 C. Figure 7b-e exhibit the first 6 cycles charge-discharge profiles of Co3O4 with different ratio of Br- and Cl- at a current density of 0.1 C (1 C = 890 mA h g-1) in the voltages between 0.2 V and 3.0 V. The four kinds of porous Co3O4 display the similar discharge plateaus at about 1.0 V in the first cycle and the plateaus shift to about 1.2 V in the following 5 cycles, which is consistent with the CV results. Meanwhile, the charge profiles show plateaus at 2.0 V in agreement with the CV anodic peak. It should be noted that the SEI film formation plateaus at ~0.8 V showed a tendency to be unnoticeable. This may be caused by the increase of specific surface area which enhances the lithium ion transport.14 The initial discharge profiles of CO-BrPue, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure display the initial discharge capacity of 753.2, 835.8, 876.4 and 890.1 mA h g-1 and the reverse initial charge capacity of 629.6, 648.7, 665.8 and 714.7 mA h g-1 with the initial coulombic efficiency of 83.6 %, 77.6 %, 76.0 % and 80.3 %, respectively. The initial capacities of both discharge and charge process exhibit an increase tendency with the increment of specific surface area demonstrating that the higher specific area could increase the apparent capacities due to the promoted lithium ion transport and electrochemical activity.14 The initial capacity losses of CO-BrPue, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure are 123.6, 187.1, 210.6 and 175.3 mA h g-1 which is in accordance with the variation tendency of pore volumes. Noting that the capacity losses are derived from the formation of SEI films,10, 11, 14, 33, 54 thus larger pore volumes could storage more SEI films and then cause more capacity losses. In the following 5 cycles, the discharge and charge capacities are increased. The accretion of capacities


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can be attributed to the activation of porous Co3O4 and reversible formation of organic layers surrounding the Co3O4 particles.14, 54 The rate and cycling performances of porous Co3O4 are shown in Figure 8. The voltage ranges were chosen between 0.2 and 3.0 V to minimize the influence of graphene foams on the capacities. As shown in Figure 8a, the porous Co3O4 nanosheets deliver capacities higher than 600 mA h g-1 at 0.1 C with the highest capacity of CO-ClPure 810.4 mA h g-1 and lowest capacity of CO-BrPure 687.1 mA h g-1 after 10 cycles. As mentioned above, the larger specific surface area is crucial to the higher capacity. This tendency can be applied to the high current density of 2 C. At a current density of 0.2 C, the average capacities of CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure are 652.4, 656.7, 670.2 and 781.9 mA h g-1, respectively. Compared with the capacity at 10th cycle, the capacity losses are 5.0 %, 7.1 %, 8.0 % and 3.5 %, demonstrating the quick response of porous Co3O4 to the current density change. The porous structure can enlarge the electrolyte/electrode contact area, shorten the lithium ion transport path and accumulate the volume changes during the electrochemical reaction and thus shows the excellent rate capacities. Meanwhile, due to the nonuse of binder in the electrode preparation and the high integrity of electrodes, low resistances are obtained and the lithium ion insertion is little affected. As the rate restored to 0.1 C, the capacities of porous Co3O4 are recovered and sustainable with the highest capacity of 696.9 mA h g-1. The high capacity recovery demonstrates the stability of porous Co3O4.


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Figure 8 a) Rate capability performance of porous CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure nanosheets on graphene foams within the voltage range of 0.2 - 3.0 V; b) Cycling performance of porous CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure nanosheets on graphene foams within the voltage range of 0.2 - 3.0 V at 0.1 C and the correlated coulombic efficiency. The cyclic performances at 0.1 C and the corresponding coulombic efficiency (CE) are displayed in Figure 8b. The initial capacities of CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and COClPure are 753.2, 835.8, 876.4 and 890.1 mA h g-1 with the CE of 83.6 %, 77.6 %, 76.0 % and 80.3 %. The irreversible capacity loss is attributed to the decomposition of electrolyte and the formation of SEI films. After the second cycle, the CEs gradually achieve ~100 %, demonstrating the good reaction reversibility of porous Co3O4 nanosheets. Furthermore, the capacities of porous Co3O4 nanosheets are increased with the cycles prolonged. The consequent increment of capacity is due to the gradually activation of porous Co3O4 nanosheets.14, 54 After 20 cycles, the capacity of CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure are 646.7, 695.3, 739.3 and 794 mA h g-1. In consideration of the cut-off voltage of 0.2 - 3.0 V, the capacities should be higher when the low cut-off voltage is 0.01 V. Interestingly, the capacity of


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CO-ClPure is 874.9 mA h g-1 which is almost the theoretical capacity of Co3O4. To demonstrate the origin of the capacity, the rate and cycle performances of graphene foams were tested and shown in Figure S6. The current density was the same as porous Co3O4 nanosheets. As shown in Figure S6, the reversible capacity of graphene foams is about 50 mA h g-1 which could be lower when porous Co3O4 nanosheets are loaded on the foams due to the higher current intensity. Besides the contribution of graphene foams, the high capacity is also derived from the reversible formation of the organic layers.14 Combined with the Co3O4 nanoparticles, the myriad pores existing on the porous Co3O4 can boost the reversible formation of the organic layer on the Li2O with cobalt metal nanoparticles.57 Hence, the additional capacities are derived from the uncalculated capacity of graphene foams and the organic layer. All the porous Co3O4 naonsheets electrodes exhibit good cyclic stability with the capacity higher than 600 mA h g-1 after 100 cycles. The high stability of electrodes arises from the pores which buffer the volume change during the discharge and charge process.6, 8, 9, 14 As shown in Figure S7, after 100 cycles, the nanosheet structure is well maintained. Moreover, the high integrity of electrodes, high lithium ion transport and high conductivity could also benefit the stability of electrodes.4,

10

The

demonstration was further affirmed by the CV scans at various sweep rates and electrochemical impedance. Figure 9a and b show the dependence of the cathodic and anodic peak currents on the square root of scan rate (v1/2) calculated from Figure S8. Both cathodic and anodic peaks current intensities are linear with the square root of scan rates, evincing the diffusion control of the lithiation/delithiation reaction rate. The Randles-Sevchik equation for a semi-infinite lithium ion diffusion can demonstrate this phenomenon.10, 58 ip - 269000n3/2AD1/2Cv1/2


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As mentioned in the studies, AD1/2 can be defined as the apparent diffusion coefficient and parameters of n and C are the same in the Co3O4 anodes.10, 59 As shown in the linear fit results, the slope of CO-BrPure is the smallest while the slope of CO-0Br20Cl is the highest demonstrating the lowest apparent diffusion coefficient of CO-BrPure and the highest apparent diffusion coefficient of CO-0Br20Cl. Particle sizes and pore properties can affect the diffusion of lithium ions.14 However, the particle sizes of Co3O4 can be regarded as the same according to the calculation of XRD. Thus, the pore properties are the main factors. Interestingly, the changing trend of apparent diffusion coefficient is similar to the trend of pore volume. With the increment of Cl-, the surface area of Co3O4 is increased and mesopores are developed. Although smaller pores can increase the surface area, the pore volumes may be decreased due to the smaller pore increase. As a consequence, it is essential to control the pore structures to achieve higher pore volume and then to induce better lithium ion diffusion.


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Figure 9 a) Linear fitting of the cathodic peak current and the square root of the scan rate (v1/2) and b) Linear fitting of the anodic peak current and the square root of the scan rate (v1/2) for porous CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure nanosheets on graphene foams; c) Electrochemical impedance spectra of porous CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure nanosheets on graphene foams after 10 cycles at 0.1 C over the frequency range from 100 kHz to 0.01 Hz; d) Charge transfer resistance of porous CO-BrPure, CO-8Br12Cl, CO0Br20Cl and CO-ClPure nanosheets on graphene foams.


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To demonstrate the different conductivity of Co3O4 nanosheets from different halogen ratio, electrochemical impendence (Figure 9c) was conducted under the open circle potential (OCP) in the frequency range of 100 k Hz to 10 mHz. All the electrodes show a profile consisted of two semicircles and a linear part. The two semicircles correspond to the SEI film and charge transfer resistance of porous Co3O4 nanosheets while the linear part is the diffusion resistance.10 The fitted equivalent circuit is shown in Figure S9. The circuit is composed of six components with RSEI and Rct corresponding to SEI film resistance and charge transfer resistance of porous Co3O4 nanosheets, respectively. The Warburg components are replaced by a constant phase element to achieve better fitting results.60 As shown in Figure S9 , the Rct are 198.9 Ω for CO-BrPure, 192.0 Ω for CO-8Br12Cl, 152.9 Ω for CO-0Br20Cl and 136.7 Ω for CO-ClPure. In Figure 9d, the Rct values exhibit a decrease tendency while the specific area values show an increase tendency, demonstrating the higher specific area corresponding to the lower charge transfer resistance. Meanwhile, the CO-ClPure nanosheets, which have the lowest charge transfer resistance, deliver the highest capacity in both cycling performances and rate performances. Thus, it is essential to control the specific area of porous Co3O4 nanosheets to achieve higher charge transfer resistance and then to attain better battery performances. The different conductivities at diverse charge states of CO-BrPure, CO-8Br12Cl, CO-0Br20Cl and CO-ClPure were also investigated as shown in Figure S10. From the charge voltage of 0.4 V to 2.6 V, the radiuses of the semicircles increase with the lithium ion extraction due to the regeneration of Co3O4 nanostructures. The excellent electrochemical performances of the porous Co3O4 nanosheets were investigated by the full cells. Commercial LiCoO2 was employed as the cathode material and was bought with the aluminum foils. Figure 10a shows the charge-discharge profiles of LiCoO2/CO-0Br20Cl full cell with excessive LiCoO2. In the first cycle, two charge and discharge plateaus can be claimed


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at ~2.7/4.0 V and ~3.7/1.7 V corresponding to the cathodic peaks at ~0.8/0.1 V and anodic peaks at ~0.3/2.2 V of the half cell in the first CV cycle. In the second cycle, except of the plateau at ~2.7 V which is variant but consist with the CV cycles, the remained three plateaus are the same demonstrating the excellent reversibility of the porous Co3O4 and graphene foams. The cyclic performance of the soft-package full cell is illustrated in Figure 10b. The full cell delivers a reversible capacity of 8.9 mA h after three cycles with the specific capacity of 428.6 mA h g-1. After 50 cycles, the capacity loss is still around 400 mA h g-1 testified the high stability and reversibility of the anode electrode. To further attest the admirable performance of the electrode, a cell phone was powered. As shown in Figure 10c and the video in supporting information, a cell phone is powered with one layer of CO-0Br20Cl and the electrode size is 25 × 30 mm. The graphene foam electrode also shows excellent flexibility. It is believed that more functions would be realized if another soft-package full cell is in series connection or more slices are in parallel connection.


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Figure 10 a) Discharge-charge curves of CO-0Br20Cl full cell at the current density of 50 mA g1

; b) Cycling performance of CO-0Br20Cl full cell; c) Optical photograph of the CO-0Br20Cl on

graphene foams with the size of 25 × 30 mm and applied to power a cell phone. 4. CONCLUSIONS Co3O4 nanosheets were anchored on the graphene foams due to the partial oxidation of the surface of graphene foams which avoid the usage of binders and avail the entirety of the electrodes to achieve better electrochemical performances. The particle sizes of Co3O4 were assumed to be the same based on the refinement results. Thus, the porous nanosheet structures benefited the lithium ion transport and prevented the aggregation of Co3O4 to enhance the


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structure stability. The porosity of Co3O4 could be tuned by adjusting the ratio of Br- and Cl- with the highest tunable specific capacity of 790 mA h g-1 after 100 cycles which may be due to the largest amount of pores smaller than 10 nm. The pore volumes could also affect the lithium ion diffusion. The charge transfer resistances were associated with the specific surface areas. For the higher specific surface area, a lower charge transfer resistance can be realized. A full cell was assembled to demonstrate the practical potential of the electrode. With one slice of the electrode, a cell phone could be powered and more function could be achieved with another slice. ASSOCIATED CONTENT Supporting Information The video of powering a cell phone by one piece of Co3O4/graphene foams and the bend test, contact angles of graphene foams and different Co3O4 nanosheets, C and O 1s spectra of CHO8Br12Cl and CO-8Br12Cl, CV curves of CO-0Br20Cl in 0.01 - 3.0 V, rate and cycling performance of graphene foams, CV curves of different Co3O4 nanosheets from 0.1 mV s-1 to 3.0 mV s-1 and the equivalent circuit and the corresponding values of different Co3O4 nanosheets AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This work was supported by the National Natural Science Foundation of China (21471056, 21676093, 91534202, 21776092), the Basic Research Program of Shanghai (15JC1401300, 17JC1402300), the Social Development Program of Shanghai (17DZ1200900), Innovation Program of Shanghai Municipal Education Commission, and the Fundamental Research Funds for the Central Universities (222201718002). REFERENCES (1) Mo, R. W.; Rooney, D.; Sun, K. N.; Yang, H. Y. 3D Nitrogen-Doped Graphene Foam with Encapsulated Germanium/Nitrogen-Doped Graphene Yolk-Shell Nanoarchitecture for HighPerformance Flexible Li-Ion Battery. Nat. Commun. 2017, 8, 1-9. (2) Lee, S. Y.; Choi, K. H.; Choi, W. S.; Kwon, Y. H.; Jung, H. R.; Shin, H. C.; Kim, J. Y. Progress in Flexible Energy Storage and Conversion Systems, with a Focus on Cable-Type Lithium-Ion Batteries. Energ. Environ. Sci. 2013, 6, 2414-2423. (3) Balogun, M. S.; Wu, Z. P.; Luo, Y.; Qiu, W. T.; Fan, X. L.; Long, B.; Huang, M.; Liu, P.; Tong, Y. X. High Power Density Nitridated Hematite (Alpha-Fe2O3) Nanorods as Anode for High-Performance Flexible Lithium Ion Batteries. J. Power Sources 2016, 308, 7-17. (4) Blake, A. J.; Kohlmeyer, R. R.; Hardin, J. O.; Carmona, E. A.; Maruyama, B.; Berrigan, J. D.; Huang, H.; Durstock, M. F. 3D Printable Ceramic-Polymer Electrolytes for Flexible HighPerformance Li-Ion Batteries with Enhanced Thermal Stability. Adv. Energy Mater. 2017, 1602920.


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Halide Ion Intercalated Electrodeposition Synthesis of Co3O4

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