eletroquimica e pilha litio

eletroquimica e pilha litio

(Parte 1 de 2)

Journal of Power Sources 189 (2009) 620–623 Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: w.elsevier.com/locate/jpowsour

Short communication Electrochemical properties of LiMnO2 for lithium polymer battery

En Mei Jina,B oJ inb,a, Yeon-Su Jeona, Kyung-Hee Parka, Hal-Bon Gua,∗Department of Electrical Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of KoreaCollege of Materials Science and Engineering, Jilin University, Changchun 130025, China article info

Keywords: Quenching method LiMnO Solid polymer electrolyte Liquid electrolyte abstract

Well-defined o-LiMnO cathode materials were synthesized by quenching method at 1050 Ci na na rgon flow. The synthesized LiMnO particle was characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). LiMnO /solid polymer electrolyte (SPE)/Li batteries were characterized electrochemically by charge/discharge experiments, cyclic voltammetry (CV) and ac impedance spectroscopy. The charge/discharge results show that the discharge capacities of LiMnO are 62mAhg at the first cycle and 124mAhg after 70 cycles, respectively. Moreover, we evaluated batteries using liquid electrolyte and SPE. From the charge/discharge results, the discharge capacity of LiMnO /SPE/Li battery is greater than that of LiMnO /Li battery with liquid electrolyte. © 2008 Published by Elsevier B.V.

1. Introduction

Lithium polymer batteries have been utilized in a wide range of applications, such as cellular phones, notebooks, camcorders and digital cameras [1,2]. The successful commercialization of Liion gel polymer batteries for portable electronic devices has led to the other applications where the size and weight of batteries are important. A considerable investment in this battery technol- ogy that utilizes LiCoO2 cathodes has been made [3–8]. However, low-costcathodematerialsarerequiredformanyapplicationssuch as in electrical vehicles (EVs) and hybrid electric vehicles (HEVs) [9,10]. 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 4V region, but also exhibits a significant capacity loss in the 3–4V region as well as at high-temperature [1,12].I n

contrast to LiMn2O4, the trivalent manganese compounds LiMnO2 (both orthorhombic and monoclinic) exhibited a better cyclability even between 2 and 4.5V vs. Li+/Li [13,14]. Orthorhombic LiMnO2 (hereafterreferredtoaso-LiMnO2)shouldbethebestsubstitutefor spinel LiMn2O4. Meanwhile, it becomes clear that o-LiMnO2 is particularlyattractivebecauseofitspotentialtoofferahightheoretical

bic LiMnO2 can be synthesized by various methods such as solid-state method [4], hydrothermal method [17] and quenching method [18].

In this study, o-LiMnO2 particles were prepared by quenching method. The electrochemical properties of the as-synthesized o-

LiMnO2 for lithium ion batteries and lithium polymer batteries are presented.

2. Experimental

Orthorhombic LiMnO2 was prepared with the starting materi- als of LiOH·H2O (Aldrich, 9.95%) and Mn3O4 (Aldrich, 97%) by quenching method. The precursors were mixed. After the mixture was pelleted and heated at 1050◦C for 15h, the obtained sample was cooled by liquid state of nitrogen. The heating rate was

The crystalline phases of the obtained o-LiMnO2 powders were identified with X-ray diffraction (XRD, Dmax/1200, Rigaku). The

XRD pattern was collected by a step-scanning mode in the range of 10–80◦ withas tept imeo f5◦ min−1. Powder morphologies were observed by FE-SEM.

The composite electrodes were prepared by mixing o-LiMnO2, acetyleneblack(AB)andpolyvinylidenefluoride(PVDF)binderdis- solved in N-mehtylpyrrolidinone in different weight ratios. The obtained slurry was ball-milled for 1h, and coated onto an Alfoil, and dried at 90◦C for 1h. The resulting electrode films were pressed with a twin roller, cut into a round plate (=15.958mm), and dried at 110◦C for 24h under vacuum. A lithium foil was used

as an anode. Liquid electrolyte was 1M LiPF6 dissolved in ethy- lene carbonate (EC)/dimethyl carbonate (DMC) (EC:DMC=1:1).

25PVDFLiClO4 EC10PC10 was used as SPE and its synthesis was described in detail previously [19]. The coin-type cells (CR2032) were fabricated for the electrochemical tests. LiMnO2/SPE/Li bat- teries and LiMnO2/Li batteries were fabricated in an argon-filled glove box. The charge/discharge testing was performed using auto- matic charge/discharge equipment (WBCS3000, WonATech Co.) in a potential range of 2.0–4.3V at a constant current density of 0.2mAcm−2 at 25◦C.

Electrochemical impedance measurements were performed using an IM6 impedance system (Zahner Elektrik Co.). The spectrum was potentiostatically measured by applying an ac voltage of 10mV over the frequency range from 10 to 1MHz. The WBCS3000 (WonATech Co.) Battery Tester System was also used for the measurements of cyclic voltammetry in a potential range of 2.0–4.3V at a scan rate of 0.1mVs−1.

3. Results and discussion 3.1. Crystal structure

The XRD pattern of o-LiMnO2 powders is shown in Fig. 1. The

XRDpatternofthecompoundo-LiMnO2 canbeindexedtoasinglephase material having an orthorhombic structure, which is the same as the works of Myung et al. [20].

3.2. Morphology analysis

The FE-SEM image of o-LiMnO2 powders is shown in Fig. 2.A s can be seen from Fig. 2, the average particle size of o-LiMnO2 powders is around 1.5 m in the length and 0.5 m in the width. The morphologyofo-LiMnO2 isbar-type,whichisthetypicalcrystallite pattern [21,2].

3.3. Charge/discharge properties

The cycling performance of LiMnO2/SPE/Li batteries with dif- ferent electrode combination ratios (LiMnO2:AB:PVDF=85:10:5, 80:15:5 and 75:20:5) is shown in Fig. 3. The batteries were cycled between 2.0 and 4.3V at a current density of 0.2mAcm−2.A sc an beseenfromFig.3,theinitialdischargecapacitiesofLiMnO2/SPE/Li batteries with electrode combination ratio of 75:20:5, 80:15:5 and

Fig. 1. The XRD pattern of the o-LiMnO powders.

Fig. 2. The FE-SEM image of prepared o-LiMnO powders by quenching method.

85:10:5are52,62and54mAhg−1,respectively.Themaximumdischargecapacitiesare76,124and104mAhg−1 atthe40th,50thand 60th cycle, respectively. It is demonstrated that the cycling perfor- mance of LiMnO2/SPE/Li cell with electrode combination ratio of 80:15:5 is better than that of other batteries. Moreover, we evalu- ated liquid electrolyte and solid polymer electrolyte. Fig. 4 shows the cycling performance of LiMnO2/SPE/Li battery and LiMnO2/Li battery with liquid electrolyte with electrode combination ratio of 80:15:5. As can be seen from Fig. 4, the discharge capacities of LiMnO2/SPE/Li battery are 62mAhg−1 at the first cycle and 124mAhg−1 after 70 cycles, respectively. The cycling performance of LiMnO2/SPE/Li battery is better than that of LiMnO2/Li battery with liquid electrolyte.

3.4. Cyclic voltammetry

The obtained o-LiMnO2 sample heated at 1050◦C was exam- ined by cyclic voltammetry. The LiMnO2/SPE/Li battery was tested between 2.0 and 4.3V at a scanning rate of 0.1mVs−1. The cyclic voltammetry can also be used to reveal the reactions occuring during the transformation. Fig. 5 shows the cyclic voltammograms. As

Fig. 3. The cycling performance of LiMnO /SPE/Li batteries with different electrode combination ratios.

Fig. 4. The cycling performance of LiMnO /Li battery with liquid electrolyte and LiMnO /SPE/Li battery with electrode combination ratio of 80:15:5.

shown from Fig. 5, there are one distinct oxidation peak at 3.2V, and three small reduction peaks around at 4.0V. The characteris- tic redox peaks of spinel LiMn2O4 at around 3.95 and 4.10V are observed in the plot. The height of the redox peaks around 3.1 and 4V increase with cycling, indicating a progressive formation of cycle induced spinel phase, and become saturated after cycles [23]. These results are in consistent with those shown in Fig. 4, though capacity saturation is found at higher cycle number than that obtained from capacity retention study.

3.5. Impedance properties

Impedance spectra of LiMnO2/SPE/Li upon cycling are shown in Fig. 6. It is noted that the ac impedance response of the cell forms a broad semicircle and a line to the real axis in the lowest frequency range. The semicircle in the high frequency is mainly related to

Fig. 5. The cyclic voltammetry of LiMnO /SPE/Li in a potential range of 2.0 and 4.3V at a scanning rate of 0.1mVs at 25 C.

Fig. 6. The ac impedance spectra of LiMnO /SPE/Li upon cycling.

the complex reaction process at the electrolyte/cathode interface including resistance of SEI film formed on the surface o-LiMnO2 particles,theparticle-to-particlecontactresistance,chargetransfer resistance,andcorrespondingcapacitances.Theinclinedlineinthe lower frequency is attributed to the Warburg impedance, which is associated with lithium-ion diffusion in o-LiMnO2 electrode. As can be seen from Fig. 6, the resistances are 150 at the first cycle,

120 after 2 cycles, 110 after 10 cycles, respectively. It is obvious that LiMnO2/SPE/Li battery resistance slacks up upon cycling. This result is consistent with the cycling performance of LiMnO2/SPE/Li battery with electrode combination ratio of 80:15:5 (as shown in

4. Conclusions

Orthorhombic LiMnO2 cathode materials were prepared by quenching reaction. The prepared o-LiMnO2 can be indexed to a single-phase material having the orthorhombic structure.

Among LiMnO2/SPE/Li batteries with different electrode combination ratios (LiMnO2:AB:PVDF=85:10:5, 80:15:5 and 75:20:5), LiMnO2/SPE/Li battery with electrode combination ratio of 80:15:5 is the best. The cycling performance of LiMnO2/SPE/Li battery is better than that of LiMnO2/Li battery with liquid electrolyte. The initial discharge capacity of LiMnO2/SPE/Li battery is 62mAhg−1 and the maximum discharge capacity is 124mAhg−1 at the 70th cycle.

Acknowledgment

This research project received supporting funds from the second-stage Brain Korea 21.

References

[4] Y.J. Wei, H. Ehrenberg, N.N. Bramnik, K. Nikolowski, C. Baehtz, H. Fuess, Solid

[7] M. Park, S. Hyun, S. Nam, S. Cho, Electrochim. Acta 53 (2008) 5523.

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