121 Journal of New Materials for Electrochemical Systems 4, 121-125 (2001) © J. New Mat. Electrochem. Systems
Charge-discharge characteristics and phase transitions of mixed LiNi0.8Co0.2O2 and LiMn2O4 cathode materials for lithium-ion batteries
Z. F. Maa,*, X. Q. Yangb, X. Sunb and J. McBreenb aDepartment
of Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, P. R.China bBrookhaven National Laboratory, Upton NY 11973, USA (Received March 15, 2001; received in revised form April 4, 2001)
Abstract: The charge-discharge characteristics of the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 was studied in a battery cell using a lithium foil as an anode. Three plateaus were observed at 3.85 V, 4.0 V and 4.2 V during charge. The operating voltage of the composite cathode is higher than the single-component cathode LiNi0.8Co0.2O2. Compared to the other single-component LiMn2O4 cathode, a smaller capacity fading was obtained for the composite cathode. The synchrotron based In situ x-ray diffraction (XRD) was used to study the phase transitions of the composite cathode during charge-discharge. In the composite cathode, the phase transitions of each component during chargedischarge can be clearly identified. These phase transitions are basically the same as observed in single-component cathode: the phase transitions from hexagonal I (H1) to hexagonal II (H2), and then to hexagonal III (H3) were observed for the component of LiNi0.8Co0.2O2, and the phase transitions from cubic I (C1) to cubic II (C2), and then to cubic III (C3) were observed for the component of LiMn2O4. However, some interesting new kinetic effects, which are different from the single-component cathode, were observed. Key words: Composite cathode, lithium ion batteries, cycle life, phase transitions.
1. INTRODUCTION
batches with the ideal layered structure [1]. A more serious problem is the poorer thermal stability of LiNiO2 in the overcharged state. This problem causes safety concerns for the cell. The structural limitations of LiNiO2 cathode material can be partly overcome by using a cobalt-substituted LiNi1-xCoxO2 structure [2-6]. The third type of cathode material, LiMn2O4, has a spinel-type structure, A[B2]X4. Materials with this structure have become the subject of intensive research over the past ten years. The spinel compounds LiMn2O4 and other manganese oxide based compounds (layer structured LiMnO2, for example) are potential cathode materials for the lithiumion batteries in the future [7-9]. Numerous papers have been published about the synthesis and characterization of these three types of cathode materials for lithium-ion batteries. Because the electrochemical performance and cycle life of lithium-ion batteries depend on the structure integrity of the insertion electrodes during repeated insertion/extraction of
The rapid growth of new portable electronic devices such as laptop computers, mobile phones and camcorders has generated huge demands for advanced rechargeable batteries. Lithium ion batteries are the most widely used power source for these devices because of its high energy density. The transition metal oxide insertion cathode materials such as LiCoO2, LiNiO2, and LiMn2O4 are three important types of cathode materials. Among them, LiCoO2 is the most widely used cathode material for commercial lithium-ion cells because of its high power density. Although LiNiO2 is cheaper than LiCoO2, and is capable of delivering a rechargeable capacity greater than the LiCoO2 material when charged to 4.2V, it is difficult to prepare large and reproducible LiNiO2
*To
whom correspondence should be addressed. Fax: +86-2154741297, email:
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lithium cation, x-ray diffraction (XRD) is often used to characterize the crystal structures of these cathode materials. Most reported structural studies of intercalation cathode materials are based on ex situ XRD data. Several in situ XRD studies on lithium intercalation materials such as LiNiO2 [10, 12], LiNi1-yCoyO2 [13, 14], 11], LiCoO2 [3, LiMg0.125Ti0.125Ni0.75O2 [15] and LixMn2O4 [16] have been reported. The in situ XRD is a much more powerful tool to study the phase transitions during charge-discharge, especially when kinetic effects and multi-phases are involved. Recently, some battery companies have demonstrated a new concept of mixing two different types of insertion compounds to make a composite cathode, aimed at reducing cost and improving self-discharge [17, 18]. However, not much work has been published on the charge-discharge characteristics and phase transitions for these composite cathodes. In this paper, the study of the charge-discharge characteristics for a composite cathode made of mixture of LiNi0.8Co0.2O2 and LiMn2O4 are reported. In situ synchrotron based XRD was used to study the phase transitions of the composite cathode during the charge and discharge processes.
Fig. 1. Charge and discharge curves of composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 (c1 and d1) during in situ XRD data collection. The c2 and d2 curves are the charge and discharge curves of the single-component LiNi0.8Co0.2O2 cathode.
2 EXPERIMENTAL The samples of high purity LiNi0.8Co0.2O2 and LiMn2O4 were purchased from FMC Corporation (FMC Corp., Gastonia, North Carolina). The composite cathode material was prepared by mixing LiNi0.8Co0.2O2 and LiMn2O4 with a ratio of 1:1(w/w), in a mixer (SPEX Mixer/Mill) for 2 hours. This mixture was further mixed with polyvinylidene fluoride (PVDF) binder (Kynar Flex 2801, Atochem), and acetylene black in a weight ratio of 80:10:10. N-methyl1-2-pyrrolidone (NMP) solvent was trickled into the mixture to make a slurry. Then the slurry was coated onto an aluminum foil. After vacuum drying at 100°C for 12h, the electrode disks (2.8 cm2) were punched and weighed. The average weight of active material was 20mg. The test cathodes were incorporated into cells with a Li metal foil anode, a Celgard@-2500 separator, and an electrolyte of 1M LiPF6 in a 1:1 EC:DMC solvent (LP 10 from EM Industries Inc.). Charge-discharge cycle testing was carried out on a battery test system (BT-2043 model, Arbin Instruments Inc.). In order to avoid corrosion of the beryllium window at voltage above 4.3 V, Mylar windows were used to replace the beryllium windows in these in situ cells. In situ XRD spectra were collected on beam line X18A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The energy of x-ray radiation is 10.375 k eV (λ=1.195 Å). The step size of 2θ scan was 0.02 degree. The XRD spectra were collected in the transmission mode. The design of the in situ cell is described in reference 14.
3. RESULTS AND DISCUSSION 3. 1. Charge discharge characteristics For the assessment of the electrochemical performance and
cycle life of the composite cathode made of LiNi0.8Co0.2O2 and LiMn2O4, the charge-discharge characteristics and cycle lives of these mixed cathode materials were measured. The first charge and discharge curves of the composite cathode are plotted in Fig. 1 as c1 and d1 respectively. (These curves were recorded during the in situ XRD data collection, which will be presented later in this paper.) The cell was charged at a constant current of 0.3 mA (a C/10 rate). For comparison purpose, the first charge and discharge curves of the singlecomponent LiNi0.8Co0.2O2 cathode are also plotted in Fig. 1 (curve c2 and d2 respectively). The same C/10 rate was used for this reference cell. The charge curve c1 for the composite cathode combines the features of the two components in the mixture. (The charge-discharge curves of the singlecomponent LiMn2O4 cathode are not plotted). The first plateau at 3.85 V is attributed to the dilithiuation of the LiNi0.8Co0.2O2 component while the second plateau at 4.0 V and the third plateau at 4.2 V are attributed to the LiMn2O4 component [19]. However, compared with curve c2 for the single-component cathode LiNi0.8Co0.2O2, the first plateau in c1 is at a higher voltage due to the contribution of the LiMn2O4 component. This contribution is also responsible for the higher voltage in the discharge curve d1 (comparing with the d2 curve). The higher-voltage discharge curve has the advantage of high energy density. In addition, maybe more importantly, it provides more usable capacity for powering mobile phones, which require a cut-off voltage above 3.8 V. The third plateau at 4.2 V (4.15 V for discharge) is also beneficial. Because it provides more usable capacity within the safety range. Without this plateau, the charging process would have been terminated earlier at 4.3 V, as indicated in curve c2, in order to avoid over-charge of the LiNi0.8Co0.2O2
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materials. Five cycles for this cell have been taken. The first charge/discharge capacity was 153 and 140 mAh/g, respectively. The first and second charge processes have a higher plateaus. After the third cycle, the charge/discharge plateaus became stable, and three plateaus were observed. The more electrochemical performances of the composite cathode for different ratios of LiNi0.8Co0.2O2/ LiMn2O4 were investigated by means of step potential sweep techniques and cycle lives testing techniques [20].
3. 2. Phase transitions of the composite cathode during charge and discharge Fig. 2. Voltage vs. capacity for the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 at the C/4 rate.
cathode. Fig. 2 shows the charge/discharge characteristics and cycle lives of a new testing cell with the composite cathode
Fig. 3. In situ XRD patterns of the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 during the first charge in the 2θ-angle range from 13° to 16°.
In order to eliminate any unpredictable factors introduced by the cycling history, a fresh cell was used for synchrotron based in situ XRD studies. A slower charge-discharge rate (a C/10 rate at a constant current of 0.3 mA) was applied for the in situ cell. The cell was charged from 3.5 V to 4.5 V and discharged from 4.5 V to 3.0 V. The charge and discharge curves recorded during in situ XRD data collection are plotted in Fig. 1 as c1 and d1. The in situ XRD spectra during first charge are plotted in Fig. 3 and 4. Some corresponding voltages of the cell are marked in Fig. 3. The spectra are also numbered in the order as they were taken. The label “R” means the cell was at rest after the charging current was cut off at 4.5 V. Part of the data in scan 7 and 8 are missing due to the unavailability of the x-ray beam. Since the charging process continued during this period, incomplete spectra are kept in the plot. The XRD spectra
Fig. 4. In situ XRD patterns of the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 during the first charge in the 2θ-angle range from 27° to 53°.
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Fig. 6. In situ XRD patterns of the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 during the first discharge in the 2θ-angle range from 27° to 53°.
Fig. 5. In situ XRD patterns of the composite cathode made of the mixture of LiNi0.8Co0.2O2 and LiMn2O4 during the first discharge in the 2θ-angle range from 13° to 16°.
plotted in Fig. 3 cover a 2θ angle range from 13 to 16 degrees. The (003) reflections of three hexagonal structures (H1, H2, and H3) for LixNi1-yCoy type layered materials [14] are located in this range. The (111) reflections of the three cubic structures (C1, C2, and C3) for the spinel LixMn2O4 are also located in this range. (due to the shorter wave length used here, the corresponding 2θ angles for the same Bragg peaks are smaller here than those when a radiation with λ =1.54 Å wave length were used). The scan 1 in Fig. 3 shows an asymmetric single peak, centered around 14.4º labeled as (003)H1 for LiNi0.8Co0.2O2 and (111)C1 for LiMn2O4. No significant changes occurred in scan 2 and 3. In scan 4, a new peak emerged at a lower angle as the original peak, moved to an even lower angle in scan 5, and then moved to higher angles from scan 6 to scan 9. At the last scan when the cell was resting (charge current had been cut off) after the 4.5 V limit had been reached, a new peak centered at 15º was emerged. This new peak is identified as the (003) reflection of the H3 hexagonal structure for LiNi0.8Co0.2O2. The three cubic structures of LiMn2O4 can also be identified from Fig. 3. In scan 5, The (111)C1 peak started to move to a higher angle, indicating the delithiation of LiMn2O4. A new peak emerged
at a higher angle in scan 6, and became a sharp dominant peak located at 14.52º in scan 7. This new peak is labeled as (111)C2. The peak located at 14.7 º in scan 9 and scan R was identified as (111) reflection for C3 of LiMn2O4. The twophase coexistent region between C2 and C3 can be clearly identified in scan 9. The XRD spectra at higher 2θ angles are plotted in Fig. 4. The phase transitions from H1 to H2 and then to H3 for LiNi0.8Co0.2O2 can be clearly identified by the (104)H1, (104)H2, and (104)H3 peaks. The two-phase coexistence region between C1 and C2 for Li1-xMn2O4 [16] can be seen clearly in these spectra. The contraction of the lattice constant for C2 is indicated by the increasing angles of the (311)C2 and (440)C2 peaks. The separation of the (311)C1 and (311)C2 peaks clearly indicates the two-phase coexistence of C1 and C2 in scans 5 and 6. This can be further confirmed by the separation of (440)C1 and (440)C2 peaks in scan 5 and 6. This is different than the XRD spectra of singlecomponent Li1-xMn2O4 cathode, where the coexistence of C2 and C3 is widely observed, while the coexistence of C1 and C2 has not been so clearly identified. The reversibility of these structural changes of this composite cathode was studied during discharge. The cell was discharged from 4.5 V to 3.5 V at a D/10 rate at a constant current of 0.3 mA. The in situ XRD spectra are plotted in Fig. 5 and 6. It can be seen from these spectra, all the structural changes during charging are reversible. No irreversible damage of the structure was observed.
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4. CONCLUSIONS The charge-discharge behavior and capacity fading of the composite cathode, made of a mixture of LiNi0.8Co0.2O2 and LiMn2O4 were studied. It can be seen that the mixed cathode materials have three plateaus at 3.85 V, 4.0 V and 4.2 V during charging. Both the charge and discharge plateaus of the composite cathode were higher than the corresponding ones for the single component cathode, LiNi0.8Co0.2O2. In situ synchrotron XRD was used to study structural changes of the composite cathode during first charge-discharge cycle. The results of XRD pattern shows the phase transitions of this composite cathode basically follow the same routes as for the two components from which they were made. All of the three hexagonal phases, H1, H2, and H3 for the LiNi0.8Co0.2O2, and all of the three cubic phases, C1, C2, and C3 for LiMn2O4 were observed. The formation of H3 structure during rest indicates that the voltage cut-off limit should be set lower than 4.5 V, in order to avoid the formation of an over-charged structure.
AKNOWLEDGEMENTS Z. F. Ma gratefully acknowledges the financial support by the Natural Science Foundation of China under contract No. 20076026. The work performed at Brookhaven National Laboratory is supported by U.S. Department of Energy, Division of Materials Science of the Office of Basic Energy Sciences, and the Office of Energy Research, Laboratory Technology Research Program, under Contract No. DE-AC0298CH10886.
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