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Fluorine-doped LiNi0.5Mn1.5O4 for 5 V cathode materials of lithium-ion battery
Materials Research Bulletin 43 (2008) 3607–3613 www.elsevier.com/locate/matresbu
Short communication
Fluorine-doped LiNi0.5Mn1.5O4 for 5 V cathode materials of lithium-ion battery Guodong Du, Yanna NuLi, Jun Yang *, Jiulin Wang Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 7 August 2007; received in revised form 2 February 2008; accepted 29 February 2008 Available online 8 March 2008
Abstract Fluorine-doped 5 V cathode materials LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) have been prepared by sol–gel and post-annealing treatment method. The results from X-ray diffraction and scanning electron microscopy (SEM) indicate that the spinel structure changes little after fluorine doping, but the particle size varies with fluorine doping and the preparation conditions. The electrochemical measurements show that stable cycling performance can be obtained when the fluorine amount x is higher than 0.1, but the specific capacity is decreased and 4 V plateau capacity resulting from a conversion of Mn4+/Mn3+ remains. Moreover, influence of the particle size on the reversible capacity of the electrode, especially on the kinetic property, has been examined. # 2008 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Sol-gel chemistry; C. Electrochemical measurements; D. Energy storage
1. Introduction In order to increase the cell voltage and allow the use of anode materials working significantly above 0 V versus Li/Li+, which avoid the safety problems related to lithium intercalation compounds formed near 0 V [1], more and more attention has been recently devoted to the investigation of 5 V cathode materials for lithium-ion batteries. Owing to a high capacity of 130–150 mAh g 1, numerous researches have focused on cation-substituted LiMn2O4 spinel compounds with general formula LiMxMn2 xO4, where manganese is partially replaced by other transition metals: M = Ni, Co, Cu, Cr, Ti, and Fe [2–7]. There are 5 and 4 V two voltage plateau regions during Li+ extraction and insertion. The 5 V plateau is originated from oxidation/reduction of the substituted transition metal M, while 4 V plateau is associated with the reaction of Mn3+/ Mn4+. It has been suggested that 5 V plateau character and the discharge capacity are strongly dependent on the kind of transition metals and their substituted molar ratios [8,9]. Moreover, 4 V plateau capacity is decreased with increasing the amount of doped transition metal, and cycle life is improved for Co- and Ni-doped spinels [4]. It has been found that the potential profile of LiNixMn2 xO4 changes according to x value and the voltages appeared around 4.4–4.7 and 4.7–5.0 V correspond to the redox reactions of Ni2+/Ni3+ and Ni3+/Ni4+couples, respectively [10]. With the increase of nickel in LiNixMn2 xO4, 5 V plateau capacity rises and finally the 4 V plateau disappears basically at x = 0.5. The LiNi0.5Mn1.5O4 material is usually obtained through sol–gel, solid-state reaction, molten salt, emulsion drying, and polymer-pyrolysis methods [8,11–15]. The effects of the structure and particle size on the electrochemical response of the material have been widely investigated [14,16]. Many attempts have been made to * Corresponding author. Tel.: +86 21 5474 7667; fax: +86 21 5474 1297. E-mail address: [email protected] (J. Yang). 0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.02.025
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improve both the cyclability and the rate capability at room and high temperature, and some positive effects obtained by surface treatments [1,17,18] and anion doping [19–21]. Recently, it has been reported that a small amount of fluorine substitution for oxygen can reduce Ni and Mn dissolution from HF attack, enhancing the electrochemical properties and thermal stability [22]. As we know, fluorine-doping method has been widely introduced to the layered cathode material LiNi1/3Co1/3Mn1/ O 3 2 and the ways of doping can cause different effects [23–25]. In this study, LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) spinel powders are prepared by sol–gel and post-annealing treatment method and the fluorine effect on the structure, morphology and electrochemical properties are investigated. 2. Experimental The compounds of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) were synthesized by sol–gel and post-annealing treatment method. The stoichiometric amount of Li(CH3COO)2H2O, Ni(CH3COO)24H2O, Mn(CH3COO)24H2O and LiF were added to 80 ml distilled water and stirred at about 70 8C to form gel. Then the gel precursor was decomposed at 450 8C for 5 h. The resulted residue was ground in a mortar and pressed into pellets, followed by a heat-treatment at 850 8C in oxygen for 12 h. By controlling sol–gel and subsequent heattreatment temperature, the powder granularity can be regulated. The XRD patterns were obtained by a Bruker D8 ADVANCE X-ray diffractometer using Cu Ka radiation (wavelength: 1.5406 Å; voltage: 40 kV; scan speed: 1.28/min), with silicon powder as an internal reference for the measurement of lattice parameter. Scanning electron microscopy (SEM) micrographs were obtained on a Hitachi S 2150 microscope. The electrochemical tests were carried out via CR2016 coin-type cells. The cathodes were prepared by blending active material, acetylene black, and polyvinylidene fluoride (weight ratio 85:7:8) dissolved in N-methyl-2pyrrolidinone. Then the slurry was coated on an aluminum foil and dried under vacuum at 120 8C over 4 h. Electrode disks were punched from the foil and weighed. Celgard 2700 membrane was used as separator and metallic lithium as the anode, 1 M LiPF6 dissolved in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight ratio) was used as the electrolyte. The cells were assembled in an argon-filled glove box with each of oxygen and moisture is less than 1 ppm. Charge and discharge measurements of the coin cells were carried out at a current density of 29.4 mA g 1 (0.2 C) with voltage cut-off of 5.2 V/3.5 V versus Li/Li+ at 20 8C. 3. Results and discussion Fig. 1 shows the powder X-ray diffraction patterns of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2). It can be observed that all XRD peaks are quite narrow, indicating their high crystallinity. There is no obvious
Fig. 1. XRD patterns of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2).
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Table 1 Refinement results of lattice parameters for LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) active materials Active materials
Lattice parameter a (Å)
LiNi0.5Mn1.5O4 LiNi0.5Mn1.5O3.95F0.05 LiNi0.5Mn1.5O3.9F0.1 LiNi0.5Mn1.5O3.85F0.15 LiNi0.5Mn1.5O3.8F0.2
8.1815 8.1783 8.1829 8.1914 8.1983
peak change when the different amount of fluorine is added. All the samples can be indexed as a cubic spinel structure ¯ It means that Li+ occupies the tetrahedral (8a) sites, Ni2+and Mn4+ are randomly located with a space group of Fd 3m. at the octahedral (16d) sites, and O2 and F are located at the octahedral (32e) sites [22,26]. As reported that impurities are often produced in the calcination process [8], a little amount of NiO secondary phase is also observed in the patterns. Lattice parameters of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2) are further calculated from the XRD data by a least squares method and the results are shown in Table 1. The fluorine-free LiNi0.5Mn1.5O4 has a lattice parameter of a = 8.1845 Å. For LiNi0.5Mn1.5O3.95F0.05, this value is slightly smaller. It may be related to the stronger bond of M–F than M–O and substitution of the small amount of F with relatively small ionic radius (r = 1.33 Å) for O2 (r = 1.40 Å) [22]. However, the lattice parameter turns to increase when fluorine content is further enhanced. Here the existence of more Mn3+ in the material may become the dominating factor. Because fluorine substitution changes the oxidation state of transition metal components [24] and more Mn3+ ions with larger ionic radius (r = 0.645 Å) will replace partial Mn4+ ions (r = 0.53 Å) for electro-neutrality. The variation in lattice parameters suggests that fluorine ions are successfully substituted for oxygen site in LiNi0.5Mn1.5O4. SEM micrographs in Fig. 2 display octahedral shape characteristic of cubic spinel samples. The LiNi0.5Mn1.5O3.95F0.05 powder exhibits a wide particle size distribution. The particles become larger and the distribution more uniform with the increase of the fluorine amount. The initial charge and discharge curves of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O3.95F0.05 are illustrated in Fig. 3. Both the samples exhibit two similar voltage plateaus at around 4.0 and 4.7 V. It is noted that 4 V capacity becomes larger with increasing fluorine content. This result supports the previous assumption about existence of more Mn3+ due to fluorine substitution in LiNi0.5Mn1.5O4 xFx. The total discharge capacity increases after limited fluorine doping (x = 0.05) by this preparation route. Fig. 4 shows steady-state scan cyclic voltammograms of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.2) with a scan rate of 50 mV s 1. It can be found that the intensity and location of peaks are related to the amount of fluorine in the samples. The enhanced fluorine amount results in the decrease of the peak current. The redox couple located at around 4 V is associated with the reaction between Mn3+ and Mn4+, and the charge amount for this reaction increases when more fluorine is doped. Two oxidation peaks between 4.4 and 5 V, which are clearly shown in the insert of Fig. 4 for LiNi0.5Mn1.5O3.8F0.2 after 100 charge/discharge cycles, can be attributed to redox reactions of Ni2+/Ni3+ and Ni3+/ Ni4+. Moreover, because of the inductive effect of the doped fluorine, the peak voltage shifts slightly toward positive direction with the increase of the fluorine amount. This phenomenon can be also observed in the initial charging curve. The cyclic performance was measured for the LiNi0.5Mn1.5O4 xFx with different fluorine amounts. The cycling results shown in Fig. 5 indicate that discharge capacity decreases with enhanced fluorine content. Although LiNi0.5Mn1.5O3.95F0.05 exhibits a high initial capacity, the capacity fade is relatively fast. When the fluorine content is increased to higher than 0.1, the sample gives better cyclic stability. It appears that strong Li–F bonding may hinder Li+ extraction, leading to a lower reversible capacity. On the other hand, fluorine doping makes lattice parameter larger (see Table 1) and spinel structure more stable due to the strong M–F bonding. These are favorable for the cyclic stability. In view of the balance between the capacity and cyclic stability, LiNi0.5Mn1.5O3.9F0.1 gives the best performance. The initial capacity of 122 mAh g 1 and the capacity retention of 91% after 100 cycles are obtainable. Furthermore, the influence of the powder granularity of fluorine-doped LiNi0.5Mn1.5O4 on the discharge capacity and rate capability was investigated. SEM images in Fig. 6 show powder morphologies of two LiNi0.5Mn1.5O3.9F0.1 samples. The sample (a) exhibits dense and aggregated particle character. The particle size is small and uniform (ca. 0.1–0.2 mm) with some aggregations up to 0.5 mm. In contrast, the particle size of sample (b) is much larger, mostly more than 1 mm.
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Fig. 2. Scanning electron micrographs of (a) LiNi0.5Mn1.5O3.95F0.05, (b) LiNi0.5Mn1.5O3.9F0.1 and (c) LiNi0.5Mn1.5O3.8F0.2.
Fig. 3. Initial charge and discharge curves of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O3.95F0.05.
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Fig. 4. Cyclic voltammograms of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.2). The insert exhibits oxidation peaks of LiNi0.5Mn1.5O3.8F0.2 after 100 cycles.
Fig. 5. Cyclic performances of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2).
Fig. 7 shows discharge capacity upon cycle number for the samples with different particle sizes demonstrated in Fig. 6. At low discharge rate of 0.2 C, the large particle sample delivers a larger capacity than the small one and their cycling stability is quite similar. When the discharge rate is enhanced to 1 C, the cycling capacity drops significantly for both the samples. However, the large particle sample exhibits lower capacity in the initial cycles and faster capacity fade. Such a contrary electrode behavior at different discharge rate may be explained as follows: the large LiNi0.5Mn1.5O3.9F0.1 particles have better crystallinity and less surface defects. Thereby, the material could provide a capacity closer to theoretic value under a quasi-reversible condition (i.e. low discharge rate). The small particle sample possesses fast kinetic advantage due to the larger surface area and shorter diffusion length of Li+ ions within the particles. It is favorable for high rate discharge. The limitations of both lithium diffusion speed and interfacial reaction region for the large particle sample could cause strong electrochemical polarization at high charge and discharge rate, leading to side reactions such as electrolyte decomposition. Therefore, the cycling stability becomes poor.
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Fig. 6. Scanning electron micrographs of LiNi0.5Mn1.5O3.9F0.1 with different particle sizes. The insert in (a) exhibits a higher resolution image.
Fig. 7. Influence of LiNi0.5Mn1.5O3.9F0.1 particle size on the cyclic performance.
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4. Conclusions 5 V LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) cathode materials all with the spinel structure were prepared by sol–gel and post-annealing treatment method. Fluorine doping for a substitution for oxygen sites in LiNi0.5Mn1.5O4 results in a variation of the lattice parameters and bonding energy. For the materials with x beyond 0.1, the capacity decreases, but the cyclability is significantly improved. This is probably due to more stable fine-structure arising from fluorine doping. On the other hand, depending on the discharge rate, the effect of the LiNi0.5Mn1.5O3.9F0.1 particle size on the electrode performance is greatly different. Owing to shorter Li+ diffusion length and larger specific surface area, the powder material with smaller particle size exhibits larger cycle capacity and better cyclability at high charge and discharge rate. But the larger particle sample shows the advantage in the Li-storage capacity at a low reaction rate, where the crystallinity can become a dominant factor. Acknowledgements This work was supported by National 973 Program (No. 2007CB209700) and NCET Fund from Education Ministry of PR China. Authors are greatly indebted to Mr. Haibo Han in the Instrumental Analysis Center of Shanghai Jiao Tong University for discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, J. Electroanal. Chem. 566 (2004) 187. C. Sigala, D. Guyomard, A. Verbaere, Y. Piffard, M. Tournorx, Solid State Ionics 81 (1995) 167. K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Electrochem. Soc. 143 (1996) 1607. G. Li, H. Ikuta, T. Uchida, M. Wakihara, J. Electrochem. Soc. 143 (1996) 178. H. Kawai, M. Nagata, H. Kageyama, H. Tsukamoto, A.R. West, Electrochim. Acta 45 (1999) 315. H. Shigemura, H. Sakaebe, H. Kageyama, H. Koboyashi, A.R. West, R. Kanno, S. Morinoto, S. Nasu, M. Tabuchi, J. Electrochem. Chem. 148 (2001) A730. K. Kanamura, W. Hoshikawa, T. Umegaki, J. Electrochem. Soc. 149 (2002) A339. Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 205. T. Ohzuku, S. Takeda, M. Iwanaga, J. Power Sources 81–82 (1999) 90. K. Dokko, M. Mohamedi, N. Anzue, T. Itoh, I. Uchida, J. Mater. Chem. 12 (2002) 3688. X. Wu, B.K. Seung, J. Power Sources 109 (2002) 53. Y. Idemoto, H. Narai, N. Koura, J. Power Sources 119–121 (2003) 125. J.H. Kim, S.T. Myung, C.S. Yoon, S.G. Kang, Y.K. Sun, Chem. Mater. 16 (2004) 906. S.T. Myung, S. Komaba, N. Kumakai, H. Yashiro, H.T. Chung, T.H. Cho, Electrochim. Acta 47 (2002) 2543. L. Yu, Y. Cao, H. Yang, X. Ai, J. Solid State Electrochem. 10 (2006) 283. D. Kovacheva, B. Markovsky, G. Salitra, Y. Talyosef, M. Gorova, E. Levi, M. Riboch, J.K. Hyeong, D. Aurbach, Electrochim. Acta 50 (2005) 5553. Y.K. Sun, C.S. Yoon, I.H. Oh, Electrochim. Acta 48 (2003) 503. A. Eftekhari, Chem. Lett. 33 (5) (2004) 616. T.A. Arunkumar, A. Manthiram, Electrochim. Acta 50 (2005) 5568. B. León, J.M. Lloris, C.P. Vicente, J.L. Tirado, Electrochem. Solid-State Lett. 9 (2) (2006) A96. S.H. Oh, S.H. Jeon, W.I. Cho, C.S. Kim, B.W. Cho, J. Alloys Compd. 452 (2008) 389. S.W. Oh, S.H. Park, J.H. Kim, Y.C. Bae, Y.K. Sun, J. Power Sources 157 (2006) 464. S. Jouanneau, J.R. Dahn, J. Electrochem. Soc. 151 (2004) A1749. G.H. Kim, J.H. Kim, S.T. Myung, C.S. Yoon, Y.K. Sun, J. Electrochem. Soc. 152 (2005) A1707. D. Li, Y. Sasaki, K. Kobayakawa, H. Noguchi, Y. Sato, Electrochim. Acta 52 (2006) 643. S. Nieto, S.B. Majumder, R.S. Katiyar, J. Power Sources 136 (2004) 88.
Short communication
Fluorine-doped LiNi0.5Mn1.5O4 for 5 V cathode materials of lithium-ion battery Guodong Du, Yanna NuLi, Jun Yang *, Jiulin Wang Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 7 August 2007; received in revised form 2 February 2008; accepted 29 February 2008 Available online 8 March 2008
Abstract Fluorine-doped 5 V cathode materials LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) have been prepared by sol–gel and post-annealing treatment method. The results from X-ray diffraction and scanning electron microscopy (SEM) indicate that the spinel structure changes little after fluorine doping, but the particle size varies with fluorine doping and the preparation conditions. The electrochemical measurements show that stable cycling performance can be obtained when the fluorine amount x is higher than 0.1, but the specific capacity is decreased and 4 V plateau capacity resulting from a conversion of Mn4+/Mn3+ remains. Moreover, influence of the particle size on the reversible capacity of the electrode, especially on the kinetic property, has been examined. # 2008 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Sol-gel chemistry; C. Electrochemical measurements; D. Energy storage
1. Introduction In order to increase the cell voltage and allow the use of anode materials working significantly above 0 V versus Li/Li+, which avoid the safety problems related to lithium intercalation compounds formed near 0 V [1], more and more attention has been recently devoted to the investigation of 5 V cathode materials for lithium-ion batteries. Owing to a high capacity of 130–150 mAh g 1, numerous researches have focused on cation-substituted LiMn2O4 spinel compounds with general formula LiMxMn2 xO4, where manganese is partially replaced by other transition metals: M = Ni, Co, Cu, Cr, Ti, and Fe [2–7]. There are 5 and 4 V two voltage plateau regions during Li+ extraction and insertion. The 5 V plateau is originated from oxidation/reduction of the substituted transition metal M, while 4 V plateau is associated with the reaction of Mn3+/ Mn4+. It has been suggested that 5 V plateau character and the discharge capacity are strongly dependent on the kind of transition metals and their substituted molar ratios [8,9]. Moreover, 4 V plateau capacity is decreased with increasing the amount of doped transition metal, and cycle life is improved for Co- and Ni-doped spinels [4]. It has been found that the potential profile of LiNixMn2 xO4 changes according to x value and the voltages appeared around 4.4–4.7 and 4.7–5.0 V correspond to the redox reactions of Ni2+/Ni3+ and Ni3+/Ni4+couples, respectively [10]. With the increase of nickel in LiNixMn2 xO4, 5 V plateau capacity rises and finally the 4 V plateau disappears basically at x = 0.5. The LiNi0.5Mn1.5O4 material is usually obtained through sol–gel, solid-state reaction, molten salt, emulsion drying, and polymer-pyrolysis methods [8,11–15]. The effects of the structure and particle size on the electrochemical response of the material have been widely investigated [14,16]. Many attempts have been made to * Corresponding author. Tel.: +86 21 5474 7667; fax: +86 21 5474 1297. E-mail address: [email protected] (J. Yang). 0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.02.025
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improve both the cyclability and the rate capability at room and high temperature, and some positive effects obtained by surface treatments [1,17,18] and anion doping [19–21]. Recently, it has been reported that a small amount of fluorine substitution for oxygen can reduce Ni and Mn dissolution from HF attack, enhancing the electrochemical properties and thermal stability [22]. As we know, fluorine-doping method has been widely introduced to the layered cathode material LiNi1/3Co1/3Mn1/ O 3 2 and the ways of doping can cause different effects [23–25]. In this study, LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) spinel powders are prepared by sol–gel and post-annealing treatment method and the fluorine effect on the structure, morphology and electrochemical properties are investigated. 2. Experimental The compounds of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) were synthesized by sol–gel and post-annealing treatment method. The stoichiometric amount of Li(CH3COO)2H2O, Ni(CH3COO)24H2O, Mn(CH3COO)24H2O and LiF were added to 80 ml distilled water and stirred at about 70 8C to form gel. Then the gel precursor was decomposed at 450 8C for 5 h. The resulted residue was ground in a mortar and pressed into pellets, followed by a heat-treatment at 850 8C in oxygen for 12 h. By controlling sol–gel and subsequent heattreatment temperature, the powder granularity can be regulated. The XRD patterns were obtained by a Bruker D8 ADVANCE X-ray diffractometer using Cu Ka radiation (wavelength: 1.5406 Å; voltage: 40 kV; scan speed: 1.28/min), with silicon powder as an internal reference for the measurement of lattice parameter. Scanning electron microscopy (SEM) micrographs were obtained on a Hitachi S 2150 microscope. The electrochemical tests were carried out via CR2016 coin-type cells. The cathodes were prepared by blending active material, acetylene black, and polyvinylidene fluoride (weight ratio 85:7:8) dissolved in N-methyl-2pyrrolidinone. Then the slurry was coated on an aluminum foil and dried under vacuum at 120 8C over 4 h. Electrode disks were punched from the foil and weighed. Celgard 2700 membrane was used as separator and metallic lithium as the anode, 1 M LiPF6 dissolved in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight ratio) was used as the electrolyte. The cells were assembled in an argon-filled glove box with each of oxygen and moisture is less than 1 ppm. Charge and discharge measurements of the coin cells were carried out at a current density of 29.4 mA g 1 (0.2 C) with voltage cut-off of 5.2 V/3.5 V versus Li/Li+ at 20 8C. 3. Results and discussion Fig. 1 shows the powder X-ray diffraction patterns of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2). It can be observed that all XRD peaks are quite narrow, indicating their high crystallinity. There is no obvious
Fig. 1. XRD patterns of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2).
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Table 1 Refinement results of lattice parameters for LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) active materials Active materials
Lattice parameter a (Å)
LiNi0.5Mn1.5O4 LiNi0.5Mn1.5O3.95F0.05 LiNi0.5Mn1.5O3.9F0.1 LiNi0.5Mn1.5O3.85F0.15 LiNi0.5Mn1.5O3.8F0.2
8.1815 8.1783 8.1829 8.1914 8.1983
peak change when the different amount of fluorine is added. All the samples can be indexed as a cubic spinel structure ¯ It means that Li+ occupies the tetrahedral (8a) sites, Ni2+and Mn4+ are randomly located with a space group of Fd 3m. at the octahedral (16d) sites, and O2 and F are located at the octahedral (32e) sites [22,26]. As reported that impurities are often produced in the calcination process [8], a little amount of NiO secondary phase is also observed in the patterns. Lattice parameters of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2) are further calculated from the XRD data by a least squares method and the results are shown in Table 1. The fluorine-free LiNi0.5Mn1.5O4 has a lattice parameter of a = 8.1845 Å. For LiNi0.5Mn1.5O3.95F0.05, this value is slightly smaller. It may be related to the stronger bond of M–F than M–O and substitution of the small amount of F with relatively small ionic radius (r = 1.33 Å) for O2 (r = 1.40 Å) [22]. However, the lattice parameter turns to increase when fluorine content is further enhanced. Here the existence of more Mn3+ in the material may become the dominating factor. Because fluorine substitution changes the oxidation state of transition metal components [24] and more Mn3+ ions with larger ionic radius (r = 0.645 Å) will replace partial Mn4+ ions (r = 0.53 Å) for electro-neutrality. The variation in lattice parameters suggests that fluorine ions are successfully substituted for oxygen site in LiNi0.5Mn1.5O4. SEM micrographs in Fig. 2 display octahedral shape characteristic of cubic spinel samples. The LiNi0.5Mn1.5O3.95F0.05 powder exhibits a wide particle size distribution. The particles become larger and the distribution more uniform with the increase of the fluorine amount. The initial charge and discharge curves of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O3.95F0.05 are illustrated in Fig. 3. Both the samples exhibit two similar voltage plateaus at around 4.0 and 4.7 V. It is noted that 4 V capacity becomes larger with increasing fluorine content. This result supports the previous assumption about existence of more Mn3+ due to fluorine substitution in LiNi0.5Mn1.5O4 xFx. The total discharge capacity increases after limited fluorine doping (x = 0.05) by this preparation route. Fig. 4 shows steady-state scan cyclic voltammograms of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.2) with a scan rate of 50 mV s 1. It can be found that the intensity and location of peaks are related to the amount of fluorine in the samples. The enhanced fluorine amount results in the decrease of the peak current. The redox couple located at around 4 V is associated with the reaction between Mn3+ and Mn4+, and the charge amount for this reaction increases when more fluorine is doped. Two oxidation peaks between 4.4 and 5 V, which are clearly shown in the insert of Fig. 4 for LiNi0.5Mn1.5O3.8F0.2 after 100 charge/discharge cycles, can be attributed to redox reactions of Ni2+/Ni3+ and Ni3+/ Ni4+. Moreover, because of the inductive effect of the doped fluorine, the peak voltage shifts slightly toward positive direction with the increase of the fluorine amount. This phenomenon can be also observed in the initial charging curve. The cyclic performance was measured for the LiNi0.5Mn1.5O4 xFx with different fluorine amounts. The cycling results shown in Fig. 5 indicate that discharge capacity decreases with enhanced fluorine content. Although LiNi0.5Mn1.5O3.95F0.05 exhibits a high initial capacity, the capacity fade is relatively fast. When the fluorine content is increased to higher than 0.1, the sample gives better cyclic stability. It appears that strong Li–F bonding may hinder Li+ extraction, leading to a lower reversible capacity. On the other hand, fluorine doping makes lattice parameter larger (see Table 1) and spinel structure more stable due to the strong M–F bonding. These are favorable for the cyclic stability. In view of the balance between the capacity and cyclic stability, LiNi0.5Mn1.5O3.9F0.1 gives the best performance. The initial capacity of 122 mAh g 1 and the capacity retention of 91% after 100 cycles are obtainable. Furthermore, the influence of the powder granularity of fluorine-doped LiNi0.5Mn1.5O4 on the discharge capacity and rate capability was investigated. SEM images in Fig. 6 show powder morphologies of two LiNi0.5Mn1.5O3.9F0.1 samples. The sample (a) exhibits dense and aggregated particle character. The particle size is small and uniform (ca. 0.1–0.2 mm) with some aggregations up to 0.5 mm. In contrast, the particle size of sample (b) is much larger, mostly more than 1 mm.
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Fig. 2. Scanning electron micrographs of (a) LiNi0.5Mn1.5O3.95F0.05, (b) LiNi0.5Mn1.5O3.9F0.1 and (c) LiNi0.5Mn1.5O3.8F0.2.
Fig. 3. Initial charge and discharge curves of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O3.95F0.05.
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Fig. 4. Cyclic voltammograms of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.2). The insert exhibits oxidation peaks of LiNi0.5Mn1.5O3.8F0.2 after 100 cycles.
Fig. 5. Cyclic performances of LiNi0.5Mn1.5O4 xFx (x = 0.05, 0.1, 0.15, 0.2).
Fig. 7 shows discharge capacity upon cycle number for the samples with different particle sizes demonstrated in Fig. 6. At low discharge rate of 0.2 C, the large particle sample delivers a larger capacity than the small one and their cycling stability is quite similar. When the discharge rate is enhanced to 1 C, the cycling capacity drops significantly for both the samples. However, the large particle sample exhibits lower capacity in the initial cycles and faster capacity fade. Such a contrary electrode behavior at different discharge rate may be explained as follows: the large LiNi0.5Mn1.5O3.9F0.1 particles have better crystallinity and less surface defects. Thereby, the material could provide a capacity closer to theoretic value under a quasi-reversible condition (i.e. low discharge rate). The small particle sample possesses fast kinetic advantage due to the larger surface area and shorter diffusion length of Li+ ions within the particles. It is favorable for high rate discharge. The limitations of both lithium diffusion speed and interfacial reaction region for the large particle sample could cause strong electrochemical polarization at high charge and discharge rate, leading to side reactions such as electrolyte decomposition. Therefore, the cycling stability becomes poor.
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Fig. 6. Scanning electron micrographs of LiNi0.5Mn1.5O3.9F0.1 with different particle sizes. The insert in (a) exhibits a higher resolution image.
Fig. 7. Influence of LiNi0.5Mn1.5O3.9F0.1 particle size on the cyclic performance.
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4. Conclusions 5 V LiNi0.5Mn1.5O4 xFx (0.05 x 0.2) cathode materials all with the spinel structure were prepared by sol–gel and post-annealing treatment method. Fluorine doping for a substitution for oxygen sites in LiNi0.5Mn1.5O4 results in a variation of the lattice parameters and bonding energy. For the materials with x beyond 0.1, the capacity decreases, but the cyclability is significantly improved. This is probably due to more stable fine-structure arising from fluorine doping. On the other hand, depending on the discharge rate, the effect of the LiNi0.5Mn1.5O3.9F0.1 particle size on the electrode performance is greatly different. Owing to shorter Li+ diffusion length and larger specific surface area, the powder material with smaller particle size exhibits larger cycle capacity and better cyclability at high charge and discharge rate. But the larger particle sample shows the advantage in the Li-storage capacity at a low reaction rate, where the crystallinity can become a dominant factor. Acknowledgements This work was supported by National 973 Program (No. 2007CB209700) and NCET Fund from Education Ministry of PR China. Authors are greatly indebted to Mr. Haibo Han in the Instrumental Analysis Center of Shanghai Jiao Tong University for discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, J. Electroanal. Chem. 566 (2004) 187. C. Sigala, D. Guyomard, A. Verbaere, Y. Piffard, M. Tournorx, Solid State Ionics 81 (1995) 167. K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Electrochem. Soc. 143 (1996) 1607. G. Li, H. Ikuta, T. Uchida, M. Wakihara, J. Electrochem. Soc. 143 (1996) 178. H. Kawai, M. Nagata, H. Kageyama, H. Tsukamoto, A.R. West, Electrochim. Acta 45 (1999) 315. H. Shigemura, H. Sakaebe, H. Kageyama, H. Koboyashi, A.R. West, R. Kanno, S. Morinoto, S. Nasu, M. Tabuchi, J. Electrochem. Chem. 148 (2001) A730. K. Kanamura, W. Hoshikawa, T. Umegaki, J. Electrochem. Soc. 149 (2002) A339. Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 205. T. Ohzuku, S. Takeda, M. Iwanaga, J. Power Sources 81–82 (1999) 90. K. Dokko, M. Mohamedi, N. Anzue, T. Itoh, I. Uchida, J. Mater. Chem. 12 (2002) 3688. X. Wu, B.K. Seung, J. Power Sources 109 (2002) 53. Y. Idemoto, H. Narai, N. Koura, J. Power Sources 119–121 (2003) 125. J.H. Kim, S.T. Myung, C.S. Yoon, S.G. Kang, Y.K. Sun, Chem. Mater. 16 (2004) 906. S.T. Myung, S. Komaba, N. Kumakai, H. Yashiro, H.T. Chung, T.H. Cho, Electrochim. Acta 47 (2002) 2543. L. Yu, Y. Cao, H. Yang, X. Ai, J. Solid State Electrochem. 10 (2006) 283. D. Kovacheva, B. Markovsky, G. Salitra, Y. Talyosef, M. Gorova, E. Levi, M. Riboch, J.K. Hyeong, D. Aurbach, Electrochim. Acta 50 (2005) 5553. Y.K. Sun, C.S. Yoon, I.H. Oh, Electrochim. Acta 48 (2003) 503. A. Eftekhari, Chem. Lett. 33 (5) (2004) 616. T.A. Arunkumar, A. Manthiram, Electrochim. Acta 50 (2005) 5568. B. León, J.M. Lloris, C.P. Vicente, J.L. Tirado, Electrochem. Solid-State Lett. 9 (2) (2006) A96. S.H. Oh, S.H. Jeon, W.I. Cho, C.S. Kim, B.W. Cho, J. Alloys Compd. 452 (2008) 389. S.W. Oh, S.H. Park, J.H. Kim, Y.C. Bae, Y.K. Sun, J. Power Sources 157 (2006) 464. S. Jouanneau, J.R. Dahn, J. Electrochem. Soc. 151 (2004) A1749. G.H. Kim, J.H. Kim, S.T. Myung, C.S. Yoon, Y.K. Sun, J. Electrochem. Soc. 152 (2005) A1707. D. Li, Y. Sasaki, K. Kobayakawa, H. Noguchi, Y. Sato, Electrochim. Acta 52 (2006) 643. S. Nieto, S.B. Majumder, R.S. Katiyar, J. Power Sources 136 (2004) 88.