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Electrochemistry Communications 9 (2007) 2116–2120

Highly electroactive nanosized a-LiFeO2

Julian Morales, Jesus Santos-Pena *

Departamento de Quımica Inorganica e Ingenierıa Quımica, Edificio Marie Curie, Campus de Rabanales, Universidad de Cordoba, 14071 Cordoba, Spain

Received 9 May 2007; received in revised form 29 May 2007; accepted 4 June 2007Available online 13 June 2007

Abstract

We obtained pure nanosized a-LiFeO2 by a simple and quick method involving the reaction between goethite, lithium hydroxide andlithium nitrate at temperatures as low as 523 K. Unlike the known limited electrochemical activity of this polymorph, cells based on ournanosized electrode provided a remarkable high capacity in the first cycles and showed a good cycling life, due to its nanosized characterand adequate morphology to enhance lithium insertion/deinsertion. Capacity values when cycling in the 1.5–4.5 V range are up to150 mAh g�1 in the 50th cycle. To our knowledge, the capacity values and its retention upon cycling are the best reported for this poly-morph in an un-doped state.� 2007 Elsevier B.V. All rights reserved.

Keywords: a-LiFeO2; Lithium ion batteries; Positive electrode; Nanosized materials; Goethite

1. Introduction

Lithium cobalt oxide, LiCoO2, is the most widely usedpositive electrode material in commercial Li-ion batteriesat present [1] by virtue to its high reversible capacity(130–150 mAh g�1), long cycle life (300–500 cycles) andeasy preparation. The cell charging reaction is

LiCoIIIO2 ! xLiþ þ xe� þ Li1�xCoIII1�xCoIV

x O2: ð1ÞA voltage plateau at 3.8 V is obtained during the extractionof 1/2 mole of lithium per mole of oxide [2]. However, Cocompounds are toxic and expensive. Moreover, LiCoO2-based cells are subject to safety problems associated tothe instability of the delithiated phase, which containsCo(IV); this is a strong oxidant which can give a highlyexothermic reaction upon contact with the electrolyte sol-vent [3]. Various strategies have been proposed and testedto avoid some of the previous drawbacks, including replac-ing cobalt with another transition metal [4–6] or using of aprotective coating consisting of some inert matrix such as

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.06.013

* Corresponding author. Tel.: +34 957218620; fax: +34 957218620.E-mail address: iq2sanpe@uco.es (J. Santos-Pena).

an oxide (ZrO2 [3], Al2O3 [7], SiOx [8] or AlPO4 [3]).One interesting alternative is the use of LiFeO2 given thelow cost and environmental friendliness of iron. LiFeO2

crystallizes in a Na–Cl type structure where Li and Featoms occupy the octahedral sites in a cubic close packingof oxygen atoms. Four polymorphs have been identifiedfrom cation arrangement [9,10]. In the a-NaFeO2 typestructure, alternate layers of trigonally distorted MO6

and LiO6 octahedra share edges. In the a-LiFeO2 structure,which crystallizes in the cubic system, Li+ and Fe3+ ran-domly occupy the octahedral sites; in the LiMnO2-typestructure, however, the oxygen anion array is distortedand alternating zigzag layers of Li+ and Fe3+ cations resultin a reduced symmetry (orthorrombic system, corrugatedlayered structure). Finally, in the c-LiFeO2 (a goethite-structure), metal ions are ordered and the resulting symme-try is tetragonal. Recently, a tunnel structure bearing somesimilarities to hollandite a-MnO2 was reported [11]. Thiscompound contains FeO6 octahedra creating tunnels withoxygen in the center. Lithium ions surround the oxygensand strongly coordinate to other oxygens in the frameworkstructure.

The hypothetical reaction taking place at the electrodeduring the charge process in a LiFeO2 based cell is

J. Morales, J. Santos-Pena / Electrochemistry Communications 9 (2007) 2116–2120 2117

LiFeIIIO2 ! xLiþ þ xe� þ Li1�xFeIII1�xFeIV

x O2: ð2Þ

With x = 1, this reaction provides a capacity of283 mAh g�1. The four materials have been reported to ex-hibit disparate electroactivity. Thus, Kanno [12] reported amaximum value of x = 0.1 for the a-NaFeO2-type struc-ture. LiFeO2 with a corrugated structure obtained fromLiOH and c-FeOOH using a conventional ceramic method[13] was reported to yield 150 mAh g�1 (x = 0.53). How-ever, the capacity decayed over the next few cycles by effectof the transformation of the layered structure into theLiFe5O8 spinel. This value was higher than the firstreported for the same material by Kanno et al. [12] (lithiumremoved x = 0.4); also the lithium ferrite becomes anamorphous phase upon cycling. Goethite-structure LiFeO2

has been reported to provide similar capacities [14]. Thetunnel structure [11], with a maximum lithium removalvalue, x, close to 0.7, was found to exhibit higher electroac-tivity. However, there is some controversy concerning thereaction behind the electrochemical activity as Mossbauerspectra for this charged electrode suggest the release ofoxygen from the structural framework rather than reaction(2). Finally, there is little literature on the electrochemistryof the a-LiFeO2 polymorph. A preliminary x-value of 0.2was reported for a low temperature form [15]. Later, anx-value close to 0.3 was reported for the compound inthe form of nanorods [16]. Higher reactivity was observedin a Li4/3Ti2/3O2–LiFeO2 solid solution prepared by Tabu-chi et al. [17] in Fe/Fe + Ti ratios between 0.25 and 0.75. Inany case, the previous three studies revealed a low revers-ibility in reaction (2). In this work we have prepared nano-sized a-LiFeO2 by means of a simple and quick method.After the first half cycle, the nanomaterial showed a highelectroactivity with a value of x ranging between 0.53(C/4) and 0.62 (C/2.5). These values are much higher thanthose initially observed for this polymorph. However themost interesting result shown there is the high reversibilityof the reaction, as at 50th cycle the values of x are 0.52 and0.56 for C/4 and C/2.5, respectively.

Fig. 1. XRD pattern of the lithium ferrite. Inset: TEM image of thecompound. White bar corresponds to 100 nm.

2. Experimental

The lithium ferrite was obtained from a-FeOOH (goe-thite) as iron source. LiNO3 (Panreac, 98%), LiOH (Merck,98%) and goethite freshly prepared were mixed in a 2:2:1mole proportion and ground for 30 min. The resultingslurry was heated to 523 K at 3 K min�1 in 3 h. The brownproduct obtained was thoroughly washed with distilledwater in order to remove excess lithium compounds, thencentrifugated and dried at 333 K for 2 h. The synthesis pro-cedure was repeated in order to check its validity to obtainthe lithium ferrite. X-ray diffraction (XRD) patterns wererecorded on a Siemens D5000 X-ray diffractometer, usingCu Ka radiation (k = 1.54059 A) and a graphite mono-chromator. Transmission electron microscopy (TEM)images were obtained on a Jeol 2010 microscope operatingat 200 keV. Samples were prepared in an ultrasonic bath,

using hexane as dispersing agent and a nickel grid coveredwith a carbon perforated film.

The electrode was prepared from a mixture of activematerial, carbon black and Teflon in a 75:17:8 weight pro-portion. Electrochemical measurements were made in atwo-electrode cell, using lithium as counter-electrode. Theelectrolyte used was Merck battery electrolyte LP 40, whichconsists of 1 M LiPF6 in ethylene carbonate (EC) anddimethyl carbonate (DMC) in a 1:1 w/w ratio. The cellswere galvanostatically charged and discharged over the1.5–4.5 voltage range and under two different regimes(viz C/4 and C/2.5, which are referred to the content in lith-ium ferrite, C representing 1 Li+ ion exchanged in 1 h).Electrochemical measurements were controlled via a Mac-Pile II or Arbin potentiostat–galvanostat. Tests were car-ried out at least twice.

3. Results and discussion

The XRD pattern corresponding to the material isshown in Fig. 1. This pattern exhibited several peaks thatwere ascribed to a-LiFeO2 and was indexed in the cubicsystem with a = 4.176(1) A, which is quite consistent withthe results reported by several authors [15–17]. The TEMimages of the sample (inset of Fig. 1) revealed that the sam-ple consist of very thin particles with a rounded shape andmore than 50 nm in size. These particles tended to agglom-erate adopting a bunch-like morphology. Examinations inother zones of the holder yielded the same features:agglomerate nanoparticles in this particular way and short-age of isolated particles. Mossbauer spectra (not shownhere) were recorded at 295 K. A doublet was observed,which is indicative of a paramagnetic state. The hyperfineparameters for the major iron site (IS = 0.323 ± 0.003mm s�1 and QS = 0.602 ± 0.002 mm s�1) are reasonablyconsistent with the results of a comprehensive study con-ducted by Cox et al. [18]. Therefore, the method usedtherein is adequate to obtain pure a-LiFeO2 in a fast andsimple way.

2118 J. Morales, J. Santos-Pena / Electrochemistry Communications 9 (2007) 2116–2120

Fig. 2a shows the galvanostatic curves for the first cycle.On charging the cell, the voltage exhibited an abruptincrease from 2.6 (OCV) to 3.9 V; this accounts barelyfor 0.1 lithium ion removed from the structure. From thisvalue, a significant slope change was observed and 0.5 Liions were removed up to 4.5 V (the upper limit recorded).This is a common behavior for other LiFeO2 with differentstructures and the amount of lithium removed depends onseveral factors among other of electrolyte and the cell con-figuration used. In fact, tests on swagelock and coin cellsprovided disparate results. Coin cells used therein (2032model) have two advantages on swagelock cells (internaldiameter 12 mm), namely (i) the bigger diameter of the coincell allows the use of a greater amount of electrolyte,increasing the electrolyte–electrode interface and (ii) thecell components of coin cells are more tightly packed thanthose of the swagelock cells, thus decreasing the cell imped-ance. As a result, the cell activity in the first charge was bet-ter for the coin cells, adopting this configuration forsubsequent cycles. There was a pronounced drop in voltageat the beginning of the discharge process. At 3.0 V, theslope changed with the presence of a pseudo-plateau thatextended to ca. 1.7 V.

Fig. 2b shows the ex situ XRD patterns for the reactionproducts obtained in the charged (4.5 V) and discharged(1.5 V) states. The electrochemical process occurring atthe electrode degrades the crystallinity of the componentsalthough the peaks for lithium ferrite retained their sameposition, as previously reported by Sakurai et al. [15].However, we found the I(220)/I(200) ratio to be higher duringcharge (0.37) than at OCV (0.17). Moreover, the intensityratio recovered its initial value, 0.17, during the discharge.These results are suggestive of a structural rearrangement

Fig. 2. (a) First charge/discharge curve. (b) XRD patterns of the a-LiFeOss = stainless steel.

upon lithium removal. Similar results were reported for aLi4/3Ti2/3 O2–LiFeO2 solid solution prepared by Tabuchiet al. [17]. In this work, the authors proposed that FeIV

atoms formed during reaction (2) are displaced from octa-hedral 4a sites to tetrahedral 8c positions, and also thatlithium ions must use the 8c sites as a conduction pathway,similarly to Na+ and Ag+ in the cubic rock-salt structuresNaCl and AgCl [19]. Therefore, iron (IV) must hinder lith-ium diffusion during charge, which accounts for the dispa-rate profiles of the charge–discharge curves. The amount ofLi inserted in the first discharge was close to 1.4 Li per for-mula unit (Fig. 2a). This means an excess of Li over theavailable octahedral sites. In this context, Sakurai et al.[15] previously reported amounts of inserted Li higher thanone after the first cycle. The location of the Li excess isunknown but, assuming the maintenance of the ions intheir original positions, tetrahedral positions 8c are for-mally empty and disposable for their occupancy. Actually,the discharge curve exhibits a pseudo-plateau close to 1.7 Vin this compositional interval (from 1 < x < 1.4) the originof which could be due to this occupancy, but this modelneeds further confirmation. Nevertheless, other authorsbelieve that no clear free spaces exist for lithium diffusionas the metal ions are randomly arranged in the a-LiFeO2

structure [15].Subsequent charge/discharge curves, Fig. 3, were

s-shaped, similarly to iron oxides with different structuressuch as a-LiFeO2 [15,16], corrugated layer LiFeO2 [12–14], goethite-type LiFeO2 [14], spinel-type LiFe5O8 [20] oreven FeOOH nanorods [21]. In this respect, Sakurai et al.[13,14] noted that unusual FeIV ions generated duringcharging may play an important role in the developmentof voltage hysteresis in these systems. However, this model

2 electrodes at the different states represented in the charge/discharge.

Fig. 3. First galvanostatic cycles of the cells based on a-LiFeO2

nanoparticles recorded at different rates. Black and red lines correspondto charge and discharge process, respectively (For interpretation of thereferences to colour in this figure legend, the reader is referred to the webversion of this article.) .

J. Morales, J. Santos-Pena / Electrochemistry Communications 9 (2007) 2116–2120 2119

must be inappropriate for the latter compound, the electro-chemical activity of which is independent of the presence ofthis oxidation state. A small plateau at 4.2 V was observedduring the second charge (Fig. 3), the width of whichdecreased on cycling until completely vanished at thefourth cycle. Then, there was substantial profile retentionin the charge curves.

Fig. 4. Capacity values as a function of cycle number for the cells basedon a-LiFeO2 at different rates.

Fig. 4 shows the variation of the capacity as a functionof the number of cycles at C/4 and C/2.5. The cells, whichwere cycled between 4.5 and 1.5 V, exhibited a similartrend, namely a capacity drop in the first cycles, followedby a gradual increase on further cycling. Thus, the capacityincreased to ca. 150 and 130 mAh g�1, at the 9th (C/4) and14th (C/2.5) cycles, respectively. These values are muchhigher than those reported for other electroactive a-LiFeO2 forms [12,15,16] (particularly those for the nano-rods based cells, which provide only 80 mAh g�1 at the50th cycle [16]). These nanorods were 80 nm in diameterand 900 nm in length on average. Our sample consistedof not aligned nanocrystals sized of more than 50 nm onaverage. Therefore, we believe that the origin for the dis-crepancy between the electrochemical response of thenanorods prepared by Wang et al. [16] and our nanocom-posites must reside on their different size and morpholo-gies. The original bunch architecture of our material canprovide a not-interrupted pathway for lithium ion diffu-sion, enhancing the reaction (2). Thus, our nanocompos-ites possess good properties as positive electrodes for lowvoltage batteries. The combined effect of the nanometricparticle size and the bunch morphology leads to theremarkable electrochemical performance shown by thislithium ferrite. Future work is in progress to have a moreaccurate knowledge on the textural and structural modifi-cations undergone by the ferrite upon cycling, responsiblefor its good performance.

Acknowledgements

This work was supported by CICyT (MAT2005-03069)and Junta de Andalucıa (Group FQM 175). The authorsacknowledge the help of Prof. R. H. Herber and Dr. I.Nowik (Racah Institute of Physics, The Hebrew Universityof Jerusalem, Israel), for recording and discussing theMossbauer spectra. JSP is also grateful to Junta de And-alucıa (Spain) for inclusion in its Researcher ReturnProgram.

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