Synthesis and characterization of iron − doped Li4Ti5O12 microspheres as anode for lithium-ion batteries

Synthesis and characterization of iron − doped Li4Ti5O12 microspheres as anode for lithium-ion batteries

Journal of Alloys and Compounds 735 (2018) 1871e1877 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: htt...

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Journal of Alloys and Compounds 735 (2018) 1871e1877

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis and characterization of iron  doped Li4Ti5O12 microspheres as anode for lithium-ion batteries ndez-Carrillo a, G. Ramos-Sa nchez b, **, 1, G. Guzma n-Gonza lez b, R.A. Herna lez b, E.M. Sanchez-Cervantes a, * N.A. García-Gomez a, I. Gonza a s de los Garza, Nuevo Leo noma de Nuevo Leo n, Facultad de Ciencias Químicas, Av. Universidad S/N Ciudad Universitaria, San Nicola n, Universidad Auto 66451, Mexico b noma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No.186. Col. Vicentina, Iztapalapa, CDMX, 09340, Universidad Auto Mexico

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2017 Received in revised form 14 November 2017 Accepted 19 November 2017 Available online 21 November 2017

Li4Ti5O12 microspheres and the equivalent iron doped materials, with 0.1 and 0.2 mol of Fe per unit formula, were synthesized by a solvothermal method. The presence of the dopant was verified by X-Ray diffraction with Rietveld refinement and XPS experiments. It was found that the dopant is included in the lattice structure as Fe (III), at lower concentration the dopant is found primarily on Ti (IV) sites while at higher concentration it occupies both Ti and Li sites. Conductivity measurements indicate that the higher iron concentration increases the material intrinsic electronic conductivity; however, the effect of Fe doping at lower concentration on the conductivity is almost null. Because of the Fe doping, the charge/ discharge plateaus are obtained at higher/lower voltages, which might be correlated to lower charge transfer resistance, even though the electrochemical measurements show slightly lower capacities and very similar rate capabilities. Additionally, EIS experiments indicate that the presence of the dopant causes lower charge transfer resistance and enhanced finite lithium diffusion. However, as the size of the particles is still very large, the improved Liþ intercalation properties do not influence the rate capability since this property should be more related to shortened diffusion paths. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lithium-ion batteries Anode material Solvothermal Lithium titanate Iron-doping

1. Introduction Lithium Ion batteries (LIB) have become one of the most promising technologies for energy storage, both, to power portable systems and to store energy generated from renewable sources. To this end, one of the main objectives of LIB research is the development of high capacity anodes with high reversibility and durability. Owed to their intrinsic characteristics, excellent safety and extended lifetime, lithium titanate spinel (Li4Ti5O12, LTO) has been considered a promising material for LIB anodes [1,2]. LTO can accommodate up to three Li ions in its structure with a theoretical capacity of 175 mAhg1; differently from graphite anodes which suffer a 10% expansion along with lithium intercalation, the lithium intercalation in LTO occurs with negligible size changes. The

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. [email protected] (E.M. Sanchez-Cervantes). 1 CONACYT fellow. https://doi.org/10.1016/j.jallcom.2017.11.218 0925-8388/© 2017 Elsevier B.V. All rights reserved.

Ramos-S anchez),

eduardo.

discharge process occurs at a very flat potential of 1.5 V vs Li0/Liþ which represents another advantage since the reduction potential for most organic compounds lays below this potential; therefore, the loss of capacity associated to the formation of the Solid Electrolyte interface (SEI) can be avoided [3]. Among other advantages the low cost, easy fabrication process and plentifulness of the base materials make LTO one of the most promising anodic materials. Despite its adequate properties, LTO suffers from intrinsic low electron conductivity, and slow Liþ diffusion; therefore, in order to increase their transport properties, several modification routes have been studied. Similarly to other materials, to increase the electron conductivity, the active material is mixed with a conductive graphite phase, or even better, the carbon precursor is added during the synthesis in order to form a conductive carbon/LTO composite; for instance, composites with graphene [4], Carbon Nanotubes [5], carbon nanofibers [6], etc. These modifications have resulted on very high rate capability with capacity as high as 160 mAhg1 at 10C [7,8]. Another modification method, intended to increase the electron conductivity is the selective doping with metallic ions such as Al3þ [9], Cr3þ [10], Co3þ [11], the effect of the

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2. Experimental 2.1. Synthesis of Li4Ti5-xFexO12 (x ¼ 0, 0.1 y 0.2) micro-spheres

Fig. 1. X-ray diffraction pattern of as prepared a) LTO, b) LTO-Fe01 and c) LTO-Fe02. At the bottom, the JCPDF corresponding to spinel LTO showing the main diffraction peaks.

dopant has resulted on the improvement of the capacity at high C rates which has been attributed to the improvement on the electric conductivity on the LTO bulk and enhanced diffusion coefficient of Liþ. However, the effect of Feþ3 as dopant on the capacity and rate capability has not been sufficiently discussed and still is a controversial topic. For instance, it has been reported that the surface modifications of LTO with Fe resulted not only in higher rate capability but also in enhanced specific capacity [12], similarly, Fe doping of Zn titanates have resulted in improved rate capability [13]. On the other hand, theoretical calculations have indicated that Fe doping might have no effect on the band gap and conductivity [14]. Thus a careful investigation of the effect of iron as dopant in LTO is of importance; moreover, the influence of the dopant on the Liþ diffusion coefficient and materials resistivity variations as result of the presence of the dopant is of primordial importance in order to investigate the relation between rate capability and dopant concentration. In this work, we report the characterization of pure LTO microspheres and LTO doped with 0.1 and 0.2 mol of Fe per unit formula synthesized by a solvothermal method. The microsphere morphology is intended to create good contact between particles increasing the intraparticle conductivity while doping is intended to increase the intra particle conductivity. The results indicate that the presence of Fe improves the electric conductivity and Liþ diffusion coefficient, which, however, only represents a minor improvement on the discharge voltage and cycling performance but not in the capacity and rate capability.

All chemicals are analytical grade and are used without any further purification. In a typical process, stoichiometric amounts of LiOH$H2O (1.8 mmol, 98% purity), and Ti [OCH (CH3)2]4 (2.0 mmol, 99.999% purity) were dissolved in 40 ml of ethanol to form a homogeneous solution under ultrasonic bath. The resulting solution was transferred into a 60 mL Teflon lined stainless steel autoclave and heated to 180  C for 24 h, and then cooled to room temperature. The white precipitate was collected, washed and centrifugated with ethanol and distilled water for three times, and finally dried at 80  C in vacuum. Afterwards the as-obtained particles were thermally treated at 700  C during 4 h under air atmosphere. The procedure used for the preparation of Fe (III) doped Li4Ti5O12 microspheres was similar to that of undoped LTO, except for the addition of Fe (NO3)3$9H2O (99.99% purity) into the precursor solution, according to the stoichiometric formulas Li4Ti4.9Fe0.1O12 (0.004 mol of Fe) and Li4Ti4.8Fe0.2O12 (0.008 mol of Fe), in the subsequent LTOFe01 and LTOFe02, respectively. 2.2. Characterization of Li4Ti5-xFexO12 micro-spheres The crystal structure and phase identification of the obtained powders were carried out by X-ray diffraction (XRD) using a D2Phaser (Bruker) with Cu Ka radiation source (l ¼ 1.5418 Å). The high quality data for Rietveld refinements were recorded between 10 and 70 with a step of 0.05 and a counting time of 1s per step. Rietveld refinements were carried out using the GSAS program with the EXPGUI interface [15,16]. The chemical composition and the oxidation valence states of the elements in the samples were obtained through X-ray photoelectron spectroscopy (XPS) experiments using an Al Ka source (Thermo Scientific K-Alpha). The morphology and elemental analyses of the compounds were investigated using a FESEM (JEOL ISM6701F, JEOL Company) equipped with an EDXS detector (INCA X-Act, Oxford Inst.). 2.3. Conductivity measurements The samples electric conductivity was measured by a DC-four probe technique, using a Keithley 2400 Source Meter system. The powder materials LTO, LTO-Fe01 and LTO-Fe02, were pressed into pellets (12 mm diameter and 1.5 mm thickness) and sintered at 700  C for 4 h. Electrical contacts were made using Cu wires and Ag paste, placed over the edges of the pellet facets, ensuring a homogeneous current flow. The conductivity was calculated from the resistance and specimen dimensions.

Fig. 2. Experimental (black), calculated (red) and error profiles (blue) obtained from Rietveld analyses for (a) LTO, (b) LTO-Fe01 and (c) LTO-Fe02.

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2.4. Electrochemical measurements For the electrochemical measurements, the working electrode was prepared from a slurry composed of Li4Ti5-xFexO12 as active material (70%wt), conductive carbon black (20%wt), polyvinylidene fluoride as binder (10%wt) and N-methyl-2-pyrrolidone as solvent. The slurry was coated on an aluminum foil as current collector by the doctor-blade technique and dried at 100  C for 5 h under vacuum; finally the foil was cut into disks (18 mm in diameter). Lithium metal foil was employed as anode, Whatman GF/F glass microfiber paper was used as separator, and 1 M LiPF6 in EC þ DMC þ DEC (1:1:1) solution as electrolyte. CR2032 coin-type half cells were assembled inside an argon filled glove box with O2 and moisture levels below 0.5 ppm. Galvanostatic charge/discharge performances were evaluated in the voltage range of 1e3 V vs Liþ/ Li0 at different current rates (1C ¼ 175 mAhg1). Electrochemical Impedance Spectroscopy (EIS) measurements were performed by applying an AC voltage perturbation of 10 mV amplitude over the frequency range from 1 MHz to 10 mHz after cell equilibration at open circuit conditions. Cycling performance of the assembled coin cells with the three synthesized materials was carried out at 1C for 200 cycles. All electrochemical analyses were carried out in a VMP3 (BioLogic) electrochemical workstation. 3. Results and discussion Fig. 1 shows the XRD powder patterns of pristine Li4Ti5O12 microspheres and the iron doped samples. All diffraction peaks can be indexed to a cubic spinel Li4Ti5O12 with a Fd3m space group (JCPDS No. 49-0207). The presence of sharp peaks indicates high crystallinity for all samples while no impurities or precursor phases were detected, demonstrating that pure phases of lithium titanates were successfully synthesized by the solvothermal method. In order to understand the crystal structure, Rietveld refinements were employed. Fig. 2a-c shows the experimental and calculated XRD patterns of the samples, as observed the introduction of Fe (III) in the structure of LTO did not affect the spinel phase. However, the lattice parameters increased upon Fe substitution from 8.4250 to 8.4378 Å, as x increased from 0 to 0.2. This trend can be explained by the fact that Fe (III) ion is larger than Li or Ti (IV) ions, thus the introduction of iron into the crystal structure lead to the expansion of the unit cells. The small deviation between the observed and calculated XRD patterns demonstrates that Li, Ti, and O are located at their original sites [17]; however, in the LTO-Fe02 sample, a small reflection at 20.43 is observed, suggesting that Fe (III) has the tendency to occupy both Ti (IV) and Li sites [18]. XPS is a well-suited technique for the evaluation of valence and electronic states of metal/non-metal ions. Fig. 3a shows the XPS surveys in which it is readily visible the presence of signals corresponding to Li, Ti, O, C and Fe, the last one is only present on Fe doped samples. Fig. 3b and c shows Li1s and Ti2p XPS spectra, respectively. The Li1s spectra consist of a single peak at 54.5 eV corresponding to the LieO bonds [19], Fe doped samples show a slight shift toward lower binding energy; this behavior can be related to the introduction of Fe in the LTO spinel structure [20]. The Ti2p spectra show two peaks at 458 eV and 464 eV corresponding to Ti2p3/2 and Ti2p1/2 respectively [21,22]; both signals reveal the presence of Ti (IV) ions, bonded to oxygen in an octahedral environment [23]. Additionally, the observed peaks in the high resolution XPS spectrum of Fe-doped samples are broadened, indicating the influence of Fe addition on the electronic states of Ti; probably some of the Ti ions are replaced with Fe ions in the lattices. On the other hand, the O1s spectra (Fig. 3d) show mainly the peak at 529 eV corresponding to lattice oxygen (O in TieO2 bonds) [24]; additionally, in the LTO sample a small signal is observed at

Fig. 3. (a) XPS survey for the synthesized samples LTO, LTO-Fe01 and LTO-Fe02. High resolution XPS spectra for b) Li1s, c) Ti2p, d) O2p and e) Fe2p.

531 eV, attributed to surface adsorbed eOH and CO2 groups. For doped samples, these signals are overlapped, also the binding energies shifts slightly from 529 eV to 528.7 eV, and the peak become broader. This indicates the presence of non-lattice oxygen due to the formation of oxygen vacancies, by the Ti (IV) substitution for Fe (III) in the Li4Ti5O12 structure [25e27]. Lastly, the Fe2p XPS spectra

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confirm the presence of Fe (III) only on the doped samples (Fig. 3e). The intense peak at 712 eV represents the Fe2p3/2; while the peak observed at 723 eV corresponds to Fe2p1/2 photoelectrons [28e30]. The introduction of Fe (III) in the LTO structure induces some changes in its electronic structure, as revealed by changes in the intensity of the Fe2p band, at higher iron content. Also, the iron doping changes the surface bonding and the chemical nature of the atomic interactions between Fe and the other elements in the lithium titanate. Fig. 4 shows the SEM micrographs of the resulting powders before (a-c) and after (d-f) calcination. The morphology of all compounds consists of regular micro-spheres with smooth surface and regular size. After the calcination process the average size of the micro-spheres was reduced due to the elimination of volatile organic compounds and the pore collapse inside the micro-spheres. After heat treatment, the particle size of all samples was 800 nm approximately. After the physicochemical characterization and the determination of the presence of iron in the crystalline structure of LTO, its effect on the electronic conductivity was evaluated by four-point DC tests. The electrical conductivity of LTO-doped samples is higher than the pristine LTO, as the amount of iron is increased, the electric conductivity is almost doubled with 0.2 mol of iron. This behavior is attributed to the fact that doping with the metal promotes greater electron mobility due to changes in its band structure [31,32]. Also, the introducction of Fe (III) ions into Li sites promote the reduction of Ti (IV) to Ti (III) to balance the charge. The Ti (III) ions in LTO can effectively improve the concentration of electronehole; therefore, a larger proportion of Ti (III) improves the bulk electricconductivity [33], as shown in Table 1. For the electrochemical characterization, all samples were prepared according to the aforementioned procedure. Special care was

taken during sample preparation in order to obtain homogeneous coatings, discarding all samples with visible defects, in the same way similar care was taken during the electrode cutting process (EL-CELL cutting machine) discarding all samples with peeled coatings on the edge. Several reports have addressed the charge/ discharge characteristics of LTO, the properties for the electrodes here reported are depicted in Fig. 5a. The performance of the obtained materials is very similar to previous reports [34e36], briefly, during the charging process a plateau around 1.5 V is observed up to around the 80% SOC then the potential rises quickly reaching the maximum capacity at 3 V; during discharge, the inverse phenomena is observed. The capacity values here described are in line with previous reports of pure LTO [37,38], indicative of an adequate electrode preparation process. However, the specific capacity of doped and undoped samples is very similar, in disagreement with others reports in which the effect of doping causes higher specific capacity [39,40]. It has been observed that Fe (III) can replace Ti (IV) in the crystal lattice, but also might occupy some lithium atoms in the octahedral sites; therefore, the positions available for Liþ intercalation capacity are decreased and the capacity cannot be higher [41,42]; moreover, the presence of Fe on Li sites might represent an obstacle which can limit the number of Liþ reaching their position in the crystalline structure. Discharge rate capabilities of the samples were investigated by increasing the C-rate every 6 cycles (Fig. 5b). At first sight the effect of dopant seems null, since the capacity is similar and no effect on the retention of the capacity is observed. A closer analysis of the discharge process is depicted in Fig. 5c, in this, two differences are observed: the first one indicates that the discharge potential is higher in the LTO-Fe02 in comparison to the all other samples particularly at high discharge rates, thus, the charge transfer resistance for doped samples is lower; the second difference is that

Fig. 4. Scanning Electron Microscopy images of materials before heat treatment: (a) LTO, (b) LTO-Fe01 and (c) LTO-Fe02; and after heat treatment at 700 C/4 h: (d) LTO, (e) LTO-Fe01 and (f) LTO-Fe02.

ndez-Carrillo et al. / Journal of Alloys and Compounds 735 (2018) 1871e1877 R.A. Herna Table 1 DC Electrical conductivity for each synthesized material. Sample

Conductivity (S/cm)

LTO LTO-Fe01 LTO-Fe02

1.22  109 ± 1.2  1010 1.25  109 ± 2.5  1010 1.89  109 ± 2.4  1011

the trend of higher discharge voltage changes rapidly at the end of discharge, leading to a lower capacity on LTO-Fe02 samples, i.e. it indicates a lower amount of sites for Liþ intercalation. Differently from LTO-Fe02 samples the discharge potential of LTO-Fe01 is lower than LTO and the maximum capacity is lower as well, indicating that the lower amount of Fe did not increase the samples conductivity to the required levels. On the other hand, the higher Fe amount indeed increased the conductivity which might be directly related to the more positive discharge voltage at higher C-rates, Fig. 5d shows the discharge potential at two different SOD, portrayed on the figure as function of the C-rate. The direct relationship between discharge voltage and c-rate indicates that the electronic conductivity inside the microspheres might be the responsible for the changes [43]. The poor capacity and low discharge voltage of LTO-Fe01 indicate that Fe not only occupies Li sites which diminishes the capacity, but that the mere presence of Fe is not enough to enhance the conductivity inside the microsphere; instead, on the LTO-Fe02 the conductivity might be increased inside the microsphere thus facilitating electronic transport. The improved discharge potential is small at slow discharge rate but reaches around 50 mV at 2C, which might have an important effect on the specific power. Fig. 5e shows the charging voltage of a battery with an ideal cathode of 3.5 V with an anode composed of the materials here reported, the lower charging voltages using LTO-Fe02 as anode are readily visible specially at higher c-rates thus confirming the improvement in comparison to

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pure LTO sample, on the contrary the LTO-Fe01 presents a much higher voltage which is directly related to a lower energy efficiency. On the other hand, cycling performance experiments indicate that the inclusion of Fe on LTO influences the capacity fade behavior. As previously described, at 1C all samples present a similar initial capacity, around 110 mAhg1; however, during the initial 40 cycles the capacity continuously decreases, the capacity fade for LTO, LTO-Fe01 and LTO-Fe02 is 16, 20 and 12% respectively. These results indicate that the lower concentration of Fe influences negatively both the capacity and cycling performance; however, the higher Fe concentration promotes the increment of the cycling performance. After the first cycles of capacity fade all samples reach a stable behavior as the number of cycles is increased up to 200. EIS experiments were carried out at the beginning of the charge/ discharge experiments in order to obtain insights on the electrodes properties, the experiments were carried out as explained in the experimental section, it is worth noting that the experiment was repeated several times giving similar results and confirming stationary state during the experiments. A qualitative description of the Nyquist spectra depicted on Fig. 6a indicates that the system is composed by three distinctive zones, the high frequency semicircle, medium frequency semicircle and low frequency line. The high frequency zone is composed by the cooperative mechanism between double layer capacitance and charge transfer resistance, in this zone it is evident that the charge transfer resistance is lower for Fe doped samples. The Nyquist diagram for LTO presents a linear behavior at lower frequencies associated to the Liþ diffusion on the solid which is very similar to previous reports, however the doped samples present a second time constant just distinguishable at medium frequencies. A careful inspection of the Bode diagram (Fig. 6b) indicates that effectively, a second time constant is present; this second process can be associated to a second path for charge transfer or Lithium diffusion related to the presence of Fe as dopant, which could then be the responsible for the slight

Fig. 5. Charge discharge properties of pure and doped LTO, (a) Capacity versus voltage during discharge and charge of LTO half-cell at the indicated C-rate, (b) Specific capacity (31 V) at the indicated C-rate, (c) Comparison of specific capacity during discharge for all samples, (d) Discharge potential at SOD ¼ 0.3 and 0.7 (open and closed symbols respectively) for LTO and Fe doped LTO and (e) Charging potential of a battery with a fictitious 3.5 V cathode and anode shown in the legend.

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Fig. 6. Analysis of the EIS experiments (a) Nyquist plot and (b) Bode phase diagram.

improvement on the electrochemical properties of Fe-doped LTO as LIB anode material. 4. Conclusions Microspheres composed by Li4Ti5-xFexO12 (x ¼ 0, 0.1 and 0.2)/C have been successfully prepared through the solvothermal method. We have studied the doping mechanism and the effects of Fe (III) on the crystal structure, morphology, and electrochemical performance of titanates. The introducction of Fe (III) improves the bulk electrical conductivity; however, the electrochemical reaction mechanism has not changed by Fe-doping and the charge transfer reaction is favored at higher content of iron, according to the EIS results. This kind of microstructured materials can be considered as a promising candidate for applications such as anode material for lithium ion batteries, due to the morphological and structural features. Although the inclusion of Fe as dopant has led to changes in the samples conductivity and diminished charge transfer resistance, the microsphere structure needs to be improved in order to reach nanometric size for which the doping effect of Fe would have a much higher impact. Acknowledgments This research was supported by the project SEP-CONACyT 151587 and PN-CONACyT 2015-01-595. Also, RAHC is grateful for the academic scholarship #339506 provided by CONACyT and for the technical support provided by Dr. Domingo I. Garcia Gutierrez  Francisco Go mez Garcia for XPS and Rietveld meaand Dr. Jose surements, respectively. References [1] T. Ohzuku, A. Ueda, N. Yamamoto, Zero-strain insertion material of Li[ Li1/3Ti5/ 3 ] O 4 for rechargeable lithium cells, J. Electrochem. Soc. 142 (1995) 1431e1435. [2] K. Nakahara, R. Nakajima, T. Matsushima, H. Majima, Preparation of particulate Li4Ti5O12 having excellent characteristics as an electrode active material for power storage cells, J. Power Sources 117 (2003) 131e136. [3] X. Sun, P.V. Radovanovic, B. Cui, Advances in spinel Li4Ti5O12 anode materials for lithium-ion batteries, New J. Chem. 39 (2015) 38e63. [4] Y. Tang, F. Huang, W. Zhao, Z. Liu, D. Wan, Synthesis of graphene-supported Li4Ti5O12 nanosheets for high rate battery application, J. Mater. Chem. 22 (2012) 11257e11260. [5] S. Cao, X. Feng, Y. Song, X. Xue, H. Liu, M. Miao, J. Fang, L. Shi, Integrated fast assembly of free-standing lithium titanate/carbon nanotube/cellulose nanofiber hybrid network film as flexible paper-electrode for lithium-ion batteries, ACS Appl. Mater. Interfaces. 7 (2015) 10695e10701.

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