Improving the lithium storage properties of Li4Ti5O12 anodes by facile two-phase formation and nanostructure engineering strategy

Journal of Alloys and Compounds 705 (2017) 638e644

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Improving the lithium storage properties of Li4Ti5O12 anodes by facile two-phase formation and nanostructure engineering strategy Qinghua Tian a, *, Zhengxi Zhang b, Li Yang b, **, Yixin Xiang c a

Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China c School of Biological and Chemical Engineering, Anhui Polytechnic University, Wuhu, 241000, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 December 2016 Received in revised form 5 February 2017 Accepted 17 February 2017 Available online 20 February 2017

The main issues spinel lithium titanate (Li4Ti5O12) anodes suffered from are poor electrical conductivity and low theoretical capacity, which impede the practical application of Li4Ti5O12 anodes in power lithium-ion batteries. Herein, the improvement in rate capability and specific capacity of Li4Ti5O12 anodes has been achieved by facile two-phase formation and nanostructure engineering strategy. When evaluated as anode material for lithium-ion batteries, the as-prepared TiO2 in-situ decorated Li4Ti5O12 nanobelts exhibit impressive performance, delivering a high reversible specific capacity of 185.1 and 161.2 mAh g
1 at 20 and 200 mA g
1 after 250 and 2000 cycles, respectively. More importantly, a capacity of 135.6 mAh g
1 could be retained at a high rate of 2000 mA g
1 even after 5000 cycles, showing excellent rate property and cycle life. Thus excellent performance may make them a promising anode material for advanced lithium-ion batteries. © 2017 Elsevier B.V. All rights reserved.

Keywords: Li4Ti5O12 In-situ decoration Dual-phase Anode materials Lithium-ion batteries

1. Introduction Search and development of new anode materials is one of the current research hotspots in lithium-ion batteries (LIBs) due to the predominantly used graphite cannot meet the urgent need of next generation LIBs for high power density and safety [1e5]. With the intrinsic characteristics of zero strain and safe operation potential, spinel Li4Ti5O12 (denoted as LTO) has attracted extensive research interest as a promisingly alternative anode material for high power LIBs [6e14]. However, its practical application is yet impeded by poor electrical conductivity and low theoretical capacity of 175 mAh g
1 [15e21]. To promote the development of LTO, great efforts have been devoted to design and explore effective strategies to improve the electrical conductivity and lithium storage of LTO anodes. It has been demonstrated that LTO anodes with high performance can be achieved by fabrication of proper nanostructures, because the elaborate low-dimensional nanostructure active materials have larger contact area with electrolyte, shorter diffusion distance of lithium-ions and electrons and special excess near-

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Tian), [email protected] (L. Yang). http://dx.doi.org/10.1016/j.jallcom.2017.02.175 0925-8388/© 2017 Elsevier B.V. All rights reserved.

surface lithium storage when compared with bulk LTO anodes [22e25]. Thus, design and preparation of nanoscale LTO materials with novel nanostructures had been regarded as an effective strategy for improving electrochemical performance of LTO anodes and, at the same time, became a hot research topic in LIBs anode field. For example, lots of low-dimensional nanostructure LTO materials have been reported, such as LTO nanowires [26], LTO nanotubes [27], LTO nanosheets [28], and so on. As a result, all of them exhibited higher lithium storage and better rate than pristine bulk LTO materials. Currently, it was found that engineering nanoscale grain boundaries by introduction of trace second phase TiO2 into desirable nanostructure LTO to form dual-phase LTO-TiO2 nanostructures could further enhance the electronic transport properties and lithium storage of electrodes due to the two aspects: one hand, the LTO-TiO2 nanostructures could reserve the characteristics of nanostructure LTO, namely large contact surface area with electrolyte, shorten diffusion path of ions and electrons and excess near-surface lithium storage; on the other hand, the generated grain boundaries between two phases would act as channels to transport electrons in the active materials, as well as provide more sites for lithium storage [29e32]. This was demonstrated in a number of studies that the electrochemical performance of LTO electrodes with dual-phase LTO-TiO2 nanostructures, in term of capacities and rate capabilities in particular, was significantly

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improved compared with the pure LTO electrodes, such as mesoporous spherical Li4Ti5O12/TiO2 [33], hierarchical carambola-like Li4Ti5O12/TiO2 composites [34], high grain boundary density Li4Ti5O12/anatase-TiO2 nanocomposites [35], Ag quantum dots promoted Li4Ti5O12/TiO2 nanosheets [36], porous Li4Ti5O12/TiO2 nanosheet arrays [37], copper-doped dual phase Li4Ti5O12/TiO2 nanosheets [38], and petal-like Li4Ti5O12/TiO2 nanosheets [30]. Thus, fabrication of two-phase LTO-TiO2 nanostructures with abundant grain boundaries is a wise strategy for effectively overcoming the issues LTO materials suffered from. However, facile preparation of LTO-based materials with proper dual-phase nanostructures is still a challenge. Herein, we present a facile approach for preparing TiO2 in-situ decorated LTO nanobelts (denoted as T-LTO NBs) by ionic exchange and followed by calcination in air. The as-prepared T-LTO NBs would have two main merits when used as anode material for LIBs: (1) The nanobelt structures have shortened diffusion distance of lithium-ions and electrons, increased contact interface with electrolyte and abundant surface lithium storage sites (pseudocapacitance or excess near-surface lithium storage); (2) the abundant grain boundaries between TiO2 and LTO would act as channels to transport electrons in the active materials to enhance electronic transport properties and lithium storage of electrodes. In consequence, the as-prepared T-LTO NBs exhibits impressive performance, delivering a high reversible specific capacity of 185.1 and 161.2 mAh g
1 at 20 and 200 mA g
1 after 250 and 2000 cycles, respectively. More importantly, a capacity of 135.6 mAh g
1 could be retained at a high rate of 2000 mA g
1 even after 5000 cycles, showing excellent rate property and cycle life. Thus excellent performance may make T-LTO NBs a promising anode material for advanced LIBs.

during hydrothermal process and subsequent washed with one time of anhydrous ethanol and two times of deionized water orderly, and finally calcination at air: 0.5 g of as-prepared nanobelt precursor was dispersed in 50 mL of 0.63 M LiOH by ultrasound for 0.5 h. The suspension was transferred into a Teflon-lined stainless steel autoclave, and then placed in an oven at 100  C for 24 h. After cooled down to room temperature naturally, the as-prepared white product was collected by centrifugation, washed with one time of anhydrous ethanol and two times of deionized water orderly, and dried in an oven at 60  C under vacuum overnight. Finally, the TLTO NBs was obtained via second ionic exchange product calcination at 500  C for 2 h under a ramping rate of 3  C min
1 in air.

2. Experimental section

The 2016-type coin cells, assembled in an argon-filled glove box (German, M. Braun Co., [O2] < 1 ppm, [H2O] < 1 ppm), were used to evaluate the electrochemical performances of as-prepared T-LTO NBs samples. The working electrodes were prepared via fully mixing active material (T-LTO NBs), conductive material (acetylene black, AB), and binder (sodium carboxymethyl cellulose, CMC) in a weight ratio of active material/AB/CMC ¼ 70/20/10 in right amount of deionized water, and then pasting the mixture on Cu foil to dry in a vacuum oven at 110  C overnight. The counter electrode and separator respectively used pure lithium disc and Cellgard 2400 membrane. The electrolyte used in this work is composed of a solution of LiPF6 (1 M) in dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 by volume). The CT2001a cell test instrument (LAND Electronic Co.) was used to test the galvanostatic discharge/ charge cycling performance of cells with a voltage range of 1.0e3.0 V at room temperature. The CHI660D electrochemical workstation was applied to record Cyclic Voltammograms (CVs) of cells at a certain scan rate between 1.0 and 3.0 V.

2.1. Preparation of T-LTO NBs sample First, the nanobelt precursor was prepared by hydrothermal reaction and subsequent proton exchange. Typically, a 3 mL of tetrabutyl titanate (TBT) was slowly dropped into 60 mL of 10 M NaOH aqueous solution. After continuously stirred for 30 min, the as-prepared white suspension was transferred into a Teflon-lined stainless steel autoclave, and then placed in an oven at 180  C for 24 h. After cooled down to room temperature naturally, the white product was collected by centrifugation, washed with deionized water and ethanol thoroughly, followed by dispersed in 50 mL of 0.1 M HCl and stirred for 24 h to exchange Naþ by Hþ completely. Then, the precursor obtained after ion exchange was collected by centrifugation, washed with deionized water and ethanol thoroughly, and dried in an oven at 60  C under vacuum overnight. Finally, the T-LTO NBs was prepared by second ionic exchange

2.2. Materials characterizations The field emission scanning electron microscopy (FE-SEM, JEOL JSM-7401F) and transmission electron microscopy (TEM, JEOL JEM2010) coupled with a selected area electron diffraction measurement (SAED) were applied to observe the microstructures of asprepared T-LTO NBs. The X-ray diffraction measurement (XRD, Rigaku, D/max-Rb using Cu K radiation) and X-ray photoelectron spectroscopy (XPS, carried out on an AXIS ULTRA DLD instrument with using aluminum K X-ray radiation) were used to measure the crystal structure and surface chemical states of T-LTO NBs samples. The nitrogen (N2) adsorption/desorption isotherms (Micromeritics ASAP 2010 instrument) were adopted to study the specific surface area of T-LTO NBs.

2.3. Electrochemical characterization

Fig. 1. (a, b) SEM and (c) TEM images of the as-prepared precursor.

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Fig. 2. (aec) SEM, (d, e) TEM and (f) HRTEM images of T-LTO NBs.

3. Results and discussion The morphologies and microstructures of as-prepared precursor were first studied by SEM and TEM characterizations, as shown in Fig. 1. It can be clearly seen that the precursor is composed of long one-dimensional nanobelts with smooth surface and a maximal diameter of less than 500 nm. Then, the SEM and TEM characterizations were adopted to investigate the nanostructure of T-LTO NBs. Fig. 2a, c show the different magnified SEM images of T-LTO NBs, it is clearly observed that the T-LTO NBs well reserves the morphology of precursor but the surface of nanobelts is rough. The change of surface indicates that the transform from precursor into LTO has happened. So, the TEM characterization was used to deeply insight into the nanostructure of T-LTO NBs, as displayed in Fig. 2d, e. Distinctly, the T-LTO NBs well reserves the nanobelt structure of precursor but the surface visibly becomes rough. The maximal diameters of T-LTO NBs are less than 500 nm, which is in good agreement with the size of precursor. Fig. 2f further gives the HRTEM image of T-LTO NBs

nanobelts, two sets of visible lattice fringes with spacing of 0.48 and 0.25 nm are observed and could be assigned to the d111spacing and d311-spacing of spinel Li4Ti5O12, respectively, which implies the high crystalline nature of T-LTO NBs after ionic exchange and calcination process [33]. The XRD and XPS measurements were applied to study the crystal structure and surface chemical states of T-LTO NBs. Fig. 3a gives the XRD patterns, besides one clear but weak diffraction peak is assigned to anatase TiO2 phase (JCPDS card no. 21-1272, marked as A), all the other diffraction peaks can be indexed into the standard peaks of spinel LTO (PDF no. 49-0207) [39,40]. The roundly relative content of anatase TiO2 in T-LTO NBs can be calculated from the XRD results according to the following equations [34]:

WA ¼

IA IA þ

IB KBA

Fig. 3. (a) XRD pattern, (b) high resolution XPS spectra and (c) N2 adsorption and desorption isotherms of T-LTO NBs.

(1)

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Fig. 4. (a) Cycling performance and (b) Galvanostatic discharge/charge profiles of T-LTO NBs electrodes at 20 mA g
1; (c) Cyclic voltammograms and (d) rate capability of T-LTO NBs electrodes.

KBA ¼

RB RA

(2)

Where WA is the weight percentage of anatase TiO2 in T-LTO NBs, and the IA and IB are obtained by calculation of the peak areas of anatase TiO2 (101) and spinel Li4Ti5O12 (111) peaks [41]. And the RA and RB represent for the K-value of anatase TiO2 and spinel Li4Ti5O12 crystal forms. In consequence, the roundly relative weight content of anatase TiO2 in T-LTO NBs is ca. 16.2%. The TiO2 may be derived from residual titanate analogue precursor which prone to transform into anatase TiO2 during calcination process under 500  C. It is worthy of noting that a number of studies demonstrated that abundant grain boundaries could be generated between two phases of LTO and TiO2, which would act as channels to transport electrons in the active materials, as well as provide more sites for lithium storage [33e38]. Besides, it was found that the ion diffusion coefficients for composite are higher than that of the pure constituents, which able to improve the electrochemical kinetics of electrodes [42e44]. The XPS characterization is used to study the surface chemical states of T-LTONBs. Fig. 3b shows the high resolution XPS spectra of Ti 2p element of T-LTO NBs, the peaks can be perfectly deconvoluted into two peaks centred at 464.2 and 458.5 eV, which correspond to the Ti 2p1/2 and Ti 2p3/2 core level binding energies of Ti4þ of spinel Li4Ti5O12, respectively [45,46]. As is well known, the surface area is one of the important characteristics of nanostructure materials. The BET surface area of T-LTO NBs

was measured by N2 adsorption and desorption isotherms, as shown in Fig. 3c. Based on the N2 adsorption and desorption isotherms, the BET surface area of T-LTO NBs is estimated to be 30.82 m2 g
1. After that, the electrochemical properties of as-prepared T-LTO NBs were studied by using the 2016-type coin cells assembled in an argon-filled glove box. Fig. 4a gives the cycling performance of TLTO NBs at a low current density of 20 mA g
1 (1 C is 175 mA g
1). It can be clearly seen from Fig. 4a that the T-LTO NBs electrode exhibits an outstanding lithium storage performance, delivering a capacity of 185.1 mAh g
1 after 250 cycles coupled with stable Coulombic efficiencies which almost constant at about 100% during this long-term cycle period. Fig. 4b gives the corresponding discharge/charge profiles of T-LTO NBs electrodes. They show a typical electrochemical behavior of dual-phase Li4Ti5O12-TiO2, the charge and discharge flat plateaus respectively at around 1.59 and 1.56 V corresponding to a two-phase reaction during electrochemical lithium extraction/insertion from/into Li4Ti5O12 (Li4Ti5O12 þ 3Liþ þ 3ee 4 Li7Ti5O12). The other weak charge/ discharge flat plateaus located at around 1.98/1.76 V corresponding to the redox reactions of the Ti4þ/Ti3þ couple in TiO2 (TiO2 þ xLiþ þ xe
4 LixTiO2), which is consistent with XRD characterization, namely achievement of in-situ introduction of TiO2 into LTO phase. After second cycle, the almost overlapped discharge/charge profiles imply that the T-LTO NBs has outstanding electrochemical reversibility. However, it is found that the flat

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Fig. 5. The long-term cycling performances of T-LTO NBs electrodes at different current densities.

plateaus of second discharge is greatly shortened compared with first discharge, which indicates a number of lithium-ions could not reversible extraction from Li4þxTi5O12 after one cycle, resulting in a low initial Coulombic efficiency of 55.1%. It was suggested that the tiny water adsorbed on specific surface area of active material and irreversible Liþ insertion sites (i.e., surface defects and voids) could be partially responsible for the large initial capacity loss, for example, the adsorbed water would generate oxygen by electrolytic reaction during charge/discharge process which would irreversibly consume a certain amount of lithium [47]. In fact, lots of previous work have demonstrated that the nanoscale LTO generally suffer from high irreversible capacity loss due to the excess near-surface lithium storage in nanoscale LTO resulting from lithium cointercalation at 8a-16c sites because of the likely enlarged

overpotential needed for spinel/rock-salt transformation to proceed [48e55]. Currently, Demopoulos et al. found that excessively intercalated lithium-ion at both 8a and 16c sites trigger nucleation of the relaxed LTO structure in the near-surface region, which hinders lithium-ion diffusion and causes the increasing polarization of LTO nanosheet electrodes, finally resulting in large capacity loss [56]. Fig. 4c further gives the Cyclic Voltammograms of T-LTO NBs electrode at a scan rate of 0.3 mV s
1 between 1.0 and 3.0 V vs. Li/Liþ. Obviously, it is in good agreement with galvanostatic discharge/charge profiles, showing a typical electrochemical lithium storage behavior of dual-phase Li4Ti5O12-TiO2 anodes. One pair of visible and dominant peaks is observed at 1.44/1.69 V besides a pair of weak peaks at 1.73/2.05 V, the former corresponding to the redox reactions of the Ti4þ/Ti3þ couple in Li4Ti5O12, and the

Table 1 Comparison of our T-LTO NBs with other reported dual-phase Li4Ti5O12/TiO2 anode materials. Anode materials

Potential cutoff (V)

Current density (mA g
1) (1C ¼ 175 mA g
1)

Cycles

Capacity (mAh g
1)

Reference

As-prepared T-LTO NBs As-prepared T-LTO NBs As-prepared T-LTO NBs Hierarchical Li4Ti5O12/TiO2 composite microsphere consisting of nanocrystals Dual-phase Li4Ti5O12/TiO2 nanowire arrays Mesoporous spherical Li4Ti5O12/TiO2 composites Hierarchical carambola-like Li4Ti5O12/TiO2 composites Li4Ti5O12/TiO2 composite Porous Li4Ti5O12/TiO2 nanosheet arrays 3 D rutile-TiO2 decorated Li4Ti5O12 nanosheet arrays Li4Ti5O12-TiO2-C

3e1 3e1 3e1 2.5e1

20 200 2000 10 C

250 2000 5000 100

185.1 161.2 135.6 120

This work This work This work [17]

3e1 3e1 3e1 2.5e1 3e1 3e1 3e1

10 C 2C 1C 5C 200 1C 10 C

100 120 200 100 100 130 100

129.3 145.3 171.7 140 172.4 183.6 110

[26] [33] [34] [57] [58] [59] [60]

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latter is assigned to the redox reactions of the Ti4þ/Ti3þ couple in TiO2. Fig. 4d shows the rate capabilities of T-LTO NBs at current density range from 50 to 4000 mA g
1. As a result, the capacity of 197.3, 169.4, 159.4, 150 and 145.8 mAh g
1 can be retained in fifth cycle at current density of 50, 300, 1000, 2000 and 4000 mA g
1, respectively. And then, a high specific capacity of 184.2 mAh g
1 can be restored at 100th cycle when the current density returns to 50 mA g
1, revealing superior rate performance. The excellent structure stability is one of the famous merits of LTO, so the ultralong term cycling performance of T-LTO NBs electrodes were evaluated at 200 and 2000 (ca. 11.4 C) mA g
1, as shown in Fig. 5a, b. Commendably, they respectively deliver high specific capacity of 161.2 and 135.6 mAh g
1 after 2000 and even 5000 cycles, demonstrating excellent cycling stability and outstanding rate capacity. In addition, compared with other reported dual-phase LTOTiO2 electrodes, our prepared T-LTO NBs also shows improved performance, as shown in Table 1, which indicates that the synergistic effect of nanobelt structure of T-LTO NBs and the TiO2 in-situ decoration should be responsible for thus outstanding performance. To effectively demonstrate the positive effect of TiO2 in-situ decoration on the electrochemical performance of T-LTO NBs, we compared the performance of T-LTO NBs with pure LTO nanobelts (P-LTO NBs). The preparation process of P-LTO NBs can be seen in Supporting Information. Fig. S1a shows the TEM image of P-LTO NBs, it can be found from that the P-LTO NBs has a similar structure with T-LTO NBs, composed of nanobelts. The XRD (Fig. S1c) and SAED (Fig. S1b) patterns demonstrate that the P-LTO NBs has a pure spinel Li4Ti5O12 phase. Fig. S2 gives the cycling performance of PLTO NBs electrodes at 20 mA g
1 between 1.0 and 3.0 V. After 100 cycles, the P-LTO NBs delivers a capacity of 154.3 mAh g
1, is much lower than that of T-LTO NBs. It is well demonstrated that the TiO2 in-situ decoration play a positive effect on improvement of lithium storage performance of T-LTO NBs. Thus, the excellent performance of T-LTO NBs electrode should be attributed to the synergistic effect between nanostructure and TiO2 in-situ introduction: one hand, the low-dimensional nanobelt structures have shortened diffusion distance of lithium-ions and electrons, increased contact interface with electrolyte and abundant surface lithium storage sites (pseudocapacitance or excess near-surface lithium storage); on the other hand, the abundant grain boundaries generated in interface between TiO2 and LTO would generate high ion diffusion coefficients and act as channels to transport electrons in the active materials to enhance electronic transport properties of electrodes.

4. Conclusions Herein, a facile approach is introduced to preparation of TiO2 insitu decorated LTO nanobelts by ionic exchange and followed by calcination in air. Thanks to the synergistic effect of lowdimensional nanostructure and anatase TiO2 in-situ decoration, the T-LTO NBs exhibits an outstanding electrochemical performance, delivering a high reversible specific capacity of 185.1 and 161.2 mAh g
1 at 20 and 200 mA g
1 after 250 and 2000 cycles, respectively. More importantly, a capacity of 135.6 mAh g
1 could be retained at a high rate of 2000 mA g
1 even after 5000 cycles, showing excellent rate property and cycle life. Thus excellent performance coupled with facile preparation process may make T-LTO NBs a promising anode material for advanced LIBs.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.02.175.

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