Electrochimica Acta 45 (2000) 3141–3149

Synthesis and host properties of tetragonal Li2Mn2O4 andLi2Co0.4Mn1.6O4

K. West a,*, G. Vitins a,b, R. Koksbang c

a Department of Chemistry, Technical Uni6ersity of Denmark, DK-2800 Lyngby, Denmarkb Institute of Solid State Physics, Uni6ersity of Lat6ia, 8 Kengara St., LV-1063 Riga, Lat6ia

c Danionics A/S, Hesteha6en 21-J, DK-5260 Odense S, Denmark

Received 15 November 1999; received in revised form 9 February 2000

Abstract

This paper presents synthesis and electrochemical properties of tetragonal lithium manganese dioxide, Li2Mn2O4

and its cobalt doped analogue Li2Co0.4Mn1.6O4. The materials are compared as host materials for lithium insertionand the behavior during the initial lithium extraction as well as on repeated cycling is presented. These materials showan initial lithium extraction capacity between 200 and 270 mAh/g. On repeated cycling, they are converted intospinel-like lattices with reversible capacities in the range 82–90 mAh/g. As they are chemically compatible with themanganese spinel, they will be well suited as additives compensating for the capacity loss during the initial formingcycle of spinel-based lithium-ion cells. © 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Lithium insertion; Lithium manganese dioxide; Tetragonal lithium manganite

www.elsevier.nl/locate/electacta

1. Introduction

A characteristic feature of the spinel structure is theaccommodation of cations on both tetrahedral sites andoctahedral sites. In the lithium manganese spinel,LiMn2O4, manganese is located on octahedral siteswhereas lithium is preferentially located on tetrahedralsites, from which it can be reversibly extracted andreinserted at a rather high voltage, 4 V versus Li. Thisvoltage is considerably higher than the voltage associ-ated with lithium insertion into octahedral sites incomparable manganese oxides. For a similar interval ofmanganese oxidation states, voltages in the range from3.0 to 3.4 V versus Li are observed [1]. The number oftetrahedral sites available for lithium insertion is limitedto one site per four oxygen, as only sites that do notshare faces with occupied coordination polyhedra can

be populated (Pauling’s third rule). Introduction ofcrystallographic defects will generally subtract from thisnumber, as manganese interstitials block one or twopotential lithium sites, and all attempts to ‘tune’ thismaterial to yield a higher capacity have been futile.

A generally accepted strategy for improving the cy-cling performance of lithium manganese spinel elec-trodes is to increase the average oxidation state ofmanganese in the fully lithiated spinel by doping with alower valence ion (typically lithium) on manganese sites[2]. Unfortunately, this also decreases the charge capac-ity of the material. The rationale for this strategy is todecrease the internal stresses in the material resultingfrom the disposition for a Jahn–Teller distortion of thecoordination octahedra around Mn(III) ions. By reduc-ing the number of Mn(III) ions, the tendency for alattice distortion is also reduced. In the stoichiometricspinel, the forces are closely balanced. At room temper-ature the structure is cubic, but undergoes a change toa tetragonal phase on cooling to about 280 K [3]. It is

* Corresponding author. Fax: +45-45-252438.E-mail address: west@kemi.dtu.dk (K. West)

0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 3 9 5 - 9

K. West et al. / Electrochimica Acta 45 (2000) 3141–31493142

thus very likely, that during cycling regions of thecrystal will temporarily be forced into the tetragonalstate as a consequence of the unidirectional stress fieldcaused by gradients in lithium concentration. Theunidirectional stress field will lower the symmetry lo-cally, and the entropic drive opposing the Jahn–Tellerdistortion will be partially lifted. The distortion may bethus favored even without an increase in the number ofMn(III) ions. The repeated shifts in unit cell parameterswill wear down the electrode particles causing loss ofcapacity. This effect is sometimes referred to as ‘electro-chemical milling’.

The high voltage for lithium insertion taken togetherwith a lower price and a better environmental compli-ance compared to alternative materials makes Li�Mnspinels attractive for use as the positive electrode inlithium-ion batteries. The charge capacity is, however,not fully satisfactory, and for the reasons outlinedabove, it is unlikely that it can be improved muchbeyond the 120 mAh/g available today with materialsoptimized for cycling. It is therefore of importance tofind other ways of increasing the cycling capacity oflithium ion cells based on the manganese spinel. Onepossible strategy [4] is to add to the positive electrode ahigh-capacity material that is compatible with the man-ganese spinel. The purpose of this material would be todeliver the excess capacity spent as irreversible capacityloss in the initial conditioning of the carbon electrode.For this purpose, neither a particularly high voltage nor

good reversibility is required. The main requirement isthat the additive does not interfere with the reversibilityor the cyclability of the spinel active material.

We have undertaken a survey of the host propertiesof a large number of manganese oxides and lithiummanganese oxides. In the present paper, we presentresults for tetragonal LiMnO2 (Li2Mn2O4) and thecobalt doped analogue Li2Co0.4Mn1.6O4. Special atten-tion will be given to the application of these materialsas electrode additives delivering primary capacity in theconditioning cycle of lithium-ion batteries as well asproviding rechargeable capacity in the 4 V versus Lirange common for spinel systems. The principal charac-terization method used in this investigation is analysisof voltage curves (voltage, E, versus composition, x)and derivative voltage curves (differential capacity, dx/dE versus x). In a number of investigations on interca-lation materials, this method has proven to be verysensitive to details in the local structure around the siteswhere lithium is inserted regardless of the overall crys-tallinity of the host material.

In the literature, several polymorphs having the com-position LiMnO2 have been described. Of these, fourstructures are based on the same cubic close packedarray of oxygen atoms as the spinel, differing mainlywith respect to the cation arrangement. At high pres-sure and high temperature (65 kbar and 1000°C) a rocksalt modification of LiMnO2 having a random distribu-tion of Li and Mn over the octahedral sites in the cubicclose oxygen packing is stabilized [5]. The cation ar-rangements of the other three polymorphs (character-ized as orthorhombic, monoclinic and tetragonalLiMnO2) are illustrated in Fig. 1. Structural aspectstogether with previous results concerning the propertiesof these materials as hosts for lithium insertion areoutlined in e.g. [6].

Tetragonal lithium manganese dioxide, Li2Mn2O4 isthe phase resulting from lithium intercalation into thelithium manganese spinel, LiMn2O4. Based on X-rayand neutron diffraction studies it has been described ashaving an ordered rock salt structure with both lithiumand manganese ions occupying octahedral sites in aclose packed oxygen lattice [7–9]. Available data indi-cate, however, that the displacement of lithium fromtetrahedral to octahedral sites may not be complete.

Li2Mn2O4 can be prepared by a number of chimiedouce techniques. Both electrochemical lithium inser-tion into LiMn2O4 at 3.0 V [7] and chemical lithiationof MnO2 or LiMn2O4 using n-butyl lithium [8–10] orLiI [11] at near ambient temperatures have been de-scribed. The material prepared by the LiI route isreported to have the better stability in air [11]. Fuchsand Kemmler-Sack have performed chemical lithiationof LiMn2O4 in molten LiI at 460°C [12], but withoutreporting electrochemical data for the product.Li2Mn2O4 is considered to be metastable and to trans-

Fig. 1. Cation arrangement in lithium manganese oxides basedon a cubic close packed array of oxygen anions. The tetrahe-dral sites occupied by Li and the octahedral sites occupied bymanganese in the spinel LiMn2O4 is shown in (A). In the otherLi�Mn�O compounds, lithium is in octahedral coordinationand is shown as balls in (B) orthorhombic LiMnO2, (C)monoclinic LiMnO2, and (D) tetragonal Li2Mn2O4.

K. West et al. / Electrochimica Acta 45 (2000) 3141–3149 3143

form irreversibly into orthorhombic LiMnO2

(O�LiMnO2) at temperatures above 500°C [11,12]. Te-tragonal Li2Mn2O4 prepared by these methods will bedenoted low temperature (LT�Li2Mn2O4) compoundsin the following.

In a short communication, Reimers et al. [13] re-ported the preparation of Li2Mn2O4 at 1000°C byreacting O-LiMnO2 for 2 h with a 25% excess of Li inthe form of Li2CO3 in an argon flow. The resultingproduct had the Li2Mn2O4 XRD pattern. Extraction oflithium from this high temperature (HT) phase showscharacteristic differences compared to the LT�Li2-Mn2O4 compounds. Instead of a plateau close to 3 Vversus Li, the potential starts off at 3.2 V and enters aplateau region at 3.8 V. At the end of this plateau 0.5Li/Mn is removed, and the remainder of the voltagecurve has the appearance typical of a spinel [13]. It wassuggested that the differences in electrochemical behav-ior were due to lithium ions being situated in differentcrystallographic positions in the HT and the LT phases.It is interesting to note that the tetragonal phase thathas been believed to be only meta-stable apparently hasa high temperature stability interval. This HT phaseseems to be stabilized by substitution with cobalt formanganese, as a similar tetragonal phase has beenreported for Li2Co0.4Mn1.6O4 prepared at 800°C [14].Lithium extraction by acid treatment of this compoundcauses a phase change from tetragonal to a structurerelated to the cubic spinel-and at very low lithiumcontents, to a l-MnO2 type phase. The limited electro-chemical data presented by Stoyanova et al. [14], sug-gests that in the case of the acid treated compound,approximately equal amounts of lithium are availablein a reversible fashion at two distinct 4 and 3 Vplateaus very similar to those of the LiMn2O4 spinel.Similarly, doping with chromium has been shown tostabilize the tetragonal Li2Mn2O4 structure. TetragonalLi2CrxMn2-xO4, in the composition interval 0.1BxB1.25 can be prepared directly at 1000°C by solid-statesynthesis [15]. This material as well can be cycled inlithium cells, showing a step in the voltage curve froma 4 V plateau to a 3 V plateau at a compositiondependent on the degree of Cr–doping.

2. Experimental

2.1. Synthesis

Since Li2Mn2O4 becomes oxidized in air at tempera-tures above 200°C [11,16], all syntheses at elevatedtemperatures were carried out in a flow of nitrogen(:2 ml/min). The compositions of starting materialsand products were determined by chemical analysis forlithium (flame photometry) and manganese (totalamount of Mn and average oxidation state of Mn bypotentiometric titration).

LT�Li2Mn2O4 was obtained by chemical lithiation ofLiMn2O4 in molten LiI (20% excess) at 460°C for 5 h[12]. The reaction temperature was determined by twofactors: The melting point of LiI at 449°C and thephase transition of metastable Li2Mn2O4 to O�LiMnO2

starting at 500–600°C [12,17]. The product, a light-brown powder was washed in n-hexanol and acetoni-trile and dried at 60°C in air. Subsequently it was driedat 150°C in vacuum to remove traces of n-hexanol (b.p.155°C).

HT�Li2Mn2O4 was obtained by reaction ofO�LiMnO2 and Li2CO3 at 1005–1010°C for 1.5 to 2 hin a steel tube flushed with nitrogen. A surplus oflithium corresponding to a Li/Mn ratio between 1.06and 1.08 was used. In order to optimize the synthesisconditions several starting compositions and reactiontemperatures were tried. In all cases, the product wasquenched from 950–1000°C by immersing the steeltube in water. To ensure accurate control of thermalconditions, the tube furnace was checked for tempera-ture gradients and a thermocouple was kept close to thereaction zone.

Various Li2CoxMn2-xO4 samples were synthesisedfrom orthorhombic LiMnO2 and LiCoO2 by a methodderived from [14]. Tablets of the appropriate powdermixtures were annealed at 780–800°C for 12–24 h withintermediate regrinds to ensure phase purity. Since thetetragonal Co compounds are (meta) stable at lowtemperature, as opposed to the corresponding puremanganese oxide, quenching was not necessary.

2.2. Electrochemical characterization

Electrodes for the cycling tests were made usingpolytetrafluoroethylene (PTFE) as binder. The PTFEbased electrodes are formulated with a relatively smallamount of binder: 70–80% oxide, 15–25% carbon, and:5% PTFE by weight. Electrode foils are made bymanual kneading of the components, followed by arolling procedure where the thickness is gradually re-duced to 75–150 mm. This technique yields electrodesof good consistency and with a high utilization of theactive components. The reproducibility and uniformityof electrodes cut from these foils is high, and theamount of active material present in the electrodesdetermined either from the weight of the electrodes orfrom chemical analysis deviates less than a few percent.

All electrodes were cut into circular discs of diameter1 cm (0.79 cm2), weighed, and dried in high vacuum at100–120°C prior to use. The exact amounts of man-ganese in the electrodes were determined by spec-trophotometric analysis after termination of cyclingexperiments.The electrodes were cycled in spring loadedtest cells with metallic lithium as the negative electrodeand a measured amount of liquid electrolyte absorbedin porous polypropylene discs (Celgard 2400) and/or

K. West et al. / Electrochimica Acta 45 (2000) 3141–31493144

Table 1Unit cell parameters and average stoichiometry of tetragonalLi2Mn2O4 and Li2Co0.4Mn1.6O4 specimens

Unit cellMaterial Average stoichiometryparameters

LT-Li2Mn2O4 a=5.645(1) A, Li2.14Mn2+3.06O4.16

c=9.262(1) A,

Li2.08Mn2+3.02O4.06HT-Li2Mn2O4 a=5.654(2) A,

c=9.306(4) A,

Li2Co0.4Mn1.6O4 a=5.659(1) A, Li1.94Co0.41Mn1.59O4.02

c=9.283(3) A,

imbalance between capacities during charge and dis-charge caused by parasitic reactions of the electrolytebecame noticeable.

3. Results and discussion

The phase purity of the synthesis products was con-trolled by X-ray diffraction. Weak lines characteristicof O�LiMnO2 were always present in HT�Li2Mn2O4.In the product prepared at 1005°C the relative intensityof these lines were B3%. Lattice parameters and chem-ical analysis of the products are given in Table 1. Unitcell parameters and chemical compositions are close tothose expected and previously reported.

Difficulties were encountered in the attempt to repro-duce Reimers et al’s synthesis of HT�Li2Mn2O4 at hightemperature, as a very precise control of synthesisconditions seems to be imperative. To find the optimumsynthesis conditions, several starting compositions (Li/Mn ratios) were annealed at temperatures from 950 to1025°C followed by XRD and chemical analysis. Apictorial representation of the results of the syntheses isgiven in Fig. 2. This graph is not intended as anequilibrium phase diagram because full equilibrationcould not be ensured in the open system used for thesesyntheses. Under operating conditions, lithium is lostgradually by sublimation, which must be taken intoaccount in the formulation of the reaction mixture.

The HT�Li2Mn2O4 phase occurs as a nearly purephase (a dark brown powder with shiny particles) onlywithin a rather narrow temperature and compositionwindow. The optimum temperature range is between1005 and 1015°C, and the Li/Mn ratio of the resultingHT�Li2Mn2O4 phase falls in the interval 1.00 to 1.08.Starting with a Li/Mn ratio of 1.06 and annealing at1005°C for 1.5 h yields a product of the compositiongiven in Table 1.The amount of O�LiMnO2 phasepresent in the product was estimated to be B3% fromthe relative peak intensities. The X-ray diffraction pat-tern is shown in Fig. 3 together with a diffractogram ofO�LiMnO2 prepared below 950°C and a diffractogramof LiMnO2 annealed at 1025°C. The latter materialapparently has a disordered orthorhombic structure asindicated by the broad, low-intensity peaks. The oxida-tion state of Mn in this product was slightly below +3and the Li/Mn ratio below 1. At high temperature, lossof lithium is accompanied by a loss of oxygen, andprolonged heat treatment in an open system as the oneused here will eventually lead to the formation ofMn3O4.

The tetragonal LT�Li2Mn2O4 prepared in reactionwith LiI at 460°C shows XRD patterns and electro-chemical properties identical to the material preparedby chemical lithiation of LiMn2O4 with LiI at 150°C[11] or by electrochemical lithiation. We conclude that

glass fiber membranes serving as the separator. In orderto isolate possible electrolyte effects, electrodes werecycled with different electrolytes: 1 M LiPF6 or 1 MLiBF4 in 1:1 mixtures of ethylene carbonate (EC) anddiethyl carbonate (DEC); EC and propylene carbonate(PC); or EC and dimethyl carbonate (DMC). Theresults presented are all representative of a larger (\5)number of similar cells tested under similar conditions.

As in our previous work, we use galvanostatic dis-charge and charge to fixed voltage limits followed by aperiod of potentiostatic charge at the upper voltage inorder to bring the electrode into a reproducible statebefore each discharge. The differential capacity (dx/dE)is calculated numerically from the voltage, current, andtime relationship taken together with the amount ofoxide present in the electrode. Currents in the rangebetween 50 and 200 mA, corresponding to dischargetimes larger than 10 h/(Dx=1) (typically 20 h/(Dx=1))were chosen in order to minimize concentration polar-ization effects. At even lower currents, the systematic

Fig. 2. Phase and chemical analysis of products obtained byannealing mixtures of O�LiMnO2 and Li2CO3 at differenttemperatures between 950 and 1040°C for 1.5 to 2 h in a N2

purged steel tube. The orthorhombic material obtained at thehighest temperatures (\1015°C) is partially disordered.

K. West et al. / Electrochimica Acta 45 (2000) 3141–3149 3145

Fig. 3. X-ray powder diffractogram of tetragonal Li2Mn2O4 compared with diffractograms of O-LiMnO2 prepared at 780°C and thedisordered O-LiMnO2 appearing after heating to 1025°C.

synthesis at this relatively high temperature does notlead to O�LiMnO2 expected, but to the meta-stableLT�Li2Mn2O4. Whether this implies that the tetragonalphase in fact is thermodynamically favored at lowertemperatures, or that the energetic difference between

these phases is small can not be decided on the basis ofthe present data.

Voltage and capacity data for LT�Li2Mn2O4 areshown in Fig. 4. In the initial charge to 4.5 V versus Li,95% of the lithium present in the oxide (270 mAh/g)

Fig. 4. Potential (E) vs. composition (x) and differential capacity (dx/dE) for cycling of LT�LiMnO2 in EC/PC LiBF4 electrolyte.Charge and discharge current 200 mA, corresponding to a stoichiometric discharge time of 14 h/(Dx=1).

K. West et al. / Electrochimica Acta 45 (2000) 3141–31493146

Fig. 5. Potential (E) vs. composition (x) and differential capacity (dx/dE) for cycling of HT�LiMnO2 in EC/DEC LiPF6 electrolyte.Charge and discharge current 200 mA, corresponding to a stoichiometric discharge time of 15 h/(Dx=1).

can be extracted electrochemically with an associatedvoltage curve showing the features characteristic ofcycling of well-crystallized spinel (LiMn2O4) electrodes.The capacity between the peaks is very low, showingthat essentially all lithium ions are situated on well-defined, spinel-type sites. However, a few minor peaksthat are not part of the basic spinel pattern are seen,among these a small oxidation peak at 3.9 V 6s Li. Weconsistently observe an oxidation peak close to 3.8 Vversus Li in LiMnO2 materials that have been dis-charged to 2 V. We hypothesize that these peaks mayresult from the formation of Li�Mn defect clusters athigh lithium concentrations. In order to accommodatean excess of lithium ions in the structure, manganeseions may move off their equilibrium positions andcreate sites in which lithium ions are stabilized. Becauseof the stabilization, a higher voltage is required toextract these ions. The energy used to create the defectsis subsequently dissipated as heat. It would be of greatinterest to compare the voltages of these minor peakswith site energies based on defect calculations.

On repeated cycling, LT�Li2Mn2O4 showed the well-known cycling properties of an non-optimized lithiummanganese spinel electrode: good capacity retention(98.5–99.5%) in the 3.5 to 4.5 V versus Li window(:100 mAh/g), and a slightly higher rate of capacityfade when the 3 V plateau was included in the cyclingregime. It is most likely that this material will beamenable to the same types of optimization of cycle lifeas the spinel material.

As discussed above, we did not succeed in gettingsingle-phase HT�Li2Mn2O4 as even the best synthesescontained a few percent O�LiMnO2. Fig. 5 showsvoltage and capacity curves for cycling of this material.Despite the very small differences in XRD pattern forthe low temperature and the high temperature modifi-cations of HT�Li2Mn2O4, the voltage curves during theinitial lithium extraction step are remarkably different.In the first charge to 4.5 V versus Li, approximately80% of the lithium initially present in HT�Li2Mn2O4

could be extracted (232 mAh/g). The extraction takesplace at much higher voltages than seen with the lowtemperature modification (Fig. 4). It is especially note-worthy that nearly half the extraction capacity is foundat voltages higher than the 4 V double peak associatedwith extraction of lithium from tetrahedral sites. In thesubsequent cycles, the voltage and capacity curves fol-lows the spinel pattern closely, although the baselinecapacity at high voltages is considerably higher, and thetotal capacity is only 60% of the theoretical value. Thehigher baseline capacity indicates that not all lithiumions in the cycled material are situated on ideal spinelsites. On repeated cycling, this material yields a verystable capacity of 0.2 Li/fu (:30 mAh/g) in the voltagerange 3.5–4.7 V versus Li. When cycled in the range1.5–4.7 V versus Li the capacity fades gradually, start-ing from 0.6 Li/fu (:90 mAh/g).

Comparison of the powder XRD diagrams of tetrag-onal Li2Mn2O4, obtained by reaction between LiI and

K. West et al. / Electrochimica Acta 45 (2000) 3141–3149 3147

Fig. 6. Comparison of the powder XRD diagrams of tetrago-nal Li2Mn2O4, obtained by reaction between LiI and LiMn2O4

at 460°C, and Li2Co0.4Mn1.6O4 obtained by solid-state reac-tion at 700°C.

and the sample is thus considered sufficiently pure forthe electrochemical measurements.

Refinement of the tetragonal unit cell shows thatLi2Co0.4Mn1.6O4 has lattice parameters very similar toboth LT�Li2Mn2O4 and HT�Li2Mn2O4 (Table 1).Comparing the two materials prepared at high tempera-ture, Co substitution is seen to lead to a shortening ofthe c-axis as expected from the lower ionic radius of theCo3+ ion compared to the Mn3+ ion (0.61 versus 0.65A, [18]).

Attempts to prepare cobalt substituted compounds ofdifferent compositions, showed that upon reduction ofthe Co content, increasing amounts of cubic and or-thorhombic LiMnO2 phases appeared. However, it waspossible to alleviate this effect somewhat by increasingthe synthesis temperature, until, in the extreme case ofno Co present, a temperature of 1010°C is necessary toprepare the tetragonal phase and maintain reasonablephase purity. In the open systems used here for synthe-ses, high synthesis temperature leads to an increasedloss of lithium by evaporation, which must be compen-sated for in the formulation of the reaction mixture. Inthe following, focus is on the Li2Co0.4Mn1.6O4 com-pound. This particular composition is stable at lowertemperatures and rapid quenching is not necessary topreserve the structure and phase purity as in the case ofHT�Li2Mn2O4.

Fig. 7 shows the cell voltage (E) versus composition(x) for the first charge of a number of Li/LixCo0.4Mn1.6O4 cells with two different electrolytes,LiPF6 in EC/DMC, and LiBF4 in EC/PC. Previously, ithas been shown [19,20] that Co3+ ions in Co-dopedLiMn2O4 spinels do not change their redox state duringlithium insertion in the potential range where lithiummanganese oxide electrodes are normally operated. Thisis consistent with the reduced capacity observed in thefirst charge of Li2Co0.4Mn1.6O4. The capacity, :1.4 Liper formula unit (200 mAh/g), corresponds to theoxidation of nearly all Mn to Mn4+ — based on theanalysis in Table 1, full oxidation of Mn to Mn+4

LiMn2O4, with that of the dark brown Li2Co0.4Mn1.6O4

shows that the two diagrams are virtually identical withrespect to line positions and relative intensities. Smallamounts of O�LiMnO2 and a cubic disordered phase,presumably cubic LiCoxMn1−xO2 [14] are present inthe sample. The impurities are denoted by O and C,respectively, in Fig. 6. Based on Rietveld calculations,the phase purity was estimated to be better than 97%

Fig. 7. Potential (E) vs. composition (x) for the initial charge (lithium extraction) of Li2Co0.4Mn1.6O4 cells with different electrolytes.Charge current 150–200 mA (12–16 h/(Dx=1)). Full curves: EC/DMC LiPF6, broken curves: EC/PC LiBF4.

K. West et al. / Electrochimica Acta 45 (2000) 3141–31493148

Fig. 8. Potential (E) vs. composition (x) for the second cycle of Li2Co0.4Mn1.6O4 cells at 150–200 mA (12–16 h/(Dx=1)) Full curves:EC/DMC LiPF6, broken curves: EC/PC LiBF4.

would require 1.49 Li/f.u.. Only a small part of thiscapacity is extracted close to 3 V versus Li, whereas inthe EC/DMC LiPF6 electrolyte nearly 0.6 Li/f.u. isextracted at a voltage plateau centered around 3.75 Vversus Li, i.e. at a potential intermediate between thatof Li extraction from octahedral sites (‘3 V plateau’)and tetrahedral sites (‘4 V plateau’) in pure lithiummanganese oxides. The shape of the plateau is depen-dent on the electrolyte used. In the EC/PC LiBF4

electrolyte, it splits into two: a smaller plateau at 3.45and a larger at 3.75 V versus Li. Furthermore, thecapacity in this composition interval is not recovered inthe following discharges. Although cobalt is not partic-ipating directly in the redox reaction, it is clear fromthese observations that the presence of cobalt alters theredox chemistry of the doped material. The irreversibil-ity of the initial lithium extraction indicates that astructural rearrangement is occurring, maybe involvinga partial segregation of Mn and Co ions.

After the initial lithium extraction, the voltage curvesshow the 3 V plateau that is characteristic of thevoltage curve of the LiMn2O4 spinel, Fig. 8. The 3.75 Vplateau is now only seen in cells that have been dis-charged well below 2 V, and most clearly in cells with

the PC/EC LiBF4 electrolyte — note the parallel to thesmall capacity peak mentioned above for deeply cycledLi2Mn2O4! The capacity at the 4 V plateau (:0.6Li/f.u.) is lower than expected if all Mn ions wereparticipating (82 mAh/g instead of 113 mAh/g), illus-trating that in the doped material, not all Mn atomscan participate in the formation of the electronic bandsthat are occupied during lithium insertion. On deepdischarge, a very large overvoltage is developed as seenfrom the sharp rise in voltage when charging is ini-tiated. On repeated cycling, the Co doped materialshows a good capacity retention, although not as goodas the undoped material.

4. Conclusion

The materials studied are all well suited as primarycapacity additives for lithium ion cells. LT�LiMnO2 hasthe highest initial capacity (270 mAh/g) and a reversiblecapacity of 100 mAh/g in the following cycles. Usingthis material as additive will not detract much from thereversible gravimetric of the electrode, but increase itsinitial, primary capacity. This effect is illustrated in

Table 2Calculated capacities of lithium ion cell using a spinel electrode (120 mAh/g) with different additives (assuming 10% irreversiblecapacity loss)

Additive Reversible capacity (mAh/g)Amount (%) 1st charge capacity (mAh/g)

No additive 0 1081201307 118LT-Li2Mn2O4

1141266HT-Li2Mn2O4

11612810Li2Co0.4Mn1.6O4

K. West et al. / Electrochimica Acta 45 (2000) 3141–3149 3149

Table 2, where the cell capacity per g active cathodematerial is calculated for a hypothetical cell assumingthat 10% of the initial capacity is lost irreversibly. Theamount of additive is chosen so the irreversible capacityloss is balanced by the difference between the initialand the reversible capacity of the additive. In case of noadditive, the irreversible capacity loss is covered by thespinel material itself, leading to a reduction in reversiblecapacity from 120 to 108 mAh/g.

LT�LiMnO2 is modestly air-stable, and can be usedin electrodes fabricated in ambient atmosphere as longas care is taken to avoid an excessive exposure tohumidity. The main disadvantage of this material is thehigh production cost that can be foreseen due to the useof LiI in the synthesis. Recycling of the iodine gener-ated in the synthesis using scrap lithium metal may bea way to reduce the expenses, but will probably notbring down the cost to an acceptable level.

Synthesis of HT�LiMnO2 does not require expensivematerials, but the process must be minutely controlled,and scale-up to industry scale is definitely not straight-forward. Addition of HT�LiMnO2 to spinel electrodeswill increase the initial capacity, but as the reversiblecapacity of this material is less than that of the spinel,the cycling capacity will not be as high as whenLT�LiMnO2 is used as the additive.The starting materi-als used for synthesis of Li2Co0.4Mn1.6O4 are moreexpensive than for HT�LiMnO2. The synthesis is, how-ever, straightforward and can easily be scaled up to anindustrial process. This material is fully air stable, butthe amount of additive required to balance a givenirreversible capacity loss is slightly higher than of thetwo other materials.

It is clear that the choice of additive is a balancebetween cost and performance. LT�LiMnO2 will proba-bly only be of interest for applications where a highpremium is put on capacity improvements, whereasLi2Co0.4Mn1.6O4 for most applications will be the opti-mal compromise between price and performance.

AcknowledgementsA scholarship from The Danish Rectors Conference

to G.V. is gratefully acknowledged. Sedema is thanked

for their kind supply of orthorhombic lithium man-ganese dioxide.

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