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Mechanochemical synthesis of LiNiO2
Materials Science and Engineering B 116 (2005) 341–345
Mechanochemical synthesis of LiNiO2 Chun-Chieh Chang, Prashant N. Kumta∗ Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Received 5 March 2004; accepted 14 May 2004
Abstract The mechanochemical process has been used to synthesize precursors and evaluate their efficacy in generating the layered structure of LiNiO2 at lower heat treatment temperatures. A comprehensive study of the as-milled precursor has been conducted using simultaneous TG/DTA. It is observed that the milling process is efficient in generating a disordered LiNiO2 using Li2 O2 and NiO as the starting materials. The transformation of the disordered LiNiO2 to the ordered form of LiNiO2 however requires a minimum temperature of 700 ◦ C. This observation suggests that the formation temperature of well-ordered LiNiO2 (∼8% disorder) is limited not only by the chemical reaction kinetics of the starting materials but also the kinetics of ordering of Li and Ni in LiNiO2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Stoichiometry; Sol–gel; Alumina
1. Introduction Lithiated transition metal oxides LiMO2 (M = Ni, Nix Co1−x ) are technically important cathode materials for lithium-ion battery applications because they possess high energy density and capacity [1–5]. These materials are 2D intercalation compounds which have a layered structure with Li+ cations inserted in between the MO2 − (M = transition metal cations) slabs. LiNiO2 is especially important not only because of its higher reversible capacity in comparison to LiCoO2 [2], but also due to the larger abundance of Ni that makes it a candidate material to potentially replace LiCoO2 as the cathode material for rechargeable Li-ion battery applications. Many attempts have been made to synthesize defectfree stoichiometric LiNiO2 [6–12]. The importance of the need to generate stoichiometric LiNiO2 stems from the point that the disorder of Ni on Li site decreases the Li diffusivity and therefore the capacity [6]. It is well known that different starting materials and various synthesis protocols (e.g. solid-state processes and sol–gel processes) alter the reaction mechanisms [11,12]. Consequently, the reaction kinetics is also affected by the post ∗
Corresponding author. Tel.: +1 412 268 8739; fax: +1 412 268 7596. E-mail address: [email protected] (P.N. Kumta).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.042
heat-treatment of the precursors generated using different synthesis protocols and starting materials that leads to considerable variations in defect concentrations (disorder of Li and Ni) and therefore, the electrochemical properties of the resultant materials. Identification of efficient precursors that promotes the kinetics of formation of defect-free LiNiO2 at low temperature is thus, very important both from scientific and technological fronts. Our earlier report also concluded that the reaction temperatures should be sufficiently high to accelerate the reaction kinetics for the formation of LiNiO2 while preventing the decomposition of LiNiO2 [11]. This conclusion also validates the purpose of investigating more efficient precursors for synthesizing defect free LiNiO2 at low temperatures. In the present work, the mechanochemical process has been investigated for the generation of more efficient precursors for synthesizing LiNiO2 . Mechanical milling process has been studied very intensively as a viable process for generating nanostructured materials [13–18]. The technique is extremely useful for promoting intimate mixing by pulverization of the starting materials during high-energy milling. The high-energy impact forces provided by the milling media enhance diffusion of the various components of the starting materials, thus promoting chemical reactions resulting in metastable structures. In addition, the milling
342
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
action has been reported to generate nanostructured as well as metastable phases of various compounds. These advantages could, therefore, be very useful in generating more efficient precursors for synthesizing LiNiO2 at low temperatures. Li2 O2 and NiO are particularly chosen as the starting materials in this case since Li2 O2 offers the possibility of reacting with NiO directly and has the necessary amount of oxygen to generate stoichiometric LiNiO2 without the need for any additional treatments in oxygen-rich environments. The structure of the as-milled precursor as well as the reaction pathways to the formation of LiNiO2 have been characterized and analyzed through a series of experiments. Details of the experimental procedure and results of the studies are reported in the following sections.
2. Experimental In general, 6 g of starting materials containing NiO (Alfa Aesar, 99%) and Li2 O2 (Aldrich, 90% assay) with a molar ratio of Ni:Li equals 1:1 were first placed in a steel vial. The vial was then sealed and subjected to the milling process using the SPEX-8000 Mixer/Mill. A weight ratio of balls/powder of 8:1 and an agitation frequency of 60 Hz were used as the milling conditions. Steel balls with a diameter of 10 mm were used in the milling process. In order to investigate the evolution of phases during heattreatment, the as-milled samples were heat-treated in air in the temperature range of 400–700 ◦ C at increments of 100 ◦ C each, using alumina boats as sample carriers. The dwell time for all the as-prepared powders was set at 5 h for each of the temperatures in the range of 400–700 ◦ C. The as-milled and heat-treated powders were subjected to X-ray diffraction characterization (XRD, Rigaku θ/θ diffractometer). A Cu X-ray tube was constantly used as the radiation source. The general X-ray diffraction parameters consisted of a 0.05◦ step size at 35 kV and 20 mA with a duration time of 2 s. Gravimetric/Differential thermal analysis was also conducted on the as-milled, as well as some of the other samples prepared for comparison using the simultaneous TG/DTA instrument (TA Instruments, Model 2960). The nature and type of the different starting materials will be specified in the Section 3. A constant heating rate of 10 ◦ C/min was used for all the samples subjected to TG/DTA analysis. All the experiments were conducted in air unless stated otherwise.
3. Results and discussion Fig. 1 shows the XRD results of the precursor materials before and after milling. Fig. 1(a) shows distinct diffraction peaks originating from crystalline NiO and Li2 O2 prior to milling. Compared to Fig. 1(a), the as-milled precursor shown in Fig. 1(b) shows only broadened NiO peaks with decreased intensity. From results shown in Fig. 1(a) and (b),
Fig. 1. The XRD analysis results of the precursor materials before and after the milling process. (a) Diffraction peaks originating from crystalline NiO and Li2 O2 prior to milling. (*) represents the existence of NiO, other peaks are originated from Li2 O2 . (b) The as-milled precursor shows only broadened NiO peaks with decreased intensity.
it is clear that the milling process pulverizes the precursor materials. However, the disappearance of the Li2 O2 peaks in the as-milled precursor suggests that the components of the precursor can be described by one of the following variations: (1) The as-milled precursor consists of amorphous Li2 O2 that was pulverized during the milling process. (2) The as-milled precursor consists of amorphous Li2 O that was formed as the consequence of the decomposition of Li2 O2 . (3) The as-milled precursor consists of disordered LiNiO2 alone. In order to verify the constituents of the as-milled precursor, simultaneous TG/DTA was conducted on the following samples. The first sample consisted of crystalline NiO and Li2 O2 . The second sample was composed of crystalline NiO and Li2 O. The third sample is comprised of the as-milled mixture of NiO and Li2 O2 . Fig. 2 shows the results of the simultaneous TG/DTA analysis. In Fig. 2(a), a drastic weight loss is observed at 341.5 ◦ C. A subsequent weight gain is observed that appears to continue beyond 800 ◦ C. The observed weight loss and weight gain can be attributed to the decomposition of Li2 O2 to Li2 O at 341.5 ◦ C, followed by the subsequent reaction between NiO and Li2 O that continues to
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
343
800 ◦ C. The thermal response of the precursors and the observations above could be described by the following chemical reactions: Li2 O2 → Li2 O + 21 O2 (g) (theoretical weight loss 34.78%) 1 2 O2 (g) + Li2 O + 2NiO
→ 2LiNiO2
(theoretical weight gain 8.89%)
Fig. 2. The results of the simultaneous TG/DTA analysis. (a) The sample consists of crystalline NiO and Li2 O2 . (b) The sample consists of crystalline NiO and Li2 O. (c) The sample consisting of as-milled NiO and Li2 O2 .
(1)
(2)
Incomparison to Fig. 2(a), the sample containing crystalline NiO and Li2 O as shown in Fig. 2(b) does not possess any weight loss. Instead, a weight gain is observed that begins at ∼440 ◦ C and continues beyond 800 ◦ C. The weight gain of 5.40% in comparison to the original sample weight corresponds to 61% completion of the reaction shown in Eq. (2) if the decomposition of the materials beyond 750 ◦ C [11] is not considered. Fig. 2(c) shows the weight change profile of the as-milled sample. A slight weight loss of 2.65% is observed in the initial state. This weight loss can be attributed to the loss of moisture adsorbed on the surface of the material. Upon heating, a slight weight gain is observed until 800 ◦ C, which is similar to the result observed in the case of NiO and Li2 O reaction as shown in Fig. 2(b). If we analyze the weight gain of the as-milled sample more carefully by considering the weight at 300 ◦ C as the initial weight after removal of moisture and the weight at 800 ◦ C as the final weight, the weight gain during heat treatment is 0.12%. This weight gain is much smaller in comparison to the weight gain of the sample consisting of NiO and Li2 O reported as 5.40% earlier. If we assume that the as-milled sample consists of intimately mixed nanocrystalline NiO and Li2 O only, the reaction should be much faster in comparison to the reaction of crystalline NiO and Li2 O. This implies that a more substantial weight gain should be observed at 800 ◦ C in the as-milled sample in comparison to the crystalline NiO and Li2 O sample. However, the weight gain observed in the as-milled sample is much smaller than the mixture of crystalline NiO and Li2 O. The results up to this point lead us to conclude that the as-milled sample consists of primarily disordered LiNiO2 . This conclusion is important in the sense of investigating whether the milling process can actually lower the formation temperature of ordered LiNiO2 by using disordered LiNiO2 as the starting precursor material. In order to answer this question, a through investigation of the phase evolution with temperature using the as-milled sample as the precursor material was conducted. The results of this phase evolution study are shown in Fig. 3. Meanwhile, for the purpose of strengthening the interpretations of the phase evolution study, results of simulated XRD pattern for different extents of disorder of Ni on Li site are also shown in Fig. 4 for comparison. In Fig. 3 it is observed that a diffraction peak at a 2-theta angle of ∼18◦ begins to evolve at 500 ◦ C. More peaks (36, 48, 59◦ , . . ., etc.) are observed to
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C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
Fig. 3. The results of the phase evolution study. The as-milled samples were heat treated in air in the temperature range of 400–700 ◦ C at increments of 100 ◦ C each. () Represents well-ordered LiNiO2 and (* ) Represents disordered LiNiO2 .
form at the heat treatment temperature of 600 ◦ C. At 700 ◦ C, distinct peaks can be observed implying a well-crystallized nature of the heat-treated material. This result can be comprehended from the simulation results shown in Fig. 4. Fig. 4 shows three XRD pattern simulations for full-ordered LiNiO2 (shown in Fig. 4(a)), 25% disordered (shown in Fig. 4(b)) and 50% disordered LiNiO2 (shown in Fig. 4(c)) all assuming the ¯ symmetry. The implication of 25% disordered LiNiO2 R3m in this case is 25% of Li on Ni site and vice versa. The lattice parameters and atom positions used for the simulations are tabulated in Table 1. By comparing the phase evolution study results and the simulation results shown in Fig. 4, it can be concluded that the progressive ordering of LiNiO2 becomes more significant at a temperature of 500 ◦ C (close to 50% of Li and Ni ordering). However, a well-ordered LiNiO2 is obtainable only at 700 ◦ C, which is comparable with the results reported by us and others using different precursor materials Table 1 The lattice parameters and atom positions used for the simulations Bravais lattice Hexagonal Space group ¯ R3m Atom positions Li (3a) Ni (3b) O (6c)
(0 0 0) (0 0 1/2) (0 0 z) z = 0.2417
Lattice parameters ˚ a (A) ˚ c (A) ˚ 3) Cell volume (A
2.87562 14.20137 101.7007
Fig. 4. The structural XRD patterns for (a) well-ordered LiNiO2 , (b) 25% disordered and (c) 50% disordered LiNiO2 . All simulations were assuming ¯ symmetry. A 25 and 50% disordered LiNiO2 implies 25 and 50% the R3m of Li on Ni site and vice versa.
[9,10]. The extent of disorder for the well-ordered LiNiO2 obtained at 700 ◦ C is also determined to be ∼8% disorder using an experimental formula reported by Dahn et. al [7] based on the peak intensity ratio of (1 0 1), (0 0 6) and (1 0 2) peaks. Thus, it appears that although the milling process results in disordered LiNiO2 , its conversion to the ordered form is only possible by heat treatment at 700 ◦ C. Since the formation temperature of well-ordered LiNiO2 is not lowered by using disordered LiNiO2 as the pre-
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
cursor, one can conclude that the formation of ordered LiNiO2 is limited not only by the chemical reactions that are dependent on the chemical species of starting materials, but also the ordering kinetics of formation of LiNiO2 .
4. Conclusions High-energy milling process is efficient in generating disordered LiNiO2 using NiO and Li2 O2 as the starting materials. This can be concluded from the simultaneous TG/DTA analysis on the as-milled sample as no obvious weight change can be observed during heat treatment. Although the disordered LiNiO2 can be generated using the milling process, the formation of well-ordered LiNiO2 can only be obtained at elevated temperatures of 700 ◦ C. This suggests that the formation of well-ordered LiNiO2 is limited not only by the chemical reaction kinetics of starting materials, but also the ordering kinetics of LiNiO2 itself.
Acknowledgment The authors gratefully acknowledge the support of NSF (Grants CTS-9700343 and CTS-0000563). Financial support of Changs Ascending, Taiwan, and the technical assistance of Eveready Battery Company is also acknowledged.
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References [1] M. Broussely, F. Perton, P. Biensan, J.M. Bodet, J. Labat, A. Lecerf, C. Delmas, A. Rougier, J.P. Peres, J. Power Sources 54 (1995) 109. [2] T. Ohzuku, A. Ueda, M. Nagayama, Y. Iwakoshi, H. Komori, Electrochim. Acta 38 (1993) 1159. [3] B. Banov, J. Bourilkov, M. Mladenov, J. Power Sources 54 (1995) 268. [4] D. Caurant, N. Baffier, B. Garcia, J.P. Pereira-Ramos, Solid State Ionics 91 (1996) 45. [5] T. Ohzuku, A. Ueda, M. Nagayama, J. Electrochem. Soc. 140 (1993) 1862. [6] A. Hirano, R. Kanno, Y. Kawamoto, Y. Takeda, K. Yamaura, M. Takano, K. Ohyama, M. Ohashi, Y. Yamaguchi, Solid State Ionics 78 (1995) 123. [7] J.R. Dahn, U. von Sacken, C.A. Michal, Solid State Ionics. 44 (1990) 87. [8] R.V. Moshtev, P. Zlatilova, V. Manev, A. Sato, J. Power Sources 54 (1995) 329. [9] C.C. Chang, P.N. Kumta, J. Power Sources 75 (1998) 44. [10] C.C. Chang, P.N. Kumta, Solid State Ionics. 112 (1998) 329. [11] C.C. Chang, J.Y. Kim, P.N. Kumta, J. Electrochem. Soc. 149 (2002) A331. [12] C.C. Chang, J.Y. Kim, P.N. Kumta, J. Electrochem. Soc. 149 (2002) A1114. [13] M. Jurczyk, M. Nowak, E. Jankowska, J. Alloys Compd. 340 (2002) 281. [14] F. Sun, F.H. Froes, J. Alloys Compd. 340 (2002) 220. [15] S. Zghal, R. Twesten, F. Wu, P. Bellon, Acta Mater. 50 (2002) 4711. [16] H.-Y. Lee, S.-M. Lee, J. Power Sources 112 (2002) 649. [17] M. Tatsumisago, S. Hama, A. Hayashi, H. Morimoto, T. Minami, Solid State Ionics 635 (2002) 154–155. [18] M. Sannia, R. Orru, J.E. Garay, G. Gao, Z.A. Munir, Mater. Sci. Eng. 270 (2003) 345.
Mechanochemical synthesis of LiNiO2 Chun-Chieh Chang, Prashant N. Kumta∗ Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Received 5 March 2004; accepted 14 May 2004
Abstract The mechanochemical process has been used to synthesize precursors and evaluate their efficacy in generating the layered structure of LiNiO2 at lower heat treatment temperatures. A comprehensive study of the as-milled precursor has been conducted using simultaneous TG/DTA. It is observed that the milling process is efficient in generating a disordered LiNiO2 using Li2 O2 and NiO as the starting materials. The transformation of the disordered LiNiO2 to the ordered form of LiNiO2 however requires a minimum temperature of 700 ◦ C. This observation suggests that the formation temperature of well-ordered LiNiO2 (∼8% disorder) is limited not only by the chemical reaction kinetics of the starting materials but also the kinetics of ordering of Li and Ni in LiNiO2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Stoichiometry; Sol–gel; Alumina
1. Introduction Lithiated transition metal oxides LiMO2 (M = Ni, Nix Co1−x ) are technically important cathode materials for lithium-ion battery applications because they possess high energy density and capacity [1–5]. These materials are 2D intercalation compounds which have a layered structure with Li+ cations inserted in between the MO2 − (M = transition metal cations) slabs. LiNiO2 is especially important not only because of its higher reversible capacity in comparison to LiCoO2 [2], but also due to the larger abundance of Ni that makes it a candidate material to potentially replace LiCoO2 as the cathode material for rechargeable Li-ion battery applications. Many attempts have been made to synthesize defectfree stoichiometric LiNiO2 [6–12]. The importance of the need to generate stoichiometric LiNiO2 stems from the point that the disorder of Ni on Li site decreases the Li diffusivity and therefore the capacity [6]. It is well known that different starting materials and various synthesis protocols (e.g. solid-state processes and sol–gel processes) alter the reaction mechanisms [11,12]. Consequently, the reaction kinetics is also affected by the post ∗
Corresponding author. Tel.: +1 412 268 8739; fax: +1 412 268 7596. E-mail address: [email protected] (P.N. Kumta).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.042
heat-treatment of the precursors generated using different synthesis protocols and starting materials that leads to considerable variations in defect concentrations (disorder of Li and Ni) and therefore, the electrochemical properties of the resultant materials. Identification of efficient precursors that promotes the kinetics of formation of defect-free LiNiO2 at low temperature is thus, very important both from scientific and technological fronts. Our earlier report also concluded that the reaction temperatures should be sufficiently high to accelerate the reaction kinetics for the formation of LiNiO2 while preventing the decomposition of LiNiO2 [11]. This conclusion also validates the purpose of investigating more efficient precursors for synthesizing defect free LiNiO2 at low temperatures. In the present work, the mechanochemical process has been investigated for the generation of more efficient precursors for synthesizing LiNiO2 . Mechanical milling process has been studied very intensively as a viable process for generating nanostructured materials [13–18]. The technique is extremely useful for promoting intimate mixing by pulverization of the starting materials during high-energy milling. The high-energy impact forces provided by the milling media enhance diffusion of the various components of the starting materials, thus promoting chemical reactions resulting in metastable structures. In addition, the milling
342
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
action has been reported to generate nanostructured as well as metastable phases of various compounds. These advantages could, therefore, be very useful in generating more efficient precursors for synthesizing LiNiO2 at low temperatures. Li2 O2 and NiO are particularly chosen as the starting materials in this case since Li2 O2 offers the possibility of reacting with NiO directly and has the necessary amount of oxygen to generate stoichiometric LiNiO2 without the need for any additional treatments in oxygen-rich environments. The structure of the as-milled precursor as well as the reaction pathways to the formation of LiNiO2 have been characterized and analyzed through a series of experiments. Details of the experimental procedure and results of the studies are reported in the following sections.
2. Experimental In general, 6 g of starting materials containing NiO (Alfa Aesar, 99%) and Li2 O2 (Aldrich, 90% assay) with a molar ratio of Ni:Li equals 1:1 were first placed in a steel vial. The vial was then sealed and subjected to the milling process using the SPEX-8000 Mixer/Mill. A weight ratio of balls/powder of 8:1 and an agitation frequency of 60 Hz were used as the milling conditions. Steel balls with a diameter of 10 mm were used in the milling process. In order to investigate the evolution of phases during heattreatment, the as-milled samples were heat-treated in air in the temperature range of 400–700 ◦ C at increments of 100 ◦ C each, using alumina boats as sample carriers. The dwell time for all the as-prepared powders was set at 5 h for each of the temperatures in the range of 400–700 ◦ C. The as-milled and heat-treated powders were subjected to X-ray diffraction characterization (XRD, Rigaku θ/θ diffractometer). A Cu X-ray tube was constantly used as the radiation source. The general X-ray diffraction parameters consisted of a 0.05◦ step size at 35 kV and 20 mA with a duration time of 2 s. Gravimetric/Differential thermal analysis was also conducted on the as-milled, as well as some of the other samples prepared for comparison using the simultaneous TG/DTA instrument (TA Instruments, Model 2960). The nature and type of the different starting materials will be specified in the Section 3. A constant heating rate of 10 ◦ C/min was used for all the samples subjected to TG/DTA analysis. All the experiments were conducted in air unless stated otherwise.
3. Results and discussion Fig. 1 shows the XRD results of the precursor materials before and after milling. Fig. 1(a) shows distinct diffraction peaks originating from crystalline NiO and Li2 O2 prior to milling. Compared to Fig. 1(a), the as-milled precursor shown in Fig. 1(b) shows only broadened NiO peaks with decreased intensity. From results shown in Fig. 1(a) and (b),
Fig. 1. The XRD analysis results of the precursor materials before and after the milling process. (a) Diffraction peaks originating from crystalline NiO and Li2 O2 prior to milling. (*) represents the existence of NiO, other peaks are originated from Li2 O2 . (b) The as-milled precursor shows only broadened NiO peaks with decreased intensity.
it is clear that the milling process pulverizes the precursor materials. However, the disappearance of the Li2 O2 peaks in the as-milled precursor suggests that the components of the precursor can be described by one of the following variations: (1) The as-milled precursor consists of amorphous Li2 O2 that was pulverized during the milling process. (2) The as-milled precursor consists of amorphous Li2 O that was formed as the consequence of the decomposition of Li2 O2 . (3) The as-milled precursor consists of disordered LiNiO2 alone. In order to verify the constituents of the as-milled precursor, simultaneous TG/DTA was conducted on the following samples. The first sample consisted of crystalline NiO and Li2 O2 . The second sample was composed of crystalline NiO and Li2 O. The third sample is comprised of the as-milled mixture of NiO and Li2 O2 . Fig. 2 shows the results of the simultaneous TG/DTA analysis. In Fig. 2(a), a drastic weight loss is observed at 341.5 ◦ C. A subsequent weight gain is observed that appears to continue beyond 800 ◦ C. The observed weight loss and weight gain can be attributed to the decomposition of Li2 O2 to Li2 O at 341.5 ◦ C, followed by the subsequent reaction between NiO and Li2 O that continues to
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
343
800 ◦ C. The thermal response of the precursors and the observations above could be described by the following chemical reactions: Li2 O2 → Li2 O + 21 O2 (g) (theoretical weight loss 34.78%) 1 2 O2 (g) + Li2 O + 2NiO
→ 2LiNiO2
(theoretical weight gain 8.89%)
Fig. 2. The results of the simultaneous TG/DTA analysis. (a) The sample consists of crystalline NiO and Li2 O2 . (b) The sample consists of crystalline NiO and Li2 O. (c) The sample consisting of as-milled NiO and Li2 O2 .
(1)
(2)
Incomparison to Fig. 2(a), the sample containing crystalline NiO and Li2 O as shown in Fig. 2(b) does not possess any weight loss. Instead, a weight gain is observed that begins at ∼440 ◦ C and continues beyond 800 ◦ C. The weight gain of 5.40% in comparison to the original sample weight corresponds to 61% completion of the reaction shown in Eq. (2) if the decomposition of the materials beyond 750 ◦ C [11] is not considered. Fig. 2(c) shows the weight change profile of the as-milled sample. A slight weight loss of 2.65% is observed in the initial state. This weight loss can be attributed to the loss of moisture adsorbed on the surface of the material. Upon heating, a slight weight gain is observed until 800 ◦ C, which is similar to the result observed in the case of NiO and Li2 O reaction as shown in Fig. 2(b). If we analyze the weight gain of the as-milled sample more carefully by considering the weight at 300 ◦ C as the initial weight after removal of moisture and the weight at 800 ◦ C as the final weight, the weight gain during heat treatment is 0.12%. This weight gain is much smaller in comparison to the weight gain of the sample consisting of NiO and Li2 O reported as 5.40% earlier. If we assume that the as-milled sample consists of intimately mixed nanocrystalline NiO and Li2 O only, the reaction should be much faster in comparison to the reaction of crystalline NiO and Li2 O. This implies that a more substantial weight gain should be observed at 800 ◦ C in the as-milled sample in comparison to the crystalline NiO and Li2 O sample. However, the weight gain observed in the as-milled sample is much smaller than the mixture of crystalline NiO and Li2 O. The results up to this point lead us to conclude that the as-milled sample consists of primarily disordered LiNiO2 . This conclusion is important in the sense of investigating whether the milling process can actually lower the formation temperature of ordered LiNiO2 by using disordered LiNiO2 as the starting precursor material. In order to answer this question, a through investigation of the phase evolution with temperature using the as-milled sample as the precursor material was conducted. The results of this phase evolution study are shown in Fig. 3. Meanwhile, for the purpose of strengthening the interpretations of the phase evolution study, results of simulated XRD pattern for different extents of disorder of Ni on Li site are also shown in Fig. 4 for comparison. In Fig. 3 it is observed that a diffraction peak at a 2-theta angle of ∼18◦ begins to evolve at 500 ◦ C. More peaks (36, 48, 59◦ , . . ., etc.) are observed to
344
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
Fig. 3. The results of the phase evolution study. The as-milled samples were heat treated in air in the temperature range of 400–700 ◦ C at increments of 100 ◦ C each. () Represents well-ordered LiNiO2 and (* ) Represents disordered LiNiO2 .
form at the heat treatment temperature of 600 ◦ C. At 700 ◦ C, distinct peaks can be observed implying a well-crystallized nature of the heat-treated material. This result can be comprehended from the simulation results shown in Fig. 4. Fig. 4 shows three XRD pattern simulations for full-ordered LiNiO2 (shown in Fig. 4(a)), 25% disordered (shown in Fig. 4(b)) and 50% disordered LiNiO2 (shown in Fig. 4(c)) all assuming the ¯ symmetry. The implication of 25% disordered LiNiO2 R3m in this case is 25% of Li on Ni site and vice versa. The lattice parameters and atom positions used for the simulations are tabulated in Table 1. By comparing the phase evolution study results and the simulation results shown in Fig. 4, it can be concluded that the progressive ordering of LiNiO2 becomes more significant at a temperature of 500 ◦ C (close to 50% of Li and Ni ordering). However, a well-ordered LiNiO2 is obtainable only at 700 ◦ C, which is comparable with the results reported by us and others using different precursor materials Table 1 The lattice parameters and atom positions used for the simulations Bravais lattice Hexagonal Space group ¯ R3m Atom positions Li (3a) Ni (3b) O (6c)
(0 0 0) (0 0 1/2) (0 0 z) z = 0.2417
Lattice parameters ˚ a (A) ˚ c (A) ˚ 3) Cell volume (A
2.87562 14.20137 101.7007
Fig. 4. The structural XRD patterns for (a) well-ordered LiNiO2 , (b) 25% disordered and (c) 50% disordered LiNiO2 . All simulations were assuming ¯ symmetry. A 25 and 50% disordered LiNiO2 implies 25 and 50% the R3m of Li on Ni site and vice versa.
[9,10]. The extent of disorder for the well-ordered LiNiO2 obtained at 700 ◦ C is also determined to be ∼8% disorder using an experimental formula reported by Dahn et. al [7] based on the peak intensity ratio of (1 0 1), (0 0 6) and (1 0 2) peaks. Thus, it appears that although the milling process results in disordered LiNiO2 , its conversion to the ordered form is only possible by heat treatment at 700 ◦ C. Since the formation temperature of well-ordered LiNiO2 is not lowered by using disordered LiNiO2 as the pre-
C.-C. Chang, P.N. Kumta / Materials Science and Engineering B 116 (2005) 341–345
cursor, one can conclude that the formation of ordered LiNiO2 is limited not only by the chemical reactions that are dependent on the chemical species of starting materials, but also the ordering kinetics of formation of LiNiO2 .
4. Conclusions High-energy milling process is efficient in generating disordered LiNiO2 using NiO and Li2 O2 as the starting materials. This can be concluded from the simultaneous TG/DTA analysis on the as-milled sample as no obvious weight change can be observed during heat treatment. Although the disordered LiNiO2 can be generated using the milling process, the formation of well-ordered LiNiO2 can only be obtained at elevated temperatures of 700 ◦ C. This suggests that the formation of well-ordered LiNiO2 is limited not only by the chemical reaction kinetics of starting materials, but also the ordering kinetics of LiNiO2 itself.
Acknowledgment The authors gratefully acknowledge the support of NSF (Grants CTS-9700343 and CTS-0000563). Financial support of Changs Ascending, Taiwan, and the technical assistance of Eveready Battery Company is also acknowledged.
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