Growth of Lithium Lanthanum Titanate Nanosheets and Their

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Growth of Lithium Lanthanum Titanate Nanosheets and Their Application in Lithium Ions Batteries Xi Lin, Hongqiang Wang, Haiwei Du, Xinrun Xiong, Bo Qu, Zaiping Guo, and Dewei Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10877 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

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ACS Applied Materials & Interfaces

Growth of Lithium Lanthanum Titanate Nanosheets and Their Application in Lithium Ions Batteries Xi Lin a,#, Hongqiang Wang b,#, Haiwei Du a, Xinrun Xiong a, Bo Qu a, Zaiping Guo b,*, Dewei Chu a,∗ a

School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, NSW, Australia b

Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia

Abstract In this work, lithium doped lanthanum titanate (LLTO) nanosheets have been prepared by a facile hydrothermal approach. It is found that with the incorporation of lithium ions, the morphology of the product transfers from rectangular nanosheets to irregular nanosheets, along with a transition from La2Ti2O7 to Li0.5La0.5TiO3. The as-prepared LLTO nanosheets are used to enhance electrochemical performance of the LiCo1/3Ni1/3Mn1/3O2 (CNM) electrode by forming higher lithium ion conductive network. The LiCo1/3Ni1/3Mn1/3O2-Li0.5La0.5TiO3 (CNM-LLTO) electrode shows better lithium diffusion coefficient of 1.5 × 10-15 cm2 S-1, resulting from higher lithium ion conductivity of LLTO and shorter lithium diffusion path, compared with the lithium diffusion coefficient of CNM electrode (5.44 × 10-16 cm2 S-1). Superior reversibility and stability are also found in the CNM-LLTO electrode, which retains a capacity at 198 mAh/g after 100 cycles at a rate of 0.1 C. Therefore, it can be confirmed that the existence of LLTO nanosheets can act as bridges to facilitate the lithium ion diffusion between the active materials and electrolytes.

Keywords: Perovskite oxides, hydrothermal, lithium lanthanum titanate, nanosheets, lithium ions battery, composite electrode, ionic conductivity

Introduction Recently, lithium ion conductors have attracted wide research interests in battery applications. For example, lithium lanthanum titanate, Li3xLa2/3 − xTiO3 (LLTO), has shown fast and high conductivity at *

Corresponding author: Email: [email protected] (Z. Guo) Email: [email protected] (D. Chu) # Authors have equal contribution

1 Environment ACS Paragon Plus


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room temperature.1-2 In general, the ionic conductivity of LLTO can be enhanced through two approaches: structure design and size and/or morphology control. Firstly, the ionic conductivity of LLTO is sensitive to crystal defects, e.g. the ion vacancy, and deformations such as bottleneck, and TiO6 octahedra tilting,3 which are related to structural characteristics. Notably, LLTO with various crystal structures, such as tetragonal and orthorhombic, exhibit ionic conductivity on the order of 10-3 S cm-1.4 Secondly, although the electrical properties of bulk LLTO have been studied since 1990s, only several reports with regarding to the morphology control of LLTO5-6 and their ionic transportation characteristics. Recently, it has been reported that the ionic conductivity can be enhanced by reducing the particles size to nanoscale because of the large ratio of surface and grain boundary which provides faster diffusion pathways.4, 7 Compared with other nanostructures, two dimensional nanosheets may be one of the ideal nanostructures for ionic transportation because of their unique advantages to improve the performance of lithium battery, including the large active surface area to increase contact with electrolyte and shorter lithium ion diffusion path.8-10 Therefore, it is expected that LLTO with controllable nanosheet morphology may show improved ionic transportation properties for energy storage applications. Up to date, several approaches have been utilized to synthesize LLTO-based nanomaterials, including sol-gel,11 hydrothermal,12 solid-state reaction13 and atomic layer deposition.14 However, there are less reports regarding to the morphology control of LLTO nanomaterials, especially the nanosheets. As to hydrothermal/solvothermal process, it is also difficult to control the morphology of the LLTO nanoparticles due to the rapid hydrolysis of Ti precursors. Also, LLTO are usually formed at very high pH value, where the precursor solution has very high viscosity and thus results in difficulties in control of the distribution of the particle shape and size. In this study, we developed a modified hydrothermal approach to synthesize LLTO nanosheets, and Bis (ammonium lactate) titanium dihydroxide (TALH) was used as a stable Ti precursors. Besides, the hydrothermal reaction was conducted under stirring to prevent agglomeration. The growth mechanism as well as the effect of lithium doping level on the morphology was discussed. On the other hand, lithium-rich layered LiCo1/3Ni1/3Mn1/3O2 (CNM) materials have been regarded as


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alternatives for LiCoO2 batteries.15-16 However, there are still some challenging issues such as its large irreversible capacity and poor rate capability. Herein, the as-prepared LLTO nanosheets were used as additives to enhance the electrochemical performance of the LiCo1/3Ni1/3Mn1/3O2 (CNM) electrode by improving their lithium ion conductivity via a very simple hand-balling process. The resulting LiCo1/3Ni1/3Mn1/3O2-Li0.5La0.5TiO3 composite electrode also shows better capacity, improved reversibility and stability and excellent rate capability.

Experimental procedure All chemicals were purchased from Sigma without further puriﬁcation. In detail, Bis (ammonium lactate) titanium dihydroxide (TALH) (Sigma-Aldrich, C6H18N2O8Ti, 50 wt% in H2O, 0.05 mol/L) was added into a aqueous solution of lanthanum (III) nitrate hexahydrate (Sigma-Aldrich La(NO3)3·6H2O, ≥ 99.0%), La:Ti = 1:1. And lithium nitrate (Sigma-Aldrich LiNO3, ≥ 99.0%) was added into solution based on different nominal doping levels (Molar ratio: La:Li = 9.5:0.5, 9:1, 8:2, 7.5:2.5 and 6.5:3.5). Then a 5 mol/L aqueous NaOH solution was added into the mixture with mechanical stirring in order to achieve a final NaOH concentration of 0.6mol/L. The mixture was transferred to a 50 ml autoclave and Hexadecyltrimethylammonium bromide (Sigma-Aldrich CTAB, ≥ 99.0%) (Molar ratio: Ti:CTAB = 1:2) was added into the mixture. The sealed autoclave was heated to 260 oC for 24 hours and cooled to room temperature. After that, the products were collected by centrifugation at 4000 rpm for 4 minutes and washed by ethanol. The products were dried in oven at 100 oC for electrical characterization. Structural analysis of as-synthesized LLTO nanosheets was carried out by using an X-ray diffractometer with Cu Ka radiation. The morphologies and microstructure of the LLTO nanosheets were characterized by scanning electron microscopy (Nova Nano SEM 230) and transmission electron microscopy (Philips CM200). The commercial LiCo1/3Ni1/3Mn1/3O2 (CNM) and LiCo1/3Ni1/3Mn1/3O212.5% Li0.5La0.5TiO3 (CNM-LLTO35) composites (80% by weight) were mixed with carbon black (10%) and polyvinylidene difluoride (10%) in N-methyl-2-pyrrolidinone (NMP) to form slurry, respectively. After that, the electrode was prepared by coating the slurry onto aluminum foil using a



roll press and then dried under vacuum at 100 °C for 24 h. Coin-type (CR2032) cells were assembled in an argon-filled glove box. The electrolyte used was 1.0 M LiPF6 dissolved in a 1:1 mixture of ethylene carbonate (EC)/diethyl carbonate (DEC). The coin cells were galvanostatically chargeddischarged between 3.0 and 4.0 V (vs. Li/Li+) by using a cell test instrument (CT2001A, LAND, China).

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Li 10

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Li 5

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Li 20

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Li 25

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(400) (-212) (020) (212)

(210) (002)

Results and Discussion

Intensity (a.u)

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Li 35 •

80

Position [o2 Theta] Figure 1. X-ray diffraction of as prepared LLTO with different nominal Li doping concentration (5 at% to 35 at%).

Figure 1 shows the XRD patterns of the LLTO with different nominal doping concentrations. For the low Li doping concentration (5 at% to 15 at%), the majority of diffracted peaks can be indexed as the monoclinic phase [space group P21/m] of La2Ti2O7 (JCPDS 00-027-1182). As the nominal lithium -

doping concentration increases from 20 at% to 35 at%, the cubic phase [space group Pm3m ] of Li0.5La0.5TiO3 (JCPDS 01-089-4928) can be observed and the monoclinic phase of La2Ti2O7 disappeared, indicating that a phase transition takes place.


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The TEM images of as-prepared LLTO samples are shown in Figure 2. Figure 2(a-b) indicate that the morphology of low nominal doping level of lithium (5 at% to 10 at%) is rectangular nanosheets with the edge size of ~100 nm to ~200 nm. Figure 2(b) shows that the lattice spacing of these particles is 0.29 nm which corresponds to the (212) spacing of La2Ti2O7. It can be seen that the samples with high nominal doping level of lithium (20 at% to 35 at%) are mainly irregular nanosheets (as shown in Figure 2(e-f)). The lattice spacing of these particles is 0.27 nm which corresponds to the (110) plane of Li0.5La0.5TiO3. Similar phase transition has also been found in other perovskite materials, such as Li-modified Bi0.5Na0.5TiO3 and Li-modified K0.5Na0.5NbO3-based ceramics with phase transformation from rhombohedral or orthorhombic to tetragonal respectively,17,18 indicating a Li-doping driven polar to weakly polar phase transition. In this work, transformation from monoclinic to cubic with the increasing Li content can also be regarded as a polar to weakly polar transition. It has been reported that diffusion of lithium into perovskite structure may result in lattice distortion, increased degree of local disorder, tilting of octahedral and then the phase transformation since Li shows different characteristics such as ionic radius and valence state when comparing with other A-site ions. Thus, the phase transition can be attributed to the doping-induced lattice distortion and tilting of octahedral by incorporation of lithium ions during the hydrothermal process.



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Figure 2. TEM images of different levels of nominal lithium doped LLTO: (a) 5% nominal Li-doped (b) HRTEM image of 5% nominal Li-doped (c) 10% nominal Li-doped (d) 20% nominal Li-doped (e) 25% nominal Li-doped (f) HRTEM image of 35% nominal Li-doped.

Figure 3. Growth mechanism of La2Ti2O7 and Li0.5La0.5TiO3. The growth mechanism of lanthanum titanate and lithium lanthanum titanate has been shown in Figure 3. During the hydrothermal process, the TALH can hydrolyse as TiO6 octahedra at the high temperature and La3+ ions can incorporate into Ti-O structure to facilitate the nucleation of La2Ti2O7. Under the typical growth conditions, the growth of LTO is along to the a and b directions, thereby forming as the rectangular nanosheet structure. When Li+ is introduced, the Li+ and La3+ can incorporate into TiO6 octahedra simultaneously and the crystal structure will transform from monoclinic phase to cubic phase. LLTO shows nanosheets structure which can be observed in the TEM image (nominal 30% Li doped LLTO) in Figure 3. Subsequently, the cubic nanosheets further form as irregular nanosheets. The detailed shape evolution of LLTO has been shown in Supporting Information (Figure S1). Although the LLTO can exhibit a theoretical ideal capacity at 160 mAh/g,3, 19 some studies have reported that lithium lanthanum titanate only showed low reversible capacity and poor cycle performance.20 Therefore, LLTO nanomaterials are usually used as additives to improve the


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electrochemical performance of lithium battery because of their high ionic conductivity.21-22 Therefore, the pure LLTO35 nanosheets (nominal 35% Li-doped, Li0.5La0.5TiO3) were mixed with CNM to act as an electrode material to study the properties improvement. The SEM images of CNM and CNMLLTO35 powders are shown in Figure 6. The elementals mapping results show that the LLTO nanosheets are uniformly distributed in the CNM matrix, which connected with each other to form a network with higher lithium ion conductivity. This will play a critical role in the electrochemical properties of the CNM-LLTO35 composite for lithium ions batteries.

Figure 4. (a) Cycle performance of CNM and CNM-LLTO35 electrode at the rate of 0.1 C, (b) Discharge-charge voltage profiles of CNM and CNM-LLTO35 electrodes, (c) Rate capabilities of CNM and CNM-LLTO35 electrodes and (d) Cycle performance of CNM and CNM-LLTO35 electrodes at the rate of 1 C.

The cycle performance of CNM and CNM-LLTO35 electrodes can be observed in Figure 4(a). It shows that the initial discharge capacities of the CNM and CNM-LLTO35 electrodes were 178 and 200 mAh/g at 0.1 C, respectively. After 100 cycles, the CNM-LLTO35 electrode can still retain a



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capacity of 198 mAh/g, indicating an excellent cycling stability. The gap between the charge and discharge plateau regions is displayed in Figure 4(b). The gap for the CNM-LLTO35 electrode is smaller than that for the pristine CNM electrode, which indicates that the CNM materials modified by LLTO have lower polarization. The rate capabilities of CNM and CNM-LLTO35 electrodes have been presented in Figure 4(c). It can be found that the discharge capacities of two electrodes gradually reduced with the increase of rate from 0.05 C to 5 C. Especially, CNM electrode could only retain at the capacity of 99 mAh/g at current rate of 5 C, while CNM-LLTO35 electrode delivered a capacity of 135 mAh/g, which is increased by 35% compared with the pristine CNM electrode. To further determine the cyclic performance of CNM-LLTO35 electrode at high current density, the cells were tested at 1 C for 300 cycles in Figure 4(d). The CNM-LLTO35 electrode can deliver 170 mAh/g even after 300 cycles at current density of 1 C. These results indicate that CNM-LLTO35 material possesses excellent cyclic performance and rate capability because of the high lithium ion conductivity of LLTO.

Figure 5 shows the typical CV profiles of CNM and CNM-LLTO35 electrodes. The sharp redox peaks of CNM and CNM-LLTO35 electrode can be observed in Figure 5(a-b), in which the cathodic peak near 3.7 V corresponds to the voltage platform of the discharge process (Li+ insertion into electrode materials) while the anodic peak around 3.9 V corresponds to the voltage platform of the charge process (Li+ extraction from electrode materials). Compared with the CV curves of the pristine CNM electrode, the sharp redox peaks of CNM-LLTO35 electrode show more stable overlap after the first cycle, implying that the existence of LLTO nanosheets improve the electrochemical reversibility and stability of CNM-LLTO35 electrode.23 Moreover, Figure 5(b) shows that the voltage gap (∆E) between oxidation and reduction peaks of CNM-LLTO35 electrode is much smaller than that of CNM electrode, which also indicates the CNM-LLTO35 cell has better electrochemical reversibility, since the ∆E is determined by the polarization degree of the active materials during the charge and discharge process.23 The smaller electrochemical polarization indicates a better conductivity of the CNM-LLTO35 electrode due to the good dispersion of LLTO nanosheets in the CNM matrix.


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Figure 5. Cyclic voltammograms of (a) CNM-LLTO35 electrode, inset shows CNM electrode, (b) Comparison between the CV curves of CNM and CNM-LLTO35 in the 2nd cycle (c) EIS spectra of CNM-LLTO35 and CNM electrodes and the equivalent circuit used to fit the EIS. (d) The real parts of the complex impedance vs. ω-1/2 from CNM-LLTO35 and CNM electrodes.

To further study the influences of the LLTO on the CNM electrode, the Electrochemical Impedance Spectroscopy (EIS) spectra of CNM-LLTO35 and CNM electrodes were carried out and can been seen in Figure 5(c). Both of two samples display a semicircle in the high frequency related to the charge transfer resistance and a straight line in the low frequency region linked to the Warburg impedance (Wo), which is related to the lithium ion diffusion.24 In the equivalent circuit, Rs represents for uncompensated resistance between the working electrode and reference electrode, Rct refers to charge transfer resistance.25,26 The CPE in the equivalent circuit refers to the constant phase-angle element that is used to describe the capacitance of the surface layer. It can be clearly seen that with



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the addition of LLTO, the charge transfer resistance (Rct) has been significantly decreased, thereby improving the ionic conductivity of electrode.25 Moreover, the LLTO nanosheets can also constrain the formation of SEI.26 It is also known that lithium lanthanum titanate is the A-site cation deficient perovskite oxide with high ionic conductivity. Therefore, the improvement of lithium ion conductivity in electrodes with LLTO can be expected. Many studies have used lithium diffusion coefficient to consider the lithium ion transport in lithium batteries.27-29 Here, the EIS spectra can be used to calculate the lithium diffusion coefficients by using the equations blow

Z re = Rct + Rs + σω−1/ 2 R 2T 2 DLi = 2 A2 n 4 F 4 CLi2 σ 2

(1)

(2)

where DLi is the diffusion coefficient of electrodes (in the unit of cm2 S-1), R is the gas constant, T is the absolute temperature, A is surface area of the cathode (0.9025 cm2), n is the number of electros transferred in the half-reaction for the redox couple, which is 1, F is the Faraday’s constant, CLi is the concentration of lithium ion in solid (1.77 × 10-2 mol cm-3 for CNM30), ω is the angular frequency and the σ is the Warburg factor, which has the relationship with Zre (as shown in Eq. 1) and can be obtained from the slopes in Figure 5(d). It can be found that the lithium diffusion coefficients of CNM and CNM-LLTO35 electrodes are 5.44 × 10-16 and 1.5 × 10-15 cm2 S-1, respectively. The improved lithium diffusion coefficient of the CNM-LLTO35 electrode is related to the existence of LLTO35 nanosheets, which can act as bridge to facilitate the transportation of the lithium ions and thus make contribution to the excellent electrochemical performance.


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Figure 6. SEM images of (a) CNM powders, (b) CNM-LLTO35 mixture, (c) CNM-LLTO and corresponding elementals mapping images of Ti, La, Co, Ni, and Mn.

Figure 7. Schematic illustration of CNM-LLTO structure, and the role of LLTO to improve the ion conductivity.

The Schematic illustration of CNM-LLTO35 structure in Figure 7 clearly explains the mechanism of improving its electrochemical performance. The addition of LLTO nanosheets can absorb on the surface of CNM particles, as shown in Figure 6. Based on the electrochemical properties of CNMLLTO35 above, it can be found that LLTO nanosheets connected each other to form networks with higher lithium ion diffusion in the CNM matrix. Compared with pure CNM electrode, the network



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with higher lithium ion diffusion is much less. The network in CNM-LLTO35 electrode can act as bridges for Li+ ions and accelerates the transportation of Li+ between the active materials and electrolyte. This mechanism can be confirmed by the higher lithium diffusion coefficient and lower charge transfer resistance of CNM-LLTO35 electrode.

Conclusion In summary, Li0.5La0.5TiO3 nanosheets were successfully fabricated through a facile hydrothermal method. By changing the nominal lithium doping, the phase transition from La2Ti2O7 to Li0.5La0.5TiO3 can be observed from the changes in XRD patterns and lattice spacing. The growth mechanism of LTO and LLTO was proposed as the incorporation of lithium ions into A-sites. Furthermore, because of high lithium ion conductivity and ultrathin thickness, the as-prepared LLTO nanosheets are also acted as the additive materials to improve the reversibility and stability of CNM-LLTO35 electrode. The existence of LLTO also shows the improvement of lithium ion conductivity of electrode by generating bridges to accelerate lithium ion diffusion between the active materials and electrolyte, resulting in excellent electrochemical performance.

Acknowledgement The authors would like to acknowledge the financial support from the Australian Research Council Project of FT140100032.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM images of the lithium lanthanum titanate’s morphology change with the increase of reaction time.

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Noh, S.; Kim, J.; Eom, M.; Shin, D., Surface Modification of LiCoO2 with Li3xLa2/3-xTiO3 for

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Qian, D.; Xu, B.; Cho, H.-M.; Hatsukade, T.; Carroll, K. J.; Meng, Y. S., Lithium Lanthanum

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Sha, Y.; Yuan, T.; Zhao, B.; Cai, R.; Wang, H.; Shao, Z., Solid Lithium Electrolyte-Li4Ti5O12

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Gao, P.; Yang, G.; Liu, H.; Wang, L.; Zhou, H., Lithium Diffusion Behavior and Improved

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Zhou, L. Z.; Xu, Q. J.; Liu, M. S.; Jin, X., Novel Solid-state Preparation and Electrochemical

Properties of Li1.13[Ni0.2Co0.2Mn0.47]O2 Material with a High Capacity by Acetate Precursor for Li-ion Batteries. Solid State Ionics 2013, 249-250 (1), 134-138.



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Growth of Lithium Lanthanum Titanate Nanosheets and Their

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