A New Look at Lithium Cobalt Oxide in a Broad Voltage Range for

Feb 2, 2010 - (1.0-4.3 V) are studied by charge-discharge cycling, XRD, XPS, Raman, and HRTEM. It is found that ... Battery Preparation and Electroche...
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J. Phys. Chem. C 2010, 114, 3323–3328

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A New Look at Lithium Cobalt Oxide in a Broad Voltage Range for Lithium-Ion Batteries Jie Shu,* Miao Shui, Fengtao Huang, Yuanlong Ren, Qingchun Wang, Dan Xu, and Lu Hou Faculty of Materials Science and Chemical Engineering, Ningbo UniVersity, Ningbo, Zhejiang 315211, People’s Republic of China ReceiVed: December 18, 2009; ReVised Manuscript ReceiVed: January 17, 2010

The electrochemical behaviors and lithium-storage mechanism of LiCoO2 in a broad voltage window (1.0-4.3 V) are studied by charge-discharge cycling, XRD, XPS, Raman, and HRTEM. It is found that the reduction mechanism of LiCoO2 with lithium is associated with the irreversible formation of metastable phase Li1+xCoII IIIO2-y and then the final products of Li2O and Co metal. During the charging process, the Li2O/Co mixture can be oxidized into CoO, and then the Li2O/CoO mixture can result in the formation of Co3O4 in the higher-voltage region. LixCoOy is the final product when the active material is charged to 4.3 V. During the subsequent cycles, the lithium uptake/release reactions are related to the reversible conversion of Co T CoO T Co3O4 T LixCoOy. 1. Introduction In recent years, various metal oxides, such as tin oxides (SnO, SnO2), cobalt oxides (CoO, Co3O4), iron oxides (Fe3O4), etc.,1-3 have aroused great interest as anode materials for high Li-storage capacity and excellent calendar life. Among these metal oxides, transition-metal oxides (such as Co3O4) show excellent capacity maintenance (>90%, 50 cycles), high Coulombic efficiency (>95%), and large reversible capacity (600-800 mAh g-1). On the basis of the previous research reports of Y. M. Kang’s and J. M. Tarascon’s groups,2,4,5 it is known that the low binding energy of Li2O for the catalysis of Co and well-dispersed nanosized Li2O particles can bring high-degree reversible Li2O formation/decomposition coupled reactions. This indicates that the redox process between Co3O4/Li and Co/Li2O mixtures is feasible. Therefore, splendid electrochemical properties for these transition-metal oxides come from the reversible formation/ decomposition of Li2O. On the basis of the above analysis and former studies, it is believed that the catalytic effect of transitionmetal particles (Co, Ni, Fe, Cu, etc.) is the main driving force to increase the electrochemical reaction activity of transitionmetal oxides. As is well-known, LiCoO2, LiNiO2, LiMn2O4, and LiFePO4 are cathode materials for lithium-ion batteries. Reversible capacities about 110-160 mAh g-1 can be delivered for these electrode materials when they were cycled in high-voltage regions. On the other hand, they are transition-metal composite oxides with a catalytic element in the structure. However, lithium element is one of the components in these compounds, which is different from common transition-metal oxides (such as Co3O4, NiO, MnO2, Fe3O4, etc.). In recent years, Sony laptop batteries have presented a risk of explosion from possible inner short circuit. Upon short circuit or abuse, partial cathode materials in the electrode are overlithiated and may hold more lithium ions than those in the initial materials. Therefore, it is necessary to study the electrochemical behaviors for these common cathode materials in a broad electrochemical window, especially for safety. On comparison with common transitionmetal oxides, these special oxides probably have different * To whom correspondence should be addressed. E-mail: sergio_shu@ hotmail.com, [email protected]. Tel: +86-574-87600787. Fax: +86-57487600734.

electrochemical behaviors upon lithium-ion insertion/extraction reactions in 0.0-4.3 V (vs Li+/Li). In this paper, electrochemical characterization of LiCoO2 was investigated in broad voltage regions (1.0-4.3 V). Structural changes and lithium storage of LiCoO2 were carefully studied with different lithium contents. It is expected that systematic and thorough studies of traditional cathode materials in broad voltage regions will provide new insight to develop advanced and safe electrode materials for lithium-ion batteries. 2. Experimental Section 2.1. Synthesis of Material. In the experiment, stoichiometric amounts of high-purity lithium hydroxide (99.9 wt %, Aldrich) and cobalt acetate (99.9 wt %, Aldrich) were mixed by highenergy ball milling for 5 h as the precursors. Microsized LiCoO2 was synthesized from a solid-state reaction in the air. The starting materials were initially heated at 400 °C for 12 h and then calcined at 850 °C for 24 h to obtain the product. 2.2. Battery Preparation and Electrochemical Testing. To fabricate the electrode, a slurry was prepared by mixing the LiCoO2 (85 wt %), acetylene black (10 wt %), and polyvinylidene fluoride (5 wt %) with N-methyl-2-pyrrolidone as solvent. The slurry was spread on Ti foil (10 µm) and dried at 120 °C for 12 h, then cut into disks. The electrodes for repeated cycling were assembled with 2016 coin cells in an Ar-filled glovebox (M. Braun, Germany). The electrodes for physical characterization were fabricated with Swagelock batteries. The coin cell or Swagelock battery was composed of the Ti piece with LiCoO2 as working electrode, a Whatman glass fiber filter as separator, a lithium metal disk as counter electrode, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v) as electrolyte. Galvanostatic charge-discharge (here, “discharge” and “charge” correspond to lithium insertion and extraction, respectively) cycling was tested on a multichannel Land Battery Test System (Wuhan, China). All batteries were charged and discharged at a constant current of 30 mA g-1. All the electrochemical tests were carried out at room temperature. 2.3. Physical Characterization. The X-ray diffraction (XRD) patterns collected on samples were carried out with a Rigaku X-ray diffractometer using nickel-filtered Cu KR radiation (λ

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Figure 1. (a) Charge-discharge curves of LiCoO2 in the voltage ranges of 0.0-4.3 V and 0.0-3.0 V. (b) Corresponding ex situ XRD patterns of oxides in the dotted position (a, original; b, discharged to 1.0 V; c, discharged to 0.0 V; d, recharged to 4.3 V).

) 1.5418 Å), operating at 40 kV and 100 mA (Rigaku D/maxRB, Japan). The surface morphology for the sample was characterized by means of scanning electron microscopy (SEM, Philips Co., XL30). The grain morphology was observed with high-resolution transmission electron microscopy (HRTEM), performing at 200 kV (JEM-2010, JEOL, Japan). The electrodes for HRTEM investigation were taken out of the batteries and rinsed with anhydrous DMC, then evacuated overnight. Before the investigation, powders scraped from the electrodes were dispersed in DMC using ultrasonic technique. After that, the sample was dropped onto a copper grid and then evacuated for 5 h. The transfer of the sample to the HRTEM chamber was performed within several seconds under Ar blowing. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB MKII photoelectron spectrometer (VG, Great Britain). The XPS experiments were performed in the spectroscopy chamber using a standard Mg anode X-ray source (Mg KR X-ray line at 1253.6 eV) and a 150 mm hemispherical electron energy analyzer. The samples for XPS study were taken out of the batteries and rinsed with anhydrous DMC, then evacuated overnight. Before operation, samples were adhered to nickel sample holders, then sealed in a bottle purged with Ar atmosphere. The transfer of the samples to the XPS chamber was operated within several seconds under Ar blowing. Raman behaviors were taken on a SPEX-1403 Raman spectrophotometer (SPEX, U.S.A.) from 100 to 1000 cm-1. Samples for Raman analysis were taken out of the batteries and rinsed with anhydrous DMC, then evacuated overnight. Before study, samples were sealed in two quartz disks with vacuum ester. All the samples were prepared in the Ar-filled glovebox. 3. Results and Discussion LiCoO2 is the most widely commercial material for lithiumion batteries. When it is used as cathode material, LiCoO2 shows excellent cycle calendar life and has a reversible capacity of 135 mAh g-1 (0.5 Li). However, as exhibited in Figure 1a, there is a flat voltage plateau that appears at 1.25 V and the lithiation capacity is 1055 mAh g-1 (3.98 Li) after it is discharged to 0.0 V. It is different from the electrochemical behavior of Co3O4 (or CoO), which shows a Li uptake platform at 0.7 V. It indicates that the reaction of LiCoO2 with Li may have its own special characteristics and mechanism. During the lithium-ion extraction process, lithiated active material can deliver a delithiation capacity of 1182 mAh g-1 (4.32 Li). Viewed from charge-discharge curves, there are four plateaus (1.96, 2.29, 2.60, and 3.87

V) for lithium release between 0.0 and 4.3 V, corresponding to four peaks in the dQ/dV data. For comparison, Co3O4 merely exhibits two plateaus (1.36 and 2.19 V) during lithium-ion extraction. Moreover, one voltage plateau appears at ∼3.87 V in the charge curve of LiCoO2, corresponding to a capacity of 250 mAh g-1 from lithium-ion extraction. This plateau can also be observed when LiCoO2 is cycled between 1.0 and 4.3 V. Therefore, it may not be entirely attributed to solid electrolyte interphase (SEI) film decomposition at high voltage. According to the capacity evaluation in Figure 2a, it is believed that the reversible capacity of 180-190 mAh g-1 comes from this extraction plateau during the subsequent cycling. Partial lithium ions may be released from the formation of high-valence cobalt compounds, here, named as LixCoOy. As a result, the first Coulombic efficiency is as high as 117.8%. This suggests that the reaction of LiCoO2 with lithium ion has its own special characteristics and mechanism in this broad electrochemical window. Figure 1b shows the structural changes (ex situ XRD results) of active material during the first lithium-ion insertion and extraction processes in 0.0-4.3 V. After 2.64 Li insertion (point b in Figure 1a), all the diffraction peaks of original LiCoO2 (point a) disappear and new reflection shoulders appear at 20.2-25.2, 33.7, 43.3-45.2, and 56.2° in the ex situ XRD pattern b. According to the JCPDS file, the new phase may not be cobalt oxides (CoO, Co2O3, and Co3O4) but Li2O (JCPDS No. 77-2144). It is the product of the reaction of LiCoO2 with Li. Moreover, it is difficult to find obvious differences in XRD patterns after it went down to 0.0 V, which suggests that the b-c process is probably correlated to the formation of organic conductive polymer,6 SEI layer, and possible lithium ion further intercalation. From the XRD pattern with the recharged state (point d) in Figure 1b, it can be observed that, while the lithium ions are extracted, the Li2O peaks disappear and unknown diffraction shoulders show at 20.2, 30.04, 45.05, and 66.15°. This delithiated phase is different from the charged product of Co3O4 or CoO,7,8 indicative of a new electrochemical mechanism. In addition, SEI film generated at low voltage can decompose upon charge, which is responsible for the abnormal plateau at 3.8 V in the second discharge curve. As a result, it exhibits poor cycling performance because of structural breakdown and electrolyte exhaustion in the subsequent cycles. To improve the cycle calendar life, capacity limitation is a favorable method for suppressing the particle pulverization. Limitation of upper cutoff voltage is one of the choices for capacity

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Figure 2. (a) Charge-discharge curves of LiCoO2 in the voltage range of 1.0-4.3 V. Corresponding ex situ XRD (b), Co 2p XPS (c), and Raman (d) patterns are recorded in the dotted position (a, original; b, first discharged for 24 h; c, first discharged to 1.0 V; d, first charged to 2.5 V; e, first charged to 4.3 V; f, second discharged to 1.38 V; g, second discharged to 1.0 V; h, second charged to 2.73 V; i, second charged to 4.3 V).

restriction.9 The voltage curves of the cell cycled with an upper voltage limitation of 3.0 V are shown in Figure 1. Unfortunately, the cycle calendar life is still poor in the voltage range from 0.0 to 3.0 V. Particle pulverization during lithium-ion insertion in the low-voltage range, especially below 1.0 V, may be the key factor to account for the irreversibility of LiCoO2 electrodes. This indicates that limitation of delithiation capacity cannot take great effect to suppress the cracking and pulverization of the electrode. Therefore, limitation of lithium-ion accommodation is very important to decrease volume variation. This technique is very effective to enhance the reversible cycling of crystalline silicon powder.10 To limit the insertion capacity of LiCoO2, the lower cutoff voltage is adjusted to 1.0 V. Herein, the microstructure of oxide is expected to be maintained in the voltage range from 1.0 to 4.3 V. As shown in Figure 2a, it is obvious that LiCoO2 shows excellent electrochemical cycleability in this voltage range. Different with the degradation curves cycled between 0.0 and 4.3 V, there is a dramatic improvement in the reversibility of LiCoO2 with the lower voltage limitation increasing from 0.0 to 1.0 V. Therefore, four characteristic delithiation plateaus during repeated cycling (first to sixth) still can be seen. The phenomenon suggests that the structural change of active material is effectively restricted with an increase of the lower cutoff voltage from 0.0 to 1.0 V. Furthermore, it fails to observe SEI film decomposition at high voltage (>3.5 V). This indicates that the interface of the LiCoO2/electrolyte is stable when this oxide was cycled above 1.0 V. Therefore, electrolyte will not

be exhausted for repeated SEI film formation and decomposition. When the possibility of SEI film decomposition is excluded, it suggests that the delithiation plateau at 3.8 V in Figure 2a is probably attributed to the formation of LixCoOy. In addition, the initial slope (1.8-3.7 V) in the subsequent discharge curve (second to sixth) probably corresponded to the reversed reversible lithium-ion insertion/extraction of LixCoOy. However, this plateau gradually shortens upon cycling. It indicates that the microstructure of oxide undergoes irreversible transformation slightly with each cycle. To clearly investigate the Li-storage/release mechanism between 1.0 and 4.0 V, HRTEM, XPS, Raman, and XRD were performed on LiCoO2 with different lithiated states. The crystal structure and phase changes in the initial two cycles (1.0-4.3 V) were observed by ex situ XRD, as exhibited in Figure 2b. Compared with the results in 0.0-4.3 V (Figure 1b), it is obvious that the ex situ XRD patterns show more detailed information about structural evolution upon cycling. There was no metastable phase other than a layered R-NaFeO2 structure, but Li2O was observed by XRD upon initial Li insertion; see pattern b shown in Figure 2b. It seems that the reaction of LiCoO2 with Li results in active material direct decomposition into Li2O and metal Co. However, XPS and Raman results show that the metastable phase appears during the initial insertion. Generally, the Co 2p spectrum is split by the spin-orbital interaction into Co 2p1/2 and Co 2p3/2 regions. As displayed in Figure 2c, the Co 2p3/2 and Co 2p1/2 peaks corresponding to trivalent Co3+ are observed at 778.87 and 793.57 eV for the

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Figure 3. SEM and TEM images of LiCoO2: (a) original; (b, c) first discharged to 1.0 V; (d-i) first charged to 4.3 V, characterization of Co3O4; (j-l) first charged to 4.3 V, structure of LixCoOy.

starting LiCoO2, respectively. The original two main peaks almost disappear after the initial reaction between LiCoO2 and Li, while another four new peaks appear at 780.53 (Co 2p3/2), 785.96 (satellite, Co 2p3/2), 796.73 (Co 2p1/2), and 803.06 eV (satellite, Co 2p1/2) when there is initial 1.16 Li storage in the system. This is different from Co 2p bands in Co metal.11 Usually, Co2+ in an oxygen environment is characterized by a strong broadening of the main line and a very intense satellite peak at 785.96 (Co 2p3/2) and 803.06 eV (Co 2p1/2); see pattern b shown in Figure 2c.12 Furthermore, ex situ XRD (pattern b, Figure 2b) shows that partial particles still maintain the R-NaFeO2-like structure, except for the appearance of the Li2O phase. However, no shoulder of cobalt oxides, especially for CoO or possible Li2CoO2 (similar as spinel Li2NiO2),13 is detected. In addition, Debart et al reported that the reaction mechanism of CuO with lithium involved the formation of a solid solution of Cu1-xIICuxIO1-x/2 (0 < x < 0.4).14 Therefore, it can be concluded that the metastable phase may be a compound with a postulated chemical formula of Li1+xCoII IIIO2-y (0 < x, 0 e y), which probably has a similar structure with LiCoO2. Besides, ex situ Raman displays the blue shifts of two main stretching bands (479 f 483 cm-1, 590 f 592 cm-1) after a lithium intercalation capacity of 317 mAh g-1. For the pristine LiCoO2 (pattern a exhibited in Figure 2d), the high-frequency band at 590 cm-1 originates from the asymmetric stretching mode (A1g) of the CoO6 octahedron, whereas the mediumfrequency band at 479 cm-1 is assigned to the bending mode (Eg) of the O-Co-O chemical bond. The observed upward shifts of both bands are probably attributed to the change of cell parameters, which contributes to a change in electrostatic repulsion with the insertion of lithium ion. Another two new bands are also observed at 515 and 693 cm-1, corresponding to Li-O stretching of Li2O and probable Co-O (or Co-O-Li) bonding of Li1+xCoII IIIO2-y,15 respectively. When the electrode is discharged to 1.0 V, only Li2O and Co metal can be observed from XRD, XPS, and Raman spectrum. It indicates that the structure of Li1+xCoII IIIO2-y is totally destroyed and it decom-

poses into Li2O and Co at the end of discharge process. All this evidence reveals that the reaction of LiCoO2 with lithium ion comes through several processes that do not reach the final by one step. Therefore, the first discharging process can be represented as follows:

LiCoO2 + (x + 2y)Li+ + (x + 2y)e- f Li1+xCoO2-y + yLi2O (1) Li1+xCoO2-y + (3 - x - 2y)Li+ + (3 - x - 2y)e- f (2 - y)Li2O + Co (2) As is well-known, the electrochemical oxidation of the Li2O and Co mixture normally can deliver the product of CoO.2,15 After about two Li extractions (550 mAh g-1), the Li2O Bragg reflections disappear and the new diffraction peaks at 34.4 and 57.6° can be identified to CoO (JCPDS No. 75-0419); see pattern d shown in Figure 2b. Moreover, a weak characteristic diffraction peak at 44.95° probably accounts for the further electrochemical oxidation of the partial CoO/Li2O mixture into Co3O4 (JCPDS No. 78-1969). According to the previous literature,15-18 it is obvious that XPS and Raman spectra (pattern d in Figure 2, panels c and d) also confirm the biphase products composed of CoO and Co3O4. When the electrode is charged to 4.3 V, it is noticed that the indexing lines for CoO completely vanish and the relative intensity of the Bragg reflections for Co3O4 increases, indicative of all the CoO conversion into Co3O4. On the other hand, several extra diffraction peaks (18.7, 20.6, 21.7, 22.3, 36.29, and 66.5°) arise from the XRD pattern of the charge-end products (pattern e displayed in Figure 2b), corresponding to the probable electrochemical oxidation of partial Co3O4 into a new compound. In addition, the XRD spectra of Co2O3 in the JCPDS database reveal different characteristic reflections with those of the new phase. Thackeray et al found

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Figure 4. (a) Charge-discharge curves of LiCoO2 in 1.0-3.0 V. (b) Corresponding curves about electrochemical performance vs cycle number.

that the charging process of Li2O/Co3O4 cathode can lead to the postulated formation of Li2xCo2O3+x in a high-temperature Li-ion cell.19 Therefore, it indicates that the final product of lithium extraction to 4.3 V is probably the mixture of Co3O4 and LixCoOy (1.34 + 0.5x e y e 1.5 + 0.5x) in our experiment. The XPS spectra (780.56, 786.49, 796.26, and 802.53 eV) shift downward after a capacity of 350 mAh g-1 is released from point d to e in Figure 2a. Furthermore, the Raman spectra (185, 461, 505, 609, 667, and 679 cm-1) are similar with those of Co3O4 but stretching at a lower wavenumber. All the information shows that the end-point product is different from any other known cobalt oxide and their mixtures. Hence, the charging reaction can be described by the following equations:

Li2O + Co f CoO + 2Li+ + 2e-

(3)

3CoO + Li2O f Co3O4 + 2Li+ + 2e-

(4)

Co3O4 + (3y - 4)Li2O f 3LixCoOy + (6y - 3x - 8)Li+ + (6y - 3x - 8)e- (5) For comparison, ex situ techniques (XRD, XPS, and Raman) show that the redischarged compound (point f in Figure 2a) exhibits the characterizations of Co3O4. Raman spectroscopy is a more sensitive probe of structural distortions, short-range order, and symmetry in solids in comparison with XRD, which usually reveals structural information on the long-range order of materials. It is worth noticing that the Raman spectrum (pattern f in Figure 2d) taken from the redischarged sample clearly exhibits five well-defined Raman peaks at 193 (F2g), 480 (Eg), 521 (F2g), 618 (F2g), and 689 (A1g) cm-1, assigned to the Raman-active modes of Co3O4.20 When the sample is further discharged to 1.0 V, Li2O and Co metal can be observed by XRD and XPS again (pattern g in Figure 2, panels b and c). More Co3O4 can form after the electrode is recharged to the plateau at 2.73 V in the second cycle, as revealed by pattern h in Figure 2, panels b and c. With repeated cycling, ultrafine delithiated products account for the weakened or disappeared diffraction peaks (pattern i displayed in Figure 2b), whereas pattern i in the XPS spectra confirms that the reversibility of the Co3O4 T LixCoOy conversion reaction process. The subsequent charge-discharge cycles are similar with the behavior of the second one. On the basis of these results, the subsequent reaction mechanism of active material with lithium ion in a

broad voltage range (1.0-4.3 V) can be expressed by the following steps:

Li2O + Co T CoO + 2Li+ + 2e-

(6)

3CoO + Li2O T Co3O4 + 2Li+ + 2e-

(7)

Co3O4 + (3y - 4)Li2O T 3LixCoOy + (6y - 3x - 8)Li+ + (6y - 3x - 8)e-

(8)

The HRTEM technique is used to further confirm the reaction mechanism between LiCoO2 and Li. Original LiCoO2 is a kind of particle with a diameter of 2 µm (SEM photograph in Figure 3a). As shown in Figure 3b,c, the bulk sample pulverizes into smaller particles after the electrode is initially discharged to 1.0 V. With results obtained above by XRD, XPS, and Raman, it is known that the products are composed of Li2O and Co metal. Similar with the reduction of CuO,13 the final products are related to the formation of Co nanosized grains dispersed into the Li2O matrix, as shown in Figure 3b,c. When the active material is charged to 4.3 V, hundreds of nanosized grains aggregate into one large particle, as exhibited in Figure 3d, indicating that the Li2O/Co mixture is oxidized into cobalt oxide with a large size. To make a clear description, various high-resolution TEM pictures are taken from different parts of the same particle. TEM photographs in Figure 3e,f are observed and recorded from the bottom corner of the particle. The lattice distance is measured to be 0.466 nm for the bottom part, as the HRTEM picture revealed in Figure 3f. It is similar with that of original LiCoO2. However, all the pristine samples involved in the charge-discharge reaction are completely destroyed, corresponding to the disappearance of basic characterization for LiCoO2 in Figure 2. Considering that the d spacing of other cobalt oxides, such as Co2O3 and CoO, is far smaller than 0.466 nm, the observed d spacing can be attributed to the index of the (111) plane of Co3O4. On the other hand, a close investigation at the right corner of the particle in Figure 3d also confirms the existence of Co3O4 in the final charged products. As the HRTEM images show in Figure 3h,i, it is obvious that the characteristic (220) and (311) planes that contributed to Co3O4 can be clearly seen and correspond to d values of 0.285 and 0.243 nm, respectively. These results are also consistent with those achieved by XRD, XPS, and Raman analysis. If all the cobalt oxides are Co3O4, it is impossible to deliver an

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insertion capacity of 200 mAh g-1 above 1.5 V in the subsequent discharge, as the curve displayed in Figure 2a.4,5,7 It is found that the distance between the two adjacent planes, d, is measured to be 0.474 nm on another particle, as given in Figure 3j-l. This value is different from that of any other cobalt oxide (Co3O4, Co2O3, and CoO) and LiCoO2, but similar with the d111 (0.4797 nm) of Li1.34Co3O4 (JCPDS No. 39-0846), which can be obtained by the reduction reaction between Co3O4 and Li,5,21 indicative of the formation of a new compound during further oxidation. Therefore, it is believed that this plane is attributed to the former postulated new phase LixCoOy, which is also proved by XRD, XPS, and Raman techniques. To further improve the reversibility of the conversion reaction, the upper and lower cutoff voltages are set to 3.0 and 1.0 V, respectively. It is expected that this technique can suppress the structural change of active material upon lithium uptake/release. As shown in Figure 4a,b, a dramatic improvement in cyclic performance is found when the LiCoO2 electrode is cycled between 1.0 and 3.0 V. Comparing with the discharge behavior in the voltage range from 1.0 to 4.3 V, no lithiation slope (1.8-3.7 V) appears in the initial region of the second to fifth discharge profiles, implying that limitation of charging upper voltage may restrict the formation of LixCoOy. Moreover, the cycle calendar life becomes better with the limitation of upper and lower voltages. As a result, it can be concluded that repeated phase transitions of active material are a main cause for capacity deterioration during cycling. 4. Conclusions LiCoO2 shows abnormal lithium uptake/release behaviors in 1.0-4.3 V, indicating that this active material has a peculiar Li-storage story. A series of reaction mechanisms between LiCoO2 and lithium ion are investigated by various electrochemical and physical characterization techniques. With charge-discharge time after time, the active material experiences several irreversible and reversible phase transformations. In the initial discharge, the reactions are irreversible and can be described by the process of LiCoO2 f Li1+xCoII IIIO2-y f Co. After that, the insertion/extraction mechanism during repeated cycling is related to the reversible conversion of Co T CoO T Co3O4 T LixCoOy. Through adjusting the cycling voltage windows, it is known that the capacity retention of active material is associated with the restriction of phase changes during repeated charge-discharge processes. In addition, the formation and decomposition of SEI film can be effectively

Shu et al. suppressed in the high working voltage region, which prevents the exhaustion of electrolyte after many cycles. Acknowledgment. The authors truly thank Prof. Tingfeng Yi of Anhui University of Technology, Dr. Yu Shi of CANMET Energy Technology Centre-Devon, Dr. Ying Wang of Japan Advanced Institute of Science & Technology, Prof. Shuyan Gao of the National Institute of Advanced Industrial Science & Technology and Henan Normal University, Dr. Xiaodong Zhu of Harbin Institute of Technology, Dr. Ying Bai of Henan University, Prof. Yueqing Zheng and Mr. Wei Xu of Ningbo University for their help on the experimental techniques and suggestions. The work is sponsored by K. C. Wong Magna Fund in Ningbo University. References and Notes (1) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (2) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 499. (3) Taberna, P. L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J. M. Nat. Mater. 2006, 5, 573. (4) Kang, Y. M.; Kim, K. T.; Kim, J. H.; Kim, H. S.; Lee, P. S.; Lee, J. Y.; Liu, H. K.; Dou, S. X. J. Power Sources 2004, 133, 259. (5) Larcher, D.; Sudant, G.; Leriche, J. B.; Chabre, Y.; Tarascon, J. M. J. Electrochem. Soc. 2002, 149, A241. (6) Laruelle, S.; Grugeon, S.; Poizot, P.; Dolle, M.; Dupont, L.; Tarascon, J. M. J. Electrochem. Soc. 2002, 149, A634. (7) Kang, Y. M.; Song, M. S.; Kim, J. H.; Kim, H. S.; Park, M. S.; Lee, J. Y.; Liu, H. K.; Dou, S. X. Electrochim. Acta 2005, 50, 3667. (8) Yu, Y.; Chen, C. H.; Shui, J. L.; Xie, S. Angew. Chem., Int. Ed. 2005, 44, 7085. (9) Obrovac, M. N.; Dunlap, R. A.; Sanderson, R. J.; Dahn, J. R. J. Electrochem. Soc. 2001, 148, A576. (10) Li, H.; Huang, X. J.; Chen, L. Q.; Wu, Z. G.; Liang, Y. Electrochem. Solid-State Lett. 1999, 2, 549. (11) Moulder, F. F.; Stickle, W. F.; Sobol, F. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer: Eden Prairie, MN, 1993. (12) Kim, K. S. Phys. ReV. B 1975, 11, 2177. (13) Dahn, J. R.; Von Sacken, U.; Michal, C. A. Solid State Ionics 1990, 44, 87. (14) Debart, A.; Dupont, L.; Poizot, P.; Leriche, J.-B.; Tarascon, J. M. J. Electrochem. Soc. 2001, 148, A1266. (15) Liu, H. C.; Yen, S. K. J. Power Sources 2007, 166, 478. (16) Gallant, D.; Pezolet, M.; Simard, S. J. Phys. Chem. B 2006, 110, 6871. (17) Yi, J. B.; Ding, J. J. Magn. Magn. Mater. 2006, 303, e160. (18) Jiang, J.; Li, L. C. Mater. Lett. 2007, 61, 4894. (19) Thackeray, M. M.; Baker, S. D.; Coetzer, J. Mater. Res. Bull. 1982, 17, 405. (20) Hadjiev, V. G.; Iliev, M. N.; Vergilov, I. V. J. Phys. C: Solid State Phys. 1988, 21, L201. (21) Thackeray, M. M.; Baker, S. D.; Adendorff, K. T.; Goodenough, J. B. Solid State Ionics 1985, 17, 175.

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