CuO

ARTICLE pubs.acs.org/JPCC

Reversible Electrochemical Conversion Reaction of Li2O/CuO Nanocomposites and Their Application as High-Capacity Cathode Materials for Li-Ion Batteries Ting Li, Xin P. Ai, and Han X. Yang* Department of Chemistry, Wuhan University, Wuhan 430072, China ABSTRACT: Novel Li2O/CuO nanocomposites were prepared by ball-milling the CuO and Li2O nanoparticles with rigid TiN nanopowders, where TiN nanopowders act as a conductive substrate to immobilize the electroactive nanolayer of a biphasic Li2O/CuO mixture. The as-prepared samples demonstrate a superior electrochemical capacity of 560 mAh g
1 at a moderate charge
discharge rate of 50 mA g
1 and also exhibit a quite high reversible capacity of ∼438 mAh g
1 even at a very high rate of 500 mA g
1 at room temperature. CV and XRD analyses revealed that the Li2O/CuO nanocomposite can realize nearly a three-electron transfer through electrochemical conversion of Cu2O/Cu to LiCuO2 and vice versa, involving lithium intercalation and deintercalation. This conversion reaction can proceed reversibly and rapidly as long as the different phases of the cathode-active particles are well-dispersed and closely contacted to create electrochemically favorable nanodomains in the electrode. The experimental results demonstrated in this study suggest the possibility to use inexpensive multivalent metal oxides as high-capacity cathode materials for construction of future-generation lithium-ion batteries through an electrochemical conversion mechanism.

’ INTRODUCTION Rechargeable Li-ion batteries are now considered as an attractive power source for a variety of electric energy storage applications, such as in electric vehicles, smart grids, or power storage from solar or wind electricity.1
4 However, present commercialized Li-ion batteries use scarce transition-metal oxide cathodes, such as cobalt and nickel compounds, which are difficult to support large-scale electric storage applications because of their global resource shortfall and high costs. In addition, lithium battery technologies are now mostly based on intercalation chemistry, which allows no more than one Li ion per formula unit to insert reversibly into the cathodic host lattices and, therefore, provides insufficient utilization of the electrochemical capacity of the cathode materials.5,6 Hence, the search for high storage capacity and cost-effective cathode materials has been intensified in recent years, and a number of new redox mechanisms are developed for building next-generation lithium batteries with substantially enhanced energy densities. Electrochemical conversion reactions seem to be a possible approach to bring about the significant breakthrough in the storage capacity of the electroactive materials for Li-ion batteries through full utilization of all the charge states of a multivalent transition-metal compound. Since Tarascon et al. first reported the reversible Li-storage reaction for transition-metal oxides through the heterogeneous conversion, Li þ MO T Li2O þ M (M = Co, Fe, Ni, Cu, etc.),7 a vast array of compounds, such as metal fluorides,8
12 oxides,13
15 sulfides,16,17 and nitrides,18,19 have been demonstrated to produce a large multielectron redox capacity through reversible electrochemical conversion r 2011 American Chemical Society

reactions. However, the conversion electrode materials reported so far are mostly around anodic Li-storage reactions, except a few metal fluorides and sulfides were studied as possible alternatives for the cathode-active materials of Li-ion batteries.8
10,17 From the electrochemical point of view, some transition-metal oxides (MOx) have their metallic cations in various high oxidation states and the M
O bonds in these compounds have a similar ionic character as the M
F bonds in metal fluorides. Thus, it is expected that some of the multivalent metal oxides may possibly serve as the conversion cathodes with a high reversible electrochemical capacity through multielectron redox reactions as long as the Li2O and MOx phases are uniformly interspersed and intimately contacted at the atomic or nanometer scale, as required for electrochemical conversion reactions.7,8,20
22 To search for appropriate metal oxides for the conversion cathode material, we found that copper oxide seems to be an attractive choice because Cu is a fairly abundant, inexpensive, and less toxic transition metal and can exist in the oxidation states of Cu1þ, Cu2þ, and Cu3þ in its oxides, enabling a possible utilization of a three-electron redox capacity. Previous studies of the conversion reaction of CuO were mostly focused on the electrochemical utilization of its lower oxidation states (CuO T Cu2O/ Cu). Though this conversion reaction can be realized to deliver a quite high reversible capacity of about 700 mAh g
1, the conversion potential is around an inappropriate region of ∼1.2 V Received: December 30, 2010 Revised: February 19, 2011 Published: March 11, 2011 6167

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The Journal of Physical Chemistry C (vs Liþ/Li), which is too low for the cathode and fairly high for the anode in Li-ion battery applications.23
26 Several attempts were earlier made to develop layered LixCuO2 compounds as positive electrode materials; however, the Li-storage capacities of these cathodes are all about 130 mAh g
1, corresponding to an reversible intercalation of only 0.5 electrons.27,28 In consideration of possible high-voltage and high-capacity output of copper oxides through a full three-electron conversion reaction, we synthesized four types of Li2O/CuO nanocomposites by different chemical and mechanochemical routes and examined the charge
discharge properties of these materials. In this paper, we report the electrochemical conversion behaviors of the Li2O/CuO nanocomposites with our focus on their cycleability and high rate utilization at room temperature. Also, the electrochemical redox mechanism and structural evolution of the Li2O/CuO nanocomposites during the discharge/charge process are also described.

’ EXPERIMENTAL SECTION Materials Preparation. CuO nanoparticles used in this work for preparing the Li2O/CuO nanocomposites were synthesized through four different chemical routes. The first synthetic method was to hydrolyze organic cupric salts simply by hydrothermal treatment of 80 mL of copper acetate ethanol solution (Cu(OAc)2, 0.05 mol 3 L
1) in a 100 mL Teflon-lined stainless steel autoclave at 120 °C for about 20 h. After the autoclave cooled naturally to room temperature, the hydrolytic deposits were separated by centrifugation, washed with distilled water and ethanol, and then dried under vacuum at room temperature. The final sample thus prepared was denoted as CuOa. The second CuO sample was made by pyrolytic decomposition of CuCO3 nanopowders (CuCO3 T CuO þ CO2). The CuCO3 powders was prepared by rapidly adding 50 mL of aqueous ammonium carbonate solution ((NH4)2CO3, 0.3 mol 3 L
1) into 300 mL of aqueous copper acetate solution (Cu(OAc)2, 0.05 mol 3 L
1). After several minutes of chemical reaction, the precipitate was separated by centrifugation, purified with distilled water and ethanol, then dried at 60 °C in air, and finally heated at 250 °C for 45 min under an argon atmosphere. The CuO nanopowder such prepared was marked as CuOb. Two other CuO samples were synthesized by dropwise adding aqueous ammonia (NH3 3 H2O) as a complexing agent to 150 mL of copper(II) chloride ethanol solution (CuCl2, 0.5 mol 3 L
1) to form a clear, dark blue [Cu(NH3)4]2þ solution then adding 150 mL of sodium hydroxide ethanol solution (NaOH, 2 mol 3 L
1) into the [Cu(NH3)4]2þ solution to deposit Cu(OH)2. The Cu(OH)2 precipitate was filtrated, washed with distilled water and ethanol, and then dried under vacuum at 70 °C. The CuO sample was obtained by heat treatment of the Cu(OH)2 precipitate at 250 °C for 15 min under argon and at 400 °C for 2 h in air. The former and the latter were denoted as CuOc and CuOd, correspondingly. The Li2O/CuO nanocomposites were prepared by high-energy ball-milling (8000 M Mixer/Mill, SPEX, USA) of a stoichiometric mixture of lithium oxide (Li2O, 99.5%, Alfa-Aesar) with nanotitanium nitride (TiN, 99.2%, ∼20 nm, Hefei Kaier Nanomaterials Co., Ltd., China) for 6 h and then ball-milling the TiN/Li2O nanoparticles with the as-prepared CuO powders and graphite using a planetary mill (QM-1SP04, Nanjing, China) with the rotation speed of 240 rpm for 6 h. All the TiN/ Li2O/CuO/graphite nanocomposites had the following mass fraction: CuO, 36.36%; Li2O, 13.64%; TiN, 30%; graphite, 20%.

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The molar ratio of CuO to Li2O in these composites was designed to be 1:1. Structural Characterization. Powder X-ray diffraction (XRD) was used to characterize the crystalline structures of the ball-milled nanocomposite samples using a Shimadzu XRD-6000 diffractometer equipped with Cu KR radiation. The XRD spectra were collected in a range of 2θ values from 10° to 80° at a scanning rate of 2°/min and a step size of 0.02°. To characterize the structural changes during cycling, the electrode samples at different depths of charge and discharge (1.0
4.4 V) were taken out from the disassembled cells and rinsed with pure dimethyl carbonate solvent in an Ar-filled glovebox and then immediately sent for ex situ XRD analysis. The morphologies of CuO nanoparticles and as-prepared nanocomposites were observed using a high-resolution transmission electron microscope (HRTEM, JEM-2100FEF). The samples for the TEM analysis were prepared by dispersing the sample powders in ethanol and releasing a few drops of the dispersed solution on a carbon film supported on a copper grid. Electrochemical Characterization. The cathodes were prepared by mixing 80 wt % active material, 10 wt % acetylene black, and 10 wt % polytetrafluoroethylene (PTFE) into ca. 0.1 mm thick films and pressing the cathode films onto an aluminum net. Coin cells (2032 type) were assembled in an argon-filled glovebox with Li metal foil as the anode, a microporous membrane (Celgard 2400) as the separator, and 1 mol 3 L
1 solution of LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylene methyl carbonate (EMC) (1:1:1 by wt) as the electrolyte. The cells were controlled by an automatic battery tester (Land CT2001A, Wuhan, China) between 1 and 4.4 V at different current densities. Cyclic voltammograms (CVs) were measured using an electrochemical station (CHI660a, Shanghai, China) by the three-electrode cell at a scanning rate of 0.1 mV s
1 with the voltage ranges of 1.0
4.4 V.

’ RESULTS AND DISCUSSION Structural and Morphological Features. Electrochemical conversion reactions require the electrode-active materials of different phases to be downsized as much as possible and to be contacted as closely as possible in order to enhance the conversion activity of the phase transformation reaction.21,22 Keeping this in mind, we tried different chemical routes to synthesize nanosized CuO at first. The XRD patterns of the CuO nanoparticles synthesized from different methods in this work are shown in Figure 1. All the as-prepared samples show a similar diffraction pattern, which can be well indexed to a monoclinic CuO phase (C2/c space group, JCPDS No. 41-0254). Compared with the standard diffraction lines, the XRD signals of the three samples, CuOa, CuOb, and CuOc, appear as relatively weak and broad peaks, implying a poor crystallization and a sufficiently small size of the samples. Nevertheless, the nonexistence of any residual XRD peaks suggests a high purity of the as-obtained samples. By Lorentzian fitting of the XRD lines of the samples and calculating from the Scherrer equation, the average crystalline size of the CuO particles is about 4.3 nm (CuOa), 9.4 nm (CuOb), and 15 nm (CuOc). In contrast, the sample CuOd, derived from a higher temperature sintering at 400 °C, shows a group of intense and sharp XRD lines, from which the average size of the sample is calculated to be ∼23 nm using the Debye
Scherrer formula. The morphological structures and size distributions of the CuO nanoparticles can be clearly visualized from their TEM images, as shown in Figure 2. Both the CuOa and the CuOb 6168

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samples display as spherelike crystallites with a fairly uniform size distribution and show an average size of ∼5 and ∼10 nm (Figure 2a,b), respectively. On the other hand, the samples of CuOc and CuOd heat-treated at elevated temperature show a rodlike shape composed of aggregated crystal grains (Figure 2c, d), and the sizes of the individual crystallites are ∼15 and ∼25 nm, respectively, which are much larger than the CuOa and CuOb particles. This TEM observation agrees very well with the calculated values from the XRD data, further revealing the structural features of the as-prepared CuO nanoparticles.

To create an appropriate interface for the conversion reaction of the Li2O/CuO composite, we downsized the Li2O particles using conductive TiN nanoparticles as a grinding powder by high-energy ball-milling and then mixing the CuO nanoparticles and graphite with TiN/Li2O by planetary ball-milling. XRD analysis clearly reveals the crystalline phases of the Li2O/ CuO nanocomposites as prepared by the ball-milling process. As shown in Figure 3, the phases of CuO, Li2O, TiN, and graphite are clearly distinguished, indicating that there were no

Figure 3. XRD patterns of Li2O/CuO nanocomposites prepared by ball-milling: (a) Li2O/CuOa, (b) Li2O/CuOb, (c) Li2O/CuOc, and (d) Li2O/CuOd.

Figure 1. XRD patterns of the as-synthesized CuO samples: (a) CuOa, (b) CuOb, (c) CuOc, and (d) CuOd.

Figure 2. TEM images of (a) CuOa, (b) CuOb, (c) CuOc, and (d) CuOd. 6169

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Figure 4. (a) TEM image and (b) expanded TEM image of the TiN/Li2O particles. (c) TEM image and (d) expanded TEM image of the TiN/Li2O/ CuOa/C nanopowders.

chemical reactions occurring between the phases during ballmilling. High-resolution transmission electron microscopy (HRTEM) can provide insight into the structural details in the nanodomains of the nanocomposites formed at different stages of material preparation. As shown in Figure 4a, the TiN/Li2O nanocomposite appears as well-distributed particles with an average size of 10
20 nm, indicating that the Li2O particles were well downsized after being ball-milled with the rigid TiN nanoparticles. From the magnified HRTEM image in Figure 4b, it can be seen that the TiN nanoparticles are actually embedded in the Li2O matrix as reflected by two different nanodomains: the darker areas dotted in the image have a regular lattice fingerprint of d = 0.212 nm, corresponding to the (200) plane of cubic TiN (JCPDS No. 38-1420), and all the surrounding regions show a lattice fringe with the d-spacing of 0.266 nm, characteristic of the (111) plane of cubic Li2O (JCPDS No. 12-0254). Ball-milled together with CuO and graphite, the final composite appears to be irregular nanoparticles with the TiN particles (∼20 nm) dispersed in the mixed Li2O/CuO matrix, as shown in Figure 4c. The high-resolution TEM image from an edge part of a single particle clearly shows that, all around the TiN particles, the crystalline phases are composed of nanosized CuO and graphite, as seen by the nanodomains with the two lattice fringes corresponding to the (200) plane (d = 0.231 nm) of monoclinic CuO (JCPDS No. 41-0254) and the (002) plane (d = 0.331 nm) of hexagonal graphite (JCPDS No. 41-1487), respectively. The

Figure 5. Discharge/charge profiles of the Li2O/CuOa nanocomposite electrodes at 50 mA g
1 with the voltage regions of 1.0
4.4 and 2.0
4.4 V.

lattice fringe of Li2O can hardly be discerned in the HRTEM image of Figure 4d, possibly because the Li2O particles were severely pulverized to lose their crystalline structure. Nevertheless, such a composite structure is no doubt favorable for an electrochemical conversion reaction, because the well-contacted 6170

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Figure 6. Discharge/charge profiles of the different types of Li2O/CuO electrodes at the second cycle and at a constant current of 50 mA g
1.

Figure 7. Discharge capacities of the different types of Li2O/CuO electrodes at the first 15 cycles and at a constant cycling current of 100 mA g
1.

phases confined in the nanodomains can promote the phase transformation and also prevent the electrode-active nanoparticles from aggregation. Moreover, because the rigid TiN nanoparticles are deeply embedded in the electroactive Li2O/CuO matrix, their impact on the separator is greatly suppressed. Electrochemical Performances. The specific capacity and cycleability of the Li2O/CuO nanocomposites were directly evaluated by charge
discharge measurements at constant currents at room temperature. Figure 5 gives the voltage profiles of the Li2O/CuOa nanocomposite electrodes cycled between the voltage regions of 1.0
4.4 and 2.0
4.4 V at a current density of 50 mA g
1. At the first discharge from its open-circuit potential of 2.8 V to a very low potential of 1.0 V, the Li2O/CuOa electrode delivered an initial discharge capacity of 420 mAh g
1, corresponding to an electrical capacity of 578 mAh g
1 produced per gram of CuO, which is much higher than a one-electron discharge capacity of CuO (337 mAh g
1) but less than a twoelectron reduction capacity of 670 mAh g
1, suggesting that the CuO particles were all electrochemically converted to Cu2O and some of Cu2O was continuously reduced to metallic Cu as the discharge potential went down to 1 V.25,26 During the subsequent charge, the electrode displayed a three-stage voltage profile with different slopes at 1.0
2.0, 2.0
3.6, and 3.6
4.4 V, respectively, reflecting a three-step oxidation from Cu0 to Cu3þ judging from the possible electrochemical reactions of Cu. After this initial cycle, the electrode still demonstrated the three-stage discharge
charge profiles with a reversible capacity of ∼560 mAh g
1, corresponding to a 77% utilization of the theoretical capacity (∼731 mAh g
1) as expected from a complete three-electron reaction. The stepwise charge
discharge profiles and exclusive three-electron redox capacities suggest that the copper ions in the Li2O/CuO composite can be discharged to their metallic state and also recharged to their highest oxidation state through the electrochemical conversion reaction, releasing a remarkably high capacity. However, the observed capacity for the composite cathode is ∼23% lower than the theoretical capacity expected from a full three-electron conversion reaction. This capacity gap may originate from the following two reasons: On the one hand, the CuO particles cannot be fully transformed into metallic Cu at the potential over 1.0 V, as discussed previously,23,25,26 because the deposition of

Cu involves a huge polarization, causing the deposition potential to decrease to a more negative value. On the other hand, it is difficult for electrochemically inactive Li2O to go through a complete conversion due to its low reactivity. From the viewpoint of lithium battery applications, only the discharge capacity at the high-voltage plateau is usable as a cathode-active material. The charge
discharge curves of the Li2O/CuOa electrode at the high-voltage region are illustrated as the inset in Figure 5. As is shown, the Li2O/CuOa nanocomposite can deliver a quite high capacity of ∼300 mAh g
1 at a highvoltage region of 2.0
4.4 V, which is about twice higher than current commercialized high-capacity LiCoO2 (∼150 mAh g
1) and LiFePO4 (∼160 mAh g
1). This high redox capacity observed from the Li2O/CuO nanocomposite at room temperature and at moderately high rates suggests a potential possibility to create a high-capacity conversion cathode for lithium-ion batteries using fairly inexpensive CuO materials. Figure 6 compares the discharge/charge behaviors of the Li2O/CuO electrodes made from different types of CuO. All the Li2O/CuO samples show very similar stepwise charge and discharge profiles, implying a conversion mechanism governing the electrode process. In comparison, the Li2O/CuOa nanocomposite gives a reversible capacity of 564 mAh g
1, remarkably higher than that of the Li2O/CuOb (503 mAh g
1), Li2O/CuOc (460 mAh g
1), and Li2O/CuOd (444 mAh g
1). Moreover, the Li2O/CuOa nanocomposite exhibits a longer high-voltage discharge plateau and a lower charging voltage, characterizing a smaller polarization and higher rate capability. This phenomenon can be apparently accounted for by a downsizing effect of the CuOa particles, the extremely small size (∼5 nm) of which is kinetically favorable for lithium diffusion and phase-transformation reactions. Figure 7 shows the cycling performances of these Li2O/CuO electrodes at a constant cycling current of 100 mA g
1. All the Li2O/CuO electrodes, made from different types of CuO, as labeled in the figure, display a stable reversible capacity of 446, 427, 420, and 390 mAh g
1, respectively, and show only a slight capacity decay after 15 cycles. This good cycleability suggests that the electrochemical conversion reaction of Li2O/ CuO nanocomposites could occur very reversibly as long as the CuO and Li2O particles are sufficiently downsized, uniformly dispersed, and electrically wired. 6171

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Figure 8. Cycling performance of the Li2O/CuOa electrodes at various current rates as labeled in the figure.

To evaluate their applicability as a practical cathode-active material, we cycled the as-prepared nanocomposites at different current rates at room temperature. As an example, Figure 8 shows the high-rate performance of the Li2O/CuOa electrodes cycled at various current densities. At a considerable high rate of 500 mA g
1, the Li2O/CuOa electrode delivered a discharge capacity of 438 mAh g
1 at the second cycle and maintained a value of ∼370 mAh g
1 after 15 cycles. When the current density was increased to a very high value of 1000 mA g
1, the discharge capacity of this cathode could still reach ∼330 mAh g
1 at cycling. The excellent high-rate capability and cycling stability observed from the Li2O/ CuO nanocomposite in this work demonstrate that the conversion materials could serve not only as a high-capacity but also as a high-rate cathode material as long as appropriate nanodomains are created to facilitate the phase-transformation reactions occurring in the discharge and charge of the composite electrode. Electrochemical Conversion Mechanism. To understand the conversion mechanism of the Li2O/CuO composite, we measured the cyclic voltammetric (CV) response of the composite electrodes using a three-electrode cell. Figure 9 shows typical CV curves of the Li2O/CuOa electrode at a slow scan rate of 0.1 mV s
1 in a wide voltage range of 1.0
4.4 V. In the first negative scan, there was a remarkable reduction peak at 2.2 V, followed by a pair of closely located peaks at 1.3 and 1.1 V. These CV features are basically in accordance with the differential capacity
voltage curve25 and the step potential electrochemical spectroscopy26 previously reported for the CuO anodes and could also be assigned to Liþ reaction with CuO to form Cu2O and successive reductive decomposition of Cu2O into Cu and Li2O, respectively, as in the earlier mechanistic studies of CuO reduction.25,26 In the reversed scan, the first distinct oxidation peak appeared at 2.7 V and a broad band emerged at 3.1
3.6 V, followed by a small peak at 4.0 V. The first oxidation peak is likely brought about by the conversion reaction of Cu2O and Cu with additional Li2O to form lithium cupric oxide Li2CuO2. The second broad band at 3.1
3.6 V is attributed to consecutive Liþ deintercalation from the Li2CuO2 phase to form LiCuO2.27,28 The third oxidation peak at 4.0 V is possibly due to the electrochemical decomposition of LiCuO2 at this high potential, as reported in previous studies on the Li-intercalation behaviors of LiCuO2.27 However, in the second cathodic scan, the CV features appeared very different from those in the first negative scan, showing an overlapped reduction band at ∼2.8 V and a broad band at 1.6

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Figure 9. Cyclic voltammograms of the Li2O/CuOa sample in 1 mol L
1 LiPF6 þ EC
DMC
EMC. Scan rate = 0.1 mV s
1.

V, followed by a small peak at 1.1 V. This difference in the CV curves is well in accordance with the difference in the potential profiles of the Li2O/CuO electrode at the first and second discharge, as shown in Figure 5, indicative of different reduction mechanisms operating in the first two scans. As discussed above, CuO is almost transformed to a biphasic mixture of Cu2O and Cu at the first discharge and the latter is successively converted to LiCuO2 on the participation of Li2O at the first charge. As a result, the CV bands observed from the Li2O/CuO electrode since the second scan could be considered as a reflection of electrochemical redox behaviors of LiCuO2. That is, the cathodic process of the Li2O/CuO electrode proceeds through electrochemical reduction of LiCuO2 into Li2CuO2 with Liþ insertion, followed by a further reduction of Li2CuO2 into a Cu2O, Cu, and Li2O nanomixture, and on the contrary, the nanomixture of Cu2O, Cu, and Li2O is oxidatively transformed to LiCuO2 at subsequent anodic scans. On the basis of the CV evidence and in light of previous mechanistic studies of the electrochemical behaviors of LiCuO2,27,28 the overall charge
discharge reactions of the Li2O/CuO nanocomposite electrode could be described as follows: At the first discharge from 2.8 to 1.0 V 1 1 CuO þ Liþ þ e
f Cu2 O þ Li2 O 2 2

ð1Þ

1 1 Cu2 O þ Liþ þ e
f Cu þ Li2 O ð2Þ 2 2 At subsequent charge
discharge cycles in the potential region of 1.0
4.4 V 1 1 Cu þ Li2 O T Cu2 O þ Liþ þ e
2 2

ð3Þ

1 3 Cu2 O þ Li2 O T Li2 CuO2 þ Liþ þ e
2 2

ð4Þ

Li2 CuO2 T LiCuO2 þ Liþ þ e


ð5Þ

To further confirm the conversion mechanism described above, we also carried out an XRD characterization of the Li2O/CuO nanocomposite electrode at different states of charge and discharge. To avoid the interference from graphite for XRD signals, the Li2O/CuO electrodes were prepared by use of less 6172

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Li2O/CuO nanocomposite to serve as a conversion electrode with cycling stability and high energy storage capacity.

Figure 10. Ex situ XRD patterns of the Li2O/CuO electrodes at different discharge and charge states as labeled in the inset figure.

’ CONCLUSIONS In summary, we prepared four types of Li2O/CuO nanocomposites by ball-milling Li2O and nanosized CuO using conductive TiN nanoparticles as grinding powders, and investigated the nanocomposites as the conversion materials for high-capacity rechargeable lithium batteries. Electrochemical measurements demonstrated that the Li2O/CuO nanocomposite as prepared can deliver a reversible high capacity of 560 mAh g
1 at 50 mA g
1 and also shows a strong power capability with a quite high rate output of about 438 mAh g
1 even at 500 mA g
1 at room temperature. It was found from CV and XRD analyses that the superior high capacity and cycling stability arise exclusively from the electrochemical conversion of the Li2O/CuO nanocomposite, which enables the three-electron redox to proceed reversibly along with the reversible phase transformation of the cathodeactive phases. These results may suggest the possibility to utilize the inexpensive metal oxides as high-capacity cathode materials for future-generation Li-ion batteries through an electrochemical conversion mechanism. ’ AUTHOR INFORMATION

diffractive acetylene black and polyvinylidene fluoride (PVDF) as conductive and adhesive agents instead of graphite and PTFE. As shown in Figure 10, the XRD pattern of the Li2O/CuO composite at open circuit reflects only pristine CuO and TiN phases, but not any diffraction from Li2O, possibly due to the poor diffractions of well-dispersed Li2O nanoparticles. When first discharged to 1.0 V, the electrode shows a very different XRD pattern from its initial phases with the disappearance of the CuO signals (Figure 10, pattern b) and appearance of some new diffraction peaks at 36.6, 42.5, and 43.5°, all of which can be well indexed to Cu2O and Cu, respectively. Although the diffractions of Cu2O at 36.6° and 42.5° are partially overlapped with those of TiN, both of them are still distinguishable from the peak splitting and potential dependence of the XRD bands. Once recharged to 3 V (Figure 10, pattern c), a weak XRD peak emerges at 26.3°, while the Cu2O peaks at 36.6° and 42.5° and the Cu peak at 43.5° decrease their intensities, revealing the formation of Li2CuO2 through the electrochemical conversion of Cu2O/Cu involving Li2O. Because the conversion reaction can only occur in the nanodomains of well-mixed phases, the intermediate phases generated during the reaction appear in a nanosized and well-dispersed state with low crystallinity, which usually leads to weak diffraction intensities in their XRD spectra. Nevertheless, if the electrode is charged to 3.8 V (Figure 10, pattern d), a new XRD peak comes out at 18.3°, characterizing the formation of LiCuO2 through a Liþ deintercalation reaction from Li2CuO2. With a continuous charge to a high voltage of 4.2 V, two weak peaks at 35.5° and 38.7° appear, indicative of the CuO phase produced from the electrochemical decomposition of LiCuO2 at the high oxidation potential.27 During subsequent discharge, the XRD peak at 26.3°, characteristic of Li2CuO2, reappears at 2.7
2.3 V, reflecting Liþ reinsertion into the LiCuO2 phase. As the discharge proceeds to a low potential region of 1.6 and 1 V, the XRD signals of Cu2O and Cu reemerge evidently, demonstrating a reversible conversion of the cathode-active phases from their highest oxidation state to a completely discharged state. This reversible phase transformation together with a concomitant multielectron redox enables the

Corresponding Author

*Tel: þ86-27-68754526. Fax: þ86-27-87884476. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the financial support by the 973 Program, China (Grant No. 2009CB220100). ’ REFERENCES (1) Kang, K.; Meng, Y. S.; Breger, J.; Grey, C. P.; Ceder, G. Science 2006, 311, 977. (2) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652. (3) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587. (4) Scrosati, B.; Garche, J. J. Power Sources 2010, 195, 2419. (5) Whittingham, M. S. Chem. Rev. 2004, 104, 4271. (6) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mater. 2010, 22, 691. (7) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496. (8) Li, H.; Richter, G.; Maier, J. Adv. Mater. 2003, 15, 736. (9) Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150, A1209. (10) Badway, F.; Mansour, A. N.; Pereira, N.; Al-Sharab, J. F.; Cosandey, F.; Plitz, I.; Amatucci, G. G. Chem. Mater. 2007, 19, 4129. (11) Yamakawa, N.; Jiang, M.; Key, B.; Grey, C. P. J. Am. Chem. Soc. 2009, 131, 10525. (12) Li, T.; Li, L.; Cao, Y. L.; Ai, X. P.; Yang, H. X. J. Phys. Chem. C 2010, 114, 3190. (13) Grugeon, S.; Laruelle, S.; Herrera-Urbina, R.; Dupont, L.; Poizot, P.; Tarascon, J. M. J. Electrochem. Soc. 2001, 148, A285. (14) Taberna, P. L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J. M. Nat. Mater. 2006, 5, 567. (15) Li, Y.; Tan, B.; Wu, Y. Nano Lett. 2008, 8, 26. (16) Debart, A.; Dupont, L.; Patrice, R.; Tarascon, J. M. Solid State Sci. 2006, 8, 640. (17) Lai, C. H.; Huang, K. W.; Cheng, J. H.; Lee, C. Y.; Hwang, B. J.; Chen, L. J. J. Mater. Chem. 2010, 20, 6638. (18) Pereira, N.; Klein, L. C.; Amatucci, G. G. J. Electrochem. Soc. 2002, 149, A262. 6173

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(19) Pereira, N.; Balasubramanian, M.; Dupont, L.; McBreen, J.; Klein, L. C.; Amatucci, G. G. J. Electrochem. Soc. 2003, 150, A1118. (20) Yu, X. Q.; Sun, J. P.; Tang, K.; Li, H.; Huang, X. J.; Dupont, L.; Maier, J. Phys. Chem. Chem. Phys. 2009, 11, 9497. (21) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem., Int. Ed. 2008, 47, 2930. (22) Cabana, J.; Monconduit, L.; Larcher, D.; Palacín, M. R. Adv. Mater. 2010, 22, E170. (23) Gao, X. P.; Bao, J. L.; Pan, G. L.; Zhu, H. Y.; Huang, P. X.; Wu, F.; Song, D. Y. J. Phys. Chem. B 2004, 108, 5547. (24) Yu, Y.; Shi, Y.; Chen, C. H. Nanotechnology 2007, 18, 055706. (25) Debart, A.; Dupont, L.; Poizot, P.; Leriche, J. B.; Tarascon, J. M. J. Electrochem. Soc. 2001, 148, A1266. (26) Morales, J.; Sanchez, L.; Martin, F.; Ramos-Barrado, J. R.; Sanchez, M. Electrochim. Acta 2004, 49, 4589. (27) Arai, H.; Okada, S.; Sakurai, Y.; Yamaki, J. Solid State Ionics 1998, 106, 45. (28) Prakash, A. S.; Larcher, D.; Morcrette, M.; Hegde, M. S.; Leriche, J. B.; Masquelier, C. Chem. Mater. 2005, 17, 4406.

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