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Hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2
Electrochimica Acta 47 (2002) 3287 /3295 www.elsevier.com/locate/electacta
Hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2 Seung-Taek Myung, Shinichi Komaba *, Naoaki Kumagai Department of Chemical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan Received 20 March 2002
Abstract The most highlighted point of this work to emphasize is that it is the first trial to use Mn3O4 oxide as a precursor to synthesize orthorhombic LiMnO2 by the hydrothermal method. A well-ordered orthorhombic LiMnO2 phase was formed by the hydrothermal treatment of Mn3O4 with excess LiOH aqueous solution at 170 8C. According to TEM observation, the as-synthesized powder was single crystalline particle oxide. Comparing with other orthorhombic LiMnO2 prepared by low temperature synthetic route and by high temperature calcination, the orthorhombic LiMnO2 prepared by the hydrothermal route showed enhanced battery performance as a lithium battery cathode material. We believe that the new hydrothermal synthesis is expected as an excellent alternative of powder preparation method of high capacity cathode material to be used for Li-ion secondary battery. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Orthorhombic; LiMnO2; Hydrothermal synthesis: cathode; Li-ion battery
1. Introduction The Mn-based materials have attracted attention as intercalation cathode materials because of their low cost and nontoxicity. The LiMn2O4 has shown excellent cycle performance at room temperature in the 4 V region [1 /3]. However, it presented a significant capacity loss when cycled in the 3 V region. The trivalent manganese compound LiMnO2 (both orthorhombic and monoclinic) offers a better cycle performances than the LiMn2O4 spinel when it cycled over a wide voltage region between 2 and 4.5 V versus Li [4,5]. Therefore, it seemed likely that the use of orthorhombic LiMnO2 would be equivalent of the use of spinel Li2Mn2O4. Orthorhombic unit cell of LiMnO2 was assigned by Johnston and Keikes [6] as early as 1956, and the detailed structural information was given by Hoppe et al. [7]. Electrochemical behaviors of the orthorhombic LiMnO2 (hereafter referred as to o -LiMnO2) are greatly
* Corresponding author. Fax: /81-196216328 E-mail address: [email protected] (S. Komaba).
dependent on the synthetic routes. Therefore, the oLiMnO2 can classified to three categories. 1.1. Low-temperature syntheses (5/450 8C) Ohzuku et al. [8] have discovered a low temperature synthesis of o-LiMnO2 by the reaction of LiOH ×/H2O and g-MnOOH at 450 8C. Reimers et al. [9] reported a new synthetic route of o -LiMnO2 below 100 8C. The prepared material showed higher capacity about 200 mAh g 1, and in-situ XRD study demonstrated that gradual phase transformation to a spinel structure after the first cycle or so. Gummow et al. [10,11] also synthesized o -LiMnO2 in a range from 300 to 600 8C. Their suggestion was that the material prepared at 300 8C showed better electrochemical properties on cycling. Furthermore, they confirmed the suggestion of Reimer et al. [9] that electrochemical cycling of oLiMnO2 resulted in a gradual phase formation to spinel phase and suggested that the newly formed spinel phase is significantly more tolerant to repeated lithium insertion and extraction, when it cycled over both 4 and 3 V regions versus Li. Initial capacities were greatly en-
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 4 8 - 7
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hanced by low temperature syntheses, but cyclability was still remained unsolved. 1.2. Intermediate-temperature syntheses (600 /800 8C) Croguennec et al. [12 /15] correlated the electrochemical properties with crystalline size and stacking fault. In the case of stacking fault, they explained using the full width of the half maximum of (110) Bragg peak. According to their suggestion, small particle size below 1 mm in diameter delivered high capacities. The increased capacities were also observed in monoclinically distorted form of o -LiMnO2. This means that higher stacking faulted o -LiMnO2 showing a broad (110) peak contributed to much faster spinel phase transformation, leading to a high capacity.
method dealt with battery performances for initial several cycles only and the dependence of starting materials for hydrothermal reaction on the products was unclear. What we would like to emphasize in the present work is the synthesis of o-LiMnO2 using a spinel type crystalline Mn3O4, [MnII ]tet [MnIII 2 ]oct O4 ; oxide as a hydrothermal precursor, as we reported previously [25]. To progress this experiment, Mn3O4 was synthesized through a oxidation of Mn(OH)2 in alkaline solution by O2 gas bubbling at 80 8C. Finally, we have obtained a high crystalline orthorhombic LiMnO2 by hydrothermal method and present its physical and electrochemical properties.
2. Experimental 1.3. High-temperature syntheses ( ]/800 8C) 2.1. Hydrothermal synthesis Recently, Jang et al. [16 /21] have successfully prepared o -LiMnO2 by high temperature calcination route using LiOH and Mn3O4 under a well-controlled oxygen atmosphere. The prepared high crystalline oxide powders that have a lower stacking fault in the respect of the report of Croguennec et al. [15] exhibited a high capacity as well as good cyclability upon cycling. According to their suggestion, repeated Li insertion and extraction are significantly dependent on applying current density. As mentioned above, cycling of o -LiMnO2 induces a gradual phase transformation to spinel phase. In this regard, for the first time, they revealed that the cycleinduced spinel phase consisted of nanodomains confirmed by observation of high-resolution transmission electron microscopy (HRTEM) so that they played a significant role to maintain a higher capacity in 3 V region [20,21]. From above reviews, we postulated that if o -LiMnO2 has hybrid properties of low and high temperature ones, those are, higher initial capacity (low temperature form) as well as good cyclability (high temperature form), this material will show better electrochemical performances. This point prompted us to consider a hydrothermal preparation route to synthesize o -LiMnO2 oxide. Because this method is particularly suited for the synthesis of phases that are unstable at higher temperatures and is also a useful technique for preparation of ceramic powders and for growth of single crystal. Furthermore, bulk synthesis at low temperature at B/200 8C is significantly cheaper than synthesis at the higher temperature. It was reported earlier that o -LiMnO2 could be prepared by hydrothermal reaction between MnO2 or Mn2O3 and LiOH at 600 8C under a pressure of 100 MPa [22]. Recently, hydrothermal reaction between excess of LiOH and g-MnOOH also resulted in the formation of o-LiMnO2 [23,24]. Nonetheless, the reports about the o-LiMnO2 prepared by hydrothermal
Mn3O4 as a hydrothermal precursor was synthesized through a mild autoxidation route [26]. Manganese acetate tetrahydrate (Mn(CH3COO)2 ×/4H2O, Kanto Chemicals) was dissolved in distilled water. Then, the Mn aqueous solution was mixed with 10 M KOH aqueous solution, resulting in Mn(OH)2 precipitation, and the aqueous suspension was stirred at 80 8C for 1 day with bubbling by O2 gas to oxidize Mn(OH)2. Finally, dark brown powders of Mn3O4 precipitated on the bottom of reactor. The precipitates were continuously washed with deionized water, and then dried at 80 8C in air. The prepared Mn3O4 powder (0.2 g) was hydrothermally treated with 40 ml of LiOH aq. solution at 170 8C for 4 days in an autoclave under autogeneous pressure [25]. A teflon beaker was used to avoid any reaction with the vessel. After hydrothermal reaction, the produced powders were washed with deionized water, and the obtained powders were dried to remove water at 120 8C in air. 2.2. Powder characterization X-ray powder diffraction (XRD) measurement was carried out using a Cu Ka radiation of Rigaku Rint 2200 diffractormeter. Atomic absorption spectroscopy (Aanalyst 300, Perkin /Elmer) was employed to analyze chemical composition after dissolving the products in HCl. Morphology of the prepared powders was observed by a transmission electron microscope (TEM; acceleration voltage; 200 kV, Hitachi, H-800). 2.3. Electrochemical measurements For fabrication of working electrodes, the prepared oLiMnO2 powder was mixed with graphite, acetylene black and polyvinylidene fluoride (80:5:10:5) in N -
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methylpyrrolidinon. The slurry thus obtained was pasted on nickel ex-met (1 cm2), and the ex-met was dried at 80 8C for 1 day, and then dried again at 120 8C for 4 days in vacuum state prior to use. A beaker-type cell consisted of the oxide of working electrode and lithium foil as a counter electrode was assembled in an Ar-filled glove box. The cells were firstly charged (oxidized) and discharged (reduced) between 2.0 and 1 4.3 V (C/6.45) Li=Li with current densities of 45 mA g 1 and 22.5 mA g (C/12.9) at 30 8C. Electrochemical impedance measurements using a three-electrode cell were carried out by applying alternating voltage in the frequency range of 100 kHz to 1 mHz with an acamplitude of 10 mVrms.
3. Results and discussion 3.1. Powder characteristics The XRD pattern of the oxide precursor formed from the mixture of manganese acetate aqueous solution and potassium hydroxide aqueous solution in bubbling oxygen at 80 8C is shown in Fig. 1(a). After washing
Fig. 1. XRD patterns indicating phase evolution from Mn3O4 to o LiMnO2 by hydrothermal reactions between Mn3O4 and various concentrations of LiOH aqueous solution at 170 8C for 4 days: (a) as-prepared Mn3O4; (b) 0.1; (c) 1; (d) 2; (e) 3; and (f) 3.5 M.
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the precipitates by deionized water until the pH reaches to neutral, Mn3O4 appears as the major phase with no other Mn-containing minor phases being detectable, since the all peaks (hkl ) could be indexed for the tetragonal phase with a space group of I 41/amd in Fig. 1. The calculated lattice parameters from the XRD ˚ , which are well pattern are a /5.7676, and c /9.4889 A coincident with the JCPDS card [27]. And, the c/a ratio of the prepared material was about 1.64, indicating that the Mn3O4 (spinel [MnII ]tet [MnIII 2 ]oct O4 ) oxide could be easily prepared from the autoxidation route. The as-prepared Mn3O4 oxide was hydrothermally treated with various LiOH aqueous solutions in a teflon sealed autoclave at 170 8C for 4 days. Figs. 1 and 2 show the XRD patterns and TEM bright-field images of phase evolution steps as a function of lithium aqueous concentration, respectively. It is obvious that the hydrothermal reaction between Mn3O4 and LiOH resulted in orthorhombic LiMnO2, although the products depended on the LiOH aqueous concentration. The Mn3O4 particle before the hydrothermal treatment showed highly developed edges and corners in all particles in Fig. 2(a), and the size distribution was between 0.1 and 1 mm in diameter. Comparing with the as-prepared Mn3O4, there is no significant difference after Mn3O4 was hydrothermally treated with 0.1 M LiOH aqueous solution. As can be seen in Fig. 1(c /f), the o -LiMnO2 phase marked with open circle begin to appear as a minor phase, and during hydrothermal reaction, the o-LiMnO2 phase was gradually developed with increasing lithium concentration. Here, we can imagine that some parts of the as-prepared Mn3O4 which is not soluble in LiOH solution in common condition allow lithium ion to be incorporated into its original structure during the hydrothermal treatment. As given in Fig. 2(b), the well-developed corners and their sharpness of Mn3O4 lost somewhat, comparing to Fig. 2(a). When the concentration was 3 M, the XRD pattern shows the o -LiMnO2 phase as a major phase and Mn3O4 as a minor phase. In the case, the original shape of Mn3O4 particle is hardly observed in the TEM image. As shown in the selected area diffraction (SAD) and bright-field image of Fig. 2(c), we could obtain the o -LiMnO2 fine single crystallite particle powders when the LiOH concentration was only 3.5 M. There is a possible way to explain how the o -LiMnO2 phase could be formed with the LiOH aqueous solution; partial dissolution of Mn2 from the Mn3O4 precursor which consists of spinel [MnII ]tet [MnIII 2 ]oct O4 (average valence of Mn: 2.67) into the strong basic aqueous LiOH solution with combination of lithium ions incorporation into the tetragonal Mn3O4 structure. Only when the concentration of lithium aqueous solution is 3.5 M, the pure o -LiMnO2 phase could be formed, as confirmed by XRD in Figs. 1 and 2. Here, we supposed that the aqueous concentration is enough to dissolve all
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Fig. 3. Variation in dissolved Mn in LiOH aqueous solution after hydrothermal reaction. The used solution for AAS was 10 ml.
Fig. 2. TEM bright-field images of morphology evolution from Mn3O4 to o -LiMnO2 by hydrothermal reaction between Mn3O4 and various concentrations of LiOH aqueous solution at 170 8C: (a) asprepared Mn3O4; (b) 2; and (c) 3.5 M.
of the Mn2 in [MnII ]tet [MnIII 2 ]oct O4 during the hydrothermal reaction. To confirm that the manganese component was dissolved during the hydrothermal reaction, determination of the dissolved Mn amount in the aqueous solution after the hydrothermal reaction was carried out by AAS. According to the AAS results, manganese ingredient was included in the reacted solution as shown in Fig. 3. The dissolved Mn in LiOH aqueous solution increased
monotonously. This trend agrees with the increase in LiMnO2 formation as mentioned in Figs. 1 and 2. It is most likely that the Mn2 from [MnII ]tet [MnIII 2 ]oct O4 was mostly dissolved in 3.5 M of LiOH aqueous solution, because the value 31% in (dissolved Mn)/(total Mn in the Mn3O4) coincides well with the theoretical value of 33%. So, we believe that the most appropriate concentration of lithium aqueous solution to prepare o LiMnO2 is 3.5 M. Like this way, the layered orthorhombic phase could be readily formed by the selective dissolution of Mn2 from the spinel type precursor [MnII ]tet [MnIII 2 ]oct O4 by the hydrothermal treatment without any oxidant at low temperature as low as 170 8C. In the case of Mn2O3 or g-MnOOH instead of Mn3O4 as a precursor, the product (not shown here) under the same hydrothermal condition showed much lower crystallinity. We concluded that using Mn3O4 as a hydrothermal precursor is greatly advantageous to obtain high crystalline o -LiMnO2. The XRD pattern presented in Fig. 1(f) is similar to that prepared by calcination of 950 8C of freeze-dried precursor reported in Ref. [7]. In the case, they controlled sensitively the oxygen partial pressure to reach the manganese valence from 2.67 to 3 at around 950 8C. Table 1 shows the chemical formula and lattice parameters determined from AAS and XRD, respectively. The lattice parameters are consistent with those reported in the literatures [7,12,17,28]. Croguennec et al. [15] suggested that stacking fault is possible to observe
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Table 1 Crystallographic parameters of o -LiMnO2 synthesized by the hydrothermal treaatment at 170 8C Formula Crystal system Space group Cell parameters ˚) a (A ˚) b (A ˚) c (A ˚ 3) Volume (A
Li1.03Mn1.00O2 Orthorhombic Pmnm 4.5495(1) 5.7550(4) 2.8105(7) 74.0799
from XRD pattern and directed to monoclinic LiMnO2 structure. In case of occurrence of the stacking fault, the (110) Bragg peak shows very broaden XRD pattern. As can be seen in Fig. 1(f), the narrow full width of half maximum (FWHM) of the (110) peak (0.188 2u ) depicts that the o -LiMnO2 oxide synthesized by the hydrothermal route is considered to have a well-ordered orthorhombic structure. As shown in Fig. 2, TEM observation gave the confirmation that the observed average particle size of the o -LiMnO2 powder is 0.1 /0.5 mm in diameter. From the selected area electron diffraction patterns of Mn3O4 and o -LiMnO2, single crystallite patterns were observed in most of the particles observed. The observed shape was nearly cubic-shaped oxide particles. The color of fresh Mn3O4 powder was a brown. During hydrothermal reaction, the color began to change from brown into dark green. Finally, the powder color reached olive green, when the LiOH concentration of aqueous solution was 3.5 M. Consequently, we found that the hydrothermal route using Mn3O4 is greatly beneficial to obtain the fine powders of o -LiMnO2 which consist of single crystallite particles having highly ordered structure. 3.2. Electrochemical characteristics We investigated the o -LiMnO2 powder obtained at 170 8C from 3.5 M LiOH aqueous solution. The Li/o LiMnO2 cells were galvanostatically cycled at current densities of 45 and 22.5 mA g 1 in the range between 2.0 and 4.3 V at 30 8C. The first charge and following discharge curves of these cells are shown in Fig. 4. In the case of the current density of 45 mA g 1, a simple potential plateau at 3.7 V appeared in the initial charge curve. As observed previously by Shu et al. [29], a very large capacity loss of about 95 mAh g 1 was observed during the first cycling in Fig. 4(a). They claimed that the irreversible capacity is from phase transition from o LiMnO2 to spinel LiMn2O4 and that the phase transition is greatly dependent on cycling rate. The phase transformation was confirmed by in-situ XRD measurement during several cycles applying 14 mA g 1 by Ko¨tschau and Dahn [30]. Applying lower current
Fig. 4. Charge and discharge profiles of o -LiMnO2: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
density of 22.5 mA g 1, the initial several charge / discharge curves were much different from that of the current density of 45 mA g 1. The charge curve at 22.5 mA g 1 in Fig. 4(b) showed the charge potential plateau in the similar potential region around 3.7 V. In the initial discharge process, more spinel-like behavior was observed, compared to that of the current density of 45 mA g 1. Further, the phase transformation of the orthorhombic phase into spinel one is much more progressive than that of higher current density one so that a highly reversible electrochemical reaction around 3 and 4 V was clearly observed from the first cycle. With further cycling, voltage plateaus develop more clearly around 4 and 2.9 V in Fig. 4, which are indicative of Li intercalation on different sites, tetrahedral site over 4 V and octahedral site over 3 V in the cycled-induced spinel LiMn2O4 [31]. The length of the 4 V plateau is getting longer, while that of the 3 V plateau is simultaneously getting shorter in further cycling. In more detail, the 4 V discharge plateau is gradually divided into two subplateaus, meaning that reordering of Li in the structure toward the cycle-induced spinel is in progress during the initial several cycling. The intensity of the differentiated peak located at around 2.9 V increased and was saturated as cycling
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Applying higher current density (45 mA g 1), the XRD pattern was consistent with o -LiMnO2 (Fig. 6(a)). When a lower current density was applied (22.5 mA g 1), the structural reordering was observed much clearly in Fig. 6(b). The XRD pattern shows mainly disordered cycleinduced spinel, o-LiMnO2 and tetragonal spinel phases. In this case, about 0.75 mol of Li was extracted or inserted into the structure, leading to formation of tetragonal spinel Li2Mn2O4 phase formation as observed in Fig. 6(b). This result confirms that the slow current density allows a more complete de-/intercalation process (higher capacity). Fig. 7 shows the cyclabilities of o -LiMnO2 oxide electrodes at current densities of 45 and 22.5 mA g 1 at 30 8C. When a higher current is applied, it is seen that the present material can be cycled over both 4 and 3 V ranges without any significant capacity fading in a long term, as can be seen in Figs. 4 and 7. The behavior is remarkably different from conventional spinel LiMn2O4 whose the capacity decreases drastically within a few cyclings below 3 V region. This means that the electrochemically formed spinel-like phase originated from the o -LiMnO2 is more tolerant to cycling than conventional LiMn2O4. As shown in Fig. 4(a) and Fig. 5(a), the spinel phase began to develop during the initial several cycles, and the clear 4 V subplateaus appeared from the 20th Fig. 5. Incremental discharge capacities of o -LiMnO2: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
goes by in Fig. 5. And, two sharp 4 V subplateaus beyond the fifth cycle were seen in Fig. 5(b). XRD patterns on the fifth cycle (discharge) against different current densities give more comparative information about the phase transformation. A very broad hill in lower angle is attributed from XRD holder in Fig. 6.
Fig. 6. Ex-situ XRD patterns of o -LiMnO2 after the fifth cycle. Applied densities were: (a) 45; and (b) 22.5 mAh g 1. The XRDs were precisely taken with a step of 0.0038 s 1 (o , o -LiMnO2; s, cycleinduced cubic spinel; t, tetragonal spinel; j, Ni-exmet; and Gr, graphite).
Fig. 7. Cyclability of o -LiMnO2 between 2.0 and 4.3 VLi=Li at current densities of: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
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cycle in Fig. 5(a). The appearance of the 4 V plateaus contributes to the increase in capacities. Like this way, the obtained capacities increase progressively with further cycling and stabilize after 40 cycles with showing a discharge capacity of about 135 mAh g 1. In the case of 22.5 mA g 1, the electrochemical behavior shows different tendencies comparing with the case of 45 mA g 1, that is, the capacity of the cell is near 220 mAh g 1 in earlier several cycles. As shown in Fig. 5(b) and Fig. 7(b), the sufficient charge and discharge capacities are obtained reversibly from the early cycles, revealing that the phase transformation from orthorhombic to spinel is much faster and more complete, when the lower current density was applied, even though it showed a slow capacity fading in the 3 V region due to the collective the Jahn-Teller distortion. From the obtained capacities and cyclability, it was found that the material prepared by the hydrothermal method at low temperature shows the close properties as the o -LiMnO2 synthesized at high temperature [16,17]. The open circuit voltage (OCV) and closed circuit voltage (CCV) were measured as a function of lithium content, d , in Lid MnO2. The OCV curve was taken by electrochemical reduction with potential interval of 0.1 V, as shown in Fig. 8. The CCV was obtained from Fig. 4(b), the 40th discharge curve. The difference between the OCV and CCV is considerably small till d /0.33. It seems that the structural reordering to cycle-induced spinel phase after the 40th cycles is almost finished, so that the discharge behavior showed a small resistance in the range, being very similar to that of the conventional spinel compound. Interestingly, as the amount of Li ions more than d /0.33 is incorporated into the structure, the difference becomes relatively larger. The difference between OCV and CCV increases more, when d in Lid MnO2 exceeded 0.72.
Fig. 8. Open-circuit voltages as a function of Li content in Lid MnO2 prepared from electrochemical reduction of the 40th discharge process. OCVs as a function of Li contents in Lid MnO2 were measured by electrochemical reduction in the 40th discharge at 22.5 mA g 1 (C/ 12.9). Equilibrating time was more than 48 h so that the rate of OCVs change was less than 1 mV h 1. CCVs were obtained from Fig. 4(b).
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Ac-impedance measurements were employed to observe the Li intercalation reaction occurring in the interface between cathode and electrolyte. The impedance was taken by using the three points, d /0.33, 0.52 and 0.72 in Lid MnO2 marked by arrows in Fig. 8 at 30 8C. As can be seen in Fig. 9, the whole impedance of the semi-circle in the higher frequency region increases as lithium ions are intercalated into the structure. The chemical diffusivity of lithium ion was calculated from the fT at which a transition occurs from semi-infinite to finite diffusion behavior lengths as shown in typical impedance spectra of Fig. 9(b) [32 /37]. The value of the chemical diffusion coefficients, D˜ Li ; in the Lid MnO2 composite electrode was calculated by the following equation [33,35/37]; D˜ Li
pfT r2 1:94
(1)
Fig. 9. Nyquist impedance plots of Lid MnO2 as a function of lithium content; (o, d /0.33; I, d /0.52; and \, d /0.72) at 30 8C.
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where r means the radius of a particle. The average particle size of 0.3 mm in diameter was obtained from the TEM picture in Fig. 2(c). The diffusivities at d /0.33 and 0.52 in Lid MnO2 were calculated to be 1.8 /10 13 and 5.8 /10 14 cm s 1, respectively. In the inset plot, however, there is an obvious difference in the low frequency region. At d /0.72, especially, where the difference between OCV and CCV was relatively higher than at d /0.33 and 0.52 in Lid MnO2, the plot clearly showed another huge Rct component. For the case of d /0.72, we could not calculate the diffusion coefficient, because of no showing any diffusitive component. To examine the potential dependence of the structure, X-rays were radiated across the cycled electrodes. As shown in Fig. 10, cycle-induced cubic spinel was observed as a major phase at d /0.33 in Lid MnO2. Compared to the conventional spinel, the relative intensities of the electrochemically formed spinel phase is somewhat different. The broad widths and different intensities in XRD pattern are ascribed to disordering of the host structure. At d /0.52, tetragonal spinel phase began to appear at 2.9 V of the 40th cycle. The tetragonal phase was a little more intensified after 50 cycles. As described about the diffusivity at 4.0 and 2.9 V in Fig. 9, the difference in the structure may lead to the change in the diffusivity, because Li can diffuse through 8a /16c /8a after the phase transformation to spinel phase (d /0.33 in Lid MnO2). As Li intercalated more into the transformed structure (d /0.52), the 16c sites for Li diffusion passway are gradually occupied by Li, causing the decrease in Li diffusion coefficient. At d /0.52, furthermore, the existence of two phases which are composed by the cycle-induced cubic spinel and tetragonal one may hinder Li diffusion. Therefore, a significant difference in the diffusion coefficient would
Fig. 10. Ex-situ XRD patterns of o -LiMnO2 at various potentials. The samples were prepared from electrochemical reduction of the 40th discharge process: (a) d /0.33; (b) 0.52 in Lid MnO2 as shown with arrows in Fig. 8; and (c) 50 cycles. The XRDs were precisely taken with a step of 0.0038 s 1. (s, cycle-induced cubic spinel; t, tetragonal spinel; j, Ni-exmet; and Gr, graphite.)
Fig. 11. (a) TEM bright-field photo of o -LiMnO2; and (b) magnified image of the squared part on (a) after extensive cycling between 2.0 and 4.3 V Li=Li :/
be seen at the each potential (d /0.33 and 0.52 in Lid MnO2). The extensively cycled particles were directly observed by TEM. Fig. 11 shows a typical image of the cycled particles. Most of the particles observed gave the similar patterns. The cycled particle was severely damaged by cyclings. Comparing with Fig. 2(c) of which the edges of particles are well developed, it can be interpreted that the shapes of the edges and corners were degraded to zigzag shaped or even severely cracked (marked by arrows) by electrochemical shock. It is thought that the material would take much stress from the phase transformation. In general, the collective Jahn-Teller distortion is notorious for degradation of manganese oxide matrix. Nonetheless, the o -LiMnO2 material showed relative higher capacity on 3 /4 V region, especially the 3 V region where the Jahn-Teller effect exists. Recently, it was explored to elucidate the mechanism why cycle-induced spinel phase can show higher capacity on 3 V region. From the results of Wang
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and Chiang et al. [20,21], it is concluded that ferroelastic nanodomains derived from structural changes on cycling played a role to keep the higher capacity, even though electrochemical cycling changed the particle shapes.
4. Conclusion In attempts to prepare o -LiMnO2, a high synthetic temperature leads to an increased loss of lithium by evaporation, which must be compensated for in the formulation of the reaction mixture with controlling oxygen partial pressure sensitively. The absolute necessity can be negligible by employing a new hydrothermal condition. Here, the hydrothermal condition using the Mn3O4, [MnII ]tet [MnIII 2 ]oct O4 ; as a hydrothermal precursor gave the highly ordered o -LiMnO2 single crystallite oxide. Its electrochemical properties are excellent comparing to high temperature calcined powders. At 45 mA g 1 of current density, the obtained capacities increase progressively with further cycling and stabilize after 100 cycles with showing a discharge capacity of about 133 mAh g 1. The capacity increased more with applying somewhat lower current density of 22.5 mA g 1. The new hydrothermal condition is greatly beneficial to prepare highly fine and a well-ordered single crystallite o -LiMnO2 oxide.
Acknowledgements The authors would like to thank Nobuko Kumagai and K. Kurihara, Iwate University, for helpful assistance in the experimental work. This study was supported NEDO of Japan, Yazaki Memorial Foundation for Science and Technology, and Saneyoshi Scholarship Foundation. S.T.M. personally acknowledges Yoneyama Studentship.
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[6] W.D. Johnston, R.R. Keikes, J. Am. Chem. Soc. 78 (1956) 3255. [7] V.R. Hoppe, G. Brachtel, M. Jansen, Z. Anorg. Allg. Chem. 417 (1975) 1. [8] T. Ohzuku, A. Ueda, T. Hirai, Chem. Expr. 7 (1992) 193. [9] J.N. Reimers, E.W. Fuller, E. Rossen, J.R. Dahn, J. Electrochem. Soc. 140 (1993) 3396. [10] R.J. Gummow, D.C. Liles, M.M. Thackeray, Mat. Res. Bull. 28 (1993) 1249. [11] R.J. Gummow, M.M. Thackeray, J. Electrochem. Soc. 141 (1994) 1178. [12] L. Croguennec, P. Deniard, R. Brec, A. Lecerf, J. Mater. Chem. 5 (1919) 1995. [13] L. Croguennec, P. Deniard, R. Brec, P. Biensan, M. Broussely, Solid State Ionics 89 (1996) 127. [14] L. Croguennec, P. Deniard, R. Brec, J. Electrochem. Soc. 144 (1997) 3323. [15] L. Croguennec, P. Deniard, R. Brec, A. Lecerf, J. Mater. Chem. 7 (1997) 511. [16] Y.-I. Jang, B. Huang, Y.-M. Chiang, D.R. Sadoway, Electrochem. Solid-State Lett. 1 (1998) 13. [17] Y.-I. Jang, B. Huang, H. Wang, D.R. Sadoway, Y.-M. Chiang, J. Electrochem. Soc. 146 (1999) 3217. [18] Y.-M. Chiang, D.R. Sadoway, Y.-I. Jang, B. Huang, H. Wang, Electrochem. Solid-State Lett. 2 (1999) 107. [19] Y.-I. Jang, Y.-M. Chiang, Solid State Ionics 130 (2000) 53. [20] H. Wang, Y.-I. Jang, Y.-M. Chiang, Electrochem. Solid-State Lett. 2 (1999) 490. [21] Y.-M. Chiang, H. Wang, Y.-I. Jang, Chem. Mater. 13 (2001) 53. [22] P. Strobel, J.P. Levy, J.C. Joubert, J. Cryst. Growth 66 (1984) 257. [23] M. Tabuchi, K. Ado, C. Masquelier, I. Matsubara, H. Sakaebe, H. Kageyama, H. Kobayashi, R. Kanno, O. Nakamura, Solid State Ionics 89 (1996) 53. [24] Y. Nitta, M. Nagayama, H. Miyake, A. Ohta, J. Power Sources 81 /82 (1999) 49. [25] S.-T. Myung, S. Komaba, N. Kumagai, Chem. Lett. (2001) 80 / 81. [26] A.R. Nichols, J.H. Walton, J. Am. Chem. Soc. 64 (1866) 1942. [27] Joint Committee on Powder Diffraction Standards, File no. 240734. [28] Joint Committee on Powder Diffraction Standards, File no. 350794. [29] Z.X. Shu, I.J. Davidson, R.S. McMillan, J.J. Murray, J. Power Sources 68 (1997) 618. [30] I.M. Ko¨tschau, J.R. Dahn, J. Electrochem. Soc. 145 (1998) 2672. [31] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769. [32] A.J. Vaccaro, T. Polanisamy, R.L. Huggins, J. Electrochem. Soc. 127 (1982) 343. [33] R. Cabanel, G. Barral, J.-P. Diard, B. Le Gorrec, C. Montella, J. Appl. Electrochem. 23 (1993) 93. [34] M.G.S.R. Thomas, P.G. Bruce, J.B. Goodenough, Solid State Ionics 17 (1985) 13. [35] Y.-M. Choi, S.-I. Pyun, S.-I. Moon, Solid State Ionics 89 (1996) 43. [36] S.-T. Myung, N. Kumagai, S. Komaba, H.-T. Chung, Solid State Ionics 139 (2001) 47. [37] S.-T. Myung, S. Komaba, N. Kumagai, J. Electrochem. Soc. 148 (2001) A482.
Hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2 Seung-Taek Myung, Shinichi Komaba *, Naoaki Kumagai Department of Chemical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan Received 20 March 2002
Abstract The most highlighted point of this work to emphasize is that it is the first trial to use Mn3O4 oxide as a precursor to synthesize orthorhombic LiMnO2 by the hydrothermal method. A well-ordered orthorhombic LiMnO2 phase was formed by the hydrothermal treatment of Mn3O4 with excess LiOH aqueous solution at 170 8C. According to TEM observation, the as-synthesized powder was single crystalline particle oxide. Comparing with other orthorhombic LiMnO2 prepared by low temperature synthetic route and by high temperature calcination, the orthorhombic LiMnO2 prepared by the hydrothermal route showed enhanced battery performance as a lithium battery cathode material. We believe that the new hydrothermal synthesis is expected as an excellent alternative of powder preparation method of high capacity cathode material to be used for Li-ion secondary battery. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Orthorhombic; LiMnO2; Hydrothermal synthesis: cathode; Li-ion battery
1. Introduction The Mn-based materials have attracted attention as intercalation cathode materials because of their low cost and nontoxicity. The LiMn2O4 has shown excellent cycle performance at room temperature in the 4 V region [1 /3]. However, it presented a significant capacity loss when cycled in the 3 V region. The trivalent manganese compound LiMnO2 (both orthorhombic and monoclinic) offers a better cycle performances than the LiMn2O4 spinel when it cycled over a wide voltage region between 2 and 4.5 V versus Li [4,5]. Therefore, it seemed likely that the use of orthorhombic LiMnO2 would be equivalent of the use of spinel Li2Mn2O4. Orthorhombic unit cell of LiMnO2 was assigned by Johnston and Keikes [6] as early as 1956, and the detailed structural information was given by Hoppe et al. [7]. Electrochemical behaviors of the orthorhombic LiMnO2 (hereafter referred as to o -LiMnO2) are greatly
* Corresponding author. Fax: /81-196216328 E-mail address: [email protected] (S. Komaba).
dependent on the synthetic routes. Therefore, the oLiMnO2 can classified to three categories. 1.1. Low-temperature syntheses (5/450 8C) Ohzuku et al. [8] have discovered a low temperature synthesis of o-LiMnO2 by the reaction of LiOH ×/H2O and g-MnOOH at 450 8C. Reimers et al. [9] reported a new synthetic route of o -LiMnO2 below 100 8C. The prepared material showed higher capacity about 200 mAh g 1, and in-situ XRD study demonstrated that gradual phase transformation to a spinel structure after the first cycle or so. Gummow et al. [10,11] also synthesized o -LiMnO2 in a range from 300 to 600 8C. Their suggestion was that the material prepared at 300 8C showed better electrochemical properties on cycling. Furthermore, they confirmed the suggestion of Reimer et al. [9] that electrochemical cycling of oLiMnO2 resulted in a gradual phase formation to spinel phase and suggested that the newly formed spinel phase is significantly more tolerant to repeated lithium insertion and extraction, when it cycled over both 4 and 3 V regions versus Li. Initial capacities were greatly en-
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 2 4 8 - 7
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hanced by low temperature syntheses, but cyclability was still remained unsolved. 1.2. Intermediate-temperature syntheses (600 /800 8C) Croguennec et al. [12 /15] correlated the electrochemical properties with crystalline size and stacking fault. In the case of stacking fault, they explained using the full width of the half maximum of (110) Bragg peak. According to their suggestion, small particle size below 1 mm in diameter delivered high capacities. The increased capacities were also observed in monoclinically distorted form of o -LiMnO2. This means that higher stacking faulted o -LiMnO2 showing a broad (110) peak contributed to much faster spinel phase transformation, leading to a high capacity.
method dealt with battery performances for initial several cycles only and the dependence of starting materials for hydrothermal reaction on the products was unclear. What we would like to emphasize in the present work is the synthesis of o-LiMnO2 using a spinel type crystalline Mn3O4, [MnII ]tet [MnIII 2 ]oct O4 ; oxide as a hydrothermal precursor, as we reported previously [25]. To progress this experiment, Mn3O4 was synthesized through a oxidation of Mn(OH)2 in alkaline solution by O2 gas bubbling at 80 8C. Finally, we have obtained a high crystalline orthorhombic LiMnO2 by hydrothermal method and present its physical and electrochemical properties.
2. Experimental 1.3. High-temperature syntheses ( ]/800 8C) 2.1. Hydrothermal synthesis Recently, Jang et al. [16 /21] have successfully prepared o -LiMnO2 by high temperature calcination route using LiOH and Mn3O4 under a well-controlled oxygen atmosphere. The prepared high crystalline oxide powders that have a lower stacking fault in the respect of the report of Croguennec et al. [15] exhibited a high capacity as well as good cyclability upon cycling. According to their suggestion, repeated Li insertion and extraction are significantly dependent on applying current density. As mentioned above, cycling of o -LiMnO2 induces a gradual phase transformation to spinel phase. In this regard, for the first time, they revealed that the cycleinduced spinel phase consisted of nanodomains confirmed by observation of high-resolution transmission electron microscopy (HRTEM) so that they played a significant role to maintain a higher capacity in 3 V region [20,21]. From above reviews, we postulated that if o -LiMnO2 has hybrid properties of low and high temperature ones, those are, higher initial capacity (low temperature form) as well as good cyclability (high temperature form), this material will show better electrochemical performances. This point prompted us to consider a hydrothermal preparation route to synthesize o -LiMnO2 oxide. Because this method is particularly suited for the synthesis of phases that are unstable at higher temperatures and is also a useful technique for preparation of ceramic powders and for growth of single crystal. Furthermore, bulk synthesis at low temperature at B/200 8C is significantly cheaper than synthesis at the higher temperature. It was reported earlier that o -LiMnO2 could be prepared by hydrothermal reaction between MnO2 or Mn2O3 and LiOH at 600 8C under a pressure of 100 MPa [22]. Recently, hydrothermal reaction between excess of LiOH and g-MnOOH also resulted in the formation of o-LiMnO2 [23,24]. Nonetheless, the reports about the o-LiMnO2 prepared by hydrothermal
Mn3O4 as a hydrothermal precursor was synthesized through a mild autoxidation route [26]. Manganese acetate tetrahydrate (Mn(CH3COO)2 ×/4H2O, Kanto Chemicals) was dissolved in distilled water. Then, the Mn aqueous solution was mixed with 10 M KOH aqueous solution, resulting in Mn(OH)2 precipitation, and the aqueous suspension was stirred at 80 8C for 1 day with bubbling by O2 gas to oxidize Mn(OH)2. Finally, dark brown powders of Mn3O4 precipitated on the bottom of reactor. The precipitates were continuously washed with deionized water, and then dried at 80 8C in air. The prepared Mn3O4 powder (0.2 g) was hydrothermally treated with 40 ml of LiOH aq. solution at 170 8C for 4 days in an autoclave under autogeneous pressure [25]. A teflon beaker was used to avoid any reaction with the vessel. After hydrothermal reaction, the produced powders were washed with deionized water, and the obtained powders were dried to remove water at 120 8C in air. 2.2. Powder characterization X-ray powder diffraction (XRD) measurement was carried out using a Cu Ka radiation of Rigaku Rint 2200 diffractormeter. Atomic absorption spectroscopy (Aanalyst 300, Perkin /Elmer) was employed to analyze chemical composition after dissolving the products in HCl. Morphology of the prepared powders was observed by a transmission electron microscope (TEM; acceleration voltage; 200 kV, Hitachi, H-800). 2.3. Electrochemical measurements For fabrication of working electrodes, the prepared oLiMnO2 powder was mixed with graphite, acetylene black and polyvinylidene fluoride (80:5:10:5) in N -
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methylpyrrolidinon. The slurry thus obtained was pasted on nickel ex-met (1 cm2), and the ex-met was dried at 80 8C for 1 day, and then dried again at 120 8C for 4 days in vacuum state prior to use. A beaker-type cell consisted of the oxide of working electrode and lithium foil as a counter electrode was assembled in an Ar-filled glove box. The cells were firstly charged (oxidized) and discharged (reduced) between 2.0 and 1 4.3 V (C/6.45) Li=Li with current densities of 45 mA g 1 and 22.5 mA g (C/12.9) at 30 8C. Electrochemical impedance measurements using a three-electrode cell were carried out by applying alternating voltage in the frequency range of 100 kHz to 1 mHz with an acamplitude of 10 mVrms.
3. Results and discussion 3.1. Powder characteristics The XRD pattern of the oxide precursor formed from the mixture of manganese acetate aqueous solution and potassium hydroxide aqueous solution in bubbling oxygen at 80 8C is shown in Fig. 1(a). After washing
Fig. 1. XRD patterns indicating phase evolution from Mn3O4 to o LiMnO2 by hydrothermal reactions between Mn3O4 and various concentrations of LiOH aqueous solution at 170 8C for 4 days: (a) as-prepared Mn3O4; (b) 0.1; (c) 1; (d) 2; (e) 3; and (f) 3.5 M.
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the precipitates by deionized water until the pH reaches to neutral, Mn3O4 appears as the major phase with no other Mn-containing minor phases being detectable, since the all peaks (hkl ) could be indexed for the tetragonal phase with a space group of I 41/amd in Fig. 1. The calculated lattice parameters from the XRD ˚ , which are well pattern are a /5.7676, and c /9.4889 A coincident with the JCPDS card [27]. And, the c/a ratio of the prepared material was about 1.64, indicating that the Mn3O4 (spinel [MnII ]tet [MnIII 2 ]oct O4 ) oxide could be easily prepared from the autoxidation route. The as-prepared Mn3O4 oxide was hydrothermally treated with various LiOH aqueous solutions in a teflon sealed autoclave at 170 8C for 4 days. Figs. 1 and 2 show the XRD patterns and TEM bright-field images of phase evolution steps as a function of lithium aqueous concentration, respectively. It is obvious that the hydrothermal reaction between Mn3O4 and LiOH resulted in orthorhombic LiMnO2, although the products depended on the LiOH aqueous concentration. The Mn3O4 particle before the hydrothermal treatment showed highly developed edges and corners in all particles in Fig. 2(a), and the size distribution was between 0.1 and 1 mm in diameter. Comparing with the as-prepared Mn3O4, there is no significant difference after Mn3O4 was hydrothermally treated with 0.1 M LiOH aqueous solution. As can be seen in Fig. 1(c /f), the o -LiMnO2 phase marked with open circle begin to appear as a minor phase, and during hydrothermal reaction, the o-LiMnO2 phase was gradually developed with increasing lithium concentration. Here, we can imagine that some parts of the as-prepared Mn3O4 which is not soluble in LiOH solution in common condition allow lithium ion to be incorporated into its original structure during the hydrothermal treatment. As given in Fig. 2(b), the well-developed corners and their sharpness of Mn3O4 lost somewhat, comparing to Fig. 2(a). When the concentration was 3 M, the XRD pattern shows the o -LiMnO2 phase as a major phase and Mn3O4 as a minor phase. In the case, the original shape of Mn3O4 particle is hardly observed in the TEM image. As shown in the selected area diffraction (SAD) and bright-field image of Fig. 2(c), we could obtain the o -LiMnO2 fine single crystallite particle powders when the LiOH concentration was only 3.5 M. There is a possible way to explain how the o -LiMnO2 phase could be formed with the LiOH aqueous solution; partial dissolution of Mn2 from the Mn3O4 precursor which consists of spinel [MnII ]tet [MnIII 2 ]oct O4 (average valence of Mn: 2.67) into the strong basic aqueous LiOH solution with combination of lithium ions incorporation into the tetragonal Mn3O4 structure. Only when the concentration of lithium aqueous solution is 3.5 M, the pure o -LiMnO2 phase could be formed, as confirmed by XRD in Figs. 1 and 2. Here, we supposed that the aqueous concentration is enough to dissolve all
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Fig. 3. Variation in dissolved Mn in LiOH aqueous solution after hydrothermal reaction. The used solution for AAS was 10 ml.
Fig. 2. TEM bright-field images of morphology evolution from Mn3O4 to o -LiMnO2 by hydrothermal reaction between Mn3O4 and various concentrations of LiOH aqueous solution at 170 8C: (a) asprepared Mn3O4; (b) 2; and (c) 3.5 M.
of the Mn2 in [MnII ]tet [MnIII 2 ]oct O4 during the hydrothermal reaction. To confirm that the manganese component was dissolved during the hydrothermal reaction, determination of the dissolved Mn amount in the aqueous solution after the hydrothermal reaction was carried out by AAS. According to the AAS results, manganese ingredient was included in the reacted solution as shown in Fig. 3. The dissolved Mn in LiOH aqueous solution increased
monotonously. This trend agrees with the increase in LiMnO2 formation as mentioned in Figs. 1 and 2. It is most likely that the Mn2 from [MnII ]tet [MnIII 2 ]oct O4 was mostly dissolved in 3.5 M of LiOH aqueous solution, because the value 31% in (dissolved Mn)/(total Mn in the Mn3O4) coincides well with the theoretical value of 33%. So, we believe that the most appropriate concentration of lithium aqueous solution to prepare o LiMnO2 is 3.5 M. Like this way, the layered orthorhombic phase could be readily formed by the selective dissolution of Mn2 from the spinel type precursor [MnII ]tet [MnIII 2 ]oct O4 by the hydrothermal treatment without any oxidant at low temperature as low as 170 8C. In the case of Mn2O3 or g-MnOOH instead of Mn3O4 as a precursor, the product (not shown here) under the same hydrothermal condition showed much lower crystallinity. We concluded that using Mn3O4 as a hydrothermal precursor is greatly advantageous to obtain high crystalline o -LiMnO2. The XRD pattern presented in Fig. 1(f) is similar to that prepared by calcination of 950 8C of freeze-dried precursor reported in Ref. [7]. In the case, they controlled sensitively the oxygen partial pressure to reach the manganese valence from 2.67 to 3 at around 950 8C. Table 1 shows the chemical formula and lattice parameters determined from AAS and XRD, respectively. The lattice parameters are consistent with those reported in the literatures [7,12,17,28]. Croguennec et al. [15] suggested that stacking fault is possible to observe
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Table 1 Crystallographic parameters of o -LiMnO2 synthesized by the hydrothermal treaatment at 170 8C Formula Crystal system Space group Cell parameters ˚) a (A ˚) b (A ˚) c (A ˚ 3) Volume (A
Li1.03Mn1.00O2 Orthorhombic Pmnm 4.5495(1) 5.7550(4) 2.8105(7) 74.0799
from XRD pattern and directed to monoclinic LiMnO2 structure. In case of occurrence of the stacking fault, the (110) Bragg peak shows very broaden XRD pattern. As can be seen in Fig. 1(f), the narrow full width of half maximum (FWHM) of the (110) peak (0.188 2u ) depicts that the o -LiMnO2 oxide synthesized by the hydrothermal route is considered to have a well-ordered orthorhombic structure. As shown in Fig. 2, TEM observation gave the confirmation that the observed average particle size of the o -LiMnO2 powder is 0.1 /0.5 mm in diameter. From the selected area electron diffraction patterns of Mn3O4 and o -LiMnO2, single crystallite patterns were observed in most of the particles observed. The observed shape was nearly cubic-shaped oxide particles. The color of fresh Mn3O4 powder was a brown. During hydrothermal reaction, the color began to change from brown into dark green. Finally, the powder color reached olive green, when the LiOH concentration of aqueous solution was 3.5 M. Consequently, we found that the hydrothermal route using Mn3O4 is greatly beneficial to obtain the fine powders of o -LiMnO2 which consist of single crystallite particles having highly ordered structure. 3.2. Electrochemical characteristics We investigated the o -LiMnO2 powder obtained at 170 8C from 3.5 M LiOH aqueous solution. The Li/o LiMnO2 cells were galvanostatically cycled at current densities of 45 and 22.5 mA g 1 in the range between 2.0 and 4.3 V at 30 8C. The first charge and following discharge curves of these cells are shown in Fig. 4. In the case of the current density of 45 mA g 1, a simple potential plateau at 3.7 V appeared in the initial charge curve. As observed previously by Shu et al. [29], a very large capacity loss of about 95 mAh g 1 was observed during the first cycling in Fig. 4(a). They claimed that the irreversible capacity is from phase transition from o LiMnO2 to spinel LiMn2O4 and that the phase transition is greatly dependent on cycling rate. The phase transformation was confirmed by in-situ XRD measurement during several cycles applying 14 mA g 1 by Ko¨tschau and Dahn [30]. Applying lower current
Fig. 4. Charge and discharge profiles of o -LiMnO2: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
density of 22.5 mA g 1, the initial several charge / discharge curves were much different from that of the current density of 45 mA g 1. The charge curve at 22.5 mA g 1 in Fig. 4(b) showed the charge potential plateau in the similar potential region around 3.7 V. In the initial discharge process, more spinel-like behavior was observed, compared to that of the current density of 45 mA g 1. Further, the phase transformation of the orthorhombic phase into spinel one is much more progressive than that of higher current density one so that a highly reversible electrochemical reaction around 3 and 4 V was clearly observed from the first cycle. With further cycling, voltage plateaus develop more clearly around 4 and 2.9 V in Fig. 4, which are indicative of Li intercalation on different sites, tetrahedral site over 4 V and octahedral site over 3 V in the cycled-induced spinel LiMn2O4 [31]. The length of the 4 V plateau is getting longer, while that of the 3 V plateau is simultaneously getting shorter in further cycling. In more detail, the 4 V discharge plateau is gradually divided into two subplateaus, meaning that reordering of Li in the structure toward the cycle-induced spinel is in progress during the initial several cycling. The intensity of the differentiated peak located at around 2.9 V increased and was saturated as cycling
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Applying higher current density (45 mA g 1), the XRD pattern was consistent with o -LiMnO2 (Fig. 6(a)). When a lower current density was applied (22.5 mA g 1), the structural reordering was observed much clearly in Fig. 6(b). The XRD pattern shows mainly disordered cycleinduced spinel, o-LiMnO2 and tetragonal spinel phases. In this case, about 0.75 mol of Li was extracted or inserted into the structure, leading to formation of tetragonal spinel Li2Mn2O4 phase formation as observed in Fig. 6(b). This result confirms that the slow current density allows a more complete de-/intercalation process (higher capacity). Fig. 7 shows the cyclabilities of o -LiMnO2 oxide electrodes at current densities of 45 and 22.5 mA g 1 at 30 8C. When a higher current is applied, it is seen that the present material can be cycled over both 4 and 3 V ranges without any significant capacity fading in a long term, as can be seen in Figs. 4 and 7. The behavior is remarkably different from conventional spinel LiMn2O4 whose the capacity decreases drastically within a few cyclings below 3 V region. This means that the electrochemically formed spinel-like phase originated from the o -LiMnO2 is more tolerant to cycling than conventional LiMn2O4. As shown in Fig. 4(a) and Fig. 5(a), the spinel phase began to develop during the initial several cycles, and the clear 4 V subplateaus appeared from the 20th Fig. 5. Incremental discharge capacities of o -LiMnO2: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
goes by in Fig. 5. And, two sharp 4 V subplateaus beyond the fifth cycle were seen in Fig. 5(b). XRD patterns on the fifth cycle (discharge) against different current densities give more comparative information about the phase transformation. A very broad hill in lower angle is attributed from XRD holder in Fig. 6.
Fig. 6. Ex-situ XRD patterns of o -LiMnO2 after the fifth cycle. Applied densities were: (a) 45; and (b) 22.5 mAh g 1. The XRDs were precisely taken with a step of 0.0038 s 1 (o , o -LiMnO2; s, cycleinduced cubic spinel; t, tetragonal spinel; j, Ni-exmet; and Gr, graphite).
Fig. 7. Cyclability of o -LiMnO2 between 2.0 and 4.3 VLi=Li at current densities of: (a) 45; and (b) 22.5 mA g 1 at 30 8C.
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cycle in Fig. 5(a). The appearance of the 4 V plateaus contributes to the increase in capacities. Like this way, the obtained capacities increase progressively with further cycling and stabilize after 40 cycles with showing a discharge capacity of about 135 mAh g 1. In the case of 22.5 mA g 1, the electrochemical behavior shows different tendencies comparing with the case of 45 mA g 1, that is, the capacity of the cell is near 220 mAh g 1 in earlier several cycles. As shown in Fig. 5(b) and Fig. 7(b), the sufficient charge and discharge capacities are obtained reversibly from the early cycles, revealing that the phase transformation from orthorhombic to spinel is much faster and more complete, when the lower current density was applied, even though it showed a slow capacity fading in the 3 V region due to the collective the Jahn-Teller distortion. From the obtained capacities and cyclability, it was found that the material prepared by the hydrothermal method at low temperature shows the close properties as the o -LiMnO2 synthesized at high temperature [16,17]. The open circuit voltage (OCV) and closed circuit voltage (CCV) were measured as a function of lithium content, d , in Lid MnO2. The OCV curve was taken by electrochemical reduction with potential interval of 0.1 V, as shown in Fig. 8. The CCV was obtained from Fig. 4(b), the 40th discharge curve. The difference between the OCV and CCV is considerably small till d /0.33. It seems that the structural reordering to cycle-induced spinel phase after the 40th cycles is almost finished, so that the discharge behavior showed a small resistance in the range, being very similar to that of the conventional spinel compound. Interestingly, as the amount of Li ions more than d /0.33 is incorporated into the structure, the difference becomes relatively larger. The difference between OCV and CCV increases more, when d in Lid MnO2 exceeded 0.72.
Fig. 8. Open-circuit voltages as a function of Li content in Lid MnO2 prepared from electrochemical reduction of the 40th discharge process. OCVs as a function of Li contents in Lid MnO2 were measured by electrochemical reduction in the 40th discharge at 22.5 mA g 1 (C/ 12.9). Equilibrating time was more than 48 h so that the rate of OCVs change was less than 1 mV h 1. CCVs were obtained from Fig. 4(b).
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Ac-impedance measurements were employed to observe the Li intercalation reaction occurring in the interface between cathode and electrolyte. The impedance was taken by using the three points, d /0.33, 0.52 and 0.72 in Lid MnO2 marked by arrows in Fig. 8 at 30 8C. As can be seen in Fig. 9, the whole impedance of the semi-circle in the higher frequency region increases as lithium ions are intercalated into the structure. The chemical diffusivity of lithium ion was calculated from the fT at which a transition occurs from semi-infinite to finite diffusion behavior lengths as shown in typical impedance spectra of Fig. 9(b) [32 /37]. The value of the chemical diffusion coefficients, D˜ Li ; in the Lid MnO2 composite electrode was calculated by the following equation [33,35/37]; D˜ Li
pfT r2 1:94
(1)
Fig. 9. Nyquist impedance plots of Lid MnO2 as a function of lithium content; (o, d /0.33; I, d /0.52; and \, d /0.72) at 30 8C.
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where r means the radius of a particle. The average particle size of 0.3 mm in diameter was obtained from the TEM picture in Fig. 2(c). The diffusivities at d /0.33 and 0.52 in Lid MnO2 were calculated to be 1.8 /10 13 and 5.8 /10 14 cm s 1, respectively. In the inset plot, however, there is an obvious difference in the low frequency region. At d /0.72, especially, where the difference between OCV and CCV was relatively higher than at d /0.33 and 0.52 in Lid MnO2, the plot clearly showed another huge Rct component. For the case of d /0.72, we could not calculate the diffusion coefficient, because of no showing any diffusitive component. To examine the potential dependence of the structure, X-rays were radiated across the cycled electrodes. As shown in Fig. 10, cycle-induced cubic spinel was observed as a major phase at d /0.33 in Lid MnO2. Compared to the conventional spinel, the relative intensities of the electrochemically formed spinel phase is somewhat different. The broad widths and different intensities in XRD pattern are ascribed to disordering of the host structure. At d /0.52, tetragonal spinel phase began to appear at 2.9 V of the 40th cycle. The tetragonal phase was a little more intensified after 50 cycles. As described about the diffusivity at 4.0 and 2.9 V in Fig. 9, the difference in the structure may lead to the change in the diffusivity, because Li can diffuse through 8a /16c /8a after the phase transformation to spinel phase (d /0.33 in Lid MnO2). As Li intercalated more into the transformed structure (d /0.52), the 16c sites for Li diffusion passway are gradually occupied by Li, causing the decrease in Li diffusion coefficient. At d /0.52, furthermore, the existence of two phases which are composed by the cycle-induced cubic spinel and tetragonal one may hinder Li diffusion. Therefore, a significant difference in the diffusion coefficient would
Fig. 10. Ex-situ XRD patterns of o -LiMnO2 at various potentials. The samples were prepared from electrochemical reduction of the 40th discharge process: (a) d /0.33; (b) 0.52 in Lid MnO2 as shown with arrows in Fig. 8; and (c) 50 cycles. The XRDs were precisely taken with a step of 0.0038 s 1. (s, cycle-induced cubic spinel; t, tetragonal spinel; j, Ni-exmet; and Gr, graphite.)
Fig. 11. (a) TEM bright-field photo of o -LiMnO2; and (b) magnified image of the squared part on (a) after extensive cycling between 2.0 and 4.3 V Li=Li :/
be seen at the each potential (d /0.33 and 0.52 in Lid MnO2). The extensively cycled particles were directly observed by TEM. Fig. 11 shows a typical image of the cycled particles. Most of the particles observed gave the similar patterns. The cycled particle was severely damaged by cyclings. Comparing with Fig. 2(c) of which the edges of particles are well developed, it can be interpreted that the shapes of the edges and corners were degraded to zigzag shaped or even severely cracked (marked by arrows) by electrochemical shock. It is thought that the material would take much stress from the phase transformation. In general, the collective Jahn-Teller distortion is notorious for degradation of manganese oxide matrix. Nonetheless, the o -LiMnO2 material showed relative higher capacity on 3 /4 V region, especially the 3 V region where the Jahn-Teller effect exists. Recently, it was explored to elucidate the mechanism why cycle-induced spinel phase can show higher capacity on 3 V region. From the results of Wang
S.-T. Myung et al. / Electrochimica Acta 47 (2002) 3287 /3295
and Chiang et al. [20,21], it is concluded that ferroelastic nanodomains derived from structural changes on cycling played a role to keep the higher capacity, even though electrochemical cycling changed the particle shapes.
4. Conclusion In attempts to prepare o -LiMnO2, a high synthetic temperature leads to an increased loss of lithium by evaporation, which must be compensated for in the formulation of the reaction mixture with controlling oxygen partial pressure sensitively. The absolute necessity can be negligible by employing a new hydrothermal condition. Here, the hydrothermal condition using the Mn3O4, [MnII ]tet [MnIII 2 ]oct O4 ; as a hydrothermal precursor gave the highly ordered o -LiMnO2 single crystallite oxide. Its electrochemical properties are excellent comparing to high temperature calcined powders. At 45 mA g 1 of current density, the obtained capacities increase progressively with further cycling and stabilize after 100 cycles with showing a discharge capacity of about 133 mAh g 1. The capacity increased more with applying somewhat lower current density of 22.5 mA g 1. The new hydrothermal condition is greatly beneficial to prepare highly fine and a well-ordered single crystallite o -LiMnO2 oxide.
Acknowledgements The authors would like to thank Nobuko Kumagai and K. Kurihara, Iwate University, for helpful assistance in the experimental work. This study was supported NEDO of Japan, Yazaki Memorial Foundation for Science and Technology, and Saneyoshi Scholarship Foundation. S.T.M. personally acknowledges Yoneyama Studentship.
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