Effect of Annealing on the First-Cycle Performance and Reversible

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Effect of Annealing on the First-Cycle Performance and Reversible Capabilities of Lithium-Rich Layered Oxide Cathodes Yufang Chen, Zhongxue Chen, and Kai Xie* College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, 410073, China ABSTRACT: The cathode materials of lithium-rich 0.4Li 2 MnO 3 ·0.6LiNi1/3Co1/3Mn1/3O2 have been synthesized via coprecipitation method. To propose a better understanding of the effects of annealing on the cathode material, the bulk and surface structures of the pristine and cycled samples of 0.4Li2MnO3· 0.6LiNi1/3Co1/3Mn1/3O2 are characterized by X-ray diffraction (XRD) together with transmission electron microscopy (TEM). According to XRD and TEM observations, the surface structure of the sample annealed at 800 °C (LMCNO-800) tends to transform from layer to spinel after the first cycle, whereas the sample annealed at 950 °C (LMCNO-950) inclines to turn amorphous. Combined with the electrochemical characterizations, it was found that the LMCNO-800 cathode has a high activation degree, which enables a high discharge capacity of 274 mAh g−1. However, the spinel surface structure caused by activation is apt to suffer from corrosion by the electrolyte during cycling, which leads to rapid capacity decay. On the contrary, the surface amorphous structure of the first cycled LMCNO-950 sample may serve as a barrier layer between the bulk structure and the electrolyte, thereby guaranteeing extremely high capacity retention of 98.5% after 100 cycles. Further, the effects of different annealing temperatures on electrochemical performance of the Li-rich compounds are investigated. It is thus concluded that the structure stability and electrochemical performance of the Li-rich cathode is strongly dependent on the annealing temperature, and the results suggest that the sample annealed at 900 °C exhibits the best electrochemical performance.

1. INTRODUCTION The demand of lithium-ion batteries is rapidly growing for the power source of portable electronics and electric vehicles. Layered transition metal oxides, spinels, and olivines are currently used as the cathodes for lithium-ion batteries. However, the available energy density of these electrode materials is difficult to be enhanced because of the limited lithium intercalation stoichiometry. Therefore, great efforts have been devoted to explore alternative cathodes, such as high voltage spinel1 and silicates,2 in order to improve the energy density of large-scale rechargeable lithium-ion batteries. Among numerous cathode materials, Li-rich layered oxides (nominal xLi2MnO3·(1 − x) LiMO2) have gained particular attention because of their extremely high capacity.3 This extraordinary capacity is obtained from an abnormal electrochemical reaction, which involves more than one lithium-ion transfer and the electrochemical activation of Li2MnO 3 compound. During the activation process, lithium and oxide ions are simultaneously extracted from the lattice of Li2MnO34,5 and thus release a high capacity over 250 mAh g−1.6 However, as we know, although the activation of Li2MnO3 is favorable for high specific capacity, still the continuous activation process is harmful to the stability of structure and finally leads to capacity decay and voltage decline. It has been reported that the activation of Li-rich cathode is strongly related to the crystal structure, which comprises the surface activation at first charge−discharge and progressive activation of the bulk structure.7 In previous papers, most © 2014 American Chemical Society

researchers only focused on the construction of a passive layer to enhance cycling stability on Li-rich cathode by electrochemical or chemical methods, such as limiting the voltage window8 and former treating by acid,9 but the synthesis condition, especially the annealing effect on the structure and electrochemical performance of Li-rich cathode, was seldom reported. In this work, we synthesized a variety of Li-rich layered oxide compounds 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 by a coprecipitation method at different temperatures. To investigate the annealing effect on the structure of 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the bulk and surface structures of the pristine and cycled samples. Electrochemical tests were conducted to further explore the relationship between the electrochemical performance and the activation process. The results suggest that the sample annealed at 900 °C exhibits the best electrochemical performance, delivering a capacity of 255 mAh g−1 with capacity retention of 94.6% after 100 cycles. Received: January 6, 2014 Revised: May 7, 2014 Published: May 12, 2014 11505

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2. EXPERIMENTAL SECTION 2.1. Materials Preparation. To synthesize Li-rich layered oxide compounds 0.4Li 2 MnO 3 ·0.6LiNi 1/3 Co 1/3 Mn 1/3 O 2 , NiSO4·5H2O, CoSO4·7H2O, and MnSO4·H2O were used as the starting material for the coprecipitation process. The obtained metal hydroxide precursors were mixed with LiOH· H2O at the required stoichiometry. After that, the mixture was thermal treated first at 480 °C for 10 h and then was followed with further sintering at 800, 850, 900, and 950 °C for 15 h in air. The final products were denoted as LMCNO-800, LMCNO-850, LMCNO-900, and LMCNO-950, respectively. The heating rate was controlled at 1 °C min−1. 2.2. Morphology and Structure Investigation. X-ray diffraction (XRD) of these samples was performed with a Bruker D8 advanced powder X-ray diffractometer with Cu−Kα radiation. The morphologies of the samples were observed with scanning electron microscopy (SEM, Histachi S-4800) and transmission electron microscopy (JEM-2100F). Inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses were conducted on a varian 725-ES spectrometer to determine the relative amounts of Li, Ni, Co, and Mn in the samples. The atomic ratio of Ni/Co/Mn in the sample was measured to be 1:1:3. ICP measurements revealed that the Li: (Ni + Co + Mn) ratios are 1.440, 1.438, 1.422, and 1.393 for LMCNO-800, LMCNO-850, LMCNO-900, LMCNO-950, respectively, which are close to the value of stoichiometry (1.4). 2.3. Electrochemical Studies. The electrochemical measurements of the cathode materials were performed using 2016-type coin cells with lithium foil as a counter electrode. The electrolyte was 1.2 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (3:7 by weight). Cathodes were prepared by mixing the 0.4Li2MnO3· 0.6LiNi1/3Co1/3Mn1/3O2 with 10 wt % acetylene black and 10 wt % poly(vinylidene fluoride) (PVDF) in N-methyl pyrrolidone (NMP) solution. The slurry was cast onto an Al foil and was dried in a vacuum oven at 80 °C. The coin cells were cycled galvanostatically in the voltage range of 4.8−2.0 V at a rate of 0.1 C (25 mA g−1) at room temperature. Cyclic voltammetric measurement was carried out with the coin cells at the scan rate of 0.05 mV s−1, and electrochemical impedance spectroscopy (EIS) tests were carried out using a threeelectrode system.

LMCNO-850, LMCNO-900, and LMCNO-950 refer to the samples sintered at 800, 850, 900, and 950 °C, respectively. As can be seen, all peaks in the pattern can be indexed to the R3̅m space group, except for the weak superlattice reflections at 2θ = 20−25° which are attributed to Li2MnO3 (C2/m symmetry). From the enlarged image of this region, we can find that the intensity of the peaks increases as the annealing temperature increases, which demonstrates that the Li2MnO3-like features are more obvious after high-temperature annealing. As can also be seen, both the (006)/(102) and the (108)/(110) peaks are well split, suggesting that a well-defined layered structure with good crystallinity was formed in the lattice. The crystal refinement results showed that the lattice parameter a gradually increases from 2.846795 (LMCNO-800) to 2.852058 Å (LMCNO-950) and c increases from 14.212464 to 14.239626 Å as the annealing temperature increases, indicating that the crystalline size grows bigger when the annealing temperature goes up. In addition, the relative intensity ratios of the (003) to (104) lines in four samples are about 1.51 (LMCNO-800), 1.65 (LMCNO-850), 1.92 (LMCNO-900), and 2.04 (LMCNO950), suggesting that the cation disorder decreases with increasing sinter temperature. The SEM images of the 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 sintered at different temperatures are shown in Figure 2. It can be seen that LMCNO-800 (Figure

3. RESULTS AND DISCUSSION Figure 1 presents the XRD patterns of the as-prepared 0.4Li 2 MnO 3 ·0.6LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 . LMCNO-800,

Figure 2. SEM images of (a) LMCNO-800, (b) LMCNO-850, (c) LMCNO-900, and (d) LMCNO-950.

2a), LMCNO-850 (Figure 2b), and LMCNO-900 (Figure 2c) are composed of uniformly distributed particles with an average size of 100−200 nm. When increasing the annealing temperature to 950 °C (Figure 2d), the particles tend to aggregate and exhibit a sudden increase in particle size, which is located in the range of 400−500 nm. To deeply study the effects of annealing on the bulk and surface structures of the 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 cathode materials, two representative samples, LMCNO-800 (Figure 3a) and LMCNO-950 (Figure 3b), were investigated by using transmission electron microscope (TEM). As presented in Figure 3a, the LMCNO-800 sample has shortrange order arrangement. From the local images, we can easily find that the distance between the fringes of the regions A and B is 0.315 and 0.202 nm, corresponding to the d spacing values of the (022) plane of monoclinic structure (C2/m) and the (104) plane of layered structure (R3̅m), respectively. These two

Figure 1. XRD patterns of 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 materials calcined at different temperatures. 11506

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Figure 3. TEM images of pristine (a) LMCNO-800 and (b) LMCNO-950.

particle size and crystalline size for the Li-rich compound when the annealing temperature is raised to 950 °C, indicating that the capacity of the LiMn1/3Ni1/3Co1/3O2 is closely related to the crystalline size. When the Li-rich cathodes are charged above 4.45 V, we can see that the capacity generated by Li2MnO3 increased rapidly from LMCNO-800 to LMCNO-900 and then decreased slightly for the LMCNO-950. These observations suggest that the activation of Li2MnO3 is determined by both crystallinity and crystalline size; when the annealing temperature is lower than 900 °C, crystallinity is the major factor to affect the activation degree, and when the temperature is above 900 °C, crystalline size becomes the dominant factor. To visualize the detail structure changes of the Li-rich compounds after cycling, the TEM images of the LMCNO-800 (Figure 5a) and LMCNO-950 (Figure 5b) taken from the cycled electrode are presented in Figure 5. Compared to the TEM image of uncycled particles in Figure 3a, the bulk structure of the cycled LMCNO-800 (Figure 5a) still retained its layered phase. However, the surface structure was electrochemically etched, and a 5−8 nm thick area with obviously different crystalline structure can be detected. From the local magnified image, we can determine that the structure transformed from layered to spinel, which is similar to the reported finding by Robertson and Bruce.3 By comparison, the surface structure of cycled LMCNO-950 transformed from layered to disordered structure as amorphous phase (Figure 5b) after the first cycle, which can be seen from the enlarged image and the corresponding electron diffraction (ED) pattern of the selected surface area. A ∼2 nm thick amorphous layer was formed on the surface of the particle, which may serve as a barrier layer between the bulk structure and the electrolyte. Figure 6 presents the XRD patterns of the cycled 0.4Li 2 MnO 3 ·0.6LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 . LMCNO-800f, LMCNO-850f, LMCNO-900f, and LMCNO-950f refer to the samples sintered at 800, 850, 900, and 950 °C after the first cycle, respectively. Compared with the pristine samples (Figure 1), the cycled samples still maintain their layered structure, except that the characteristic peaks at 20−25° for Li2MnO3 get weaker. Furthermore, some extra peaks appear around 44° in the patterns of LMCNO-800 and LMCNO-850 but were not observed in the other two samples; these peaks can be attributed to the spinel phase. These results further demonstrate that the Li-rich sample annealed at lower temperature has more surface structural transition potential. Figure 7 compares the EIS of the four cathodes before and after the initial cycle. The EIS data can be well fitted by the equivalent circuit shown in the inset of Figure 7d, where Rs

phases were embedded in each other to form complex longrange order. As the annealing temperature increases to 950 °C, the lattice fringe of the LMCNO-950 sample in Figure 3b becomes more visible. The lattice fringe with a basal distance of 0.478 nm can be clearly observed from the locally magnified image of region B, which is consistent with the (003) lattice spacing of rhombohedra LiNi1/3Co1/3Mn1/3O2 (R3̅m). The lattice fringe distance of 0.203 nm in region A can be indexed to the (202) lattice spacing of Li2MnO3 phase. We can see that the region A was embedded in the long-term-ordered region B to form a “mixed structure”, which refers to the solid solution structure. Interestingly, region A does not show any long-term order arrangement, which is consistent with the XRD results that no superlattice structure was observed in the pattern. These results reveal that the structure of Li-rich compounds is strongly related with the annealing temperature. Figure 4 shows the initial charge/discharge curves of the LMCNO-800, LMCNO-850, LMCNO-900, and LMCNO-950

Figure 4. Charge−discharge curves of LMCNO-800, LMCNO-850, LMCNO-900, and LMCNO-950 in first cycle at a cycling current of 0.1 C (=25 mA g−1).

samples. As mentioned by previous work, the initial charge capacity of the Li-rich cathode can be divided into two parts. The first part below 4.45 V can be ascribed to the reversible lithium intercalation/deintercalation in LiMn1/3Ni1/3Co1/3O2, and the second part above 4.45 V should be attributed to the electrochemical activation of Li2MnO3.10−13 As we can see, the capacity that originated from LiMn1/3Ni1/3Co1/3O2 remained at ∼170 mAh g−1 for the LMCNO-800, LMCNO-850, and LMCNO-900 samples but dropped to 140 mAh g−1 for the LMCNO-950 sample. From the XRD and SEM characterizations, we have known that there is a sudden increase in 11507

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Figure 5. TEM images of the (a) LMCNO-800 and (b) LMCNO-950 after the first cycle.

represents the electrolyte resistance, Rf refers to the surface resistance, Csl refers to the capacitance, Rct and Cdl refer to the charge-transfer resistance and double-layer capacitance, and Zw refers to the diffusion impedance of lithium ion in the solid phase. As is shown in Figure 7, all the EIS spectra of the four electrodes display two overlapped semicircles at high- to medium-frequency regions. The left semicircle relates to the ohmic portion of the impedance while the right one refers to the charge-transport resistance. In previous work, it has been investigated that the Rct is actually the main resistance contribution of the whole cell system,14−16 especially for Lirich cathode-based half cells. As can be seen, after the first cycle, the charge-transfer resistance of the LMCNO-800 (Figure 7a) and LMCNO-850 (Figure 7b) electrode increased greatly, which indicates that the surface of the cycled sample after phase transformation may be easily corroded by the electrolyte. By comparison, the diameters of the semicircles for the other two spectra almost remain unchanged, indicating that the sample annealed at higher temperature may form a stable surface layer

Figure 6. Powder XRD patterns for 0.4Li 2 MnO 3 ·0.6LiNi1/3Co1/3Mn1/3O2 materials within different sinter temperatures after first cycle (#, indexed to Al foil).

Figure 7. EIS of the four cathodes before and after the initial cycle. 11508

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Figure 8. Charge−discharge profiles of four samples at 1st, 2nd, 10th, 50th, and 100th cycles.

Figure 9. Cyclic voltammograms of (a) LMCNO-800, (b) LMCNO-850, (c) LMCNO-900, and (d) LMCNO-950.

after the first cycle. In addition, the relatively lower chargetransfer resistance of the LMCNO-900 (Figure 7c) and LMCNO-950 (Figure 7d) cathodes may lead to much faster lithium ion transport and smaller polarization. This has already been demonstrated in Figure 4 as we can see that the gap between the charge and discharge plateau potentials of

LMCNO-900 and LMCNO-950 is much smaller than that of LMCNO-800 and LMCNO-850. The charge−discharge profiles of four samples at 1st, 2nd, 10th, 50th, and 100th cycles are given in Figure 8. As is shown, the LMCNO-800 (Figure 8a), LMCNO-850 (Figure 8b), LMCNO-900 (Figure 8c), and LMCNO-950 (Figure 8d) electrodes delivered the initial reversible capacities of 80.3, 11509

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138.4, 109.9, and 104 mAh g−1, respectively. It seems that an increase in annealing temperature from 800 to 900 °C can bring out gradually decreased discharge capacity, and the capacity difference emerges only in the potential region below 2.8 V; this is presumably because the reversible capacity of Li2MnO3 is related to the crystalline size. When the annealing temperature goes up to 950 °C, the discharge capacity of LMCNO-950 decreases suddenly, which may be attributed to the decreased charge capacity of LiMn 1/3 Ni 1/3 Co 1/3 O 2 mentioned in Figure 4. In addition, a discharge plateau at ∼2.5 V for LMCNO-800 appeared in the first cycle and disappeared in subsequent cycles, which may attributed to the reduction of Mn4+ in spinel impurities.17,18 After 100 cycles, the LMCNO-800, LMCNO-850, LMCNO-900, and LMCNO-950 electrodes retain 73.0%, 84.1%, 94.6%, and 98.5% of the initial capacity, respectively. Apparently, the capacity retention of the Li-rich cathodes is affected by the crystalline structure. The surface spinel structures of cycled LMCNO-800 and LMCNO850 may provide a convenient channel for the progressive activation of the bulk structure and thus may lead to capacity deterioration, whereas the amorphous surface structure of the cycled LMCNO-900 and LMCNO-950 sample may serve as a barrier layer between the bulk structure and the electrolyte, thereby guaranteeing an extremely high capacity retention. In addition, we can see that the voltage decay still exists for the four cathodes. The decay of the discharge midpoint voltage for all the samples is quite similar, reaching ∼400 mV after 100 cycles, indicating that the voltage decay has little relationship with the surface structure of Li-rich compounds. Recently, Croy et al.19 demonstrated that lithium and manganese ordering plays a significant role in the voltage degradation mechanisms of high-capacity lithium- and manganese-rich composite electrode structures. In general, xLi2MnO3·(1 − x) LiMO2 (M = Mn, Ni, Co) electrode material with more content of Li2MnO3 has a larger voltage decay during cycling, and electrode materials with the same amount of Li2MnO3 possess the same voltage decay feature and similar degradation after activation. This finding may account for the similar voltage decay observed from the four cathodes in our work. Figure 9 presents the cyclic voltammogram (CV) curves of the four samples. As can be seen, all the CV curves are quite similar to those reported in previous papers.20−23 Two anodic peaks appeared in the first positive scan, which correspond to the reversible lithium intercalation/deintercalation in LiMn1/3Ni1/3Co1/3O2 and the electrochemical activation of Li2MnO3. The oxidation peak at ∼4.6 V shifts to lower potential gradually from LMCNO-800 (Figure 9a) and LMCNO-850 (Figure 9b) to LMCNO-900 (Figure 9c) and then shifts to slightly higher potential (Figure 9d); this trend is consistent with the variation tendency of the charge capacity in Figure 4. In the reverse negative scan for the four samples, two broad cathodic peaks were observed in the potential range of 3.8−3.0 V, which correspond to the reduction of Ni4+ and Co4+.17 Moreover, a noticeable peak at ∼2.5 V was also detected for LMCNO-800 and disappeared subsequently; this result is in good agreement with the initial discharge behavior in Figure 6. In the fifth scan, the anodic peak at ∼4.6 V almost disappeared, and only one visible and reversible anodic peak at ∼3.8 V remained, suggesting that the structure of the Li-rich cathodes tends to become stable after the structural rearrangement.24−26 Meanwhile, the cathodic peaks of all the four samples shift to lower potential gradually, further indicating that the voltage decay for Li-rich cathode still exists.

4. CONCLUSIONS In summary, we have prepared a variety of Li-rich layered oxide compounds 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 by a coprecipitation method at different temperatures. The bulk and surface structures of the pristine and cycled samples of Li-rich compounds are characterized by XRD and TEM to evaluate the effects of annealing on the cathode material. The results show that the surface structure of the compound annealed at 800 °C (LMCNO-800) tends to transform from layer to spinel after the first cycle, whereas the sample annealed at 950 °C (LMCNO-950) inclines to turn amorphous. XRD and EIS characterizations further demonstrate the relationship between the surface structural transformation and the annealing temperature for the Li-rich cathodes. Electrochemical characterizations show that the structure transformation has strong effect on the electrochemical performance of the Li-rich cathode. The LMCNO-800 cathode delivers a high initial capacity with a poor cycling performance while the LMCNO-950 cathode exhibits a relatively lower capacity but with an extremely high capacity retention. The sample annealed at 900 °C (LMCNO900) displays the best electrochemical performance, delivering an initial capacity of 255 mAh g−1 with capacity retention of 94.5% after 100 cycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel/fax: 86-731-84573150. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the financial support from the program of science and technology innovative research team in higher education institutions of Hunan province and the national science foundation of China (21303262) and Hunan Provincial natural science foundation of China (13jj4004).

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REFERENCES

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