Facile fabrication of cuprous oxide nanocomposite anode films for flexible Li-ion batteries via thermal oxidation

Electrochimica Acta 86 (2012) 323–329

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Facile fabrication of cuprous oxide nanocomposite anode films for flexible Li-ion batteries via thermal oxidation夽 A. Lamberti a,b , M. Destro b , S. Bianco a , M. Quaglio a , A. Chiodoni a , C.F. Pirri a,b , C. Gerbaldi a,b,∗ a b

Center for Space Human Robotics @Polito, Istituto Italiano di Tecnologia, Corso Trento, 21, 10129 Torino, Italy Department of Applied Science and Technology – DISAT, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy

a r t i c l e

i n f o

Article history: Received 30 November 2011 Received in revised form 16 February 2012 Accepted 4 March 2012

Keywords: Copper oxide Nanocomposite Anode Polymer electrolyte Lithium ion battery

a b s t r a c t In the present work, nanostructured Cu2O films are directly grown from a Cu metal foil by means of a rapid thermal oxidation process. The structural characteristics of the films are investigated by field emission scanning electron microscopy, X-ray diffraction and attenuated total reflectance Fourier-transformed infrared spectroscopy. The electrochemical behaviour is investigated in both lithium (liquid electrolyte) and all-solid lithium polymer cells. At a discharge/charge rate of C/5, the films can provide a specific capacity greater than 220 mAh g-1 in the all-solid configuration, with excellent cycling stability and capacity retention after prolonged cycling. High surface area, short diffusion path and good conduction of the Cu2O films are considered to be responsible for the good electrochemical performance, along with the use of the polymeric electrolyte which is directly formed in situ on the electrode film surface. The present findings can provide a new and easy approach to fabricate nanocomposite films with interesting performance as negative electrode particularly for the next generation of flexible all-solid-state Li-ion microbatteries. © 2012 Published by Elsevier Ltd.

1. Introduction Nowadays, lithium-ion batteries (LIBs) are the most popular type of rechargeable battery for consumer electronics, with very high energy densities, no memory effect, and a slow loss of charge when not in use [1]. Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle, and aerospace applications [2,3]. Research is currently yielding a stream of improvements to traditional LIB technology, focusing on energy density, durability, cost, and intrinsic safety. Transition metal oxides (MOs, where M = Cu, Co, Fe, etc.) are being attracting considerable interest in the last decade, as novel anode materials for Li-ion batteries. Poizot et al. [4] discovered the mechanism of Li reactivity in oxides of 3d transition metals (MOs, where M = Cu, Co, Fe, V, etc.), which involves the formation and decomposition of Li2O; thus, it differs from the classical lithium intercalation/deintercalation or Li alloying/dealloying processes. MO anodes show the advantages of high electrochemical capacities,

夽 This article is a reprint of a previously published article. For citation purposes, please use the original publication details; Electrochimica Acta, 70, pp. 62–8. DOI of original item: http://dx.doi.org/10.1016/j.electacta.2012.03.025. ∗ Corresponding author. Department of Applied Science and Technology –DISAT, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy. Tel.: +39 11 090 4638; fax: +39 11 090 4699. E-mail addresses: [email protected], [email protected] (C. Gerbaldi). 0013-4686/$ – see front matter © 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2012.11.036

excellent cycle retention and good safety compared to conventional carbonaceous materials. However, the powder oxides still suffer from some limitations, such as poor electronic conductivity; moreover, large volume change of MO particles during reaction with Li+ ions often results in the pulverization and strong polarization of the electrodes, which limit their practical application in thin film Liion batteries. An interesting solution to overcome these problems is their preparation in the form of nanostructured film electrodes [5]. In this context, Jiang et al. [6] reported that CuO nanowires can be easily grown on a copper foil as a thin film by simple thermal oxidation. However, the adhesion of the carpet of nanowires on the Cu substrate was poor and lithographic patterning steps were required to obtain a metal oxide film that could be processed for device fabrication [7]. Different authors reported on the growth of these nanowires on a CuxO multilayer structure [8–10]. Mittiga et al. [11] reported on the fabrication of heterojunction solar cells made by deposition of transparent conducting oxide (TCO) films on good quality Cu2O sheets prepared by oxidizing copper at a high temperature. Huang et al. [12] investigated the growth of both Cu2O and CuO nanowires by thermal oxidation and suggested that it can be described as a vapour–solid mechanism. From the perspective of battery design, improving performance revolves around raising the fraction of the electrochemically active components (namely, the electrodes) to as close to unity as possible, while decreasing the fraction of the supporting components (e.g. electrolyte, current collectors, and packaging) to as near zero

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as possible. Regarding this last point, reducing the thickness of the electrolyte and eliminating the need for binders and conductivity additives are desirable strategies. Materials research has an important role to play in attaining these goals. In the present work, nanostructured Cu2O films are directly grown, through a rapid thermal oxidation process, from a Cu metal foil. The perspectives of such kind of materials are illustrated by the experimental data on the electrochemical response in different laboratory scale lithium test cells with both liquid and solid electrolyte, respectively. Results obtained, particularly in the all-solid configuration, demonstrate the promising implementation of this material as Li-ion battery anode. All the synthesis and technological steps involved appear highly suitable for large scale production, for instance by means of roll to roll type procedure of solid state batteries, which could decrease production costs. Moreover, since the electrode characteristics, such as conductivity and adhesion strength, constrain the flexibility of the battery, the development of flexible electro active self-standing multiphase electrode/electrolyte thin films is here demonstrated. High cycling stability is obtained and good overall electrochemical performances are observed, enlightening that these kind of materials are interesting candidates for future applications in thin and flexible polymeric lithium-based batteries.

ethoxylate (15 EO/phenol) dimethacrylate (BEMA, average Mn = 1700, Aldrich), poly(ethylene glycol) methyl ether methacrylate (PEGMA, avg. Mn: 475, Aldrich), lithium bis(trifluoromethane) sulphonimide (LiTFSI, Ferro Corp., USA, battery grade) a mixed ethylene carbonate/diethyl carbonate solution (EC-DEC, Ferro Corp., battery grade) and 2-hydroxy-2-methyl-1-phenyl-1-propanon (Darocur1173, Ciba Specialty Chemicals) as the free radical photoinitiator. This liquid reactive mixture (exact composition of BEMA:PEGMA:EC-DEC:LiTFSI = 15:15:40:30, along with 3 wt% of photo-initiator), which constitutes the polymer electrolyte precursor, was drawn onto a thin film over the Cu2O electrode material film with a calibrated, wire-wound applicator to obtain a final thickness of about 50 m. The process was carried out inside an Ar-filled dry glove box and the coated sheets were transferred into a UV transparent sealed quartz glass tube. This set up was then exposed to UV-irradiation for approx. 3 min. The photochemical curing was performed by using a medium vapour pressure Hg UV lamp (Helios Italquartz, Italy), with a radiation intensity on the surface of the sample of 35 mW cm-2 . These conditions assured maximum curing (disappearance of the acrylic/methacrylic double bonds, checked by FT-IR) [13]. After the UV exposure, the box was taken back inside the dry glove box and, later, free multiphase electrode/electrolyte disks of 0.785 cm2 were punched out of the foil and used for electrochemical testing.

2. Experimental 2.1. Synthesis A 0.0125 mm thick copper foil (Cu coil 99.9% purity degree, Goodfellow Cambridge Ltd., Italian distributor Prodotti Gianni S.p.A.) of about 4cm×4cm was used as the starting material. Prior to use, it was cleaned with acetone in an ultrasonic bath for 20 min, followed by rinsing with ethanol and drying by N2 flow. The Cu foil was then immersed into a 10% vol HCl aqueous solution at room temperature for 10 min in order to remove native oxide layer and obtain a fresh surface for thermal oxidation. Then, it was adequately rinsed with doubly distilled water and blow-dried with N2. The copper foil was then heated on a hotplate under ambient conditions (static air atmosphere in a class 1000 cleanroom) employing small glass slides keeping flat the extremity of the foil to reduce stress releasing during annealing. The oxidation treatment was performed at 400◦ C for 30 min and, after cooling, the surface was covered by a dark oxide thin film. The weight of the nanostructured oxide surface layer was found to be 1.4 mg cm-2 , measured weighing the oxidized sample before and after superficial oxide layer removal in HCl 10% vol aqueous solution. 2.2. Characterization techniques Different kinds of characterisation techniques were used to assess the properties of the nanostructured film prepared. The structure and morphology were analysed by X-ray diffraction (XRD, Panalytical PW1140–PW3020, Cu K X-ray source), Attenuated total reflectance Fourier-transformed infrared spectroscopy (ATR FT-IR, Nicolet 5700 FTIR Spectrometer) and field emission scanning electron microscopy (FESEM, ZEISS Supra 40 Field Emission Scanning Electron Microscope), respectively. After characterization the oxidized bottom of the samples was gently etched employing paper soaked in HCl solution and tested in lithium cells to verify the effective employment as anode material in lithium-based battery. 2.3. Multiphase electrode/electrolyte preparation The reactive formulation for the preparation of the polymer electrolyte consisted of: the dimethacrylic oligomer Bisphenol A

2.4. Assembly of lithium cells and electrochemical tests The electrochemical response in liquid electrolyte of the samples was tested in a polypropylene three-electrode T-cell assembled as follows: a Cu2O disk (area 0.785 cm2) as the working electrode, a 1.0 M lithium perchlorate (LiClO4, Aldrich) in a 1:1 (w/w) % mixture of ethylene carbonate (EC, Fluka) and diethyl carbonate (DEC, Aldrich) electrolyte solution soaked on a Whatman® GF/A separator and a lithium metal foil (high purity lithium foils, Chemetall Foote Corporation) as the counter electrode. When needed (i.e. cyclic voltammetry), a lithium foil was added at the third opening, in direct contact to the electrolyte, acting as the reference electrode. In the case of all-solid lithium polymer cells, free multiphase electrode/electrolyte disks of 0.785 cm2 were contacted with a Li metal anode disk, assembled and tested in a complete laboratoryscale polypropylene (PP)-made two electrode lithium cell equipped with two stainless-steel (SS-316) current collectors. The evaluation of the electrochemical performances was carried out by galvanostatic discharge/charge cycling (cut off potentials: 0.02–3.0 V vs. Li/Li+) and cyclic voltammetry (between 0.02 and 3.0 V vs. Li/Li+, scan rate of 0.100 mV s-1 ) at ambient temperature, using an Arbin Instrument Testing System model BT-2000 as the controlling instrument. The discharge/charge cycles were set at the same rate at C/10 and C/5, respectively. Note that C/10 corresponds to 0.2 mA with respect to a Cu2O active mass of about 1 mg. Clean electrodes and fresh samples were used for each test. To confirm the results obtained, the tests were performed at least twice on different fresh samples. Both electrodes fabrication and cell assembly were performed in the inert atmosphere of a dry glove box (MBraun Labstar, O2 and H2O content < 0.1 ppm) filled with extra pure Ar 6.0. 3. Results and discussion In order to unravel the composition and structure of the prepared samples, powder X-ray diffraction (XRD) experiments were carried out. XRD analysis of the samples shows good correspondence with the reference Cu2O pattern [14,15]. The XRD spectrum of the Cu2O is depicted in Fig. 1 and shows seven peaks that are

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630

1000

900

800

700

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Wavenumber / cm 30

40

50

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Fig. 2. ATR FT-IR spectrum of the as-synthesized nanostructured Cu2O film.

2 / degree Fig. 1. X-ray diffraction patterns of the as-prepared sample, obtained by simple thermal oxidation process in ambient atmosphere, together with the diffraction pattern of the Cu foil for comparison; in the inset, weak diffraction peaks ascribable to the CuO phase.

clearly distinguishable. The peaks with 2 values of 28.56, 36.41, 42.28, 61.38, 73.52, 77.39 and 92.45 correspond to the crystal planes of (1 1 0), (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0) respectively, of cubic Cu2O with space group of Pn-3m (JCPDS 78 2076). In the same Fig. 1, the XRD reflection of the metallic copper on which the thin oxide film was grown was reported. Besides the feature peaks ascribed to the Cu2O phase, weak signals for the two main diffraction peaks of cupric oxide (CuO) crystalline phase are detectable in the XRD patterns (JCPDS 89 5899, see the inset in Fig. 1). As shown in the scanning electron micrographs (see Fig. 3) CuO nucleates on the top surface of the cuprous oxide film in the form of short incipient nanowires. Huang et al. [12] investigated the growth of CuO nanowires by thermal oxidation and their results demonstrated the presence of two oxide layers on the copper foil surface after thermal treatment: a bottom layer consisting of Cu2O and a very thin top layer consisting of CuO. The proposed hypothesis was that, by heating the copper foil in oxidative atmosphere, its surface evolved to form Cu2O and, afterwards, CuO appeared as a second step of the oxidation of Cu2O. Finally, if the oxidation time or the temperature were further increased, the growth of CuO nanowires took place. Our experimental work confirms that the oxidation time exerts some impact on the composition, morphology and surface area of the films. In fact, an increase in the oxidation time, will lead to higher content of CuO phase in the composite films due to the incipient growth of nanowires (results not shown here). Therefore, we found out that the optimal oxidation time for a pure Cu2O film should be lower than 30 min. The above results demonstrate that Cu2O nanostructured films can be directly grown on metallic copper foils via a simple and rapid thermal oxidation process. FT-IR spectroscopy is often used as a powerful tool for the characterisation of Cu-based oxides [16,17]. Fig. 2 shows the ATR FT-IR spectrum of the as-synthesized nanostructured Cu2O film in the wavenumber range between 500 and 1000 cm-1 . It is well consistent with those of Cu2O reported in the literature [16,18,19] displaying an absorption peak at around 630 cm-1 , which can be attributed to the Cu(I)–O vibration mode in the Cu2O phase. No obvious absorption peaks is clearly visible at lower frequency, which can be assigned to the vibrations of Cu(II) O; however, from

the results obtained by XRD, the presence of a marginal fraction of cupric oxide (CuO) impurity cannot be excluded. The surface morphology of the films was investigated by field emission scanning electron microscopy (FESEM), and typical micrographs at different magnification of the material obtained are shown in Fig. 3. The FESEM micrographs show that a superficial nanostructured Cu2O film was successfully grown from the metallic Cu foil. The lower magnification image demonstrates the homogeneity of the nanostructured film obtained, while the higher magnification images clearly reveal that it mainly consists of nanostructured particles with irregular shape coexisting with a few short superficial nanowires. The average size of the particles is about 90 nm under the oxidation conditions of 400◦ C for 30 min. A certain degree of porosity is also evident: these non-uniform nanosized pores should be beneficial to the electrochemical properties of the material by providing space and sites for the electrolyte to be loaded into. From the micrograph at higher magnification, it can be seen that there is a very small amount of nanowire formed after the thermal treatment. We found out that long and uniform CuO nanowires (not shown here) can be formed in large amount on the copper thin film after prolonged thermal treatment. 3.1. Electrochemical characterisation In the followings, we present the results obtained from the tests of the films as electrodes in laboratory-scale Li-based test cells. The electrochemical properties of the Cu2O film were firstly investigated by cyclic voltammetry (CV). Typical cyclic voltammograms obtained at a scan rate of 0.1 mV s-1 between 0.02 and 3.0 V vs. Li/Li+ are displayed in Fig. 4(a); in the inset, the initial CV cycle. The shapes and potentials of the CV peaks are generally consistent with the results reported in the literature for Cu2O-based electrodes [14,20]. The cathodic peak centred at about 1.5 V vs. Li/Li+ is attributed to the reaction of Cu2O with Li+, corresponding to the transformation from Cu2O to Cu and Li2O. The broad peak located below 0.7 V vs. Li/Li+ can be ascribed to the formation of the solid electrolyte interface (SEI) due to the decomposition on the surface of the electrode of some of the electrolyte components; the peak is clearly observable only in the first cycle and, then, it disappears in the following. During the anodic scans, correspondingly, two broad peaks centred at about 1.4 and 2.4 V vs. Li/Li+ can be observed. The main reaction in the potential range between 0.9 and 1.6 V vs. Li/Li+ would be the decomposition of the formed SEI, while the anodic peak between

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Fig. 3. FESEM images at different magnifications of the nanostructured cuprous oxide obtained by simple thermal oxidation process in ambient atmosphere.

2.1 and 2.7 V vs. Li/Li+ corresponds to the recombination of Cu and Li2O to produce Cu2O and Li+ [14]. The noticeable difference in the characteristics and shape of the CV profiles between the first and the following CV cycles is also consistent with the results reported in the literature [14,20]. The electrochemical behaviour in terms of discharge/charge galvanostatic cycling of the nanostructured Cu2O film was tested in a cell using lithium metal as the counter electrode and a 1.0 M LiClO4 in EC-DEC solution as the electrolyte. Fig. 4(b) shows the first and the second discharge/charge cycling profiles between 0.02 and 3.0 V vs. Li/Li+ at 0.02 mA cm-2 . The potential profiles have the characteristic shape of Cu2O-based materials already described in the literature [21–23], consistent with the reaction: chargedischarge

Cu2 O + 2e− + 2Li+ ⇐

⇒

2Cu + Li2 O

The difference between the initial and the subsequent discharge profile (over-discharge) is likely due to the series of irreversible processes already discussed in the CV, mainly the decomposition of the electrolyte with the formation of a solid electrolyte interphase (SEI) layer surrounding the metallic particles [24]. The amount of electricity involved in the first discharge was partially extracted in the subsequent charge process, when Cu2O (poor semiconductor) was formed.

Fig. 4(c) shows the specific capacity vs. cycle number at ambient temperature and at different current rates of C/10 and C/5. The first discharge and charge capacities were found to be about 475 and 314 mAh g-1 , respectively, at C/10 current rate, with a low initial Coulombic efficiency of about 66%. The irreversible capacity loss during the first cycle can be ascribed to the above discussed side reaction with the electrolyte. The initial reversible specific capacity was found to be about 250 mAh g-1 . After the first cycle, the Coulombic efficiency rapidly increases to above 90% and, subsequently, remains highly stable throughout the cycles. Unfortunately, despite the interesting initial specific capacity values, the nanostructured films suffered from poor cyclability and cycle life in liquid electrolyte: in fact, in less than 50 cycles, the specific capacity underwent a huge drop down to less than 60 mAh g-1 . As already stated by other research groups [21], the expansion and shrinkage of the structure caused by the lithium insertion/extraction processes are difficult to accommodate, leading to strong stresses that may affect the electrode integrity and, consequently, the electrochemical performance. In fact, as it is shown in the FESEM image of Fig. 6, the electrode tested in liquid electrolyte, after only 50 cycles, appeared to be dramatically fractured. The mechanical degradation of the electrode is thought to be determinant for the huge drop in electrochemical yielding, as it happens in bulk electrodes. Most probably, there is a loss of electrical contact between all the isolated

E vs. Li/Li / V

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Cycle number Fig. 4. (a) Typical cyclic voltammograms of the nanostructured Cu2O with a scan rate of 0.1 mV s-1 between 0.02 and 3.0 V vs. Li/Li+; in the inset, the initial CV cycle. (b and c) Constant current discharge/charge cycling test of the Cu2O film in liquid electrolyte at ambient temperature and at different current regimes: (b) initial discharge/charge potential profiles, (c) cycling performance along 50 discharge/charge cycles.

Generally, all-solid-state thin-film type batteries are fabricated using sequential deposition of current collectors, electrode active materials, and solid electrolytes by various deposition processes. Simplification of the fabrication process would expedite practical application and reduce manufacturing costs. One of the most effective methods for simplifying fabrication process will be to use in situ prepared materials. To this purpose, due to the versatile nature of free radical photo-polymerisation (UV-curing technique) a multiphase electrode-electrolyte was produced in the present work, in which a methacrylic-based polymer electrolyte was directly formed in situ at the interface with the Cu2O film electrode. In our opinion, this should improve the cycling performance of the nanostructured material by overcoming the kinetic limitations due to the ionic exchange between the electrode and the polymer electrolyte surfaces. The polymeric electrolyte surrounding the nanostructured active material particles should also act as a buffer, thus absorbing the mechanical stress associated with the reaction between Cu2O and Li+ and, consequently, improve the cycle life of the electrode. In this process, an appropriate liquid reactive mixture (see Section 2.2), which constitutes the polymer electrolyte precursor, was

600

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Specific Capacity / mAh g

3.2. Multiphase electrode/electrolyte production and testing in lithium polymer cell

deposited as a thin film over the Cu2O electrode film. Therefore, due to its low viscosity, the liquid reactive mixture could penetrate into the electrode voids to achieve an intimate contact at the electrode/electrolyte interface. The film was then in situ polymerised

-1

islands and the Cu collector substrate on which the nanostructures were grown.

400

C/10 C/5

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Charge Discharge

0 0

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Cycle number Fig. 5. Specific capacity vs. cycle number of the lithium polymer cell assembled by contacting the Cu2O-based multiphase electrode/electrolyte and a Li metal counter electrode, at ambient temperature and at different current rates of C/10 and C/5.

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Fig. 6. FESEM images at different magnifications of: (a) Cu2O electrode after 50 complete discharge/charge cycles in liquid electrolyte; (b–d) Cu2O-based multiphase electrode/electrolyte after 50 complete discharge/charge cycles in the all-solid Li cell configuration.

by UV-curing to form, in a single step, a self-standing electrode intimately connected to the Li+-ion conducting electrolyte membrane, with an efficient interpenetration of the two surfaces. Thorough analysis and details on the characteristics of the polymeric electrolyte and reactive monomers used can be found in [13,25]. The discharge/charge tests were performed at the same C/10 and C/5 rates as for the lithium cell with liquid electrolyte. Fig. 5 shows the resulting plot of specific capacity versus cycle number at ambient temperature. Results obtained demonstrated that, by using a Cu2O film intimately connected to the polymeric electrolyte, at a discharge/charge rate of C/10, the cell can provide an average specific capacity of about 300 mAh g-1 , greater than the one obtained in liquid electrolyte, with very high Coulombic efficiency and capacity retention after prolonged cycling. Reproducible voltage profiles were obtained where the initial specific discharge capacity at C/5 rate approaches the value of 252 mAh g-1 . The capacity retention is satisfactory: after 50 cycles the cell still delivers specific discharge capacity values around 220 mAh g-1 , with a drop in capacity of about 13% with respect to the initial value at C/5. Thus, the cell operates with the expected voltage profiles delivering a good fraction of the theoretical capacity (about 375 mAh g-1 for Cu2O). The cycle response is also encouraging since no decay in capacity is shown during this initial test and the charge–discharge efficiency following the first cycles, where rearrangements in the structure of the electrode take place, is maintained to be always higher than 95%. If compared with the results reported in Fig. 4(c) for the Li cell with liquid electrolyte, a huge effect of stabilization is evident, and the overall specific capacity is also enhanced. This is a convincing indication of a good interfacial contact between the electrode and the polymeric electrolyte. These effects can be ascribed to the use of the polymeric electrolyte (e.g. mechanical stress absorption,

enhanced electrode/electrolyte interface) and the polymerization process used which allows to produce in situ the polymeric electrolyte directly onto the surface of the electrode film. To corroborate the above reported considerations, the surface of both the electrodes after the cycling process was examined by FESEM. In Fig. 6, image (a) corresponds to the Cu2O electrode after 50 discharge/charge cycles in liquid electrolyte, while images (b–d) correspond to the multiphase Cu2O-based electrode/electrolyte after 50 discharge/charge cycles in Li-polymer cell. In Fig. 6(a) it is clearly evident that the original film morphology has disappeared (see for comparison Fig. 3(a)) and huge fractures are present. Therefore, the deterioration of the electrical contact between the Cu2O layer and the underlying metallic Cu substrate after the discharge/charge process can be considered the main responsible for the poor cycle life of the electrode. On the contrary, as clearly evident in Fig. 6(b–d), the surface of the electrode intimately connected to the polymeric electrolyte is smooth and suffered no major damage; no pulverization of the electrode can be observed, thus accounting for the very good stability during cycling of the multiphase electrode/electrolyte produced. 4. Conclusions The thermal oxidation technique demonstrated to be an excellent tool for preparing homogeneous cuprous oxide films with a controlled oxidation state and, highly homogeneous and clean surface. The electrochemical capacity of the Cu2O/Li test cell with liquid electrolyte, even if initially satisfactory, after few discharge/charge cycles underwent dramatic drop due to the huge cracks observed by FESEM. On the contrary, good cycling stability and capacity retention was obtained by testing the Cu2O-based

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electrode in all-solid Li test cell configuration after 50 cycles. High surface area, short diffusion path, and good conduction of the Cu2O films are considered to be responsible for the interesting electrochemical characteristics obtained. Moreover, the deposition of the liquid reactive mixture onto an electrode surface followed by UV polymerization appears an effective technique for upgrading the properties of the nanostructured electrodes, by obtaining in a single-step a high performing multiphase electrode/electrolyte. The present findings demonstrate that this simple thermal oxidation method is compatible with IC fabrication and can provide a new, easy and low cost approach to fabricate nanocomposite films with remarkable performance for the next generation of advanced flexible 3D Li-ion microbatteries. Work is in progress and preliminary results in this direction are interesting and encouraging. Acknowledgement The authors would like to thank Dr. Salvatore Guastella (Department of Materials Science and Chemical Engineering-Politecnico di Torino) for the FESEM analysis of the samples. References [1] M. Armand, J.-M. Tarascon, Nature 451 (2008) 652. [2] B. Scrosati, J. Garche, J. Power Sources 195 (2010) 2419. [3] B.V. Ratnakumar, M.C. Smart, C.K. Huang, D. Perrone, S. Surampudi, S.G. Greenbaum, Electrochim. Acta 45 (2000) 1513.

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