10-LiO2-AM

www.advmat.de

COMMUNICATION

Crystal Habit-Tuned Nanoplate Material of Li[Li1/3–2x/3NixMn2/3–x/3]O2 for High-Rate Performance Lithium-Ion BatteriesBy Guo-Zhen Wei, Xia Lu, Fu-Sheng Ke, Ling Huang, Jun-Tao Li, Zhao-Xiang Wang, Zhi-You Zhou, and Shi-Gang Sun*The increasing demand for high-energy and high-power batteries, especially in the development of electric vehicles (EVs), has stimulated great research interest focused on lithium ion batteries (LIBs).[1–5] Olivine LiFePO4 has recently attracted particular attention as high-power batteries for EVs. In spite of the material’s low electronic conductivity, its fabrication as a battery cathode has been engineered to give quite high power levels.[6] However, its low theoretical capacity and low volume energy density compared with layered LiMO2 (M= Co, Ni, Mn, or/and Cr) composite materials make it less attractive for high-energy LIBs. Lithium Mn-rich metal oxides such as Li[Li1/3–2x/3MxMn2/3–x/3]O2 (M= Ni, Co, or/and Cr) are currently receiving signi cant attention for use as modern cathode materials for LIBs, owing to their high capacity of over 200 mA h g 1 when charged to 4.5 V or higher.[5,7–14] Because of the rapidly fading capacity and the poor rate-capability of Li[Li1/3–2x/3MxMn2/3–x/3]O2 materials,[15] extensive efforts have been made in recent years to improve their rate-capability,[16–20] such as fabrication of nanoparticles, nanowires, and nanoplates, which possess a short Li+ transportation path due to their reduced dimensions. Although the move to nanometer-sized materials can improve performance to a certain extent,[16–23] the structure of the materials, especially the surface structure, is a crucial factor that determines the rate for Li+ deintercalation/intercalation. Our research group has recently demonstrated that the catalytic properties of Pt nanoparticles can be signi cantly enhanced by tuning the surface of Pt nanoparticles from closest-packed to an open structure, such as from{111} or{100} to{730} and vicinal high-index facets.[24,25] During our research into nanomaterials applied in LIBs,[21,22] we further noticed that the nanomaterials are generally bounded by closest-packing facets, which intrinsically present dif culties in furnishing suitable channels for fast Li+ transportation. Recent reports have also evidenced that the surface structure for Li+ transportation is critical to the rate capability.[6,26–30] Li+ can only intercalate into the bulk of a crystal along the direction parallel to the Li+ layers in a layered cathode material with anα-NaFeO2 structure, such as Li[Li1/3–2x/3NixMn2/3–x/3]O2. As illustrated in Figure 1, each layer perpendicular to the c-axis is indexed as a (001) plane; the planes perpendicular to a (001) plane and parallel to the a- (or b-) axis are indexed as (010) planes (or (100) planes). The (010) plane is equivalent to the (100) plane in a typical perfect crystal. When a Li[

Li1/3–2x/3NixMn2/3–x/3]O2 nanoplate grows perpendicular to the[001] direction (c-axis of the crystal), its surface is preferentially dominated by (001) planes ((001) nanoplates), which are not electrochemically active for Li+ transportation because they cannot provide an appropriate path for Li+ transportation.[26] In contrast, the (010) (or (100)) plane that is perpendicular to the (001) plane is an active plane for Li+ deintercalation/intercalation.[31] If a Li[Li1/3–2x/3NixMn2/3–x/3]O2 nanoplate grows perpendicular to the[010] (or[100]) direction, its surface will be consequently dominated by the (010) (or (100)) planes ((010) nanoplates). Therefore, the rate-capability of LIBs employing Li[Li1/3–2x/3NixMn2/3–x/3]O2 nanoplates as cathode materials can be signi cantly improved by tuning the crystal habit to obtain (010)-nanoplate material. To the best of our knowledge, such a (010)-nanoplate material of Li[Li1/3–2x/3NixMn2/3–x/3]O2 has not been reported before. Based on the structural analysis above, we calculated the surface energy of the (001) and (010) planes of Li(Li0.17Ni0.25Mn0.58)O2 materials withα-NaFeO2 structure to quantify their stability (see Supporting Information, Figure S1). The result indicates that the surface energy of the (001) plane is lower than that of the (010) plane. As the crystal growth rate of a high-energy plane is faster than that of a low-energy plane, the high-energy planes have the tendency to disappear during growth and the surface of the grown crystal will be dominated by low-energy planes. As a consequence, the (001) nanoplates are thermodynamic equilibrium products in most synthesis route when the synthesis reaction is under hydrothermal conditions for a long enough time.[19,20,23] Therefore, the nanoplates were always less attractive in terms of rate-performance in previous reports. In the present Communication, we report a crystal habit-tuned nanoplate material of Li(Li0.17Ni0.25Mn0.58)O2 (HTN-LNMO), in

[ ] Dr. G.-Z. Wei, F.-S. Ke, L. Huang, Z.-Y. Zhou, Prof. S.-G. Sun State Key Laboratory of Physical Chemistry of Solid Surfaces Department of Chemistry College of Chemistry and Chemical Engineering Xiamen University Xiamen, 361005 (China) E-mail: sgsun@ Dr. X. Lu, Prof. Z.-X. Wang Laboratory for Solid State Ionics Institute of Physics Chinese Academy of Sciences Beijing, 100190 (China) Dr. J.-T. Li School of Energy Research Xiamen University, Xiamen, 361005 (China)

DOI: 10.1002/adma.201001578

4364

© 2010 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

Adv. Mater. 2010, 22, 4364–4367

www.advmat.de

COMMUNICATION

Figure 1. Schematic illustration of two kinds of nanoplates and the microstructure of their surfaces.

which the number of (010) nanoplates has been signi cantly increased in comparison with the conventional thermodynamic equilibrium nanoplate material of Li(Li0.15Ni0.25Mn0.6)O2 (CN-LNMO). In our synthesis route,

the precursor, which is different from that reported in the literature,[19] was preheated under hydrothermal conditions for a shorter time. Both the different precursor and the short time of hydrothermal conditions led to the nanoplate growing simultaneously in both the[010] (or[100]) and[001] directions, which increases the yield of (010) nanoplates. It was demonstrated that HTN-LNMO exhibits high-rate performance when used as the cathode in LIBs. Figure 2a shows a scanning electron microscopy (SEM) image of the as-prepared HTN-LNMO material. Nanoplates of several nanometers (5–9 nm) thick are observed. The crystal structure of the HTN-LNMO was studied by powder X-ray diffraction (XRD). Most re ections in XRD patterns of HTN-LNMO (Figure 2b) can be indexed as theα-NaFeO2 phase. Shortranged superlattice ordering, which is indicated by the weak re ections around 21° and 66°, exists in the transition metal layers because of the Li(Li1/3Mn2/3)O2 structure in the lithium metal oxide solid solution.[32] It is interesting that HTN-LNMO yielded XRD re ections with wider width and weaker intensity in comparison with those produced by Li[Li1/3–2x/3NixMn2/3–x/3]O2 particle materials (LNMO particles), except for the 003 re ection and the distinctly enhanced 100 re ection (namely 010 re ection). In contrast, the 100 re ection almost disappeared in the XRD pattern of the CN-LNMO. The markedly enhanced 100 re ection in the XRD pattern indicates that the number of (010) nanoplates in HTN-LNMO increased signi cantly in comparison with that in CN-LNMO. The transmission electron microscopy (TEM) characterization of HTN-LNMO is displayed in Figure 3. The thickness of a typical nanoplate is measured as 6 nm in Figure 3b. Two kinds of nanoplates are shown in the bottom inset to Figure 3a. The frontal plane of the left one is the (010) plane (Figure 3c), which is the plane normal to the set of (001) planes with a lattice spacing of 0.48 nm (d003). The set of (100) planes is also shown with a crossing lattice spacing of 0.25 nm (d100). It is therefore con rmed as a (010) nanoplate. The frontal plane of the right one is

Figure 2. SEM and XRD characterizations. a) SEM characterization of the crystal habit-tuned nanoplate material of HTN-LNMO). b) XRD patterns of (1) LNMO particles, (2) CN-LNMO, and (3) HTN-LNMO. Inset: Magni ed area in the 35°–40° region.

the (001) plane (Figure 3d), which is the plane normal to both the set of (110) planes with a crossing lattice spacing of 0.14 nm (d110) and the set of (100) planes with a lattice spacing of 0.25 nm (d100). The ratio of the number of (001) nanoplates to that of (010) nanoplates is found to be approximately 6:1 by sorting out the nanoplates from the XRD analysis and the statistical results of high-resolution TEM (HRTEM) characterizations (see Supporting Information, Section 2), which signi es that the quantity of (010) nanoplates is increased to 1/7 of the total amount of HTN-L

NMO. If the (010) and (001) nanoplates have the same volume, the proportion of the active surface area, that is, the (100)+(010) planes, on the (001) nanoplates is only 21.7%, while it is increased to 93.7% on the (010) nanoplates (see Supporting Information, Section 3). Such analysis implies that the active surface area for Li+ transportation on the (010) nanoplate can be as high as 4.3 times that of the (001) nanoplates. In the present case, although the proportion of the (010) nanoplates in HTN-LNMO is about 1/7, the active surface area in the HTN-LNMO sample

Adv. Mater. 2010, 22, 4364–4367

© 2010 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

4365

www.advmat.de

COMMUNICATION

a)

b)

d003

d 100

c)

d003 d100 d100[100][100]

d)d110

[110][001]

Figure 3. HRTEM results. a) TEM image of HTN-LNMO; the scale bars in the insets both represent 20 nm. b) HRTEM image of the lateral view of HTN-LNMO. c) HRTEM image of the frontal view of a (010) nanoplate. d) HRTEM image of the frontal view of a (001) nanoplate.

has been increased by about 50% in comparison with that in CN-LNMO (see Supporting Information, Section 3). The charge/discharge cycling performance of the HTN-LNMO is illustrated in Figure 4a. The rst charge and discharge capacities were measured as 260 mA h g 1 and 190 mA h g 1, respectively. After 5 cycles, the charge capacity had decreased slightly to 226 mA h g 1, while the discharge capacity had increased to 221 mA h g 1. The discharge capacity increased further to 242 mA h g 1 after 60 cycles due to the tendency of stabilization of the nanoplates after several cycles. The discharge capacity remained at 238 mA h g 1 after 100 cycles. The high-rate performance of the HTN-LNMO materials is evidenced by Figure 4b. A speci c capacity around 197 mA h g 1 is retained even when the discharge rate approaches 6 C, that is, 80% of the capacity at a rate of 0.1 C is retained at a rate of 6 C. The rate capability of the HTN-LNMO materials is further illuminated in Figure 4c, in which the discharge capacity at a 6 C rate and cycleability of the HTN-LNMO are compared with those of the CN-LNMO and LNMO particles. The discharge capacity after 50 cycles at a 6 C rate is measured as 186 mA h g 1 for HTN-LNMO, but only 106 mA h g 1 for CN-LNMO and 40 mA h g 1 for LNMO particles. The capacity of the HTN-LNMO in the 50th cycle is 1.7 times that of the CN-LNMO, and 4.6 times that of the LNMO particles. That is, the HTN-LNMO exhibits excellent capacity and superior cycleability at high-rate charge/discharge, in comparison withCN-LNMO and LNMO particles. It is obvious that the excellent rate-capability of HTN-LNMO originates from the nanometer-size effect on one hand, but most importantly from the signi cantly increased amount of (010) nanoplates, on the other. In conclusion, we have developed a crystal habit-tuned nanoplate material of Li(Li0.17Ni0.25Mn0.58)O2, in which the (0

10) 4366

Figure 4. Cycleability and rate capability of HTN-LNMO. a) Plots of speci c capacity vs cycle number for electrodes prepared from HTN-LNMO. Test conditions: current density 60 mA g 1 (about 0.2 C), voltage window 2.0–4.8 V. b) Stabilized discharge voltage pro les of HTN-LNMO cycled at different rates: 6, 3, 1, 0.5, 0.1 C from bottom to top. c) Discharge capacity at a 6 C rate and cycleability of HTN-LNMO compared with CN-LNMO and LNMO particles.

nanoplates have been signi cantly increased, to be used as a cathode for high-rate performance LIBs.This material exhibits not only a high reversible capacity but also anexcellent cycleability. At a 6 C rate, the reversible capacity is measured as around 200 mA h g 1, and 186 mA h g 1 after 50 cycles. The excellent high-rate performance has been attributed to the increased active surface area for Li+ transportation in the HTN-LNMO sample, by about 50% in comparison with

© 2010 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

Adv. Mater. 2010, 22, 4364–4367

www.advmat.de

COMMUNICATION

CN-LNMO. The results demonstratethat the proportion of electrochemically active surface for Li+ transportation is a key criterion for evaluating different nanostructures for high-rate performance of LIB materials. Therefore, further increasing the yield of the (010)-nanoplate materials could enhance greatly the rate-performance. Based on these ndings, HTN-LNMO is a promising candidate as a cathode material in LIBs with high energy and high power for application in EVs.

National Science Foundation of China (grant nos. 20833005, 20773102, and 20931160426). Received: April 29, 2010 Published online: August 30, 2010

Experimental SectionPreparation of Materials: HTN-LNMO was prepared by stirring stoichiometric amounts of Ni(CH3COO)2, Mn(CH3COO)2, and Li(CH3COO), and adding oxalic acid as a precipitating agent and acetic acid as an additive. The precursor was pretreated in a poly(tetra uoroethylene) (Te on) container at 150–200°C for normally 6–12 h. Then the mixture was stirred vigorously until dry. The dried mixture was heated at 450°C for 4–5 h, and then calcined in air using a step procedure: at 500°C for 3–5 h, at 750°C for 3–5 h, at 900°C for 8–16 h. All the raw materials of transition metal salts were of analytical purity grade, and the amount of lithium salt was in excess in the synthesis, so the LNMO nanoplates could reach a high yield (ca. 100%) with reference to the quantity of raw materials of transition metal salt. The CN-LNMO samples were prepared under hydrothermal conditions.[19] The Li(Li0.17Ni0.25Mn0.58)O2 particles used in this study were prepared with a conventional co-precipitation method using LiOH, NiSO4, and MnSO4. (More details are available in the Supporting Information, Section 4) Sample Characterization: Products were thoroughly characterized by XRD (Philips X’Pert Pro Super X-ray diffractometer,

Cu Kα radiation), SEM (Hitachi S-4800 SEM), and TEM (Tecnai F30 microscope). Electrochemical Performance Tests: Electrochemical properties of the Li(Li0.17Ni0.25Mn0.58)O2 nanoplate electrodes were measured by assembling them into coin cells (type CR2025) in an argon- lled glove box. The cathode was prepared by spreading a mixture of 80% Li(Li0.17Ni0.25Mn0.58)O2 powder, 10% acetylene black (Shanshan Limited Co., Shanghai, P.R. China), and 10% poly(vinylidene uoride) (Kynar FLEX 2801, Elf-atochem, USA) binder dissolved in N-methyl pyrrolidone (Fluka Inc.) onto an aluminum foil current collector. The cathode was separated from the lithium anode by a separator material (Celgard 2400). The electrolyte consisted of a solution of 1 mol L 1 LiPF6 in a mixture of ethylene carbonate (EC)/ dimethyl carbonate (DMC)/ diethyl carbonate (DEC) 1:1:1 (vol%) obtained from Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd, P.R. China. The cells were galvanostatically charged and discharged on a battery test system (NEWARE BTS-610, Neware Technology Co., Ltd., P.R. China) between 2.0 and 4.8 V at room temperature. Calculation: Calculation methods in this work were based on density functional theory (see Supporting Information, Section 1).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the Major State Basic Research Development Program of China (grant no. 2009CB220102) and the

[1] M. Armand, J. M. Tarascon, Nature 2008, 451, 652.[2] K. Kang, Y. S. Meng, J. Bréger, C. P. Gray, G. Ceder, Science 2006, 311, 977.[3] S. B. Schougaard, J. Bréger, M. Jiang, C. P. Grey, J. B. Goodenough, Adv. Mater. 2006, 18, 905.[4] C. M. Park, H. J. Sohn, Adv. Mater. 2007, 19, 2465.[5] Y.-K Sun, S.-T Myung, B.-C Park, J. Prakash, I. Belharouak, K. Amine, Nat. Mater. 2009, 8, 320.[6] B. Kang, G. Ceder, Nature 2009, 458, 190.[7] Z. Lu, J. R. Dahn, J. Electrochem. Soc. 2002, 149, A815.[8] A. R. Armstrong, M. Holzapfel, P. Novak, C. S. Johnson, S.-H. Kang, M. M. Thackeray, P. G. Bruce, J. Am. Chem. Soc. 2006, 128, 8694.[9] S.-H. Park, S.-H. Kang, C. S. Johnson, K. Amine, M. M. Thackeray, Electrochem. Commun. 2007, 9, 262.[10] M. M. Thackeray, C. S. Johnson, J. T. Vaughey, N. Li, S. A. Hackney, J. Mater. Chem. 2005, 15, 2257.[11] C. S. Johnson, N. Li, C. Le ef, M. M. Thackeray, Electrochem. Commun. 2007, 9, 787.[12] Z. Lu, J. R. Dahn, J. Electrochem. Soc. 2002, 149, A1454.[13] B. Ammundsen, J. Paulsen, Adv. Mater. 2001, 13, 943.[14] Y.-S. Hong, Y. J. Park, X. Wu, K. S. Ryu, S. H. Chang, Electrochem. Solid State Lett. 2003, 6, A166.[15] Y.-S. Hong, Y. J. Park, K. S. Ryu, S. H. Chang, M. G. Kim, J. Mater. Chem. 2004, 14, 1424.[16] P. G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed. 2008, 47, 2930.[17] A. S. Aricò, P. G. Bruce, B. Scrosati, J.-M. Tarascon, W. Schalkwijc, Nat. Mater. 2005, 4, 366.[18] H. Lee, J. Cho, Nano Lett. 2007, 7, 2

638.[19] M. G. Kim, M. Jo, Y.-S. Hong, J. Cho, Chem. Commun. 2009, 218.[20] J. Cho, Y. Kim, M. G. Kim, J. Phys. Chem. C 2007, 111, 3192.[21] F. S. Ke, L. Huang, G. Z. Wei, L. J. Xue, J. T. Li, B. Zhang, S. R. Chen, X. Y. Fan, S. G. Sun, Electrochim. Acta 2009, 54, 5825.[22] F. S. Ke, L. Huang, H. H. Jiang, H. B. Wei, F. Z. Yang, S. G. Sun, Electrochem. Commun. 2007, 9, 228.[23] X. Qian, X. Cheng, Z. Y. Wang, X. J. Huang, R. Guo, D. L. Mao, C. K. Chang, W. J. Song, Nanotechnology 2009, 20, 115608.[24] N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding, Z. L. Wang, Science 2007, 316, 732.[25] N. Tian, Z. Y. Zhou, S. G. Sun, J. Phys. Chem. C 2008, 112, 19801.[26] J. B. Bates, N. J. Dudney, B. J. Neudecker, F. X. Hart, H. P. Jun, S. A. Hackney, J. Electrochem. Soc. 2000, 147, 59.[27] M. S. Islam, D. J. Driscoll, C. A. J. Fisher, P. R. Slater, Chem. Mater. 2005, 17, 5085.[28] D. Morgan, A. Van der Ven, G. Ceder, Electrochem. Solid State Lett. 2004, 7, A30.[29] G. Y. Chen, X. Y. Song, T. J. Richardson, Electrochem. Solid State Lett. 2006, 9, A295.[30] K. Kang, D. Morgan, G. Ceder, Phys. Rev. B 2009, 79, 014305.[31] H. L. Chen, C. P. Grey, Adv. Mater. 2008, 20, 2206.[32] Y. Wu, A. Manthiram, J. Power Sources 2008, 183, 749.

Adv. Mater. 2010, 22, 4364–4367

© 2010 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

4367

本文来源：https://www.bwwdw.com/article/h6tq.html