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Journal of Power Sources 244 (2013) 272e279

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Structure and high rate performance of Ni2þ doped Li4Ti5O12for lithium ion battery

Chunfu Lin a, Man On Lai a, Li Lu a,*, Henghui Zhou b,**, Yuelong Xin b

aDepartment of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, SingaporebCollege of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China

h i g h l i g h t s

* Corresponding author. Tel.: þ65 65162236; fax: þ** Corresponding author. Tel./fax: þ86 10 62757908

E-mail addresses: luli@nus.edu.sg (L. Lu), hhzhou@

0378-7753/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.jpowsour.2013.01.056

g r a p h i c a l a b s t r a c t

< Li4�2xNi3xTi5�xO12 (0 � x � 0.25)from solid-state reaction is system-atically studied.

< The effects of material structure onelectrochemical properties areinvestigated.

< The electronic conductivity islargely improved through Ni2þ

doping.< Li3.9Ni0.15Ti4.95O12 anode exhibits

high rate performance.

a r t i c l e i n f o

Article history:Received 1 November 2012Received in revised form10 January 2013Accepted 11 January 2013Available online 18 January 2013

Keywords:Lithium ion batterySpinel structureAnode materialDoping

a b s t r a c t

Li4�2xNi3xTi5�xO12 (0 � x � 0.25) has been synthesized via solid-state reaction. X-ray diffractions (XRD)demonstrate that all doped samples have a spinel structure with Fd3m space group without anyimpurities. Through further Rietveld refinements, it is shown that both lattice parameter and occupancyof non-Liþ ions in the 8a sites negligibly change with the amount of Ni2þ dopants. Scanning electronmicroscope reveals that Ni2þ doping does not change the morphology of Li4Ti5O12. The best electronicconductivity of Ni2þ doped Li4Ti5O12 is at least one order of magnitude higher than that of the pristineone, while all samples have similar Liþ ion diffusion coefficients. The electrochemical performance ofNi2þ doped Li4Ti5O12 shows good rate capability. The specific capacity of Li3.9Ni0.15Ti4.95O12 at 5 C is ashigh as 72 mAh g�1, while that of the pristine one can only achieve 33 mAh g�1. This improved rateperformance can be ascribed to its enhanced electronic conductivity.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Stimulated by the urgency of environmental protection and theexhaustion of fossil fuel reserves, more and more attention havebeen paid to the development of lithium ion batteries with highpower density for the applications of large format energy storagesystem, such as electric vehicles (EVs) or hybrid electrical vehicles(HEVs) [1]. At present, one of the main factors that hinders the

65 67791459..pku.edu.cn (H. Zhou).

All rights reserved.

commercialization of EVs/HEVs is the low rate performance ofbatteries. Therefore, an urgent step for the significant marketpenetration of EVs/HEVs is the development of a high rate batterysystem.

Graphite has been proven to be a reliable anode material incommercial lithium ion batteries [2]. However, its rate capability islimited. When the operation current density is large, it suffers fromsevere polarization causing metallic lithium to deposit on its sur-face [3,4]. Safety problems will emerge if such process is repeatedmany times due to the growth of lithium dendrites.

Li4Ti5O12 has been regarded as an attracting substitute forcarbon-based anode in batteries. It has a theoretical capacity of175 mAh g�1 and a high discharge plateau of 1.55e1.56 V vs Li/Liþ

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279 273

[5,6], avoiding the growth of lithium dendrites. It has a spinelstructure with Fd3m space group [7]. O2� ions at the 32e sites forma cubic closest packed structure. The tetrahedral 8a sites areoccupied by Liþ ions, while Liþ and Ti4þ ions are disordered, fillingthe octahedral 16d sites in the molar ratio of 1:5. It is noted that theremaining half of the octahedral cation sites in the cubic closestpacked structure are vacant (the 16c sites). During discharging,three external Liþ ions and the three Liþ ions in the 8a sites move tothe 16c sites. During charging, the process reverses. During thecharging and discharging processes, the robust three-dimensionalframework ðLiTi5Þ16dðO12Þ32e is not changed and its volumechange is smaller than 0.1% [8]. Thus, it is considered as a “zero-strain” anodematerial. Consequently, it has demonstrated excellentcycle life and reversibility.

Nevertheless, a main obstacle that impedes the widespreadapplications of Li4Ti5O12 is its poor conductionwith associated poorrate performance [9e11]. During the charge and discharge pro-cesses, Liþ ions intercalate and de-intercalate into the framework ofLi4Ti5O12 lattice accompanied by electrons. Therefore, the conduc-tion depends on Liþ ion diffusion coefficient, electronic conduc-tivity and particle size. Larger Liþ ion diffusion coefficient, largerelectronic conductivity and smaller particle size are favourable fora better conduction. To enhance electronic conductivity, Zr4þ [9],Mg2þ [10,11], Zn2þ [12], Ni2þ [13], Al3þ [14], Nb5þ [15], V5þ [16],Ta5þ [17], Cr3þ [18] and Ni3þ [19] doping have been used, andconducting carbon [20], Cu [21] and Ag [22] have also been incor-porated to increase electronic conductivity. Zr4þ [9], Mg2þ [10],Nb5þ [15], V5þ [16] and Cr3þ [18] doping have been shown toimprove Liþ ion diffusion coefficient. In addition, to reduce Liþ iontransportation distance, various types of nanosize Li4Ti5O12 havebeen synthesized such as nanoparticles [23e27], nanowires [28],nanorods [29], nanotubes [30], nanosheets [31], porous particles[32,33] and hollow spheres [34]. With suitable doping includingZr4þ [9], Nb5þ [15] and V5þ [16] the particle size can also bereduced.

In this work, we aim to increase intrinsic conductivity by dopingmethodology. The 3d electrons in Ni2þ ion have a t2g6 eg3 electronicconfiguration [35]. The electrons in the 3d orbitals can contribute tothe improvement in electronic conductivity. Therefore, Ni2þ ion isexpected to be a promising dopant for Li4Ti5O12 to improve its rateperformance. Thus far, only one paper has focused on the electro-chemical characteristics of Ni2þ doped Li4Ti5O12. Kim et al. haveinvestigated the rate performance of Li11/3Ni1/2Ti29/6O12 [13]. Theyhave shown that the specific capacity of this heavily Ni2þ dopedLi4Ti5O12 is almost double that of pristine material at 5 C. Never-theless, in this type of Ni2þ doped Li4Ti5O12, every two Liþ ions andevery one Ti4þ ion are substituted by three Ni2þ ions. Obviously,when the amount of dopant increases, the molecular weight in-creases, and the theoretical capacity decreases. Moreover, the studyon this doping is not complete. It is still unclear how electro-chemical properties can be related to the material structure.Therefore, it is highly desirable to systematically research on theelectrochemical performance of Li4Ti5O12 with light Ni2þ doping.Li4�2xNi3xTi5�xO12 (0 � x � 0.25) synthesized via a facile solid-statereaction method is systematically studied in the current research.The effects of doping on electrochemical properties are alsoinvestigated.

2. Experimental

2.1. Preparation of materials

All samples were prepared using a solid-state reaction method.In a typical fabrication processing, Li2CO3 (Merck, 99.99%), TiO2(SigmaeAldrich, 99.9%) and NiO (Aldrich, 76e77%Ni) powders were

employed as Liþ, Ti4þ and Ni2þ sources respectively. Appropriateamounts of these sources were firstly ball-milled for 0.5 h in a Spexball-milling machine to homogenously mix the powders. In allcases, 3 mol% excessive Li2CO3 was added to compensate for Li2Oevaporation during the synthesis at high temperature. Then, themilled mixtures were calcined at 800 �C for 4 h in a muffle furnacein air.

2.2. Characterization

The structures of the as-sintered powders were determinedusing an X-ray diffractometer on a Shimazu XRD-6000/7000 witha Cu Ka radiation. The continuous-scan data were collected be-tween 15� and 70� (2q) at a scan speed of 2�/min. The high qualitydata for Rietveld refinements were recorded from Shimazu XRD-7000 in an angle interval 15�e125� using step scan with 8 s perstep. Rietveld refinements were carried out using the GSAS pro-gram with the EXPGUI interface [36,37]. The refined instrumentaland structural parameters were: scale factor, background parame-ters, zero-shift, unit cell parameters, atomic fractional coordinates,atomic occupancies, atomic isotropic displacement parameters andprofile parameters. The site occupancies were constrained to thedesigned chemical formulas. The site occupancy of oxygen atomswas fixed to be 1.

Surface solid-state chemistry of particles was characterizedby X-ray photoelectron spectroscopy (XPS, Kratos Ultra DLD,Shimadzu, Japan) in fixed transmission mode with a pass energy of80 mV and the binding energy ranged from 0 to 1100 eV. Themorphologies and microstructures of the samples were examinedby a field emission scanning electron microscopy (FESEM, JEOLJSM-6700F) operating at 5 kV.

2.3. Electrochemical tests

Electrochemical performances of the as-synthesized materialswere tested using a two-electrode coin-type half-cell at roomtemperature. Electrodes were prepared by mixing 80 wt% activematerial with 10 wt% super P conductive carbon (TIMCAL Ltd.), and10 wt% polyvinylidene fluoride (PVDF, Aldrich) in N-methyl-pyrrolidone (NMP, SigmaeAldrich) solvent at a rotational speed of1500 rpm for 12 h to form a homogeneous slurry. The slurry wascoated on the Al foil. The coated electrodes were dried at 120 �C ina vacuum chamber for overnight and then pressed by a rollingmachine. Thereafter, 2016 coin cells (MTI corporation) wereassembled in a glove box filled with ultra-pure argon gas usingthe as-prepared working electrodes, Li metal as the counter andreference electrode, Celgard 2400 as the separator, and 1 M LiPF6 inethylene carbonate (EC)-dimethyl carbonate (DMC)-diethylenecarbonate (DEC) (1:1:1 in weight, DAN VEC) as the electrolyte. Thedischarge-charge tests of the cells were carried out between 1.0 and2.5 V using a Newware Battery Test System. The cells were firstlydischarged/charged at 0.1 C where current density of 1 C is equal to175 mA g�1 for 1 cycle, and then cycled at 0.5 C, 1 C, 2 C and 5 C for10 cycles with a 5 min step-interval between each cycle.

To measure Liþ ion diffusion coefficient, electrochemical impe-dance spectroscope (EIS) measurements were performed with5 mV ac signal and a frequency range from 105 to 0.1 Hz usinga Solartron Analytical 1470E CellTest System coupled with a Solar-tron Analytical 1400 CellTest System. All measurements were car-ried out after the cells being discharged to 50% state of dischargefollowed by two cycles.

The specimens used for conductivity measurements wereprepared by pressing the same precursor powders in Section 2.1into pellets of 10.25 mm diameter at 50 MPa. Cold compactswere then calcined at 650 �C for 5 h followed by sintering at

Fig. 1. (a) X-ray diffraction patterns of Li4�2xNi3xTi5�xO12 (0 � x � 0.075); (b) comparison of (220) peaks of Li4�2xNi3xTi5�xO12 (0 � x � 0.25).

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279274

900 �C for 48 h for densification. The thickness of these pellets wasabout 0.94 mm. All the sintered pellets had similar density ofapproximately 85e90%. Gold was sputtered to both sides of thepellets after grinding of the surfaces using sand papers. Electronicconductivity was measured using an Analytical 1470E CellTestSystem under a constant voltage of 50 mV for 3 h until thecurrents became stabilized.

3. Results and discussion

3.1. Crystal structure analysis

XRD spectra of Li4�2xNi3xTi5�xO12 (0 � x � 0.25) samples withenlarged (220) peaks are shown in Fig. 1(a) and (b). All peaks inFig. 1(a) can be indexed as cubic spinel structure with Fd3m spacegroup (JCPDS 26-1198). The sharp peaks indicate good crystallinityof all the samples and no impurity-phase were detected, demon-strating that Ni2þ ions have been successfully introduced into the

Fig. 2. Final observed, calculated, and error profiles with Rietveld refinements for L

lattice structure of Li4Ti5O12. In this spinel structure, the intensity ofthe (220) peak at approximately 30.2� is determined by the scat-tering power of the cations in the 8a sites. For all the samples, the(220) peak cannot be observed, indicating that the 8a sites arealmost full of Liþ ions with very small scattering factor and thatvery few non-Liþ ions occupy the 8a sites.

To better understand the structure in detail, Rietveldrefinements were employed. Fig. 2 presents the final observed,calculated, and error profiles for Li4e2xNi3xTi5�xO12 (x ¼ 0, 0.025,0.05 and 0.075) while the results of the crystal structure analysisare listed in Table 1. Several constrains for the refinements havebeen assumed. First, the site occupancies fulfilled the stoichio-metric compositions of Li4�2xNi3xTi5�xO12 in which all Ni-ionshave þ2 oxidation state. The XPS results of Li4.85Ni0.225Ti4.925O12

given in Fig. 3 show that the 2p3/2 peak of Ni3þ is much smallerthan that of Ni2þ, which is well consistent with existing results[13]. Therefore, it is reasonable to assume that Ni3þ ions inLi4�2xNi3xTi5�xO12 (0 � x � 0.25) can be negligible. Second, all ions

i4�2xNi3xTi5�xO12 with (a) x ¼ 0, (b) x ¼ 0.025, (c) x ¼ 0.05 and (d) x ¼ 0.075.

Table 1Results of crystal structure analysis by Rietveld refinements in Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and 0.075).

Spinel Li4�2xNi2xTi5�xO12, space group: Fd3m (cubic)

Composition x 0 0.025 0.05 0.075

Structure order (Li2.997Ti0.003)8a

(Li1.003Ti4.997)16dO1232e

(Li3)8a(Li0.95Ni0.075Ti4.975)16dO1232e (Li2.997Ti0.003)8a

(Li0.903Ni0.15Ti4.947)16dO1232e

(Li2.997Ti0.003)8a

(Li0.853Ni0.225Ti4.922)16dO1232e

Lattice parameter a/�A 8.36107(5) 8.36121(5) 8.36093(6) 8.36111(5)8a Li1 f 0.999(3) 1.000(2) 0.999(2) 0.999(2)

Ti1 0.001(3) 0.000(2) 0.001(2) 0.001(2)16d Li2 0.167(2) 0.158(1) 0.151(1) 0.142(1)

Ti2 0.833(2) 0.829(1) 0.824(1) 0.821(1)Ni e 0.0125(�) 0.025(�) 0.0375(�)

32e O 1(e) 1(e) 1(e) 1(e)Rwp 0.1032 0.1011 0.1054 0.0943Rp 0.0782 0.783 0.0815 0.0730c2 3.012 2.999 3.378 2.826

f: site occupancy, Rwp: weighted profile residual, Rp: profile residual, and c2: goodness of fit.

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279 275

had the same isotropic temperature factors. Finally, the distribu-tion of ions in the spinel structure was fixed as follows: O2� ionswere fixed at the 32e sites; Liþ and Ti4þ ions were located at boththe 8a and the 16d sites while Ni2þ ions only occupied the 16dsites. In principle, Ni2þ ions may be distributed at both the 8a andthe 16d sites. However, the refinements in this case could notproceed due to the similarity in scattering factors of Ni2þ and Ti4þ

ions. Ni2þ ion has larger octahedral site preference energy (OSPE,86.25 kJ mol�1) than Ti4þ ion (0 kJ mol�1) [35], indicating thatNi2þ ion has higher tendency to stay at the 16d sites than Ti4þ ion.Therefore, it is reasonable to assume that Ni2þ ions only occupiedthe 16d sites, while Ti4þ ions occupied both the 8a and the 16dsites.

In all the four samples, the occupancy of Ti4þ ions in the 8asites is negligible, confirming that Ni2þ doping hardly introducesnon-Liþ ions into the 8a sites. Pristine Li4Ti5O12 has a latticeparameter of 8.3611 �A, consistent with that previously reported[16e18]. The variation of lattice parameter is within 0.01%, indi-cating that Ni2þ doping cannot significantly impact the latticeparameter. This phenomenonmay be ascribed to the ion-size effect.For Li4�2xNi3xTi5�xO12, almost all the substitutions occur in the 16dsites. In the 16d sites, every two Liþ ions (0.76�A) and every one Ti4þ

ion (0.605 �A) are replaced by three Ni2þ ions (0.69 �A) [38].2� 0.76þ 1�0.605z 3� 0.69, resulting in negligible difference inlattice parameter.

Fig. 3. XPS spectra of Ni 2p core levels of Li4.85Ni0.225Ti4.925O12.

3.2. Particle morphology

The powder morphology of Li4�2xNi3xTi5�xO12 (x¼ 0, 0.025, 0.05and 0.075) calcined at 800 �C for 4 h in air is displayed in Fig. 4. Allthe samples have similar morphologies. There is almost no changein morphologies after Ni2þ doping. The particle size, with a widedistribution from less than 100 nm to more than 1000 nm, hasa medium value of approximately 500 nm.

3.3. Liþ ion diffusion coefficient measurement

The conductivity of electrode material is a very important factorin evaluating the rate performance of cell. Electrochemical impe-dance spectroscope (EIS) was employed to study the influence atdifferent levels of doping in Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05and 0.075) samples. Fig. 5(a) shows the Nyquist plots of theimpedance response of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and0.075) electrodes. To better understand the plots, the AC impedancespectra were fitted using an equivalent circuit shown in Fig. 5(b)[9,10]. The circuit is comprised of a series resistance (Rs), a charge-transfer resistance (Rct), a constant phase element (CPE), anda Warburg impedance (W). Rs indicates the ohmic resistance of thecell. Rct is the electron transfer resistance at the active interface. CPEreflects the interfacial capacitance. W is described as Warburgimpedance caused by a semi-infinite diffusion of Liþ ion in theelectrolyte. The fitted results are tabulated in Table 2. It can be seenthat the Rs values of all the samples investigated are similar,whereas the Rct values vary largely with different samples.Ni2þ doped samples exhibit lower Rct values than that ofpristine Li4Ti5O12, indicating that the doped samples have higherconductivity than pristine counterpart. Among all the samples,Li3.9Ni0.15Ti4.95O12 has the lowest charge-transfer resistance.

The plot of real impedance versus reciprocal square root of thelower angular frequency u�0.5 is illustrated in Fig. 5(c), from whichthe value of Warburg impedance coefficient sw can be obtainedaccording to Equation 1. Liþ ion diffusion coefficient D can becalculated from Equation 2 [9,10]. The dependency tendency ofLiþ ion diffusion coefficient D on composition x is given in Fig. 6.Pristine Li4Ti5O12 electrode has a Liþ ion diffusion coefficient of4.92 � 10�13 cm2 s�1, consistent with previous reports [9,10]. All thedoped samples exhibit Liþ ion diffusion coefficients similar to pris-tine one, although the Li3.9Ni0.15Ti4.95O12 sample has the largestvalue. These similar D values may be ascribed to their crystallinecharacteristics. It is known that Liþ ions transport in 3-dimensional8ae16ce8a pathways in this spinel structurewith Fd3m space groupduring the discharging and charging processes [39]. Liþ ions have to

Fig. 4. FESEM images of as-synthesized Li4�2xNi3xTi5�xO12 with (a) x ¼ 0, (b) x ¼ 0.025, (c) x ¼ 0.05 and (d) x ¼ 0.075.

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279276

migrate through the oxygen planes located between the 8a sites andthe 16c sites, which is the Liþ ion transportation bottle neck. In thisoxygen closest packed structure, small variation in lattice parameterimplies that the distance between the nearest neighbouring O2� ions

Fig. 5. (a) Nyquist plots for impedance response of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 andbattery; Rct: charge-transfer resistance; CPE: interfacial capacitance; W: Warburg impedanc(x ¼ 0, 0.025, 0.05 and 0.075) electrodes.

is hardly changed, which can lead to the similar values of Liþ iondiffusion coefficient of the active materials in the cell.

Z0 ¼ Rs þ Rct þ swu�0:5 (1)

0.075) electrodes. (b) Equivalent circuit used to fit the EIS. Rs: ohmic resistance of thee. (c) Relationship between real impedance with low frequency for Li4�2xNi3xTi5�xO12

Table 2Impedance parameters of the Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and 0.075)electrodes.

Samples Rs (U) Rct (U) sw (U s�0.5) D (cm2 s�1)

Li4Ti5O12 3.23 89.3 10.9 4.92 � 10�13

Li3.95Ni0.075Ti4.975O12 2.81 64.3 10.7 5.46 � 10�13

Li3.9Ni0.15Ti4.95O12 2.55 55.7 9.25 6.61 � 10�13

Li3.85Ni0.225Ti4.925O12 2.81 70.1 10.6 5.26 � 10�13

Fig. 6. Variation in Liþ ion diffusion coefficient and electronic conductivity as a func-tion of composition x in Li4�2xNi3xTi5�xO12 (0 � x � 0.075).

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279 277

D ¼ R2T2

2A2F4s2wC2 (2)

where Z0is the real part of the impedance, sw, the Warburg impe-

dance coefficient, u, the angular frequency, R, the gas constant, T,the absolute temperature, A, the surface area, F, the Faraday’sconstant, and CLiþ , the molar concentration of Liþ ions.

3.4. Electronic conductivity

Fig. 6 also illustrates the dramatic increase in electronic con-ductivity that can be imparted to Li4�2xNi3xTi5�xO12 by Ni2þ doping.The dense pellets were achieved by sintering the precursor pow-ders at 900 �C. The as-prepared samples from 900 �C and 800 �Chave similar structural characteristics (See SupportingInformation). Thus the electronic conductivity obtained from thepellets sintered at 900 �C is a good approximation to that at 800 �C.The electronic conductivity of pristine Li4Ti5O12 is very low(<1 � 10�9 S cm�1) so that it could not be accurately determinedwithin the resolution of the Solartron Analytical 1470E CellTestSystem. The electronic conductivitymonotonically increases with x,reaching a large value of 3.1 � 10�8 S cm�1 (x ¼ 0.075), in a goodagreement with Kim et al.’s results [13]. This value is larger thanthose of Al3þ doped, Ga3þ doped, Co3þ doped, Mg2þ and Al3þ co-doped, and Ta5þ doped Li4Ti5O12 with similar doping levelsshown in Table 3 [17,40]. It is obvious that 3d transition metal ionNi2þ doping can greatly improve the electronic conductivity.

Table 3Comparison of electronic conductivity using various dopings. Electronic conductivity wa

Nominal formula Li4Ti5O12 Li3.95Ni0.075Ti4.975O12

Li3.9Ni0.15Ti4.95O12

Li3.85Ni0.225Ti4.925O12

Electronic conductivity(S cm�1)

<1 � 10�9 1.5 � 10�8 2.2 � 10�8 3.1 � 10�8

Previous reports showed that electronic conduction in spinel ox-ides containing transition metal ions is dominated by localizedd electrons hopping among the octahedral (16d) cations [41e44].Pristine Li4Ti5O12 is an insulator since there are no electrons in theTi4þ (t2g0 eg0) 3d orbitals. In contrast, Ni2þ (t2g6 eg3) ions in the 16dsites supply the charge carriers (electrons) in the doped Li4Ti5O12.Therefore, the electronic conductivity of Li4Ti5O12 is greatlyenhanced upon Ni2þ doping. More Ni2þ ions can supply moreelectrons, leading to larger electronic conductivity.

3.5. Charge/discharge performance at 0.5 C

Fig. 7 plots the second specific charge and discharge capacitiesof the cells prepared using Li4�2xNi3xTi5�xO12 (x¼ 0, 0.025, 0.05 and0.075) in the potential range of 1e2.5 V vs Li/Liþ at 0.5 C. During thedischarge, the potential drops quickly down to a flat dischargeplateau of approximately 1.56 V vs Li/Liþ and a flat charge plateau atabout 1.6 V vs Li/Liþ can also be observed. This suggests a two-phase reaction based on Ti3þ/Ti4þ redox couple [31]. No othercharge/discharge plateaus were observed. The 1.56/1.6 V vs Li/Liþ

plateau contributes more than 90% of the total discharge/chargecapacity. The specific capacity decreases when the doping amountincreases. At such low rate, electrons and Liþ ions have adequatetime to conduct and diffuse respectively. Thus, the maximumaccessible capacity of the active material can be achieved, whichmay be determined by the theoretical capacity of the active ma-terial. The theoretical capacity of Li4�2xNi3xTi5�xO12 is calculated as80400/(460 þ 114x) mAh g�1. Obviously, when x increases, thetheoretical capacity decreases. Therefore, pristine Li4Ti5O12 has thelargest capacity of 164 mAh g�1, while Li3.85Ni0.225Ti4.925O12 has150 mAh g�1.

3.6. Rate performance

High rate performance is one of the most important electro-chemical characteristics of batteries for EVs/HEVs. The seconddischarge-charge profiles of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05and 0.075) samples at different rates from 0.5 C to 5 C are presentedin Fig. 8. It can be clearly observed that all discharge plateausmonotonically drop due to increasing polarization when the rateincreases. For pristine Li4Ti5O12, with increasing rate, the dischargeplateau slowly becomes inconspicuous so that no obvious dis-charge plateau can be found at 5 C. For each Ni2þ doped Li4Ti5O12,however, an obvious discharge plateau is always observed even ata high rate of 5 C. Meanwhile, all discharge and charge capacitiesdecrease with rates.

Fig. 9 reveals the rate and cyclic performance of the four samplesat different rates. It can be seen that the variation in capacity for allthe samples is not apparent at low rates (under 1 C). As discussedin Section 3.5, when the rate is low, Liþ ions and electrons haveadequate time to diffuse and conduct respectively, which minimizethe difference among the capacities at low rates. It can be observedthat pristine Li4Ti5O12 displays the largest capacity among all thefour samples at 0.5 C.When the rate increases, however, its capacityquickly decreases. At 1 C, it is 143 mAh g�1, 105 mAh g�1 at 2 C; andat 5 C, its remained capacity is only 33 mAh g�1. In contrast, asrevealed in Fig. 9, in spite of their relatively lower capacities at 0.5 C,

s measured using the same method [17,40].

Li3.95Al0.15Ti4.9O12

Li3.95Ga0.15Ti4.9O12

Li3.95Co0.15Ti4.9O12

Li3.9Mg0.1Al0.15Ti4.85O12

Li4Ta0.05Ti4.95O12

1.1 � 10�8 2.0 � 10�9 1.3 � 10�9 7.9 � 10�9 1 � 10�9

Fig. 7. Second dischargeecharge curves of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and0.075) samples at 0.5 C (identical discharge/charge rates were used). Fig. 9. Rate and cyclic performance of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and 0.075)

samples at different rates: the first 10 cycles at 0.5 C, second at 1 C, third at 2 C andfourth at 5 C (identical discharge/charge rates were used).

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279278

the doped samples reveal much less capacity degradation thanthat of pristine one with the rate increases. For example, at 2 C,the capacities of Li3.95Ni0.075Ti4.975O12, Li3.9Ni0.15Ti4.95O12 andLi3.85Ni0.225Ti4.925O12 are 123, 121 and 115 mAh g�1 respectively;and at 5 C, they are 63, 72 and 56 mAh g�1. For clearer observation,the relative capacities of all the samples as a function of rate areprovided in Fig. 10, in which the capacities determined at 0.5 C aretaken as standard.

These results indicate that Ni2þ doping impairs the capacityof Li4Ti5O12 at low rates, but can obviously enhance its rate per-formance. In this research, since all the samples have similarmorphologies and Liþ ion diffusion coefficients, the rate perfor-mance of the active materials is determined by their electronic

Fig. 8. Second dischargeecharge curves of Li4�2xNi3xTi5�xO12 samples with (a) x ¼ 0, (b) x ¼rates were used).

conductivity. The electronic conductivity in the doped materialscan facilitate the charge-transfer reaction in the cathodes, resultingin good rate performance. It is worth noting that, as shown inFig. 10, Li3.9Ni0.15Ti4.95O12 has the best rate performance, indicatingthat the x¼ 0.05 dopant amount is appropriate. This result could beexplained by its best combination of Liþ ion diffusion coefficientand electronic conductivity. It is also consistent with EIS resultsshowing the smallest Rct value for Li3.9Ni0.15Ti4.95O12 in Table 2.At 5 C, the capacity of Li3.9Ni0.15Ti4.95O12 is 118% higher than that ofthe pristine value, indicating that Li3.9Ni0.15Ti4.95O12 is superiorcompared to the heavily Ni2þ doped Li11/3Ni1/2Ti29/6O12 samplestudied by Kim et al. [13] at this high rate.

0.025, (c) x ¼ 0.05 and (d) x ¼ 0.075 at 0.5 C, 1 C, 2 C and 5 C (identical discharge/charge

Fig. 10. Capacity retention of Li4�2xNi3xTi5�xO12 (x ¼ 0, 0.025, 0.05 and 0.075) samplesat 0.5 C, 1 C, 2 C and 5 C (identical discharge/charge rates were used).

C. Lin et al. / Journal of Power Sources 244 (2013) 272e279 279

4. Conclusions

In this research, Li4�2xNi3xTi5�xO12 (0 � x � 0.25) powders havebeen prepared through solid-state reaction method. All Ni2þ dopedmaterials possess spinel structure with Fd3m space group withoutany impurities. They have similar lattice parameters and freeblockages of Ni2þ in the 8ae16ce8a Liþ transportation paths. Ni2þ

doped samples have similar Liþ ion diffusion coefficients to pristineone due to the negligible change in lattice parameter. The electronsin the 3d orbitals of Ni2þ ions contribute to the improved electronicconductivity. As a result, Ni2þ doped samples have good rate per-formance, of which Li3.9Ni0.15Ti4.95O12 is the best. At 5 C, it hasa high capacity of 72 mAh g�1 which is 1.2 times larger than that ofpristine one. Therefore, the novel Ni2þ doped Li4Ti5O12 may findpromising applications in EVs/HEVs due to its good rate perfor-mance and the simple synthesis route.

Acknowledgement

This research is supported by Agency for Science, Technologyand Research, Singapore through the research grant R265-000-442-305 and A* Star Singapore-China Joint Research Programme(No. 2012DFG52130).

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://dx.doi.org/10.1016/j.jpowsour.2013.01.056.

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