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Freeze drying synthesis of LiNi0.5Mn0.5O2 cathode materials
Electrochimica Acta 50 (2004) 505–509
Freeze drying synthesis of LiNi0.5Mn0.5O2 cathode materials O.A. Shlyakhtina,b , Young Soo Yoonc,∗ , Sun Hee Choia , Young-Jei Oha a
Materials Science and Technology Division, Korea Institute of Science and Technology, Seoul 139-791, Republic of Korea b Institute of Chemical Physics, Russian Academy of Sciences, 117334 Moscow, Russia c Department of Advanced Fusion Technology, Konkuk University, Seoul 143-701, Republic of Korea Received 2 June 2003; received in revised form 1 May 2004; accepted 1 May 2004 Available online 5 August 2004
Abstract The influence of several processing conditions on the phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 powders, obtained by freeze drying method, is studied. Thermal processing in pellets at maximum heating rate promotes better crystallographic ordering of hexagonal LiNi0.5 Mn0.5 O2 and maximum capacity values irrespectively of chemical composition of the precursor. Instead, intense mechanical processing of precursors exerts considerable negative effect on the electrochemical performance. Cathode materials containing superstoichiometric amount of lithium (Li1.3 Mn0.5 Ni0.5 O2+␦ ) demonstrate reversible capacity values up to 190 mAh/g between 2.5 and 4.6 V. © 2004 Elsevier Ltd. All rights reserved. Keywords: Lithium nickel manganese oxide; Cathode materials; Li-ion batteries; Freeze drying; Chemical prehistory
1. Introduction Lithium nickel manganese oxide Li(Ni0.5 Mn0.5 )O2 was recently proposed as promising cathode material for new generation of secondary Li-ion batteries [1]. Good electrochemical performance of Li(Ni0.5 Mn0.5 )O2 , combined with reduced cost of raw materials, is promoted by the easy formation of solid solutions with layered structure in the Li–Ni–Mn–O system [2–4]. Along with this advantage, the closeness of crystallochemical radii of Li+ and Ni2+ leads to the low driving force of crystallographic ordering, necessary for the free transport of Li ions in layered ABO2 phases. The most common point defects in Li(Ni,Mn)O2 are antisite defects formed by Li+ localized in B-positions and by Ni2+ in A-positions of hexagonal ABO2 lattice [5,6]. The ability of Li+ ions to be localized in B-positions is used in another series of recently discovered cathode materials Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 [5–8]. At the same time the electrochemical behavior of compositions Li1+x (Ni0.5 Mn0.5 )O2 with equimolar amount of Mn and Ni is not described yet. ∗
Corresponding author. Tel.: +82 2 2049 6042; fax: +82 2 452 5558. E-mail address: [email protected] (Y.S. Yoon).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.05.051
Owing to easy formation of Li2 MnO3 at reduced temperatures, the synthesis of Li(Ni,Mn)O2 often proceeds through initial formation of Li2 MnO3 and LiNiO2 followed by solid state reaction between them [9]. Both topochemical processes are rather sensitive to the variations of processing conditions. It was also found that the use of MnO2 instead of -␥-MnOOH or MnO prevents the formation of single phase Li(Mn,Ni)O2 at 850–900 ◦ C [2,9]. Recent observations demonstrated the existence of localized clusters, resembling local environment of Li in Li2 MnO3 and LiNiO2 , in the single phase Li–Ni–Mn oxides [10]. Due to recent discovery of Li(Ni,Mn)O2 -based materials, little is known about the influence of preparation technique on the crystallographic ordering, micromorphology and electrochemical performance of Li(Ni0.5 Mn0.5 )O2 powders. Most of the authors use hydroxide synthesis scheme [5,6,11–13] while other groups use precursor mixtures of LiOH, NiO and MnO [2], Mn2 O3 , LiOH and Ni metal [14]; Me acetates and nitrates [8,12,15]. Final processing at T < 900 ◦ C usually resulted in poorer electrochemical performance [1,3,4], but even at comparable processing temperatures the Li(Ni0.5 Mn0.5 )O2 powders, obtained by different synthesis methods, often demonstrate rather different electrochemical behavior. It shows at the substantial influence of
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processing variables on the starting capacity and, especially, on its fade rate of Li(Ni,Mn)O2 based cathode materials. Some of the mentioned processing variables are common for the various synthesis techniques, while the information on their influence on the properties of Li(Ni0.5 Mn0.5 )O2 is insufficient or controversial. In order to simplify the comparison of materials obtained by different methods we estimated the effect of several frequently used processing approaches on the phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 powders. Freeze drying method, often applied for the synthesis of cathode materials, was used for powder preparation.
2. Experimental Freeze drying synthesis of Li(Ni,Mn)O2 powders was performed via solution and hydroxide routes. In the first case the aqueous solutions of Li acetate or nitrate and Ni and Mn acetates in the ratio corresponding to LiNi0.5 Mn0.5 O2 were frozen by spraying to liquid nitrogen and freeze dried for 2 days at P = 5 × 10−2 mbar (Alpha 2–4, Christ). In order to estimate the influence of lithium excess on the electrochemical performance of LiNi0.5 Mn0.5 O2 , Ni and Mn hydroxides in the equimolar ratio were coprecipitated by NaOH, carefully washed, freeze dried and mixed with LiOH, taken in 30% excess to LiNi0.5 Mn0.5 O2 stoichiometry and dissolved in the minimum amount of water. As-obtained paste was frozen by liquid nitrogen and freeze dried. The products of freeze drying were subjected to thermal processing at 500 ◦ C and/or at 900 ◦ C in air using various processing schedules. In several experiments the products of freeze drying before further thermal treatment were mixed with K2 SO4 and processed in the Fritsch planetary mill for
12 h at 600 rpm in ZrO2 media. In this case the thermal decomposition product was washed by distilled water several times and separated by centrifuge. Samples with Li excess were also carefully washed before SEM measurements in order to remove possible Li2 CO3 contaminations. Obtained LiNi0.5 Mn0.5 O2 powders have been studied by XRD (Geigerflex, Rigaku, 2◦ /min, Cu K␣) and SEM (Philips ESEM). The electrochemical characterizations were performed using CR2032 coin-type cell; cut-off voltage 2.5–4.6 V; I = 0.1 mA. The cathode was fabricated with 20 mg of active material and 12 mg of conductive binder (8 mg of teflonized acetylene black (TAB) and 4 mg of graphite). It was pressed on 200 mm2 copper mesh used as the current collector under a pressure of 300 kg/cm2 and dried at 180 ◦ C for 24 h in a vacuum oven. The test cell was made of cathode and a lithium metal separated by a porous polypropylene film (Celgard 3401). The electrolyte used was a mixture of 1 M LiPF6 –ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:2 by vol., Merck).
3. Results and discussion According to thermal analysis data (Fig. 1), the decomposition of freeze-dried precursors is completed at 500 ◦ C with no traces of further solid state reaction in decomposition products. Lower decomposition temperature of nitrate-containing precursor compared to acetate precursor is caused by internal redox reaction while steady decomposition of hydroxide precursor is typical for thermolysis of amorphous hydroxides. The formation of complex oxide with layered structure occurs at thermal decomposition for all kind of cryoprecursors, though crystallographic ordering proceeds only at T > 800 ◦ C.
Fig. 1. Thermogravimetric curves of freeze dried precursors, obtained using (A) Li, Ni and Mn acetates; (B) Li nitrate, Mn and Ni acetates; (C) Li, Ni and Mn hydroxides.
O.A. Shlyakhtin et al. / Electrochimica Acta 50 (2004) 505–509
Heating rate during thermal decomposition is proved to be the crucial factor for thermal processing of Li(Ni,Mn)O2 precursors. Usually applied heating rates <10 K/min lead to poorly crystallized products, containing traces of thermolysis intermediates even after further annealing of powders at 900 ◦ C. Meanwhile, fast heating of the same precursors by their insertion to the preheated to 900 ◦ C muffle furnace allows to obtain rather crystalline single phase Li(Ni0.5 Mn0.5 )O2 powders in one step (Fig. 2; curves I and II). Considerable broadening of XRD peaks of decomposition products and close structural similarity of LiNiO2 , Li2 MnO3 and Li(Ni0.5 Mn0.5 )O2 give poor chances for direct XRD detection of Li2 MnO3 formation at the intermediate stages of thermal processing. Meanwhile, taking into account the data of the previous decomposition studies [9], the formation of stable Li2 MnO3 intermediate could be a good explanation for observed heating rate dependence of Li(Ni0.5 Mn0.5 )O2 formation processes. Moderate heating rates (5–10 K/min) stimulate Li2 MnO3 crystallization and hinder the following solid solution formation, while fast heating leads to the direct formation of Li(Ni0.5 Mn0.5 )O2 . Similar accelerated formation of complex oxides at enhanced heating rates is typical for the system with stable intermediate products of thermal decomposition and has been observed before during freeze drying synthesis of YBa2 Cu3 Ox , YBa2 Cu4 O8 and (Bi,Pb)Sr2 CaCu2 Oy [17–19].
Fig. 2. XRD patterns of LiNi0.5 Mn0.5 O2 powders, obtained by annealing at 900 ◦ C for 12 h in powder or in pellets at various heating rates: (I) 5 K/min, powder; (II) 1000 K/min, powder; (III) 1000 K/min, pellet.
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Pelletizing of thermal decomposition products before final annealing at 900 ◦ C, as it was anticipated, results in smaller FWHM of the main peaks and more pronounced splitting of
Fig. 3. SEM micrographs of Li(Ni0.5 Mn0.5 )O2 powders at various stages of thermal processing: (A) 500 ◦ C, 6 h; (B) 900 ◦ C, 1 h; (C and D) 900 ◦ C, 12 h.
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1 0 8/1 1 0 reflections (Fig. 2, curve III), characterizing welldefined hexagonal structure [1,12]. Domination of the layered structure usually correlates with smaller capacity fade rate, so that these features make preferable final annealing in pellets and allow to explain reduced capacity values, observed by several authors after final annealing of LiNi0.5 Mn0.5 O2 at 900–950 ◦ C in powder form [8]. The need in pelletizing differentiates LiNi0.5 Mn0.5 O2 from LiCoO2 where similar ordering processes occur in powders and at lower temperatures (800–850 ◦ C). Intense mechanical processing has profound influence on the properties of Li(Ni0.5 Mn0.5 )O2 powders in spite of the fact, that external force was applied not to final product but to the precursor powder. The most important features of Li(Ni0.5 Mn0.5 )O2 response on the mechanical processing are absence or decreasing the splitting of 1 0 8/1 1 0 reflec-
tions and reverse 0 0 3/1 0 4 peak intensity ratio compared to reference samples, obtained without planetary milling. Both features, according to [1], are indicative to high concentration of rock-salt domains in the layered matrix and, hence, to the poorer electrochemical reactivity of these materials. Similar reasons might explain unusual (up to 68 h) increase in thermal processing duration after mixing the precursor powders in highly powered Spex mill, observed in [16]. SEM analysis of obtained powders shows that the thermal decomposition of salt precursor results in the formation of 20–30 nm crystallites of Li(Ni,Mn)O2 (Fig. 3A). Grain growth at 900 ◦ C proceeds rather slowly, so that only continuous annealing in pellets results in complete rearrangement of primary grain ensembles (Fig. 3B and C). Nevertheless, even in that case many samples still demonstrate nanocrystalline features, showing at the residual
Fig. 4. The influence of processing conditions on the voltage vs. capacity profiles for the first discharge cycle (A) and cycling performance (B and C) of the cells LiNi0.5 Mn0.5 O2 cathodes. (1, 2 and 6) two stage thermal processing (500 ◦ C + 900 ◦ C); (3 and 7) single-stage heat treatment at 900 ◦ C; (4, 5 and 8) samples with preliminary mechanical processing before thermal decomposition. (A and B) hydroxide precursors; (C) acetate precursors.
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chemical inhomogeneities within newly formed large crystallites (Fig. 3D). The sensitivity of initial capacity and fade rate of Li(Ni0.5 Mn0.5 )O2 cathodes to the variation of processing conditions is proved to be higher than the sensitivity of common powder XRD method, though the most substantial changes are indicated both by XRD and by electrochemical capacity measurements. As it was mentioned before, the XRD patterns of Li(Ni0.5 Mn0.5 )O2 samples, subjected to mechanical processing before decomposition of precursors, have the traces of crystallographic disorder even after continuous annealing. Electrochemical capacity of these powders is also much lower than for usually processed samples (Fig. 4A and B, curve 4). Additional thermal processing can lead to the small increase in capacity values, but their level remains to be rather low (Fig. 4A and B, curve 5; Fig. 4C, curve 8). These data show at rather limited perspectives of mechanical activation methods in the Li(Ni,Mn)O2 synthesis and production, as the negative effect of this kind of processing is rather stable even at elevated temperatures. Another important processing feature, poorly detectable by XRD, is a strong influence of the Li(Ni0.5 Mn0.5 )O2 formation temperature on the electrochemical performance of cathode materials. Li(Ni0.5 Mn0.5 )O2 cathodes, obtained by single-stage thermal decomposition at 900 ◦ C, demonstrate substantially lower capacity values than similar samples, subjected to preliminary thermal processing at 500 ◦ C (Fig. 4A and B; curves 2 and 3; Fig. 4C, curves 6 and 7). Better homogeneity of reaction mixture, obtained by thermolysis at reduced temperatures, promotes faster completion of phase formation and better ordering of Li(Ni0.5 Mn0.5 )O2 during further annealing at 900 ◦ C. Mechanical processing of precursor powder and thermal processing schedule have similar effect on Li(Ni0.5 Mn0.5 )O2 powders obtained from acetate and from hydroxide precursors (Fig. 4B and C). At the same time the maximum capacity values are proved to be different in these two series. After initial falling down the electrochemical capacity of Li(Ni0.5 Mn0.5 )O2 sample, obtained from acetate precursor, is stabilized at the value of 125–130 mAh/g, usual for Li(Ni0.5 Mn0.5 )O2 -based materials (Fig. 4C, curve 6). Meanwhile, the samples with hydroxide prehistory demonstrate negligible capacity fade after first cycle and stabilization at 186–192 mAh/g (Fig. 4B; curves 1 and 2). These values are not usual for Li(Ni0.5 Mn0.5 )O2 , but close to the capacity of Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 -based materials [5–8]. Li/Ni/Mn ratio in our single phase samples with hydroxide prehistory is rather different from this stoichiometry, but they also contain considerable Li excess (Li1.3 Ni0.5 Mn0.5 O2 ). Similarly to Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 , observed capacity enhancement can be connected with partial localization of excess lithium in the B-positions of ABO2 lattice.
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4. Conclusions Phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 -based cathode materials are considerably affected by thermal and mechanical processing conditions. Maximum heating rate, pelletizing and preliminary thermal decomposition of precursors at moderate temperatures before final annealing promote faster formation of perfectly ordered hexagonal structure and enhancement of electrochemical capacity values. On the contrary, intense mechanical processing of precursor complicates long-range ordering during high temperature annealing and results in considerably reduced capacity values at usual annealing time. Introduction of lithium excess during synthesis of LiNi0.5 Mn0.5 O2 is accompanied by enhancement of reversible capacity values up to 185–190 mAh/g. Acknowledgements The work is supported by National Research Laboratory (Development of monolithic high power hybrid battery) and by KISTEP fellowship. References [1] T. Ohzuku, Y. Makimura, Chem. Lett. (2001) 744. [2] E. Rossen, C.D.W. Jones, J.R. Dahn, Solid State Ionics 57 (1992) 311. [3] D. Caurant, N. Baffler, V. Bianchi, G. Gregoire, S. Bach, J. Mater. Chem. 6 (1996) 1149. [4] M.E. Spahr, P. Novak, O. Haas, R. Nesper, J. Power Sources 68 (1997) 629. [5] Z. Lu, L.Y. Beaulieu, R.A. Donaberger, C.L. Thomas, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A778. [6] Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid State Lett. 4 (2001) A191. [7] S.-S. Shin, Y.-K. Sun, K. Amine, J. Power Sources 112 (2002) 634. [8] Y.J. Park, M.G. Kim, Y.-S. Hong, X. Wu, K.S. Ryu, S.H. Chang, Solid State Commun. 127 (2003) 509. [9] M. Yoshio, Y. Todorov, K. Yamato, H. Noguchi, J. Itoh, M. Okada, T. Mouri, J. Power Sources 74 (1998) 46. [10] W.-S. Yoon, N. Kim, X.-Q. Yang, J. McBreen, C.P. Gray, J. Power Sources 119–121 (2003) 649. [11] Z. Lu, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A815. [12] K.M. Shaju, G.V. Subba Rao, B.V.R. Choudhari, Electrochim. Acta 48 (2003) 1505. [13] X.-Q. Yang, J. McBreen, W.-S. Yoon, C.P. Grey, Electrochem. Commun. 4 (2002) 649. [14] B.L. Cushing, J.B. Goodenough, Solid State Sci. 4 (2002) 1487. [15] R. Chitrakar, S. Kasaishi, A. Umeno, K. Sakane, N. Takagi, Y.-S. Kim, K. Ooi, J. Solid State Chem. 169 (2002) 35. [16] Y.-K. Sun, C.S. Yoon, Y.S. Lee, Electrochim. Acta 48 (2003) 2589. [17] O.A. Shlyakhtin, A.L. Vinokurov, A.N. Baranov, Yu.D. Tretyakov, J. Supercond. 11 (1998) 507. [18] K. Takahashi, T. Ito, H. Yoshikawa, A. Hiraki, Jpn. J. Appl. Phys. 32 (1993) 1211. [19] Yu.D. Tretyakov, N.N. Oleynikov, O.A. Shlyakhtin, Cryochemical Technology of Advanced Materials, Chapman & Hall, London, 1997, p. 260.
Freeze drying synthesis of LiNi0.5Mn0.5O2 cathode materials O.A. Shlyakhtina,b , Young Soo Yoonc,∗ , Sun Hee Choia , Young-Jei Oha a
Materials Science and Technology Division, Korea Institute of Science and Technology, Seoul 139-791, Republic of Korea b Institute of Chemical Physics, Russian Academy of Sciences, 117334 Moscow, Russia c Department of Advanced Fusion Technology, Konkuk University, Seoul 143-701, Republic of Korea Received 2 June 2003; received in revised form 1 May 2004; accepted 1 May 2004 Available online 5 August 2004
Abstract The influence of several processing conditions on the phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 powders, obtained by freeze drying method, is studied. Thermal processing in pellets at maximum heating rate promotes better crystallographic ordering of hexagonal LiNi0.5 Mn0.5 O2 and maximum capacity values irrespectively of chemical composition of the precursor. Instead, intense mechanical processing of precursors exerts considerable negative effect on the electrochemical performance. Cathode materials containing superstoichiometric amount of lithium (Li1.3 Mn0.5 Ni0.5 O2+␦ ) demonstrate reversible capacity values up to 190 mAh/g between 2.5 and 4.6 V. © 2004 Elsevier Ltd. All rights reserved. Keywords: Lithium nickel manganese oxide; Cathode materials; Li-ion batteries; Freeze drying; Chemical prehistory
1. Introduction Lithium nickel manganese oxide Li(Ni0.5 Mn0.5 )O2 was recently proposed as promising cathode material for new generation of secondary Li-ion batteries [1]. Good electrochemical performance of Li(Ni0.5 Mn0.5 )O2 , combined with reduced cost of raw materials, is promoted by the easy formation of solid solutions with layered structure in the Li–Ni–Mn–O system [2–4]. Along with this advantage, the closeness of crystallochemical radii of Li+ and Ni2+ leads to the low driving force of crystallographic ordering, necessary for the free transport of Li ions in layered ABO2 phases. The most common point defects in Li(Ni,Mn)O2 are antisite defects formed by Li+ localized in B-positions and by Ni2+ in A-positions of hexagonal ABO2 lattice [5,6]. The ability of Li+ ions to be localized in B-positions is used in another series of recently discovered cathode materials Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 [5–8]. At the same time the electrochemical behavior of compositions Li1+x (Ni0.5 Mn0.5 )O2 with equimolar amount of Mn and Ni is not described yet. ∗
Corresponding author. Tel.: +82 2 2049 6042; fax: +82 2 452 5558. E-mail address: [email protected] (Y.S. Yoon).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.05.051
Owing to easy formation of Li2 MnO3 at reduced temperatures, the synthesis of Li(Ni,Mn)O2 often proceeds through initial formation of Li2 MnO3 and LiNiO2 followed by solid state reaction between them [9]. Both topochemical processes are rather sensitive to the variations of processing conditions. It was also found that the use of MnO2 instead of -␥-MnOOH or MnO prevents the formation of single phase Li(Mn,Ni)O2 at 850–900 ◦ C [2,9]. Recent observations demonstrated the existence of localized clusters, resembling local environment of Li in Li2 MnO3 and LiNiO2 , in the single phase Li–Ni–Mn oxides [10]. Due to recent discovery of Li(Ni,Mn)O2 -based materials, little is known about the influence of preparation technique on the crystallographic ordering, micromorphology and electrochemical performance of Li(Ni0.5 Mn0.5 )O2 powders. Most of the authors use hydroxide synthesis scheme [5,6,11–13] while other groups use precursor mixtures of LiOH, NiO and MnO [2], Mn2 O3 , LiOH and Ni metal [14]; Me acetates and nitrates [8,12,15]. Final processing at T < 900 ◦ C usually resulted in poorer electrochemical performance [1,3,4], but even at comparable processing temperatures the Li(Ni0.5 Mn0.5 )O2 powders, obtained by different synthesis methods, often demonstrate rather different electrochemical behavior. It shows at the substantial influence of
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processing variables on the starting capacity and, especially, on its fade rate of Li(Ni,Mn)O2 based cathode materials. Some of the mentioned processing variables are common for the various synthesis techniques, while the information on their influence on the properties of Li(Ni0.5 Mn0.5 )O2 is insufficient or controversial. In order to simplify the comparison of materials obtained by different methods we estimated the effect of several frequently used processing approaches on the phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 powders. Freeze drying method, often applied for the synthesis of cathode materials, was used for powder preparation.
2. Experimental Freeze drying synthesis of Li(Ni,Mn)O2 powders was performed via solution and hydroxide routes. In the first case the aqueous solutions of Li acetate or nitrate and Ni and Mn acetates in the ratio corresponding to LiNi0.5 Mn0.5 O2 were frozen by spraying to liquid nitrogen and freeze dried for 2 days at P = 5 × 10−2 mbar (Alpha 2–4, Christ). In order to estimate the influence of lithium excess on the electrochemical performance of LiNi0.5 Mn0.5 O2 , Ni and Mn hydroxides in the equimolar ratio were coprecipitated by NaOH, carefully washed, freeze dried and mixed with LiOH, taken in 30% excess to LiNi0.5 Mn0.5 O2 stoichiometry and dissolved in the minimum amount of water. As-obtained paste was frozen by liquid nitrogen and freeze dried. The products of freeze drying were subjected to thermal processing at 500 ◦ C and/or at 900 ◦ C in air using various processing schedules. In several experiments the products of freeze drying before further thermal treatment were mixed with K2 SO4 and processed in the Fritsch planetary mill for
12 h at 600 rpm in ZrO2 media. In this case the thermal decomposition product was washed by distilled water several times and separated by centrifuge. Samples with Li excess were also carefully washed before SEM measurements in order to remove possible Li2 CO3 contaminations. Obtained LiNi0.5 Mn0.5 O2 powders have been studied by XRD (Geigerflex, Rigaku, 2◦ /min, Cu K␣) and SEM (Philips ESEM). The electrochemical characterizations were performed using CR2032 coin-type cell; cut-off voltage 2.5–4.6 V; I = 0.1 mA. The cathode was fabricated with 20 mg of active material and 12 mg of conductive binder (8 mg of teflonized acetylene black (TAB) and 4 mg of graphite). It was pressed on 200 mm2 copper mesh used as the current collector under a pressure of 300 kg/cm2 and dried at 180 ◦ C for 24 h in a vacuum oven. The test cell was made of cathode and a lithium metal separated by a porous polypropylene film (Celgard 3401). The electrolyte used was a mixture of 1 M LiPF6 –ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:2 by vol., Merck).
3. Results and discussion According to thermal analysis data (Fig. 1), the decomposition of freeze-dried precursors is completed at 500 ◦ C with no traces of further solid state reaction in decomposition products. Lower decomposition temperature of nitrate-containing precursor compared to acetate precursor is caused by internal redox reaction while steady decomposition of hydroxide precursor is typical for thermolysis of amorphous hydroxides. The formation of complex oxide with layered structure occurs at thermal decomposition for all kind of cryoprecursors, though crystallographic ordering proceeds only at T > 800 ◦ C.
Fig. 1. Thermogravimetric curves of freeze dried precursors, obtained using (A) Li, Ni and Mn acetates; (B) Li nitrate, Mn and Ni acetates; (C) Li, Ni and Mn hydroxides.
O.A. Shlyakhtin et al. / Electrochimica Acta 50 (2004) 505–509
Heating rate during thermal decomposition is proved to be the crucial factor for thermal processing of Li(Ni,Mn)O2 precursors. Usually applied heating rates <10 K/min lead to poorly crystallized products, containing traces of thermolysis intermediates even after further annealing of powders at 900 ◦ C. Meanwhile, fast heating of the same precursors by their insertion to the preheated to 900 ◦ C muffle furnace allows to obtain rather crystalline single phase Li(Ni0.5 Mn0.5 )O2 powders in one step (Fig. 2; curves I and II). Considerable broadening of XRD peaks of decomposition products and close structural similarity of LiNiO2 , Li2 MnO3 and Li(Ni0.5 Mn0.5 )O2 give poor chances for direct XRD detection of Li2 MnO3 formation at the intermediate stages of thermal processing. Meanwhile, taking into account the data of the previous decomposition studies [9], the formation of stable Li2 MnO3 intermediate could be a good explanation for observed heating rate dependence of Li(Ni0.5 Mn0.5 )O2 formation processes. Moderate heating rates (5–10 K/min) stimulate Li2 MnO3 crystallization and hinder the following solid solution formation, while fast heating leads to the direct formation of Li(Ni0.5 Mn0.5 )O2 . Similar accelerated formation of complex oxides at enhanced heating rates is typical for the system with stable intermediate products of thermal decomposition and has been observed before during freeze drying synthesis of YBa2 Cu3 Ox , YBa2 Cu4 O8 and (Bi,Pb)Sr2 CaCu2 Oy [17–19].
Fig. 2. XRD patterns of LiNi0.5 Mn0.5 O2 powders, obtained by annealing at 900 ◦ C for 12 h in powder or in pellets at various heating rates: (I) 5 K/min, powder; (II) 1000 K/min, powder; (III) 1000 K/min, pellet.
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Pelletizing of thermal decomposition products before final annealing at 900 ◦ C, as it was anticipated, results in smaller FWHM of the main peaks and more pronounced splitting of
Fig. 3. SEM micrographs of Li(Ni0.5 Mn0.5 )O2 powders at various stages of thermal processing: (A) 500 ◦ C, 6 h; (B) 900 ◦ C, 1 h; (C and D) 900 ◦ C, 12 h.
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1 0 8/1 1 0 reflections (Fig. 2, curve III), characterizing welldefined hexagonal structure [1,12]. Domination of the layered structure usually correlates with smaller capacity fade rate, so that these features make preferable final annealing in pellets and allow to explain reduced capacity values, observed by several authors after final annealing of LiNi0.5 Mn0.5 O2 at 900–950 ◦ C in powder form [8]. The need in pelletizing differentiates LiNi0.5 Mn0.5 O2 from LiCoO2 where similar ordering processes occur in powders and at lower temperatures (800–850 ◦ C). Intense mechanical processing has profound influence on the properties of Li(Ni0.5 Mn0.5 )O2 powders in spite of the fact, that external force was applied not to final product but to the precursor powder. The most important features of Li(Ni0.5 Mn0.5 )O2 response on the mechanical processing are absence or decreasing the splitting of 1 0 8/1 1 0 reflec-
tions and reverse 0 0 3/1 0 4 peak intensity ratio compared to reference samples, obtained without planetary milling. Both features, according to [1], are indicative to high concentration of rock-salt domains in the layered matrix and, hence, to the poorer electrochemical reactivity of these materials. Similar reasons might explain unusual (up to 68 h) increase in thermal processing duration after mixing the precursor powders in highly powered Spex mill, observed in [16]. SEM analysis of obtained powders shows that the thermal decomposition of salt precursor results in the formation of 20–30 nm crystallites of Li(Ni,Mn)O2 (Fig. 3A). Grain growth at 900 ◦ C proceeds rather slowly, so that only continuous annealing in pellets results in complete rearrangement of primary grain ensembles (Fig. 3B and C). Nevertheless, even in that case many samples still demonstrate nanocrystalline features, showing at the residual
Fig. 4. The influence of processing conditions on the voltage vs. capacity profiles for the first discharge cycle (A) and cycling performance (B and C) of the cells LiNi0.5 Mn0.5 O2 cathodes. (1, 2 and 6) two stage thermal processing (500 ◦ C + 900 ◦ C); (3 and 7) single-stage heat treatment at 900 ◦ C; (4, 5 and 8) samples with preliminary mechanical processing before thermal decomposition. (A and B) hydroxide precursors; (C) acetate precursors.
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chemical inhomogeneities within newly formed large crystallites (Fig. 3D). The sensitivity of initial capacity and fade rate of Li(Ni0.5 Mn0.5 )O2 cathodes to the variation of processing conditions is proved to be higher than the sensitivity of common powder XRD method, though the most substantial changes are indicated both by XRD and by electrochemical capacity measurements. As it was mentioned before, the XRD patterns of Li(Ni0.5 Mn0.5 )O2 samples, subjected to mechanical processing before decomposition of precursors, have the traces of crystallographic disorder even after continuous annealing. Electrochemical capacity of these powders is also much lower than for usually processed samples (Fig. 4A and B, curve 4). Additional thermal processing can lead to the small increase in capacity values, but their level remains to be rather low (Fig. 4A and B, curve 5; Fig. 4C, curve 8). These data show at rather limited perspectives of mechanical activation methods in the Li(Ni,Mn)O2 synthesis and production, as the negative effect of this kind of processing is rather stable even at elevated temperatures. Another important processing feature, poorly detectable by XRD, is a strong influence of the Li(Ni0.5 Mn0.5 )O2 formation temperature on the electrochemical performance of cathode materials. Li(Ni0.5 Mn0.5 )O2 cathodes, obtained by single-stage thermal decomposition at 900 ◦ C, demonstrate substantially lower capacity values than similar samples, subjected to preliminary thermal processing at 500 ◦ C (Fig. 4A and B; curves 2 and 3; Fig. 4C, curves 6 and 7). Better homogeneity of reaction mixture, obtained by thermolysis at reduced temperatures, promotes faster completion of phase formation and better ordering of Li(Ni0.5 Mn0.5 )O2 during further annealing at 900 ◦ C. Mechanical processing of precursor powder and thermal processing schedule have similar effect on Li(Ni0.5 Mn0.5 )O2 powders obtained from acetate and from hydroxide precursors (Fig. 4B and C). At the same time the maximum capacity values are proved to be different in these two series. After initial falling down the electrochemical capacity of Li(Ni0.5 Mn0.5 )O2 sample, obtained from acetate precursor, is stabilized at the value of 125–130 mAh/g, usual for Li(Ni0.5 Mn0.5 )O2 -based materials (Fig. 4C, curve 6). Meanwhile, the samples with hydroxide prehistory demonstrate negligible capacity fade after first cycle and stabilization at 186–192 mAh/g (Fig. 4B; curves 1 and 2). These values are not usual for Li(Ni0.5 Mn0.5 )O2 , but close to the capacity of Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 -based materials [5–8]. Li/Ni/Mn ratio in our single phase samples with hydroxide prehistory is rather different from this stoichiometry, but they also contain considerable Li excess (Li1.3 Ni0.5 Mn0.5 O2 ). Similarly to Li[Nix Li(1/3−2x/3) Mn(2/3−x/3) ]O2 , observed capacity enhancement can be connected with partial localization of excess lithium in the B-positions of ABO2 lattice.
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4. Conclusions Phase formation and electrochemical performance of LiNi0.5 Mn0.5 O2 -based cathode materials are considerably affected by thermal and mechanical processing conditions. Maximum heating rate, pelletizing and preliminary thermal decomposition of precursors at moderate temperatures before final annealing promote faster formation of perfectly ordered hexagonal structure and enhancement of electrochemical capacity values. On the contrary, intense mechanical processing of precursor complicates long-range ordering during high temperature annealing and results in considerably reduced capacity values at usual annealing time. Introduction of lithium excess during synthesis of LiNi0.5 Mn0.5 O2 is accompanied by enhancement of reversible capacity values up to 185–190 mAh/g. Acknowledgements The work is supported by National Research Laboratory (Development of monolithic high power hybrid battery) and by KISTEP fellowship. References [1] T. Ohzuku, Y. Makimura, Chem. Lett. (2001) 744. [2] E. Rossen, C.D.W. Jones, J.R. Dahn, Solid State Ionics 57 (1992) 311. [3] D. Caurant, N. Baffler, V. Bianchi, G. Gregoire, S. Bach, J. Mater. Chem. 6 (1996) 1149. [4] M.E. Spahr, P. Novak, O. Haas, R. Nesper, J. Power Sources 68 (1997) 629. [5] Z. Lu, L.Y. Beaulieu, R.A. Donaberger, C.L. Thomas, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A778. [6] Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid State Lett. 4 (2001) A191. [7] S.-S. Shin, Y.-K. Sun, K. Amine, J. Power Sources 112 (2002) 634. [8] Y.J. Park, M.G. Kim, Y.-S. Hong, X. Wu, K.S. Ryu, S.H. Chang, Solid State Commun. 127 (2003) 509. [9] M. Yoshio, Y. Todorov, K. Yamato, H. Noguchi, J. Itoh, M. Okada, T. Mouri, J. Power Sources 74 (1998) 46. [10] W.-S. Yoon, N. Kim, X.-Q. Yang, J. McBreen, C.P. Gray, J. Power Sources 119–121 (2003) 649. [11] Z. Lu, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A815. [12] K.M. Shaju, G.V. Subba Rao, B.V.R. Choudhari, Electrochim. Acta 48 (2003) 1505. [13] X.-Q. Yang, J. McBreen, W.-S. Yoon, C.P. Grey, Electrochem. Commun. 4 (2002) 649. [14] B.L. Cushing, J.B. Goodenough, Solid State Sci. 4 (2002) 1487. [15] R. Chitrakar, S. Kasaishi, A. Umeno, K. Sakane, N. Takagi, Y.-S. Kim, K. Ooi, J. Solid State Chem. 169 (2002) 35. [16] Y.-K. Sun, C.S. Yoon, Y.S. Lee, Electrochim. Acta 48 (2003) 2589. [17] O.A. Shlyakhtin, A.L. Vinokurov, A.N. Baranov, Yu.D. Tretyakov, J. Supercond. 11 (1998) 507. [18] K. Takahashi, T. Ito, H. Yoshikawa, A. Hiraki, Jpn. J. Appl. Phys. 32 (1993) 1211. [19] Yu.D. Tretyakov, N.N. Oleynikov, O.A. Shlyakhtin, Cryochemical Technology of Advanced Materials, Chapman & Hall, London, 1997, p. 260.