Electrochimica Acta 55 (2010) 832837

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Electrochimica Acta

journa l homepage: www.e lsev ier .com

Electro -sivia the ic L

X. Fang, YCAS Key Labora ing, U

a r t i c l

Article history:Received 4 JulReceived in re15 SeptemberAccepted 15 SAvailable onlin

Keywords:Lithium nickelEutecticLow temperatCapacity retenLithium batter

partcetatC. Aft

perd at 9hemacity

ized Lt 10capac

1. Introduction

Lithium-ion batteries are one of the most successful powersources and have dominated the portable electronic device mar-ket for thefast develowide appliccontinuedcapacities arials, such aspinel LiNi0LiNi0.5Mn1.5Amine et avoltage plaLiNi0.5Mn1.5tial, the eneLiCoO2. Thufor use in el

A varieLiNi0.5Mn1.5tion [4], soelectrophorKimet al. [1by the mo

CorresponE-mail add

and Li/Ni/Mn hydroxides; the synthesized LiNi0.5Mn1.5O4 pow-ders show excellent cycling performance. In this study, we choseto use acetates as starting materials; these can form a ternaryeutectic LiNiMn acetate below 80 C. Though acetates have been

0013-4686/$ doi:10.1016/j.past two decades. Nonetheless, to keep up with thepment of the laptop central processing unit and theation of 3G techniques to cell phones, people haveto search for new electrode materials with higherndmore power. Compared to traditional cathodemate-s LiCoO2 (3.9V), LiMn2O4 (4.1V) and LiFePO4 (3.5V),.5Mn1.5O4 has a higher voltage (4.7V) [1]. In 1996,O4 was rst reported to be a 3V cathode material by

l. [2]. Later, Dahn and coworkers discovered the 4.7Vteau of LiNi0.5Mn1.5O4 [3]. The theoretical capacity ofO4 is 146.7mAhg1; due to its high working poten-rgy density of LiNi0.5Mn1.5O4 is 20% higher than that ofs, LiNi0.5Mn1.5O4 is seen as a potential cathodematerialectric vehicles and energy storage systems in the future.ty of synthetic methods for the preparation ofO4 have been reported; these include solid state reac-lgel [5], co-precipitation [6,7], spray pyrolysis [8,9],etic deposition [10] and pulsed laser deposition [11].2] prepared thewell-dened octahedral LiNi0.5Mn1.5O4lten salt method starting with the mixture of LiCl

ding author. Tel.: +86 551 3606971; fax: +86 551 3601592.ress: cchchen@ustc.edu.cn (C.H. Chen).

widely used in the literature, most studies used a wet-chemicalroute assisted by organic materials, such as acrylic acid [13], citricacid [14], poly(ethylene glycol) [15] and poly(methyl methacry-late) [16]. There are also some reports of studies using the so-calledsolgelmethod. Thismethod involvesrst dissolving the acetatesin water and then evaporating the water to obtain a gel [17,18].Webelieve that the abovemethod shouldproduce the same ternaryLiNiMn acetate eutectic instead of the real gel. It should be men-tioned thatprior toourwork, Lafont et al. [19]observed this eutecticphenomenonof LiNiMnacetate,which theycalled greenslurry,but no extensive investigationwas conducted. In Lafonts work, thecapacity at 2Cwas observed to be only about 60mAhg1. Ourworkshows that by simply optimizing the sintering temperature of theternary eutectic LiNiMn acetate, we can obtain nano- and micro-sized LiNi0.5Mn1.5O4 particleswith a capacity of about 100mAhg1

at 8C.

2. Experimental procedures

We mixed 2.488g (10mmol) nickel acetate (Ni(Ac)24H2O),7.352g (30mmol) manganese acetate (Mn(Ac)24H2O) and 2.146g(21mmol) lithium acetate (LiAc2H2O) and milled the mixture byhand in a mortar. Then, the mixture was calcined at 300 C for 5h.Aftermilling byhand again, the powderswere sintered in air at 300,

see front matter 2009 Elsevier Ltd. All rights reserved.electacta.2009.09.046chemical properties of nano- and micrormal decomposition of a ternary eutect

. Lu, N. Ding, X.Y. Feng, C. Liu, C.H. Chen

tory of Materials for Energy Conversion, Department of Materials Science and Engineer

e i n f o

y 2009vised form2009eptember 2009e 22 September 2009

manganese oxide

ure performancetiony

a b s t r a c t

Nano- and micro-sized LiNi0.5Mn1.5O4eutectic LiNiMn acetate. Lithium aeutectic LiNiMn acetate below 80

be obtained at an extremely low tem700 C, the particle size increases, anof about 4m) are obtained. Electroc(sinteredat900 C)exhibit thebest capcapacity can still be reached. Nano-sat low temperatures; when cycled aLiNi0.5Mn1.5O4 powders can deliver a/ locate /e lec tac ta

zed LiNi0.5Mn1.5O4 synthesizediNiMn acetate

niversity of Science and Technology of China, Anhui, Hefei 230026, China

icles are prepared via the thermal decomposition of a ternarye, nickel acetate and manganese acetate can form a ternaryer further calcination, nano-sized LiNi0.5Mn1.5O4 particles canature (500 C). When the sintering temperature goes above00 C micro-sized LiNi0.5Mn1.5O4 particles (with a diameter

ical tests show that the micro-sized LiNi0.5Mn1.5O4 powdersretentionat25 C, andafter100cycles, 97%of initial dischargeiNi0.5Mn1.5O4 powders (sintered at 700 C) perform the bestC and charged and discharged at a rate of 1C, nano-sizedity as high as 110mAhg1.

2009 Elsevier Ltd. All rights reserved.

X. Fang et al. / Electrochimica Acta 55 (2010) 832837 833

Fig. 1. Images f LiNportion); (c) th bottle

400, 500, 6rate of 3 Cstructurestion (XRD,the range fthe powder(SEM, JEOL-ucts were ein ethyleneweight ratiglovebox (Mwas compoand poly(vion a multi-cbetween 2.8

3. Results

3.1. Ternary

The meand 80 C,NiAc24H2Owith an incthat amixtua ternary euand d). On tput in a botmelts at an(Fig. 1b andof the formation of a ternary eutectic LiNiMn acetate: (a) mechanical mixture oe mechanical mixture (left) and MnNi acetates without mixing at 80 C; (d) lying00, 700, 800, 900 and 1000 C for 10h (with a heatingmin1) and allowed to cool naturally. The crystallineof the powders were characterized by X-ray diffrac-Philips XPert Pro Super, Cu K radiation) with 2 inrom 10 to 80. The morphology and composition ofs were determined by scanning electronic microscopy6970). The electrochemical characteristics of the prod-valuated with coin cells (CR2032 size) of Li/1M LiPF6carbonate (EC) and dimethyl carbonate (DMC, with ao of 1:1)/LiNi0.5Mn1.5O4 assembled in an argon-lledBraunLabmaster 130). Thepositive electrode laminate

sed of LiNi0.5Mn1.5O4 (84wt.%), acetylene black (8wt.%)nylidene uoride) (PVDF, 8wt.%). The cells were testedhannel battery test system (Shenzhen Neware Co. Ltd.)and 5.1V (vs. Li+/Li).

and discussion

eutectic LiNiMn acetate

lting points of LiAc2H2O and MnAc24H2O are 70respectively. Unlike lithium and manganese acetates,can be directly decomposed, rather than being melted,

rease in temperature. In our experiment, we observedreof LiAc2H2O,MnAc24H2OandNiAc24H2Ocan formtectic solution of LiNiMn acetate at 80 C (Fig. 1a, che other hand, when MnAc24H2O and NiAc24H2O aretle one after another without mixing, only MnAc24H2Oelevated temperaturewhileNiAc24H2O remains a solidc). Thus, the melting of the LiNiMn acetate-mixture

can be attrwhich guarlevel.

The TG-in Fig. 2. That 54 C is athe endothetion water,weight lossto the decoreticalweigthe temper

Fig.iMn acetates; (b) Mn acetate (bottom portion) and Ni acetate (topof LiNiMn acetate-mixture at 80 C.ibuted to the formation of a ternary eutectic system,antees the mixing of Li, Ni and Mn atoms at the atomic

DTA curves of the LiNiMn acetate-mixture are shownere are several thermal steps: (i) the endothermic peakresult of the formation of a ternary eutectic solution; (ii)rmic peak at 118 C results from the loss of crystalliza-accompanied by a weight loss of 26% (the theoreticalis 27.7%); (iii) the exothermic peak at 345 C is related

mposition of acetate, with a 41.4% weigh loss (the theo-ht loss is 37.2%); (iv) theweight is nearly constantwhenature is above 370 C; (v) the small endothermic reac-

2. TG-DTA curves of the ternary eutectic LiNiMn acetate.

834 X. Fang et al. / Electrochimica Acta 55 (2010) 832837

tion at 750 C is a result of the emission of oxygen in LiNi0.5Mn1.5O4(LiNi0.5Mn1.5O4) [20].

3.2. Structure and morphology

The XRD patterns of the samples sintered at different tempera-tures (from 200 to 1000 C) are shown in Fig. 3. Due to the similardiffraction patterns of the spinel and layered structures, it is dif-cult to differentiate these structures for the samples sintered at lowtemperatures (below 500 C). We believe that the samples sinteredat high temperatures (beyond 500 C) are of a pure spinel structure.This conclusion is supported by the electrochemical test, as shownbelow. The peak width at half-height decreases with an increase inthe sintering temperature, which indicates improved crystallinity.

The SEM images of the as-synthesized samples are shown inFig. 4. When the temperature is below 700 C, nanoparticles areobtained. When the temperature increases to 800 C, the particlesgrow; at 900 C, the particle size is about 4m. When the tem-perature increases to 1000 C, the particle size further increasesto about 10m. There are some small particles on the surface ofthe sample sintered at 1000 C; these particles may have arisen

Fig. 3. XRD patterns of the samples sintered at different temperatures (from 200 to1000 C).

ized samples.Fig. 4. SEM images of the as-synthes

X. Fang et al. / Electrochimica Acta 55 (2010) 832837 835

Fi ples (c

from the d[21].

3.3. Electro

The galvas-synthesiat 300 C haode (e.g. LiThis result i(LiMnO2) a400 C, a voof LiNi0.5Mthat might[24,25]. TheMn ions. It sshould be tto 500 C, thimpurity inshort voltaThis plateauin LiNi0.5Mnten as LiNi0in the sampthe plateaudeliver a catemperaturto a submic800 C canbeenrepeatat 800 C isthe lengththe plateauions to Mn3

+ ionto ae ofadveredu, theg. 5. Galvanostatic chargedischarge curves (2nd cycle) of the as-synthesized sam

ecomposition of LiNi0.5Mn1.5O4 at high temperature

chemical performance

anostatic chargedischarge curves (2nd cycle) of the

of Mn4

leadingincreasfor thewhich900 Czed samples are shown in Fig. 5. The sample sintereds a typical voltage prole of a layered-structure cath-

MnO2) with a voltage plateau at around 4.0V [22,23].ndicates that Mnn+ ions are only oxidized to trivalencet 300 C. When the sintering temperature increases toltage plateau at 4.7V appears, indicating the formationn1.5O4. At around 3V another voltage plateau appearsbe due to the transition from LiMn2O4 to Li2Mn2O4powder sintered at 400 Cmay still have some trivalenthould be mentioned that in LiNi0.5Mn1.5O4, all Mn ionsetravalent. When the sintering temperature increasese plateau at 3V almost disappears, indicating that theLiMn2O4 has been converted into LiNi0.5Mn1.5O4. A

ge plateau at around 4.1V is still observed, however.may be attributed to the transition from Mn3+ to Mn4+

1.5O4 [26]; thus, the real composition should be writ-.5Mn1.5O4. The plateau at 3V completely disappearsle sintered at 600 C and due to the oxidation of Mn3+,at 4.1V shortens. The sample sintered at 700 C canpacity of as high as 127mAhg1. When the sinteringe is above700 C, theparticle size increases signicantlyron-size (as shown in Fig. 4). The sample sintered atonly deliver a capacity of 121mAhg1. This result hasedbysintering thesampleagain. Thecapacity reductionlikely due to the growth of particles, which increasesof the lithium diffusion path. It is also observed thatat 4.1V lengthens, due to the reduction of some Mn4++ ions at high temperatures [21]. During the reduction

129.2mAhAs a result,the samplefrom the 4.similar volt125mAhg

Fig. 6 shLiNi0.5Mn1.discharged

Fig. 6. Cycled at 25 C, charged and discharged at a rate of 1/3C).

s, the concentration of oxygen vacancy also increases,n increase in electronic conductivity. Nevertheless, theelectronic conductivity cannot completely compensaterse effect of increasing the length of the diffusion path,ces capacity. As the temperature further increases toas-synthesized sample delivers the highest capacity of1g , yet 16.6% capacity comes from the 4.1V plateau.the overall output energy is still lower than that fromsintered at 700 C, for which only 7.2% capacity comes1V plateau. The sample sintered at 1000 C exhibits aage prole as that sintered at 900 C, with a capacity of1.ows the cycling performance of the nano- and micro-5O4 sintered at different temperatures, charged andat a rate of 1C, cycled between 2.8 and 5.1V at room

ycling performance of nano- and micro-LiNi0.5Mn1.5O4 (at 1C).

836 X. Fang et al. / Electrochimica Acta 55 (2010) 832837

Fig.

temperaturexhibits theing cycling;still be reaccycle. Althoers a high cmicro-900.dissolutionLiNi0.5Mn1.5during cyc600. As narelativelybeinvestigated900 (chargemicro-sizedbility, withmicro-900increases tofactor for thcan be enhasintering tetion; at theLiNi0.5Mn1.5path, is bettcapacity ret

As a higseen as a poelectric vehformance atnano-700 achargediscThe voltagenicant drorate performat a rate ofcells werewas foundwas still abLiNi0.5Mn1.5traditionalLiFePO4 [31to be fast Lwas foundthat nano-stures.

alvanled at

Fig. 9. Rate performance of nano-700 and micro-900 at 10 C.

clusions

o- and micro-sized LiNi0.5Mn1.5O4 particles are successfullysized via the thermal decomposition of a ternary eutecticMnacetate.When the sintering temperature is below800 C,ized LiNi0.5Mn1.5O4 powders are obtained. When the sinter-perature further increases, the particles grow, and nally,

sized LiNi Mn O powders are obtained. Electrochemical7. Rate performance of nano-700 and micro-900 at 25 C.

e. Obviously, the sample sintered at 900 C (micro-900)highest capacity and the best capacity retention dur-after 100 cycles, 97% of initial discharge capacity canhed, with a capacity loss of less than 0.04mAhg1 perugh the sample sintered at 700 C (nano-700) alsodeliv-apacity, the capacity retention is not as good as that ofThe large surface area of the nano-700 accelerates theofNi andMn ions,which results in the loss of capacity ofO4 during cycling [27,28]. The ability to retain capacity

ling is: micro-900 submicro-800>nano-700>nano-no-700 and micro-900 exhibit higher capacities andtter capacity retention, their rate capabilitywas further. The rate performance at 25 C of nano-700 andmicro-d at a rate of 1C) is shown in Fig. 7. Both nano- andLiNi0.5Mn1.5O4 particles exhibit an excellent rate capa-a capacity of about 100mAhg1 at 8C. At the low rate,displays a better rate capability, though when the rate6C,nano-700 isbetter. At the lowrate, the rate-limitinge rate performance is the electronic conductivity,whichnced by the formation of oxygen vacancy at the highermperature; thus, micro-900 is better under this condi-high rate, the limiting factor is the lithium diffusion inO4, and thus nano-700, which has a shorter diffusioner. Again, for long-time cycling, micro-900 has a betterention than nano-700.h energy density cathode material, LiNi0.5Mn1.5O4 istential cathodematerial for electric vehicles and hybridicles in the future. Thus, it is necessary to test its per-

Fig. 8. G900 (cyc

4. Con

NansyntheLiNinano-sing temmicro-low temperatures.We investigated the performance ofnd micro-900 at 10 C. Fig. 8 shows the galvanostaticharge curves (2nd cycle) of nano-700 and micro-900.plateau still remains at about 4.6V, without a sig-

p like that found in LiMn2O4 [29]. Fig. 9 shows theance of the two samples cycled at 10 C, charged1C (except for the rst 10 cycles, during which the

charged and discharged at a rate of 1/3C). Nano-700to have a higher capacity, and at 1C the capacityout 110mAhg1. The capacity retention of nano-sizedO4 at low temperatures is much better than thosecathode materials such as LiMn2O4 [29], LiCoO2 [30],,32], andevenV2O5 nanoberswhichhasbeen reportedi-ion conductor [33,34]. For micro-900, the capacityto be only about 70mAhg1 at 1C. Thus, we believeized LiNi0.5Mn1.5O4 perform better at low tempera-

tests at roopowders sincycled at 2ity can stillpowders siperatures,110mAhg

particles ex100mAhg

Acknowled

This stuChina (granProvince (gPlan of Acaostatic chargedischarge curves (2nd cycle) of nano-700 and micro-10 C, charged and discharged at a rate of 1/3C).0.5 1.5 4m temperature show that micro-sized LiNi0.5Mn1.5O4tered at 900 C have the best capacity retention when

5 C; after 100 cycles, 97% of initial discharge capac-be reached. Nevertheless, nano-sized LiNi0.5Mn1.5O4

ntered at 700 C exhibit a higher capacity at low tem-and at a rate of 1C it can still deliver a capacity of1 at 10 C. Both nano- and micro-sized LiNi0.5Mn1.5O4hibit excellent rate capabilities, with a capacity of about1 at 8C (at room temperature).

gements

dy was supported by National Science Foundation oft no. 20971117), the Education Department of Anhuirant no. KJ2009A142) and the Solar Energy Operationdemia Sinica.

X. Fang et al. / Electrochimica Acta 55 (2010) 832837 837

References

[1] S. Patoux, L. Daniel, C. Bourbon, H. Lignier, C. Pagano, F. Le Cras, S. Jouanneau,S. Martinet, J. Power Sources 189 (2009) 344.

[2] K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Electrochem. Soc. 143 (1996)1607.

[3] Q.M. Zhong, A. Bonakdarpour, M.J. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc.144 (1997) 205.

[4] H. Fang, Z. Wang, X. Li, H. Guo, W. Peng, J. Power Sources 153 (2006) 174.[5] H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, J. Solid State Electrochem. 8 (2004)

450.[6] Y.S. Lee, Y.K. Sun, S. Ota, T. Miyashita, M. Yoshio, Electrochem. Commun. 4

(2002) 989.[7] Y. Fan, J. Wang, X. Ye, J. Zhang, Mater. Chem. Phys. 103 (2007) 19.[8] S.-H. Park, Y.-K. Sun, Electrochim. Acta 50 (2004) 431.[9] D. Li, A. Ito, K. Kobayakawa, H. Noguchi, Y. Sato, Electrochim. Acta 52 (2007)

1919.[10] A. Caballero, L. Hernn, M. Melero, J. Morales, R. Moreno, B. Ferrari, J. Power

Sources 158 (2006) 583.[11] H. Xia, S.B. Tang, L. Lu, Y.S. Meng, G. Ceder, Electrochim. Acta 52 (2007) 2822.[12] J.-H. Kim, S.-T. Myung, Y.-K. Sun, Electrochim. Acta 49 (2004) 219.[13] S.B. Park, W.S. Eorn, W.I. Cho I, H. Jang, J. Power Sources 159 (2006) 679.[14] N. Amdouni, K. Zaghib, F. Gendron, A. Mauger, C.M. Julien, Ionics 12 (2006) 117.[15] J.C. Arrebola, A. Caballero, M. Cruz, L. Hernn, J. Morales, E.R. Castelln, Adv.

Funct. Mater. 16 (2006) 1904.[16] J.C. Arrebola, A. Caballero, L. Hernn, J. Morales, J. Power Sources 180 (2008)

852.

[17] T. Nakamura, H. Demidzu, Y. Yamada, J. Phys. Chem. Solids 69 (2008) 2349.[18] G. Du, Y. NuLi, J. Yang, J. Wang, Mater. Res. Bull. 43 (2008) 3607.[19] U. Lafont, C. Locati, E.M. Kelder, Solid State Ionics 177 (2006) 3023.[20] M.G. Lazarraga, L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, J.M. Amarilla,

R.M. Rojas, M.A. Martin-Luengo, J.M. Rojo, J. Mater. Chem. 14 (2004) 1640.[21] M.I. Zaki, M.A. Hasan, L. Pasupulety, K. Kumari, Thermochim. Acta 2 (1997) 171.[22] T. Nohma, H. Kurokawa, M. Uehara, M. Takahashi, K. Nishio, T. Saito, J. Power

Sources 54 (1995) 522.[23] A.R. Armstrong, P.G. Bruce, Nature 381 (1996) 499.[24] M.M. Thackeray, W.I.F. David, P.G. Bruce, J.B. Goodenough, Mater. Res. Bull. 18

(1983) 461.[25] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769.[26] J.-H. Kim, S.-T. Myung, C.S. Yoon, S.G. Kang, Y.-K. Sun, Chem. Mater. 16 (2004)

906.[27] Y. Talyosef, B. Markovsky, G. Salitra, D. Aurbach, H.-J. Kim, S. Choi, J. Power

Sources 146 (2005) 664.[28] X. Fang, N. Ding, X.Y. Feng, Y. Lu, C.H. Chen, Electrochim. Acta,

doi:10.1016/j.electacta.2009.09.046.[29] R.A. Marsh, S. Vukson, S. Surampudi, B.V. Ratnakumar, M.C. Smart, M. Manzo,

P.J. Dalton, J. Power Sources 97 (2001) 25.[30] Y. Ein-Eli, R.C. Urian, W. Wen, S. Mukerjee, Electrochim. Acta 50 (2005) 1931.[31] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 159 (2006) 702.[32] X.Z. Liao, Z.F. Ma, Q. Gong, Y.S. He, L. Pei, L.J. Zeng, Electrochem. Commun. 10

(2008) 691.[33] C.R. Sider, C.R. Martin, Adv. Mater. 17 (2005) 125.[34] C.K. Chan, H. Peng, R.D. Twesten, K. Jarausch, X.F. Zhang, Y. Cui, Nano Lett. 7

(2007) 490.