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Electrochimica Acta 93 (2013) 163– 172

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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nfluence of the synthesis method on the electrochemical properties of thei4Ti5O12 spinel in Li-half and Li-ion full-cells. A systematic comparison

bdelfattah Mahmouda,b, José Manuel Amarillaa,∗, Karima Lasrib, Ismael Saadouneb

Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (CSIC), c/Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, SpainLaboratoire de Chimie des Matériaux et de l’Environnement, Université Cadi Ayyad, Avenue A. Khattabi, BP 549 Marrakech, Morocco

r t i c l e i n f o

rticle history:eceived 29 November 2012eceived in revised form 10 January 2013ccepted 17 January 2013vailable online xxx

eywords:ithium batterynode materialithium titanium oxide

a b s t r a c t

To study the influence of the synthesis method on the electrochemical performances of the Li4Ti5O12

spinel (LTO) several samples have been prepared by three different synthesis procedures (sol–gel, self-combustion and solid-state). The use of the same Ti-source (TiCl4) and strictly the same experimentalconditions has allowed performing a meaningful comparison analysis. The structural characterization byX-ray diffraction shown that cubic spinels, with similar cell parameter (ac ∼ 8.35 A), were obtained for thethree methods. The purity of the samples prepared by sol–gel (LTO-SG) and combustion (LTO-CB) wasvery high, >96%. FE-SEM studies revealed that the particle size (ps) and the particle aggregation notablychanged with the synthesis method. The ps-values grow up from 301 nm for LTO-SG to 1580 nm for thesample synthesized by solid-state (LTO-SS). The comparison of the galvanostatic results permitted to

i4Ti5O12 spineli-ion full-cell

demonstrate that LTO-electrochemical performances (capacity, cycleability and rate capability) stronglydepend on the synthesis method. The LTO-SG sample exhibited the best electrochemical performances,with a discharge capacity of 146 mAh/g at 0.2 C, elevated cycleability and the highest rate capability.New formulation Li-ion full-cells with the synthesized LTO-samples as anode and LiCo2/3Ni1/6Mn1/6O2

as cathode have been assembled and studied. They have remarkable cycling performances with capacityretentions >90% after 50 cycles.

. Introduction

Li-ion batteries (LIBs) are attracting more and more interestince the development of technologies called “nomadic” becausehey can store a maximum energy for minimum weight and bulkith very good reliability [1,2]. In recent years, the interest in LIBsas awfully grown up because they have been the technology ofhoice for most of the automakers to power their hybrid (HEVs)nd electrical vehicles (EVs) [3]. Nowadays, the most used formu-ation in commercial LIBs is: LiC6//LiCoO2. It has a nominal voltagef 3.9 V and can reach specific energy of 160 Wh/kg. One of the keyssues in HEVs and EVs is the development of safety Li-ion batter-es. Unfortunately, LIBs using carbonaceous compounds as active

aterial for the negative electrode (simplifying, anode) have therawback of dendritic lithium growth on the anode surface. Thisangerous process is favored by the low Li+-insertion voltage ofarbon anodes during the battery charge. To circumvent this short-

oming, a number of researches are being carried out to developew electrode materials able to improve the safety of the LIBs. Onef the most following ways proposed the use of anode materials

∗ Corresponding author. Tel.: +34 913349074; fax: +34 913720623.E-mail address: amarilla@icmm.csic.es (J.M. Amarilla).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.01.083

© 2013 Elsevier Ltd. All rights reserved.

working at safe voltages, close to 1.5 V vs. Li+/Li, for instance, singleand mixed titanium oxides [4–7].

Li4Ti5O12 spinel (referred to as LTO) is one of the most promisinganode materials to replace the carbonaceous materials in large-size LIBs. It is characterized by an extremely flat working potentialat ∼1.5 V. At this moderate voltage, lithium dendrite growth iscompletely avoided and hence, the safety of the battery is greatlyenhanced. In fact, high charge currents can be applied in LTO-basedLi-ion cells without risk [8–10]. The voltage of LTO is also slightlyhigher than those at which most of the electrolytes are reduced.Then, degradation of the electrolyte on the LTO-electrode surfacedoes not take place during the charge/discharge processes. LTO alsoexhibits exceptional cycling performances as a result of the verysmall change of the lattice parameter on cycling [5,11]. Indeed, itsunit cell volume undergoes a variation lower than 1% in the wholeLi-composition range [12]. For this feature, LTO-spinel is consideredas zero strain insertion compound. In addition to its excellent elec-trochemical performances, LTO-based anodes are environmentalfriendliness and they have low cost. For all these outstanding per-formances, Li4Ti5O12 spinel is probably the safest anodic material

for large-size LIBs.

In order to optimize the electrochemical performances ofLTO-based anodes many synthesis procedures have been devel-oped [5,13–19]. In general, papers dealt with the preparation of

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TO-samples by one specific synthesis procedure for which, theptimization of the electrochemical performances was achievedy changing the synthesis parameters, e.g., starting reagents ornnealing temperature [17,20,21]. Only in few papers, the com-arison between the electrochemical properties of LTO-samplesrepared by different synthesis methods has been reported. Inhose papers, no longer that two synthesis procedures were com-ared, one of which was always the conventional solid-stateethod [18,20]In order to evaluate the influence of the synthesis proce-

ure used to prepare the Li4Ti5O12 spinel on its electrochemicalerformances, we have synthesized LTO-samples following threeriginal procedures based on: (i) solid-state, (ii) sol–gel and (iii)ucrose self-combustion methods. To ensure a meaningful com-arison, the same Ti-source was used for the three methods. Itas TiCl4 which is a reagent cheaper than others commonly used

o prepare LTO-samples, e.g., titanium isopropoxide [16]. More-ver, the “as prepared” precursors of each method were submittedo the same thermal treatment. The LTO-samples were struc-urally and morphologically characterized by X-ray diffraction, fieldmission scanning electron microscopy and BET analysis. For thehree methods, composite electrodes using the synthesized LTO-amples as active materials were processed in the same way.heir electrochemical performances were determined in Li half-ells using the same experimental conditions. Finally, to indicatehat a new-formulation Li-ion cells comprising one of the synthe-ized LTO-samples as anode and the layered LiCo2/3Ni1/6Mn1/6O2 asathode have been assembled and studied. Comparison of the elec-rochemical performances of these Li-ion full-cells has been carriedut.

. Experimental

.1. Synthesis of Li4Ti5O12

For the three synthesis methods, TiCl4 was used as titaniumource. In contact with air, TiCl4 forms an opaque cloud of titaniumxide and hydrogen chloride. To stabilize the Ti-source; 100 ml ofiCl4 was mixed with 900 ml of ethanol at 0 ◦C. To accurately deter-ine the concentration of titanium in the solution, 10 ml of it was

eated at 100 ◦C overnight and then at 600 ◦C for 6 h. The obtainedhite powder (TiO2) was recovered and weighted. This solution isereafter referred to as “Ti-source solution”. In order to compen-ate the partial volatility of lithium on heating, a small excess of 5%as used for the three methods.

.1.1. Solid-state based synthesisIn order to guarantee a meaningful comparison, the Ti-source

olution was used instead of others solid Ti-reactives usuallymployed in the solid-state synthesis of LTO [14,15]. The Ti-sourceolution was added drop by drop to an appropriate amount ofi2CO3 (Panreac, 98%). The slow addition prevents vigorous reac-ion with Li2CO3 powder. The resulting solution, which keeps theright yellow color, was dried 8 h at 100 ◦C. This “as prepared” pre-ursor was precalcined at 500 ◦C for 12 h. After regrinding it wasnnealed at 900 ◦C for 1 h in air. The heating up and cooling downates were fixed to 4 ◦C/min. The color of the final product washite. Hereafter this sample was referred to as LTO-SS.

.1.2. Sol–gel synthesisConcentrated nitric acid and Ti-source solution were firstly

ixed (1:1 nitric acid/Ti molar ratio). Then, appropriate amount of

iOH·H2O (Rieldel-de Haën, 98.5%) was dissolved in the obtainedolution by magnetic stirring. This solution keeps the brightellow color characteristic of the titanium source solution. Tri-thanolamine (TEA, Fluka >97%) was introduced as complexing

a Acta 93 (2013) 163– 172

agent. Based on previously reported data, the Ti/TEA molar ratiowas fixed at 0.8 [22,23]. After stirring for 2 h, a white gel wasobtained at pH = 4. It was dried at 100 ◦C for 8 h. This precursor wasthermal treated using the same conditions as for LTO-SS. Hereafterthis sample was referred to as LTO-SG.

2.1.3. Combustion synthesisLTO-samples were also synthesized by an original sucrose

assisted combustion method using LiNO3 and Ti-source solutionas reagents precursors and sucrose (C12H22O11) as fuel. Lithiumnitrate (Aldrich, 99%) and the Ti-source solution were mixed withthe sucrose (1:1 sucrose/Li molar ratio). After drying at 100 ◦C, thesolid precursor was auto-combusted by heating at 120 ◦C. This “as-prepared” precursor was submitted to the same heat treatmentused for solid-state and sol–gel synthesis, i.e. precalcination at500 ◦C for 12 h, regrinding and annealing at 900 ◦C for 1 h. Hereafterthis sample was referred to as LTO-CB.

For the Li-ion full-cells, LiCo2/3Ni1/6Mn1/6O2 oxide (referredto as LCNMO) was used as active material of the positive elec-trode (cathode). This layered material was synthesized followinga combustion method previously described [24]. Lithium, cobalt,manganese and nickel nitrates were used as oxidants and sucrose(C12H22O11) as fuel. The “as-prepared” precursor obtained after theself-combustion was further heated at 900 ◦C for 12 h.

2.2. Characterization

Powder X-ray diffraction, using a Bruker D8 diffractometerequipped with a Super Speed Vantec-1 detector and Cu-K� radi-ation, was employed to check the purity of the synthesized phasesand to evidence their structural features. The patterns were regis-tered at the 2�: 10–80◦, with a step size of 0.02◦ and a counting timeof 2 s/step. X-ray Rietveld refinement was performed by FULLPROF[25] from the powder diffraction data.

The study of the particle size and morphology of the synthe-sized LTO-samples was carried out by using a Novasem 230 fieldemission scanning electron microscope (FE-SEM). To determine theparticle size distribution, more than 10 micrographs were taken foreach LTO-sample in different regions of the holder. For each sample,about 100 particles were analyzed using the commercial softwareIMAGEJ [26].

The specific surface area of the synthesized LTO-sampleswas determined by the single point method using theBrunauer–Emmet–Teller equation (BET) from a MicromeriticsFlowsorb II 2300 apparatus. The gas for analysis was a mixture of10 vol% N2 in He as inert carrier. Before the analysis, 200 mg ofeach powder sample was outgassed at 150 ◦C for 30 min.

2.3. Electrochemical measurements

The electrochemical properties of LTO-SG, LTO-CB and LTO-SSsamples were evaluated in two-electrode Li//LTO half cells andin new-formulation LTO//LiCo2/3Ni1/6Mn1/6O12 full-cells with anArbin-BT4 battery system. The Li half-cells consisted of synthe-sized LTO-samples as cathode active material, a disc of metalliclithium as anode and reference electrode, and 1 M LiPF6 in EC:DMC(1:1 in volume) solution as electrolyte. Whatman BSF80 paper wasused as separator. The composite cathode was prepared by thedoctor-blade technique spreading on a cooper foil current collector,a slurry composed of 80% (weight percent) LTO active material, 10%acetylene black (AB) as conducting agent and 10% polyvinylidenefluoride (PVDF) as binder in N-methyl pyrrolidinone. The electrode

was dried at 90 ◦C for 2 h and at 80 ◦C overnight. Disc electrodesof 1.13 cm2 were cut from the so-obtained electrode tapes. Theresulting electrodes had an active material loading of ∼2.5 mg/cm2.For Li-ion full-cells, the composite cathode was formed by 85%

A. Mahmoud et al. / Electrochimica Acta 93 (2013) 163– 172 165

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JCPDS 26-1198). The Rietveld refinement of the diffraction data byconsidering multiphase systems leads the presence of 7.3% of TiO2in the case of LTO-SS, while 3.9% in the case of LTO-CB and only 1.7%of TiO2 impurity is present in LTO-SG. For LTO-SS, an additional

ig. 1. XRD patterns of Li4Ti5O12 samples prepared by sol–gel (LTO-SG), self-ombustion (LTO-CB) and solid-state (LTO-SS) based synthesis.

f LiCo2/3Ni1/6Mn1/6O2 active material, 10% AB and 5% PVDF. Theame processing followed for LTO-electrode was used to prepareCNMO-composite cathode. Al foil was used as current collector.

Li half and Li-ion full-cell components were assembled into aR2032 coin cell in a dry Ar-filled glove box. Good interfacial con-acts were obtained by pressing the sandwich between two metalliclates with a spring. All the electrochemical tests were carriedut at 25 ◦C (±1 ◦C). For the cycling galvanostatic measurements,i//LTO half-cells were tested at a constant charge/discharge cur-ent of 1 C rate (175 mA/g) in the voltage windows from 1.0 to.5 V vs. Li+/Li. To compare the rate capability of the synthesizedTO-samples, the coin cells were first cycled five times at 0.2 Charge/discharge rate then they were discharged under differenturrent intensities from 0.5 C (87.5 mA/g) to 10 C (1750 mA/g) with

fixed charge rate of 0.5 C. Five cycles was performed for every rate.fter the rate capability test, the cells were cycled two times at the

nitial low current of 0.2 C. In order to check the reproducibility ofhe electrochemical data, at least two measurements were made forach sample. Excellent consistency of the cell measurements wasbtained. LTO//LiCo2/3Ni1/6Mn1/6O2 full-cells were cycled at 0.5 C138 mA/g of LCNMO) charge/discharge rate in the voltage windowrom 1.0 to 3.2 V. Stoichiometric amounts of both active electrode

aterials, which were determined from the corresponding theo-etical capacities (175 mAh/g for LTO and 276 mAh/g for LCNMO)ere used.

. Results and discussion

.1. Structural and morphological characterization

X-ray diffraction patterns of LTO-SS, LTO-SG and LTO-CBbtained after heating at 900 ◦C for 1 h are shown in Fig. 1. Forhe three samples, the main X-ray diffraction peaks were assignedo the Li4Ti5O12 spinel (JCPDS 26-1198). K˛1/K˛2 splitting waslearly observed for almost all these diffraction lines, indicatinghe high degree of crystallinity of the LTO-samples synthesized. Ast is shown in Fig. 1, the X-ray pattern of LTO-SS prepared by aased solid-state synthesis shows additional peaks revealing thexistence of rutile TiO2 (JCPDS 00-001-1292) and Li2TiO3 (JCPDS8-0416) impurities. The sol–gel and the combustion synthesis

ead to samples with notably higher purity. Indeed, no Li2TiO3 wasetected in the X-ray diffraction patterns of LTO-SG and LTO-CBnd the amount of TiO2 was clearly reduced. The crystal structuref the Li4Ti5O12 spinel (space group Fd-3m) was first determined

y Deschanvres et al. in 1971 [27]. It could be described as a cubiclose packing of oxygen ions where the 8a tetrahedral sites areccupied by Li+ and the 16d octahedral sites were randomly occu-ied by titanium and lithium (5/3 Ti4+ + 1/3 Li+). The same atomic

Fig. 2. Rietveld refinement results of X-ray diffraction pattern of the LTO-SG sample.

positions were selected as starting model for our structural refine-ment. At the same time, we introduced in this structural model,the atomic positions of rutile TiO2 in order to quantify the percent-age of this impurity in the synthesized samples. As an example,the refinement results for LTO-SG are shown in Fig. 2. The smalldifference between calculated/experimental patterns and the lowvalues of the agreement parameters (see Rwp and RB in the insetof Fig. 2) demonstrated the successfully refinement of the experi-mental data. Similar results were obtained for LTO-CB and LTO-SS.The cubic cell parameter (ac) calculated by Rietveld refinement was8.3588(2) (Å) for LTO-SG, 8.3592(9) (Å) for LTO-SS and 8.3576(1)(Å) for LTO-CB. It is shown that ac is similar for the three samplesand very close to those reported for Li4Ti5O12 spinel (ac = 8.3554 A,

Fig. 3. (a) XRD pattern and (b) FE-SEM micrograph of the layeredLiCo2/3Ni1/6Mn1/6O2 (LCNMO) oxide synthesized by sucrose assisted combustionsynthesis.

166 A. Mahmoud et al. / Electrochimica Acta 93 (2013) 163– 172

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ig. 4. FE-SEM micrographs of (a) LTO-SG, (c) LTO-CB and (e) LTO-SS. The correspo

4% of Li2TiO3 impurity, as determined by Rietveld refinement,as detected. These results clearly show that (i) the purity of the

TO-samples annealed at 900 ◦C depends on the synthesis proce-ure and (ii) the LTO-SG and LTO-CB samples have very high purity98.3% and 96.4%, respectively). It should be noticed that titaniumxide impurities are usually present in most of the reported LTO-amples being really difficult to prepare single-phase Li4Ti5O12pinel [16,18,28]. On the other hand, Fig. 3a shows the XRD patternf the layered LiCo2/3Ni1/6Mn1/6O2 used as cathode active materialor Li-ion full-cells. It was synthesized by a self-combustion proce-ure similar to those of LTO-CB. All diffraction peaks were indexedccording to the �-NaFeO2 type structure, R-3m space group. Itonfirms the synthesis of a single-phase. This result, together withur previous studies on LiMn2O4-based cathodes confirms that theucrose assisted combustion method is a suitable synthesis pro-edure to prepare electrode materials for LIBs [29,30]. In Fig. 3b,E-SEM micrograph of the LCNMO oxide is presented. The pic-ure shows that the LCNMO is characterized by a homogeneous

orphology with an average particle size of 300 nm.Beside the crystallographic structure, size and morphology

f the electrode particles have a strong influence on the Li+

e-/insertion reactions [29,31,32]. For these reason, we have stud-

ed the morphological characteristics of the LTO-spinels. FE-SEM

icrographs of LTO-SG, LTO-CB and LTO-SS are shown in Fig. 4a, c,nd e, respectively. The three samples are constituted by facetedarticles with well-defined edges. This morphological feature,

particle size histograms (b), (d) and (f) are presented in the right part of the figure.

together with the X-ray data (Fig. 1) confirms the high crys-tallinity of the studied LTO-samples, independently of the synthesismethod. The micrograph of LTO-SG shown that it was constitutedby sub-micrometric particles slightly aggregated (Fig. 4a). How-ever, the primary particles are clearly agglomerated for LTO-CB(Fig. 4c) and particles are even sintered in the case of LTO-SS (Fig. 4e). The comparison of the three micrographs clearlyevidenced that the size of the primary particles was very affectedby the synthesis procedure. To determine the average value of theprimary particle size (ps) as well as the particle size distribution,about 100 particles from several micrographs were measured forevery sample. Due to the high sintering of the LTO-SS (Fig. 4e)the ps-measurement for this sample was less accurate. The his-tograms of LTO-SG, LTO-CB and LTO-SS are shown in Fig. 4b, d, andf, respectively. They were fitted to Gaussian profiles and ps-valueswere determined from the maximum of the curve. The full widthat half maximum (FWHM) was calculated and the correspondingvalues are also summarized in Table 1. Among the synthesizedsamples, LTO-SG has the lowest particle size (301 nm) and thehighest homogeneity. The ps-value is notably smaller than thoseof LTO-CB (817 nm) and five times lower than the particle size ofLTO-SS (1580 nm). The specific surface area was determined by

the BET method. The values rose from 0.79 m2/g for LTO-SS to2.48 m2/g for LTO-SG (Table 1). The highest BET surface area ofLTO-SG was justified by its smallest particle size and lowest aggre-gation (Fig. 4). As conclusion of the morphological studies it can

A. Mahmoud et al. / Electrochimica Acta 93 (2013) 163– 172 167

Table 1Cubic cell parameter (ac), average particle size (ps), FWHM from Gaussian fit andspecific surface area (BET) for Li4Ti5O12 samples prepared by sol–gel (LTO-SG), com-bustion (LTO-CB) and solid-state (LTO-SS) based synthesis.

ac (Å) ps (nm) FWHM (nm) BET (m2/g)

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e claim that the synthesis method used to prepare the Li4Ti5O12pinel has a great influence on the particle size, the particle aggre-ation and the specific surface area of the synthesized sample.mong the studied LTO-samples, those synthesized by sol–gelhows the smallest particle size, the lowest particle aggregationnd the highest BET-surface area. All these features are very suit-ble for Li-electrochemical insertion reactions, especially at highurrents.

.2. Li4Ti5O12 electrochemical performances in Li half-cells

In order to determine the influence of the synthesis methodn the Li4Ti5O12 electrochemical properties, Li half-cells were gal-anostatically tested at 25 ◦C. Fig. 5 shows the charge/dischargeurves of LTO-SG, LTO-CB and LTO-SS registered at 0.2 C (87.5 mA/g)n the potential window 1–2.5 V. For the three samples, the curveshown the typical profile reported for the LTO-spinel [5,11,17,18].t consists to an extremely flat plateau between 1.5 and 1.6 VTable 2) corresponding to the reversible electrochemical reaction:i4Ti5O12 + 3(Li+ + e−) ↔ Li7Ti5O12.

Ohzuku et al. [12] reported that LTO can insert Li+-ionsn discharge resulting in a structural transition from thei[8a](Li1/3Ti5/3)[16d]O4 spinel to the Li2[16c](Li1/3Ti5/3)[16d]O4 rock-alt phase without a noticeable change in the lattice parameter. Dueo this important feature, LTO is considered as a zero-strain elec-rode material. The discharge capacity (Qdch) of LTO-SG, LTO-CB andTO-SS determined from the corresponding galvanostatic curvet 0.2 C was 146, 152 and 87 mAh/g, respectively. The observedifferences demonstrated that Qdch remarkably depended on theynthesis method. The lowest capacity drained for LTO-SS in com-arison to LTO-SG and LTO-CB could be explain because it has theighest particle size (Table 1 and Fig. 4.) and the largest amount of

mpurities (Fig. 1). Both features decrease the Li+-insertion reactioninetics, i.e. the lithium diffusion between Li4Ti5O12 and Li7Ti5O12

hases. After a deeply revision of data reported in the literature itas realize that the discharge capacity of most of the studied LTO-

amples ranged from 120 to 170 mAh/g when they were tested atbout 0.2 C constant rate [12,15,33,34]. Synthesized LTO-SG and

ig. 5. First discharge/charge curves for Li//Li4Ti5O12 half-cells registered at 0.2 C35 mA/g). Ta

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168 A. Mahmoud et al. / Electrochimica Acta 93 (2013) 163– 172

Fig. 6. (a) Selected discharge/charge curves registered during the cycling of Li//LTO-SG half-cell at 26(±1) ◦C. (b) Evolution of discharge capacity vs. cycle number forLTO-SG, LTO-CB, LTO-SS at 1 C (175 mA/g) and Coulombic efficiency in percent (openrtv

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Fig. 7. Evolution of the discharge capacity drained for LTO-SG, LTO-CB and LTO-

ed circle) for LTO-SG. The voltage window explored was 1.0–2.5 V. (For interpreta-ion of the references to color in this figure legend, the reader is referred to the webersion of this article.)

TO-CB samples, with elevates capacities (146 and 152 mAh/g,espectively) are fully included in this Qdch-range.

The cycling performances of the synthesized LTO-samples wereested at the elevate current of 1 C discharge/charge rate in theoltage range from 1 to 2.5 V and at 25 ◦C. As an example, pro-les for selected cycles of the Li//LTO-SG cell are shown in Fig. 6a.ll curves have the same shape which is characterized by thextremely flat plateau at ∼1.55 V. After the first discharge, for whichdch = 1.511 V, the voltage of the plateau remained almost constantt 1.60 V in charge and 1.54 V in discharge. This result indicatedhat the cell impedance did not noticeably change on cycling andence detrimental cell polarization effects were not detected dur-

ng the galvanostatic cycling. The curves in Fig. 6a also shown thatdch of LTO-SG significantly decreased during the first 15 cycles

rom 133 (1st cycle) to 102 mAh/g (15th cycle). Nevertheless, theapacity remained practically unchanged after the 15th cycle. Inact, Qdch only decreased from 102 (15th cycle) to 97 mAh/g (100thycle). In Fig. 6b, the evolution of Qdch vs. cycle number for LTO-SB,TO-CB and LTO-SS at 1 C is compared. The first result to comments that the shape of the evolution is similar for the three samples.he first discharge capacity was of 133, 122 and 64 mAh/g for LTO-G, LTO-CB and LTO-SS, respectively. Unfortunately, a significantapacity loss during the first 15 cycles taken place. The fall was 21%,5% and 39% for LTO-SG, LTO-CB and LTO-SS, respectively. Capac-

ty loss during the first cycles was previously reported for manyTO-samples prepared by different synthesis procedures [35–37].ll these results indicate that many LTO-samples need severalischarge/charge cycles before they reached a stable cycling

SS on cycling at increasing rates from 0.2 C to 10 C. For discharge rates ≥0.5 C, theconstant charge current was 0.5 C. Only for 0.2 C, the charge current was 0.2 C. Thevoltage window explored was 1.0–2.5 V.

regime. We also analyzed the evolution of the Coulombic efficiency(CEff) with the cycle number for the three LTO-samples synthe-sized. As an example, CEff vs. cycle number for LTO-SG is shownin Fig. 6b. Similar evolutions were obtained for LTO-CB and LTO-SSsamples. As for the discharge capacity, CEff significantly decreasedduring the first 20 cycles. Afterwards, it remained almost constanton cycling with high values close to 100%. The decrease of CEff indi-cated that the capacity drained during the first 15 discharges wasnot fully recovered during the charge process. This phenomenonseems to be the responsible of the capacity fade observed for thesynthesized LTO-samples when they are cycled at 1 C ch/dch ratein Li half-cell. After the first 15–20 cycles, all LTO-samples shownexcellent cycling performances (Fig. 6b). Their capacity retentionfrom 15th to 100th cycles (QRt-15,100) was 95.2%, 97.8% and 88.2%for LTO-SG, LTO-CB and LTO-SS, respectively. We have also cal-culated the normalized capacity loss by cycle (NQL) in percentaccording to the equation: NQL = (slope × 100 / Q15). The slope wasdetermined by linear fit of the Qdch-values from the 15th to 100thcycles, and Q15 was the capacity for the 15th cycle. For the threesamples, the NQL-values were very small, 0.23% for LTO-SS and only0.14% and 0.09% for LTO-SG and LTO-CB, respectively. The com-parison of cycling data shown that LTO-SG had the best cyclingperformance because it shown the highest reversible capacity (Qrev

∼100 mAh/g at 1 C).Rate capability is other important electrochemical parameter

because it informs about the specific power that electrodes areable to drain. For this reason, we decided to study the influenceof synthesis method on the rate capability of the Li4Ti5O12 spinel.Fig. 7 gives the evolution of Qdch on cycling at increasing rates from0.2 C to 10 C. For each discharge rate, five cycles were performed inthe voltage windows of 1.0–2.5 V. As can easily deduced from Fig. 7,there are significant differences between the three LTO-samplesconcerning the evolution of Qdch with the applied rate. Theseresults allowed us demonstrating that the rate capability of theLi4Ti5O12 spinel strongly depends on the synthesis procedure usedfor its preparation. At moderate rates (≤0.5 C) the Qdch-evolutionfor LTO-SG and LTO-CB is almost similar (Qdch ∼145 mAh/g at 0.2 Cand ∼120 mAh/g at 0.5 C). Nevertheless, LTO-SS showed the small-est capacities (87 mAh/g at 0.2 C and 45 mAh/g at 0.5 C) because ithad the highest particle size, the lowest surface area and the largest

amount of impurities (Table 1). For rates >1 C, it should to remarkthat the capacities drained for LTO-SG (107, 75, 50 mAh/g at 1 C, 4 Cand 10 C, respectively) were always higher than those of LTO-CB(89, 42, 27 mAh/g at 1 C, 4 C and 10 C, respectively) and almost

A. Mahmoud et al. / Electrochimica Acta 93 (2013) 163– 172 169

Fi

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ig. 8. Discharge curves of LTO-SG, LTO-CB and LTO-SS spinels registered at increas-ng discharges rates.

hree times higher than those of LTO-SS (33, 18, 13 mAh/g at 1 C, C and 10 C, respectively). These results showed that the LTO-SGample synthesized by an original sol–gel procedure exhibited theest rate capability.

For every tested rate, it should be indicated that Qdch wasetained during the five performed cycles (Fig. 7). This resultuggested that the synthesized LTO-samples exhibited highycleability, even for the fastest rate of 10 C (1750 mA/g). It shoulde important to indicate that the severe capacity fade observeduring the first cycles at 1 C (Fig. 6b) is much lower when cyclingas performed at 0.2 C ch/dch rates. Thus, the capacity retention

fter the first five cycles notably increased from 84.9%, 82.7% and1.2% at 1 C to 95.1%, 93.4% and 80.2% at 0.2 C for LT-SG, LTO-B and LTO-SS, respectively. A similar behavior was reported byin et al. [38] for a porous LTO-sample synthesized by a modi-ed solid state method. Altogether, these results seem to indicatehat the cycling performances of the Li4Ti5O12 spinel depend onhe intensity current used in the electrochemical tests. Further-

ore, it should be also indicated that the capacity was recoveredo 140, 141 and 64 mAh/g for LTO-SG, LTO-CB and LTO-SS, respec-ively, when the charge/discharge rate was lowered at 0.2 C (Fig. 7).his result demonstrated that the spinel structure of the Li4Ti5O12as not destroyed during the rate capability studies. Consequently,

he Qdch-drop on increasing the discharge rate was due to kineticimitations of Li+-insertion processes into the Li4Ti5O12 spinel.

To deepen on the rate capability analysis, one discharge pro-le of LTO-SG, LTO-CB and LTO-SS recorded for each tested rate

s plotted in Fig. 8. The curves showed as the discharge capacityf the three samples decreases on increasing the rate being LTO-G the sample with the highest capacity retention. Refer to theoltage of the discharge plateau (Edch), it also decreased with theaise of the intensity current. In fact, the voltage drop from 2 C to0 C (�Edch referred) clearly depended on the synthesis method.or LTO-SS, Edch decreased from 1.552 V (0.2 C) to 1.492 V (2 C) and.288 V (10 C), �Edch = 0.264 V. For LTO-CB, the voltage diminishedrom 1.552 V at 0.2 C to 1.370 V at 10 C (�Edch = 0.182 V). The low-st voltage drop was for LTO-SG for which Edch only decreased from.555 V (0.2 C) to 1.520 V (2 C) and 1.405 V (10 C), �Edch = 0.150 V.he analysis of the shape of discharge curves showed that for LTO-G, the flat plateau was preserved for all rates. However, for rates

4 C, the profile of LTO-CB was more and more sloped. This effectas much higher for LTO-SS, for which deterioration started from

C. Voltage drop and sloping of the discharge curves on raisinghe current intensity is mainly due to ohmic and electrochemical

Fig. 9. Comparison of the evolution of normalized capacity as a function of thedischarge rate (Peukert plot) of LTO-SG, LTO-CB and LTO-SS.

polarizations. Thus, the observed differences between the dis-charge curves of the three LTO-spinels indicated that detrimentalpolarization effects also depended on the synthesis method. Exper-imental results clearly showed that these effects were markedlysmaller for LTO-SG synthesized by the original sol–gel proceduredeveloped.

With the aim to complete the comparative study on rate capabil-ity, Peukert plot (normalized discharge capacity (Qnor) vs. current)for LTO-SG, LTO-CB and LTO-SS is drawn in Fig. 9. Normalized dis-charge capacity was determined as Qnor = QNC × 100/Q0.2 C, whereQNC and Q0.2 C are the discharge capacities at NC and 0.2 C rates,respectively. Data in Fig. 9 show as the shape of the Qnor-evolutionwas similar for the three LTO-samples. It is characterized by tworegions between 0.2–1 C and 4–10 C in which Qnor diminishesalmost linearly on increasing the rate. In the first region (0.2–1 C),Qnor quickly fell down following slopes of −32, −48 and −73 forLTO-SG, LTO-CB and LTO-SS, respectively. The remarkable differ-ences in the slope values indicated that the variation of Qnor withcurrent, i.e., the rate capability, clearly depended on the synthesismethods. The lowest slope (in absolute value) of LTO-SG confirmedthat this sample exhibits the best rate capability. In fact, it wasable to retain 73.2% of Qdch at the elevate current of 1 C. This valueis almost three times higher than those of LTO-SS for which theretention at 1 C was only 37.4%. A gradual transition between thetwo linear regions takes place from 1 C to 4 C rates. The second linealregion, developed at very fast rates from 4 C to 10 C, is character-ized by a smooth decrease of Qnor. The corresponding slopes were−2.8, −1.9 and −0.9 for LTO-SG, LTO-CB and LTO-SS, respectively.

We have reviewed previous papers focused on the rate capabil-ity studies of the LTO-spinel and we have realized that most of thepapers reported the influence of different synthesis parameters onthe rate capability of LTO-samples which was synthesized by onlyone specific synthesis procedure. To the best of our knowledge thecomparison between samples prepared by three different synthe-sis procedures, as it is given in the present manuscript, has notbeen reported until now. Jin et al. [20] synthesized several LTO-samples by calcination of TiO2 nanoparticles and LiOH at severaltemperatures in air. They found a remarkable dependency of therate capability with the thermal treatment. Indeed, LTO-samplesannealed at 700 and 800 ◦C shown similar capacity (∼150 mAh/g)at 1 C; but, the capacity retention at 10 C was 77.4% for the sample

at 700 ◦C and only 37.9% for those at 800 ◦C. Le et al. [21] studiedthe influence of the particle size of TiO2 used as Ti-source. Theyreported that the LTO-sample synthesized using micro-sized TiO2

1 chimica Acta 93 (2013) 163– 172

dttwstaehLsZpft(Lse7orLibf(rasht

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Fig. 10. (a) First charge/discharge profiles of Li//LiCo2/3Ni1/6Mn1/6O2 and Li//LTO-SG

70 A. Mahmoud et al. / Electro

elivered 148 mAh/g at 0.1 C and 64 mAh/g at 4 C (capacity reten-ion = 43.2%). However, when nano-sized TiO2 was used as reagent,he capacity at 0.1 C was 162 mAh/g and the capacity retention at 4 Cas 58%. All these results point out that in addition to the synthe-

is method used to prepare the Li4Ti5O12, as it is demonstrated inhe present manuscript, the synthesis conditions (heating temper-ture, Ti-source, etc.) also play an important role to optimize thelectrochemical performances of LTO-based anodes, especially atigh power. It should be to indicate that the rate capability of ourTO-SG is close to those reported for other samples prepared byynthesis procedures also based on sol–gel processes. For instance,hu et al. [16] synthesized LTO-samples using titanium (IV) iso-ropoxide and lithium acetylacetonate as sol–gel reagents. Theyound that the capacity at low current (0.2 C) was 157 mAh/g andhe sample retained 38% of this capacity at the high current of 10 C34% for our LTO-SG). Khomane et al. [17] prepared nano-sizedTO-samples using the CTAB-assisted sol–gel technique. The bestample, which was synthesized at 800 ◦C in presence of CTAB, deliv-red 175 mAh/g during the first discharge at 0.2 C and it retained5% of the initial capacity at 2 C (64% for our LTO-SG). As summaryf all results achieved for the comparative study of Qdch vs. cur-ent evolution, it can be conclude that the rate capability of thei4Ti5O12 spinel notably depends on the synthesis method used tots preparation. Among the synthesized samples, LTO-SG has theest rate capability; in fact (i) it delivered the highest Qdch-valuesor each tested rate; (ii) it had the lowest polarization effects; andiii) it showed the largest capacity retention on increasing the cur-ent intensity. These better rate performances compared to LTO-CBnd LTO-SS are explained because it has: (i) the smallest particleize; (ii) the lowest aggregation of the primary particles; (iii) theighest homogeneity; (iv) the largest specific surface area; and (v)he highest purity.

.3. Li4Ti5O12//LiCo2/3Ni1/6Mn1/6O2 Li-ion full-cell

In the last few years, growing interest to test new electrodeaterials in Li-ion full-cells is being observed [8–10,14,34,39,40].

special interest has full-cells using Li4Ti5O12 as anode material dueo their greater safety and long-term cycleability [9,10,14,34,39,40].efer to the positive electrode, it must be said that, although

ayered LiCoO2-based oxides are the most used cathode mate-ials in commercial LIBs, only few papers have been publishedn LTO//LCO-based full-cells [8,41]. In this paper, we present

new-formulation of Li-ion full-cell assembled with one ofhe study LTO-samples as anode material and the layerediCo2/3Ni1/6Mn1/6O2 as cathode material. To indicate that layerediCo2−x−yNixMnyO2 oxides are being extensively studied becausehey have higher capacity, higher working voltage, improved ther-

al stability and lower cost than LiCoO2 [41–43].Previous to test the full-cells, we studied the electrochemical

erformances of the layered LiCo2/3Ni1/6Mn1/6O2 in Li half-cells.ig. 10a shows the first charge/discharge profiles registered at.5 C between 2.7 and 4.5 V. The shape of the curves is the usu-lly reported for LiCo1–2yNiyMnyO2 oxides [8,41–43]. It showed aontinuous variation of the voltage during the charge/dischargerocesses. The average working voltage was ∼3.88 V being the dis-harge capacity of 175 mAh/g. It should be noted that the capacityf LCNMO is notably higher than the practical capacity reportedor LiCoO2 (140 mAh/g) [44]. The cycling performances of LCNMOt 0.5 C are reported in Fig. 10b. The capacity slowly decreasedrom 177 to 156 mAh/g after 50 cycles. Thus, the capacity reten-ion was 87.97% and the normalized capacity loss by cycle of 0.24%.

ll these data illustrate the high cycleability of the LCNMO sampleynthesized by the sucrose assisted combustion method.

Refer to Li-ion full-cells, Fig. 11a shows a selection of dis-harge curves recorded during cycling of the LTO-SG//LCNMO cell

half-cells registered at 0.5 C. (b) Evolution of discharge capacity vs. cycle number forLCNMO at 0.5 C charge/discharge rate. The voltage window explored was 1.0–2.5 V.

at 0.5 C as an example. The shape of the curves was similar to thosereported for layered LCNMO in Li half-cells (Fig. 10a). This resultcan be explained assuming that the LTO-electrode is working inthe voltage range of its flat plateau. The first discharge capacity ofthe LTO-SG//LCNMO cell was 164 mAh/g and its average workingpotential of ∼2.2 V (Q-values for full-cells are referred to mass ofLCNMO in the composite cathode). This voltage value is close tothe difference between the average discharge potential of LCNMO(∼3.88 V) and the constant voltage of the LTO-SG on charge (1.58 V).The discharge curves in Fig. 11a also indicated that Qdch lightlydecreases on cycling. The comparison of the cycling performancesof LTO//LCNMO full-cells is displayed in Fig. 11b. Galvanostatic dataclearly showed that the three cells exhibited excellent cycleabil-ity. In all cases, the value of the capacity retention after 50 cycles(QRt-50) was higher than 90% (Table 2) being the NQL-values of0.103, 0.104 and 0.151 for LTO-SG, LTO-CB and LTO-SS, respectively.It should be stressed that cycleability of the Li-ion full-cells wasnotably better than those of Li half-cells. In fact, the detrimentalcapacity loss observed during the first 15 cycles for Li half-cells(from 61 to 85%, Table 2) is notably smaller for the Li-ion full-cells(<10%). This is a very interesting result because it shows that thecapacity loss detected in Li half-cells was not due to irreversibilityof the Li+-insertion/extraction in/from the synthesized LTO-spinels.

Finally, to indicate that Li-ion full-cells using LTO-SG and LTO-CB asactive anode materials with Qdch of ∼160 mAh/g, specific energy of∼350 Wh/kg and normalized capacity loss by cycle of only 0.10% can

A. Mahmoud et al. / Electrochimic

Fig. 11. (a) Selected charge/discharge curves of the LTO-SG//LiCo2/3Ni1/6Mn1/6O2

full-cell at 0.5 C. (b) Comparison of discharge capacity vs. cycle number forLti

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TO//LCNMO full-cells. The cycling was performed at 0.5 C charge/discharge rate inhe voltage window from 1 to 3.2 V. Capacity values are referred to mass of LCNMOn the composite cathode.

e considered a new promising candidates to be used as large-sizei-ion batteries with improved safety.

. Conclusions

The influence of the synthesis method on the electrochemi-al properties of Li4Ti5O12 spinel (LTO) has been evaluated by theystematic comparison of samples prepare by original proceduresased on sol–gel, combustion and solid-state synthesis. A meaning-ul comparison have been performed in which: (i) we have used theame Ti-source (TiCl4) for the three methods, (ii) the correspond-ng precursors were submitted to the same thermal treatment, (iii)TO-composite electrodes were processing in the same way, andiv) the same electrochemical test were performed for all cells. Thetructural characterization by X-ray diffraction allowed confirminghe synthesis of cubic LTO spinels with similar cell parameterac ∼ 8.35 A) as the major phase. Differences in the impurity amountith the synthesis method were detected. The very small amount of

iO2 impurity determined by Rietveld refinement, 1.7% for LTO-SGnd 3.9% for LTO-CB, proved the high purity of these two samplesynthesized. The morphological characterization showed that the

ynthesis method also has a high influence on the particle size,article aggregation and specific surface area of the LTO-samples.

The electrochemical performances were determined in Li half-ells and Li-ion full-cells. The comparative study has allowed us

[

a Acta 93 (2013) 163– 172 171

to certainly demonstrate the strong dependency of the capacity,cycleability and rate capability on the synthesis method used toprepare the Li4Ti5O12 spinel. The observed differences have beenexplained on the basis of morphological features and the presenceof impurities in the sample. Among the synthesized samples, LTO-SG is the one which has the best electrochemical performances. Itdelivers 146 mAh/g at 0.2 C and is able to retain 34.2% of the capacityat the high rate of 10 C (1750 mA/g). This last value was more thantwo times higher than those of LTO-SS obtained by a based solid-state synthesis. Besides, LTO-SG has an elevate cycleability with areversible capacity of ∼100 mAh/g at 1 C. These remarkable electro-chemical performances have been explained because LTO-SG has:(i) the smallest particle size (300 nm), (ii) the lowest aggregation ofthe primary particles, (iii) the highest homogeneity, (iv) the largestspecific surface area (2.48 m2/g), and (v) the highest purity (98.3%).

Finally, a new-formulation Li-ion cell using the synthe-sized LTO-samples as active anode material and the layeredLiCo2/3Ni1/6Mn1/6O2 as active cathode material has been devel-oped. Electrochemical tests have shown the excellent cycleabilityof all full-cells, with capacity retentions >90% after 50 cycles. Thiscycling behavior was notably better than those of Li half-cells (<75%after 50 cycles). Full-cells assembled with LTO-SG or LTO-CB asactive anode material, showing elevate capacity (∼160 mAh/g) andhigh cycleability, are new promising candidates to be used as safetyLi-ion batteries.

Acknowledgements

Financial support through the projects MAT2011-22969,MAT2011-24198 (MINECO), MATERYENER P2009/PPQ-1626(CAM) and 2009MA0007 (CSIC/CNRST) and CNRST (RechercheSectorielle RS03/2011) are gratefully recognized. A. Mahmoudthanks the AECID for the MAEC-AECI fellowship.

References

[1] J.M. Tarascon, M. Armand, Issues and challenges facingrechargeable lithiumbatteries, Nature 414 (2001) 359.

[2] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithiumbatteries, Angewandte Chemie International Edition 47 (2008) 2930.

[3] B.G. Pollet, I. Staffell, J.L. Shang, Current status of hybrid, battery and fuel cellelectric vehicles: From electrochemistry to market prospects, ElectrochimicaActa 84 (2012) 235.

[4] S. Yang, Y. Cai, Y. Cheng, C.V. Varanasi, J. Liu, Monolithic co-aerogels ofcarbon/titanium dioxide as three dimensional nanostructured electrodes forenergy storage, Journal of Power Sources 218 (2012) 140.

[5] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Ti-based compounds as anode materials for Li-ionbatteries, Energy & Environmental Sciences 5 (2012) 6652.

[6] P. Tartaj, J.M. Amarilla, Multifunctional Response of Anatase Nanostructuresbased on 25 nm Mesocrystal-like Porous Assemblies, Advanced Materials 23(2011) 4904.

[7] G.F. Ortiz, I. Hanzu, P. Lavela, P. Knauth, T. Djenizian, J.L. Tirado, Novel fabrica-tion technologies of 1D TiO2 nanotubes vertical tin and iron-based nanowiresfor Li-ion microbatteries, International Journal of Nanotechnology (IJNT) 9(2012) 3.

[8] M.S. Sang, Y.M. Choi, S.A. Song, Y.B. Sohn, J.S. Shin, S.G. Doo, Charge/dischargeproperty and cycle life of Li4Ti5O12/LiCoO2–LiNi1−x−yCoxAlyO2 cell for electric-ity storage application: effect of cell design parameters, NSTI-Nanotech 2010 3(2010) 785.

[9] J. Morales, R. Trócoli, S. Franger, J. Santos-Pena, Cycling-induced stress inlithium ion negative electrodes: LiAl/LiFePO4and Li4Ti5O12/LiFePO4 cells, Elec-trochimica Acta 55 (2010) 3075.

10] H.G. Jung, M.W. Jang, J. Hassoun, Y.K. Sun, B. Scrosati, A high-rate long-lifeLi4Ti5O12/Li[Ni0.45Co0.1Mn1.45]O4 lithium-ion battery, Nature Communications2 (2011) 516.

11] Z. Wang, G. Xie, L. Gao, Electrochemical Characterization of Li4Ti5O12/CAnodeMaterial Prepared by Starch-Sol-Assisted Rheological PhaseMethod for Li-Ion Battery, Journal of Nanomaterials 2012 (2012) 7,http://dx.doi.org/10.1155/2012/876197, Article ID 876197.

12] T. Ohzuku, A. Ueda, N. Yamamoto, Zero-Strain Insertion Material of

Li[Li1/3Ti5/3]O4 for Rechargeable Lithium Cells, Journal of the ElectrochemicalSociety 142 (1995) 1431.

13] T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, Recent development andapplication of Li4Ti5O12 as anode material of lithium ion battery, Journal ofPhysics and Chemistry of Solids 71 (2010) 1236.

1 chimic

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

72 A. Mahmoud et al. / Electro

14] J.M. Amarilla, Batería de iones litio: Li4/3Ti5/3O4/LiClO4(PC)/LiMn2O4. Influen-cia de los reactivos utilizados en la síntesis de la espinela LiMn2O4, Boletin DeLa Sociedad Espanola De Ceramica y Vidrio 34 (5–6) (1995) 463.

15] H.G. Jung, S.T. Myung, C.S. Yoon, S.B. Son, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun,Microscalesphericalcarbon-coated Li4Ti5O12 as ultra highpoweranode materialforlithiumbatteries, Energy & Environmental Sciences 4 (2011) 463.

16] N. Zhu, W. Liu, M. Xue, Z. Xie, D. Zhao, M. Zhang, J. Chen, T. Cao, Grapheneas a conductive additive to enhance the high-rate capabilities ofelectrospunLi4Ti5O12 for lithium-ion batteries, Electrochimica Acta 55 (2010) 5813.

17] R.B. Khomane, A.S. Prakash, K. Ramesha, M. Sathiya, CTAB-assisted sol–gel syn-thesis of Li4Ti5O12 and its performance as anodematerial for Li-ion batteries,Materials Research Bulletin 46 (2011) 1139.

18] T. Yuan, R. Cai, P. Gu, Z. Shao, Synthesis of lithium insertion material Li4Ti5O12

from rutileTiO2 via surface activation, Journal of Power Sources 195 (2010)2883.

19] T. Yuan, R. Cai, K. Wang, R. Ran, S. Liu, Z. Shao, Combustion synthesis of high-performance Li4Ti5O12 for secondary Li-ion battery, Ceramics International 35(2009) 1757.

20] Y.H. Jin, K.M. Min, H.W. Shim, S.D. Seo, I.S. Hwang, K.S. Park, D.W. Kim, Facilesynthesis of nano-Li4Ti5O12 for high-rate Li-ion battery anodes, NanoscaleResearch Letters 7 (2012) 1.

21] W. Le, S.R.K. Su-rong, L. Shi-gang, Z. Xiang-jun, Effect of particle size andagglomeration of TiO2 on synthesis and electrochemical properties of Li4Ti5O12,Transactions of Nonferrous Metals Society of China 17 (2007) S117.

22] A.B. Panda, A. Pathak, P. Pramanik, Low temperature preparation of nanocrys-talline solid solution of strontium–barium–niobate by chemical process,Materials Letters 52 (2002) 180.

23] Y. Hao, Q. Lait, Z. Xu, X. Liu, X. Ji, Synthesis by TEA sol–gel method and electro-chemical properties of Li4Ti5O12 anode material for lithium-ion battery, SolidState Ionics 176 (2005) 1201.

24] A. Mahmoud, I. Saadoune, J.M. Amarilla, R. Hakkou, Onthe LiCo2/3Ni1/6Mn1/6O2

positive electrode material, Electrochimica Acta 56 (2011) 4081.25] J.R. Carvajal, Laboratoire Léon Brillouin Fullprof: A Rietveld Refinement and

Pattern Matching Analysis Program, CEA-CNRS, Grenoble, France, 2010.26] M.D. Abramoff, P.J. Magelhaes, S.J. Ram, Image processing with Image J, Bio-

photonics International 11 (2004) 36.27] A. Deschanvres, B. Raveau, Z. Sekkal, Mise en évidence et étude cristallo-

graphique d’une nouvelle solution solde de type spinelle Lil + xTi2−x O4 O ≤x ≤0,Materials Research Bulletin 6 (1971) 699.

28] C. Pecharroman, J.M. Amarilla, Thermal evolution of infrared vibrational prop-erties of Li4/3Ti5/3O4 measured by specular reflectance, Physical Review B62

(2000) 12062.

29] L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, P. Herrero, J.M. Amarilla, R.M.Rojas, J.M. Rojo, Effect of the Thermal Treatment on the Particle Size and Elec-trochemical Response of LiCr0.2Mn1.8O4 Spinel, Journal of the ElectrochemicalSociety 152 (2005) A301.

[

[

a Acta 93 (2013) 163– 172

30] R.M. Rojas, J.M. Amarilla, L. Pascual, R.M. Rojas, J.M. Rojo, D. Kovacheva, K.Petrov, Combustion synthesis of nanocrystalline LiNiYCo1−2YMn1 + YO4 spinelsfor 5 V cathode materials: Characterization and electrochemical properties,Journal of Power Sources 160 (2006) 529.

31] L. Kavana, M. Gratzel, Facile Synthesis of NanocrystallineLi4Ti5O12 SpinelEx-hibiting Fast Li Insertion, Electrochemical and Solid-State Letters 5 (2002)A39.

32] Z. Liu, N. Zhang, K. Sun, A novel grain restraint strategy to synthesize highlycrystallized Li4Ti5O12 (∼20 nm) for lithium ion batteries with superior high-rate performance, Journal of Materials Chemistry 22 (2012) 11688.

33] K. Zaghib, M. Simoneau, M. Armand, M. Gauthier, Electrochemical study ofLi4Ti5O12 as negative electrode for Li-ion polymer rechargeable batteries, Jour-nal of Power Sources 81–82 (1999) 300.

34] A. Jaiswal, C.R. Horne, O. Chang, W. Zhang, W. Kong, E. Wang, T. Chern, M.M.Doeff, Nanoscale LiFePO4 and Li4Ti5O12 for High Rate Li-Ion Batteries, Journalof the Electrochemical Society 156 (2009) A1041.

35] Y.F. Tang, L. Yang, Z. Qiu, J.S. Huang, Preparation and electrochemical lithiumstorage of flower-like spinelLi4Ti5O12 consisting of nanosheets, Electrochem-istry Communications 10 (2008) 1513.

36] Y. Tang, L. Yang, S. Fang, Z. Qiu, Li4Ti5O12 hollow microspheres assembledby nanosheets as an anode material for high-rate lithium ion batteries, Elec-trochimica Acta 54 (2009) 6244.

37] S.L. Chou, J.Z. Wang, H.K. Liu, S.X. Dou, Rapid Synthesis of Li4Ti5O12 Micro-spheres as Anode Materials and ItsBinder Effect for Lithium-Ion Battery, Journalof Physical Chemistry 115 (2011) 16220.

38] C.-Y. Lin, J.-G. Duh, Porous Li4Ti5O12 anode material synthesized by one-stepsolid state method for electrochemical properties enhancement, Journal ofAlloys and Compounds 509 (2011) 3682.

39] S. Panero, D. Satolli, M. Salomon, B. Scrosati, A new type of lithium-ion cell basedon the Li4Ti5O12/Li2Co0.4Fe0.4Mn3.2O8 high-voltage, Electrochemistry Commu-nications 2 (2000) 810.

40] S.K. Martha, O. Haik, V. Borgel, E. Zinigrad, I. Exnar, T. Drezen, J.H. Min-ers, D. Aurbach, Li4Ti5O12/LiMnPO4 Lithium-ion battery systems for loadleveling application, Journal of the Electrochemical Society 158 (2011)A790.

41] W. Lu, J. Liu, Y.K. Sun, K. Amine, Electrochemical performance ofLi4/3Ti5/3O4/Li1 + x(Ni1/3Co1/3Mn1/3)1 − xO2 cell for high power applications, Jour-nal of Power Sources 167 (2007) 212.

42] M. Dahbi, I. Saadoune, J.M. Amarilla, LixNi0.7Co0.3O2 electrode material: Struc-tural physical and electrochemical investigations, Electrochimica Acta 53(2008) 5266.

43] A. Mahmoud, M. Yoshita, I. Saadoune, J. Broetz, K. Fujimoto, S. Ito,LixCo0.4Ni0.3Mn0.3O2 electrode materials: Electrochemical and structural stud-ies, Materials Research Bulletin 47 (2012) 1936.

44] J. Cho, G. Kim, Enhancement of thermal stability of LiCoO2 by LiMn2O4 coating,Electrochemical and Solid-State Letters 2 (1999) 253.