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Solid State Sciences 4 (2002) 1031–1038 www.elsevier.com/locate/ssscie Synthesis, structure and electrochemistry of LiMn 2y Cr y/2 Cu y/2 O 4 (0.0 y 0.5) prepared by wet chemistry C. Julien a,, I. Ruth Mangani b , S. Selladurai b , M. Massot c a Laboratoire des Milieux Désordonnés et Hétérogènes, CNRS-UMR 7603, Université Pierre et Marie Curie, 4 place Jussieu, case 86, 75252 Paris cedex 05, France b Solid State Ionics Laboratory, MIT, Anna University, Chennai 600044, India c Laboratoire de Physique des Milieux Condensés, CNRS-UMR 7602, Université Pierre et Marie Curie, 4 place Jussieu, case 77, 75252 Paris cedex 05, France Received 13 March 2002; received in revised form 15 May 2002; accepted 22 May 2002 Abstract The LiMn 2 O 4 co-doped with copper and chromium forming LiMn 2y Cr y/2 Cu y/2 O 4 spinel phases have been synthesized by wet chemistry technique using an aqueous solution of metal acetates and dicarboxylic acid (succinic acid) as a complexing agent. The structural properties of the synthesized products have been investigated by X-ray powder diffraction, Raman scattering, and Fourier- transform infrared spectroscopy. To improve the rechargeable capacity of Li//LiMn 2y Cr y/2 Cu y/2 O 4 cells, the electrochemical features of LiMn 2y Cr y/2 Cu y/2 O 4 compounds have been evaluated as positive electrode materials. The structural properties of these oxides are very similar to LiMn 2 O 4 , their electrochemical performances show that the capacity is maintained 95% of the initial value at the 36th cycle for y = 0.1, this being explained by the change of Mn 3+ /Mn 4+ ratio in doped phases. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Spinel-LiMn 2y Cr y/2 Cu y/2 O 4 ; Soft-chemistry technique; Lithium-ion batteries; Intercalation 1. Introduction Application of the spinel-type manganese oxide mate- rials based on Li–Mn–O phases as positive electrodes in rechargeable lithium-ion batteries is due to their low cost and high environmental acceptability. The use of LiMn 2 O 4 stems from the fact that the Li + ions can be removed and rein- serted into this compound topotactically [1–5]. Reversible insertion–extraction occurs in the 4 V range and gives rise to two voltage plateaus located at 4.05 and 4.15 V vs. Li/Li + . Instead, its moderate specific gravimetric capacity of 148 mAh g 1 tends to decline upon cycling. This behav- iour has been connected to such factors as (i) a slow dissolution of the material as a consequence of a disproportionation reaction, (ii) a relative structural instability when the limit λ-MnO 2 (spinel structure) is approached at the end of charge, * Correspondence and reprints. E-mail address: [email protected] (C. Julien). (iii) the Jahn–Teller effect at the end of discharge when, especially the Mn 3+ /Mn 4+ ratio is higher than unity [6]. Progress has already been made to overcome the capac- ity fading by doping with several cations [6–14]. In princi- ple, the reduction of Mn 3+ concentration can be obtained by substitution of monovalent, divalent, trivalent ions for Mn ions in LiMn 2y M y O 4 spinels (M = Al, Co, Mg, Cr, Ni, Fe, Ti and Zn). This principle was early demonstrated by Gummow et al. [6] who have pointed out that doping with Li + or Mg 2+ enhanced the stability of the LiMn 2 O 4 spinel phase. Pistoia’s work on LiMn 2y Cr y O 4 has shown that Cr substitution considerably enhances the stability of the 3 V plateau [7]. Guohua et al. [13] have suggested that the im- provement of the stability is due to stronger M–O bonding of the MO 6 octahedron of partially substituted LiMn 2y M y O 4 (M = Co, Cr, Ni) in comparison with that of Mn–O of the parent LiMn 2 O 4 spinel. The lithium intercalation properties of LiMn 2y Cu y O 4 have been reported by Ein-Eli et al. [14], while LiMn 2y Cr y O 4 compounds grown by solid-state re- action have been studied by Guyomard et al. [15,16]. These 1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1293-2558(02)01357-2

Synthesis, structure and electrochemistry of LiMn2−yCry/2Cuy/2O4 (0.0⩽y⩽0.5) prepared by wet chemistry

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Page 1: Synthesis, structure and electrochemistry of LiMn2−yCry/2Cuy/2O4 (0.0⩽y⩽0.5) prepared by wet chemistry

Solid State Sciences 4 (2002) 1031–1038www.elsevier.com/locate/ssscie

Synthesis, structure and electrochemistry of LiMn2−yCry/2Cuy/2O4(0.0� y � 0.5) prepared by wet chemistry

C. Juliena,∗, I. Ruth Manganib, S. Selladuraib, M. Massotc

a Laboratoire des Milieux Désordonnés et Hétérogènes, CNRS-UMR 7603, Université Pierre et Marie Curie,4 place Jussieu, case 86, 75252 Paris cedex 05, France

b Solid State Ionics Laboratory, MIT, Anna University, Chennai 600044, Indiac Laboratoire de Physique des Milieux Condensés, CNRS-UMR 7602, Université Pierre et Marie Curie,

4 place Jussieu, case 77, 75252 Paris cedex 05, France

Received 13 March 2002; received in revised form 15 May 2002; accepted 22 May 2002

Abstract

The LiMn2O4 co-doped with copper and chromium forming LiMn2−yCry/2Cuy/2O4 spinel phases have been synthesized by wetchemistry technique using an aqueous solution of metal acetates and dicarboxylic acid (succinic acid) as a complexing agent. Thestructural properties of the synthesized products have been investigated by X-ray powder diffraction, Raman scattering, and Fourier-transform infrared spectroscopy. To improve the rechargeable capacity of Li//LiMn2−yCry/2Cuy/2O4 cells, the electrochemical features ofLiMn2−yCry/2Cuy/2O4 compounds have been evaluated as positive electrode materials. The structural properties of these oxides are verysimilar to LiMn2O4, their electrochemical performances show that the capacity is maintained 95% of the initial value at the 36th cycle fory = 0.1, this being explained by the change of Mn3+/Mn4+ ratio in doped phases. 2002 Éditions scientifiques et médicales Elsevier SAS.All rights reserved.

Keywords: Spinel-LiMn2−yCry/2Cuy/2O4; Soft-chemistry technique; Lithium-ion batteries; Intercalation

1. Introduction

Application of the spinel-type manganese oxide mate-rials based on Li–Mn–O phases as positive electrodes inrechargeable lithium-ion batteries is due to their low cost andhigh environmental acceptability. The use of LiMn2O4 stemsfrom the fact that the Li+ ions can be removed and rein-serted into this compound topotactically [1–5]. Reversibleinsertion–extraction occurs in the 4 V range and gives riseto two voltage plateaus located at 4.05 and 4.15 V vs.Li/Li+. Instead, its moderate specific gravimetric capacityof 148 mAh g−1 tends to decline upon cycling. This behav-iour has been connected to such factors as

(i) a slow dissolution of the material as a consequence of adisproportionation reaction,

(ii) a relative structural instability when the limitλ-MnO2

(spinel structure) is approached at the end of charge,

* Correspondence and reprints.E-mail address: [email protected] (C. Julien).

(iii) the Jahn–Teller effect at the end of discharge when,especially the Mn3+/Mn4+ ratio is higher than unity[6].

Progress has already been made to overcome the capac-ity fading by doping with several cations [6–14]. In princi-ple, the reduction of Mn3+ concentration can be obtained bysubstitution of monovalent, divalent, trivalent ions for Mnions in LiMn2−yMyO4 spinels (M= Al, Co, Mg, Cr, Ni,Fe, Ti and Zn). This principle was early demonstrated byGummow et al. [6] who have pointed out that doping withLi+ or Mg2+ enhanced the stability of the LiMn2O4 spinelphase. Pistoia’s work on LiMn2−yCryO4 has shown that Crsubstitution considerably enhances the stability of the 3 Vplateau [7]. Guohua et al. [13] have suggested that the im-provement of the stability is due to stronger M–O bonding ofthe MO6 octahedron of partially substituted LiMn2−yMyO4(M = Co, Cr, Ni) in comparison with that of Mn–O of theparent LiMn2O4 spinel. The lithium intercalation propertiesof LiMn2−yCuyO4 have been reported by Ein-Eli et al. [14],while LiMn2−yCryO4 compounds grown by solid-state re-action have been studied by Guyomard et al. [15,16]. These

1293-2558/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.PII: S1293-2558(02 )01357-2

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1032 C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038

spinels constitute a class of positive electrode materials withhigh operating voltage for lithium batteries.

Recently, Julien et al. [12] have shown that lithium in-tercalation properties of LiMn2−yAlyO4 phases synthesizedby wet chemistry were maintaining their electrochemical ca-pacity at 95% of the initial value at the 50th cycle. In thiscontext, we have synthesized the LiMn2−yCry/2Cuy/2O4

spinel phases adopting the same approach, which providesgood stoichiometric control of the spinel electrochemically-active materials. Obviously, the morphology and microstruc-ture of the samples, i.e., nanostructured materials, can be ex-perimentally controlled in contrast to the conventional solid-state reaction products obtained at high temperature, whichdo not ensure a high level of homogeneity in the final com-position. Thus, the consequence of the submicronic charac-ter of powders prepared fromchimie douce is a modificationof the spinel electrochemical properties.

This paper presents the synthesis of spinel LiMn2−y-Cry/2Cuy/2O4 (0.0 � y � 0.5) by a wet chemistry method.The effect of partial (Cr,Cu) co-substitution for Mn3+ inLiMn2O4 positive electrode on its electrochemical behaviorin Li cells has been investigated. Structural properties havebeen studied by X-ray powder diffraction (XRPD), Ramanscattering (RS) and Fourier transform infrared (FTIR) spec-troscopy. The electrochemical activity of the synthesizedLiMn2−yCry/2Cuy/2O4 powders has been tested in lithium-containing test cells that include the above spinels as posi-tive electrode materials by means of galvanostatic (multiplecharge–discharge) technique.

2. Experimental

The undoped LiMn2O4 and LiMn2−yCry/2Cuy/2O4

(0.1� y � 0.5) powders were prepared using the same syn-thesis method, i.e., the succinic acid-assisted wet chem-istry. In this technique, the dicarboxylic acid (C4H6O4, m.w.118.09) plays the role of chelating agent. A stoichiometricproportion of high purity manganese, chromium, copper, andlithium acetates (AR-grade from Aldrich) are dissolved in aminimum volume of distilled water. An equal volume of a1 mol per liter aqueous solution of succinic acid is added asa complexing agent in the above aqueous solution. The con-centration of the complexing agent is adjusted carefully toget a solution with the pH in the range between 3–4. Uponadding the above complexing agent, homogeneous precipi-tates are obtained owing to the poor solubility of manganeseand lithium succinates, which are finely dispersed in the so-lution medium. It is believed that the carboxylic (COOH)groups on the succinic acid form a chemical bond with themetal ions and these mixtures develop the extremely vis-cous paste-like substance upon slow evaporation. It is alsopresumed that the lithium and transition-metal cations aretrapped homogeneously within the paste; this ensures themolecular level mixing and eliminates the need for long-

range diffusion during the subsequent formation of spinellithium manganate.

The paste is further dried at 120◦C to obtain the driedprecursor mass. The precursor is then allowed to decomposein air at around 300◦C. The decomposition results in a largeexothermic reaction which originates from the combustionof organic species present in the precursor mass. Thisexothermic process enhances the oxidation reaction andonset of the phase formation of the spinel compound.The process yields a bluish black coloured voluminousmass of spinel LiMn2−yCry/2Cuy/2O4. Experimental detailsregarding the access of oxygen to the reactants during thecombustion stage, as well as specification of the time of heattreatment and oxidation have been described in a previouspaper [17].

Elemental analysis of the final products was determinedusing an induced-coupled-plasma mass (ICPM) spectrom-eter (model VG Plasma Quad II-S). The structure of theLiMn2−yCry/2Cuy/2O4 samples was characterized by X-raypowder diffraction (XRPD) using a diffractometer (Philipsmodel PW1830) with nickel-filtered CuKα radiation (λ =1.5406 Å). The diffraction patterns were taken at room tem-perature in the range of 5� 2θ � 80◦ using step scans. TheXRPD data were tentatively analyzed by the Rietveld profilerefinements using the FULLPROF program. Raman scatter-ing (RS) spectra were taken at room temperature in a quasi-backscattering configuration. A Jobin–Yvon (model U1000)double monochromator with holographic gratings and acomputer-controlled photon-counting system was used. Thelaser light source was the 514.5 nm line radiation from aSpectra-Physics 2020 argon-ion laser. RS spectra are theaverage of 12 scans obtained with a spectral resolution of2 cm−1. To avoid sample photodecomposition or denatu-ration, RS spectra were recorded using a low excitationpower of 10 mW. Infrared absorption spectra were recordedat room temperature using a Fourier-transform interferom-eter (model Bruker IFS113v). In 100–1200 cm−1 region,this vacuum bench apparatus was equipped with 3.5 µmthick Mylar and KBr/Ge beamsplitter, a global source,and DTGS/PE far-infrared and MCT/KBr detector. Sampleswere ground to fine powders dispersed in ICs pellet, whichis a non-absorbing medium in the wavenumber range stud-ied. Data were collected in transmission mode at a spectralresolution of 2 cm−1 after 256 scans in vacuum atmosphere.The curve analysis was based on the original algorithm ofnon-linear peak fitting described by Marquardt and knownas the Levenberg–Marquardt method [18]. The fitting calcu-lation were done assuming a linear baseline for the spectraand that the bands introduced in the fit have a mixed Gauss–Lorentz line shape for Raman lines, while FTIR bands wereLorentz-type line shape.

Electrochemical studies were carried out on the synthe-sized products annealed at 600◦C in order to test their suit-ability as positive electrode-active materials in high volt-age lithium-containing batteries. The above tests were per-formed to measure quantitatively the electrochemical ca-

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C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038 1033

pacity of the synthesized spinel product. The laboratory-scale Li//LiMn2−yCry/2Cuy/2O4 cells were housed in aTeflon laboratory-cell hardware employing a non-aqueousLi+-ion conducting organic electrolyte. The electrolyte wasa 1 M LiPF6 in EC-DMC in 2 : 3 volumetric ratio. Thiselectrolyte was selected because of its reported stabilityto oxidation up to 5 V [19]. A microporous polypropy-lene film (Celgard 2500) was used as a separator. The typ-ical composite cathode consisted of the mixture of spinel-LiMn2−yCry/2Cuy/2O4 powders (600◦C), acetylene blackand colloidal PTFE binder in the 90: 5 : 5 weight ratio. ThePTFE-acetylene black was used to provide good electricalconductivity as well as mechanical toughness between activegrains. To assess the quasi-open-circuit voltage profiles, gal-vanostatic charge–discharge cycles were recorded at a slowscan mode using a Mac-Pile system as follows. A currentdensity of 0.1 mA cm−2 was supplied for 1 h, correspond-ing to a lithium extraction of about 0.01 mol from 1 molof the electrode material. This was followed by a relaxationtime of 0.6 h before the next charging started. The chargingwas stopped when the closed-circuit voltage reached 4.5 V.The apparent lithium content of the charge–discharge com-pounds was estimated using the current passed and the massof the electrode material.

3. Results and discussion

3.1. Synthesis and structural characterization

The exact temperature of the phase formation reactionof the spinel LiMn2−yCry/2Cuy/2O4 has been studied usingthermogravimetric analysis (DTA / TG) on the precursorcomplex in the range from 30 to 600◦C. These resultsdisplay two discrete weight-loss regions occurring at ca.110 and 300◦C. The DTA curve shows two distinguishabletransformation enthalpies. An endothermic peak observedat about 110◦C is accompanied by a noticeable weightloss in the TG curve. This is attributed to the superficialwater loss due to the hygroscopic nature of the precursorcomplex. As the process of heating continues, an exothermictransformation begins to appear at 300◦C indicating theonset of the decomposition and/or the soft oxidation of themolecular precursor. The huge exothermic reaction indicatesthe decomposition of organic species present in the precursorcomplex, which burn in the presence of atmospheric oxygen.More than 30% of the weight loss occurring during this stagearises from the violent oxidation–decomposition reaction.It appears that succinic acid acts as a fuel in the pyrolysisof the molecular precursors and accelerates the process ofdecomposition as mentioned above. This process leads to theformation of the spinel-type structure when the precursor isheated above 350◦C.

Elemental analysis of LiMn2−yCry/2Cuy/2O4 oxides wascarried out by ICPM measurements. All specimens have acomposition near the nominal one measured with an accu-

Fig. 1. X-ray powder diffraction patterns of LiMn2−yCry/2Cuy/2O4(0.1 � y � 0.5) samples annealed at 600◦C. Bragg lines were indexedusing the cubic lattice with Fd̄3m space group.

racy of ±0.5%. These results prove that the wet-chemicalsynthesis assisted by succinic acid provides stoichiometricsamples and that no significant loss of lithium oxide has oc-curred during the thermal treatment of precursors.

The structure of the resulting LiMn2−yCry/2Cuy/2O4products was investigated by XRPD. The results show well-defined peaks even at temperature as low as 350◦C, indi-cating that the product obtained immediately after decom-position has gained the single-phase spinel structure with-out any residual impurities observable from XRPD measure-ments. XRPD diagrams of LiMn2−yCry/2Cuy/2O4 (0.1 �y � 0.5) materials calcined at 600◦C exhibit patterns withthe strongest (1 1 1) Bragg line, as presented in Fig. 1. TheBragg peaks of the LiMn2−yCry/2Cuy/2O4 spinels measuredafter annealing the decomposed product at 600◦C were in-dexed to a cubic system with a lattice parameter, which wasrefined with space group Fd3̄m. Unit cell parameters in thecubic setting were determined by the least squared methodusing thirteen diffraction lines. For the undoped materialheated at 600◦C, we founda = 8.2360± 0.0017 Å, in goodagreement with the literature value for the spinel LiMn2O4[8]. The structure of LiMn2−yCry/2Cuy/2O4 (0.1� y � 0.5)nominal composition materials was analyzed by Rietveldrefinement of their XRPD patterns, but structural refine-ments of such complex system are rather difficult, particu-larly when three different cation types are disordered overone crystallographically independent site. Despite the diffi-culties that were encountered (Rp = 9.8%), the results showthat LiMn2−yCry/2Cuy/2O4 samples exhibit patterns, whichare characteristic of the normal spinel structure (Fd3̄m spacegroup) with an appropriate site occupancy for cations tak-ing into account the structural model proposed by Ein-Eli etal. [14]. Here, we assume that copper and chromium ions

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1034 C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038

Table 1Refined crystallographic parameters for LiMn1.5Cr0.25Cu0.25O4 samplesas determined from X-ray diffraction dataa

Atom Site Position B (Å2) Site occupancy (%)

Li 8a 0.125 1.29 95Cu(1) 8a 0.125 0.38 5Mn 16d 0.5 0.41 78Cr 16d 0.5 0.44 15Cu(2) 16d 0.5 0.55 7O 32e 0.265 0.95 100

a Rp = 9.8%,RBragg= 7.4%.

Fig. 2. Variation of the cubic lattice parameter of LiMn2−yCry/2Cuy/2O4as a function of the dopant fractiony.

are in their Cu2+ and Cr3+ state, respectively, occupyingtetrahedral and octahedral sites of the spinel lattice. OurXRPD structure profile refinement suggests that most of the(Cr,Cu) ions replace Mn3+ in the 16d octahedral site with asmall fraction of Cu2+ in the 8a tetrahedral holes currentlyoccupied by lithium ions in the A[B2]O4 spinel structure.Since, we observe the appearance of the(2 2 0) Bragg lineat ca. 2θ = 30◦, which is extremely sensitive to the occu-pancy of the 8a tetrahedral site. It is possible for lithiumand copper ions to exchange on the tetrahedral sites. There-fore, subsequent refinement was carried out on the spinel[Li 0.95Cu0.05]8a[Mn1.5Cr0.25Cu0.2]16dO4 which yielded thebest fit listed in Table 1. The important issue of cationic dis-tribution will be discussed in Section 3.2.

The cubic lattice parameter of LiMn2−yCry/2Cuy/2O4spinel structures decreases with increasing the substituting(Cr,Cu) content. The(4 4 0) Bragg line and those in the in-terval 56� 2θ � 72◦ display a slight displacement due tothe decrease of the lattice parameter. Fig. 2 shows the varia-tion of the cubic lattice parameter of LiMn2−yCry/2Cuy/2O4as a function of the degree of substitution. It is obvious thatthe slight lattice parameter shift observed is concordant to anincreasing concentration of Mn4+ ions in the spinel struc-ture which results from the substitution of Mn3+ ions bytransition metal having lower oxidation state, for example,Cu2+ ions. This trend is consistent with the results of previ-ous workers [9,14].

Fig. 3. Raman scattering spectra of LiMn2−yCry/2Cuy/2O4 spinel oxidesannealed at 600◦C with (a) y = 0.0, (b) y = 0.1 and (c)y = 0.5. Spectrawere recorded with the 514.5 nm line of an Ar+ laser line at 10 mW powerexcitation.

The LiMn2−yCry/2Cuy/2O4 powders prepared by wet-chemical method possess grains of average diameter smallerthan 1 µm with a fairly narrow particle size distribution.The grains are almost connected and ensure the high specificsurface area of 3.0–4.5 m2 g−1 measured by BET technique.

3.2. Local structure studies

The LiMn2−yCry/2Cuy/2O4 powders annealed at 600◦Cwere also characterized by means of vibrational spec-troscopy, namely Raman scattering and FTIR, to determinethe local structure and the nature of the cationic environ-ment in the spinel phases. Fig. 3 shows the room temper-ature RS spectrum of undoped LiMn2O4 and the RS spec-tra of the synthesized LiMn2−yCry/2Cuy/2O4 samples an-nealed at 600◦C for y = 0.0, 0.1, andy = 0.5 (curves (a)–(c), respectively). As reported before, the RS spectrum ofLiMn2O4 is dominated by a strong and broad band at ca.625 cm−1 with a shoulder at 580 cm−1 [20–22]. A band witha medium intensity appears at ca. 482 cm−1, while two weakbands are observed at ca. 370 and 298 cm−1. The RS spec-tra of LiMn2−yCry/2Cuy/2O4 powders are similar to theirLiMn2O4 counterpart.

The cubic spinel LiMn2O4 possesses the O7h spectro-

scopic symmetry. It has a general structural formula Li-[Mn2]O4, where the manganese cations reside on the octa-hedral 16d sites, the oxygen anions on the 32e sites, andthe lithium ions occupy the tetrahedral 8a sites. Analy-sis of the vibrational spectra of LiMn2O4 with O7

h (Fd3̄mspace group) yields five Raman-active modes (A1g + Eg +

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C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038 1035

Fig. 4. FTIR absorption spectra of samples LiMn2−yCry/2Cuy/2O4(0.1 � y � 0.5) annealed at 600◦C. Spectra were recorded using the ICsdispersion technique.

3F2g) [23]. It is also convenient to analyze these spectra interms of localized vibrations, considering the spinel struc-ture built of MnO6 octahedra and LiO4 tetrahedra [25].For LiMn1.9Cr0.05Cu0.05O4, the Raman band located at ca.621 cm−1 can be viewed as the symmetric Mn–O stretchingvibration of MnO6 groups. This band is assigned to the A1gsymmetry in the O7h spectroscopic space group. Its broaden-ing is related with the cation–anion bond lengths and poly-hedral distortion occurring in LiMn2−yCry/2Cuy/2O4. Theintensity of the shoulder located at 582 cm−1 is slightly en-hanced upon (Cr,Cu) substitution. This may be due to thechange of Mn3+ and Mn4+ proportion vs. (Cr,Cu) ions inthe material. The RS peaks located at 370 and 495 cm−1

have the F2g symmetry. One can state that in the ideal cubicspinel LiMn2O4, the Mn3+ and Mn4+ cations are consideredas crystallographically equivalent (16d sites) in agreementwith XRD data. Then, occupation probabilities of 0.5 mustbe affected for each cation in 16d site. Hence, a loss of trans-lation invariance certainly occurs in LiMn2−yCry/2Cuy/2O4,due to local lattice distortion around the different Mn3+ andMn4+ cations. As a result, a breakdown in the Raman se-lection rules is observed, which explains the observation ofbroad bands (disorder in the cationic distribution) and thefact that more modes than expected are observed in cubicLiMn2−yCry/2Cuy/2O4 spinels (Fig. 3). One may also no-tice that Cu2+ is a peculiar ion for entering octahedral sites,where it forms strong covalent bonds and causes a Jahn–Teller effect [24].

Fig. 4 shows the FTIR absorption spectra of LiMn2−y-Cry/2Cuy/2O4 samples as a function of the doping con-tent. In the compositional range 0.1 � y � 0.4, the IRspectra of the doped LiMn2−yCry/2Cuy/2O4 phases havethe same shape as the initial spinel LiMn2O4. Our curvefittings were realized using four Lorentzian-type infrared

Fig. 5. Evolution of the infrared-active modes of LiMn2−yCry/2Cuy/2O4as a function of the dopant fractiony. Frequencies were determined usingthe Levenberg–Marquardt fitting method with Lorentzian lineshape.

bands, while an additional band was used to include thestrong low-wavenumber contribution below 300 cm−1 forLiMn1.5Cr0.25Cu0.25O4. The experimental IR spectra forLixMn2O4 consist of a series of broad bands between 150and 650 cm−1, which all correspond to modes with the F1usymmetry. Four infrared-active modes are expected from thefactor group analysis O7h of LiMn2O4. The FTIR spectrumof LiMn2O4 spinel is dominated by two strong absorptionbands at ca. 615 (ν1) and 510 cm−1 (ν2) involving mainlydisplacement of oxide ions. They are attributed primarily tothe asymmetric stretching modes of MnO6 octahedra [20,21]. Three weak bands are observed in the low-frequencyregion at ca. 225, 275 and 365 cm−1. A weak band is ap-peared out at ca. 420 cm−1. It is obvious that the total num-ber of experimental bands is close to that predicted by thegroup factor analysis (O7h) of the spinel phase, for which fourvibrations are infrared active modes with the F1u symmetry.Such vibrations are rather more complex because an isolatedMnO6 octahedron does not exist in the spinel framework,but octahedra are connected with other MnO6 octahedra andLiO4 tetrahedra. Therefore, the low-frequency bands at ca.225, 275 and 420 cm−1 can be primarily assigned to the de-formation vibrations of the O–Mn–O groups, i.e., bendingmodes of MnO6 octahedra.

It was recognized that the atomic displacements con-tributing to the infrared spectra of spinel phases vary fromcompound to compound, depending on the masses, charges,and chemical properties of ions [25]. Because FTIR spec-troscopy is capable of probing directly the near neigh-bor environment of the cation, we can study the localstructure around cations in LiMn2−yCry/2Cuy/2O4 materi-als. Fig. 5 shows the evolution of the infrared-band po-sition as a function of the degree of (Cr,Cu) substitutionin LiMn2−yCry/2Cuy/2O4 samples. It is experimentally ob-

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1036 C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038

Fig. 6. Electrochemical features of Li//LiMn2−yCry/2Cuy/2O4 cells during the first charge–discharge cycle: (a)y = 0.1, (b) y = 0.2, (c) y = 0.4 and (d)y = 0.5. The non-aqueous cells employing the electrolyte of composition 1 M LiPF6 in EC-DMC (2 : 3) were cycled in the voltage range 3.0–4.5 V at0.1 mA cm−2 current density.

served that the infrared-ν2 band shifts toward the low-wavenumber side upon (Cr,Cu) doping, while the high-frequency band at 615 cm−1 remains almost at the same po-sition. Similar behavior was noticed in the local structurestudy of LiMn1.8Cu0.2O4 prepared by solid-state reactionwhich shows a frequency shift from 510 to 491 cm−1 of theinfrared-ν2 band [26]. These results are in good agreementwith the XRPD data, which display the lattice softening ofLiMn2O4 associated with the increasing amount of Mn4+ions in doped samples. Subsequently, the relative stability ofthe infrared-ν1 band demonstrates that partial replacement ofMn3+ in 16d octahedral sites seems to occur without caus-ing strains in the structure.

The cation distribution on both octahedral (B) and tetra-hedral (A) sites on the A[B2]O4 spinel structure is a verycomplex issue. Considering the Rousset’s work dealing withCuMn2O4 spinel, it is shown that Cu really likes to goonto tetrahedral sites as well [27]. A very good example isCuFe2O4 and CuFe2−xCrxO4 ferrites where Cu is pushedfrom octahedral to tetrahedral sites as the Cr content in-creases [28]. The Cu2+ migration from octahedral to tetra-hedral positions has also been observed in CoxCu1−xFe2O4

ferrites [29]. The presence of copper atoms in tetrahedral (A)sites is also revealed in LiMn2−yCry/2Cuy/2O4 spinels. Theoccurrence of the(2 2 0) diffraction line, which is extremelysensitive to the occupancy of the 8a tetrahedral site, is a

first indication. This is also seen by Rietveld refinements ofthe X-ray diffraction data of LiMn1.5Cr0.25Cu0.25O4 whichhave shown the presence of interstitial Cu+ ions in tetra-hedral sites. The FTIR spectra reveal the shift of the bandν2 toward the low-wavenumber side. The change in posi-tion is due to the change in the Mn3+–O2− internuclear dis-tances for the B- and A-sites. This is also an indication thatCu2+ ions occupy mainly the B-sites and some of them goesinto A-sites. It has been reported previously [30] that in nor-mal ferrites such as MnFe2O4 (normal largely spinel), theIR absorption bands depend on the nature of the octahe-dral cation and to a less extent on tetrahedral ions. Such abehaviour has also been observed for CuyMnxFe3−y−xO4spinels [28].

3.3. Electrochemical properties

The electrochemical features of synthesized LiMn2−y -Cry/2Cuy/2O4 spinels were examined using Li//LiMn2−y -Cry/2Cuy/2O4 cells subjected to constant current cycling.The results from the electrochemical Li//LiMn2−yCry/2-Cuy/2O4 cells are presented as plot of cell voltage vs.capacity (Fig. 6(a)–(d)) as well as curve of the incrementalcapacity−(∂x/∂V ) versus cell voltage (Fig. 7). Plateaus involtage versus capacity give rise to peaks in−(∂x/∂V ); soderivative plots are useful for displaying details. Fig. 6(a)–

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C. Julien et al. / Solid State Sciences 4 (2002) 1031–1038 1037

Fig. 7. Incremental capacity(−∂x/∂V ) as a function of the cell potentialshowing the systematic trend of decreasing capacity in the upper potentialregion of (Cr,Cu)-doped LiMn2O4 spinel. Data were taken from equilib-rium measurements.

(d) shows the charge–discharge curves at the first cycle ofLi//LiMn2−yCry/2Cuy/2O4 cells for y = 0.1, 0.2, 0.4 and0.5, respectively, under galvanostatic conditions at 22◦C.The cells were charged and discharged at current densitiesof 0.1 mA cm−2, while the voltage is monitored between 3.0and 4.5 V. In this potential domain, the charge–dischargecurves correspond to the voltage profiles characteristic ofthe spinel LiMn2O4 cathode material associated with lithiumoccupancy of tetrahedral sites, in agreement with previousworks [3–9]. The shape of the voltage curves indicateswhether the delithiated LiMn2−yCry/2Cuy/2O4 exists as asingle- or a multiple-phase. In the latter case the potentialis expected to be essentially invariant with composition.As shown in Fig. 6, at low amount of dopant (y < 0.2),the variation of the cell potential for the complete cellLi//LiMn2−yCry/2Cuy/2O4 shows the presence of threeregions during the lithium insertion–extraction processes,while a single-phase behavior is observed for heavily-dopedpositive electrodes (y > 0.2).

For LiMn1.9Cr0.05Cu0.05O4 and LiMn1.8Cr0.1Cu0.1O4,the first region (I) is characterized by an S-shaped volt-age curve, whereas the second region (II) corresponds to aplateau portion. Region (III) is characterized by slight round-ing of the curve at ca. 4.4 V. This last feature is also seenin the derivative plot. In region I, the charge voltage in-creases continuously in the voltage range of 3.80–4.05 V. Inregion II, the charge voltage is stable around 4.15 V. How-ever, the phase diagram of LixMn2O4 compounds againstthe lithium content is a current debate. Recent studies haveshown that extraction/insertion reactions occur with five dif-ferent domains induced by three cubic phases separatedby two-phase regions [31,32]. From the results shown inFigs. 6(a),(b) and 7, for the region II corresponding to theupper voltage plateau, a two-phase system is recognized,whereas the region I can be attributed to a single phase char-acterized by an S-shaped voltage curve. The two regimes ofintercalation are clearly depicted when incremental capac-

Fig. 8. Discharge capacity versus cycle number of Li//LiMn2−y -Cry/2Cuy/2O4 cells fory = 0.1 andy = 0.3. The capacity of an undopedLiMn2O4 electrode synthesized by the succinic-acid method was plottedfor comparison.

ity −(∂x/∂V ) is plotted vs. cell voltage (Fig. 7). The bandcentered at 4.05 V (capacity aroundx = 0.7) is indicative ofthe one-phase system, while the sharp band at 4.15 V (ca-pacity around 0.4) is indicative of the two-phase system. Forthe Li//LiMn1.8Cr0.1Cu0.1O4 cell (Fig. 6(b)), the upper 4-Volt plateau provides over 80 mAh g−1 based on the activematerial utilization with an acceptable cyclability.

For high amount of dopant (y > 0.2) in LiMn2−yCry/2-Cuy/2O4, the discharge curves show a systematic trendof decreasing capacity in the upper potential region. Thisis better depicted in the incremental capacity versus cellvoltage plots shown in Fig. 7. The doped spinel shows asignificant broader peak indicating a single-phase processdue to the cationic disorder introduced by the presence ofCr3+ and Cu2+ ions. As pointed out by several workers[33,34], this single-phase behavior leads obviously to astabilization of the spinel lattice during extraction andinsertion of lithium ions.

Fig. 8 shows the discharge capacity versus cycle numberof Li//LiMn2−yCry/2Cuy/2O4 cells (y = 0.0, 0.1 and 0.3).The products exhibit a practical material utilization capacityof 105 mAh g−1 for LiMn1.9Cr0.05Cu0.05O4. It is obviousthat the initial capacity is decreased with increasing the(Cr,Cu) content. This is due to the decreasing amount ofMn3+ ions in the substituted spinel phase since during theintercalation–deintercalation of Li+ in the LiMn2O4 matrixonly the amount of Mn3+ contributes to the charge capacity.Obviously, the effect of dopant incorporation is a subsequentimprovement of the Li//LiMn2O4 cell cyclability. Thesubstituted LiMn2−yCry/2Cuy/2O4 spinel phases are morestable than LiMn2O4, and the capacity fading is less fory = 0.3 than fory = 0.1. Nevertheless, the capacity of theLi//LiMn1.9Cr0.05Cu0.05O4 cell is maintained 95% of theinitial capacity at the 36th cycle.

The electrochemical stability has been attributed to thestronger metal–oxygen bonding in substituted LiMn2−y -Cry/2Cuy/2O4 spinel phases [9]. In compound like LiMn1.7-

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Cr0.15Cu0.15O4, Mn would have an average oxidation num-ber of 3.59, corresponding to a Mn3+ : Mn4+ ratio of 41: 59,which would make ratios smaller than 1 more probable in thefinal discharge stage. In LiMn1.9Cr0.05Cu0.05O4, the Mn ox-idation number is 3.53 which is close to the theoretical valuefor undoped spinel. This suggests that the electrochemicalstability of this compound is less than the former one asshown in Fig. 8.

4. Conclusion

We have synthesized the LiMn2−yCry/2Cuy/2O4 spinelpowders using a simple low-temperature wet-chemistry as-sisted process using an aqueous solution of metal acetatesand dicarboxylic acid as a complexing agent. The forma-tion temperature of LiMn2−yCry/2Cuy/2O4 spinel phaseshas been found to be around 300◦C using succinic-acidmethod. This procedure readily offers a single phase com-pound suitable to be employed as a positive electrode-activematerial for Li-ion batteries. The structural properties of thematerials are very similar to LiMn2O4, a slight decreaseof the cubic cell parameter is observed with increasing the(Cr,Cu) content.

The products exhibit a practical material utilization ca-pacity of 105 mAh g−1 for LiMn1.9Cr0.05Cu0.05O4. Thespinel-LiMn2−yCry/2Cuy/2O4 powders have been proven tobe electrochemically active when employed as positive elec-trode in lithium rechargeable cells.

Acknowledgements

The authors wish to thank Mr. M. Lemal for his assistancein the XRPD measurements. Dr. S. Ziolkiewicz is gratfullyacknowledged for his fruitful comments. One of us (S.S.)thanks the UGC (New Delhi, India) for final help and theResearch Award provided.

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