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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.3 Concepción set. 2001

http://dx.doi.org/10.4067/S0366-16442001000300016 

LITHIUM INSERTION INTO Li-Mn, Li-Fe AND Li-Co OXIDES

JUAN LUIS GAUTIER*, ROXANA AHUMADA, ERIKA MEZA,
GERARD POILLERAT1

Laboratorio de Electroquímica, Departamento de Química de los Materiales, Facultad
de Química y Biología, Universidad de Santiago de Chile, Av. L.B. O´Higgins 3363, Santiago, Chile.

1Laboratoire d'Électrochimie et de Chimie Physique du Corps Solide, Faculté de Chimie,
Université Louis Pasteur ,CNRS-UMR 7512 , 67000 Strasbourg, France.

(Received: January 19, 2001 - Accepted: July 18, 2001)

ABSTRACT

We have studied the lithium insertion into LiM2O4 (M = Mn, Fe, Co) using n-BuLi and electrochemical techniques. The oxides crystallized in the pure spinel phase S.G. Oh7(Fd3m). Probable cationic distributions were proposed based on XRD, Mössbauer and pzc measurements. The results demonstrate that an important factor in the extent of the lithium insertion reaction in mixed oxides is the presence of the highest oxidation state in the transition metal cations that are located in B-sites of the spinel framework.

KEYWORDS: Lithium insertion, lithium cobalt oxide, lithium manganese oxide, lithium iron oxide, spinels, lithium batteries.

RESUMEN

Se estudió la inserción de litio en óxidos de formula LiM2O4 ( M = Mn, Fe, Co) usando n-BuLi y mediante técnicas electroquímicas. Los óxidos cristalizaron en la fase espinela pura de grupo espacial Oh7(Fd3m). Mediante medidas de difracción de RX, espectroscopía Mössbauer y de potencial de carga cero (pzc) se determinaron las probables distribuciones catiónicas. Los resultados mostraron que el factor relevante para que se logre elevada reactividad debido a la inserción de litio en cátodos formados por óxidos mixtos, lo constituye la presencia de cationes de elementos de transición que exhiban el más elevado grado de oxidación en los sitios B de la estructura espinela.

PALABRAS CLAVES: Inserción de litio, óxido de cobalto y litio, óxido de manganeso y litio, óxido de hierro y litio, espinelas, baterías de litio.

INTRODUCTION

Technological advances impose great demands on portable power. In this regard, lithium-ion batteries are experiencing a phenomenal growth in terms of uses and specific applications. The emphasis is on the exploration on new positive electrode materials, with a refocusing in recent years on low cost, low toxicity and safe compounds such as the lithium manganese oxides. Thus, compounds based on layered LiMnO2 or on spinel LiMn2O4 are of great interest.

The cathode material commercially used to date is LiCoO2(1). Layered LiCoO2 is considered to be highly stable in the rhombohedral S.G. (spatial group)R3ma-NaFeO2 structure(2) in which the LiO6 and CoO6 octahedra share their corners and stack alternatively along the c axis direction, which allows the two-dimensional diffusion of Li ions during electrochemical intercalation and deintercalation. This oxide can be synthesized at high temperature (800 - 900°C) (3-8) to give the ordered HT-LiCoO2 form, in which lithium and cobalt ions occupy the alternating {111} layers of octahedral sites in a nearly cubic close-packed oxygen lattice having typical cell parameters (3,5) a = 0.2816 nm and c = 1.4051 nm. Both crystallographic (4) and XPS (9) studies are in agreement with the presence of the low-spin Co3+ (t62g) in LiCoO2. A cubic disordered form commonly known as LT-LiCoO2 (synthesized at 400°C) is closer to the spinel rather than to an hexagonal phase (10-14). Temperatures as low as 100°C have been used to prepare this oxide, but not in pure form (15). HT-LiCoO2 shows high stability and during lithium extraction LixCoO2 undergoes phase transformations from monoclinic to hexagonal (2,7). The limit of delithiation in commercial batteries is Li0.5CoO2 (which corresponds to 140 Ah/Kg of charge capacity) and involves mechanical fracture due to a change in the c axis dimension (16). Currently, there is not sufficient evidence on the existence of the spinel form on Li-Co electrodes. However, recent XRD and TEM analyses appear to indicate the presence of the spinel ordering whereby tetrahedral sites are occupied by lithium(17). Quantum mechanical calculations have suggested that removing lithium ions from LT-LiCoO2 involves jumps of the Li ions from the octahedral sites to tetrahedral sites and the formation of a low energy LiCo2O4 spinel structure (18). This ordering phenomena associated to lithium has also been used to understand the phase transition observed when x Æ 0.5 for O2-LixCoO2 and the classical O3-LiCoO2 (19).

The layered LiMnO2, which presents a O3 structure, is not stable during charge-discharge cycling which in turn is a process involving a rapid decrease in cell capacity with the number of cycles connected with the gradual conversion to a stable spinel phase (20). During the first charge the orthorhombic phase o-LiMnO2 (S.G. Pmnm) shows an irreversible transformation to a "spinel-like structure" (11,21,22). This spinel structure has been suggested as the one responsible for the cyclic stability of a particular high-charge-capacity o-LiMnO2 (23). Some studies have proposed that the electrochemical performance of o-LiMnO2 is increased by a low-temperature synthesis due to a structural disorder between Li and Mn sites occurring in local regions of monoclinic order (24, 25). However, a significant capacity fading has been reported for o-LiMnO2 prepared at low temperatures (22,26). Due to low costs, high environmental acceptability and feasibility of synthesis, the manganese oxide LiMn2O4 has been widely studied as an insertion cathode for rechargeable lithium batteries (27-30). In Li/LixMn2O4 cells, the insertion of Li+ ions into LiMn2O4 occurs at 4V for the range 1 x >0 (31,32) and at 3V for 2 x >1(28,31). A 4V-cell slowly loses capacity which has been attributed to structural degradation of the spinel cathodes associated with the formation of Li2MnO3 (33) whereas at 3V the cubic spinel gives rise to a tetragonal compound Li2Mn2O4 (41/amd) due to a change of Li ions from tetrahedral sites to octahedral sites (28,31). This last compound could be converted into a spinel-like lattice, which shows poor capacity (34). Other authors have considered the disproportionation of the spinel electrode into soluble Mn2+ ions and compounds presenting Mn4+ in solid phase such as Li2Mn4O9 and Li2MnO3 (35).

The available evidence indicates, then, that the performance of the cathodes is dependent on the nature, structure and oxide composition that limit the degree of lithium insertion. In addition to this, the starting materials and oxide preparation conditions are relevant in order to obtain good electrochemical behavior of the electrodes.

In this study, we examine the preparation of the spinel compounds LiM2O4 ( M= Mn, Fe, Co) and the corresponding lithium insertion properties with the aim to know the effect of the metal in the performance of the cathodes.

EXPERIMENTAL

The individual polycrystalline oxides having Li/M = 0.5 ratio (M = Mn, Fe,Co) were prepared from mother aqueous solutions obtained with molar quantities controlled by AAS (atomic absorption spectroscopy) of high purity nitrate salts Mn(NO3)2 4H2O (Merck ref. 5940), Co(NO3)2 6H2O (Merck ref. 2536), LiNO3 (Merck ref. 5653), Fe(NO3)3 9H2O (Merck ref. 3883) using the thermal decomposition technique. Water evaporation from these mother solutions was carried out at 90°C followed by 48 h of heating at 150°C which effected air removing. Grounding was constant on the solid to assure the decomposition in the oxide grain. The fine powder so obtained (400 mesh) was treated in a Pt crucible at 180°C for 24h under an oxygen current which eliminated the last nitrate decomposition vapors. In the case of Li-Fe and Li-Mn oxides, calcination at 800°C in an oxygen atmosphere for 24h was necessary to obtain the desired phase. Each black solid synthesized was stored under an Ar atmosphere. Chemical analysis by AAS (Perkin Elmer 403) and by EDAX confirmed the cationic stoichiometry. The morphology was monitored using a Jeol (JMS 840) SEM apparatus. All the samples showed a similar morphology: an uniform grain texture with agglomerates can be observed in Fig. 1.

Fig. 1. Scanning electron microscopy microphotograh of LiCo2O4 powder

The oxides were analyzed by the X-ray powder diffraction technique (Siemens D 5000, 40KV/30mA) using CuKa radiation selected through a graphite monochromator (l= 0.15406nm). Interplanar spacings were determined against an internal silicon standard. Mössbauer analysis was performed at room temperature using a spectrometer with 57Co source, a 512 multichanel analyzer and a-Fe standard. The pzc (or pHz) measurements characterizing the oxide/solution interface were determined by acid-base titration(36) using a KNO3 aqueous solution as supporting electrolyte. As an example, 0.8g oxide in 40 mL of 5 10-3M KNO3 were stirred for 4h under an Ar atmosphere, then a purged 0.1M KOH solution was added to obtain pH 11. After stirring for 24h, the titration was achieved with 0.1M HNO3 under Ar gas. The KNO3 effect was studied using electrolyte concentrations from 1 10-1M to 1 10-3M. The pHz is determined from log DG± (excess surface charge) vs. pH where DG is a function of the acid volume added according to DG± = 1.08 DV 10-4 /moxide (molg-1)(37).

Electrochemical measurements were carried out using two and three-electrode SwagelockTM cells according to ref.[38]. They were assembled in a dry box under high purity Ar atmosphere and the cathode was separated from the lithium disc anode (99.9% Aldrich ref. 26,600-0) by a porous Celegar separator soaked with the electrolyte. The cathode pellet (1.2 cm diameter, 5 mm thickness) obtained at 2 ton cm-2 was a mixture formed with 70 mg oxide and 10 % w/w additives (5% of Teflon suspension Dupont 30B + 5% acetylene black Shawinigan Gulf). Lithium (Merck ref. 805662) was used as reference electrode. Cells were discharged at a constant current density, 44 mAcm-2 (performed by previous experiments) at 25°C with a potentiostat/galvanostat device (manufactured at the laboratory) and a two electrode cell of the type: Li / 1M LiBF4 in PC: DMC 1:4 ratio / oxide-Teflon bonded. Anhydrous propylene carbonate (99,7% Aldrich ref. 31,032-8), dimethyl carbonate (99% Aldrich ref. 15,292-7) and lithium tetrafluoroborate (98% Aldrich ref. 24,476-7) were used without further purification. The lithium insertion into the oxide electrode was achieved by coulometric titration and the analysis of the experimental data was performed using a Pentium microcomputer. Open-circuit voltage (OCV) readings were taken intermittently during discharge after allowing equilibration times. OCV was considered attained when it does not change more than 1mV within 1h. The cyclic voltammetry curves (CV) were obtained at 1 mVs-1 between 1 and 4.2 V vs. Li employing a Voltalab 40TM and capturing the data with a PC.

In order to compare the lithium insertion process, chemical lithiation was carried out by reacting 5 mL of n-BuLi solution (1.6M in hexane) with, typically, 0.5 g oxide using a glove box (< 10ppm H2O) at 60°C. Hexane (Aldrich) was previously dried with sodium. The reaction mixtures were stirred under an Ar atmosphere at 60°C. A gradual color change from the initial brown color of the Li-Fe oxide to black was observed during lithiation. A second chemical insertion method was used in the case of LiMn2O4 oxide in order to check the first one. In this case, chemical lithiation was obtained by the reaction between 0.3M LiI in acetonitrile (25 mL) and typically, 0.5g LiMn2O4 under constant reflux at T =100°C during five days, with evaluation of the I2 formed each 12h.

RESULTS AND DISCUSSION

Oxide characterization.

X-ray diffractograms of LiM2O4 ( M = Co, Fe, Mn) from 5° < 2Q < 90° are shown in Fig. 2. All oxides showed good crystallization and the presence of a single pure phase. No other phases were detected. The diffraction lines of LiCo2O4 were indexed and compared with the diffractogram of Co3O4 ASTM 9-418 (a = 0.8084 nm). It is clear that the oxide crystallizes in the spinel phase with S.G.Fd3m (Fig. 2a), a-cell parameter, a = 0.8067 ± 2 10-5 nm. To our knowledge, this compound has never been prepared before. The diffractogram of the iron oxide (Fig. 2b) is similar to that of LiCo2O4 and corresponds to a disordered form, inversed spinel S.G. O7n(Fd3m). Two forms of the lithium ferrite LiFe5O8 are known (39): the disordered or random form (S.G. Fd3m) and the ordered form (S.G. P4132). The order-disorder transition occurs in the 735 to 755°C temperature range (40). In the random form, the tetrahedral sites (8a) are occupied by iron, but the octahedral sites (16d) are simultaneously occupied by lithium and iron {Fe[Li0.5Fe1.5]O4}. The cell parameter is rather low (a = 0.8322 nm) if compared with the value a = 0.8331 nm for the oxide prepared at 900°C using metal carbonates (41). This oxide can be also prepared by lithium insertion into the spinel oxide Fe3O4 to give Li0.5Fe2.5O4. The Mössbauer spectrum (Fig. 3.) showed two sextets corresponding to Fe3+ ions in tetrahedral and octahedral sites as have been already detected by us for lithium ferrite prepared at 900°C (42). The lithium spinel LiMn2O4 shows the {111} index as the principal plane (Fig. 2c) and {311} for the other compounds. The cell parameter obtained (a = 0.8244 nm) is very close to the literature value, 0.824 nm - 0.825 nm (31,43-46). This lattice parameter increases gradually as the sintering temperature increases from 650 to 900oC (47).

Fig.2. XRD patterns of
(a) LiCo2O4, (b) LiFe2O4, and (c) LiMn2O4.

 

The metals M of a mixed oxide electrode are sensitive to the solution pH because the surface becomes solvated and covered by OH groups (MOH). The response to pH changes involves surface charging due to H+ adsorption (MOH+) or release of protons (MO-). Since the pH at which no net charge is present on the surface is known as the pzc, changes in the pzc with the nature of the surface reveals possible modifications occurring in the surface active sites. The pHz results obtained at three concentrations of KNO3 for the oxides appear in Table I. All cases show that the pHz increases lightly with the concentration of the inert electrolyte. However it was found that for many oxides in KNO3 the value of pHz remains practically unchanged in the 10-3 -1M concentration range (48). The shifting may due to an irreversible adsorption of potential determining ions (49) or to a low equilibration with the solution upon prolonged immersion. Nevertheless, it may also be a potential effect of the surface modifications that takes place. At the end of the measurements, LiCo2O4 pHz = 7.3 ± 0.5 is in agreement with the presence of the Co3+ ions as in the spinel Co3O4 (pHz = 7.5 ± 0.1(50)). In the case of the Li-Fe oxide, pHz = 6.5 ± 0.5 is close with a pHz = 6.7 previously reported (51). It is known that the manganese oxides exhibited a pzc considered acidic. In fact, MnO2 that contains Mn3+ (MnOOH) and Mn4+ ions in the structure presents pHz = 3.3 ± 0.5 (52) , which can be compared with LiMn2O4 of this work, pHz = 3.4 ±0 0.3.

Table I. pHz values for LiM2O4 oxides as a function of KNO3 concentration (as indicated).


Oxide

5 10-3 M

5 10-2 M

5 10-1 M


Li-Mn

3.2

3.4

3.6

Li-Fe

6.2

6.5

6.8

Li-Co

7.1

7.3

7.4


Cationic distributions.

The general formula of a binary spinel A[B2]O4, can be described as a cubic-close-packet array of oxide ions in which only one half of the octahedral (B) and one-eight of the tetrahedral (A) sites are occupied by metals. Each unit cell contains 8 molecules A[B2]O4 where the oxygens are located at the 32e position of the S.G. Fd3m. The B cations occupy octahedral sites at 16d positions and the empty octahedral sites are at 16c. Three no-equivalent positions, 8a, 8b and 48f belong to 64 tetrahedral positions where the A cations occupy 8a sites. Considering the X-ray results, the Mössbauer analysis and the pHz determinations, it is possible to accept that for Li-Mn and the Li-Fe spinel oxides, Li+[Mn3+Mn4+] and Fe3+[Li+0.5Fe 3+1.5], are the probable cationic distributions, respectively. In the case of LiCo2O4, it is necessary to admit the presence of CoIV ions (as in CoO2) according to mass balance and electrical neutrality. The cationic structure will be a normal spinel: Li+ [ Co3+ Co4+]. Normal LiCo2O4 is energetically favored (relative to the inverse spinel) as have been suggested by calculations (18). On the other hand, several authors have proposed the presence of Co4+ ions in the normal spinel CoCo2O4 prepared by pyrolysis of nitrate salts (53-55).


Fig. 3. Mössbauer spectrum at room temperature of lithium ferrite.

Li-insertion.

Using n-butyl lithium, it was possible to show that all oxides can insert into their structure approximately the same amount of Li ions by mol of oxide: LiMn2O4 (2 mol), LiFe2O4 (1.9 mol) and LiCo2O4 (2.4 mol). In the case of LiCo2O4 it is possible to admit that at same time the Li-insertion occurs, the same amount of electrolyte can be inserted. In fact, after lithiation the a-cell parameter increases from 0.8076 nm (x=0) to 0.8087 ± 0.0002nm (x = 2). This effect was not observed when lithium iodide was used as lithiation reagent.

In order to select the electrolyte system to study the lithium insertion process into LiM2O4 (M = Mn, Fe, Co) oxides, we have used the cyclic voltammetry (CV) technique. Two different systems of electrolytes were analyzed: Li/PC:EC 1:1, 1M LiClO4/oxide and Li/PC:DMC 4:1, 1M LiBF4/oxide between the potential limits 1V and 4.2 V. As an example, Fig. 4 shows the CV curves obtained on LiMn2O4 in 1M LiBF4, PC:DMC. Anodic peaks at 4.2 and 3.3 V, plus a cathodic peak at 3.9 V and, also a broad peak near 2 V can be seen. Such peaks are indicative of the insertion and extraction process of lithium associated with an overvoltage. All the CV curves have shown the same shape with currents depending on the nature of the electrode. With both electrolyte systems, the charge density involved in the lithium process was evaluated. Table II gathers these results. Li-Fe appears to be the electrode that shows better reversibility in the insertion-extraction process. In both electrolytes, the Qins/Qext ratio follows the order LiMn2O4 > LiCo2O4 > LiFe2O4. According to these results, we have chosen LiBF4 in PC:DMC as the proper electrolyte to evaluate the galvanostatic Li-insertion reaction
LiM2O4 + xLi+ + xe ® LixLiM2O4.

Fig. 4. Cyclic voltammogram of LiMn2O4 electrode at the scan rate of 1 mV s-1, 25°C. Electrolyte: 1M LiBF4 in PC:DMC 4:1.

Table II. Charge by unit of electrode surface involved in the lithium insertion (Qins) and the lithium extraction (Qext) processes using different electrolytes.


1M LiClO4 in PC : EC 1 : 1

1M LiBF4 in PC : DMC 4 : 1

oxide

Qins / As cm-2

Qext / As cm-2

Qins / As cm-2

Qext / As cm-2


Li-Mn

300

43

380

70

Li-Fe

190

180

50

49

Li-Co

230

110

40

30


Typical discharge curves at j = 44 mAcm-2 for LiM2O4 where M = Mn, Fe and Co, are reported in Fig 5. The open circuit voltage (OCV) measured during the discharge curve is very stable for the LiMn2O4 electrode whereas for the Li-Fe and the Li-Co oxides it changes continuously. Concerning the discharge curves, after an initial part characterized by several slope changes due to structural modifications, it is quite evident that the Co-spinel shows the best performance considering the lithium concentration inserted and the insertion potential values. However, taking into account the specific capacity of the electrodes studied at 2 V, the sequence will appear inverted: LiMn2O4(40 mAh/g)> LiCo2O4(10 mAh/g)> LiFe2O4 (0.1 mAh/g).

Fig. 5. Lithium insertion curves at j = 44 (Acm-2 for LiM2O4 in 1M LiBF4, PC:DMC 4:1.
(a) M =Mn, (b) M = Fe, (c) M = Co. OCV corresponds to open circuit voltage.

The reactivity of the LiM2O4 spinels studied here (in terms of the amount of lithium ions inserted) depends on the B-site cations. Since lithiation is an oxidation/reduction reaction in which both electrons and ions are transferred from guest species to the host matrix, the ability of the spinel host structure to receive extra electrons is essential to determine the reactivity in this type of reaction. If the B-site ions show high oxidation states (such as in LiCo2O4 and LiMn2O4) the Li-insertion process will occur with high efficiency. The low reactivity showed by Li-Fe oxide can be understood by the absence of M4+ ions in the spinel structure. In the case of LiMn2O4, it is possible that the Li-insertion can be quite limited by a Jahn-Teller distortion due to the presence of Mn3+ ions. One way to improve the reactivity of manganese oxides might involve replacing part of Mn3+ by other ions such as Li, Mg, Zn, Cu, etc. The results of this work suggest the possibility of tailoring the voltage of the cathode electrode in lithium batteries by a judicious choice of the B-site cations in the spinel framework.

CONCLUSIONS

Mixed oxides having the formula LiM2O4 (M = Mn, Fe, Co) crystallize in the spinel phase spatial group Fd3m, with proposed cationic distributions Li+[Mn3+Mn4+], Fe3+[Li+0.5Fe 3+1.5] and Li+[Co3+Co4+]. In terms of the amount of lithium ions inserted, the discharge curves show the following reactivity order: Li-Co > Li-Mn > Li-Fe oxides. The reactivity depends on the nature and the oxidation state of the cations placed in the B sites of the spinel structure. It should be possible to increase the amount of lithium insertion in Co oxides by doping with cations having low oxidation sates such as Ni2+, but preserving the electronic conductivity of the oxide. These experiments are currently under way.

*Corresponding author. e-mail : jgautier@lauca.usach.cl. Fax : 56-2-68 12 108

ACKNOWLEDGEMENTS

This work was supported by FONDECYT (project 1990951) and DICYT (USACH). Partial financial assistance from ECOS-CONICYT (C97E02) is gratefully acknowledged.

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