Synergistic Enhancement Effect of Al Doping and Highly Active Facets

Mar 30, 2015 - Yao Fu, Hao Jiang,* Yanjie Hu, Yihui Dai, Ling Zhang, and Chunzhong Li*. Key Laboratory for Ultrafine Materials of Ministry of Educatio...
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Synergistic Enhancement Effect of Al Doping and Highly Active Facets of LiMn2O4 Cathode Materials for Lithium-Ion Batteries Yao Fu, Hao Jiang,* Yanjie Hu, Yihui Dai, Ling Zhang, and Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: To overcome the poor cycling stability of LiMn2O4 cathode materials without sacrificing the specific capacity, we demonstrate a new strategy for synergistically enhancing their electrochemical performance by combining the advantages of Al doping and the exposure of highly active facets. Specifically, Al doping can suppress Mn dissolution in the electrolyte, leading to an outstanding cycling stability. In addition, the exposure of highly active facets can greatly enhance the specific capacity and rate capability, while also compensating for the capacity loss caused by Al doping. As a consequence, the as-prepared Al-doped LiMn2O4 truncated octahedrons exhibit a far superior performance in both rate capacity and cycling stability than the pure LiMn2O4 octahedrons and the LiMn2O4 truncated octahedrons. Our work is meaningful not only for the synthesis of highperformance LiAl0.1Mn1.9O4 truncated octahedrons, but also for providing new insight into the development of high-performance LiMn2O4 cathode materials. agreement with several reported experimental results.5,13 Nevertheless, the highly active {100} and {110} crystal facets also easily cause Mn dissolution. This fact was verified by Hirayama et al.,14 who found that the (111) surface of LiMn2O4 is more stable than the (110) surface during charge/discharge processes. Therefore, it remains a great challenge to simultaneously achieve high energy/power densities and long cycle lifetimes, which could be alleviated if the Al doping and the high exposure of active crystal facets could be simultaneously realized in the LiMn2O4 preparation process. In such a smart Al-doped LiMn2O4 material, Al doping can suppress the Mn dissolution into the electrolyte during the intercalation/deintercalation process especially at elevated temperature, leading to outstanding cycling stability, and the high exposure of active facets can greatly enhance the specific capacity and rate capability, while also compensating for the capacity loss caused by Al doping. Inspired by the above hypothesis, herein, we demonstrate the synthesis of LiAl0.1Mn1.9O4 truncated octahedrons by a template-engaged method that combines Al doping and exposure of highly active facets of LiMn2O4 nanoparticles to synergistically enhance their electrochemical performance. As expected, the as-prepared Al-doped LiMn2O4 truncated octahedrons, as cathode materials for LIBs, manifest a maximum specific capacity of 116 mAh g−1 at 0.2C with a markedly enhanced cycling stability (the capacity retentions at room temperature for 1000 cycles and at elevated temperature for 500 cycles are 88.7% and 86.1%, respectively) compared to that of the pure LiMn2O4 truncated octahedrons (75.3% and 66.6%, respectively). In addition, our Al-doped LiMn2O4

1. INTRODUCTION Lithium-ion batteries (LIBs) dominate the market of power supply for small electronic devices and also show huge potential for further applications in the fields of large energy storage and electric vehicles.1−3 The key to meeting the ever-increasing performance demands for large-scale applications is the development of high-performance cathode and anode materials. Among cathode materials, spinel LiMn2O4 is considered as one of the most promising materials by virtue of its low price, environmental friendliness, richness in natural resources, high operating potential, and good safety characteristics.4,5 However, LiMn2O4 cathodes exhibit severe capacity fading during repeated charge/discharge processes, especially at elevated temperature, which is mainly caused by Mn dissolution [2Mn3+(solid) → Mn4+(solid) + Mn2+(solution)] and Jahn− Teller distortion.4,6−9 To overcome this limitation, a major strategy is doping LiMn2O4 with Al, which can enhance its structural stability and meanwhile increase the average valence of Mn.10,11 For example, Kim et al.12 showed that Al-doped LiMn2O4 nanorods had a more stable structure than pure LiMn2O4 nanorods by directly immersing them in an electrolyte of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) at 60 °C for 9 h. Ding et al.10 synthesized Al-doped LiMn2O4 nanotubes using β-MnO2 nanotubes as self-templates. Compared with LiMn2O4 nanotubes and commercial LiMn2O4, the Al-doped LiMn2O4 nanotubes exhibited much better cyclic stability at high rates at both room and elevated temperatures. However, doping with Al to improve the cycling stability occurs at the expense of sacrificing the specific capacity. On the other hand, recent first-principles calculations showed that the electrochemical activity of LiMn2O4 largely depends on the surface lattice plane.13 The {100} and {110} surfaces of LiMn2O4 support fast Li-ion diffusion, which is beneficial for achieving high discharge rate capabilities. This viewpoint is in good © 2015 American Chemical Society

Received: Revised: Accepted: Published: 3800

November 27, 2014 February 24, 2015 March 30, 2015 March 30, 2015 DOI: 10.1021/ie504659h Ind. Eng. Chem. Res. 2015, 54, 3800−3805

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) SEM image, (b) low- and (c) high-magnification TEM images, and (d,e) high-resolution TEM images and corresponding SAED patterns of Al-doped LiMn2O4 truncated octahedrons.

coupled plasma-atomic emission spectroscopy (ICP-AES, Vanan 710). 2.4. Electrochemical Measurements. Electrochemical performance was measured using 2016 coin-type half-cells that were assembled in an argon-filled glovebox. To make cathode disks, 80 wt % active material, 10 wt % super P, and 10 wt % poly(vinylidene difluoride) (PVDF) binder were mixed together in N-methylpyrrolidone (NMP); the slurry was then casted onto an Al foil current collector and dried at 110 °C for 12 h under a vacuum. The counter electrode was pure lithium wafer, and the separator was a polypropylene (PP) membrane (Celgard 2400). The electrolyte was composed of 1 M LiPF6 in a 1:1:1 mixture (volume ratio) of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Cyclic voltammetry (CV) measurements were carried out on an electrochemical workstation (Autolab PGSTAT302N). The charge/discharge measurements were performed on a test system (LAND-CT2001C) at different current densities.

truncated octahedrons also exhibit a superior rate performance (99 mAh g−1 at 20C). Such outstanding electrochemical performances imply an impressive potential as cathode materials for LIB applications.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents used in the experiments were of analytical grade (Sigma-Aldrich) and were used without further purification. 2.2. Method for the Synthesis of LiAl0.1Mn1.9O4 and LiMn2O4 Truncated Octahedrons and LiMn2O4 Octahedrons. The Mn3O4 octahedrons and truncated octahedrons were first obtained by a simple method according to our previous work.15 For the synthesis of LiAl0.1Mn1.9O4 truncated octahedrons, the as-synthesized Mn3O4 truncated octahedrons were dispersed in 5.0 mL of hexane, subsequently mixed with LiOH·H2O and Al(NO3)3 dispersed in 5 mL of ethanol at a mole ratio of Li/Al/Mn = 1.05:0.1:1.9, and then stirred constantly at 60 °C until dried. Finally, the LiAl0.1Mn1.9O4 truncated octahedrons were obtained after annealing of the product at 700 °C for 10 h in an air atmosphere. By contrast, the LiMn2O4 octahedrons and LiMn2O4 truncated octahedrons were prepared with a similar procedure only without the addition of the aluminum source. 2.3. Characterization. The as-synthesized products were characterized by X-ray powder diffraction (XRD; Rigaku D/ Max 2550, Cu Kα radiation) at a scan rate of 1° min−1, fieldemission scanning electron microscopy (FESEM; Hitachi, S4800), and transmission electron microscopy (TEM; JEOL, JEM-2100F) at 200 kV. The chemical composition of Al-doped LiMn2O4 truncated octahedrons was detected by inductively

3. RESULTS AND DISCUSSION The Al-doped LiMn2O4 truncated octahedrons were synthesized by a simple template-engaged reaction using the Mn3O4 truncated octahedron precursor as self-sacrificial templates. The morphologies were characterized by FESEM and TEM. As shown in Figure 1a,b, the Al-doped LiMn2O4 truncated octahedrons had a uniform size distribution with typical dimensions in the range of 100−150 nm. A representative truncated octahedron is shown in Figure 1c. For the later performance comparisons, we also synthesized LiMn2O4 octahedrons and LiMn2O4 truncated octahedrons by a similar 3801

DOI: 10.1021/ie504659h Ind. Eng. Chem. Res. 2015, 54, 3800−3805

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Industrial & Engineering Chemistry Research procedure only without the addition of Al3+. As seen in Figures S1 and S2 (Supporting Information), both of these materials had uniform size distributions with typical dimensions in the range of 100−150 nm, similarly to the Al-doped LiMn2O4 truncated octahedrons. To confirm the high exposure of the active facets of the truncated octahedrons, we recorded highresolution TEM (HR-TEM) images and selected-area electron diffraction (SAED) patterns (insets) of LiMn2O4 octahedrons and LiMn2O4 and Al-doped LiMn2O4 truncated octahedrons from different orientations. From the orientation along the truncated surfaces of the Al-doped LiMn2O4 truncated octahedrons (Figure 1e) and LiMn2O4 truncated octahedrons (Figure S2d, Supporting Information), both HR-TEM images showed a clear lattice distance of 0.21 nm corresponding to the (400) plane. The corresponding SAED patterns exhibited a single-crystalline nature. The diffraction spots are easily indexed to the (440), (220), and (040) planes of spinel LiMn2O4, which also means that the zone axis is along the [001] direction. That is, the truncated surfaces are along the [001] direction and are highly active crystal facets, regardless of Al doping was performed. Similarly, from the directions perpendicular to the octahedral surface of the LiMn2O4 octahedrons (Figure S1c, Supporting Information), LiMn2O4 truncated octahedrons (Figure S2c, Supporting Information), and Al-doped LiMn2O4 truncated octahedrons (Figure 1d), the lattice fringes are along the [111] direction. These results are in good agreement with those of several reported studies.5,16 The X-ray diffraction (XRD) patterns of the three samples are shown in Figure 2. All of the peaks from the samples can be

to observe that the LiAl0.1Mn1.9O4 truncated octahedrons likewise exhibit similar phenomena to the LiMn2O4 truncated octahedrons. That is, the LiAl0.1Mn1.9O4 truncated octahedrons exhibit greater exposure of highly active crystal facets than the undoped LiMn2O4 truncated octahedrons. This fact might be related to the Al doping process. In detail, in the case of the LiMn2O4 truncated octahedrons, during postcalcination at 700 °C for 10 h, the (111) planes with the lowest solid-to-vapor surface energy have grown during annealing with the sacrifice of the (400), (440), and other planes with higher solid-to-vapor surface energies.16,19 In contrast, the LiAl0.1Mn1.9O4 truncated octahedrons will give up fewer active facets with higher solid-tovapor surface energies after postcalcination because of their more stable lattice structure after Al doping. These results are also in good agreement with the TEM observations, suggesting the greater exposure of highly active facets by the Al-doped LiMn2O4 truncated octahedrons. The electrochemical performances of the three samples were measured by assembling them into 2016 coin-type half-cells. Figure 3 shows three of the initial five CV cycles of LiMn2O4 octahedrons and LiMn2O4 and LiAl0.1Mn1.9O4 truncated octahedrons. From the results for all three samples, there are two pairs of reversible redox peaks at about 4.18/4.07 and 4.08/3.96 V, corresponding to the two-step Li-ion intercalation and deintercalation reaction of LiMn2O4. The consistency of the CV curves can reveal the cycling stability of the related material. From Figure 3a, one can see that the first five CV curves of LiMn2O4 octahedrons are fully overlapped, indicating a stable cycling performance. For LiMn 2O 4 truncated octahedrons (Figure 3b), the corresponding CV curves are not well overlapped. In addition, the area of the CV curve gradually decreases as a result of Mn dissolution from the vulnerable highly active facets in the truncated corner. It is noted that the CV curves of the LiAl0.1Mn1.9O4 truncated octahedrons are also well overlapped (Figure 3c), which means that Al doping is feasible for suppressing Mn dissolution from the vulnerable highly active facets. The rate capability of the LiAl 0.1Mn 1.9 O 4 truncated octahedrons was then evaluated, as shown in Figure 4. The assembled half-cells were charged at a constant rate of 0.2C (1C = 148 mAh g−1) and then discharged at various rates in the range of 0.2−20C. The average capacities of the LiAl0.1Mn1.9O4 truncated octahedrons were found to be 116, 110, 108, 106, 102, and 99 mAh g−1 at 0.2C, 1C, 2C, 5C, 10C, and 20C, respectively. As comparisons, the rate performances of the LiMn2O4 truncated octahedrons and LiMn2O4 octahedrons are also provided in Figure 4. Obviously, they exhibit relatively lower capacities at different rates than the LiAl0.1Mn1.9O4 truncated octahedrons. Such excellent results can be attributed mainly to the rich highly active facets of the LiAl0.1Mn1.9O4 truncated octahedrons, where the orientations of the active facets of LiMn2O4 are aligned with the Li-ion diffusion channels, hence improving the kinetic properties of the Li ions during the intercalation/deintercalation process. The rate capability of LiAl0.1Mn1.9O4 truncated octahedrons is also much better than that of some reported LiMn2O4 nanostructures;10,20 detailed comparisons of the electrochemical performance are listed in Table S1 (Supporting Information). To better understand the charge transfer, the electrochemical impedance spectroscopy (EIS) analysis of the LiMn2O4 octahedrons and LiMn2O4 and LiAl0.1Mn1.9O4 truncated octahedrons is shown in Figure 5. The LiAl0.1Mn1.9O4 truncated octahedrons show a much lower resistance than the LiMn2O4 octahedrons and

Figure 2. XRD patterns of LiMn2O4 octahedrons and LiMn2O4 and Al-doped LiMn2O4 truncated octahedrons.

easily indexed to the spinel framework with the space group Fd3m (JCPDS card 35-0782), and no other characteristic peaks from impurities are detected. The Al content was measured by ICP-AES analysis. The results showed that the Li/Al/Mn atomic ratio was 1.0:0.1:1.9. Therefore, the chemical composition was verified to be LiAl0.1Mn1.9O4. It is reported that the relative peak intensities reflect the relative exposure degrees of surface orientations.5,17 To compare the main crystal orientations of the three samples, all of the relevant peak intensities were normalized with respect to the (111) peak and consolidated, as shown in Figure S3 (Supporting Information). It is obvious that the LiMn2O4 truncated octahedrons show much sharper peaks for the (311), (400), and (440) facets than the LiMn2O4 octahedrons, which are highly active facets of LiMn2O4 nanoparticles.18 These results are in good agreement with the reports in the literature.5,16,19 Notably, it is interesting 3802

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Figure 4. Capacity retentions of LiAl0.1Mn1.9O4 and LiMn2O4 truncated octahedrons and LiMn2O4 octahedrons at rates of 0.2−20C.

Figure 5. Electrochemical impedance spectra of LiMn2O4 octahedrons and LiMn2O4 and LiAl0.1Mn1.9O4 truncated octahedrons. Inset: Corresponding equivalent circuit diagram.

observed that the LiAl0.1Mn1.9O4 truncated octahedrons maintained 88.7% of the initial capacity after 1000 cycles. Even at the elevated temperature of 55 °C, the LiAl0.1Mn1.9O4 truncated octahedrons still maintained 86.1% of the initial capacity after 500 cycles, which was much higher than both the LiMn2O4 truncated octahedrons (66.6%) and the LiMn2O4 octahedrons (70.2%). Therefore, Al doping can effectively avoid electrolyte etching, especially at elevated temperature. Such outstanding cycling stability is not only better than that of the reported LiMn2O4 nanostructures,8,21,22 but also superior to that of metal-doped LiMn 2O4 nanostructures, such as LiNi0.5Mn1.5O4 nanoparticles23 and Al-doped LiMn2O4 nanotubes.10

Figure 3. CV curves of the first, second, and fifth cycles for (a) LiMn2O4 octahedrons, (b) LiMn2O4 truncated octahedrons, and (c) LiAl0.1Mn1.9O4 truncated octahedrons at a scan rate of 0.2 mV s−1.

LiMn2O4 truncated octahedrons, suggesting a marked reduction in charge-transfer resistance for Li-ion intercalation/ deintercalation. This fact could be associated with the higher degree of exposure of highly active facets and the more stable crystal structure of the LiAl0.1Mn1.9O4 truncated octahedrons. These results are in good agreement with the above analysis; that is, Al doping increases the degree of exposure of highly active facets. The cycling stability was further evaluated at the same charge/discharge rate of 2C at 25 °C for 1000 cycles and at 55 °C for 500 cycles, as shown in Figure 6. At room temperature (25 °C), both the LiAl0.1Mn1.9O4 truncated octahedrons and the LiMn2O4 octahedrons were more stable than the LiMn2O4 truncated octahedrons, as shown in Figure 6a. It can be

4. CONCLUSIONS In conclusion, we have demonstrated the synthesis of LiAl0.1Mn1.9O4 truncated octahedrons that combine the advantages of Al doping and high exposure of the active facets of LiMn2O4 to synergistically enhance their electrochemical performance. In this structure, Al doping can greatly enhance the cycling stability of LiMn2O4 by avoiding electrolyte etching during the intercalation/deintercalation process, especially at elevated temperature. Moreover, the exposure of highly active facets can ensure the improvement of the specific capacity and rate capability. As a consequence, the as-prepared Al-doped LiMn2O4 truncated octahedrons exhibit average specific capacities of 116 mAh g−1 at 0.2C and 99 mAh g−1 at 20C 3803

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Figure 6. Cycling performance of the LiAl0.1Mn1.9O4 and LiMn2O4 truncated octahedrons and the LiMn2O4 octahedrons at (a) 25 °C for 1000 cycles and (b) 55 °C for 500 cycles at the same charge and discharge rate of 2C. high-energy and high-power lithium-ion batteries. Angew. Chem., Int. Ed. 2012, 51, 8748. (5) Kim, J. S.; Kim, K.; Cho, W.; Shin, W. H.; Kanno, R.; Choi, J. W. A truncated manganese spinel cathode for excellent power and lifetime in lithium-ion batteries. Nano Lett. 2012, 12, 6358. (6) Wang, Y.; Cao, G. Developments in nanostructured cathode materials for high-performance lithium-ion batteries. Adv. Mater. 2008, 20, 2251. (7) Jiang, H.; Fu, Y.; Hu, Y. J.; Yan, C. Y.; Zhang, L.; Lee, P. S.; Li, C. Z. Hollow LiMn2O4 nanocones as superior cathode materials for lithium-ion batteries with enhanced power and cycle performances. Small 2014, 10, 1096. (8) Ding, Y. L.; Xie, J. G.; Cao, S.; Zhu, T. J.; Yu, H. M.; Zhao, X. B. Single-crystalline LiMn2O4 nanotubes synthesized via templateengaged reaction as cathodes for high-power lithium ion batteries. Adv. Funct. Mater. 2011, 21, 348. (9) Liu, M. X.; Gan, L. H.; Xiong, W.; Xu, Z.; Zhu, D.; Chen, L. Development of MnO2/porous carbon microspheres with a partially graphitic structure for high performance supercapacitor electrodes. J. Mater. Chem. A 2014, 2, 2555. (10) Ding, Y. L.; Xie, J.; Cao, G. S.; Zhu, T. J.; Yu, H. M.; Zhao, X. B. Enhanced elevated-temperature performance of Al-doped singlecrystalline LiMn2O4 nanotubes as cathodes for lithium ion batteries. J. Phys. Chem. C 2011, 115, 9821. (11) Xiao, L. X.; Zhao, Y. Q.; Yang, Y. Y.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method. Electrochim. Acta 2008, 54, 545. (12) Kim, D. K.; Muralidharan, P.; Lee, H. W.; Ruffo, R.; Yang, Y.; Chan, C. K.; Peng, H. L.; Huggins, R. A.; Cui, Y. Spinel LiMn2O4 nanorods as lithium ion battery cathodes. Nano Lett. 2008, 8, 3948. (13) Benedek, R.; Thackeray, M. M.; Low, J.; Bucko, T. Simulation of aqueous dissolution of lithium manganate spinel from first principles. J. Phys. Chem. C 2012, 116, 4050. (14) Hirayama, M.; Ido, H.; Kim, K.; Cho, W.; Tamura, K.; Mizuki, J.; Kanno, R. Dynamic structural changes at LiMn2O4/electrolyte interface during lithium battery reaction. J. Am. Chem. Soc. 2010, 132, 15268. (15) Jiang, H.; Zhao, T.; Yan, C. Y.; Ma, J.; Li, C. Z. Hydrothermal synthesis of novel Mn3O4 nano-octahedrons with enhanced supercapacitors performances. Nanoscale 2010, 2, 2195. (16) Huang, M. R.; Lin, L. W.; Lu, H. Y. Crystallographic facetting in solid-state reacted LiMn2O4 spinel powder. Appl. Surf. Sci. 2001, 177, 103. (17) Tang, W.; Hou, Y. Y.; Wang, F. X.; Liu, L. L.; Wu, Y. P.; Zhu, K. LiMn2O4 nanotube as cathode material of second-level charge capability for aqueous rechargeable batteries. Nano Lett. 2013, 13, 2036.

with markedly enhanced cycling stability (88.7% and 86.1% capacity retention at 25 °C after 1000 cycles and at 55 °C after 500 cycles, respectively) than the pure LiMn2O4 truncated octahedrons (75.3% and 66.6% capacity retention at 25 °C after 1000 cycles and at 55 °C after 500 cycles, respectively). The present work is meaningful not only for the preparation of high-performance LiAl0.1Mn1.9O4 truncated octahedrons, but also for providing new insights into the exploitation of fascinating LiMn2O4 cathode materials.

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ASSOCIATED CONTENT

S Supporting Information *

Additional supporting data as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.J.). Tel.: +86-21-64250949. Fax: +86-21-64250624. *E-mail: [email protected] (C.L.). Tel.: +86-21-64250949. Fax: +86-21-64250624. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21206043, 21236003, and 21322607), the Research Project of the Chinese Ministry of Education (113026A), the Shanghai Shuguang Scholars Program (13SG31), and the Fundamental Research Funds for the Central Universities.

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DOI: 10.1021/ie504659h Ind. Eng. Chem. Res. 2015, 54, 3800−3805