Article pubs.acs.org/cm
Solid State Formation Mechanism of Li4Ti5O12 from an Anatase TiO2 Source Yanbin Shen, Martin Søndergaard, Mogens Christensen, Steinar Birgisson, and Bo B. Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Aarhus, Denmark S Supporting Information *
ABSTRACT: Solid state synthesis of Li4Ti5O12 anode material for Li ion batteries typically results in products containing rutile TiO2 and Li2TiO3 impurities, and subsequent high calcination temperatures lead to particle growth that reduces capacity and rate ability. Here, the formation and growth of Li4Ti5O12 particles by a solid-state reaction using anatase TiO2 with various crystallite sizes and Li2CO3 is investigated by in situ high temperature powder X-ray diffraction (HT-PXRD) and thermal gravimetry-differential thermal analysis (TG-DTA). The combined data provide insight into the origin of the impurity phases and reveal that formation of Li4Ti5O12 from anatase TiO2 and Li2CO3 is a two stage process. Initially, TiO2 and Li2CO3 react to form monoclinic Li2TiO3, followed at higher temperature by a reaction with the remaining TiO2 to yield Li4Ti5O12. Four anatase TiO2 powders with different crystallite sizes (∼50 nm, ∼30 nm, ∼20 nm, and amorphous) were explored, and decreasing crystallite sizes causes a reduced initial reaction temperature. Using anatase with a crystallite size of ∼20 nm resulted in phase pure Li4Ti5O12 at the lowest temperature (800 °C). PXRD and TG-DTA results also revealed that Li4Ti5O12 decomposes to some Ti rich phases and probably Li2O when heated above 1000 °C.
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decrease in the capacity of nanoparticulate Li4Ti5O12.31 In addition, organic residues that appear when using organic solvents result in low Coulombic efficiency.32 For practical applications, the solid-state method is the most widely used method since it is easy to scale-up, and the precursors are cheap and abundant. Anatase TiO2 and Li2CO3 are the most commonly used starting material for the solid state synthesis of Li4Ti5O12.6,12,14,33,34 However, it has proven difficult to achieve pure phase Li4Ti5O12, and rutile TiO2 and Li2TiO3 are usually observed as impurity phases in the synthesis product.29,33−35 The literature lacks a good explanation for these impurities. In addition, in most syntheses, high calcination temperatures of 850 to 950 °C are used, and the as-prepared powder exhibits decreased capacity and rate ability due to large particles sizes.6,36 Hence, a detailed study on the reaction mechanism of this system is urgently needed in order to understand the source of the impurities and potentially establish ways to avoid them. A few studies have investigated the formation mechanism of Li4Ti5O12 from anatase TiO2 and Li2CO3 by ex situ PXRD and HR-TEM,12,16,34 but in such studies, information about phase transitions during the solid state reaction may be missed, and in situ PXRD is a preferred method for following the reaction process. In the present study, in situ HT-PXRD and TG-DTA are employed to obtain detailed information on the formation of Li4Ti5O12 from Li2CO3 and anatase TiO2 with various crystallite sizes from room temperature to 1000 °C. The experiments allow us to directly follow the
INTRODUCTION Spinel Li4Ti5O12 is known as a “zero-strain” anode material for rechargeable Li-ion batteries1−3 due to the minute structural changes upon Li insertion/extraction.1 Li4Ti5O12 possesses a flat potential of about 1.55 V (vs Li+/Li) with a theoretical capacity of 175 mAh/g.1,4,5 Compared with the currently most used anode material, graphite, Li4Ti5O12 has lower energy density, but it is safer,2,6 has longer cycle life,1,7 and an excellent power density resulting from its 3D Li-ion mobility in the spinel structure.8−10 This implies that Li4Ti5O12 has huge potential as anode material in high power density and long cycle life applications, such as electric vehicles (EV) and stationary power plants. Although numerous studies show better safety properties for Li4Ti5O12 compared with that of graphite, recent studies reveal that Li4Ti5O12 will react with alkylcarbonate solvents generating gases causing swelling problems.11 This can possibly be mitigated by carbon-coating the Li4Ti5O12 particles.11 Spinel Li4Ti5O12 can be synthesized by many different techniques: solid-state reaction,5,12−16 sol−gel methods,3,17−22 microwave-assisted synthesis,23,24 and spray pyrolysis,25−28 and in hydrothermal batch reactors,29,30 flow reactors,31 and in pulsed flow reactors.32 The sol−gel method can be employed to prepare particles with uniform composition distributions at low temperature.3 The spray pyrolysis enables the direct synthesis of powder from precursor solutions,25 using low processing temperatures and resulting in high purity and homogeneity of the as-synthesized material. However, it is difficult to control the morphology and chemical composition of the powder.25 Hydrothermal synthesis is known for allowing the preparation of nanoparticles at low temperature, but the products usually have low crystallinity, which might cause fast © 2014 American Chemical Society
Received: March 16, 2014 Revised: May 8, 2014 Published: May 9, 2014 3679
dx.doi.org/10.1021/cm500934z | Chem. Mater. 2014, 26, 3679−3686
Chemistry of Materials
Article
and T2 (2 wt % excess of Ti). All of the starting materials were first heated at 600 °C for 3 h followed by 750 °C for 5 h and then naturally cooled to room temperature. The samples were studied by temperature resolved PXRD and then reheated at 850 °C for 2 h. In Situ High Temperature PXRD Measurements. For the in situ high temperature powder diffraction investigations, the prepared pellets were placed in an Anton Paar dome hot stage (DHS1100). A thin sapphire disk was used to protect the heating stage. The samples were heated in air from room temperature to 1000 °C with a heating rate of 1 °C/min, followed by a 3 h holding time at 1000 °C, before cooling to room temperature at a rate of 100 °C/min. During heating, PXRD patterns were collected using a Rigaku SmartLab diffractometer equipped with a CuKα rotating anode, CBO optics, and a D/tex Ultra detector. Diffraction patterns were measured over a 2θ range from 15 to 70° at a speed of 6°/min, resulting in a pattern collected approximately every 10 min. Thus, each scan experienced a 10 °C difference in temperature, between the beginning and end. All of the patterns are shown with the starting temperature of the scan. The data used to calculate relative intensity and crystallite sizes were obtained by integrating single peaks in the PDXL program.38 The crystallite sizes were calculated based on the Scherer formula with the peak widths corrected for instrumental broadening.39 TG-DTA Measurements. The thermal behavior of the precursor mixture was studied by TG-DTA using a Netzsch STA 449 C in an Ar/O2 atmosphere. The starting material, which was the same as that for the in situ PXRD study, was packed in a ceramic sample holder and heated from room temperature to 1100 °C. The heating rate was 1 °C/min, and a continuous Ar/O2 flow was applied, with flow rates of 30 and 20 mL/min for Ar and O2, respectively. Ar was used as a protective gas at a flow rate of 40 mL/min.
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RESULTS AND DISCUSSION The PXRD patterns of the mixed starting materials with four different TiO2 particles before heat treatment as well as data from the inorganic crystal structure database (ICSD)40 are shown in Figure 1a. All peaks can be indexed to the anatase TiO2 (red) and the monoclinic Li2CO3 (black) crystal structures. The only apparent difference between the different mixtures is the peak broadening of the anatase phase, which is attributed to the different crystallite sizes. For the mixture with amorphous TiO2, Bragg peaks are barely observed. Nevertheless, it is highly probable that amorphous TiO2 is in the reaction mixture since anatase peaks clearly appear during heating. As an example of collected data, Figure 1b shows a contour plot from the synthesis with ∼50 nm anatase. The PXRD patterns from this synthesis are further analyzed in the following sections. Contour plots of the other syntheses can be found in the Supporting Information, Figure S1−S3. From Figure 1b, there is no evidence for phase transformation below ∼550 °C, and all peaks shift to lower angles with increasing temperature due to the thermal expansion of the structure. When the temperature increases above ∼550 °C, an intermediate phase of Li2TiO3 emerges, while Li2CO3 disappears, and the intensity of anatase decreases. Above ∼800 °C, the Li2TiO3 phase starts to convert into the final Li4Ti5O12 phase, and meanwhile, anatase disappears, and rutile is observed in the temperature range between 850 and 950 °C. Figure 2a shows the PXRD patterns of the starting materials below 500 °C in a 2θ range from 20 to 40°. Noticeably, the monoclinic Li2CO3 expands quite differently in different crystallographic directions, as can be seen for the Bragg peaks (20−2), (002), and (020). Upon heating from room temperature to 504 °C, the unit cell increased from 8.36204(12) Å to 8.48042(9) Å in the a-direction (∼1.3%), from 6.20753(6) Å to 6.32634(11) Å in the c-direction (∼2%), while the expansion in the b-direction was negligible (Figure 2b). In addition,
Figure 1. (a) PXRD patterns of the starting materials with different anatase TiO2 and the simulated PXRD patterns of monoclinic Li2CO3 and anatase TiO2 determined from the inorganic crystal structure database. The indexes in red color are for anatase and black are for Li2CO3. (b) Contour plot of the PXRD patterns collected between ∼50 and 1000 °C, showing the progress of the solid state reaction with ∼50 nm anatase. The arrows in red, yellow, green, black, and white refer to anatase, Li2CO3, Li2TiO3, rutile, and Li4Ti5O12, respectively.
reaction process within the Li2CO3 −TiO2 anatase system, and a clear effect of the anatase crystallite size is revealed.
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EXPERIMENTAL SECTION
Material Preparation. Anatase TiO2 was used as the titanium source, and Li2CO3 (sigma, > 99%) was used as the lithium source. In order to investigate the effect of the TiO2 crystallite size on the reaction temperatures, four different anatase TiO2 particles were used. The sizes of nanocrystalline anatase were ∼50 nm (Riedel-de Haen, 99.5%), ∼30 nm (Inframat Advanced Materials, 99.9%), ∼20 nm (Sigma, anatase,
