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Electrochimica Acta 71 (2012) 17– 21

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

l2O3 coating on LiMn2O4 by electrostatic attraction forces and its effects on theigh temperature cyclic performance

on-Keun Kim, Dong-Wook Han, Won-Hee Ryu, Sung-Jin Lim, Hyuk-Sang Kwon ∗

epartment of Materials Science & Engineering, Korea Advanced Institute of Science & Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 8 December 2011eceived in revised form 5 March 2012ccepted 10 March 2012

a b s t r a c t

Bare LiMn2O4 was coated by a thin Al2O3 layer using electrostatic attraction forces to investigate theeffects of the Al2O3 coating on the high-temperature cyclic performance of LiMn2O4. Transmission elec-tron microscopy (TEM) studies indicated that 5-nm thin and amorphous Al2O3 layer formed uniformlyon the surface of 2 wt.% Al2O3 coated LiMn2O4. The high-temperature cyclic performance of the LiMn2O4

vailable online 28 March 2012

eywords:iMn2O4

l2O3 coatingn dissolution

was significantly improved by the coating. From the Electrochemical Impedance Spectroscopy (EIS) andX-ray Fluorescence (XRF) analysis, the coating reduced the charge transfer resistance and inhibited theMn dissolution. The improved electrochemical performance of the Al2O3 coated LiMn2O4 was attributedto the decrease in the Mn dissolution by the coating.

© 2012 Elsevier Ltd. All rights reserved.

i-ion battery

. Introduction

Recently, lithium rechargeable batteries have become attrac-ive power sources for electric vehicles (EV) and hybrid electricehicles (HEV) due to their high energy density. Many studies ofithium rechargeable batteries have been performed in attempts toevelop a nontoxic cathode material with a high power density andxcellent thermal stability. In this regard, LiMn2O4 with a spineltructure is one of the most promising cathode materials due to itsood intrinsic characteristics, such as its proper Mn4+/Mn3+ redoxotential (4 V vs. Li+/Li), low cost, environmental friendliness, andigh thermal stability when under severe environmental condi-ions [1–6]. However, the material exhibits poor capacity retentiont high temperatures, due primarily to the Mn dissolution thatccurs by the corrosive reaction between LiMn2O4 and electrolyteuring cycling. In addition, the structural instability caused by the

ahn Teller effect reduces practical capacity (≤120 mAh g−1) as wells capacity retention at high temperatures [7–12].

To overcome the aforementioned problems, many researchersave attempted to improve the high temperature cyclic perfor-ance by forming a coating layer that would inhibit the Mn

issolution of LiMn2O4 [13–20]. Han et al. [21] reported that ZnO

oated Li1.05Al0.1Mn1.85O3.95F0.05 exhibited a better cyclic perfor-ance than it did without the coating due to the suppression

f the Mn dissolution. Tu et al. [22] reported that Al2O3 coated

∗ Corresponding author. Tel.: +82 42 350 3326; fax: +82 42 350 3310.E-mail address: hskwon@kaist.ac.kr (H.-S. Kwon).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.03.090

LiMn2O4 through a melting impregnation method exhibited goodelectrochemical properties. However, the improvement in the hightemperature cyclic performance was limited due to non-uniformityand thickness of coating layer.

It is the research objective to examine the effects of thin anduniform Al2O3 coating on the high temperature cyclic performanceof LiMn2O4, in which the Al2O3 coating on LiMn2O4 was done bythe electrostatic attraction forces between them.

2. Experimental

2.1. Material preparation and characterization

Bare LiMn2O4 was prepared by a solution-based process usingLi2CO3 (Aldrich, 99%) and MnO2 (Kojundo, 99%) as the startingmaterials. Stoichiometric amounts of the precursors were stirredand mixed for 5 h in ethanol and then dried at 100 ◦C for 6 hto vaporize the ethanol solvent. The resultant mixture was thenreground by mortar and pestle. The obtained precursor mixturewas pressed into a pellet and then annealed at 900 ◦C for 12 h inan O2 gas flow for synthesis of LiMn2O4. As shown in Fig. 1, theLiMn2O4 was coated with Al2O3 using the electrostatic attractionforces between them. The point of zero charge of the synthesizedLiMn2O4 and Al2O3 particles were measured to be 4.3 and 9.2,respectively. The Al2O3 suspension was prepared by adding 2 ml

NH4OH (Aldrich, 35 wt.%) to an 0.4 mM, 200 ml Al-acetate solutionfor 2 wt.% Al2O3 coating and 1 mM, 200 ml Al-acetate solution for5 wt.% Al2O3 coating. The pH of the suspension was adjusted to6.7, which is the midpoint between the point of zero charge of

18 W.-K. Kim et al. / Electrochimica Acta 71 (2012) 17– 21

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The cyclic performances of the bare LiMn2O4 and the 2 and5 wt.% Al2O3 coated LiMn2O4 at a high temperature (55 ◦C) areshown, respectively, in Fig. 4. The charge and discharge rates

Fig. 1. Schematic diagrams of the coating process of the Al2O3 coated LiMn2O4 by

iMn2O4 and Al2O3. Then, 0.5 g LiMn2O4 was added into the pHontrolled suspension and stirred for 1 h. During this process, theurface charges of LiMn2O4 and Al2O3 were opposite in sign andhus, a junction between them was formed by the electrostaticttraction forces. The suspension was then dried at 100 ◦C for 6 h,nd the acquired powder was annealed at 500 ◦C for 2 h to form andensify the Al2O3 coating layer on the LiMn2O4. The Al2O3 coating

ayer on the LiMn2O4 was observed using transmission electronicroscopy (TEM). X-ray diffraction (XRD) patterns were collected

n a D/MAX-IIIC(3 kW) powder X-ray diffractometer with Cu-K�

adiation (� = 1.5406 A) between 15 and 50◦.

.2. Cell fabrication and electrochemical analysis

To fabricate the electrodes, a mixture of 85 wt.% of each activeaterial and 8 wt.% of acetylene black were added to an N-methyl-

-pyrrolidene (NMP) solution containing 7 wt.% of polyvinylideneuoride (PVDF), thereby forming a slurry. The slurry was pastednto an Al foil substrate and then dried at 120 ◦C for 6 h in a vacuumven. After being dried, it was punched into a disc shape with a 1.3-m diameter and then pressed. The performances of the preparedlectrodes were evaluated using 2016 coin-type cells assembled inn argon-filled glove box. A Li metal foil was used as the counterlectrode, and 1 M of LiPF6 dissolved in EC:DMC = 1:1 (v/v) was useds the electrolyte. The cells were charged and discharged galvano-tatically between 3.0 and 4.3 V at 55 ◦C at a current density of.2 C-rate.

. Results and discussion

The X-ray diffraction patterns of the bare LiMn2O4 and the 2nd 5 wt.% Al2O3 coated LiMn2O4 are shown in Fig. 2. All the peaksf the LiMn2O4 with or without a coating matched well with theCSD (Inorganic Crystal Structure Database) reference pattern (No.

rostatic attraction forces between them, and the result of zeta potential analysis.

88-1026) without impurity phases, indicating that the thin Al2O3surface coating did not change the structure of the LiMn2O4. To dis-tinguish the nanostructures on the Al2O3 coated LiMn2O4, a TEMmicrograph and FFT (Fast Fourier Transform) images were createdand are displayed in Fig. 3. The TEM micrograph indicated that a5-nm thin layer of Al2O3 was uniformly formed on the surface ofthe LiMn2O4. From the FFT images, the Al2O3 coating layer withan amorphous phase was clearly distinguishable from the crys-talline LiMn2O4. The TEM analysis demonstrated that the coatingmethod was an effective way to coat the Al2O3 layer on the surfaceof LiMn2O4.

Fig. 2. X-ray diffraction patterns for the bare LiMn2O4 and the 2 and 5 wt.% Al2O3

coated LiMn2O4.

W.-K. Kim et al. / Electrochimica Acta 71 (2012) 17– 21 19

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Fig. 3. TEM micrograph and FFT im

ere fixed at 0.2 C (1 C = 100 mA g−1). The initial discharge capac-ties of the 2 and 5 wt.% Al2O3 coated LiMn2O4 were 118.6 and16.3 mA g−1, respectively, whereas that of the bare LiMn2O4 was19.7 mAh g−1. Although the samples with or without a coating hadifferent environmental conditions at the surface of the LiMn2O4articles, there were not notable differences in the initial dischargeapacity between them. This result indicated that the 5 nm Al2O3oating layer did not affect the Li+ diffusion between the LiMn2O4articles and the electrolyte despite the intrinsic insulating charac-eristic of the Al2O3. However, there were significant differences inhe cyclic performances between the bare LiMn2O4 and the Al2O3oated LiMn2O4. After 25 cycles, the discharge capacities of the 2nd 5 wt.% Al2O3 coated LiMn2O4 were maintained at 90.2 mAh g−1

nd 85.5 mAh g−1, respectively, whereas that of the bare LiMn2O4ecreased to 68.9 mAh g−1. The improvement in the cyclic perfor-ance of the 2 and 5 wt.% Al2O3 coated LiMn2O4 compared with

hat of the bare LiMn2O4 was due presumably to an inhibitingffect of the Al2O3 coating layer on the Mn dissolution from theiMn2O4 electrode to the electrolyte. However, the 2 wt.% Al2O3

oated LiMn2O4 exhibited a slightly better cyclic performance thanhat of the 5 wt.% Al2O3 coated LiMn2O4. Thus, it appears that the

ig. 4. Cyclic performances (0.2 C-rate, 1 C = 100 mA g−1) of the bare LiMn2O4 andhe 2 and 5 wt.% Al2O3 coated LiMn2O4 at 55 ◦C.

n the 2 wt.% Al2O3 coated LiMn2O4.

5 wt.% Al2O3 coated LiMn2O4 had an excess amount of Al2O3 withreference to 2 wt.% Al2O3 that may act as an insulator.

Electrochemical impedance spectroscopy (EIS) analysis wasperformed on the bare LiMn2O4 and the Al2O3 coated LiMn2O4 atboth the 1st and the 25th cycle to gain a further understanding ofthe difference in their cyclic performances. Before the EIS analy-sis, all the samples were charged to a state of charge of 100% toset identical conditions. Fig. 5 and Table 1 show the Nyquist plotsand the result of simulation fitting based on the equivalent cir-cuit for the bare LiMn2O4 and the Al2O3 coated LiMn2O4 after thefirst cycle and then after 25 cycles, respectively. The EIS spectraconsisted of semi-circles and a slope. The semicircle in the high-middle frequency region is attributed to the lithium-ion migrationthrough the SEI film and charge transfer reaction, and the slopein the low frequency region is attributed to the lithium-ion dif-fusion in the bulk electrode. From the result of the 1st cycle, theAl2O3 coated LiMn2O4 exhibited slightly larger film resistance (Rf)than the bare LiMn2O4. This result was attributed to the combi-nation of the SEI and Al2O3 coating layer formed on the surfaceof the Al2O3 coated LiMn2O4 after 1st cycle. The charge transferresistance (Rct) at the 1st cycle increased in the following order:2 wt.% Al2O3 coated LiMn2O4 < bare LiMn2O4 < 5 wt.% Al2O3 coatedLiMn2O4. All the samples exhibited essentially the similar Rct val-ues, but Rct of the 5 wt.% Al2O3 coated LiMn2O4 is slightly greatercompared with that of the 2 wt.% Al2O3 coated LiMn2O4 due pri-marily to the increased amount of Al2O3, which acts as an insulator.At the 25th cycle, relatively small increase in Rf for the Al2O3 coatedLiMn2O4 compared with that for the bare LiMn2O4 was observeddue probably to the decrease in electrolyte decomposition at theelectrode surface by the protective Al2O3 coating layer. Similarly,small increase in Rct for the Al2O3 coated LiMn2O4 compared withthat for the bare LiMn2O4 was observed. The larger Rct exhibited bythe bare LiMn2O4 is a result of the structural instability of the hoststructure caused by the Mn dissolution and subsequent formationof vacancies.

Amine et al. [23] suggested the possible degradation mecha-nisms of LiMn2O4 at elevated temperatures; The disproportionedreaction (2Mn3+ → Mn2+ + Mn4+) and the Mn dissolution from the

LiMn2O4 particles into the electrolyte are accelerated at an ele-vated temperature. Subsequently, the dissolved Mn migrated to thelithium anode, and then the reduction of the Mn occurred on theanode. In this regard, it is expected that the dissolved Mn and/or

20 W.-K. Kim et al. / Electrochimica Acta 71 (2012) 17– 21

Table 1Rf and Rct calculation of the bare LiMn2O4 and the Al2O3 coated LiMn2O4 at both the 1st and the 25th cycle based on an equivalent circuit of the cell.

Rf/� Rct/�

1st cycle 25th cycle 1st cycle 25th cycle

37.7 170 83317.5 155 31421.9 178 351

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LiMn2O4 9.2

2 wt.% Al2O3 coated LiMn2O4 12.0

5 wt.% Al2O3 coated LiMn2O4 14.7

he Mn-containing complexes were deposited on the surface of theithium anode. Such complexes can be chemically formed by anlectrolytic decomposition combined with Mn. Hence, the lithiumnodes used as counter electrodes of the bare LiMn2O4 and thel2O3 coated LiMn2O4 were examined by X-ray Fluorescence (XRF)fter 25 cycles to measure the amounts of Mn-containing com-

lexes deposited in the SEI layer on the surface of lithium anode.s shown in Fig. 6, the Mn-containing complexes were signifi-antly reduced by coating LiMn2O4 with Al2O3, which agrees wellith the expected role of the Al2O3 coating that inhibits the Mn

ig. 5. EIS measurement of the bare LiMn2O4 and the 2 and 5 wt.% Al2O3 coatediMn2O4 at the (a) 1st cycle and (b) 25th cycle.

Fig. 6. Quantitative XRF analysis of Mn and/or the Mn-containing complexes

deposited in the SEI layer on the surface of lithium anode and dissolved from (a)bare LiMn2O4, (b) 2 wt.% Al2O3 coated LiMn2O4, and (c) 5 wt.% Al2O3 coated LiMn2O4

during 25 cycles.

dissolution. The lithium anode used as the counter electrode for the2 wt.% Al2O3 coated LiMn2O4 exhibited the lowest amount of Mn-containing complexes. Thus, it was demonstrated that a uniformand thin Al2O3 coating on the surface of LiMn2O4 was an effec-tive way to improve the high-temperature cyclic performance byinhibiting the Mn dissolution. However, the thick coating such as5 wt.% Al2O3 coating may reduce the discharge capacity and cyclicperformances due primarily to its insulating properties.

4. Conclusions

In summary, bare LiMn2O4 was coated with Al2O3 by electro-static attraction forces between them to investigate the effectsof the Al2O3 coating on the high-temperature cyclic perfor-mance of LiMn2O4. 5 nm thin and amorphous Al2O3 layer formeduniformly on the surface of 2 wt.% Al2O3 coated LiMn2O4. Thehigh-temperature cyclic performance of LiMn2O4 was significantlyenhanced by the coating. The reason for the enhancement was clar-ified by EIS and XRF analysis. From the EIS analysis, the chargetransfer resistance of the Al2O3 coated LiMn2O4 was much less thanthat without the coating after the 25th cycle, a result of better struc-tural stability in the Al2O3 coated LiMn2O4 due to the decreasein the Mn dissolution. The actual amount of the Mn dissolutionafter the 25th cycle was measured by XRF analysis. The formation

of Mn-containing complexes at the lithium anode caused by Mndissolution decreased by 50% in the Al2O3 coated LiMn2O4 sam-ples. In particular, the 2 wt.% Al2O3 coated LiMn2O4 exhibited thebest cyclic performance and the smallest amount of Mn dissolution.

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herefore, it was demonstrated that a uniform, thin Al2O3 coatingn the surface of LiMn2O4 was an effective way to improve theigh-temperature cyclic performance.

cknowledgements

This research was supported by the Basic Science Research Pro-ram through the National Research Foundation of Korea (NRF),hich is funded by the Ministry of Education, Science and Technol-

gy (NRF-2010-0024752). This work was also supported financiallyy the ILJIN Copper foil Co. and partially by the BK21 Program ofhe Korea Ministry of Knowledge Economy.

eferences

[1] M.M. Thackeray, P.J. Johnson, L.A. de Picciotto, P.G. Bruce, J.B. Goodenough,

Mater. Res. Bull. 19 (1984) 179.

[2] C. Sigala, D. Guyomard, A. Vebaere, Y. Piffard, M. Tourmoux, Solid State Ionics81 (1995) 167.

[3] R.J. Gummow, A.D. Kock, M.M. Thackery, Solid State Ionics 69 (1994) 59.[4] J.M. Tarascon, M. Armand, Nature 414 (2001) 359.

[[

[

a Acta 71 (2012) 17– 21 21

[5] B. Deng, H. Nakamura, M. Yoshio, J. Power Sources 141 (2005) 116.[6] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825.[7] H. Huang, C.A. Vincent, P.G. Bruce, J. Electrochem. Soc. 146 (1999) 3649.[8] G. Amatucci, A. Du Pasquier, A. Blyr, T. Zheng, J.M. Tarascon, Electrochim. Acta

45 (1999) 255.[9] M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1.10] Y. Shao-horn, S.A. Hackney, A.J. Kahaian, K.D. Kepler, E. Skinner, J.T. Vaughey,

M.M. Thackeray, J. Power Sources 81 (1999) 496.11] G. Amatucci, J.M. Tarascon, J. Electrochem. Soc. 149 (2002) K31.12] W.H. Ryu, J.Y. Eom, R. Yin, D.W. Han, W.K. Kim, H.S. Kwon, J. Mater. Chem. 21

(2011) 15337.13] A.R. Han, Z.T.W. Kim, D.H. Park, S.J. Hwang, J.H. Choi, J. Phys. Chem. C 111 (2007)

11347.14] J. Cho, Y. Kim, Y. Kim, B. Park, Chem. Commun. (2001) 1074.15] Z. Chen, J.R. Dahn, Electrochim. Acta 49 (2004) 1079.16] S.T. Myung, K. Izumi, S. Komaba, Y.K. Sun, H. Yashiro, N. Kumagai, Chem. Mater.

17 (2005) 3695.17] C. Qing, Y. Bai, J. Yang, W. Zhang, Electrochim. Acta 56 (2011) 6612.18] H.W. Ha, N.J. Yun, K. Kim, Electrochim. Acta 52 (2007) 3236.19] H. Sahan, H. Goktepe, S. Patat, A. Ulgen, Solid State Ionics 178 (2008) 1837.20] L.H. Yu, X.P. Qiu, J.Y. Xi, W.T. Zhu, L.Q. Chen, Electrochim. Acta 51 (2006) 6406.

21] J.M. Han, S.T. Myung, Y.K. Sun, J. Electrochem. Soc. 153 (2006) A1290.22] J. Tu, X.B. Zhao, G.S. Cao, D.G. Zhuang, T.J. Zhu, J.P. Tu, Electrochim. Acta 51

(2006) 6456.23] K. Amine, J. Liu, S. Kang, I. Belharouak, Y. Hyung, D. Vissers, G. Henriksen, J.

Power Sources 129 (2004) 14.