Graphene Oxide Involved Air-Controlled Electrospray for Uniform, Fast

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Graphene Oxide Involved Air-controlled Electrospray for Uniform, Fast, Instantly Dry, and Binder-Free Electrode Fabrication Ling Fei, Sang Ha Yoo, Rachel Ann R. Villamayor, Brian P Williams, Seon Young Gong, Sunchan Park, Kyusoon Shin, and Yong Lak Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00087 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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ACS Applied Materials & Interfaces

Graphene Oxide Involved Air-controlled Electrospray for Uniform, Fast, Instantly Dry, and Binder-Free Electrode Fabrication Ling Fei1, Sang Ha Yoo1, Rachel Ann R. Villamayor1, Brian P. Williams1, Seon Young Gong2, Sunchan Park2, Kyusoon Shin2, and Yong Lak Joo1* 1

Robert Frederick Smith School of Chemical & Biomolecular Engineering, Cornell University,

Ithaca, NY 14853 2

Advanced Institute of Research, Dongjin Semichem Co., Ltd. 35 Sampyeong-dong, Silicon

Park, Bundang-gu, Seongnam-si, Gyeonggi-do, 13486, Korea

KEYWORDS: Air-controlled electrospray, graphene oxide, silicon, binder-free, prelithiation

ABSTRACT: We report a facile air-controlled electrospray method to directly deposit binderfree active materials/ graphene oxide (GO) onto current collectors. This method is inspired from electrospinning process, and possesses all the advantages that electrospinning has such as low cost, easy scaling up, and simultaneous solvent evaporation during the spraying process. Moreover, the spray slurry is only a simple mixture of active materials and GO suspension in water, no binder polymer, organic solvent, and conductive carbon required. In our research, highcapacity Si nanoparticles (Si NP, 70~100 nm) and SiO microparticles (SiO MP, 3~10 µm) were

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selected to demonstrate the capability of this method to accommodate particles with different size. Their mixture with GO were sprayed onto collector then thermally annealed in inert gas to obtain Si NP or SiO MP/ reduced graphene oxide (RGO) binder-free electrodes. We are also able to directly deposit fairly large electrode sheet (e.g. 12 in× 21 in) upon the application requirement. To the best of our knowledge, this is the simplest approach to produce Si related materials/ RGO layered structures directly on current collector with controllable area and loading. Si and SiO / RGO are evaluated in both half and full lithium cells, showing good electrochemical performance. Prelithiation is also studied and gives a high first cycle Coulombic efficiency. In addition to Si related materials, other materials with different shape and size (e.g. MoO3 nanobelts, Sn/carbon nanofibers, and commercial sulfur particles) can also be sprayed. Beyond the preparation of battery electrodes, this approach can also be applied for other types of electrode preparation such as that of supercapacitor, fuel cell, solar cell, etc.


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1. Introduction In recent decades, graphene or reduced graphene oxide have been frequently applied as excellent host for electrode materials in lithium ion batteries, due to their unique twodimensional structure, high specific surface area, excellent electronic conductivity, and chemical resistance.1-3 When incorporated with active materials, graphene or RGO not only increases the conductivity of the nanocomposite, but also accommodates the mechanical stress caused by volume change during lithiation and delithiation.4-7 However, most of the reported active materials/graphene or RGO composites are in the form of powder and have to go through electrode fabrication process. The traditional electrode coating method not only requires binder, carbon additives, and organic solvents, but also consists of many steps: mixing, sonication, blading, calendering, and long-time vacuum drying.8 Even after all these tedious steps, the prepared electrodes may still suffer easy exfoliation of active material from current collector, which leads to poor cycling stability.9, 10 Although there are some reports using vacuum filtration to make active materials/graphene papers, it is difficult to scale up vacuum filtration method for large sheets with high active material loading.11-13 It will be significantly meaningful to have a simple method which can directly wrap active materials inside the graphene or RGO, and simultaneously deposit the nanocomposites onto current collectors without any binder or hazardous solvent involved. Namely, active materials/graphene or RGO nanocomposites preparation and their electrodes fabrication are combined in a single step.

To achieve this goal, we have developed a facile air-controlled electrospray approach from the already industrialized electrospinning process, and utilized inexpensive chemically-oxidized graphene oxide suspension as the source of RGO. The hydrophilic functional groups on GO


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make it ideal for water based processing and the liquid crystal structure of GO suspension allows them to self-assemble into many novel structures, e.g. macroscopic graphene fibers and graphene films.14-16 Moreover, the GO liquid crystals can be aligned in one direction under electric field.17, 18

For electrode preparation, GO suspension and active materials are simply mixed in a certain

amount of water to form a semisolid mixture. No additional polymer binder or conductive carbon is necessary. The aqueous slurry is then directly sprayed onto current collectors. During the spraying process, the solvent is evaporated, yielding a dry electrode layer immediately.

It is also well known that commercial Si (mostly nanoparticles) has been studied intensively due to its high theoretic capacity, and only limited progress has been made on preparing binderfree Si nanoparticles/graphene or RGO layered electrodes.6, 19, 20 For example, Ji et al. prepared graphene/Si multilayer structures through a tedious and toxic process: (a)filter graphene suspension on anodic aluminum oxide film; (b)transfer to Cu foil; (c)decompose SiH4 to form Si on graphene; (d) repeat step a-c.6 Another two groups repeated alternative dip coating of copper or nickel foams (only foam structure rough enough for trapping materials) into GO and silicon suspension to form multilayer structure. It seems simple, but very time-consuming and tedious for accurate control of loading amount.19, 20

In this research, we not only selected Si nanoparticles (70-100 nm) but also SiO micro size particles (3-10 µm) to demonstrate the facile air-controlled electrospray method for directly depositing binder-free GO and active materials layer by layer on current collectors. The prepared electrodes show excellent adhesion, high uniformity, and a layer by layer Si NP or SiO MP and GO structure. After thermal annealing, Si NP or SiO MP/RGO electrodes can be obtained. To


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the best of our knowledge, this approach is the simplest and most scalable approach reported so far for binder-free Si or SiO/graphene or RGO layered electrodes.6, 19-21 Moreover, we are also able to spray a variety of other materials, demonstrating the versatility of this spray approach. In addition to battery electrodes preparation, this approach can be applied for other types of electrode preparation such as that of supercapacitor, fuel cell, solar cell, etc. 2. Experimental details 2.1 Preparation of active materials/GO, RGO electrodes In a typical preparation procedure of active materials/GO electrodes, 3.0 g of GO aqueous suspension (Dongjin Chemical, 2wt% GO sheets, 98wt% H2O) was diluted in 5.0 g of DI water. After sonicating the suspension for 1 hr, 60 mg or 120 mg active materials (1:1 or 2:1 weight ratio with graphene oxide) were added. The main material of interest in this study is commercial silicon related materials, nanoparticle (70-100 nm, MTI.) and silicon monoxide micro size particles (3-10 µm, Sigma Aldrich Korea Ltd.). Other materials include commercial sulfur (Spectrum chemical MFG, Corp), homemade MoO3 nanobelts and Sn/carbon naofibers (Sn/CNF). Detailed preparation methods for homemade materials are available in the following paragraphs. The mixture was then sonicated for another hour and stirred overnight before spraying. air-controlled electrospray was applied for directly depositing binder-free electrodes. The electrospray was carried out under ambient condition using a Harvard Apparatus PHD 2000 Infusion syringe pump with a coaxial needle set. Solution was supplied through the inner 17 G needle and air through outer 12 G needle. The working voltage was set at 20 kV, working distance at 20 cm, solution feeding rate between 0.05 ml min-1 – 0.1 ml min-1, and air pressure at 28 psi. To obtain active materials/RGO electrodes, the as sprayed active materials/GO were annealed at 400 °C in N2 atmosphere (MTI Tube Furnace) for 1 hr to reduce GO, ramp 5 ºC/min.


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All materials were deposited on copper foil except sulfur on porous carbon nanofiber substrate or aluminum foil. The graphite electrode sheet used for volume energy density demonstration was purchased from MTI Corporation. 2.2 Synthesis of MoO3 nanobelts MoO3 nanobelts: a certain amount of (NH4)6Mo7O24 ·4H2O (99%, Alfa Aesar) was annealed at 700 ºC for 3 h in air, ramp rate 5 ˚C/min. 0.5 g of the as-prepared powder was dispersed in 25 ml H2O. Then 5 ml H2O2 (30 wt% in H2O, Sigma Aldrich.) was added. After stirring overnight and sonicating for 1hr, the yellowish solution was transferred to an autoclave and kept at 180 ºC for 20 hrs. Finally, the precipitate was washed several times and dried at 95 ºC. 2.3 Synthesis of Sn_CNF nanofibers Sn_CNF: the Sn/CNF composite was fabricated via electrospinning method. 0.5 g PVA (poly vinyl alcohol, Mw = 78,000 g/mol, 88% Hydrolyzed, Sigma Aldrich) was dissolved in 5 g DI water. Afterwards, 0.2 mL of acetic acid was added to prevent gelling effect, followed by 1 g Sn(CH3CO2)2. Three drops of surfactant (triton X-100) was also added to reduce surface tension. The electrospinning was carried out at a high voltage of 17.5 kV with a solution feed rate of 0.005 mL/min, and the aluminum foil collector was placed 15 cm away from the tip of the needle. The as-spun nanofibers were collected and treated under H2/Ar (5% H2/ 95% Ar) gas at 400 ºC for 6 hrs then 900 ºC for 5 hrs to obtain the final product. 2.4 Characterization The structure and morphology of the samples were characterized by scanning electron microscopy (SEM, LEO 1550) and transmission electron microscopy (TEM, FEI T12 Spirit). The Si and carbon ratio was decided by thermogravimetric analysis (TA instruments Q500). Electrochemical measurement was conducted using CR-2032 coin cell. The electrodes prepared


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above were directly used without any further treatment. 1 M LiPF6 in a mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (2:1:2 by vol %) with an additive of 10 wt% fluoroethylene carbonate was used as electrodes for silicon related materials. In half cell, a lithium foil was used as counter electrode. LiCoO2 electrode (MTI.) was used as cathode in full cells. Cell assembly was carried out in an argon-filled glove-box. Prelithiation method was adopted from previous report and the prelithiation time is 25 minutes.22 The galvanostatic charge/discharge measurements were performed using a Land battery testing system in the voltage cutoff window of 0.01-1.5 V for half cells, and 2.5-4.2 V for full cells. The current density and specific capacity are based on the total mass of electrode materials. Cyclic voltammetry was

measured

using

a

PARSTAT

4000(Princeton

Applied

Research)

electrochemical work station. 3. Results and Discussion 3.1. Air-controlled Electrospray

Scheme 1 Schematic illustration of (a) air-controlled electrospray for directly depositing binderfree electrodes; (b) conventional electrospray without air.


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Both previously reported conventional electrospray and the air-controlled electrospray we reported here were inspired by the electrospinning method, thus possessing all the advantages that electrospinning has such as low cost, easy scaling up, and simultaneous solvent evaporation. Conventional electrospray has been applied to prepare various novel nanomaterials for lithium ion battery electrode application.23,

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The air-controlled electrospray and conventional

electrospray are briefly demonstrated in Scheme 1. As can be seen for the air-controlled electrospray in Scheme 1a, coaxial needles were used, with the inner needle supplied with solution and outer shell with compressed building air. The air plays a crucial role in the electrospray process, as it helps atomize the big droplets into small ones so that electrostatic force is able to bring them to target. Similar to electrospinning, solvent (in this case, H2O) can be simultaneously evaporated during the process. If no air supplied (Scheme 1b), only a big droplet at the needle tip can be observed, which gradually drips down due to gravity. The slurry for electrospray is a simple aqueous mixture of active materials and GO suspension. Here, we mainly selected commercial silicon nanoparticles and silicon monoxide microparticles as active materials to demonstrate the fabrication of binder-free, highly adhesive, and flexible electrodes due to their high specific capacity. During the electrospray process, GO acts like fishnet to hold silicon nanoparticles inside. One by one these GO nets nicely packed up to form a uniform coating layer (Scheme 1a right). Large surface area deposition can be achieved by multi-nozzle system and adjusted by nozzle number. The thickness of deposited layer and loading can be controlled by time variable. Note that starting directly with thick GO semisolid suspension (Fig. 1a, b), is one of the most critical steps for obtaining uniform coating with good adhesion due to its excellent dispersion in water and unique liquid crystal property. For comparison, we also used GO dry powder from the


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same company or powder dried directly from GO suspension. The commercial dry GO powder is very fluffy and bulky so that most float on water surface (Fig. 1c); while the GO suspension diluted with the same amount of water (Fig. 1d) disperses very well. If the GO sheets dried from suspension were redispersed in H2O via sonication, the semisolid texture can’t be recovered (Fig. 1e), possibly because the GO sheets are severely aggregated. Both GO suspension and the redispersed powder are acidic (Fig. 1f). The acidity could be from a small amount of acid leftover from Hummer method preparation process.3, 25

Figure 1. (a) and (b) GO suspension, consisting of 98 wt % H2O; (c) purchased GO powder floating on water; (d) GO suspension diluted from (a); (e) GO suspension was dried then redispersed in water; (f) litmus test for the slurries; (g) trace of ammonium hydroxide added to the diluted GO suspension. In addition to silicon, a variety of other electrode materials (either pure or composite) can also be sprayed with GO. For spraying acid sensitive materials, trace of NH4OH (10 µL to 2.5 ml slurry) can be applied to neutralize the acid first. Once base is introduced, the semisolid turns into free flowing fluid (Fig. 1g). There is no obvious difference on the appearance of fresh


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electrodes sprayed from solution with or without NH4OH. But if folding the thermally treated sample made from basic solution, some area of the sample may exfoliate. In terms of good adhesion, active materials that are acid resistant (e.g. carbon, Si, Si/carbon nanocomposite, sulfur) suit this method most. For high loading electrodes, it is also important to calender sample before annealing in N2. 3.2. Si NP/RGO We first used nano silicon (70-100 nm) to demonstrate the formation of binder-free electrodes via air-controlled electrospray. For convenience, all the samples were named by the ratio and the name of active materials, followed by GO (as-prepared samples) or RGO (annealed samples). For example, 2_Si NP/GO means silicon nanoparticles and GO have 2:1 starting weight ratio and no thermal treatment applied; while 2_Si NP/RGO indicates the thermally treated counterpart. As can be seen in Fig. 2a, we are able to make fairly large 2_Si/GO sheet (12 in ×21 in) with very uniform, smooth, and crack-free surface. When a coin-size sheet was folded many times then reopened (Fig. 2b), no exfoliation of active materials was observed, indicating good adhesion of active material to the current collector. The topology of fresh and annealed samples has no distinguishable difference. Although the fresh and annealed samples show little difference in morphology, there is ~20% weight loss after annealing due to the removal of physically absorbed water and some oxygen-containing functional groups on GO. As shown in Fig 2c, the RGO sheets indeed act like “fishnets” to wrap silicon nanoparticles inside with very few nanoparticles on the surface. There are also some micrometer pores on the surface. During the electrochemical reaction, the RGO sheets can help resist mechanic stress during charge and discharge, giving outstanding battery stability, and the micrometer-size pores allow the fast penetration of electrolyte. SEM image of the cross section of the direct-deposited electrode in


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Fig. 2d shows that RGO and Si nanoparticles are nicely packed up and have a layer by layer structure. The layer by layer structure of Si/RGO electrode is further confirmed by the TEM image of the microtomed electrode cross section in Fig. 2e and its inset. TGA was carried out to determine the silicon and RGO ratio. Fig. 2f shows that 1_Si/GO has 52% percent silicon, which is close to the 1:1 starting ratio of raw materials; after annealing, silicon increases to 69%, close to 70%, the ratio of active material in the common drop cast or doctor blade slurry. The 2_Si/RGO has silicon loading as high as 82%, and still maintains very good adhesion on current collector without any assistance of binder polymer.

Figure 2. (a) Digital picture of a 12 in × 21 in direct deposited 2_Si NP/GO large electrode sheet; (b) folding test of a coin size electrode sheet; SEM images of (c) 2_Si NP/RGO electrodes; (d) cross section of the 2_Si NP/RGO sample; TEM images of (e) microtomed cross section of the


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2_Si NP/RGO sample and the inset is the magnified image of the area pointed by the arrows; (f) TGA of the samples. The directly deposited electrodes only containing active materials and RGO, are promising for application as LIB electrodes. They were first assembled into half-cells for evaluation. Fig. 3a shows the cyclic voltammetry (CV) curves of 1_Si NP/GO and 1_Si NP/RGO at the scan rate of 0.5 mV/s among the voltage window of 0.01-1.5 V vs. Li+/Li. For both samples, a cathodic peak around 0.2 V is observed, corresponding to the conversion of Si to LixSi alloy. The two anodic peaks around 0.33 V and 0.52 V are associated with the delithiation reaction with the phase transition from LixSi alloy to Si.5, 26, 27 It is also observed that the 1_Si NP/RGO sample has larger peak current density, which could be attributed to the higher conductivity of RGO than that of GO.

Fig. 3b compares the cycling performance of 1_Si NP/GO and 1_Si NP/RGO electrodes via galvanic charging/discharging at a current density of 1 A g-1 in the voltage range of 0.01-1.5 V (vs. Li+/Li). It must be noted that all the current density and specific capacity are calculated on the basis of total electrode materials rather than Si only; and the first five cycles were tested under a low current density of 0.1 A g-1 as activation cycles. As can be seen, both 1_Si NP/GO and RGO samples show excellent long term cyclic stability, and better than that of the Si sample made by conventional slurry coating. Among the three samples, 1_Si NP/RGO demonstrates the best performance, delivering a specific capacity of 997 mA h g-1, 1188 mA h g-1, and 903 mA h g-1 for the 20th, 50th, and 100th cycle, respectively. It only shows a total of 94 mA h g-1 capacity loss over 80 cycles (from the 20th to 100th cycle). While the 1_Si NP/GO sample shows 544 mA h g-1 , 586 mA h g-1, and 566 mA h g-1 for the 20th, 50th, and 100th cycle, respectively. The


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specific capacity is lower than of that of its thermal treated counterpart (1_Si NP/RGO), due to the fact that the thermal treatment helps reduce graphene oxide, thus lower down the total weight of the composite and increase conductivity.

Figure 3. (a) cyclic voltammetry curves of 1_Si NP/GO and 1_Si NP/RGO; (b) cycle performance of the 1_Si NP/GO, 1_Si NP/RGO and Si electrode prepared by conventional slurry coating; (c) cycle performance of 2_Si NP/RGO, the inset shows the thickness measurement of


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2_Si NP/RGO and commercial graphite electrode with similar capacity, blank copper foil is 9 µm thick. The first 5 cycles were tested at 100 mA g-1, afterwards, at the current density of 1 A g1

; (d) the first cycle Coulombic efficiency (CE) of 1_Si NP/GO, 1_Si NP/RGO, and 2_Si

NP/RGO; (e) voltage profile of the 2_Si NP/RGO and LiCoO2 full cells with and without prelithiation; (f) cycle performance of the 2_Si NP/RGO and LiCoO2 full cells with and without prelithiation. We further increased the loading of Si and the electrochemical result is displayed in Fig 3c. The 2_Si NP/RGO (82wt% Si) has its specific capacity remain above 1500 mA h g-1 over 100cycles, which is higher than that of other reported Si and graphene electrodes with similar layered structure. For example, the five-layer graphene/Si structures on Cu foil prepared by Ji et al. has 30th specific capacity of 1320 mA h g-1 at a low current of C/40;6 Even though multilayered Si NP/RGO on Ni foam delivered around 1500 mA h g-1 at 1 C after 100 cycle, it only has a very low total loading of active material, 0.2 mg;19 the multilayered Si NP /RGO on Cu foam also delivers similarly high specific capacity but at a low current density 0.2 A g-1, one fifth of the current density we applied.20

In addition, we compared our electrode with

commercial graphite electrode sheet to briefly demonstrate its high volume energy density. According to the provider’s information, the commercial graphite electrode has specific capacity around 330 mA h g-1. And each coin cell size electrode has active material loading ~6.5 mg, giving capacity around 2 mAh per piece. 1.0 mg of 2_ Si NP/RGO with specific capacity around 2000 mA h g-1 can give similar capacity. Delivering similar capacity, 2_Si NP/RGO has a thickness around 8 µm (copper foil is 9µm thick), which is much thinner than graphite electrode, 52 µm, as shown in the inset of Fig. 3c. Considering the simplicity of our method, and the


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competitive performance obtained under a relative high loading and current density, the Si NP/RGO we prepared is promising for next generation high performance electrodes.

For practical application, it is important to evaluate full cell performance, namely, pairing up anodes with cathodes rather than lithium foil in half cell. The performance could have some difference with the same anode in half cell and full cell, because the number of lithium ions in cathodes is very limited in comparison with the infinite reservoir of Li in half cell. Additionally, the first cycle usually involves the formation of solid-electrolyte interphase (SEI) layer that irreversibly consumes Li ions, resulting in low first cycle Coulombic efficiency. Considering the limited Li ion source in cathode, it is desired to have high first cycle Coulombic efficiency. Fig 3d shows that annealing and increasing silicon amount both help increase the Coulombic efficiency, and the 2_Si NP/RGO has CE of 82%. Although it is higher than that of many Si/carbon, the 18% capacity lost in first cycle has to be compensated in full cell.19, 20, 28-30 In our full cell experiments, the initial capacity loss issue is tackled via two ways: a) supply extra LiCoO2 cathode to compensate the 18% percent loss; b) pre-insert Li ions into 2_Si NP/RGO anodes before cell assembly via a reported prelithiation method, then match with LiCoO2 cathode at the exact capacity ratio of 1:1.22 Since the performance of full cell is influenced by many aspects including anode, cathode, matching ratio, electrolyte, prelithiation, etc., 29, 31 the optimization experiments are still going on. Some preliminary results are presented. Fig. 3e shows the voltage profile of nonprelithiated and prelithiated cells. As can be seen, the charge capacity for non- and prelithiated cell are 2871 mA h g-1 and 2398 mA h g-1, and the cells deliver discharge capacity around 2373 mA h g-1 and 2298 mA h g-1, respectively, giving the 1st cycle CE of 82% and 95% for non- and prelithiated cells. The cyclic performance (Fig. 3f) shows


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that the prelithiated cell has slightly better full cell capacity retention, suggesting that supplying extra lithium ions via prelithiation could be a good way to increase full cell CE and cyclic performance even with less LiCoO2. 3.3. SiO MP/RGO

Figure 4. SEM images of (a) SiO MP; (b) top view of 2_SiO/RGO; Side view of 2_SiO MP/RGO (c) before calendering and (d) after calendering; (e) calendered 2_SiO MP/RGO with areal loading around 1.6 mg cm-2; (f) electrode film thickness change before and after


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calendering; (g) cyclic performance of 2_SiO MP/RGO half cell; (h) cyclic performance of 2_SiO MP/RGO and LCO full cells. In addition to nanosize silicon (70-100 nm), SiO micro particles (3-10 um) were also selected to demonstrate that micro size particles smaller than GO sheets can be accommodated. As shown in Fig. 4a, the SiO microparticles have irregular shapes and a wide size distribution. After being sprayed with GO, most microparticles can be well wrapped by RGO (Fig. 4b) in the binder-free electrodes. The cross sections of the electrode in Fig. 4c and d show SiO MP and RGO sheets are alternatively stacked together. The as-prepared SiO MP and RGO electrode layer are loosely packed. After calendering, the electrode layers become much denser (Fig 4d). The quantitative result in Fig. 4f shows the electrode thickness can be reduced to half of the one before calendering. The electrochemical performance of the 2_SiO MP/RGO was also investigated. For half-cell study (Fig. 4g), the 2_SiO MP/RGO exhibits a stable cyclic specific capacity around ~1000 mA h g-1 over 100 cycles at the current density of 0.5 A g-1, and the first CE is 72%. The 2_SiO MP/RGO was also paired up LCO to make full cells. In the non-prelithiated cells, extra amount of LCO required to match the 28% first cycle capacity loss was supplied, while the prelithiated cells have the exact LCO amount required to match the capacity of 2_SiO MP/RGO, no excess supplied. The non prelithiated and prelithiated cells give a first cycle CE of 69% and 93%, respectively. The prelithiated full cell (Fig. 4h) shows slightly higher specific capacity over 100 cycles even with less LCO.


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3.4. Other materials

Figure 5. SEM images of (a) MoO3 nanobelts prepared by hydrothermal method; (b) top view and (c) side view of 2_MoO3/RGO electrodes; (d) Sn_CNF prepared by electrospinning; (e) 2_Sn/RGO; (f) 2_S/GO; Cycle performance of (g) 2_MoO3/RGO and 2_Sn_CNF/RGO, and (h) 2_S/GO (all the calculation is based on total mass loading).


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To demonstrate the versatility of this electrospray approach, other materials including homemade MoO3 nanobelt, Sn_CNF nanocomposite, and commercial sulfur particles, were also sprayed with GO. The MoO3 nanobelts (Fig. 5a) have width around 200 to 300 nm and length around 1-3 µm. After spraying with GO, GO sheets are still able to cover most of the large nanobelts underneath, with only few lying on the surface (Fig. 5b). The side view (Fig. 5c) shows MoO3/RGO are assembled into layer structure. Additionally, Sn_CNF (Fig. 5d) and micrometer size commercial sulfur particles are also able to be wrapped by GO and secured on collector (Fig. 5e and f). Their electrochemical performance is shown in Fig. 5g and h. As can be seen, 2_MoO3/RGO and 2_Sn_CNF/RGO both show good cyclic stability, and the directly deposited 2_S/GO shows encouraging performance with gradual capacity decay. The 2_S/GO electrochemical performance is largely limited by the low conductivity of both sulfur and GO, given that the electrode was assembled into cells right after spray without thermal treatment due to the low sublimation temperature of sulfur. Appling low-temperature pre-reduction of GO before spraying with sulfur may be a way to increase GO conductivity.32,

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However, low-

temperature reduction usually requires toxic chemicals. 4. Conclusion In summary, we have developed a simple, cost-effective, and scalable air-controlled electrospray method for directly fabricating active materials/GO layered structure onto current collector. The starting solution is an aqueous mixture of active materials and inexpensive chemically oxidized GO suspension, no binder, organic solvent, or extra conductive carbon required. The obtained electrodes were tested in both Li-half cells and full cells, showing excellent cyclic stability and high specific capacity. Other electrode materials (e.g. MoO3, Sn/CNF, and sulfur particles) in composite with RGO can also be prepared via the same method.


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This facile process together with the use of inexpensive GO suspension holds great potential for large scale production of environmental-friendly, binder-free, and high-performance electrodes especially silicon based ones. Beyond its application for battery electrodes, this approach can also be applied for other types of electrode preparation such as supercapacitor, fuel cell, solar cell, etc. AUTHOR INFORMATION Corresponding Author *Email: [email protected] ACKNOWLEDGMENT This work was partially supported by Axium Nanofiber, LLC (Cornell OSP No. 71984) and Dongjin Semichem, Ltd (Cornell CCMR ICP, M01-9122). All the material characterizations were obtained via facilities at the Cornell Center for Materials Research (part of NSF MRSEC Program, Grant DMR 1120296).


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Table of Content


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Graphene Oxide Involved Air-Controlled Electrospray for Uniform, Fast

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