ORIGINAL PAPER

Improvement of cycling stability of LiMn2O4 cathode by Fe2O3

surface modification for Li-ion battery

Halil Şahan & Fatma Kılıç Dokan &

Ahmet Ülgen & Şaban Patat

Received: 2 April 2013 /Revised: 19 July 2013 /Accepted: 18 August 2013 /Published online: 31 August 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The surface of spinel LiMn2O4 was modified withFe2O3 (1.0, 2.0, 3.0, 4.0, and 5.0 wt%) by a simple sol-gelmethod to improve its electrochemical performance at roomtemperature. Compared with bare LiMn2O4, surface modifi-cation improved cycling stability of the material. Among thesurface-modified cathode materials, the 3.0- and 4.0-wt%surface-modified cathodes have lesser capacity loss than theothers. While the bare LiMn2O4 showed 25.4 % capacity lossin 70 cycles at room temperature, 3.0 and 4.0 wt% of Fe2O3-modified LiMn2O4 only exhibited the capacity loss of 2.6 and2.3 % in 70 cycles at room temperature, respectively. Thestructure and phase were identified with X-ray diffractometeralong with the lattice constant calculated by a Win-Metricprogram.

Keywords Spinels . Electrodes . Li-ion batteries .

Electrochemical stabilities . Charging/discharging

Introduction

Rechargeable lithium ion batteries are becoming more andmore important at both fundamental and applied levels be-cause of their high energy density and design flexibility [1, 2].The capacity of these batteries is usually cathode (positiveelectrode)-limited, so it follows that increasing the capacity ofthe cathode is essential to raise the performance of suchbatteries. By far, the most common cathode active materialsthat are being researched and commercially used in lithiumion batteries are the lithiated transition metal oxides such asLiCoO2, LiNiO2, LiMn2O4, and the doped counterparts [1–4].

Among these, spinel LiMn2O4 as the most potential mate-rials were ascribed to its merits of easy preparation, inexpen-siveness, more abundance of manganese resources, nontoxicity,and environment-friendly nature [5–8]. However, LiMn2O4

exhibits serious capacity fading during charge and discharge,which is the major obstacle to its commercialization. Thecapacity fading mechanism on spinel LiMn2O4 is complexand has not been fully understood yet. So far, several reasonsare advanced to explain its poor cycling performance, includingstructural instability, Jahn–Teller distortion, and Mn dissolutioninto electrolyte [9–16].

Recent developments in LiMn2O4-based cathodes for lith-ium ion batteries fall into two major categories. One way issubstitution of heterogeneous atom in to the host LiMn2O4

structure of cathode materials, and the other one is surfacemodification. Several research groups have attempted to sta-bilize the structure of LiMn2O4 powders during cycling bysubstituting a small fraction of the manganese ions withseveral divalent or trivalent metal ions. There was an improve-ment of cycle performance at room temperature by partialsubstitution of transition metal instead of Mn in LiMn2O4

[17–23]. However, the improved capacity retention is usuallyrealized at the expense of the decrease of specific capacity, andLiMn2O4 still suffered from significant capacity decline atroom and elevated temperatures (50–60 °C).

On the other hand, a different approach has been reported,which involves modifying the surface of the cathode materialsby coating it with electrochemically inactive metal oxides orceramic oxide materials. Recently, surface modifications withoxides such as Li2O-B2O3 [24, 25], Al2O3 [26], Cr2O3 [27], ZnO[28, 29], TiO2 [30, 31], AlPO4 [32, 33], CeO2 [34], LiAlO2 [35],SiO2 [36], ZrO2 [37], and Co3O4 [38] have been investigated.These studies showed that the presence of oxide coating canminimize the contact area of LiMn2O4/electrolyte interface andsuppress dissolution ofMn2+. In our previous works [39, 40], wehave successfully coated LiMn2O4 via lithium borosilicat (LBS)

H. Şahan (*) : F. K. Dokan :A. Ülgen : Ş. PatatDepartment of Chemistry, Faculty of Science, Erciyes University,38039 Kayseri, Turkeye-mail: halil@erciyes.edu.tr

Ionics (2014) 20:323–333DOI 10.1007/s11581-013-0987-x

and CaCO3. These studies showed that coated cathode materialshave better capacity retention than uncoated cathode material.

To the best of our knowledge, the Fe2O3-modifiedLiMn2O4 has never been reported yet. Iron (II) oxide FeO isa gray-black compound, iron (III) oxide Fe2O3 is brown-red,and iron (II, II) oxide Fe3O4 is black-grayish ferromagneticcompound. The first oxide FeO is well dissolved in mineralacids. The two others (Fe2O3 and Fe3O4) are poorly dissolved[41]. In the present work, we report on a novel method toimprove the cyclability of LiMn2O4. In addition, the effects ofmodified Fe2O3 amount on the structural and electrochemicalproperties were discussed.

Experimental

Untreated LiMn2O4 was prepared by a glycine–nitrate com-bustion process [42]. Firstly stoichiometric amounts of LiNO3

(Riedel-de Haen) and Mn(CH3COO)2·4H2O (Sigma) weredissolved in distilled water. Glycine (Merck) was added tothe solution either as a solid or as a water solution. Its role wasto serve both as a fuel for combustion and as a complexant toprevent inhomogeneous precipitation of individual compo-nents prior to combustion. Finally, nitric acid with the samemole of acetate anions was added to the solution. The molarratio of glycine to nitrate was 1:4. The solution was heatedcontinuously without any previous thermal dehydration.Afterwards, the solution became a transparent viscous gelwhich autoignited, giving a voluminous, black, sponge-likeash product of combustion. The resulting ash was heated at800 °C for 12 h.

Fe-oxide modifying (1.0, 2.0, 3.0, 4.0, and 5.0 wt%) onLiMn2O4 material was carried out in iron (III) nitrate aqueoussolution. After dissolving the stoichiometric amount ofFe(NO3)3·9H2O (Merck) in distilled water, ascorbic acid wasadded as a complexing agent. Then, aqueous solution ofammonia was added until pH 7 to prevent the dissolution ofLiMn2O4 during the coating process. The bare LiMn2O4

powder was then added to the solution to obtain a suspension.The suspension was stirred vigorously with a magnetic stirrerfor 12 h at room temperature to make it dispersed homoge-neously and then dried in an oven at 100 °C to evaporate thesolvent. After that, the obtained powder was further heated ina muffle furnace at 250 °C for 2 h and 600 °C for 10 h in air,respectively, and then cooled to room temperature.

The cation composition of the bare and surface-treatedLiMn2O4 powders was determined by a flame atomic absorp-tion spectrometer (AAS, PerkinElmer 3110) and flame pho-tometer (FP, Jenway PFP7) after dissolving the powders in thesolution of 0.1 M oxalic acid in 1 M sulfiric acid.

The phase identification and evaluation of the lattice pa-rameters of the bare and surface-treated LiMn2O4 powderswere carried out by powder X-ray diffraction (XRD) using

CuKα radiation (Bruker AXS D8). The diffractometer wasequipped with a diffracted beam graphite monochromator.The diffraction data were collected at 40 kVand 40 mA overa 2θ range from 10° to 70° with a step size of 0.02° and acount time of 10 s per step. The DiffracPlus and Win-Metricprograms were used to obtain the lattice parameters of thepowders.

The particle morphology of the powders was examined bymeans of scanning electron microscopy (LEO 440), operatedat 20 kV. Thermogravimetry (TG) and differential thermalanalysis (DTA) measurements were conducted by thePerkinElmer (Diamond) high-temperature thermal analyzerwith 5–20-mg samples and a heating rate of 10 °C/min from50 to 700 °C in air.

To investigate the surface charge state of Fe2O3, the bareLiMn2O4, and surface-treated LiMn2O4 particles, zeta poten-tial measurements were carried out at room temperature usinga particle size analyzer (Malvern Zetasizer, Nano ZS90) infolded capillary zeta potential cells. The pH titrations wereperformed using 0.1 M NaOH and 0.1 M HCl solutions.

To determine the existence of Fe2O3 in the modified cath-ode material, Raman spectra of bare LiMn2O4 and surface-treated LiMn2O4 were measured at room temperature using aRanishaw Ramascope Raman spectrometer. The emission ofan argon-ion laser at wavelength of 488 nm was used.

The electrochemical studies were carried out in two-electrode Teflon cells. The cells were fabricated by using thebare and surface-treated LiMn2O4 as a cathode and lithium foilas an anode. A glass fiber separator soaked in electrolyteseparated the two electrodes. The electrolyte consisted of a 1-M solution of LiPF6 dissolved in ethylene carbonate (Aldrich)/diethyl carbonate (Merck) (EC/DEC, 1:1 ratio by volume). Forthe preparation of the cathode composite, a slurry mixed with86 wt% of cathode active material, 9 wt% of acetylene blackconductor (Alfa Aesar), and 5 wt% of polyvinylidene fluoride(PVDF, Fluka) binder in 1-methyl-2-pyrrolidone (NMP,Merck) was pasted on the aluminium foil current collector witha diameter of 13 mm, followed by vacuum drying at 120 °Covernight in a vacuum oven and uniaxial pressing between twoflat plates at 2 ton for 5 min. The electrode loading consisted ofabout 4 mg of cathode active material. Diethyl carbonate,ethylene carbonate, and acetylene black were used after beingpurified according to the methods given in the literature [43]:Diethyl carbonate: 100 mL DEC was washed with an aqueous10 % Na2CO3 (20 mL) solution, saturated CaCl2 (20 mL), andthen water (30 mL). After drying by letting it stand over solidCaCl2 for 1 h (note that prolonged contact should be avoidedbecause slow combination with CaCl2 occurs), it was fraction-ally distilled; Ethylene carbonate: this was dried over P2O5,then fractionally distilled at 10 mmHg of pressure, and crystal-lized from dry ethyl ether, respectively; Acetylene black: thiswas leached for 24 h with 1:1 HCl to remove oil contaminationand then washed repeatedly with distilled water. It was then

324 Ionics (2014) 20:323–333

dried in air and eluted for 1 day each with benzene and acetone.It was again dried in air at room temperature and then heated ina vacuum for 24 h at 600 °C to remove adsorbed gases.

Charge–discharge tests were performed galvanostaticallyat a current rate of 1C with cutoff voltages of 3.5–4.5 V(versus Li/Li+) at room temperature. The potential scan ratewas 100 μV/s in the cyclic voltammogram (CV) experimentbetween 3.0 and 4.4 V. All electrochemical experimentswere performed using a multichannel battery tester (PAR,VersaSTAT MC Multichannel Potentiostat/Galvanostat). Allprocesses of assembling and dismantling the cells were carriedout in an argon-filled dry glove box.

Results and discussion

The chemical analysis of the bare and surface-treated lithiummanganese oxide indicated that the stoichiometry of the ele-ments was very close to the targeted formula.

To work out the possible chemical composition of the coat-ing layer, thermal gravimetric examination for the precursorpowder of coating material, obtained from evaporation of aque-ous solution of Fe(NO3)3·9H2O reduced with ascorbic acidsolution, was carried out in Fig. 1. A drastic weight loss is seenbetween 120 and 200 °C due to evaporation of both absorbedand crystallizedwater from the precursor. Then, steady decreasein the weight is seen to 600 °C, which is attributed to decom-position and combustion of ascorbic acid and nitrate. Therefore,we decided to use the temperature for the heat treatment tem-perature after coating procedure. As can be seen in Fig. 1a,calcination of the precursor powder at 600 °C for 10 h exhibited

a well-crystallized Fe2O3. Therefore, it is believed that thespinel shown in Fig. 2c–g is coated by Fe2O3.

Figure 2 shows the XRD patterns for both the bare andFe2O3-modified (1.0, 2.0, 3.0, 4.0, and 5.0 wt%) LiMn2O4

powders. All of the diffraction peaks correspond to a well-defined spinel structure with space group Fd3m, being in goodagreement with JCPDS card 88-1030 [44]. No additionalpeaks for other phases such as Fe2O3, Li2MnO3, LiMnO2, orMnOx can be observed, if any, reaching the detection thresh-old of instrument. However, as shown in Table 1, refinementsto the XRD patterns indicate that the lattice parameter ofsurface-modified spinels was smaller than that of pure spinelLiMn2O4. In the low spin state (LS), the ionic radiuses ofMn3+ are 0.58 Å, but in the high spin state (HS), its ionicradius is 0.66 Å. Meanwhile, the LS radius of the sixthcoordinate Fe3+ is 0.55 Å, and in the HS, its ionic radius is0.64 Å [45]. This implies that Fe3+ ions in the precursor forFe2O3 may enter the crystal structure of LiMn2O4 duringthe heat treatment. Ohzuku et al. found that the lattice con-stants of Fe-doped LiMn2O4 increase as iron content in-creases. They report that Fe3+ ions (3d5) occupy both theoctahedral and tetrahedral sites, as in Fe3+[Fe2+Fe3+]O4,Fe3+[Li+0.5Fe

3+1.5]O4, and Fe3+[□0.33Fe

3+1.67]O4. They have

simply described the location of iron ions in crystal lattice.However, the solid solution mechanism is not simplyexplained by the distribution of iron ions between the tetrahe-dral and octahedral sites for 0.57<y in LiFeyMn2−yO4 [46].The effective ionic radius of Fe3+ and Li+ on tetrahedralcoordination is 0.49 and 0.59 Å, respectively [45]. Becauseof these reasons, we may explain that the effect of Fe substi-tution for Li causes a decrease of lattice parameters of surface-modified samples.

Fig. 1 Thermal gravimetriccurve of Fe2O3 precursor powderas the coating medium obtainedfrom evaporation of solution

Ionics (2014) 20:323–333 325

The surface morphology change of LiMn2O4 after modi-fying with Fe2O3 is presented in Fig. 3. It is seen that theparticle size of uncoated LiMn2O4 particles is about 200 nm,and there is no size difference between the bare and the Fe2O3-modified LiMn2O4 particles. On the other hand, the surfacemorphology of the uncoated LiMn2O4 particles is smooth, asshown in Fig. 3a, but it becomes aggregated after coating withFe2O3 (Fig. 3b–e).

Figure 4 shows energy dispersed X-ray (EDAX) analysisimage of 1.0 wt% Fe2O3-coated LiMn2O4. As can be seen inthe image, it was found that the distribution of iron on thepowder surface is fairly uniform. As a result, it can be con-firmed that the surface of LiMn2O4 was successfully coatedwith Fe2O3 particles.

Change in zeta potential of Fe2O3, the bare LiMn2O4, andFe2O3-coated LiMn2O4 powders as a function of pH is givenin Fig. 5. In Fig. 5, it can be seen that the zeta potential for allsamples is negative at above pH 3.0, but less negative forFe2O3- and Fe2O3-coated LiMn2O4 powder compared withthe bare LiMn2O4, which indicates change in the surface chargeof LiMn2O4 particles after surface modification treatment. Inaddition, this experiment showed that isoelectric points ofFe2O3- and Fe2O3-coated LiMn2O4 are between pH 2.0 and3.0; however, the isoelectric point of bare LiMn2O4 is betweenpH 1.0 and 2.0.

The Raman spectra were employed to reveal subtle infor-mation of the surface structure, which are presented in Fig. 6.The well-known Raman spectrum (RS) of LiMn2O4 can beseen, consistent with reports by other researchers [47, 48]. The

Fig. 2 X-ray diffraction patternsof (a) Fe2O3 produced fromcalcination of the precursorpowder of coating material, (b)the bare LiMn2O4, (c) 1.0 wt%modified LiMn2O4, (d) 2.0 wt%modified LiMn2O4, (e) 3.0 wt%modified LiMn2O4, (f) 4.0 wt%modified LiMn2O4, and (g)5.0 wt% modified LiMn2O4

Table 1 The cubic lattice parameters for LiMn2O4 and surface-modifiedsamples

Compounds Lattice parameter (Å)

LiMn2O4 8.239 (3)

1 wt% Fe2O3-modified LiMn2O4 8.218 (9)

2 wt% Fe2O3-modified LiMn2O4 8.226 (9)

3 wt% Fe2O3-modified LiMn2O4 8.227 (2)

4 wt% Fe2O3-modified LiMn2O4 8.225 (5)

5 wt% Fe2O3-modified LiMn2O4 8.229 (7)

326 Ionics (2014) 20:323–333

RS of bare LiMn2O4 is dominated by a strong and broad bandat approximately 640 cm−1 with a shoulder at 565 cm−1. Three

low-wave number bands having a medium intensity are ob-served at approximately 486, 437, and 354 cm−1. The Raman

Fig. 3 SEM images of the bareLiMn2O4 and Fe2O3-surface-modified LiMn2O4 samples

Fig. 4 Electron probe microanalysis image of Fe element for 1.0 wt%Fe2O3-modified spinel

Fig. 5 Zeta potential analysis of Fe2O3, LiMn2O4, and 3 wt% Fe2O3-modified LiMn2O4

Ionics (2014) 20:323–333 327

peak centered at 640 cm−1 can be assigned to the symmetricMn–O stretching vibration of MnO6 groups in LiMn2O4. TheRaman scattering peak with medium intensity located at486 cm−1 has the F2g(2) symmetry, whereas the weak bandslocated at 437 and 354 cm−1 have the Eg and F2g(3) symmetry,respectively. The F2g(3) mode is related to the Li–O motion,i.e., connected to the tetrahedral cation movements. However,the Raman scattering spectra of the 3-wt% Fe2O3-coatedLiMn2O4 spinels display a weaker band centered at 630–650 cm−1 that corresponds to the Mn–O stretching vibrations.Also, it shows that the Raman scattering peak intensitieswhich have F2g(2) and F2g(3) symmetry in the 3-wt%Fe2O3-coated LiMn2O4 spinels were weaker than the pristineone. This issue may be explained that Mn and Li concentra-tion decreases in the surface of coated materials.

Figure 7 shows charge–discharge profiles for the bareLiMn2O4 and various amounts of Fe2O3-modified LiMn2O4

at room temperature. These experiments were carried out atthe voltage range of 3.5 to 4.5 V and the current density148 mA g−1 at 1C rate. It can be clearly seen that the charge/discharge curves of all the samples have two voltage plateausat approximately 4.1 and 4.0 V, originating from the Mn3+/4+

redox couple, which indicated a remarkably well-definedLiMn2O4 spinel. Two voltage plateaus indicate that the inser-tion and extraction of lithium ions occur in two stages [49].The first plateau at about 4.02 V is attributed to the removal oflithium ions from half of the tetrahedral sites in which Li–Liinteraction occurs. The second voltage plateau observedaround at 4.15 V is ascribed to the removal of lithium ionsfrom the remaining tetrahedral sites. As can be seen in Fig. 7,the two potential plateaus for both samples were maintainedafter the 70th cycle. Figure 7 also presents that the initialdischarge capacity of surface-modified cathodes was lowerthan that of bare LiMn2O4 cathode. The capacity drop mayresult in two factors. First, introduction of electrochemically

inactive Fe3+ into LiMn2O4 definitely decreases the content ofactive material in the sample. The other factor is formation ofelectrochemically inactive Mn4+ due to the entrance of Fe3+,namely the decrease of electrochemically active Mn3+ to keepthe charge balance in the sample.

Figure 8 shows the result of discharge cycling at 1C ratebetween 3.5 and 4.5 V for bare LiMn2O4 and 1.0, 2.0, 3.0, 4.0,and 5.0 wt% of Fe2O3-surface-modified LiMn2O4 performedat room temperature, up to 70 cycles. The discharge capacityand capacity fading rates for different numbers of cycles areevaluated and presented in Table 2. The initial dischargecapacity of bare LiMn2O4 was 115.3 mAh/g and decayed to86 mAh/g after 70 cycles with capacity loss of 25.4 %.However, among the Fe2O3-modified cathodes, the 3.0 and4.0 wt% of Fe2O3-modified LiMn2O4 delivered an initialdischarge capacity of 93.2 and 91.2 mAh/g and decayed to90.8 and 89.1 mAh/g after 70 cycles, respectively. The dis-charge capacity fading of these coated cathodes have only 2.6and 2.3 %. The cycling behavior of the Fe2O3-modifiedLiMn2O4 electrodes indicated that the impact of Fe2O3 mod-ifying significantly improves the electrochemical perfor-mances at room temperature. As mentioned in other papers,surface coating of LiMn2O4 by some stable substances, e.g.,Cr2O3 [27], TiO2 [30], AlPO4 [33], CeO2 [34], SiO2 [36],La2O3 [50], and amphoteric oxides [51], enhanced the cyclingbehavior of the spinel cathode at room temperature. As shownin the literature, the capacity loss of La2O3-, SiO2-, amphotericoxide-, and AlPO4-coated cathodes were obtained at 7.7, 3.4,4.2, and 3.5 % after 70 cycles at 0.5C rate, respectively. Inaddition, the capacity loss of CeO2-, Cr2O3-, and TiO2-coatedcathodes were obtained 7.7, 5.6, and 17.5 % after 70 cycles at1C rate, respectively.

The typical cyclic voltammograms of bare LiMn2O4 andFe2O3-modified LiMn2O4 electrodes were carried out usingpure lithium foil acting as a counter and reference electrode inthe potential range between 3.0 and 4.4 V at a scan rate of100 μV/s. Two cells were freshly cycled and were then usedfor testing the galvanostatic charge/discharge studies at roomtemperature. After the completion of the 50th cycle, the twocells were again characterized by the cyclic voltammogram.As shown in Fig. 9, the cyclic voltammogram showed twocouples of redox peaks at around 4.02 and 4.15 V. The split ofredox peaks into two couples indicates that the electrochem-ical intercalation and de-intercalation reactions of lithium ionproceed in two steps. CV results of bare LiMn2O4 showed thatthe peak potential difference (ΔEp) of two redox peak around0.14 V. However, the CV curves of 3.0 and 4.0 wt% Fe2O3-modified LiMn2O4 showed that the peak potential difference(ΔEp) of two redox peak around 0.11 V. This observationindicates that the reversibility of these reactions in modifiedcathodes is high. The resulting lower value of ΔEp, whichattributed to 3.0 and 4.0 wt% Fe2O3-modified LiMn2O4 havegood reversibility at room temperature. Additionally, as shown

100 200 300 400 500 600 700 800 900 1000400

600

800

1000

1200

1400R

aman

Int

ensi

ty (

a.u.

)

Raman Shift (cm-1

)

Pristine LiMn2O

4

Fe2O

3 coated LiMn

2O

4

640

565486437354

Fig. 6 Raman scattering spectra of bare LiMn2O4 and 3.0 wt % Fe2O3-modified LiMn2O4

328 Ionics (2014) 20:323–333

in Fig. 9a, the redox peaks of bare LiMn2O4 electrode becomebroader with cycling, which implies that the structure ofLiMn2O4 is severely destroyed during the charge–discharge

process. In comparison with bare LiMn2O4, the redox peakskeep quite stable with cycling for the 3.0 and 4.0 wt% Fe2O3-surface-modified LiMn2O4. These differences indicate that the

0 20 40 60 80 100 120 1403,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5

4,6Po

tent

ial (

V)

1'st charge-discharge 70'th charge-discharge

Specific capacity (mAh/g)

a-bare LiMn2O

4

0 20 40 60 80 100 120

3,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5

1th charge-discharge70th charge-discharge

Pote

ntia

l (V

)

Specific Capacity (mAh/g)

b-1.0 wt.% Fe2O

3 modified LiMn

2O

4

0 20 40 60 80 1003,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5

1'st charge-discharge 70'th charge-discharge

Pote

ntia

l (V

)

Specific capacity (mAh/g)

c-2.0 wt.% Fe2O

3 modified LiMn

2O

4

0 20 40 60 80 1003,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5 d-3.0 wt.% Fe2O

3 modified LiMn

2O

4

1'st charge-discharge 70'th charge-discharge

Pote

ntia

l (V

)

Specific capacity (mAh/g)

0 20 40 60 80 100 1203,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5 e-4.0 wt.% Fe2O

3 modified LiMn

2O

4

1'st charge-discharge 70'th charge-discharge

Pote

ntia

l (V

)

Specific capacity (mAh/g)

0 20 40 60 80 100 1203,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

4,1

4,2

4,3

4,4

4,5 f-5.0 wt.% Fe2O

3 modified LiMn

2O

4

1'st charge-discharge 70'th charge-discharge

Pote

ntia

l (V

)

Specific capacity (mAh/g)

Fig. 7 Continuous charge–discharge curves during 70 cycles: a the bare LiMn2O4 and b–f Fe2O3-modified LiMn2O4. The applied current density is148 mA g−1 (1C rate) at room temperature. Li metal was used as the anode

Ionics (2014) 20:323–333 329

Fe2O3-modified LiMn2O4 electrode has better reversibilitythan that of the bare spinel electrode.

The changes of surface morphology before and aftercharge–discharge tests are given in Fig. 10. As shown in thefigures, the SEM micrographs of the bare LiMn2O4 electrodeafter 70 cycles at room temperature (Fig. 10b) were differentfrom those of the Fe2O3-modified LiMn2O4 electrodes(Fig. 10d, f, h, j, l). The Fe2O3-modified LiMn2O4 electrodespreserved its original surface morphology, while physicaldestruction on the surface of the bare LiMn2O4 electrodewas observed. Abnormal surfaces which formed on the barespinel after the cell test may be a by-product of the decompo-sition of the electrolyte [52].

In the literature, some research groups [53, 54] confirmedthat hydrofluoric acid (HF) generated during cycling when

using LiPF6-based electrolyte was responsible for the disso-lution of manganese. In fact, preparation of H2O-free electro-lyte containing LiPF6 in organic solvent is difficult. The smallamount of water (though the amount is less than 20 ppm)facilitates decomposition of electrolytic salt, LiPF6. Thus, HFis formed as a by-product by the following reaction [55]:

LiPRF6 þ H2O → LiF þ POF3 þ 2HF

0 10 20 30 40 50 60 70 8085

90

95

100

105

110

115

120Sp

ecif

ic d

isch

arge

cap

acity

(m

Ah/

g)

Cycle no

Bare LiMn2O4

1Wt% Fe2O3 modified LiMn2O4

2Wt% Fe2O3 modified LiMn2O4

3Wt% Fe2O3 modified LiMn2O4

4Wt% Fe2O3 modified LiMn2O4

5Wt% Fe2O3 modified LiMn2O4

Fig. 8 Cycling performances of the bare LiMn2O4 and surface-modifiedLiMn2O4 cathodes at a current level of 1C (148 mA g−1) in the voltagerange of 3.5–4.5 V at room temperature

Table 2 Discharge capacity performance of bare LiMn2O4 and surface-treated LiMn2O4 cells

Cathodematerial

Discharge capacity (mAh/g) Capacityloss (%)

1st 5th 10th 30th 50th 60th 70th

LiMn2O4 115.3 106.2 101.3 93.0 89.3 89.0 86.0 25.4

1 wt% Fe2O3-modified LiMn2O4

98.2 95.7 94.9 94.9 94.5 94.5 93.7 4.6

2 wt% Fe2O3-modified LiMn2O4

94.4 94.4 94.4 92.8 91.9 91.9 90.7 3.9

3 wt% Fe2O3-modified LiMn2O4

93.2 93.6 92.8 92.4 91.6 91.6 90.8 2.6

4 wt% Fe2O3-modified LiMn2O4

91.2 92.4 90.8 90.8 89.5 89.5 89.1 2.3

5 wt% Fe2O3-modified LiMn2O4

93.4 93.4 92.5 90.9 89.7 89.2 88.8 4.9

Loss of discharge capacity at the last cycle is compared with that atmaximum discharge capacity

3,4 3,6 3,8 4,0 4,2 4,4 4,6-8,0x10-4

-6,0x10-4

-4,0x10-4

-2,0x10-4

0,0

2,0x10-4

4,0x10-4

6,0x10-4

8,0x10-4

50'th cycle

1'st cycle

Cur

rent

/A

Potential / V

b-3.0 wt.% Fe2O

3 modified LiMn

2O

4

3,4 3,6 3,8 4,0 4,2 4,4 4,6

-6,0x10-4

-4,0x10-4

-2,0x10-4

0,0

2,0x10-4

4,0x10-4

6,0x10-4

8,0x10-4

50'th cycle

Cur

rent

/ A

Potential / V

1'st cycle a- bare LiMn

2O

4

3,4 3,6 3,8 4,0 4,2 4,4 4,6-6,0x10-4

-4,0x10-4

-2,0x10-4

0,0

2,0x10-4

4,0x10-4

6,0x10-4

50'th cycle

Cur

rent

/ A

Potential / V

1'st cyclec- 4.0 wt.% Fe2O

3 modified LiMn

2O

4

Fig. 9 Typical cyclic voltammetric curves of initial and 50th cycle for aLiMn2O4, b 3.0 wt% Fe2O3-modified LiMn2O4, and c 4.0 wt% Fe2O3-modified LiMn2O4 cycled between 3.0 and 4.5 Vat a scan rate of 100μV/s

330 Ionics (2014) 20:323–333

The formed HF attacks active material so that the followingreaction occurs [56, 57]:

MnO þ 2HF → MnF2 þ H2O

This means that after the HF attack, the outermost surface istransformed to metal fluoride layer. Meanwhile, this reactiongenerates water molecules. The formed water molecules alsoreact with electrolyte salt again, and it ceaselessly leads to

Fig. 10 SEMmicrographs of the bare LiMn2O4 a as prepared; b after 70 cycles at RTand 1.0, 2.0, 3.0, 4.0, and 5.0 wt%Fe2O3-modified LiMn2O4; c , e ,g , i , k as prepared; and d , f , h , j , l after 70 cycles at RT, respectively

Ionics (2014) 20:323–333 331

decomposition of electrolyte salt, propagating more amount ofHF. The generated acid, HF, continuously attacks the activematerial, and active material decomposes as the cycle goes by,causing capacity fading.

2LiMn2O4 þ 4Hþ → 3λ−MnO2 þ Mn2þ þ 2Liþ þ 2H2O

As seen above reaction, Mn4+ would reside with activematerial as electro-inactive Li2MnO3 that makes discontinu-ous network of electron transfer. Mn2+ would be deposited onthe surface of negative electrode, and it is spontaneouslyreduced to metallic compound. This combination, in turn,deteriorates cell performances with increased cell impedance.

In summary, the coating layer on the surface of LiMn2O4

can lead to the following effects: (1) suppressing the dissolu-tion of Mn3+, (2) removing HF from electrolyte solutions withF-containing inorganic electrolyte salts, and (3) keeping goodstructural stability [58]. The above improvements lead toimprovement of electrochemical performance of LiMn2O4

such as capacity retention and high rate capability, and there-fore the Fe2O3-surface-modified LiMn2O4 may be attractiveparticularly for portable devices due to its lower cost andimproved cycling behavior.

Conclusion

The surface of LiMn2O4 particles was modified with iron (III)oxide by sol-gel method to improve the cycling behavior ofcathode materials. The properties of the coating and the elec-trochemical performance of themodified cathodematerial werecharacterized and compared with that of the standard(uncoated) LiMn2O4-based cathode. The discharge capacityof the Fe2O3-modified spinel electrodes shows an improvedcycling behavior compared with the bare one. Electrochemicalmeasurements showed that 3.0 and 4.0 wt% Fe2O3-surface-modified cathodes have better cycling performance than theothers. These improvements are attributed to the modificationto the surface property of LiMn2O4. The SEM micrograph ofall cathodes after cycling test revealed that the changes anddecomposition on the surface morphology of bare LiMn2O4

were higher than those of the surface-modified cathodes. Theimproved performance of the surface modified samples isascribed to Fe2O3 coated on the surface of LiMn2O4, whichsuppresses dissolution of Mn3+ ions in the cathode material.Therefore, Fe2O3 surface modification is an effective way toimprove the electrochemical performance of LiMn2O4 cathodematerial.

Acknowledgments This study was financially supported by the Re-search Foundation of Erciyes University (FBA-08-439) and InternationalPost Doctoral Research Fellowship Programme (2219) of The Scientificand Technological Research Council of Turkey. The authors would like tothank Mr. İhsan Akşit for the SEM observation.

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