delithiation process

Journal of Power Sources 281 (2015) 56e68

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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Lithium storage mechanism in cubic lithium copper titanate anode material upon lithiation/delithiation process Wei Chen a, Zhengrong Zhou a, Hanfeng Liang a, Lianyi Shao b, Jie Shu b, *, Zhoucheng Wang a, * a b

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China

h i g h l i g h t s

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


Li2CuTi3O8 with the space group of Fd-3m is synthesized for the first time.
Li2CuTi3O8 exhibits stable host structure for lithium storage.
Li2CuTi3O8 can deliver a reversible capacity of 203 mAh g1 after 50 cycles.
Lithium storage mechanism in Li2CuTi3O8 is proposed for the first time.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 December 2014 Received in revised form 26 January 2015 Accepted 29 January 2015 Available online 30 January 2015

Complex spinel Li2CuTi3O8 with a space group of Fd-3m is prepared for the first time by a simple solid state reaction route. This compound has different space group from the previous reported Li2MTi3O8 (M ¼ Zn, Co, Mg, Ni and Mn) with a space group of P4332. It shows reversibility for lithium storage in the cubic structure. A reversible capacity of 203 mAh g1 can be delivered at a current density of 100 mA g1 after 50 cycles with the capacity retention of 70.7%. The electrochemical reaction mechanism between Li2CuTi3O8 and lithium is investigated by various in-situ and ex-situ observations. Rietveld refinement results show that Li2CuTi3O8 has multiple interstitials to accommodate lithium ions during the lithiation process, in which the irregular octahedral 32e sites would be occupied by lithium ions at high working potentials and the regular octahedral 16c sites would be occupied by lithium ions at low working potentials. Besides, a reversible migration of copper ions can found during discharge process. Based on the reverse delithiation process, it is known that the whole chargeedischarge process is quasi-reversible for Li2CuTi3O8. Therefore, Li2CuTi3O8 shows high structural stability as a promising lithium storage material for lithium-ion batteries. © 2015 Elsevier B.V. All rights reserved.

Keywords: Complex spinel titanate Anode material Lithium storage mechanism In-situ X-ray diffraction Lithium ion batteries

1. Introduction

* Corresponding authors. E-mail addresses: [email protected] (J. Shu), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jpowsour.2015.01.178 0378-7753/© 2015 Elsevier B.V. All rights reserved.

Nowadays lithium-ion batteries (LIBs) have dominated the electronics market due to their merits, such as high lithium storage capacity, good cycle property and outstanding safety. Today graphitic carbon is being widely used in commercial batteries as anode material [1e3]. With increasing of high power requirements,

W. Chen et al. / Journal of Power Sources 281 (2015) 56e68

Li2CuTi3O8 (Sim)

Intensity (a.u.)

Li2CuTi3O8 (Exp) Difference Observed Reflections

10

20

30

40

50

60

70

2theta (degree) Fig. 1. XRD refinement profile of as-synthesized Li2CuTi3O8. Table 1 Rietveld refinement data for Li2CuTi3O8 (a) with some empty 8a positions. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.3948(2) Å; Space group: P4332; Rp ¼ 9.46%; Rwp ¼ 11.42% 8a Li þ1 0 0 0 0.675 8a Cu þ2 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2 5/8 5/8 5/8 0.1 16d Ti þ4 5/8 5/8 5/8 0.75 32e O 2 0.382(1) 0.382(1) 0.382(1) 1

57

the low safety of graphitic carbon material induced from lithium dendrite at low potential restricts the further development of power batteries. Therefore, enormous attention is attracted in the development of new anode materials. To the best of our knowledge, Ti-based materials have been intensively investigated as alternative anode materials for LIBs due to their high safety and excellent cycling stability [4e9]. Among the typical Ti-based compounds, Li2MTi3O8 (M ¼ Zn, Co, Mg, Ni and Mn) is a novel zero-strain lithium storage anode for LIBs [10e18]. For instance, porous Li2MTi3O8 (M ¼ Zn, Co) can be prepared via a facile one-step solution-combustion and they showed a reversible capacity of >200 mAh g1 at a current density of 100 mA g1 [10]. Li2MgTi3O8 crystallized from solution has similar electrochemical behaviors and performances with those of Li2ZnTi3O8 and Li2CoTi3O8 [11]. For comparison, solid state formed Li2NiTi3O8 exhibits a higher rate capability and excellent cycling stability. As reported by Liu [12], Li2NiTi3O8 anode can provides a large reversible capacity of 212 mAh g1 at 0.1 A g1 after 10 cycles, which is close to its theoretical capacity (223 mAh g1). Even after 100 cycles, it still can deliver a quite high capacity of 204 mAh g1, with no significant capacity fading. Recently, we also have reported the preparation of complex spinel Li2MnTi3O8 by using the simple solegel and solid state reaction routes [13,14]. Li2MnTi3O8 formed from the solid state reaction route reveals excellent electrochemical property as anode material with a reversible capacity of 193.8 mAh g1 and corresponding capacity retention of 88.0% after 50 cycles. In contrast, solegel formed Li2MnTi3O8 showed the reversible capacity of 206.1 mAh g1 after 50 cycles, corresponding to 94.5% of the initial charge capacity. In addition, the electrochemical reaction mechanism between Li2MnTi3O8 and lithium is also investigated based on the results of various in-situ and ex-situ observations [13,14].

Fig. 2. (a) SEM, EDS, (b) TEM, (c) HRTEM, and (d) SAED images of Li2CuTi3O8.

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Table 2 The result of EDS.

Table 3 Equivalent circuit parameters of Li2CuTi3O8 before and after cycles.

Element

Intensity

Weight (%)

Atomic (%)

Sample

Rs (ohm)

Rf (ohm)

Rct (ohm)

OK Ti K Cu K Totals

0.4960 0.8959 0.8326

52.30 32.63 32.63 100.00

78.07 16.27 5.66

Before cycles After 50 cycles

5.93 11.98

0.00 83.61

51.79 103.00

2. Experimental 2.1. Synthesis

Based on these previous investigations, it is known that all the reported ordered spinels Li2MTi3O8 (M ¼ Zn, Co, Mg, Ni and Mn) show a space group of P4332. Few studies of disordered spinel anode materials Li2MTi3O8 with Fd-3m space group can be found in the previous reports. To our knowledge, spinel Li2CuTi3O8 as a lithium ion anode has not been reported until now. Moreover, the relation between the redox reaction in spinel Li2MTi3O8 with Fd3m space group and its structural evolution has also not been reported. In this work, a complex spinel Li2CuTi3O8 with the space group of Fd-3m is successfully synthesized by a solid phase method. The detailed structure and lithium storage properties of Li2CuTi3O8 are analyzed by Rietveld refinement and various electrochemical methods. The results of electrochemical measurements indicate that Li2CuTi3O8 exhibits high reversible capacity and excellent cycling stability. In addition, the lithium storage mechanism in Li2CuTi3O8 is presented for the first time upon lithiation/delithiation process.

The spinel Li2CuTi3O8 is prepared by a high temperature solid phase method. In a typical synthesis, stoichiometric amounts of Li2CO3, CuO and TiO2 are ground in an agate jar by ball-milling technique at a rotate speed of 400 rpm for 12 h. After being dried at 80  C, the as-obtained product is ball-milled to form powder at a speed of 400 rpm for 1 h. The powder is calcined at 750  C for 5 h in air to get Li2CuTi3O8. 2.2. Characterization The structure and phase analysis of as-obtained product is determined on a Rigaku Ultima IV X-ray diffractometer. The X-ray diffraction (XRD) pattern of the sample is collected by a step of 0.02 over the 2q range of 10e70 at room temperature. Scan electron microscope (SEM) image and energy dispersive spectrometer (EDS) are recorded on a LEO 1530 microscope to observe the morphology and element composition of material, respectively.

Fig. 3. (a) Cyclic voltammograms, (b) chargeedischarge profiles, (c) cycling performance and (d) EIS spectra of Li2CuTi3O8.

W. Chen et al. / Journal of Power Sources 281 (2015) 56e68

Transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) patterns are performed with a JEOL JEM-2100 high resolution transmission electron microscope at an accelerating voltage of 200 kV to check the morphology and structure of as-obtained material.

59

2.3. Electrode fabrication and electrochemical tests For electrode preparation, the active material, polyvinylidene binder and conductive carbon black are mixed at the mass ratio of 8:1:1. After dispersed in N-methyl-pyrrolidinone, the mixture was

Fig. 4. (a) In-situ XRD patterns of Li2CuTi3O8 during lithiation/delithiation process, and the change of diffraction peak intensity upon lithiation/delithiation process in different 2 theta ranges (b) 17.8 e 18.8 , (c) 34.0 e 36.5 and (d) 42.6 e 43.7.

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spread on the copper foil to form a film and then dried at 120  C for 10 h. The dried film is cut into discs with a diameter of 15 mm to make the working electrodes. In the coin-type cells, metal lithium disc is used as counter electrode and Whatman glass fiber filer is treated as separator. The chargeedischarge experiments are carried out at a current density of 100 mA g1 in the potential range of 0.0e3.0 V (vs. Liþ/Li). The cyclic voltammetry (CV) measurements are performed at a scan rate of 0.1 mV s1 in a potential range of 0.0e3.0 V. Electrochemical impedance spectroscopy (EIS) patterns are recorded on a CHI660 electrochemical workstation in the frequency range from 0.01 to 100000 Hz with an amplitude of 5 mV. 2.4. Mechanism research For lithium storage mechanism observation, ex-situ HRTEM and ex-situ SAED are performed with a JEOL JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. Ex-situ Xray photoelectron spectroscopy (XPS) measurements are carried out in a UHV system using a monochromatised Al Ka radiation (hn ¼ 1486.6 eV) and a Sphera II hemispherical electron energy analyzer. Ex-situ Raman spectra are determined by HORIBA XploRA employing a helium/neon laser at 532 nm. The spectra are collected in 20 s and the distinguishability is 2 cm1. 3. Results and discussion The XRD pattern of as-synthesized Li2CuTi3O8 powder is shown in Fig. 1. All the diffraction peaks can be indexed to be a cubic structure with the space group of Fd-3m (JCPDS Card No. 49e0448). The Rietveld refinement data obtained for Li2CuTi3O8 are displayed in Table 1. It can be found that the structure of Li2CuTi3O8 a spinel derivative, in which Cu, Ti and Li atoms occupy the octahedral 12d sites, and Cu and Li atoms share the tetrahedral crystallographic 8a sites. Therefore, the unit formula can be described in an expanded form as (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4. Besides, some lithium vacancies can be found in 8a sites owing to the volatilization of lithium during high temperature synthesis. The surface morphologies of as-prepared Li2CuTi3O8 are shown in Fig. 2. As shown in Fig. 2a, b, these particles are irregular bulk and their size is found to be 100e500 nm. In Fig. 2a, the Ti, Cu, O elements can be detected by EDS. Based on the analysis of the EDS, the element composition is displayed in Table 2. As shown in Table 2, the atomic ratio of Cu to Ti is nearly equal to a stoichiometric value of 1:3, which is in accord with the ideal result in Li2CuTi3O8. This indicates that the complex spinel titanate Li2CuTi3O8 has been successively synthesized. The detailed crystal structure of Li2CuTi3O8 is shown in Fig. 2c, d. A clear and continuous lattice fringe can be observed in Fig. 2c, which demonstrates that the synthesized Li2CuTi3O8 is well crystallized. The lattice fringe is found to be 0.485 nm which is corresponding to the d111-spacing in XRD pattern of Li2CuTi3O8. The result can be verified by SAED image in which the (111) plane is found in Fig. 2d. Thus, the SAED pattern further confirms the cubic structure of the synthesized Li2CuTi3O8 product. In order to understand the electrochemical behavior of Li2CuTi3O8 during lithiation and de-lithiation cycles, the CVs of Li2CuTi3O8 are presented. As shown in Fig. 3a, it can be observed that

61

five reduction peaks appear at 0.00, 0.50, 0.67, 1.47 and 2.61 V, and four oxidation peaks appear at 0.38, 0.94, 1.7 and 2.50 V in the first cycle. However, the reduction peaks at 0.50 and 2.61 V disappear in the next scan. The irreversible reduction peak at 0.50 V is attributed to the electrolyte irreversible decomposition and the formation of solid electrolyte interface (SEI) film [19e21]. The cathodic peak at 2.61 V observed for Li2CuTi3O8 in the first cycle is probably related to the occupation of lithium vacancy in the compound, which is formed due to the volatilization of lithium during high temperature synthesis. In the second and subsequent cycles, a main redox couple is observed at 1.47/1.70 V which is related to Ti4þ/Ti3þ, indicating that lithium-ion intercalation/ deintercalation into/out of the spinel Li2CuTi3O8 is highly reversible. In addition, other two weak redox couples at 0.00/0.38 V and 0.67/0.94 V can be related to lithium-ion intercalation and deintercalation in the different sites. These electrochemical behaviors are different with those of Li2MTi3O8 with the space group of P4332 [13,14]. The detailed lithium storage mechanism can be proved by various in-situ and ex-situ methods in the following section. Fig. 3b shows the chargeedischarge profiles of Li2CuTi3O8. During discharge process of the first cycle, a short slop appears at 2.6 V owing to lithium-ion storage in 8a sites. The flat platform appears at 1.55 V owing to the reduction of Ti4þ to Ti3þ in Li2CuTi3O8 and the slop appears between 0.0 and 1.5 V owing to the probable formation of solid solution and SEI film during lithiation process. This electrochemical reaction is quite different from that of Li2ZnTi3O8 or Li2CoTi3O8 [10e12,22,23]. Upon recharge process, the flat platform is maintained but the slope shortens, which suggests that the main irreversible capacity comes from the dead lithium in the structure and electrolyte irreversible decomposition at low potentials. This is similar to the electrochemical behaviors of CV result as shown in Fig. 3a. It can be found that the initial discharge and charge capacities are respectively 436 and 300 mAh g1 as shown in Fig. 3c. After 50 cycles, a reversible charge capacity of 203 mAh g1 can be retained with the capacity retention of 70.7%. The excellent cycling performance is benefited from its structure. The structure of (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4 can provide a stable frame built by TiO6 and Li(Cu)O6 octahedrons during lithiation and de-lithiation process, in which 16d octahedral sites are occupied by 1:3 cation ordering of Li(Cu)/Ti. As a result, threedimensional tunnels are formed in this structure, where 7:3 of Li to Cu ions randomly shares the 8a tetrahedral sites in the tunnels. Thus, three-dimensional tunnels could provide diffusion routes for lithium ion insertion/extraction in the spinel structure reversibly as anode material. Besides, the average cycling coulombic efficiency of over 98% verifies the excellent electrochemical performance of Li2CuTi3O8. Electrochemical impedance spectroscopy is a good tool to detect the changes of electrode kinetics of electrode material before and after cycles. Here, the EIS patterns of Li2CuTi3O8 before and after 50 cycles are shown in Fig. 3d and they are fitted by using an equivalent circuit. It is clear that the Nyquist plot of the fresh electrode is composed of a small intercept in the high frequency range, a semicircle in the medium frequency range and a straight line at the low frequency zone, which are related to the electrolyte resistance (Re), charge transfer resistance (Rct) and Warburg resistance (W), respectively [24,25]. After 50 cycles, another semi-circle at medium

Fig. 5. Rietveld refinement patterns for Li2CuTi3O8 (a) with some empty 8a positions, (b) with 8a positions fully occupied by lithium-ion, (c) with 8a positions fully occupied and 8b positions saturatedly occupied by lithium-ion, (d) with 32e positions saturatedly occupied by lithium-ion, (e) with 16c positions fully occupied by lithium-ion, (f) with 16c positions fully occupied and 32e positions partially occupied by lithium-ion during discharge process, and (g) with 16c positions fully occupied by lithium-ion, (h) with 32e positions saturatedly occupied by lithium-ion shifting from 16c position, (i) with 8a positions fully re-occupied and 8b positions saturatedly re-occupied by lithium-ion shifting from 32e positions during charge process.

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Table 4 Rietveld refinement data for Li2CuTi3O8 with 8a positions fully occupied by lithiumion during discharge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.40953(2) Å; Space group: P4332; Rp ¼ 5.32%; Rwp ¼ 7.58% 8a Li þ1 0 0 0 0.7 8a Cu þ2 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2 5/8 5/8 5/8 0.1 16d Ti þ4,þ3 5/8 5/8 5/8 0.75 32e O 2 0.387(1) 0.387(1) 0.387(1) 1

Table 5 Rietveld refinement data for Li2CuTi3O8 with 8a positions fully occupied and 8b positions saturatedly occupied by lithium-ion during discharge process. Site

Atom

Valence

x

Y

z

Occupancy

a ¼ b ¼ c ¼ 8.38688(1) Å; Space group: P4332; Rp ¼ 5.25%; Rwp ¼ 7.26% 8a Li þ1 0 0 0 0.7 8b Li þ1 0.5 0.5 0.5 0.2 8a Cu þ2 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2 5/8 5/8 5/8 0.1 16d Ti þ4,þ3 5/8 5/8 5/8 0.75 32e O 2 0.384(1) 0.384(1) 0.384(1) 1

Table 6 Rietveld refinement data for Li2CuTi3O8 with 32e positions saturatedly occupied by lithium-ion during discharge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.37057(2) Å; Space group: P4332; Rp ¼ 5.09%; Rwp ¼ 7.11% 32e Li þ1 0.105(9) 0.105(9) 0.105(9) 0.32 32e Cu þ2 0.050(6) 0.050(6) 0.050(6) 0.075 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2 5/8 5/8 5/8 0.1 16d Ti þ3 5/8 5/8 5/8 0.75 32e O 2 0.387(1) 0.387(1) 0.387(1) 1

frequency appears, which corresponds to the formation of SEI film (Rf). The detailed electrochemical parameters after fitting are listed in Table 3. Compared with data of fresh electrode, the Re, Rf and Rct of Li2CuTi3O8 electrode become larger after 50 cycles. Especially for Rf, it increases from 0.0 to 83.61 U. It indicates that the main capacity loss comes from the electrolyte irreversible decomposition. Besides, the dead lithium in the structure also makes the increase of Rct from 51.79 to 103.0 U. It indicates that the structural evolution of Li2CuTi3O8 during chargeedischarge cycles is a quasi-reversible reaction. In order to examine the structural changes of Li2CuTi3O8 during chargeedischarge cycles, in-situ XRD diffraction patterns are collected during the 1st cycle between 0.0 and 3.0 V. Till now, no insitu XRD evolution of Li2CuTi3O8 anode with space group of Fd-3m has been reported. As shown in Fig. 4a, all the XRD patterns show

Table 7 Rietveld refinement data for Li2CuTi3O8 with 16c positions fully occupied by lithiumion during discharge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.3636(4) Å; Space group: P4332; Rp ¼ 5.29%; Rwp ¼ 7.34% 16c Li þ1 0.125 0.125 0.125 1 8a Cu þ2,þ1 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2,þ1 5/8 5/8 5/8 0.1 16d Ti þ3 5/8 5/8 5/8 0.75 32e O 2 0.380(1) 0.380(1) 0.380(1) 1

Table 8 Rietveld refinement data for Li2CuTi3O8 with 16c positions fully occupied and 32e positions partially occupied by lithium-ion during discharge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.40594(3) Å; Space group: P4332; Rp ¼ 5.27%; Rwp ¼ 7.30% 16c Li þ1 0.125 0.125 0.125 1 32e Li þ1 0.105(6) 0.105(6) 0.105(6) 0.169 8a Cu þ1 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ1 5/8 5/8 5/8 0.1 16d Ti þ3 5/8 5/8 5/8 0.75 32e O 2 0.377(3) 0.377(3) 0.377(3) 1

Table 9 Rietveld refinement data for Li2CuTi3O8 with 16c positions fully occupied by lithiumion during charge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.36332(2) Å; Space group: P4332; Rp ¼ 5.20%; Rwp ¼ 7.24% 16c Li þ1 0.125 0.125 0.125 1 8a Cu þ2,þ1 0 0 0 0.3 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2,þ1 5/8 5/8 5/8 0.1 16d Ti þ3 5/8 5/8 5/8 0.75 32e O 2 0.384(2) 0.384(2) 0.384(2) 1

Table 10 Rietveld refinement data for Li2CuTi3O8 with 32e positions saturatedly occupied by lithium-ion shifting from 16c position during charge process. Site

Atom

Valence

x

y

z

Occupancy

a ¼ b ¼ c ¼ 8.36962(3) Å; Space group: P4332; Rp ¼ 5.08%; Rwp ¼ 6.98% 32e Li þ1 0.105(9) 0.105(9) 0.105(9) 0.31 32e Cu þ2 0.049(5) 0.049(5) 0.049(5) 0.075 16d Li þ1 5/8 5/8 5/8 0.15 16d Cu þ2 5/8 5/8 5/8 0.1 16d Ti þ3 5/8 5/8 5/8 0.75 32e O 2 0.381(1) 0.381(1) 0.381(1) 1

continuous shifting upon lithiation and they can return the original positions along with the same deviation path upon recharge process. However, the end product is spinel Li2CuTi3O8 without lithium-ion vacancy in the structure as the Rietveld refinement pattern for delithiated Li2CuTi3O8 shown in Fig. 5, which shows slight difference of Bragg positions with that of pristine sample. It shows that the structural change of Li2CuTi3O8 is a quasi-reversible process during lithiation and delithiation. The result also confirms that the as-obtained Li2CuTi3O8 has stable structure for lithium storage, which is in agreement with the results of the electrochemical measurements in Fig. 3. Moreover, intensity decreasing can be observed for the main peaks of (111), (311) and (400) before and after lithiation/delithiation as shown in Fig. 4bed, which

Table 11 Rietveld refinement data for Li2CuTi3O8 with 8a positions fully re-occupied and 8b positions saturatedly re-occupied by lithium-ion shifting from 32e positions during charge process. Site

Atom

Valence

x

y

z

a ¼ b ¼ c ¼ 8.37736(1) Å; Space group: P4332; Rp ¼ 5.02%; Rwp ¼ 8a Li þ1 0 0 0 8b Li þ1 0.5 0.5 0.5 8a Cu þ2 0 0 0 16d Li þ1 5/8 5/8 5/8 16d Cu þ2 5/8 5/8 5/8 16d Ti þ4 5/8 5/8 5/8 32e O 2 0.377(1) 0.377(1)) 0.377(1)

Occupancy 6.89% 0.7 0.2 0.3 0.15 0.1 0.75 1

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63

Fig. 6. Lattice parameter evolution of (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4 during lithiation and delithiation process.

suggests that partial lithium trapped in the structure can not be extracted upon recharge process. Moreover, peak shrinking can be observed at 35.2 and peak broadening can be detected at 42.8 after delithiation. The result implies that the cation in (311) plane corresponding to 35.2 may migrate to the sites in (400) plane corresponding to 42.8 . To confirm the model of lithiation/delithiation mechanism for Li2CuTi3O8, the XRD patterns taken from in-situ XRD test are chosen to perform Rietveld refinements. In Fig. 5, the diffraction peaks at 38.4 , 41.3 and 44.0 are attributed to characteristic peaks of BeO, which is produced from the electrochemical oxidation on the X-day window of Be disc during previous work. As shown in Fig. 5aei, nine XRD patterns for different lithiated and delithiated Li2CuTi3O8 samples are calculated and the detailed Rietveld refinement data are displayed in Tables 1, 4e11. According to Rietveld refinement, the lattice parameters of Li2CuTi3O8 upon lithiation/delithiation process are calculated. As described in Fig. 1, Li2CuTi3O8 can be described in an expanded form as (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4. To describe the structure clearly, (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4 is used to describe this sample. Based on the chemical formula of (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4, this sample can theoretically accommodate two lithium ions per formula at most during galvanostatic discharge process. Here, x is used to describe the lithium content in (Li0.7Cu0.3)tet [Li0.3Cu0.2Ti1.5]octO4 during lithiation/delithiation process. As shown in Fig. 6, it changes from 0.0 to 2.0 in (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4. The plot in Fig. 6 means the evolution of lattice parameter value at different lithiated and delithiated states. The red lines are used for linear fit in each region (in web version). As shown in Fig. 6, the lattice parameter evolution during lithiation and de-lithiation process performs a good mirror symmetry, indicating the electrochemical reaction in Li2CuTi3O8 between discharge and charge process is quasi-reversible. To the best of our knowledge, the lattice parameter evolution exhibits a continuous process in the solid solution transformation region but shows a non-continuous process in the two-phase transition region. In Fig. 6, there are two continuous slopes and two flat platforms of lattice parameter evolution in the discharge process, indicating that the probable appearance of two solid solution transformations (0 < x < 0.27, 1.42 < x < 2.0) and one two-phase transition reaction during the lithiation process. Upon few lithium ions insertion, the lattice parameter of Li2CuTi3O8 shows a slight increase as shown in Fig. 6. It indicates that the unit cell of Li2CuTi3O8 swells when lithium-ion occupies the as-formed 8a vacancies. Upon further lithiation, the lattice

parameter appears a continuous shrink process due to the occupation of 8b position by lithium ions. In the structure of Li2CuTi3O8, 8b position is equivalent to 8a position in chemical environment as

Fig. 7. Ex-situ XPS patterns of Li2CuTi3O8, (a) Cu spectra and (b) Ti spectra.

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shown in Tables 4 and 5 Thus, the structural transformation is a solid solution process with the maintaining of spinel phase. Due to energy barrier, 8b sites can not contain too many lithium ions in the following lithiation process. Based on the analysis on Rietveld refinement in Fig. 5d and Table 6, it can be found that the inserted lithium ions begin to take the positions in 32e sites and the lithium ions in 8a and 8b sites also move to 32e positions to form a new cubic phase (Fd-3m) with lower energy upon further lithiation

(x > 0.25) [26]. The appearance of new phase is also confirmed by the first flat platform of lattice parameter evolution in Fig. 6. Furthermore, the migration of copper ions is proposed from 8a position to 32e position based on the structural stability. Similar migration of Cu2þ can be observed in CuCr2S4 anode [27]. With x > 0.8 in (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4 during lithiation, the new formed cubic phase transforms into the rock-salt phase, in which the 16c positions are occupied by the lithium ions shifting

Fig. 8. (a) HRTEM, (c) SAED, (e) their structural simulation images of pristine Li2CuTi3O8, and (b) HRTEM, (d) SAED, (f) their structural simulation images of lithiated Li2CuTi3O8 after a discharge to 0.0 V.

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from 32e sites and counter electrode as described by the Rietveld refinement data in Table 7. In addition, a reverse migration of copper ions is proposed from 32e position to 8e position for lower energy occupation sites. As a result, the second platform appears in the plot of lattice parameter evolution versus lithium content in (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4. When x > 1.4, the second solid solution process takes place, which is associated with the full occupation of 16c sites and partial occupation of 32e sites by lithium-ion in the structure as the Rietveld refinement data shown in Table 8. The additional lithium-ion storage in 32e sites leads to the increase of the lattice parameter at the final lithiation. Upon delithiation process, lithium ions initially extract from 32e sites and then migrate from 16c to 32e positions as shown in Fig. 5geh and Tables 9 and 10 With further delithiation, the 8a positions are fully re-occupied and 8b positions are saturatedly re-occupied by lithium-ion shifting from 32e positions during charge process (Fig. 5i and Table 11). It can be found that the final product is cubic Li2CuTi3O8 with lithium occupied 8a and 8b sites, which shows slight difference of Bragg positions with that of pristine sample. It tells that the structural change of Li2CuTi3O8 is a quasi-reversible process during lithiation and delithiation. To support the conclusion obtained from in-situ XRD, Li2CuTi3O8 samples with different lithiated and delithiated states are observed by ex-situ XPS observation. In Fig. 7a, the valence change of Ti ion is analyzed according to evolution of Ti 2p3/2 peak. It is obvious that the Ti 2p3/2 peak of the initial material is located at a binding energy of 458.6 eV, indicating the existence of Ti4þ in Li2CuTi3O8. With a gradual lithiation, the shifting of Ti 2p3/2 peak can be observed from 2.2 to 0.0 V. The appearance of Ti 2p3/2 peak at 457.3 eV indicates the reduction of Ti4þ to Ti3þ in Li2CuTi3O8 during discharge process. After a discharge process to 0.75 V, almost all the Ti4þ ions in the compound convert into Ti3þ. It suggests that the further lithiation may be related to the valence change of other ions in Li2CuTi3O8. The onset evolution of Cu 2p3/2 peak from 933.6 eV (Cu2þ) to 932.8 eV (Cuþ) indicates the reduction of Cu2þ to Cuþ appearing at 0.75 V during the discharge process as shown in Fig. 7b. This phenomenon supports the migration of Cu ions between 8a and 32e position as described by Rietveld refinement in Tables 5e7 It also tells that partial lithium storage capacity comes from the reversible transformation of Cu2þ/Cuþ redox couple. Upon delithiation, Cuþ is fully oxidized into Cu2þ in the potential range of 0.0e0.6 V (Fig. 7b) and then Ti3þ starts to oxidize into Ti4þ at 0.6 V with the onset change of Ti 2p3/2 peak from 457.3 to 458.6 eV (Fig. 7a). The reverse redox couples of Ti4þ/Ti3þ and Cu2þ/Cuþ prove the reversible phase transformations during lithiation/delithiation process. This result is in agreement with the results of electrochemical measurements and in-situ XRD. To further confirm the electrochemical reaction mechanism of spinel Li2CuTi3O8, fully lithiated samples are subjected to ex-situ HRTEM and ex-situ SAED observations. In Fig. 8a, the lattice fringes of pristine Li2CuTi3O8 are found at 0.210 and 0.2531 nm, corresponding to the (400) and (311) planes of cubic compound with the space group of Fd-3m. This result can be verified by SAED image in which the (400) and (311) planes are found in Fig. 8c. After a discharge process to 0.0 V, only one lattice fringe of 0.210 nm is observed for lithiated Li2CuTi3O8 as shown in Fig. 8b. The result can be verified by SAED image in which the (400) plane is found in Fig. 8d. According to the JCPDS Card No. 49-0448, it is known that the (400) plane is corresponding to the diffraction peak at 43.2 . It indicates that the diffraction peak at 43.2 becomes the strongest peak among all the characteristic peaks of Li2CuTi3O8 after a full lithiation. In contrast, the diffraction peak at 35.5 which corresponds to (311) plane becomes weak. In unit cell of Li2CuTi3O8 as shown Fig. 8e, the (311) plane is made up of the oxygen atoms at

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32e sites, lithium and copper atoms at 8a sites. The weakening of diffraction peak at 35.5 indicates that the lithium-ion and/or copper-ion in (311) plane may migrate out from 8a sites during lithiation process, which is in consistent with the Rietveld refinement result in Table 6. By a comparison of crystal structure in Fig. 8e, f, it is clear that most of the tetrahedrons finally transform into octahedrons upon a full lithiation. This also results in the

Fig. 9. Ex-situ Raman spectra of Li2CuTi3O8. (a) pristine material, (b) discharge to 2.2 V, (c) discharge to 1.2 V, (d) discharge to 0.4 V, (e) discharge to 0.0 V, (f) charge to 0.6 V and (g) charge to 3.0 V.

Fig. 10. The evolution of structure during the lithiation process. (a) lithium-ion in 8a positions, (b) lithium-ion in 32e positions, (c) lithium-ion in 16c positions, (d) the migration path of lithium-ion and copper-ion during the discharge process.

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strengthening of (400) plane with a lattice fringe of 0.2101 nm as shown in Fig. 8f. Raman spectroscopy is a sensitive technology to distinguish structural information during chargeedischarge cycle. Here, Li2CuTi3O8 with different lithiated and delithiated states are carried out for Raman measurement as shown in Fig. 9. In the Raman spectra of Li2CuTi3O8, the frequencies in the 250e400 cm1 region are attributed to the LieO stretching in LiO6 octahedra, the frequencies in the 400550 cm1 region are contributed to the Li(Cu)eO stretching in LiO4 or CuO4 tetrahedra, and the frequencies in the 550700 cm1 region are ascribable to the Ti(Cu)eO stretching in TiO6 (CuO6) octahedra [28,29]. Upon few lithium ions insertion, a Raman red shift can be detected from 410 to 397 cm1, indicating the lithium-ion occupation at 8a and 8b tetrahedral positions induce the slight change of LiO4 tetrahedra. With further lithiation, a new band can be observed at 275 cm1, indicating a structural transformation from LiO4 (CuO4) tetrahedra to quasi LiO6 (CuO6) octahedra due to the lithium-ion and copper-ion migration from 8a (8b) to 32e positions. This also can be reflected from the evolution of the length of LieO bond during lithiation. According to the Rietveld refinement, the length of LieO bond in LiO4 tetrahedra is 1.8165 Å. After lithium-ion storage at 32e positions, the length of LieO bond changes into 1.9793 and 2.2971 Å in quasi LiO6 octahedra. Upon deeper lithiation, the blue shift of LieO band in LiO6 octahedra from 275 to 280 cm1 and Li(Cu)eO band in LiO4 (CuO4) tetrahedra from 384 to 387 cm1 confirms the lithium-ion migration from octahedral 32e positions to octahedral 16c positions and the copper-ion migration from octahedral 32e positions to tetrahedral 8a positions. After a full delithiation process, three Raman bands are detected at 391, 507 and 630 cm1, which are slight different from those of the pristine sample. It indicates that some inserted lithium ions cannot be totally extracted from the host structure. Therefore, Raman result also suggests that the structural change of Li2CuTi3O8 is a quasi-reversible process during lithiation and delithiation. Based on the above analysis, the lithium storage mechanism in (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4 can be further described by the structural models in Fig. 10. As the spinel structure shown in Fig. 10a, O atoms occupy the 32e sites and Li, Cu, Ti atoms occupy the 16d sites to form a [Li0.3Cu0.2Ti1.5]octO4 framework which provides a stable frame during lithiation and de-lithiation process. In the structure of (Li0.7Cu0.3)tet[Li0.3Cu0.2Ti1.5]octO4, face-shared tetrahedral 8a and octahedral 32e, 16c interstitial positions in the lattice can provide the 3D network channels for ion moving. Results show that Li2CuTi3O8 has multiple interstitials to accommodate lithium ions during the lithiation process, which is in agreement with the CV results (Fig. 3a). Upon lithiation, lithium ions in the tetrahedral 8a sites migrate to the irregular quasi-octahedral 32e sites at high working potentials (1.47 V) as shown in Fig. 10b. This migration makes the interstitial tetrahedron change into the quasioctahedron. As a result, 32e site is not the centre of octahedron after discharge process. Upon further lithiation, lithium ions in tetrahedral 8a sites can migrate to the regular octahedral 16c sites at low working potentials (0.67 V) as shown in Fig. 10c. Here, the 16c sites are the centre of octahedron. Actually, copper-ion at tetrahedral 8a sites can migrate accompanied by the migration of lithium-ion during lithiation process. To observe the migration path of lithium-ion and copper-ion, a planar atom arrangement structure is built, in which 8a, 32e and 16c sites are found to stand in a straight line in Fig. 10d. Moreover, the weak redox couple at 0.00/ 0.38 V in CV curves also verifies a reversible migration of copper ions. Accompanied by the lithium-ion migration from 8a to 32e sites, the migration of copper ions also takes place from 8a to 32e positions. These copper ions can return to 8a positions after a full lithiation with lithium ions holding the 16c sites. Based on the

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reversible migration of lithium-ion and copper-ion in the structure, Li2CuTi3O8 shows the potential as anode material for lithium-ion batteries. 4. Conclusions In this article, complex spinel Li2CuTi3O8 with a space group of Fd-3m is prepared successfully via a simple synthetic route. Electrochemical tests show that Li2CuTi3O8 can deliver a reversible capacity of 203 mAh g1 after 50 cycles with capacity retention of 70.7%. The structural evolution of Li2CuTi3O8 upon lithiation/delithiation is observed by in-situ XRD, ex-situ XPS, ex-situ HRTEM, exsitu SAED and ex-situ Raman techniques. It is found that the lithium storage mechanism Li2CuTi3O8 may be related to in the probable appearance of two solid solution transformations and one twophase transition reaction during the lithiation process. Structural analysis shows that lithium ions initially take the vacancies at 8a positions, and then move to 8b positions. After saturated occupation at 8b positions, the inserted lithium ions begin to take the 32e positions and the lithium ions in 8a and 8b positions also move to 32e positions. At the same time, the migration of Cu2þ takes place from 8a to 32e positions based on the structural stability. Upon further lithiation, the 16c positions are occupied by the lithium ions shifting from 32e positions and counter electrode, accompanying by a reverse migration of Cu2þ from 32e to 8e positions. During the delithiation process, almost a reverse migration for lithium ion and copper ions can be observed except for the trapped lithium in 8a vacancies and 8b positions. It tells that the structural change of Li2CuTi3O8 is a quasi-reversible process during lithiation and delithiation. Acknowledgments The authors thank to the National Natural Science Foundation of China (Grant no. 51372212), Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201308SIC) and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201408). References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359e367. [2] Y. Wu, E. Rahm, R. Holze, J. Power Sources 114 (2003) 228e236. [3] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [4] W. Lu, I. Belharouak, J. Liu, K. Amine, J. Electrochem. Soc. 154 (2007) 114e118. [5] Z. Hong, X. Zheng, X. Ding, L. Jiang, M. Wei, Energy Environ. Sci. 4 (2011) 1886e1891. [6] M. Wagemaker, E. Eck, A. Kentgens, F.M. Mulder, J. Phys. Chem. B 113 (2009) 224e230. [7] S. Scharner, W. Weppner, P.S. Beurmann, J. Electrochem. Soc. 146 (1999) 857e861. [8] G. Zhu, Y. Wang, Y. Xia, Energy Environ. Sci. 5 (2012) 6652e6667. [9] C.F. Lin, M.O. Lai, L. Lu, H.H. Zhou, J. Phys. Chem. C 118 (2014) 14246e14255. [10] X. Li, Q. Xiao, B. Liu, H.C. Lin, J.B. Zhao, J. Power Sources 273 (2015) 128e135. [11] Z. Hong, T. Lan, Y. Zheng, L. Jiang, M. Wei, Funct. Mater. Lett. 4 (2011) 65e70. [12] J. Liu, C.Q. Du, Z.Y. Tang, Ionics 20 (2014) 1495e1500. [13] W. Chen, H.F. Liang, L.Y. Shao, J. Shu, Z.C. Wang, Electrochim. Acta 152 (2015) 187e194. [14] W. Chen, R.H. Du, H.F. Liang, Z.R. Zhou, L.Y. Shao, J. Shu, Z.C. Wang, J. Power Sources 272 (2014) 622e628. [15] L. Wang, Q.Z. Xiao, Z.H. Li, G.T. Lei, L.J. Wu, P. Zhang, J. Mao, Electrochim. Acta 77 (2012) 77e82. [16] H.Q. Tang, Z.Y. Tang, C.Q. Du, F.C. Qie, J.T. Zhu, Electrochim. Acta 120 (2014) 187e192. [17] H. Kawai, M. Tabuchi, M. Nagata, H. Tukamoto, A.R. West, J. Mater. Chem. 8 (1998) 1273e1280. [18] L. Shen, H. Li, E. Uchaker, X. Zhang, G. Cao, Nano Lett. 12 (2012) 5673e5678. [19] J. Deng, G.J. Wagner, R.P. Muller, J. Electrochem. Soc. 161 (2014) 487e496. [20] Y. Tang, L. Yang, Z. Qiu, J. Huang, Electrochem. Commun. 10 (2008) 1513e1516.

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[21] W.J.H. Borghols, M. Wagemaker, U. Lafont, E.M. Kelder, F.M. Mulder, J. Am. Chem. Soc. 131 (2009) 17786e17792. [22] W. Chen, R.H. Du, W.J. Ren, H.F. Liang, B.B. Xu, J. Shu, Z.C. Wang, Ceram. Int. 40 (2014) 13757e13761. [23] W. Chen, H.F. Liang, W.J. Ren, L.Y. Shao, J. Shu, Z.C. Wang, J. Alloys Compd. 611 (2014) 65e73. [24] J. Wang, Z. Wang, X. Li, H. Guo, W. Xiao, S. Huang, Z. He, J. Solid State Chem. 17 (2013) 1e8. [25] S. Chou, J. Wang, J. Sun, D. Wexler, M. Forsyth, H. Liu, Chem. Mater. 20 (2008)

7044e7051. [26] J.P. Kartha, D.P. Tunstall, J.T.S. Irvine, J. Solid State Chem. 152 (2000) 397e402. [27] V. Bodenez, L. Dupont, L. Laffont, A.R. Armstrong, K.M. Shaju, P.G. Bruce, J.M. Tarascon, J. Mater. Chem. 17 (2007) 3238e3247. [28] S. Chou, J. Wang, H. Liu, H. Liu, S. Dou, J. Phys. Chem. C 115 (2011) 16220e16227. [29] L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J.O. Fourcade, J.L. Tirado, J.I. Corredor, C.P. Vicente, Chem. Mater. 16 (2004) 5721e5725.