Direct Fabrication of Densely Packed Idiomorphic Li4Ti5O12 Crystal

Oct 6, 2014 - Dae-wook Kim , Nobuyuki Zettsu , Yusuke Mizuno , and Katsuya Teshima ... N. Zettsu , T. Yoda , H. Onodera , N. Handa , H. Kondo , K. Tes...
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Direct Fabrication of Densely Packed Idiomorphic Li4Ti5O12 Crystal Layers on Substrates by Using a LiCl−NaCl Mixed Flux and Their Additive-Free Electrode Characteristics Nobuyuki Zettsu,†,‡,§ Yusuke Mizuno,‡,§ Hiroki Kojima,‡ Kunio Yubuta,§,∥ Takuya Sakaguchi,⊥ Toshiya Saito,⊥ Hajime Wagata,‡ Shuji Oishi,‡ and Katsuya Teshima*,†,‡,§,@ †

Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan § CREST, Japan Science and Technology Agency, Japan ∥ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ⊥ Battery Research Division, Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan ‡

S Supporting Information *

ABSTRACT: Thin layers of densely packed idiomorphic Li4Ti5O12 crystals were prepared directly on a Pt substrate by using a LiCl−NaCl mixed flux. The thin film of colloidal anatase nanoparticles with an ∼30 nm diameter was fully converted into Li4Ti5O12 crystals with a diameter of 100−200 nm having polyhedral shapes with well-defined {111} faces during the evaporation-driven flux growth. Cross-sectional structure analysis revealed that the interface between the crystals and the substrate seemed to be atomically bonded. Galvanometric charge and discharge properties strongly supported our consideration of interface formation. The Li4Ti5O12 crystal layer exhibited a large capacity close to its theoretical value under 0.1C rate with no assistance from additional electroconductive materials and binders, meaning that the interfaces provide seamless charge transportation pathways. We also addressed the formation mechanism of the Li4Ti5O12 crystal layer accompanied by the experimental results of in situ Xray diffraction, thermogravimetric and differential thermal analysis, and scanning electron microscopy observation.

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INTRODUCTION Lithium titanium oxide (Li4Ti5O12) has been attractive as the negative electrode active material for lithium ion rechargeable batteries (LIBs) with high reliability, because an absorbing and releasing Li+ in Li4Ti5O12 progresses effectively without a change in the structure or size of the crystal lattice under a charging/discharging reaction.1−5 Spinel-type Li4Ti5O12 belongs to a cubic system with a space group of Fd-3m.6,7 The lithium, titanium, and oxygen occupy site 8a and 1/6 of site 16d, 5/6 of site 16d, and site 32e, respectively, and no atom occupies site 16c. Thus, intercalated Li+ is stored in the 16c site through the three-dimensional Li+ diffusion via the 8a−16c site in discharge reactions, described as follows: Li4Ti5O12 + 3Li+ + 3e− → Li7Ti5O12.8,9 The operating potential of Li4Ti5O12 shows much novel potential (1.55 V vs Li/Li+) compared with that of conventionally used graphite carbon (0.1 V vs Li/Li+), meaning that Li4Ti5O12 potentially suppresses the growth of dendrite Li onto the electrode.10 The electrochemical redox couple with a higher equilibrium potential using Ti4+/Ti3+ makes Li formation thermodynamically less favorable. The capacity of batteries strongly depends on the difference in the operating © 2014 American Chemical Society

potential of the positive and negative electrode pair; however, the higher operating potential of Li4Ti5O12 also results in a disadvantage with respect to the energy density of the battery, simultaneously.10,11 Furthermore, both a low electron conductivity (10−9 S cm−1) and an ion diffusion constant (10−12 S cm−1) limited the performance of Li4Ti5O12-based LIBs, for instance, rapid charging/discharging.12−14 The empty Ti 3d sites with a band gap energy of ∼2 eV characterize its electronic structure and give insulating characteristics to Li4Ti5O12.15 Therefore, the high-current properties of Li4Ti5O12 might not be sufficient for high-current applications before any material modifications, including the conductive surface coating,16−18 nitridation,15,19−21 partial substitution of Ti with Mg and/or Al,22−24 and reducing the particle size.25−27 Nanostructured approaches have been particularly demonstrated to improve the electrochemical performance of electrode materials. Nanosizing decreases the diffusion length of Li+ and increases the surface Received: June 25, 2014 Revised: September 1, 2014 Published: October 6, 2014 5634

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Figure 1. Typical surface SEM images of (a) a colloidal TiO2 particle layer and (b) a Li4Ti5O12 crystal layer on a Pt substrate. All scale bars are 200 nm.

area, which improves the reaction kinetics.28−37 However, nanostructured materials are not an ultimate solution for achieving the all requirements of future advanced LIBs. One of the primary reasons is that nanosized materials lead to selfaggregation before some surface modification, resulting in a decrease in capacity caused by both an inhomogeneous distribution and a low tap density. On the basis of this background, to enhance the capacity of Li4Ti5O12 electrodes to maintain moderately high rate properties, we demonstrated additive-free Li4Ti5O12 electrodes constructed of a thin layer of densely packed idiomorphic nanosized Li4Ti5O12 single crystals. In general, conventionally used electrodes have consisted of 70 wt % active materials and 30 wt % additives, which proved reaction kinetics as well as adhesion to the substrate. In this work, we demonstrate direct fabrication of a thin film constructed of densely packed idiomorphic Li4Ti5O12 crystals on Pt substrates by conversion of a colloidal thin film of anatase nanoparticles in a LiCl−NaCl mixed flux (i.e., molten salt) as well as the studies of the mechanism of formation and their additive-free electrode characteristics for LIBs.

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JSM-7600F). Crystallographic characteristics were evaluated by scanning transmission electron microscopy (STEM) (JEOL, JEM2800) with an energy-dispersive X-ray spectrometer and TEM (TOPCON, EM-002B). Focused ion beam machining (FIB) (JEOL, JIB-4000) with a Ga ion beam source was used for the preparation of TEM/STEM samples. The crystal phases of the products were identified with an X-ray diffractometer (XRD) (Rigaku, MiniflexII) with Cu Kα radiation (λ = 0.15418 nm). We also studied the mechanism of formation of the Li4Ti5O12 crystal layer accompanied by in situ XRD (Rigaku, SmartLab), TG-DTA (Rigaku, Thermal plusII), and SEM observation. The heating conditions in the in situ XRD and TG-DTA experiments were the same as those used for crystal layer formation. The galvanometric charge/discharge properties of the Li4Ti5O12 crystal layer as additive-free electrodes of LIBs were investigated using a coin-type cell (R2032). The Li4Ti5O12 crystal layers on the Pt substrate were dried in a vacuum at 120 °C for 12 h prior to cell assembly. Lithium metal foil and polypropylene film were used as the counter electrode and the separator, respectively. A solution of 1 M LiPF6 in an ethylene (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) solution [1:1:1 (v/v)] was used as an electrolyte. The coin-type LIBs were assembled in an argonfilled glovebox with a maintained atmosphere of ≤1 ppm of H2O and O2. The charge/discharge cycles of the LIBs were performed with a potentio/galvanostat over a voltage range of 1.0−2.5 V (vs Li+/Li) at 25 °C.

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EXPERIMENTAL SECTION

A stoichiometric concentration of solutes was prepared directly on Pt substrates by sequential spin-coating. A colloidal aqueous dispersion of TiO2 anatase at a concentration of 10 wt % with an average size of 30 nm (Kasei, Co., Ltd.) was used as a Ti source, and Li2CO3 powder (reagent grade, Wako Pure Chemical Industries, Ltd.) was used as a Li source. A mixture of LiCl and NaCl powders (reagent grade, Wako Pure Chemical Industries, Ltd.) was used as a flux for Li4Ti5O12 growth. The LiCl:NaCl molar ratio was set to 72:28, resulting in a eutectic point of 552 °C. The solute concentration was adjusted to 50 mol %; 50 μL of a colloidal dispersion was spin-coated on a Pt substrate (10 mm × 10 mm × 0.02 mm, as a current collector). Subsequently, 100 μL of an aqueous solution containing Li2CO3 and flux was further dropped on the TiO2-coated Pt substrate. Because of the strong hydrophilic nature and surface roughness of the TiO2, the dropped solution was homogeneously spread on the surface, immediately. The substrates were heated in air to 800 °C at a rate of 1000 °C h−1 and held for 1 h. They were then cooled to 500 °C at a rate of 200 °C h−1, which was controlled by a cooling program, and allowed to cool to room temperature in the furnace. This cooling condition was described in our previous studies of NaCl flux growth of Li4Ti5O12 crystals in a crucible.38 The products were washed with warm water to remove the remaining flux. Surface morphological characterization of the Li4Ti5O12 crystal layer was performed by using field-emission-type scanning electron microscopy (FE-SEM) (JEOL,

RESULTS AND DISCUSSION

Densely packed Li4Ti5O12 single crystals were successfully deposited on the Pt substrates from the LiCl-NaCl flux for the first time. The loading amount of the Li4Ti5O12 crystal was evaluated by determining changes in mass of the Pt substrate before and after flux coating and found to be 0.063 mg cm−2 on average. The phases and size of Li4Ti5O12 crystals were strongly dependent on the growth conditions, for instance, holding time. Figure 1 shows the changes in the typical surface SEM images of a thin film of the colloidal TiO2 anatase nanoparticles on a Pt substrate after LiCl−NaCl flux growth, and heating at 800 °C for 1 h. The substrate surface was fully covered with numerous crystals. The average size of individual crystals was roughly estimated to be 120 nm. Their morphologies were drastically changed from spherical to truncated octahedral shapes, which seems to reflect the fact that the crystal structure of Li4Ti5O12 belongs to an Fd-3m space group. It is interesting to note that Na ions in the NaCl flux started to penetrate inside the crystal lattice of Li4Ti5O12 and ion exchange with Li to form Na4Ti5O12 when the holding time was increased until the LiCl was fully consumed. 5635

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substrate.40 Theoretical analysis based on first-principles calculations is currently in progress. As shown in Figure 3a, cross-sectional STEM images revealed that the crystal layer showed a uniform film thickness

Figure 2 shows the corresponding XRD profiles; all diffraction lines were assigned with Li4Ti5O12 (ICDD PDF 26-1198), Pt (ICDD PDF 04-0802), and minor TiO2 rutile at 2θ = 27.45°, which contributed to the 110TiO2‑rutile (ICDD PDF 21-1276). Because the diffraction intensity of TiO2 rutile was quite small in comparison to that of Li4Ti5O12, the Li4Ti5O12 crystal layer was found to contain a trace of TiO2 rutile. Note that a pure Li4Ti5O12 crystal film was infrequently obtained and was poorly reproducible. According to our empirical evidence (not published), chloride flux accelerated the phase transition of TiO2 anatase to TiO2 rutile. Furthermore, the phase transformation tended to occur preferentially at grain boundaries in the densely packed Li4Ti5O12 crystal layer. We think that the accumulation of local film stress might primarily contribute to the phase change. The relative intensity of diffraction lines at 2θ values of 18.35° and 43.25°, assigned to 111 and 400 faces, respectively, in the flux-grown Li4Ti5O12 crystal layer, was stronger than that of Li4Ti5O12 (ICDD PDF 26-1198), meaning that the direct growth of Li4Ti5O12 crystals on the Pt substrate progressed with a preferred ⟨111⟩ direction.

Figure 3. (a) Cross-sectional STEM image of the Li4Ti5O12 crystal layer on the Pt substrate. (b) Cross-sectional high-resolution TEM image and (c) corresponding SAED pattern of a Li4Ti5O12 crystal. (d) Cross-sectional high-resolution TEM image and (e) corresponding SAED patterns of the Li4Ti5O12 crystal layer in the selected regions close to the Pt substrate.

Figure 2. XRD profiles of the Li4Ti5O12 crystal layer on the Pt substrate (a) and ICDD PDF data of Li4Ti5O12 (b) and Pt (c).

of ∼120 nm. The thickness was comparable to the diameter evaluated by SEM. In fact, a multilayer, constructed of two or more layers, was hardly observed in the entire observation area. The individual Li4Ti5O12 crystals homogeneously aggregated with each other in the plane. Panels b and c of Figure 3 show a cross-sectional high-resolution TEM images and the corresponding SAED pattern, respectively, of a single Li4Ti5O12 crystal. High-resolution TEM studies clearly showed that the same fringe pattern continuously extended throughout a crystal, suggesting the formation of a single crystal. The highly ordered corresponding SAED pattern taken from the same crystal strongly supported that the Li4Ti5O12 crystals yielded high crystallinity. Panels d and e of Figure 3 show a cross-sectional high-resolution TEM image and the corresponding SAED pattern, respectively, of the Li4Ti5O12 crystal layer in the selected regions close to the Pt substrate. Even though a slight crystal lattice distortion was observed, no major changes in the fringe pattern contributed to the Li4Ti5O12 crystals were observed at the near interface. In addition, the complex diffraction pattern was observed in the vicinity of the interface,

We evaluated the order parameter of the Li4Ti5O12 crystal layer by using the Lotgerning method that provided a degree of orientation (F) using the XRD profile, which showed good correlation to the characteristics of the degree of orientation and polycrystallinity on magnetic properties.39 Thus, single crystals and random oriented particles theoretically represent F values of 1 and 0, respectively. Because the direct growth of Li4Ti5O12 crystals on the Pt substrate represented F = 0.983, each truncated octahedrally shaped Li4Ti5O12 crystal was a single crystal with well-developed {111} faces. We think that the origin of the orientation characteristics might be strongly associated with lattice-matched Pt substrate, leading to epitaxial growth of Li4Ti5O12 crystals. A commercially available Pt substrate used for this study showed high {200} orientation in its XRD profile. A lattice spacing of the Pt (200) plane was 1.961 Å, and its six time period was remarkably matched to the Li4Ti5O12 (111) plane with 11.7694 Å. The lattice mismatch was found to be only 0.29%. Flat Li4Ti5O12 films with a single orientation were grown by controlling the lattice plane of the 5636

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which has been consistent with the one obtained by simply mixing the diffraction pattern from the Pt substrate and Li4Ti5O12 crystals. The interfacial structure has not been viewed clearly enough in a strict sense, and all results strongly support the idea that the interface between the crystals and the substrate seemed to be atomically bonded and free of the formation of major impurities such as a Li-rich Li2TiO3 phase, TiO2-rutile, or Li2PtO3. We further addressed the mechanism of formation of the Li4Ti5O12 crystal layer accompanied by in situ XRD, Tg-DTA, and SEM observation. Figure 4 shows the changes in in situ XRD profiles of the Li4Ti5O12 crystal layer on the Pt substrate during heating. All the experimental conditions were the same as those used for the preparation of crystal layers in a furnace. The broad and weak peak centered at 2θ = 25.3° was observed in an as-prepared TiO2−anatase nanoparticle film. The TiO2− anatase nanoparticles began to react with Li2CO3 below 750 °C. Newly appearing peaks centered at 18.4° and 35.6° could be assigned to {111} and {311}, respectively, of Li4Ti5O12. The diffraction peaks of Li4Ti5O12 became intense with an increasing reaction time, and the peak due to TiO2−anatase nanoparticles became smaller. Eventually, the TiO2−anatase nanoparticles were fully consumed at 800 °C for 30 min. The full width at half-maximum (fwhm) diffraction line in the Li4Ti5O12 phase became sharper after the sample had cooled to room temperature because of the crystal growth driven by cooling and/or reduction of lattice spacing. Note that the Li4Ti5O12 compound was crystallized out at 800 °C prior to cooling, indicating that the growth of Li4Ti5O12 was driven by evaporation-controlled supersaturation. We further conducted the same in situ XRD experiments with no flux condition. The almost same reaction was begun >780 °C; thus, we thought the flux strongly contributed to the decrease in the reaction temperature.

Figure 5. TG-DTA curves taken from the mixture of the colloidal TiO2−anatase, Li2CO3, and LiCl−NaCl mixed flux.

melting of Li2CO3 as well as evaporation of CO2 through the reaction with TiO2. All these thermal analyses strongly suggested that the TiO2−anatase nanoparticles began to react with Li2CO3 at 660 °C in a LiCl−NaCl mixed flux. Figure 6 shows the changes in ex situ SEM images of colloidal TiO2−anatase films on the Pt substrates during the LiCl−NaCl flux growth. To halt crystallization under cooling, the flux growth process was quenched at a given temperature by rapid cooling with a water bath. As shown in Figure 6a−c, the average particle size was gradually increased from 30 to >100 nm as the temperature increased to 800 °C. This was due to volume expansion through lithioration of TiO2−anatase nanoparticles with Li2CO3. The grain growth and shape transformation from irregular to truncated octahedral shapes with sharp corners rapidly progressed in the retention process at 800 °C. It is interesting to note that the particle size distribution became narrow over time through the gradual disappearance of smaller particles. In addition, definitive grain boundaries and small voids were formed at the interface of neighboring crystals. These results imply that the mechanism of formation of a Li4Ti5O12 crystal layer was considered to be driven by grain growth via sintering on the substrate rather than a crystallization from a homogeneous solution via initial heterogeneous nucleation and subsequent growth on the substrate. It is interesting to note that the solid-state reaction condition also provided Li4Ti5O12 crystals; however, welldefined crystal facets did not appear on the surface (Supporting Information 1). We can conclude that usage of a LiCl−NaCl mixed flux plays a key role in the development of idiomorphic Li4Ti5O12 crystal growth. The truncated octahedrally shaped Li4Ti5O12 crystals have been formed by sintering through dissolution and precipitation in the molten salt as a liquid phase. The electrochemical performance of the Li4Ti5O12 crystal layer was further studied as an additive-free LIB electrode.41 Galvanostatic cycling measurements were conducted using R2032 coin-type cells. No additives for the enhancement of their electron conductivity were used in these studies. Figure 7a shows the first 10 cycles of capacity−voltage profiles measured at a current density of 1.1 μA cm−2, corresponding to 0.1C. The charge/discharge voltage profiles exhibited sloping voltage plateaus and initial charge and discharge capacities of 167 and 154 mAh g−1, respectively. The corresponding Coulombic efficiency was 92%. A small degradation in the capacity was observed during cycled charge/discharge operation. This was thought to be due to a presence of TiO2-rutile as a byproduct in

Figure 4. Changes in in situ XRD profiles of the colloidal TiO2− anatase film mixed with Li2CO3 and a LiCl−NaCl mixed flux during heating at (a) 26 °C, (b) 753 °C, (c) 800 °C for 0 min, (d) 800 °C for 30 min, or (e) 800 °C for 60 min. The range of the y-axis in each panel is 0−4000.

Figure 5 shows the TG-DTA profile of the Li2CO3/LiCl/ NaCl mixture. The mixing ratio was the same as that of the Li4Ti5O12 crystal layer fabrication. Endothermic peaks that accompany weight loss appeared at ∼100 °C and were attributed to the evaporation of absorbed water. Second sharp endothermic peaks with no weight loss were observed at 510 °C because of the melting of a LiCl−NaCl mixed flux. Furthermore, broadened endothermic peaks that accompany weight loss appeared again at 660 °C. This was attributed to the 5637

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Figure 6. Changes in surface SEM images of the colloidal TiO2−anatase film mixed with Li2CO3 and a LiCl−NaCl mixed flux during heating at (a) 26 °C, (b) 753 °C, (c) 800 °C for 0 min, (d) 800 °C for 30 min, or (e) 800 °C for 60 min. All scale bars are 200 nm.

electroconductive materials and binders, meaning that the interfaces provide seamless charge transportation pathways. We believe that the additive-free approach, demonstrated here, possibly offers an enhancement of the loading amount of active materials per unit area. The additive-free electrodes exhibited a theoretical charge/discharge capacity or closely equivalent electrochemical performances compared to those of conventional composite electrodes, and the additive-free electrode yield a large benefit with respect to the energy density per electrode body by an amount that does not include the additives. However, this Li4Ti5O12 film thickness is not enough to substantiate our conclusion. Studies on the thickness of the Li4Ti5O12 film are currently underway to further clarify the merit of the additive-free approaches.

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CONCLUSION

A Li4Ti5O12 crystal layer was successfully fabricated directly on a Pt substrate by flux coating using a LiCl−NaCl mixed flux. The layer consisted of highly crystalline, idiomorphic, nanosized Li4Ti5O12 crystals. We further demonstrated the additivefree electrode properties for LIBs. The Li4Ti5O12 crystal layer showed a large capacity close to its theoretical value under a 0.1C rate with no assistance, indicating that the active materials in the Li4Ti5O12 crystal layer were properly utilized and the interfaces provide seamless charge transportation pathways. At this stage, the Li4Ti5O12 crystal layer-based additive-free electrode showed considerably lower capacity than the theoretical value under high C-rate conditions. The poor electrochemical performance is due to low electron conductivity. The high current properties of Li4Ti5O12 can be enhanced by material modifications via nitridation and partial substitution of Ti with Mg and/or Al. Thus, we believe that newly proposed additive-free approaches combined with the material modifications described above will potentially yield advanced LIBs.

Figure 7. Voltage−capacity profiles of the Li4Ti5O12 crystal layers on the Pt substrates as an additive-free electrode for LIBs measured at Crates of (a) 0.1C and (b) 1C.

the Li4Ti5O12 crystal layer. The charge/discharge voltage profiles under 1C are shown in Figure 7b. Even though the initial discharge capacity was decreased to 80 mAh g−1, which was considerably lower than the theoretical value, the active materials in the Li4Ti5O12 crystal layer were properly utilized with no assistance of any additives. These charge/discharge properties strongly supported our consideration of interface formation, as shown in Figure 3. The Li4Ti5O12 crystal layer exhibited a large capacity with no assistance from additional 5638

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron microscope image of the Li4Ti5O12 crystal layer prepared under solid-state reaction conditions at 800 °C for 1 h. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address @

K.T.: Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency and the Research and Development Program for Innovative Energy Efficiency Technology in 2011 (23-0712004) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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