Electrochimica Acta 54 (2009) 1654–1661

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Electrochimica Acta

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Lanthanum-doped LiCoO2 cathode with high rate capability

Paromita Ghosh, S. Mahanty, R.N. Basu ∗

Fuel Cell & Battery Division, Central Glass & Ceramic Research Institute, Council of Scientific and Industrial Research, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 29 July 2008Received in revised form 5 September 2008Accepted 25 September 2008Available online 4 October 2008

Keywords:Lithium-ion batteryLithium cobalt oxideCyclic voltammetryCharge–discharge cycleImpedance spectroscopy

a b s t r a c t

Lanthanum-doped LiCoO2 composite cathode materials, containing 0.1–10 mol% of La were synthesizedby citric acid aided combustion technique. Thermal analyses showed that the sharp decomposition reac-tion for pristine LiCoO2 became sluggish upon addition of lanthanum. X-ray diffraction analyses of thecomposites revealed existence of minute quantities of lanthanum-rich perovskite phases—rhombohedralLaCoO3 (R3̄) and tetragonal La2Li0.5Co0.5O4 (14/mmm), along with rhombohedral LiCoO2 (R3̄m). Electronmicroscopy showed a distinct grain growth with increasing La content. An increase of about two orders ofmagnitude in the electrical conductivity (1.09 × 10−3 S cm−1) was observed for 1.0 mol% La-doped LiCoO2.An excellent cycling performance with capacity retention by a factor of ∼10 in comparison to the pris-tine LiCoO2 was observed for the composite cathode containing 5.0 mol% La, when 2032 type coin cellswere cycled at 5C rate. This has been ascribed to the structural stability induced by La doping and pres-ence of the ion-conducting phase La2Li0.5Co0.5O4 which acts as a solid electrolyte for Li+ ions. A negligible

growth of impedance upon repeated cycling has been observed. Cyclic voltammetry showed a remarkableimprovement in reversibility and stability of the La-doped electrodes. These composite cathodes might

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. Introduction

Lithium cobalt oxide (LiCoO2) is the most widely usedathode material for lithium-ion batteries due to its superiorlectrochemical performance in terms of high discharge capac-ty (120–160 mAh g−1) and excellent cycleability (∼1000 cycles)ompared to other cathodes known so far [1–6]. However, the prac-ically achieved capacity is only ∼50% of its theoretical capacity∼273 mAh g−1) because the amount of Li that can be extracted isimited to only 0.5 mol/mol of LiCoO2, beyond which the layeredhombohedral structure collapses and gives way to an electrochem-cally inactive monoclinic phase. Despite the commercial successn consumer electronics, there have been continuous efforts tomprove the rate performance of LiCoO2 particularly in view ofpplications in zero emission vehicle (ZEV) and hybrid vehicle (HV).he approaches that have been undertaken may be divided into tworoad categories:

1) Reducing the structural disintegration by doping with transi-tion and non-transition elements, e.g., Ti [7], Cr [8], Mn [9], Ni[10], Fe [11], Cu [12], Bi, Zr, Sn [13], Zn [12], Mo, V [14], etc. so thatmore Li can be extracted from the structure. Theoretical stud-

∗ Corresponding author. Fax: +91 33 2473 0957.E-mail address: rnbasu@cgcri.res.in (R.N. Basu).

013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.09.050

er applications.© 2008 Elsevier Ltd. All rights reserved.

ies predict that doping with transition metal will increase thecapacity whereas doping with non-transition metal will leadto increased voltage [12]. However, experimentally an averageelectrochemical performance has been found for doped LiCoO2cathode materials;

2) Modifying the surface by coating with inactive metal oxidessuch as Al2O3, ZrO2, TiO2, La2O3, Ag, etc. in order to prevent Codissolution [5,15–20]. Fey et al. [5] have reported an improve-ment in capacity retention by surface coating of LiCoO2 withLa2O3. Similar results have been observed for LiCoO2 coatedwith lanthanum aluminium garnate [21]. In the present work,we have attempted a different approach by introducing La intothe matrix of LiCoO2 and studied its effect on the physicaland electrochemical properties of the parent compound. Beinghighly mobile, it is probable that La would move to the surfacewith a fraction of them moving towards the interlayer spaces ofLiCoO2 and eventually stabilize the layered structure, therebyfacilitating the to-and-fro movement of Li into and out of thestructure [4,22]. Nevertheless, it might also increase the elec-tronic conductivity and help in the electronic charge transferduring Li intercalation–deintercalation. Consequently, it would

increase the maximum current carrying capacity, improve theLi diffusion kinetics in the bulk cathode material and preventkinetic polarization during cell cycling. Recently, Arumugam etal. [23] have reported good capacity retention and high ratecapability in La substituted LiMn2O4 where La was shown to

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0Lt(a(soiai0TsLAupon La doping, although no systematic change is observed. Thelattice parameter a indicates the metal–metal intralayer distancewhile c represents the interlayer spacing. Lattice substitution byelements with larger ionic radii often leads to expansion of the

P. Ghosh et al. / Electrochim

substitute Mn in the lattice site. To the best of our knowledge,no study has been made on the La–LiCoO2 nanocomposite cath-ode material so far. Thus, in the present work, we report on thesynthesis of La–LiCoO2 composite powder by combustion tech-nique where La is added in situ as a precursor. The influenceof La on the material properties are discussed and correlatedto the electrochemical properties of these composite cathodematerials.

. Experimental

For the synthesis of La–LiCoO2 powders (0–10 mol% of La),citrate–nitrate gel combustion process was adopted [24–29].

n aqueous solution of stoichiometric amounts of lithiumitrate (LiNO3), cobaltous nitrate hexahydrate [Co(NO3)2·6H2O]s.d. fine Chemicals, 99.5%), lanthanum nitrate hexahydrateLa(NO3)3·6H2O] (SRL, 99.5%) and citric acid monohydrate (Merck,9.0%) was heated at a temperature of ∼150 ◦C on a hot plate withonstant stirring of the solution by a magnetic needle. Here, cit-ic acid acts both as a fuel for the combustion process as well as ahelating agent for the metal ions (Li+, Co2+ and La3+) in solution.radually, the clear dark purple solution turned into a viscous gelnd ultimately burnt into a black mass containing the precursoretal oxides for the formation of LiCoO2. The as-synthesized pow-

er, thus prepared, was collected and further heat treated in air at00 ◦C to obtain the rhombohedral phase with a good crystallinity.he code names of the prepared samples are given below with theominal La-content within the parentheses: L0 (0.0 mol% La), L10.1 mol% La), L3 (0.3 mol% La), L5 (0.5 mol% La), L10 (1.0 mol% La),20 (2.0 mol% La), L50 (5.0 mol% La) and L100 (10.0 mol% La).

Thermogravimetric analyses (TGA) of the gel samples wereone in the temperature range 30–1000 ◦C by a Thermal Anal-ser (Model STA 409C, NETZSCH, Germany) with a heating rate of0 ◦C min−1. X-ray powder diffractograms were recorded in the 2�ange 15–70◦ at a scanning rate of 2◦ min−1 by a X-ray diffractome-er (Philips X’Pert, The Netherlands) with a Cu K� radiation at 40 kVnd 40 mA. Quantitative phase analyses of the X-ray diffractionrofiles were carried out using PANalytical Highscore Plus pro-ram. Microstructural studies were carried out by a field emissioncanning electron microscope (FESEM) (ZEISS Supra 35, Germany).he electrical conductivity was measured by a two-probe conduc-ivity measurement system from room temperature to 150 ◦C onircular pellets of diameter of ∼10 mm and thickness of ∼2 mm,intered at 800 ◦C. The electrochemical performance was stud-ed by assembling 2032 coin-type cells. A typical cathode wasrepared from a slurry of the synthesized La–LiCoO2 compos-

te powder (76 wt%), acetylene black (10 wt%) and PVDF binder14 wt%) in n-methyl pyrrolidinone (NMP) solvent. The slurry wasoated on to an aluminum foil (current collector) and dried at 110 ◦Cn an oven for 12 h. It was then pressed at 2 ton/sq. in. Circularisks of 16.3 mm in diameter were cut from the coated foil andsed as cathode. The weights of the material (excluding acety-

ene black and PVDF) in the cathodes were 0.0284, 0.0176 and.0091 g, respectively for L0, L10 and L50. The thickness of theathodes was 60–70 �m excluding the aluminum foil. The cellsere assembled with these cathodes using Li metal as anode and

iPF6 in EC:DMC (1:1, vol.%) as electrolyte and Celgard 2300 aseparator within an argon filled glove box (M’BRAUN, Germany)here the moisture and oxygen levels were kept below 1.0 ppm.

alvanostatic charge–discharge cycles were carried out at various

ates in the voltage range 3–4.2 V using an automatic battery testerModel: BT2000, Arbin, USA). Electrochemical impedance spectraere recorded on a galvanostat–potentiostat (Model PGSTAT 30,utolab, The Netherlands) in the frequency range 10 mHz to 1 MHz.

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cta 54 (2009) 1654–1661 1655

yclic voltammetry of the cells were recorded on the same instru-ent between 3 and 4.4 V at a scanning rate of 0.01 mV s−1.

. Results and discussion

Fig. 1 shows the TG plots for LiCoO2 precursor gel samples syn-hesized with varying lanthanum content (0.0, 0.1 and 1.0 mol%) inhe temperature range 30–1000 ◦C. All the samples show an initialeight loss at temperatures <150 ◦C due to loss of absorbed water.

he pristine gel sample shows a sudden, sharp decomposition stept ∼177 ◦C accompanied by a large weight loss, due to decomposi-ion of the organic residues and elimination of nitrates [24,29]. Nourther significant weight loss is observed beyond this temperature.n the other hand, La–LiCoO2 gels show a sluggish decomposition

eaction which goes to completion at ∼500 ◦C, suggesting moreomplex reactions due to the addition of La3+ and possible forma-ion of additional phases. As the lanthanum content is increasedrom 0.1 to 1 mol%, the decomposition reaction becomes moreluggish. However, for both L1 and L10, the weight loss becomesegligible beyond 500 ◦C indicating formation of La–LiCoO2 pow-ers.

Fig. 2 shows the X-ray diffractograms for pristine LiCoO2 (L0),.1 mol% (L1), 1 mol % (L10), 5 mol% (L50) and 10 mol% (L100) ofa–LiCoO2 composite samples. For all samples, the characteris-ic peaks of a standard rhombohedral LiCoO2 can be identifiedJCPDS File No. 050-0653). A good resolution of the (0 0 6)/(0 1 2)nd (0 1 8)/(1 1 0) peaks present even in the most heavily doped10 mol%) sample L100 reflects a dimensionally stable layeredtructure and suggests that the highly ordered array of cobalt andxygen atoms have not been perturbed by the introduction of La3+

nto the LiCoO2 matrix. However, distinct additional peaks startppearing in the diffractograms of L10, L50 and L100 which arendexed to La-containing perovskite phases LaCoO3 [JCPDS File No.1-086-1663] and La2Li0.5Co0.5O4 [JCPDS File No. 01-083-1844].he detailed phase analysis and the lattice parameters of all theamples are given in Table 1. As the amount of La is very less in1, no separate perovskite phase could be found in this sample.s shown in Table 1, both the lattice parameters a and c increase

ig. 1. TGA of the gel precursors of pristine LiCoO2 (L0) and La–LiCoO2 compositeathode materials (L1 and L10).

1656 P. Ghosh et al. / Electrochimica A

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ig. 2. X-ray diffractograms of pristine LiCoO2 (L0) and 0.1–10.0 mol% La addediCoO2 (L1, L10, L50 and L100).

nit cell parameters. Thus, considering the large difference in ionicadii of La3+ in (∼0.115 nm in octahedral coordination) comparedo Co3+

oct (∼0.063 nm), the observed increase in the a and c val-es in La–LiCoO2 composites can be explained. Further evidencef a good cation ordering is shown by a high c/a ratio of 4.99 thatemains nearly unaltered for all the samples [14].

Fig. 3(a)–(c) show the FESEM images of pristine LiCoO2 (L0)nd La–LiCoO2 composites (L5 and L100), respectively. A partiallygglomerated mass of LiCoO2 grains can be seen in the micro-raphs of all the samples. Hexagonally shaped grains measuringpproximately 100–200 nm are observed for pristine LiCoO2 (L0).

distinct morphological change evolves as lanthanum is addedn the matrix of LiCoO2 that is reflected in the increase of grainize of L5, that measures 500–800 nm. The grain size increases fur-her for L100 (500 nm to 1 �m). The grains in L100 show two kindsf features—some grains are hexagonal measuring about ∼500 nmhile others are elongated along c axis and are ∼1 �m in size. The

bserved increase in grain size upon La doping might have resultedrom the slow and sluggish nature of combustion in these sampless already shown in thermal analysis results (Fig. 1).

The electronic conductivity is a very important property of aathode material for a better charge transfer process during lithiumntercalation–deintercalation in a lithium-ion cell [19,24]. Fig. 4hows the electrical conductivity of L0 together with those of3, L5 and L10 in the temperature range of 35–150 ◦C. The room

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able 1hase analysis and lattice parameters of LiCoO2 powder: pristine and La-LiCoO2 composit

ample code La added, mol%(wt%) Phases present (wt%)

0 0.0 (0.0) LiCoO2 (100)

1 0.1 (0.14) LiCoO2 (100)

10 1.0 (1.4) LiCoO2 (99)LaCoO3 (0.6)La2Li0.5Co0.5O4 (0.4)

50 5 (7.1) LiCoO2 (97)LaCoO3 (0.8)La2Li0.5Co0.5O4 (2.2)

100 10 (14.2) LiCoO2 (88)LaCoO3 (4)La2Li0.5Co0.5O4 (8)

cta 54 (2009) 1654–1661

emperature conductivity of L0 is found to be 6 × 10−5 S cm−1.ll the La–LiCoO2 composites show higher room temperatureonductivity with respect to L0. The conductivity increases withncreasing amount of La per mole of LiCoO2 from 0.3 mol% (L3)p to 1.0 mol% (L10). A conductivity value of 1.09 × 10−3 S cm−1 isbserved for L10 which is about two orders of magnitude higherhan L0. With increasing temperature, all the samples show aypical semiconducting behaviour. XRD phase analyses showedresence of minute quantities of La2Li0.5Co0.5O4 and LaCoO3 as

mpurity phases in L10. Apart from the doping effect of La, thesedditional phases particularly, La2Li0.5Co0.5O4 might play a signif-cant role in the observed increase in conductivity with increasingmount of La. La2Li0.5Co0.5O4 is a perovskite with a K2NiF4 typetructure with remarkable oxygen diffusibilitiy similar to othererovskites like La2NiO4 or La2CoO4. Therefore, significant oxygenyperstoichiometry might occur due to substantial incorporationf interstitial oxygen into the rock-salt layers [29]. Since theseaterials are predominantly p-type semiconductors, the oxygen

yperstoichiometry further increases the number of holes. Theoles thus formed are due to incorporation of extra oxygen onhe B-site cations (cobalt in this case) that participate in a hop-ing conduction mechanism [30,31], thereby increasing the overallonductivity.

2032 type coin cells, fabricated with the pristine LiCoO2 (L0)s well as the composite samples as active cathode materials,ere used for studying the electrochemical performance by gal-

anostatic charge–discharge at about 25 ◦C. Fig. 5 shows the firstharge–discharge profiles for L0, L10, L20 and L50 measured frompen circuit voltage (Voc) to 4.2 V at a constant current density of.002 mA mg−1 for charging and 0.004 mA mg−1 for discharging.he initial discharge capacity is found to be 138 mAh g−1 for pristineiCoO2 (L0) with a high irreversible capacity loss of ∼30%. Similarischarge capacities have been reported for LiCoO2-based cathodeaterials by many authors [4,14,32–35]. At the same current rate,decrease in the initial discharge capacity is observed for all the

omposite samples (Fig. 5). It is found that the capacity graduallyecreases with increasing amount of La and the observed valuesre 135, 117 and 101 mAh g−1 for L10, L20 and L50, respectively.his may be due to the formation of increasing quantities of theerovskite phases (LaCoO3 and La2Li0.5CoO4) with increasing Laontent in the composites which effectively reduces the amountf active LiCoO2 resulting in a decrease in the discharge capacity.

t is interesting to note here that the irreversible capacity loss isignificantly lower for the composite samples and for L50, it is only20% while that of the pristine LiCoO2 (L0) is 30%. This suggests aetter reversibility of the composite cathodes despite a reduction

n the discharge capacity.

es calcined at 700 ◦C.

Lattice parameters

a b c c/a

2.813 14.041 4.99

2.833 14.119 4.98

2.819 14.063 4.995.442 5.4435.362 5.374 12.707

2.819 14.058 4.995.442 5.4435.362 5.374 12.707

2.841 14.133 4.975.442 5.4435.362 5.374 12.707

P. Ghosh et al. / Electrochimica Acta 54 (2009) 1654–1661 1657

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difference in the electrochemical performance of the La–LiCoO2composites in comparison to pristine LiCoO2. At 1C rate, L10 showsthe highest capacity of 105 mAh g−1, L50 shows a capacity of85 mAh g−1 and L0 shows a low capacity 70 mAh g−1 [Fig. 6(c)]. L50retains 85%, L10 retains ∼75% and L0 retains 50% of the respec-

Fig. 3. FESEM images of (a) L0, (b) L5 and (c) L100.

After the initial charge–discharge, each of the cells L0, L10 and50 were tested for capacity retention by cycling between 3.0 and.2 V at four different rates—C/5, C/2, C and 5C, respectively, for 20ycles at each rate. During cycling, both charging and discharging

ere carried out at equal current rate in each case. In order to check

or capacity restoration, the cells were again cycled at C/5 (for 10ycles) and then at 5C (for 20 cycles). Thus, all these cells wereycled for 111 cycles continuously. Fig. 6(a)–(f) show the electro-hemical cycling performance of pristine LiCoO2 (L0) along with

Fig. 4. Electrical conductivity profiles of L0, L3, L5 and L10.

he La–LiCoO2 composites (L10 and L50) at various current rates. Itay be observed from Fig. 6(a) that at C/5, pristine LiCoO2 showscapacity of ∼130 mAh g−1. Compared to L0, L10 shows a slightlyigher capacity of ∼140 mAh g−1 and L50 shows a lower capacity of90 mAh g−1 at the same rate. On the other hand, it can be observed

hat the capacity retention is improved for the La–LiCoO2 com-osites with L50 showing maximum capacity retention of nearly00%, followed by L0 and L10 at 90%. Similarly, at C/2, L0 and L10how almost an equal steady capacity of 115 mAh g−1 and still lowerapacities in the range 80 mAh g−1 is obtained for L50 [Fig. 6(b)].he capacity retention is again maximum for L50 (∼95%), followedy L10 (∼90%) and L0 (∼85%). Therefore, the results indicate that atower current rates up to C/2, L0 and L10 show comparable perfor-

ance with slightly better capacity retention for L10 (containing.0 mol% of La).

At higher rates (1C and 5C), however, there is a remarkable

Fig. 5. Initial charge–discharge curves of L0, L10, L20 and L50.

1658 P. Ghosh et al. / Electrochimica Acta 54 (2009) 1654–1661

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ig. 6. Cycling performance of pristine LiCoO2 (L0) and La–LiCoO2 composites (L10orresponding current densities are given within parentheses.

ive initial capacities observed at C/5 rate of discharge. At 5C, L50hows the highest capacity of ∼70 mAh g−1, retaining ∼80% of itsnitial capacity. It is followed by L10 (65 mAh g−1), retaining 50% ofts initial capacity. On the other hand, L0 suffers a severe capacityoss and shows a value of only 10 mAh g−1 [Fig. 6(d)], i.e. retentionf less than 8% of its initial capacity. Thus, improved capacity reten-ion by a factor of ∼10 is observed for L50 (containing 5 mol% of La)ver L0 at a high current rate of 5C. This result is remarkable dueo the fact that although the C-rate is same (5C), the correspondingurrent density (0.86 mA/cm2) for L50 is much higher than that for0 (0.31 mA/cm2).

For evaluating the capacity restoration performance, the cellsre then cycled again at C/5 for 10 cycles, followed by 5C for 20ycles. At C/5, L0 shows an average capacity of ∼105 mAh g−1 corre-ponding to a capacity retention of 81%, L10 shows a steady capacityf ∼120 mAh g−1 retaining about 86% of the initial capacity and L50

hows a capacity of 85 mAh g−1, i.e. retention of ∼94% of the ini-ial capacity [Fig. 6(e)]. On further cycling at 5C, the capacity of L0educes almost to zero; L10 shows a value of 65 mAh g−1 (∼50%etention) while L50 shows a capacity of 75 mAh g−1 which corre-ponds to ∼83% of capacity retention [Fig. 6(f)]. It is interesting to

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L50) at various current rates: (a) C/5, (b) C/2, (c) C, (d) 5C, (e) C/5, and (f) 5C. The

ote that although the capacity of L50 is only 90 mAh g−1 in thest cycle at C/5, it retains 83% of the capacity at 5C in the 111thycle even after being subjected to various current rates. The aboveesults clearly show that at high current rates the La–LiCoO2 com-osite with 5 mol% of La (L50) shows much better capacity retention

n comparison to pristine LiCoO2. The observed remarkable capac-ty retention for La-doped LiCoO2 might be due to the enhancementf electrical conductivity and stabilization of the layered rhombo-edral phase of LiCoO2 due to La doping.

Fig. 7(a)–(c) show the impedance spectra of L0, L10 and L50aken after 20 cycles and 60 cycles. The impedance spectra of L0how superimposition of two depressed semicircles with a sharp

arburg resistance. The solution resistance (Rs) is found to increaserom 7.8 to 10 � between 20 and 60 cycles. On the other hand, both10 and L50 show only a single semicircle with very little changen the solution resistance at the same stage. For L10, Rs increases

rom 4 to 5 � while for L50, the increase is marginal from 5 to.4 �. Rs arises due to the interactions of the oxide cathode with thelectrolyte solvent forming a thin surface film which may be influ-nced by the presence of additional ion-conducting phase [4,5]. Its observed that Rs in L0 is slightly higher than that in either L10 or

P. Ghosh et al. / Electrochimica A

Fig. 7. Complex impedance spectra (Z′ vs. Z′ ′) of (a) L0, (b) L10, and (c) L50.

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cta 54 (2009) 1654–1661 1659

50 after cycling over an equal number of cycles. It is possible thatresence of La2Li0.5Co0.5O4 as an additional phase, which could belithium-ion conductor, helps to reduce Rs. Similar observationsave been reported by Xu et al. [4] in LiCoO2 co-doped by Mg andr where presence of additional Li2MgZrO4 phase – a lithium-iononductor – resulted in reduction of Rs. Further, it is interesting toote that the growth in Rs is significantly lower in the lanthanum-oped samples in comparison to the pristine sample during cyclingrom 20 to 60 cycles. It emphasizes the role of La in the stabiliza-ion of the rhombohedral LiCoO2 phase so that the Co dissolutionnto the acidic electrolyte is prevented to a considerable extent.he low growth of impedance in La-doped cells indicates a loweregree of diffusion polarization and degradation of the cathode.harge transfer resistance (RCT) on the other hand increases for allhe samples between 20 and 60 cycles: it increases from 32 to 36 �or L0, from 40 to 45 � for L10 and from 30 to 43 � for L50. However,t is found that despite a relatively higher increase in RCT, the rateerformance of L50 is better (Fig. 6). The rate performance is mainlyontrolled by diffusion of Li ions through the bulk cathode mate-ial. As found in the X-ray diffraction studies (Table 1), La-dopingn LiCoO2 brings about a slight increase in unit cell volume thatacilitates Li intercalation–deintercalation from the oxide structure.n addition, the La-containing perovskite (La2Li0.5Co0.5O4) may actike a solid electrolyte that further improves the Li transport withinhe bulk cathode material.

Apart from the electronic conductivity and cell impedance,he other important factors that influence the rate capability andtability of the cell are steady lithium ion diffusion through theulk cathode material. It is well known that the volume changeuffered by LiCoO2/Li1−xCoO2 (x = mole fraction of Li extracted dur-ng charging) during charge–discharge is about 2% [35]. It is alsonown that when the lithium extraction limit extends beyond.5 mol/mol of LiCoO2, a rhombohedral to monoclinic phase trans-ormation occurs, accompanied by a 2.6% volume expansion alonghe c-axis. This involves cobalt dissolution in the acidic electrolytehat results in significant capacity fade and mechanical failure inells [2,5]. It has been observed previously that when lanthanumxide is used as an additive, it decreased the overpotential in theernary alkaline metal carbonate system significantly, thus reduc-ng the transition metal dissolution into the electrolyte [36,37]. Insimilar way, in addition to the structural stability induced by Laoping, it is also probable that the additional La-rich perovskitehase (La2Li0.5Co0.5O4) present in the composite La–LiCoO2 cath-de materials might aid in reducing cobalt dissolution into thecidic electrolyte, thus further increasing the effective structuraltability of the cathode material. In order to investigate the struc-ural stability and reversibility, cyclic voltammetry was recorded forhe cells L0, L10 and L50 at a scan rate of 0.01 mV s−1 in the poten-ial range 3–4.4 V after the 1st cycle and also after the 111th cyclend are shown in Fig. 8(a)–(c). When the CV was recorded after therst cycle, L0 shows two distinct peaks at 4.04 V (oxidation) and.8 V (reduction), signifying the lithium de-intercalation and inter-alation into the layered structure of parent LiCoO2. Two additionaleaks at ∼4.1 and 4.2 V appear due to the well-known hexagonal-onoclinic phase transformation which occurs at x = 0.5 in LixCoO2

7,14]. Similar oxidation–reduction peaks are also observed in theoltammogram of L10 and L50 after the 1st cycle. But, interestingly,he additional peaks signifying the rhombohedral to monoclinichase transformation during charge–discharge of L0 is absent inhe voltammograms of L10 and L50. This phase transformation is

nown to be responsible for the structural disintegration in LiCoO2.herefore, the absence of these peaks in L10 and L50 implies that thelectrochemically active rhombohedral phase is stabilized due to Laoping. However, when the CV is recorded for L0 after 111 cycles,nly broad humps are observed in the voltammogram with no dis-

1660 P. Ghosh et al. / Electrochimica A

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ig. 8. Cyclic voltammograms of (a) L0, (b) L10, and (c) L50 after cycling the respec-ive cells for 1 cycle and 111 cycles.

ernable peak. In contrast, when the CV for L10 is recorded after 111

ycles, two distinct reduction and oxidation peaks at 3.8 and 4.04 Vre retained, showing good reversibility and cycle performance.articularly, for L50, sharp oxidation and reduction peaks appearractically at the same potentials as observed in the CV after the first

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cta 54 (2009) 1654–1661

ycle with only a minor shift from 4.04 to 3.99 V. These results indi-ate an excellent reversibility, retention of good crystallinity andtability of the electrode even after 111 charge–discharge cycles.

. Conclusions

La-doped LiCoO2 composite cathode materials are synthe-ized by combustion synthesis technique. As the amount of La isncreased, the combustion becomes more sluggish. The particleize also increases with increasing La content, from 200 nm forndoped LiCoO2 to about 1.0 �m for La–LiCoO2 with 10 mol% ofa. XRD analyses show that the highly ordered array of cobalt andxygen atoms in layered LiCoO2 is retained in the doped samples,ut there is an expansion of the lattice. For heavily doped sam-les (La ≥ 1.0 mol %), minute quantities of additional La-containingerovskite phases LaCoO3 and La2Li0.5Co0.5O4 are formed. Fromlectrical conductivity measurements it is found that all the sam-les show a typical semiconducting behaviour. A very high roomemperature conductivity of ∼1.1 × 10−3 S cm−1, which is about tworders of magnitude higher than pristine LiCoO2, is observed forhe composite sample that contains 1 mol% La. The electrochem-cal studies which include galvanostatic charge–discharge cyclingrom medium to high rate, impedance spectra and cyclic voltam-

etry of 2032 type coin cells with the pristine LiCoO2 as well asa–LiCoO2 as the active cathode materials demonstrate a remark-bly improved cycling performance of the doped samples. Even at aigh rate of 5C, the cell containing 5 mol% La–LiCoO2 shows capac-

ty retention of ∼83% at cycle number 111—an improvement by aactor of 10 over the cell containing the pristine LiCoO2 as cathode.attice expansion and formation of La2Li0.5Co0.5O4, a lithium iononductor, facilitates the Li-ion diffusion within the bulk cathodeaterial leading to improved rate performance. The presence of this

hase also contributes to the reduction of solution resistance in thea-doped cathode containing cells, which further aids in Li trans-ort during cycling. Excellent reversibility and structural stability

nduced by La is also reflected in cyclic voltammetry. Therefore,he results clearly show potential for La-doped LiCoO2 as cathode

aterial in lithium-ion batteries with high rate power.

cknowledgement

The authors wish to thank Director, CGCRI for his kind permis-ion to publish this paper.

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