jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta
reparation of Cu2O–Cu anode for high performance Li-ion battery vian electrochemical corrosion method
hibing Ni, Xiaohu Lv, Tao Li, Xuelin Yang ∗, Lulu Zhangollege of Mechanical and Material Engineering, Three Gorges University, 8 Daxue Road, Yichang 443002, PR China
r t i c l e i n f o
rticle history:eceived 21 April 2013eceived in revised form 10 July 2013ccepted 11 July 2013vailable online 29 July 2013
a b s t r a c t
Cu2O was directly grown on Cu foam via a facile electrochemical corrosion method by the aid of H2O2.Galvanostatic battery testing shows that the Cu2O–Cu electrode exhibits excellent cycle stability andrate capability. It delivers charge and discharge capacity about 0.76 mA h cm−2 without attenuation over100 cycles under a charge/discharge rate of 0.15 C. After testing at various rates from 0.2 to 35 C over
60 cycles, the 5th-cycle discharge capacity can resume 98.9% when lowering the charge/discharge rate to0.2 C. The performances are due to both the fine electric contact between Cu2O and Cu foam and a possibleporous architecture of Cu2O electrode. The electrochemical reaction kinetic of Cu2O–Cu electrode wasstudied by cyclic voltammetry measurement at various scan rate, which indicates the anodic and cathodicpeak currents show linear dependence on the square root of scan rate from 0.1 to 3 mV s−1, suggesting alithium ion diffusion controlled mechanism in the charge/discharge process.
Copper has been widely used in our lives and industry fieldss interior decoration, electrical wire, high temperature electro-agnetism heater, ink-jet printing and specific alloys due to its
ighly conductivity, good thermodynamics stability, fresh metal-ic luster and low cost than Ag and Au [1]. However, Cu in ambienttmospheric temperature and pressure inevitably has surface oxideayers because the copper oxide phases are thermodynamically
ore stable than pure Cu, which can be ascribed to a chemical cor-osion effect. For Cu, the formation of copper oxide on the surfaceill lead to the loss of electronical conductivity and metallic luster
s well as the enhancement of antioxidation ability of Cu. For cop-er oxide, the Cu substrate can improve its electrical conductivitynd structure stability. As a result, CuxO–Cu composite architecturehows multifunctional properties of Cu and CuxO, which may haveotential applications in some new fields.
During the past few years, copper oxides show potential appli-ation in Li-ion batteries base on a novel redox reaction mechanism2yLi+ + 2ye− + CuxOy ↔ xCu + yLi2O) [2–5]. However, copper oxideshow poor cycling performance because they possess low electri-
al conductivity and cannot maintain their integrity over severalharge/discharge cycles. In order to improve the electrochemi-al performance of copper oxides, efforts have been devoted on
combining copper oxides with matrix phase such as carbon mate-rial and/or directly growing copper oxides on conductive substrate[6–12]. In our previous study, we prepared CuxO–Cu compositeelectrode via annealing Cu foam in air atmosphere at 400 ◦C, whichexhibits good electrochemical performance as anode for Li-ion bat-teries [13]. The preparation of CuxO–Cu anode via annealing inair is essentially a chemical corrosion, which needs relative hightemperature. In addition, the CuxO layer is usually uniform andcompact because chemical corrosion is always not sensitive to thesurface morphology of metal. As we know, electrochemical cor-rosion is another typical corrosion of metal, which occurs undersolution environment and can be easily controlled via tuning thesolution environment. Furthermore, the electrochemical corrosionof metal is usually sensitive to the surface morphology of metal,which will probably lead to the formation of porous architec-ture on metal substrate. Recently, we successfully prepared porousNi(OH)2 nanowalls on Ni foam via a facile electrochemical corro-sion method, which shows excellent electrochemical performanceas anode for Li-ion battery [14]. Considering the above researchresults and the fact that Cu shows worse anticorrosive propertythan Ni, we are inspired to envision whether the electrochemicalcorrosion of Cu foam can lead to the formation of novel compositearchitecture and have potential application in Li-ion battery. Herein this paper, we report the growth of Cu2O on Cu foam via an
electrochemical corrosion method in the presence of H2O2 at lowtemperature, and the electrochemical performance of the Cu2O–Cuas anode for Li-ion battery was studied by galvanostatic batterytesting and cyclic voltammetry (CV) measurement.
420 S. Ni et al. / Electrochimica Acta 109 (2013) 419– 425
Table 1Preparation conditions of the electrodes.
Electrode H2O2 (ml) Reaction time (h)
2
2
rdraohe
2
cKm
2
(vb1aOuvtmt3
3
tc(6((cat
2
2
fcfiI
a 0.6 12b 0 12c 0.6 24
. Experimental
.1. Fabrication procedure
Cu2O–Cu electrode was fabricated by an electrochemical cor-osion method. Cu foams (100 PPI pore size, 380 g m−2 surfaceensity, 1.5 mm thick, purchased from Changsha Lyrun New Mate-ial) were placed in a 50 ml teflonlined autoclave, an appropriatemount of H2O2 and distilled water was subsequently added to 80%f its capacity. The autoclave was then sealed and placed in an oven,eated at 90 ◦C for 12 and/or 24 h. The preparation conditions of thelectrodes were shown in Table 1.
.2. Structure and morphology characterization
The structure and morphology of the resulting products wereharacterized by X-ray powder diffraction (Rigaku Ultima IV Cu� radiation � = 1.5406 A) and field-emission scanning electronicroscopy (FE-SEM JSM 7500F, JEOL).
.3. Electrochemical characterization
For fabrication of Li-ion battery, the as-prepared Cu2O–Cu foamsdisk electrode with diameter of 14 mm) were dried (120 ◦C, 24 h,acuum). Coin-type cells (2025) of Li/1M LiPF6 in ethylene car-onate, dimethyl carbonate and diethyl carbonate (EC/DMC/DEC,:1:1 v/v/v)/Cu2O–Cu disk electrode were assembled in anrgon-filled dry box (MIKROUNA, Super 1220/750, H2O < 1.0 ppm,2 < 1.0 ppm). A Celgard 2400 microporous polypropylene wassed as the separator membrane. The cells were tested in theoltage range between 0.02 and 3 V with a multichannel bat-ery test system (LAND CT2001A). The cyclic voltammetry (CV)
easurement of the electrodes was carried out on a CHI660C elec-rochemical workstation at a scan rate of 0.2 mV s−1 between 0 and
V.
. Results and discussion
Typical XRD pattern of electrode is shown in Fig. 1. The diffrac-ion peaks located at 43.3◦, 50.4◦ and 74.1◦ (marked with �)orrespond to the (1 1 1), (2 0 0) and (2 2 0) lattice plans of Cu foamJCPDS, No. 04-0836). Diffraction peaks located at 36.5◦, 42.4◦ and1.5◦ (marked by �) can be attributed to the (1 1 1), (2 0 0) and2 2 0) lattice plans of cubic Cu2O with lattice constant a = 4.260 AJCPDS, No. 65-3288). The XRD results indicate that the electro-hemical corrosion leads to the formation of Cu2O–Cu compositerchitecture. The electrochemical and/or chemical reactions duringhe hydrothermal process are likely to be as follows:
Cu + H2O2 → 2CuOH (1)
CuOH → Cu2O + H2O (2)
Fig. 2(a) is a low magnification SEM image of electrode a. As seen,ramework of porous Cu foam is preserved after electrochemical
orrosion (the insert of Fig. 2(a) is a SEM image of Cu foam). The Cuoam after electrochemical corrosion shows coarse surface, whichndicates the growth of copper oxides on the surface of Cu foam.n addition, a large number of holes as well as a small amount of
Fig. 1. XRD pattern of electrode a.
cracks on the surface of Cu foam can be clearly seen, which is due tothe unique asymmetric electrochemical corrosion of Cu that rele-vant to the porous architecture and special morphology of Cu foam.Fig. 2(b) shows a high magnification SEM image of the Cu2O–Cuelectrode, exhibiting a large number of particles with size rangesform 200 nm to 1.5 �m on the surface of a uniform layer. The insertof Fig. 2(b) is a SEM image of Cu foam with the same magnificationas Fig. 2(b), which shows clear crystal boundaries on the surface.The surface morphology of Cu foam is much different from that ofthe uniform layer of Cu2O–Cu, indicating the formation of Cu2O filmon the surface of Cu. In addition, cracks and holes on the surface ofCu2O film suggest a possible porous architecture, which is similarto that of Cu2S–Cu [15].
Galvanostatic charge/discharge cycling was carried out in thepotential window of 0.02–3.0 V versus Li. Fig. 3(a) shows the capac-ity retention and the initial three charge/discharge voltage profilesof electrode a at a rate of 0.15 C (1 C means accomplishing dischargeor charge in an hour). It can be seen that electrode an exhibits aslope potential region from 1.25 to 0.3 V in the initial dischargecurve, which is relevant to the formation of SEI and the generationof Cu [16]. The subsequent two discharge curves are much differentfrom the initial one, showing two slope potential regions (1.75–1.0and 1.0–0.3 V). The difference between the 1st and 2nd dischargecurve can be ascribed to the activation of Cu2O electrode, whichis similar to NiO that possesses similar charge/discharge mech-anism [17]. The charge curve shows no distinct variation in theinitial three cycles, exhibiting a sloping potential range from 1.0 to2.5 V, which can be ascribed to the decomposition of the SEI and theoxidation of Cu into Cu2O [18,19]. The initial discharge capacity isabout 2.08 mA h cm−2, much higher than the initial charge capac-ity (0.68 mA h cm−2). This is mainly due to the formation of solidelectrolyte interface (SEI) resulting from electrochemically drivenelectrolyte degradation, which is a phenomenon also observed inother systems operating through conversion reactions [2,20,21].The subsequent charge and discharge capacity increase slowlyalong with cycle number in the initial 10 cycles and then gradu-ally reach stable value, being both 0.76 mA h cm−2 after 100 cycles.This observation suggests a gradual participation in electrochem-ical reaction of active Cu2O along with cycling number, which issimilar to that for CuxO–Cu electrode [13]. The cyclic voltammet-ric (CV) curve of the Cu2O–Cu electrode was tested over a voltageregion from 0 to 3.0 V at a scan rate of 0.2 mV s−1. As shown in
Fig. 3(b), the profile of the 2nd and 3rd CV curve is similar, whereasan obvious difference between the first and subsequent two isfound. In the 1st cathodic scan, a strong reduction peak ranges from1.25 to 0.3 V was observed, which corresponds to the formation of
S. Ni et al. / Electrochimica Acta 109 (2013) 419– 425 421
e inse
Stprorwpcond
Fca
Fig. 2. SEM images of electrode a with low (a) and high (b) magnification. Th
EI and the generation of Cu [16]. The voltage region of the reduc-ion peak is little lower than that of the sloping potential, which is aolarization phenomenon that relevant to scan rate [17]. The strongeduction peak shows a little upward shift in the 2nd cathodic scanwing to the activation of the electrode, becoming two continuouseduction peaks ranges from 0.3 to 1.9 V, which is in agreementith the discharge curve. In the anodic scan, two weak oxidationeaks near 0.9 and 1.5 V and a strong oxidation peak near 2.45 Van be attributed to the decomposition of the SEI and the oxidation
f Cu into Cu2O [18,19], respectively. As seen, the oxidation peakear 2.45 V increases along with scan number, which is in accor-ance with the capacity variation versus cycle number. Fig. 3(c)
ig. 3. Electrochemical performance and rate capability of electrode a. (a) Capacity retentiharge/discharge voltage profiles for the initial three cycles. (b) Cyclic voltammograms at various C rates. (d) Capacity retention at various C rates.
rt of (a) and (b) are SEM images of Cu foam with low and high magnification.
shows the discharge and charge curves of electrode at various Crates from 0.2 to 35 C. Along with the increase of charge/dischargerate, the discharge potential decreases and the charge potentialincreases due to kinetic effects of the material, rendering higheroverpotential [22,23]. As shown in the rate capability in Fig. 3(d)(normalized according to the 5th cycle of the initial 0.2 C), the 5th-cycle discharge capacity is slightly reduced to 87.4%, 76.4%, 62.1%and 46.3% at rates of 0.4, 1.5, 3.0 and 8.0 C, respectively. Even aftercycling at various rates up to 35 C, the 5th-cycle discharge capacity
can restore 98.9% when lowering the charge/discharge rate to 0.2 C,showing excellent rate capability. It can be observed that the cyclestability and rate capability of the Cu2O–Cu electrode are distinctly
on of the galvanostatic test run at a rate of 0.15 C. The inset shows the galvanostatict a scan rate of 0.2 mV s−1. (c) Representative charge and discharge voltage profiles
422 S. Ni et al. / Electrochimica Acta 109 (2013) 419– 425
. (b) D
ic[
3srdssscfmiaeC
pps(fw6(
F
Fig. 4. (a) CV curves of electrode a at different scan rates between 0 and 3 V
mproved compared with that in literature [6,7,16], and the arealapacity is bigger than that of Fe3O4 and Ni(OH)2 film electrodes14,24,25].
CV curves of the Cu2O–Cu electrode at scan rates from 0.1 to.0 mV/s were shown in Fig. 4(a). As found, the reduction peakhifts to low potential region along with the increasing of scanate, whereas the oxidation peaks shift to high potential region,emonstrating the less polarization under a low scan rate. Fig. 4(b)hows the relationship between peak current and square root ofcan rate obtained from the experimental data in Fig. 4(a). Ashown, both the anodic (delithiation) and cathodic (lithiation) peakurrents show linear dependence on the square root of the scan raterom 0.1 to 3 mV s−1, suggesting a lithium ion diffusion controlled
echanism in the charge and discharge process [26,27]. This resultndicates that the electronic conductivity of Cu2O–Cu does not acts a restricting role of the electrochemical performance of Cu2O–Culectrode owing to the enhanced electric contact between Cu2O andu foam.
The effects of H2O2 and reaction time on the structure and mor-hology of the CuxO (x = 1, 2) electrode were studied. The XRDatterns of CuxO electrodes prepared at different conditions arehown in Fig. 5. As seen, the diffraction peaks of (1 1 1), (2 0 0) and2 2 0) lattice planes of Cu foam (JCPDS, No. 04-0836) are observed
or both electrodes, which located at 43.3◦, 50.4◦ and 74.1◦ (markedith �). Electrode b exhibits diffraction peaks at 36.5◦, 42.4◦ and
1.5◦ (marked with �), corresponding to the (1 1 1), (2 0 0) and2 2 0) lattice planes of cubic Cu2O with lattice constant a = 4.260 A
ig. 5. XRD patterns of CuxO (x = 1, 2) electrodes prepared at different conditions.
ependence of peak current on the square rate of scan rate for electrode a.
(JCPDS, No. 65-3288). Electrode c exhibits additional diffractionpeaks located at 35.6◦ and 38.8◦, which correspond to the (−1 1 1)and (1 1 1) lattice planes of monoclinic CuO with lattice constanta = 4.662 A, b = 3.416 A and c = 5.118 A (JCPDS, No. 65-2309). Theelectrochemical and/or chemical reactions of the formation of Cu2Oand CuO are likely to be as follows:
4Cu + O2 + 2H2O → 4CuOH (3)
2CuOH → Cu2O + H2O (2)
Cu + H2O2 → Cu(OH)2 (4)
Cu(OH)2 → CuO + H2O (5)
Fig. 6(a) is a low magnification SEM image of electrode b, whichshows coarse surface with a large number of holes. High magnifica-tion SEM image of electrode b is shown in Fig. 6(b), consisting of alarge number of nanoparticles with mean size about 100 nm as wellas a small quantity of micro-particles with mean size about 2 �m.After comparing the observed SEM images of electrodes a and b, itcan be deduced that the presence of H2O2 can promote the electro-chemical corrosion of Cu foam. Reaction time is another factor thatmay affect the electrochemical corrosion of Cu, which leads to theformation of CuxO (x = 1, 2) with different morphology. Fig. 6(c) isa low magnification SEM image of electrode c, which shows acci-dental surface, consisting of a large number of microparticles. Highmagnification SEM image of electrode c is shown in Fig. 6(d), whichexhibits a large number of cracks on the surface. In addition, themean size of these microparticles is about 3 �m, which is much big-ger than that of electrode a. The longer the reaction time the moreviolent the electrochemical corrosion of Cu foam, which leads tothe formation of CuxO (x = 1, 2) with bigger size.
The effects of H2O2 and corrosion time on the electrochemi-cal performance of the CuxO–Cu (x = 1, 2) electrodes were studied.Fig. 7(a) is the initial charge and discharge voltage profiles of theCuxO–Cu (x = 1, 2) electrode prepared at different conditions. Asseen, when applying a low current density (0.15 C for electrodeb, 0.1 C for electrode c), electrode b shows a sloping potentialregion (1.25–0.3 V) in discharge curve and a sloping potential region(1.0–2.5 V) in charge curve, whereas electrode c shows three poten-tial regions (2.1–1.27 V, plateau near 1.25 V, 0.9–0.2 V) in dischargecurve and two sloping potential regions (1.0–2.25, 2.25–3.0 V) incharge curve. The difference in discharge curve for electrodes b
and c may be relevant to the generation of CuO under prolongedelectrochemical corrosion time. Fig. 5(b) shows the capacity reten-tion profiles of Cu2O–Cu (x = 1, 2) electrodes obtained at differentconditions. As seen, the initial discharge capacity for both electrode
S. Ni et al. / Electrochimica Acta 109 (2013) 419– 425 423
F ith lowe
iteatid1iant
e
Fc
ig. 6. SEM images of CuxO (x = 1, 2) electrodes prepared at different conditions wlectrode c).
s much higher than the initial charge capacity, which may be dueo the formation of solid electrolyte interface (SEI) resulting fromlectrochemically driven electrolyte degradation. In addition, thereal capacity of CuxO–Cu (x = 1, 2) electrodes increases along withhe increasing of corrosion time and/or the adding of H2O2, whichs due to the increased active CuxO (x = 1, 2) on Cu foam. The initialischarge/charge capacity of electrodes b and c are 1.29/0.57 and.99/1.14 mA h cm−2, respectively. The discharge and charge capac-
ty increases slowly along with cycle number in the first few cyclesnd then decreases gradually along with the increasing of cycle
−2
umber, being 0.54 and 0.94 mA h cm after 100 cycles, respec-ively.
The cyclic voltammetric (CV) curves of the CuxO–Cu (x = 1, 2)lectrodes were tested over a voltage region from 0 to 3.0 V at a
ig. 7. The initial galvanostatic charge/discharge voltage profiles (a) and capacity retentioonditions.
(a), (c) and high (b), (d) magnification. ((a) and (b) for electrode b, (c) and (d) for
scan rate of 0.2 mV s−1. Fig. 8(a) is the CV curve of electrode b, whichshows similar profiles to that in Fig. 3(b). Fig. 8(b) is the CV curve ofelectrode c. As seen, the 2nd and 3rd CV curve are similar, whereasan obvious difference between the first and subsequent two isfound. In the 1st cathodic scan, three reduction peaks located at0.68, 0.93 and 2.34 V are observed, which is attributed to the forma-tion of SEI and the generation of Cu from CuO and Cu2O. The locationof the reduction peaks is little lower than the sloping potentialregions, which can be ascribed to a polarization phenomena. Thereduction peaks at 0.68 and 0.93 V shift to 0.74 and 1.46 V in the 2nd
cathodic scan owing to the activation of the electrode. The reduc-tion peak near 2.34 V shows no obvious shift in the initial threecathodic scan, and does not appear in Fig. 8(a), which may be rel-evant to the continuous consumption of lithium ions from CuO. In
n of the galvanostatic test (b) for CuxO–Cu (x = 1, 2) electrodes obtained at different
424 S. Ni et al. / Electrochimica Acta 109 (2013) 419– 425
ig. 9. Capacity retention of CuxO–Cu (x = 1, 2) electrodes obtained at different con-itions run at various current density from 0.1 to 5.0 mA cm−2.
he anodic scan, a weak oxidation peak near 1.5 V and a strong oxi-ation peak near 2.47 V can be attributed to the decomposition ofhe SEI and the oxidation of Cu into Cu2O [13,18], respectively. TheV curves are in accordance with the charge/discharge curves inig. 7(a) and XRD results in Fig. 5.
Fig. 9 shows the capacity retention of CuxO–Cu (x = 1, 2) elec-rodes obtained at different conditions. As seen, the charge andischarge capacity decreases along with the increasing of currentensity, which can be ascribed to a polarization that results innsufficient electrochemical reactions, leading to the hindrance ofharge transfer. Even at a high charge/discharge current density of.0 mA cm−2 (30 C for electrode b, 20 C for electrode c), the 5th-ycle discharge capacity of electrodes b and c can maintain 39.4%nd 34.6% of the initial 5th-cycle discharge capacity, respectively.fter that, the 5th-cycle discharge capacity of electrodes b and can restore 100% and 92.7% when lowering the charge/dischargeurrent density to 0.1 mA cm−2. It can be observed that the rateapability of the as-prepared CuxO electrode are slightly attenu-ted along with the increasing of reaction time, which is due to thexcessive electrochemical corrosion that leads to a destroy of Cuoam substrate.
. Conclusions
In summary, the electrochemical corrosion of Cu foam has beentilized in Li-ion battery. Cu2O–Cu composite architecture that pre-ared via an electrochemical corrosion method shows excellent
ained at different conditions at a scan rate of 0.2 mV s−1.
cycling stability and rate capability as anode for Li-ion batteries.After testing at various rates from 0.2 C to 35 C over 60 cycles,the 5th-cycle discharge capacity can resume 98.9% when lower-ing the charge/discharge rate to 0.2 C. Such good electrochemicalperformance of the Cu2O electrode can be ascribed to the enhancedelectric contact between Cu2O and Cu foam and a possible hollowarchitecture of Cu2O. The results indicate that the electrochemicalcorrosion of metal may have positive application in Li-ion batteries,and the optimization of the electrochemical corrosion of metal maybe a feasible way to obtain metal oxides/metal composite architec-ture with excellent electrochemical performance.
Acknowledgements
We gratefully acknowledge the financial support from Natu-ral Science Foundation of China (NSFC, 51272128), Excellent YouthFoundation of Hubei Scientific Committee (2011CDA093), Educa-tion Office of Hubei Province (Q20111209) and Open Project ofState Key Laboratory Cultivation Base for Nonmetal Composites andFunctional Materials (12zxfk08). Moreover, the authors are grate-ful to Dr. Jianlin Li at Three Gorges University for his kind supportto our research.
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