Hydrothermal Synthesis of Nanosized LiMnO2 ...

Journal of The Electrochemical Society, 156 共3兲 A162-A168 共2009兲

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0013-4651/2009/156共3兲/A162/7/$23.00 © The Electrochemical Society

Hydrothermal Synthesis of Nanosized LiMnO2–Li2MnO3 Compounds and Their Electrochemical Performances Xingkang Huang,a,b,z Qingshun Zhang,a Haitao Chang,a Jianlong Gan,a Hongjun Yue,b and Yong Yangb,*,z a

Fujian Nanping Nanfu Battery Company, Limited, Nanping 353000, China State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

b

Nanosized LiMnO2–Li2MnO3 compounds were synthesized by a hydrothermal method. The contents of Li2MnO3 and LiMnO2 in these compounds vary with the employed molar ratio of starting materials 关i.e., 共NH4兲2S2O8 /MnSO4兴. The effects of the reaction time and temperature on the hydrothermal products were investigated. The hydrothermally prepared LiMnO2–Li2MnO3 compounds have higher electrochemical activities compared to those obtained from conventional solid-state reaction at high temperature 关e.g., during the initial discharging process at 20 mA g−1, o-LiMnO2 共sample LMO-1兲 and Li2MnO3 共sample LMO-4兲 delivered at 184 and 247 mAh g−1, respectively兴. Meanwhile, such nanosized compounds exhibited good rate capabilities 共e.g., at a current density of 200 mA g−1, sample LMO-4 delivered a capacity of 208 mAh g−1, 84% of that obtained at 20 mA g−1兲. Comparison of electrochemical performances among the four nanosized compounds obtained from our hydrothermal conditions indicates that the Li2MnO3 has the higher discharge capacity while the o-LiMnO2 shows the better cyclability. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3054397兴 All rights reserved. Manuscript submitted September 23, 2008; revised manuscript received November 20, 2008. Published January 8, 2009.

Lithium manganese oxides are of great interest as cathode materials for lithium-ion batteries for their low cost, nontoxicity, and as a safety issue under abuse condition when compared to the commercial LiCoO2 cathode.1,2 Orthorhombic lithium manganese oxide 共o-LiMnO2兲 has an ordered rocksalt structure, where LiO6 and MnO6 octahedra are arranged in corrugated layers.2 The capacity of o-LiMnO2 depends strongly on its synthetic method and conditions.3-10 Generally, o-LiMnO2 from the solid-state reaction at high temperature has a very low initial capacity but can increase its capacity upon electrochemical cycling.9-11 In contrast, those prepared from low-temperature methods can deliver high initial capacities.3,12 Rocksalt phase Li2MnO3 has an ordered structure, where layers of lithium ion alternate with layers of lithium and manganese 共IV兲 ions,13 and is generally believed to be electrochemically inert.14 In fact, a Li2MnO3 prepared by a solid-state reaction delivered only ca. 3 mAh g−1.15 Li2MnO3 was first reported as an electrochemically active electrode material by suggesting removal of lithium ion accompanied with oxidation of Mn4+.16 However, delithiation in a form of Li2O is proposed and discussed in the subsequent reports concerning the activation mechanism of Li2MnO3.14,17-19 Besides, Robertson and Bruce suggested a proton-exchange mechanism for the activation of Li2MnO3, especially at high temperature, where Li+ exchanged by proton, which resulted from the decomposition of LiPF6 electrolyte or the oxidation of solvent such as ethylene carbonate.15,20 Forming composite phase of Li2MnO3 with other polymorphs of lithium metal oxides is also of great interest. For example, Li2MnO3–LiMO2 共M = Mn, Co, Ni, Cr, etc.兲 composite electrode materials were reported where the Li2MnO3 in the composite can stabilize the monoclinic structure of LiMO2 upon cycling;14,17,19,21 composite phase of Li2MnO3 with spinel LiMn2O4 was also synthesized.22 Among those reporting Li2MnO3 and the related materials, most of them were prepared at high temperature such as by solid-state reaction,20,23-25 and by annealing the precursors from sol-gel26,27 or hydrothermal treatment.28-31 To date, few investigations are concerned about hydrothermal synthesis of Li2MnO3 as cathode materials for lithium-ion batteries. Tabuchi et al.28-31 employed the hydrothermal method as an intermediate procedure to obtain the precursors for the final annealed products of Fe-substituted Li2MnO3, where their hydrother-

* Electrochemical Society Active Member. z

E-mail: [email protected]; [email protected]

mal products have ␣-NaFeO2 type structure 共R3m兲. Very recently, we reported a hydrothermally synthesized Li2MnO3 material.32 In addition, from the above review on the composite of Li2MnO3 with other polymorphs 共e.g., spinel LiMn2O4 and monoclinic LiMnO2兲,14,17,19,22 it can be concluded that efforts for searching cathode materials should be devoted to obtaining a manganese oxide with high performance instead of synthesizing any single-phase oxide. Consequently, we focus our effort in this study on hydrothermally synthesizing Li2MnO3 and LiMnO2–Li2MnO3 composite compounds and investigating their electrochemical performances.

Experimental LiOH·H2O 共3.27 g兲 was dissolved in 30 mL deionized water and added dropwise into a Teflon-lined stainless steel autoclave containing 20 mL of 0.01 mol MnSO4 and an appropriate amount of 共NH4兲2S2O8. The autoclave was then sealed and heated, typically, at 180°C for 48 h. The obtained precipitate was filtered, washed, and finally dried at 120°C for 24 h. The as-prepared samples were denoted as LMO-1, LMO-2, LMO-3, and LMO-4 for the usages of 1.14, 1.71, 2.05, and 2.28 g 共NH4兲2S2O8, corresponding to the molar ratios of 共NH4兲2S2O8 /MnSO4 as 0.5:1, 0.75:1, 0.9:1, and 1:1, respectively. Powder X-ray diffraction 共XRD兲 was performed on a PANalytical X’Pert diffractometer with Cu K␣ radiation 共Philips兲 by a normal scanning mode 共0.0167 °/step, 15 s/step兲 or a refining scanning mode 共0.008 °/step, 25 s/step兲; most samples were characterized by the normal scanning mode, unless otherwise stated. Scanning electron microscopy 共SEM兲 studies were performed on LEO1530 to observe morphology of samples. Electrode fabrication and coin-cell assembly were carried out as described in our previous report.33 First, the active material was mixed with 10 wt % acetylene black and 10 wt % binder 关poly共vinylidene fluoride兲兴, and then ground by ballmilling. The mixture was pressed to aluminum foil and thereafter dried at 120°C under vacuum for 2 h to obtain the cathode. The typical loading in the electrodes is ca. 2–3 mg cm−2. Coin cells were fabricated with the prepared cathode, lithium anode, Celgard 2400 polypropylene separator, and 1 M LPF6 in ethylene carbonate/ dimethyl carbonate 共1:1 v/v兲 electrolyte. The cell test was carried out at current densities of 20 or 200 mA g−1 between 4.8 and 2.0 V by an Arbin BT-2043 battery test system at 30°C. Cyclic voltammogram 共CV兲 was recorded on an AutoLab 共PGSTAT30兲 electrochemical workstation.

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Figure 1. XRD patterns of samples 共a兲 LMO-1, 共b兲 LMO-2, 共c兲 LMO-3, and 共d兲 LMO-4. Note that the patterns were obtained by the refining scanning mode.

Results and Discussion Preparation of LiMnO2–Li2MnO3 compounds.— The formation of stoichiometric LiMnO2 and Li2MnO3 needs 0.5:1 and 1:1 of 共NH4兲2S2O8 /MnSO4 共in mole兲 as starting materials, according to 2MnSO4 + 共NH4兲2S2O8 + 8LiOH → 2LiMnO2 + 共NH4兲2SO4 关1兴

+ 3Li2SO4 + 4H2O MnSO4 + 共NH4兲2S2O8 + 6LiOH → Li2MnO3 + 共NH4兲2SO4 + 2Li2SO4 + 3H2O

关2兴

When 0.5:1 of 共NH4兲2S2O8 /MnSO4 was employed, the obtained product is o-LiMnO2 as a major phase, with possible existence of Mn3O4, LiMn2O4, and Li2MnO3 as minor polymorphs 共Fig. 1a兲. Formation of impurity in the case of sample LMO-1 possibly resulted from disproportionation of trivalent of manganese. In contrast, Komaba et al. prepared o-LiMnO2 by hydrothermal treatment of Mn3O4 according to disproportionation mechanism where they observed an increase of Mn dissolution with LiOH concentration.8 Hence, increasing LiOH concentration may help to obtain a pure phase of o-LiMnO2; this experiment is not carried out because it is not the main purpose of this study. As a matter of fact, increase of 共NH4兲2S2O8 eliminated the existence of Mn3O4 and formed

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o-LiMnO2–Li2MnO3 composites with various component contents 关e.g., obtaining samples LMO-2 and LMO-3 corresponding to the usages of 共NH4兲2S2O8 /MnSO4 of 0.75:1 and 0.9:1, respectively 共Fig. 1b and c兲兴. The contents of the Li2MnO3 phase in samples LMO-2 and LMO-3 are 67 and 90% 共in mole兲 in terms of the mean oxidation states of Mn, and Li and Mn contents in the samples 共Table I兲. In the case of using 1:1 of 共NH4兲2S2O8 /MnSO4, the obtained product was almost a pure Li2MnO3 phase 共Fig. 1d兲. The two peaks at ca. 20.8 and 21.7 ° due to superlattice structure, indicate that the Li2MnO3 is well crystallized and is comparable to those prepared by solid-state reaction at high temperature.20,23-25 Despite the Li2MnO3 phase for sample LMO-4, it is indeed a proton-substituted Li2MnO3 material 共i.e., Li2−xHxMnO3兲. Actually, a Li1.59H0.41MnO3 was synthesized by hydrothermally treating an amorphous MnOOH precursor, which was discussed in detail in our recent report.32 The formation of the proton-substituted Li2MnO3 is related to a dissolution-recrystallization mechanism,32,34 where the precursor dissolves first, forming Mn关共OH兲6兴3−, and then was oxidized into Mn关共OH兲6兴2−, which is the crystal growth unit for the Li2MnO3 phase. During the recrystallizing process, some of the protons partially take the place of Li+ in Li atom layer accompanied with replacement of 1/3 Mn by Li ions in Mn atom layer, resulting in the proton-substituted phase. Proton-substituted Li2MnO3 materials were investigated by acid-leaching at ambient or hydrothermal conditions,35-40 where the acid-leached products, compared to our hydrothermally synthesized proton-substituted Li2−xHxMnO3, show difference in structure, formation mechanism, and electrochemical performance as discussed in detail in our recent report.32 The four as-prepared samples have a similar thermal behavior to the previously reported Li1.59H0.41MnO3, releasing water upon heating over the temperature range of 200–400°C. Therefore, these hydrothermal products are also believed to consist of proton-substituted phases. Suppose that the four samples consist of o-LiMnO2 and Li2MnO3 共at least one component兲, we also calculated their formulas with proton-subsitituted phases 共Table I兲. Effect of reaction time on the formation of LiMnO2–Li2MnO3 compounds.— Figures 2-4 recorded polymorphic transformation processes during hydrothermal growth of samples LMO-1, LMO-2, and LMO-4, respectively. Before hydrothermal treatment, the asprecipitated product is MnOOH for usage of 共NH4兲2S2O8 /MnSO4 of 0.5:1. The precipitate after hydrothermally treating for 1 h at 180°C, transformed into a spinel phase 共Fig. 2b兲; the XRD peaks at 37 and 44 °, compared to the LiMn2O4 of JCPDS no. 89-108, shifted to the higher 2␪, which is possibly due to proton substitution for lithium ion in the spinel structure. The spinel phase was confirmed by CV scanning, where there are two pairs of redox peaks at ⬃4 V as characteristic peaks of a spinel 共not shown兲. After 6 h, some new peaks appeared, indicating formation of o-LiMnO2; the o-LiMnO2 content in the samples increased further with the hydrothermal reaction time. However, pure phase of o-LiMnO2 was not obtained at the given synthetic conditions even if we prolonged the reaction time to eight days. In the case of 共NH4兲2S2O8 /MnSO4 = 0.75:1, the sample obtained at room temperature is a mixture of MnOOH and layered

Table I. Compositions of the as-prepared samples LMO-1, LMO-2, LMO-3, and LMO-4.

a

Sample

Mn%

Li%

Li/Mn

Mean oxidation state

Composition

Composition with H

LMO-1 LMO-2 LMO-3 LMO-4

56.58 49.45 46.89 45.2

4.38 7.53 7.89 8.90

0.606 1.19 1.32 1.54

3.16 3.67 3.90 4.08

Li0.606MnO1.88 Li1.19MnO2.43 Li1.32MnO2.61 Li1.54MnO2.27

H0.556Li0.606MnO2.158a H0.48Li1.19MnO2.67 H0.58Li1.32MnO2.9 H0.46Li1.54MnO3

The composition with proton-substituted phase of LMO-1 was calculated on the base supposing it consists of the two phases of LiMnO2 and Li2MnO3.


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Figure 2. XRD patterns for tracing the hydrothermal growth process of sample LMO-1: 共a兲 0, 共b兲 1, 共c兲 3, 共d兲 6, 共e兲 12, 共f兲 24, and 共g兲 48 h; hydrothermal temperature: 180°C.

MnO2 共Fig. 3a兲, which was subsequently converted to a spinel phase after 1 h hydrothermal treatment. At 3 h, o-LiMnO2 and Li2MnO3 appeared and both grew further with hydrothermal reaction time 共Fig. 3兲.



For sample LMO-4, its precursor obtained at room temperature is a poorly crystallized layered MnO2 共Fig. 4兲. After 1 h hydrothermal treatment, like the above-mentioned two samples, the precursor transformed quickly into the spinel. The 3 h reaction made us observe the formation of Li2MnO3, particularly indicated by the XRD peak at 2␪ = 46 °. After hydrothermally treating the precursor for 12 h, all the XRD peaks of the product can be indexed to Li2MnO3, except that at 37 ° which is uncertainly assigned to any phase at this stage; this peak is likely to result from a proton-substituted polymorph or an impurity phase such as a spinel. We took sample LMO-4 as an example to show the morphology variation along with reaction time 共Fig. 5兲. The as-precipitated sample at room temperature shows a petal-like surface 共Fig. 5a兲 that disappeared and turned into small particles of ca. 25 nm after 1 h hydrothermal treatment 共Fig. 5b兲. These particles subsequently grew eventually to ca. 40 nm at 6 h and then ca. 85 nm at 48 h. The morphological variations of the other samples 共LMO-1, LMO-2, and LMO-3兲 during hydrothermal growth processes are similar to that of sample LMO-4 and are herein not shown. The morphology changes of these hydrothermal products confirm the dissolutionrecrystallization mechanism for hydrothermal growth of these LiMnO2–Li2MnO3 compounds. Effect of reaction temperature on formation of LiMnO2– Li2MnO3 compounds.— In the case of 共NH4兲2S2O8 /MnSO4⫽0.5:1, the hydrothermal product at 120°C for 48 h, is a spinel as major phase and Mn3O4 as a minor polymorph 共Fig. 6a兲, which is comparable to the two samples obtained at 180°C for 1 and 2 h 共Fig. 2b and c兲. Meanwhile, a small amount of o-LiMnO2 is also likely to exit in the sample obtained at 120 °C as suggested by the XRD peak at 44.2 °. The formation of Mn3O4 and spinel phase is attributed to disproportionation reaction of MnOOH. In contrast, at higher hydrothermal temperatures 共e.g., 150 and 180°C兲, the obtained samples are o-LiMnO2 共major polymorph兲 while containing Mn3O4 and spinel as minor phases 共Fig. 6兲. In addition, the sample formed at 180°C shows better crystallinity compared to that at 150°C.



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Figure 7. XRD patterns of samples obtained at 共a兲 120, 共b兲 150, and 共c兲 180°C in the case preparing sample LMO-2; hydrothermal time: 48 h.

Figure 5. Typical SEM images of samples for tracing the hydrothermal growth process of sample LMO-4: 共a兲 0, 共b兲 1, 共c兲 6, and 共d兲 48 h; hydrothermal temperature: 180°C.

In the case of 共NH4兲2S2O8 /MnSO4 = 0.75:1, the obtained sample is also the spinel as major at 120°C and does not contain Li2MnO3 共Fig. 7a兲. When the temperature increased to 150°C, the formation of Li2MnO3 was observed 共Fig. 7b兲. Further increasing the temperature to 180°C resulted in the better crystallinity of the obtained sample 共Fig. 7c兲. Unlike formation of spinel phases in the above-mentioned two cases with low 共NH4兲2S2O8 /MnSO4, an almost pure phase of Li2MnO3 was observed at 120°C when 共NH4兲2S2O8 /MnSO4 = 1:1 was employed 共Fig. 8兲. Meanwhile, comparison among the three samples obtained at 120–180°C indicates the better cystallinity at the higher temperature.


As a result, higher temperature is believed to benefit formation of o-LiMnO2 and Li2MnO3 as well as their cystallinity. This is in good agreement with the dissolution-recrystallization mechanism for growth of these LiMnO2–Li2MnO3 compounds, for higher temperature promotes manganese oxides to dissolve in LiOH solution. Electrochemical performance of LiMnO2–Li2MnO3 compounds.— A comparison of the initial charge/discharge behaviors among the four as-prepared samples is shown in Fig. 9a, suggesting that sample LMO-1 had a smaller polarization upon charging, while sample LMO-4 rose its voltage quickly up to above 4.4 V due to Li+ extraction accompanied by release of oxygen 共i.e., loss of Li2O in notation兲.15,17,26 A 4 V plateau was observed on the initial discharging process, which typically belongs to Li+ insetting to a tetrahedral site of the spinel structure. This plateau progressed gradually upon cycling 共Fig. 10a兲 due to o-LiMnO2 developing to spinel structure as indicated by its characteristic of two pairs of peaks at ⬃4 V on the differential capacity plot 共Fig. 10a inset兲. Such a behavior of sample LMO-1 is in good agreement with those of the previously reported o-LiMnO2.8,41,42 Our sample LMO-1 has a higher initial discharge capacity compared to those o-LiMnO2 prepared by solidstate reaction at high temperature11,41,42 关e.g., a LiMnO2 obtained by



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Figure 9. 共Color online兲 Comparison of 共a兲 charge/discharge curves and 共b兲 differentiation capacity plots between the four as-prepared samples.

annealing mixture of LiNO3 and Mn共NO3兲2 at 900°C for 12 h, delivered an initial capacity of 25 mAh g−1 at a current density of 40 mA g−1兴.42 The smaller particle size is likely to be responsible for the higher electrochemical activity of sample LMO-1 in the initial discharge process because its mean particle size is ca. 85 nm 共Fig. 5d兲, whereas conventional LiMnO2 from the solid-state reaction at high temperature is commonly sized in microns.41,43 Actually, Lee and Yoshio43 prepared o-LiMnO2 by calcining LiOH and MnOOH at 1000°C for 10 h, and compared its charge/discharge behaviors with that of the grinded sample from the as-prepared o-LiMnO2. Their results show that the initial discharge capacity of the grinded sample is remarkably higher than that of the as-prepared o-LiMnO2, which is related to their mean particle sizes of 5–15 共before grinding兲 and 0.5–3 ␮m 共after grinding兲. In addition, Myung et al. reported that around 30 cycles needed to achieve the highest capacity for a hydrothermally synthesized o-LiMnO2 with 100–500 nm in size when cycled at 45 mA g−1.44 By contrast, our o-LiMnO2 共sample LMO-1兲 with the size of ca. 85 nm 共Fig. 5d兲 was activated after several cycles at 200 mA g−1 共Fig. 11兲. The difference in particle sizes between Myung’s sample and ours is likely to result from the different reaction conditions and subsequent hydrothermal parameters.

Sample LMO-2, from the case of 0.75:1 of 共NH4兲2S2O8 /MnSO4, exhibited an additional plateau above ca. 4.4 V on its charging curve 共Fig. 10b兲; meanwhile, on its corresponding differential capacity plot 共Fig. 10b inset兲, there are two peaks at 4.4–4.8 V, indicating activation of Li2MnO3 in the sample by the delithiation in a form of Li2O or the proton exchange mechanism. Further increasing usage of 共NH4兲2S2O8 /MnSO4 to 0.9:1 共sample LMO-3兲 resulted in higher Li2MnO3 content in the sample, and its electrochemical behavior is more similar to that of pure Li2MnO3 as suggested in Fig. 10c and d, where they have no marked discharging plateau at 4 V and show high capacities at a high current density of 200 mA g−1 共i.e., 163 and 208 mAh g−1 for samples LMO-3 and LMO-4, respectively兲. Note that the initial charging curve of sample LMO-4 is significantly different with that of the subsequent charging processes 关i.e., its initial charging voltage rose quickly up to ca. 4.5 V and then increased very slowly, while the subsequent charging voltage increased slowly from 3.0 to 4.8 V 共Fig. 10d兲兴. These phenomena suggest that the activation of the Li2MnO3 electrode should not be explained as a possible oxidation process of Mn4+ to Mn5+ as mentioned in Ref. 16.

Figure 10. 共Color online兲 Charge/ discharge curves of samples 共a兲 LMO-1, 共b兲 LMO-2, 共c兲 LMO-3, and 共d兲 LMO-4 at a current density of 200 mA g−1 between 4.8 and 2.0 V. Insets show the corresponding plots of capacity differentiation vs voltage.



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Figure 11. Cyclic performances of the four LiMnO2–Li2MnO3 compounds.

A comparison of cyclic performance at 200 mA g−1 between the four LiMnO2–Li2MnO3 compounds was shown in Fig. 11. Li2MnO3 共sample LMO-4兲 had high initial discharge capacity 共208 mAh g−1兲, but several cycles later it began to decay quickly to 116 mAh g−1 共cycle 24兲. By contrast, o-LiMnO2 共sample LMO-1兲 had lower initial capacity 共114 mAh g−1兲 but progressed to ca. 140 mAh g−1 and then cycled stably. It is well known that o-LiMnO2 would develop to spinel-like phase upon cycling, which is believed to be more tolerant to cycling than conventional LiMn2O4.8 The electrochemical behaviors of the other two samples 共LMO-2 and LMO-3兲 are in good agreement with the above tendency 共i.e., the higher the content of Li2MnO3 in the sample is, the higher capacity while the faster decay and vice versa兲. Accordingly, one can choose an electrode material integrating both accepted high capacity and cyclability 共e.g., sample LMO-2 delivered 174 mAh g−1 and then declined slowly to a maintainable capacity of ca. 137 mAh g−1兲. The LiMnO2–Li2MnO3 cells were also cycled at a low current density such as 20 mA g−1, to compare the capacities obtained at the high current density of 200 mA g−1, which indicates their rate capabilities. As shown in Fig. 12, samples LMO-1, LMO-2, LMO-3, and LMO-4 delivered 184, 188, 226, and 247 mAh g−1, respectively, which are comparable to those reported composites of Li2MnO3 with monoclinic LiMnO2 or LiMn2O4.14,21,22,26 Accordingly, at 200 mA g−1 the four LiMnO2–Li2MnO3 compounds showed around 77–92% of capacity at 20 mA g−1, indicating their good rate capabilities. This may be related to their nanometer sizes, which benefit Li+ inserting and extracting quickly upon charging and discharging. In addition, their discharge capacities at the second cycle are close to the initial capacities 共Fig. 12兲, indicating they were almost activated completely during the initial charging process. Compared to those Li2MnO3 共from solid-state reaction兲 having good cyclic performance, our Li2MnO3 decayed quickly after several cycles 共Fig. 11兲. The reason is unclear at this stage and still under investigation; however, more serious Mn dissolution due to the proton-substituted phase and nanosize of our samples is likely to take charge of the quick degeneration of our Li2MnO3 material. Incorporating a coating layer may help to improve the cyclic performance of the hydrothermal products. In addition, forming composites by doping metal 共Co, Ni, Fe, Al, etc.兲 ions is also likely to benefit the cycleablilty of the hydrothermal compounds and is of great interest to be investigated. Conclusion Nanosized LiMnO2–Li2MnO3 compounds were hydrothermally synthesized. The effects of the reaction time and temperature on the hydrothermal products were investigated. The precursors obtained

Figure 12. 共Color online兲 Charge/discharge curves of the four LiMnO2–Li2MnO3 compounds at 20 mA g−1 during the first 共bottom兲 and second 共above兲 cycles.

from reaction of 共NH4兲2S2O8, MnSO4, and LiOH at room temperature, hydrothermally transformed into the spinel phase within 1 h. The spinels subsequently converted to o-LiMnO2, Li2MnO3, or their composites along with the reaction time. Higher reaction temperature benefits formations of both o-LiMnO2 and Li2MnO3, and their crystallinities as well. These nanosized LiMnO2–Li2MnO3 compounds exhibited high capacities and good rate capabilities 共e.g., sample LMO-4 delivered 247 and 208 mAh g−1 at the current densities of 20 and 200 mA g−1, respectively兲. Comparing the four hydrothermal compounds obtained under our given conditions indicates that Li2MnO3 has the higher discharge capacity while o-LiMnO2 shows the better cyclability. Integrating the advantages of Li2MnO3 and o-LiMnO2 may help to find a promising cathode material for lithium-ion batteries. Acknowledgments Y.Y. thanks the financial support from the National Basic Research Program of China 共973 Program兲共grant no. 2007CB209702兲, and the National Natural Science Foundation of China Grants 共no. 90606015, no. 20473060, no. 29925310, and no. 20021002兲. Fujian Nanping Nanfu Battery Company assisted in meeting the publication costs of this article.

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