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Carbon 142 (2019) 51e59

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Carbon

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One-step solution combustion synthesis of CuO/Cu2O/C anode for longcycle life Li-ion batteries

Chunxiao Xu a, b, Khachatur V. Manukyan c, Ryan A. Adams b, Vilas G. Pol b,Pengwan Chen a, Arvind Varma b, *

a State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, PR Chinab Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, 47907, United Statesc Department of Physics, University of Notre Dame, Notre Dame, IN, 46556, United States

a r t i c l e i n f o

Article history:Received 22 June 2018Received in revised form2 October 2018Accepted 4 October 2018Available online 5 October 2018

Keywords:Lithium ion batteryAnodesCopper oxide/carbon compositeSolution combustion synthesis

* Corresponding author.E-mail address: avarma@purdue.edu (A. Varma).

https://doi.org/10.1016/j.carbon.2018.10.0160008-6223/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Using glucose (C6H12O6) and copper nitrate (Cu(NO3)2) as fuel and oxidizing agent respectively, CuO/Cu2O/C composites with different carbon contents were successfully prepared by the solution com-bustion synthesis method. The as-obtained CuO/Cu2O nanoparticles exhibited uniform sphericalmorphology and, by changing the amount of fuel and ambient temperature, carbon was synthesized in-situ with content ranging from 3 to 36wt%. The electrochemical performance of the CuO/Cu2O/C anodein Li-ion batteries was investigated systematically, demonstrating >400 mAh g�1 capacity at 20mA g�1

current density and highly stable cycling performance with capacity 260 mAh g�1 after 600 cycles atcurrent density 0.2 A g�1. This performance is attributed to the synergistic effect of anodes porousstructure, conducting carbon coating and two-component CuO/Cu2O structure. Owing to the inexpensiveand facile solution combustion synthesis method and the resulting high electrochemical performance,the CuO/Cu2O/Carbon composite is a promising anode material for application in Li-ion batteries.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

With increasing demand of lithium-ion batteries (LIBs) forimportant commercial applications such as electric vehicles,portable electronics and stationary storage, considerable effort hasbeen devoted to developing alternative electrode materials withhigher specific capacity, better conductivity, and superior cyclingstability as compared to the traditional graphite/LiCoO2 system[1e4]. On the anode side, nanostructured transition metal oxides(MxOy, M¼ Fe, Ni, Cu, Zn, Mn, Mo, etc.) are considered among thebest choices owing to their superior theoretical capacities [5e8]. Inparticular, copper oxides, such as CuO and Cu2O, have attractedattention because of their abundance, low-cost, safety and chemicalstability. Compared with Cu2O, CuO has a higher theoretical specificcapacity of 674 mAh g�1 vs. 369 mAh g�1. Similar to other metaloxides, however, their practical application is hindered by theirrelatively poor conductivity and large volumetric variation duringcharge/discharge processes, which inevitably causes electrode

pulverization and leads to rapid capacity fading [9].In order to address these limitations, various strategies have

been explored, including preparing two-component structures orcombining with other conductive materials [10e12]. Two-component structures that integrate different functional metaloxides could display a superior synergistic effect, which improvesthe intrinsic properties of each component such as electrochemicalreactivity and electrical/ionic conductivity [13,14]. Chen et al. pre-pared CuO/Cu2O hollow polyhedrons with porous shells thatexhibited a lithium storage capacity of 740 mAh g�1 at 100mA g�1

[11]. Pan et al. showed that Cu-based MOF derived porous CuO/Cu2O hollow octahedrons yielded a reversible capacity of 415 mAhg�1 at 50mA g�1 for Na-ion batteries [15]. Nevertheless, the ratecapability and cycling stability of these anode materials should beimproved further for commercial requirements. Moreover, thecombination of metal oxides with conductive materials, such asgraphene and carbon, can accommodate the metal oxide volumeexpansion and increase the electrical conductivity, thus leading tothe full utilization of active materials with electrochemical stability.Zhang et al. demonstrated that Fe2O3/Co3O4/rGO compositesderived by the annealing of MOF/GO fibers exhibited exceptionalcycling stability, owing to the uniform distribution of porous metal

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C. Xu et al. / Carbon 142 (2019) 51e5952

oxide particles on the carbon sheets [16].To date, reports on CuO/Cu2O/carbon composites are limited.

Lee et al. synthesized graphitized carbon supporting CuO/Cu2Othrough a Cu-based metal�organic framework via a one-stepthermal transformation process, which exhibited good electro-chemical stability with excellent capacities of 887 mAh g�1 at60mA g�1 and 303 mAh g�1 at 50mA g�1 for Li and Na-ion battery,respectively [12]. Using graphene oxide as additional carbonsource, Du et al. prepared CuO/Cu2O nanospheres/graphene com-posites by microwave irradiation with subsequent annealingtreatment, which exhibited reversible capacity as high as 487 mAhg�1 at 200 mAg�1 after 60 cycles as Li-ion battery anode materials[17]. In this regard, a combination of favorable two-componentstructure and multiple compositions with carbon is desirable forexcellent cycling performance of electrode materials. In this work,we demonstrate a facile solution combustion method for the syn-thesis of CuO/Cu2O/carbon composite as an anode material for Li-ion batteries.

Solution combustion synthesis (SCS) is a one-step method forthe fabrication of metal oxide nanomaterials with controllablephase structure, composition, oxidation state, and grain size[18e21]. The process involves self-sustained exothermic reactionsbetween metal nitrates oxidizers, and fuels such as urea, glycine,and glucose. In SCS, the liquid state of the precursors results inuniform mixing of the reactants at the molecular level, thuspermitting precise formulation of the desired composition. Thehigh reaction temperature also ensures high product purity andcrystallinity. In addition, the short reaction duration and formationof large quantities of gases such as CO2, N2 and H2O favor thesynthesis of nanoscale materials with porous structure and highspecific surface area [19,21,22]. Benefiting from its many features,various transition metal oxides (e.g., Fe3O4, Co3O4, NiO, MnO) andcomplex materials (e.g., Li4Ti5O12, NiO/Ni) have been prepared aspotential anode materials for rechargeable lithium-ion batteries[23e27], and we have also reported the solution combustion syn-thesis of ZnCo2O4 anode materials in an earlier study. The productexhibited a mesoporous structure with a tap density 1.48 g cm�3,which accommodated volumetric changes during lithiation,resulting in reversible lithium storage capacity of 1000 mAhg�1 at C/2 rate and 950 mAh g�1 at 1C rate after initial formationcycles [28]. As an excellent conductive support, however, nanosizedcarbon is difficult to obtain by the one-step SCSmethod, and carboncomposites coupled with metallic oxide have not been reportedpreviously. Thus, to our knowledge, this is the first study demon-strating in-situ synthesis of carbon and porous CuO/Cu2O/C com-posites via the SCS process, and notably, the carbon content rangingfrom 3 to 36wt%. can be easily tuned by varying the synthesisparameters. The parameter space such as fuel-to-oxidizer ratio andtemperature of SCS are explored, and the relationship betweensynthesis conditions, physicochemical properties, and electro-chemical performance of the as-prepared sample is developed. TheCuO/Cu2O/C composites offer excellent cyclability and rate capa-bilities in lithium-ion battery with the advantage of the porousstructure, carbon coating and the synergistic effect between CuOand Cu2O.

2. Experimental

2.1. Preparation of the CuO/Cu2O/C composites

For the typical synthesis process, copper nitrate hydrate[Cu(NO3)2.3H2O, Alfa Aesar, 98%] in stoichiometric amount wasdissolved in distilled (10ml) water as metal oxidizer, and thenadded to glucose (C6H12O6, Alfa Aesar, 99%) as fuel to form theoriginal precursor solution. After mixing of the obtained solution,

the mixture was placed in a heating jacket with a constant tem-perature of 300 �C, 350 �C and 400 �C (Defined as ambient tem-perature) respectively to induce combustion under air condition.The amount of fuel was determined by the fuel to oxidizer ratio (f),which can be represented in the combustion reaction stoichiom-etry as shown below:

CuðNO3Þ2,3H2Oþ ðf�712

ÞC6H12O6 þ16ðf� 1ÞO2/CuO

þ 6ðf� 712

ÞCO2 þ ½6ðf�712

Þ þ 3�,H2Oþ N2 (1)

Where f¼ 1 implies that all oxygen required for completecombustion of fuel derives from the oxidizer, while f> 1 ( 3)could also result in decomposition of glucose, thus generatingcarbon products. The f values ranged from 0.8 to 50 in this work,and the effects of f and ambient temperature were investigated indetail for the SCS characteristics and the carbon content of resultingproducts. The temperature-time histories of the SCS processes wasmeasured through a K-type thermocouple (0.2mm diameter,Omega) with the sampling rate of 10 data points per second. Thethermocouple was located inside the liquid phase prior to heating.The combustion behavior was also monitored by video recording.Ideal CuO/Cu2O/Carbon products are synthesized at f¼ 10 under350 �C ambient temperature, which lead to 26wt% carbon content.

2.2. Materials characterization

The morphology of the product was investigated using scanning(SEM, FEI Magellan 400) and transmission (TEM, FEI Tecnai G2 20)electron microscopy. The structures were characterized through X-ray diffraction (XRD) patterns using a Rigaku SmartLab X-RayDiffractometer with a Cu Ka radiation source (l¼ 1.541 Å) and apower 40 kV� 40mA. Nitrogen adsorption/desorption experi-ments were conducted at 77 K using a Quantachrome NOVA 2200eapparatus. Before adsorption measurements, the samples weredegassed under vacuum at 473 K for 12 h. Thermogravimetricanalysis was conducted using TGA i-1000 to determine carboncontent and phase structures with temperature ranging from 25 to800 �C. The 10 �C min�1 heating rate, and 10 cm3min�1 of air and10 cm3min�1 of argon flow rates were used during the test. Ramanspectra were recorded using a Thermo Fisher Scientific DXR Ramanmicroscope with a 633 nm laser excitation wavelength.

2.3. Electrochemical measurements

To prepare the working electrodes, 80wt% active materials(CuO/Cu2O/C composites as synthesized), 10wt% conductive addi-tive (Timcal Super C65) and 10wt% polyvinylidene fluoride binderwere dispersed in 1-methyl-2-pyrrolidinone solvent, with theresultant slurry uniformly coated on copper foil current collectorand dried at 80 �C for 12 h under vacuum. After drying, electrodes(12mm diameter) were punched out with active material loadingof ~2.5mg cm�2. Electrochemical tests were performed in coin-type 2032 half-cells using lithium metal as the counter and refer-ence electrode. The separator was polypropylene Celgard 2500 andthe electrolyte was 1M LiPF6 dissolved in an ethylene carbonateand dimethyl carbonate mixture with a volume ratio 1:1. The gal-vanostatic charge/discharge tests were conducted on an Arbincycling system at different current densities in the voltage rangefrom 0.005 to 3 V (vs Liþ/Li) at room temperature. Cyclic voltam-metry (CV) was performed on a Gamry-600 reference system at a

C. Xu et al. / Carbon 142 (2019) 51e59 53

scan rate of 0.1mV s�1 in the potential range of 0.01e3 V (vs Liþ/Li).Potentiostatic electrochemical impedance spectroscopy (EIS) wascarried out using the same instrument in the frequency range0.01 Hze106 Hz with a 5mV perturbation amplitude on fresh cellsat 3 V open circuit voltage (OCV).

3. Results and discussion

3.1. SCS reaction characterization

The typical measured temperature-time profiles and corre-sponding process images for the reactions with an ambient

Fig. 1. (a) SCS reaction temperature�time profile and (b) corresponding process images forreaction; StageⅢ: cooling. (c) SCS reaction temperature�time profile and (d) correspondievaporation; StageⅡ: solution reaction for the synthesis of Cu; StageⅢ: VCS reaction with ox(f¼ 1, 2, 3, 4, 5, 10, 30, 50) and ambient temperatures (300 �C, 350 �C and 400 �C). (A colo

temperature of 300 �C at f¼ 1 (stoichiometric) and f¼ 10 (fuel-rich) are displayed in Fig. 1. As may be seen, the SCS reactions withf¼ 1 exhibit conventional self-propagating high-temperaturesynthesis (SHS) mode, which is characterized by one temperaturepeak with maximum temperature 723 �C (Fig. 1a). The solution isignited from a middle point and then it self-propagates as a com-bustion wave along the reaction media within ~10s (Fig. 1b). Theignition temperature is ~130 �C, which corresponds to the decom-position of copper nitrate hydrate, as release of HNO3 [29e32], ornitrogen oxides (e.g. NO2, N2O) [33e35] that trigger the combustionreaction with the fuel. By contrast, volume combustion synthesis(VCS) mode is shown at f¼ 10 (Fig. 1c). Excess water is evaporated

f¼ 1 with the ambient temperature of 300 �C. StageⅠ: water evaporation; StageⅡ: SHSng process images for f¼ 10 with the ambient temperature of 300 �C. StageⅠ: waterygen in air; StageⅣ: cooling. (e) Maximum temperatures profiles for various f valuesur version of this figure can be viewed online.)

C. Xu et al. / Carbon 142 (2019) 51e5954

off and solute volume dilates significantly until the homogeneoussolution becomes viscous and gel-like intermediates. The temper-ature continues to increase and then ignition occurred at around400 �C. The combustion wave fronts start at different hotspots ofthe intermediates and propagate over the reaction precursor(Fig. 1d). In this study, the transition from the SHS to VCS modeoccurs at f¼ 3. In addition, excess fuel enables further reactioncompletion and heat generation, thus the burning duration isextended longer from seconds to minutes as f increases from 1 to10.

It is worth noting that while at f¼ 1 the temperature profile hasonly one ignition temperature, the reaction at f¼ 10 is character-ized by two ignition temperatures corresponding to different re-action zones, that is, a relatively slower temperature rise at thebeginning of the reaction (StageⅡ, Fig. 1c), followed by a rapidtemperature increase to even higher values (StageⅢ, Fig. 1c). Thefirst represents the case of the synthesis of Cu, while the secondthat of Cu reaction with oxygen from air. As such, SCS can be trig-gered at high f (f range from 5 to 50) for the first time and theambient temperature significantly influences the combustionconditions. It is observed from the maximum temperature profilesshown in Fig. 1e, that SCS is not induced in solution for f� 10 withthe ambient temperature of 300 �C, while the increase of theambient temperature from 300 �C to 400 �C can initiate the com-bustion with relatively higher maximum temperature.

XRD analysis further confirms the mechanism for the SCS ofCuO/Cu2O/Carbon composites. As shown in Fig. 2a, pure CuO issynthesized under fuel-lean condition (f¼ 0.8) as expected. Excessfuel could result in a reducing environment during the reaction,and lead to the formation of pure metal (e.g., Ni, Cu) or alloy(Ni�Cu�Fe), instead of metal oxide [33,36e38]. Accordingly, metalCu is the principal phase when f¼ 1. Reactive solutions with f¼ 1typically yield oxide-rich products in most SCS systems. We believethat decomposition of glucose generates gas (and/or solids) capableof reducing copper oxides tometallic Cu even at low fuel to oxidizerratios. With increase of f, the diffraction peaks of Cu decrease.Simultaneously, oxygen from air is involved in the reaction and Cuparticipates in the following secondary combustion with oxygenand transforms into CuO (f¼ 3, 5). The XRD pattern at f¼ 10displayed in Fig. 2b directly shows the synthesis of metal Cu duringstageⅡ, and the final CuO and Cu2O products (after stageⅢ),consistent with the temperature-time curve (Fig. 1c). In addition,SCS under oxygen-free environment leads to pure metal Cu underthe same condition, which could also reveal the secondary reactionbetween Cu and O2 indirectly (Figure S1). Thus, pure metal Cu isfirst synthesized as the intermediate product under the combustionof copper nitrate hydrate and glucose. Meanwhile, excess fuel ofglucose decomposes to produce carbon, which deposits on the

Fig. 2. (a) XRD pattern of the SCS products for various f values (f¼ 0.8, 1, 3, 5) with ambienproducts with various f values (f¼ 10, 30, 50) and ambient temperatures (300 �C, 350 �C

as-obtained Cu substrate. With development of the combustionprocess, Cu continues to react with oxygen under air condition andCuO/Cu2O/C composite is formed as the end product.

The carbon content of CuO/Cu2O/Carbon composites is esti-mated by TGA and element analysis (EA) method. As shown inFig. 2c, the weight decreases steadily within the samples from 300to 500 �C, which is attributed to carbon oxidation into gases. Thereis negligible further weight loss beyond 500 �C, and the residueamount is around 84% of the initial weight for f¼ 10 with theambient temperature of 400 �C. Considering that the CuO/Cu2O/Carbon composites are synthesized under high temperature con-dition (>700 �C) before TGA analysis, the release of water is negli-gible. Thus, the carbon content is identified as the total weight loss,which is calculated to be 16% for f¼ 10, 7.3% for f¼ 30 and 3.2% forf¼ 50with the ambient temperature of 400 �C, consistent with theEA results (Table S1). Noticeably, in addition to the f values, thecarbon content is related to the ambient temperature, with 26% and36% for f¼ 10 with the ambient temperature of 350 �C and 300 �C,respectively. Therefore, the high-temperature oxidation of Cuduring stageⅢ is accompanied by carbon oxidation, thus loweringthe ambient temperature is conducive to increasing carbon contentof the CuO/Cu2O/Carbon products.

3.2. Materials characterization

The crystalline phases of the CuO/Cu2O/Carbon composites withvarious f values and ambient temperature were characterized byXRD measurements (Fig. 2a and b). For comparison, the corre-sponding samples after acid etching were also tested (Figure S2). Asshown in Fig. 2b, the characteristic peaks of the SCS productscorrespond to Cu (JCPDS 85e1326), Cu2O (JCPDS 78e2076) and CuO(JCPDS 80e1917). Ideal CuO/Cu2O/Carbon products are synthesizedat f¼ 10 with the ambient temperature of 350 �C, while the broadpeak of the graphitic carbon located around 24� is also displayedafter etching (Figure S2). The relative contents of the CuO and Cu2Ophases were calculated to be 93wt% and 7wt%, according to thecalculated values using equation ICuO (111)/(ICuO (111) þ ICu2O (111))[12,39,40].

The SEM images for different f values with the ambient tem-perature of 350 �C are shown in Fig. 3, S3. As may be seen, theproduct materials consist of carbon nanosheets and CuO/Cu2Onanoparticles, and the solid spheroidal nanoparticles aggregate toform irregularly sized and shaped secondary clusters. Meanwhile,an overall increase in micrometer particle size with increasing ffrom 5 to 10 is observed owing to higher f along with higher heatrelease promoting grain growth (Fig. 3a), which is consistent withthe maximum temperature curve (Fig. 1e). Furthermore, the highmagnification morphology shows that the surface of CuO/Cu2O/

t temperature of 300 �C. (b) XRD pattern and (c) TGA of as-obtained CuO/Cu2O/Carbonand 400 �C), respectively. (A colour version of this figure can be viewed online.)

Fig. 3. Characterization of as-obtained CuO/Cu2O/Carbon composites at f¼ 10 with the ambient temperature of 350 �C. (a,b) SEM images, (c,d) TEM images, (e) HRTEM image, (f)SAED pattern, (g) Raman spectra, and (h) The nitrogen adsorption-desorption isotherm, respectively. Inset in (h) is the corresponding pore size distribution curve. (A colour versionof this figure can be viewed online.)

C. Xu et al. / Carbon 142 (2019) 51e59 55

C. Xu et al. / Carbon 142 (2019) 51e5956

Carbon composites is rough (Fig. 3b), the nanoparticles are stronglyanchored on the surface of in-situ formed carbon nanosheets with ahigh density during the SCS process, which could facilitate rapidelectron transport between the underlying carbon nanosheets. Thecontact between nanoparticles and carbon nanosheets can also beobserved from the TEM images. As shown in Fig. 3c,d and S4, theCuO/Cu2O nanoparticles are coated with carbon sheet, and thehigh-resolution TEM (HRTEM) image shows that the interspacebetween CuO/Cu2O grains is also covered by ultra-thin layer ofcarbon (~2 nm) (Fig. 3e). The crystal lattice fringes with a d-spacingof 0.25 nm arise from the (002) planes of CuO, which can also beconfirmed in the selected area electron diffraction (SAED) (Fig. 3f).

Further insight into the morphologies of the CuO/Cu2O/Carboncomposites at nanoscale level is obtained using Raman spectros-copy. As shown in Fig. 3g, the as-prepared sample atf¼ 10with theambient temperature of 350 �C shows the typical Cu2O peak atabout 219 cm�1 and two peaks at about 294 cm�1 and 339 cm�1

corresponding to CuO. In addition, the samples represent twotypical bands indexed at ca.1332, and 1591 cm�1, which correspondto D band (sp3 defects) and the G band (pristine sp2 lattice) of thegraphitic layers. The D band is attributed to the vibration of defectsand disorder in the hexagonal basal carbon planes [41], and thusthe strong D band with a high ID/IG ratio of 1.00 observed in theCuO/Cu2O/Carbon composite with f¼ 10 indicates the presence ofan amorphous structure during the SCS process. Furthermore, thepeaks of CuO and Cu2O completely disappear after acid etching, andthe carbon remains unchanged and stable, which could facilitate toform the conductive networkwith CuO and Cu2O nanoparticles andpromote charge transfer during the electrochemical reaction.

The BET surface area and DFT pore size distribution analysesbased on low temperature nitrogen adsorption measurements areshown in Fig. 3h. The synthesized CuO/Cu2O/Carbon compositesexhibit a typical hysteresis loop which can be assigned to type-IVisotherm in the IUPAC classification, and the sample with f¼ 10has specific surface area 41m2 g�1 and total pore volume0.024 cm3 g�1. The moderate surface area can be attributed to theeffect of volume expansion in the precursors using glucose as fuel,and the subsequent combustion process that releases gases,generating the porous structure in the CuO/Cu2O/Carbon products.Pore size distribution is observed to exhibit hierarchical porestructure with micropores centered at 1.23, 2.72 and 4.49 nm andbroad mesopores distributed around 5e8 nm, which may facilitateLiþ insertion and rapid diffusion during charging/discharging cycles[12,17].

3.3. Electrochemical characterization

Optimal carbon content is critical to obtain enhanced electro-chemical performance and the synthesized CuO/Cu2O/Carboncomposites at f¼ 10 with different ambient temperature (300 �C,350 �C and 400 �C) were investigated as LIB anode in this work(Fig. 4, S5, S6). Fig. 4a displays the charging/discharging behaviorprofiles of CuO/Cu2O/Carbon composites (at f¼ 10 with theambient temperature of 350 �C) for the first 4 cycles at a currentdensity of 0.02 A g�1 with a voltage range of 0.01e3.0 V. An initialdischarge capacity of 887 mAh g�1 is achieved, and the reversiblecapacity maintains a high level of 392 mAh g�1, corresponding to acoulombic efficiency of 44%. This first cycle capacity loss is due toformation of the solid electrolyte interphase (SEI) passivation layer,with irreversible trapping of lithium ions and electrolyte decom-position, with SEI stabilization observed after the first cycle [42,43].

Cyclic voltammograms (CV) are studied for further under-standing the Li-ion storage process at a scan rate of 0.1mV s�1 from0.01 to 3.0 V. As shown is Fig. 4b, three apparent peaks centered at1.68, 0.96, and 0.56 V are observed in the first discharge of CuO/

Cu2O/Carbon composite, which could be attributed to thesolid�solution process generating intermediate phasesCuII1�xCuIxO1�x/2, the formation of Cu2O phase, and reduction ofCu2O to Cu, respectively [44]. The current peaks at 0.96 and 1.68 Vshift to and maintain higher potentials of 1.31 and 2.17 V corre-spondingly in the following scans, which is attributed to thestructural changes of the metal oxide [45]. Meanwhile, three mainpeaks located at ca. 0.10 V, 1.25 V and 2.46 V appear simultaneouslyin the Li-extraction process. Of these, the peak at 0.1 V can beassigned to lithium extraction from graphitic layers, while those at1.25 and 2.46 V are associated with oxidation of copper particles toCu2O and CuO respectively, implying the multiple lithium storagesites in the CuO/Cu2O/Carbon composite [46,47]. The CV curves areoverlapped after the first cycle which further demonstrates theexcellent electrochemical reversibility and capacity retention of theCuO/Cu2O/Carbon electrodes.

Fig. 4c displays the rate capacity performancewith variable ratesrange from 0.02 to 2 A g�1 for CuO/Cu2O/Carbon composites atf¼ 10 with the ambient temperature of 350 �C. The reversible ca-pacities are ca. 381, 325, 292, 255, 203, 155 and 104mAh g�1 at0.02, 0.05, 0.1, 0.2, 0.5 1 and 2 A g�1, respectively. Moreover, anincreased capacity value of 491 mAh g�1 is obtained at 0.02 A g�1

after cycling at different rates, demonstrating the highly stablecycling and good reversibility even at fast charging rates for theCuO/Cu2O/Carbon anode. The capacity increase duringchargeedischarge cycles is a common feature of CuO and othermetal oxide anodes, which normally arises from the reversibleformation and dissolution of gel-like polymeric species in the SEIfilm due to the catalytic activity of metal in the anodes, which formsupon full lithiation [44].

Fig. 4d displays excellent cycling performance and Coulombicefficiency of CuO/Cu2O/Carbon anodes at a current density of0.2 A g�1. As may be seen, the reversible capacity of CuO/Cu2O/Carbon at f¼ 10 with the ambient temperature of 350 �C (26%carbon content) is stabilized at ca. 285 mAh g�1 over 150 cyclesgradually, and the Coulombic efficiency remains near 100%. Incomparison, the product with the ambient temperature of 300 �C(36% carbon content) displays relatively lower capacities (239 mAhg�1), but remains well-maintained after cycling over 150 cycles,while the product with an ambient temperature of 400 �C (16%carbon content) shows higher capacity which drops rapidly. Thus,the electrochemical performance of the composite anode dependssignificantly on the carbon content, which can improve the elec-tronic conductivity and structural integrity due to the cushioningeffect of carbon in the electrode during cycling. However, excessivecarbon dilutes the composite specific capacity of the electrode dueto its lower theoretical capacity value. Therefore, it is critical tooptimize the carbon content to obtain desired electrochemicalperformance, and the CuO/Cu2O/Carbon sample at f¼ 10 with theambient temperature of 350 �C shows the highest current density(0.2 A g�1) cycling performance and the Coulombic efficiency. Asshown in Fig. 4e, even after 600 cycles, this CuO/Cu2O/Carbonanode retains a capacity of 260mA h g�1 with a coulombic effi-ciency 99%, which is among the best results of previously reportedCuO and CuO/C materials [12,15,17,48e51]. The synergistic effect ofCuO/Cu2O structure could enhance its intrinsic electrical/ionicconductivity, electrochemical reactivity and mechanical stability.Also, the optimal carbon coating may offer a significant bufferingeffect for Liþ insertion/extraction induced local volume changes,thus provide good structural stability to the electrode and result inits excellent cycling stability.

To further understand the influence of carbon content on CuO/Cu2O/Carbon anode material for Li-ion battery performance, elec-trochemical impedance spectroscopy (EIS) measurements werecarried out (Fig. 5). The Nyquist plots for noncycled cells at OCV

Fig. 4. Electrochemical performance of CuO/Cu2O/Carbon composites anode at f¼ 10 with the ambient temperature of 350 �C. (a) Discharge/charge voltage profiles for the initialfour cycles at a current density of 0.02 A g�1 between 0.01 and 3.00 V. (b) CV curves for the initial five cycles. (c) Rate capacity from 0.02 to 2 A g�1. (d) Capacity vs cycle number andthe corresponding Coulombic efficiency of the CuO/Cu2O/Carbon electrodes with different ambient temperature at 0.2 A g�1 for 100 cycles. (e) cycling performance at a currentdensity of 0.2 A g�1. (A colour version of this figure can be viewed online.)

C. Xu et al. / Carbon 142 (2019) 51e59 57

(3 V) are shown in Fig. 5a, with the full range spectra shown inFigure S7a. The single semicircle at the high/middle frequency re-gion can be attributed to the charge-transfer process of Liþ at theCuO/Cu2O/Carbon e electrolyte interface, and is followed by thelinear region at low frequencies corresponding to the diffusion ofLiþ into CuO/Cu2O/Carbon [52]. The size of the semicircle decreaseswith increased carbon content, most significantly from 400 �C to350 �C, which describes the decreased charge transfer resistance of

the higher carbon content composites. After 50 charge-dischargecycles and SEI formation, Nyquist plots are shown in Fig. 5b, withfull range spectra shown in Figure S7b. The semicircle for resistanceof Liþ migration through the SEI occurs at high frequencies [52].After extended cycling, the 400 �C sample has two pronouncedsemicircles, with augmented SEI growth as compared to the lowercarbon content samples, explaining the rapid capacity fade inFig. 4d. Thus, the carbon architecture offers two major advantages,

Fig. 5. Electrochemical impedance spectra of CuO/Cu2O/Carbon composites anode at f¼ 10 with the ambient temperatures of 300 �C, 350 �C, and 400 �C. (a) Before cycling ofbattery at OCV of 3 V. (b) After 50 cycles at 3 V. (A colour version of this figure can be viewed online.)

C. Xu et al. / Carbon 142 (2019) 51e5958

namely improving conductivity for charge-transfer phenomena,and structural support to prevent oxide particle pulverization andSEI growth during cycling.

4. Conclusions

In this work, the one-step solution combustion synthesis pro-cess was successfully demonstrated for the preparation of CuO/Cu2O/Carbon composites. The spherical CuO/Cu2O nanoparticlesshow uniform distribution in the carbon matrix, and the in-situsynthesized carbon is rationally designed with content range from3 to 36wt%. The prepared composites exhibit highly stable cyclingperformance for application as anodes in Li-ion batteries, whichdeliver a capacity of 260mAh g�1 after 600 cycles at a currentdensity of 0.2 A g�1. The excellent reversibility of the CuO/Cu2O/Carbon composite can be attributed to the synergistic effect of itsporous structure, carbon coating and two-component CuO/Cu2Ostructure. Thus, the product phase and carbon content can be easilytuned by the inexpensive and facile solution combustion synthesismethod, and the CuO/Cu2O/Carbon composite is a potential high-performance anode material for application in LIBs.

Acknowledgements

Chunxiao Xu is grateful for financial support from the ChinaScholarship Council (No. 201606030089). The Slayter discretionaryfund at Purdue University supported purchase of laboratory sup-plies and materials characterization.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.carbon.2018.10.016.

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One-step solution combustion synthesis of CuO/Cu2O/C anode for long cycle life Li-ion batteries1. Introduction2. Experimental2.1. Preparation of the CuO/Cu2O/C composites2.2. Materials characterization2.3. Electrochemical measurements

3. Results and discussion3.1. SCS reaction characterization3.2. Materials characterization3.3. Electrochemical characterization

4. ConclusionsAcknowledgementsAppendix A. Supplementary dataReferences