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Mesoporous Hierarchical Structure of Li4Ti5O12/Graphene with High Electrochemical Performance in Lithium Ion Batteries Hong Yan, Wei Yao, Runze Fan, Yu Zhang, Juhua Luo, and Jianguang Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01211 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Sustainable Chemistry & Engineering

Mesoporous Hierarchical Structure of Li4Ti5O12/Graphene with High Electrochemical Performance in Lithium Ion Batteries Hong Yan, Wei Yao, Runze Fan, Yu Zhang, Juhua Luo, Jianguang Xu

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School of Materials Science and Engineering, Yancheng Institute of Technology, 211 East Jianjun Road, Yancheng, Jiangsu 224051, P. R. China

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The combination of nanomaterials with complementary properties in a welldesigned architecture is important to develop high-performance materials for applications in lithium-ion battery. Herein, hierarchical mesoporous lithium titanate (LTO)/graphene hybrids were in situ synthesized using MAX compounds as raw materials via a hydrothermal route followed by heat treatment in Ar. The yielded primary 6-10 nm lithium titanate nanoparticles assembled into 50-200 nm secondary particles with large number of mesopores, which were attached on the surface of carbon nanosheets with graphene-like structures. The unique hybrid architecture provided the product fast ion and electron transportation routes as well as stable crystalline structure. Thus this hybrid electrode displayed high reversible capacity, good rate performances, and excellent cycling stability. A reversible capacity of 145 mA h g−1 at 1 C was retained without any apparent capacity fading even after 600 charging-discharging cycles. In addition, more than 110 mA h g−1 was achieved at extraordinary high current rate of 50 C. Noting there are a large number of MAX phase compounds which have both Ti and C elements, this work suggests the designing of different special architecture of in situ LTO/graphene or other alkali titanate/graphene hybrids using the layered MAX compounds as raw materials.

KEYWORDS: Lithium ion battery; Lithium titanate; Graphene; Hydrothermal; Anode; MAX


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Li4Ti5O12 (LTO) has attracted great attention as promising high safety anode materials of lithium-ion batteries due to its outstanding thermal stability, cyclic stability and negligible structure change during lithium insertion/extraction.1 However, it also has many shortcomings, such as poor conductivity, low lithium ion diffusion coefficient. These disadvantages degrade its electrochemical performance and further restrict its application.1,2 Recently, some improvements on the electrochemical performance of LTO have been achieved by morphology optimization, carbon hybrization and chemical modification.3-14

Compared with the micro-sized LTO, the nano LTO particles are benefit to reduce the transport distance of lithium ions, and to increase the contact with the electrolyte. For instance, Dominic Bresser et al. prepared LTO nanoparticles with an average diameter of approximately 20-30 nm, which remained 115 mAh g-1 at 10 C, 70 mAh g-1 at 100 C, compared with the 63 mAh g-1 and 1.5 mAh g-1 of micro-sized LTO particles.4 However, nanostructured LTO particles usually endure the difficulty in manufacturing electrodes of high volumetric energy density due to the low tap density of these particles.3 Mateusz Odziomek et al. designed a porous hierarchical structure of LTO, in which primary, nanosized crystallites formed larger, porous agglomerates, showing very good electrochemical performance with no sign of capacity fading.3 In addition, porosity is benefit to the penetration of electrolyte and the migration of lithium ions. Ge et al. fabricated mesoporous spinel Li4Ti5O12 nanosheets through a hydrothermal route, which showed excellent rate ability.5 Meanwhile, if carbonaceous materials (such as CNT or graphene) are introduced, the conductivity of LTO can be significantly improved. For instance, hierarchically structured lithium titanate/nitrogen-doped porous graphene fiber nanocomposites were


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synthesized by using confined growth of LTO nanoparticles in nitrogen-doped mesoporous graphene fibers, showing a high reversible capacity, excellent rate capability, and long cycling stability.6 However, the authors also stated the electrochemical performance of LTO composites always suffered from the aggregation of graphene or CNT.6 Herein, we proposed using MAX phase materials and lithium hydroxide as the starting materials to synthesize the LTO/graphene hybrids by the hydrothermal method. To our knowledge, MAX phase compounds have never be used to synthesis LTO. The MAX phase compounds (such as Ti2AlC, Ti3SiC2), which were chosen as raw stuffs, contain both Ti and C elements. Therefore, it can be expected that the in situ synthesized LTO and C would combine very well in the product, and where carbon atoms were bonded together as graphene benefiting from the layered structure of MAX phase. In this well-designed structure as in Figure1, the mesopores among LTO grains provide fast Li-ion diffusion pathways and ensure the accessibility of electrolyte, while graphene forms a conductive network, facilitating the electron transportation. In addition, potassium hydroxide has been added to the raw mixture to control the reaction process for better porous structure. The effect of K-ions amount on the performances of LTO/graphene hybrids was also investigated.

Figure 1. Illustration of Li+ diffusion and electron transportation within porous LTO/graphene electrodes.


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The strategy for preparing the porous hierarchically structure LTO/graphene is briefly described in Figure 2 and the details can be found in the Experimental section. The process involves a few steps: First, under the attack of OH-, the Ti-Al bonds in Ti2AlC can be partially broken and Ti2C formed as a result, while the OH group may be adsorbed on Ti atoms. However, it’s difficult to detect Ti2C in the hydrothermal product because Ti2C is very unstable even at room temperature.15 Thus Ti3C2 was used instead here to confirm the possibility of this step, which was also obtained like Ti2C when the Ti-Al bonds in Ti3AlC2 was broken.16,17 Typically, KOH (0.1 mol/L) and Ti3AlC2 (10 g/L) were mixed and reacted under the same hydrothermal process as that of LTO/G, and a small amount of Ti3C2 was detected in the hydrothermal product (Figure S2). At the same time, K-ions were likely to insert into the layers of Ti2AlC or Ti2C, expanding interlayer spacing.18 Then, Ti2C could be further oxidized to TiO2 or titanate,19 and the oxidation rate may be accelerated by the expanded interlayer, which may provide fast ion transport pathways.20,21 In addition, the sample after 2 h hydrothermal reaction was analyzed by SEM and TEM (Figure S3, S4). There were some small particles can be found on the surface of the nanosheet, and the atomic ratio of Al:Ti was only about 1:10 (Figure S3b), indicating the etching and oxidation of Ti2AlC. After oxidation, the left single carbon layer may crystallize as graphene under hydrothermal condition. There was around 5% graphene-like structure detected in the “AX” products when electrochemical extracting of Ti from Ti-based MAX compounds.22 Next, potassium may be replaced by lithium, and titanium dioxide and lithium hydroxide may generate lithium titanate directly. In this experiment, potassium has the following effects: speeding up the reaction process, promoting the fracture of Ti-Al bonds and accelerating the rate of nucleation.


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What’s more, when used KOH in the mixture, the primary particles obtained had a small diameter with many pores due to the faster oxidation and crystallization rate.

Figure 2. Illustration of the preparation process of Li4Ti5O12/graphene. Finally, the LTO/graphene precursors were calcined under an Ar atmosphere at a setting temperature to achieve better crystallization. Although calcination increased the crystallinity of the sample, it also caused agglomeration of primary particles, resulting in a special hierarchical organization. Moreover, some gases such as water and carbon oxides would escape from the particles during the agglomeration and left a large number of mesopores in the final samples. Comparing with the final sample (Figure 3d,e), the particles before calcination showed no mesopores and smaller size, confirming that the mesoporous hierarchical structure of LTO/graphene was formed with calcination. Figure 3a shows the X-ray diffraction (XRD) patterns of LTO obtained by adding different amounts of K-ions. According to the XRD patterns of the products, the as-prepared samples were mostly indexed to spinel Li4Ti5O12 phase, which was in agreement with the reported JCPDS No. 49-0207.3,23


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Figure 3. a) XRD patterns of different samples; b) FE-SEM image of the 0.1K-LTO/G; c,d) TEM image of 0K-LTO/G and 0.1K-LTO/G; e,f) Representative TEM images of 0.1K-LTO/G, inserted is the corresponding SAED pattern. The morphology and structure of products prepared at different KOH concentrations were examined by FE-SEM and TEM. Figure 3b shows the FE-SEM image of the 0.1KLTO/graphene. Spherical aggregates of the primary particles shown in this figure have a


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diameter of several hundred nanometers. Compared with 0K-LTO/graphene (Figure S6a) and 0.2K-LTO/graphene (Figure S6b), with the addition of more K-ions, the material exhibited an increasing porosity, which is consistent with the BET analysis as follows. Figure 3c and d show the TEM image of 0K-LTO/graphene and 0.1K-LTO/graphene. The secondary particles which were formed by the agglomeration of spherical primary particles have a size similar to FE-SEM. In addition, as can be seen in Figure 3c and d, almost all the secondary particles were attached on the surface of a carbon nanosheet. The nanosheet is transparent and crumpled, showing a graphene-like structure. The selected area electron diffraction (SAED) pattern (inserted in Figure 3f) also proved the nanosheet has a hexagonal structure like graphene. In addition, the raman spectra (Figure S7) of the samples further confirm the presence of graphene. There are two broad peaks appeared at 1345 cm-1 and 1595 cm-1, which are characteristic peaks of the D and G bands, corresponding to the disordered and graphitic carbon nanosheets in graphene.6,24-26 The carbon content of 0.1K-LTO/C was 1.78 wt.%, determined by thermogravimetric analysis (TGA) (Figure S5). The SEM-EDS spectra (Figure S6) showed the carbon content was about 2.4 wt.% in both areas 1 and 2, indicating the well distribution of carbon in LTO/graphene. In addition, no K element was detected, proving that K-ions were replaced by Li-ions. The elemental mapping images (Figure S10) further showed the distribution of Ti, O and C. Although carbon signal was disturbed due to the existence of carbon film on the copper grid, the distribution of O proved where carbon was because there are some O-containing groups on the surface of graphene. Figure 3e shows the high magnification image of 0.1K-LTO/graphene. As can be seen from the magnified image, this porous large particle was made up of many small grains. The clearly observed lattice fringes indicated the well crystallinity of the material, and displayed lattice spacing of 0.48nm, corresponding to the (111) plane of spinel LTO.12,13,27


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In good agreement with its TEM, Brunauere Emmette Teller (BET) measurements further confirmed the porous structure of this material. The nitrogen adsorption-desorption isotherm curve (Figure 4a) corresponded to type IV according to the IUPAC classification, with a distinctive H2 type hysteresis at P/P0 of 0.8-1.0, which is characteristic for the mesopores materials.3,28-30 With the addition of more K-ions, the material exhibited an increased BET surface area and cumulative pore volume. The BET surface area of 0K/0.1K/0.2K-LTO/graphene was 30.593 33.460, 46.334 cm3 g-1, and the total pore volume was 0.1843, 0.2006, 0.2531 cm3 g-1 nm1

, respectively. As can be seen from the pore size distribution (Figure 4b), adding more K-ions

during the synthesis can lead to more mesoporous in the composite.31 Mesoporous are favorable for the contact between electrolyte and samples, proceeding the charge/discharge process of lithium ion.32-34 However, excessive mesoporosity may lead to the collapse of the pores. It can be expected that the difference in pores will have a significant effect on the electro-chemical properties of the material.

Figure 4. a) N2 adsorption-desorption isotherm loop of 0/0.1/0.2K-LTO/G; b) the pore size distribution of 0/0.1/0.2K-LTO/G. The cyclic voltammogram (CV) curves of the 0.1K-LTO/graphene electrode were shown in


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Figure 5a at the scanning rates of 0.1–5 mV s−1 in the range of 1–2.5 V. All curves demonstrated a couple of redox peaks, which corresponds to the Li-ions insertion and extraction.28,35,36 At the scanning rate of 0.1 mV s-1, the potential difference ΔEp (Epa - Epc) for 0.2K-LTO/graphene electrode is 68 mV. With the scanning rate increasing to 0.2, 0.5, 1, 2, 5 mV s-1, the ΔEp (Epa Epc) increase to 82, 123, 168, 229, 341 mV, respectively. With the acceleration of the scanning speed, the value of ∆Ep gradually becomes larger. In addition, The CV curves of the 0.2KLTO/graphene (Figure S11) showed similar performance as that of 0.1K-LTO/graphene. This phenomenon indicates that the polarization becomes more seriously.35

Figure 5. Electrochemical performance of the samples: a) CV curves of the 0.1K-LTO/G at the scanning rates of 0.1, 0.2, 0.5, 1, 2, 5 mV s-1; b) Charge-discharge profiles at 1C between 1.0


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and 2.5 V; c) Long-term cycling performance of 0/0.1/0.2K-LTO/G at 1C; d) Rate performance of 0.1K-LTO/G. LTO/graphene prepared by hydrothermal method exhibited excellent electrochemical performance (Figure 5b-d). The current rate of discharge and charge were both performed at 1 C. The initial charge and discharge capacity of 0.1K-LTO/grahene was 155.7 mAh g-1 and 157.6 mAh g-1, respectively. And the efficiency of the first-round coulomb was 98.7%. Until the end of the 600 cycle, discharge capacity still maintains 145 mAh g-1, which can be seen in Figure 5c. What’s more, the coulombic efficiency was maintained around 100% during the 600-cycle long-term cycling. However, 0K-LTO/graphene and 0.2K-LTO/graphene both have a degradation in various degrees. It is certain that the suitable number of mesoporous in samples made a positive effect on improving the cycling performance.37 Figure 5d shows the rate capability of 0.1K-LTO/graphene. The cell that was cycled 10 times at each current rate retained a stable capacity at each rate. The reversible specific discharge capacities at rates of 0.5, 1, 2, 4, 10 and 20 C were 151, 145.2, 141.8, 137.4, 133.1 and 126.9 mAh g-1, respectively. Even at very high current density of 50 C, a surprisingly high capacity of more than 110 mAh g-1 can be delivered. When the current density returned back to the initial value of 0.5 C after 70 cycles, a specific capacity of 142 mAh g-1 can be delivered, indicating a high electrochemical reversibility. Table S1 exhibits the comparison of the rate performances between 0.1K-LTO/graphene and other LTO-based materials reported previously. Although the capacity is little lower than those of the contrasts, the as-prepared 0.1K-LTO/graphene still has a competitive advantage with low capacity fading. The great superiority of 0.1K-LTO/graphene in high-rate performance may come from its special porous and hierarchical structure as well as a


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conductive network formed by graphene, which reduced the migration distance of Li-ions, promoted the penetration of electrolyte and transportation of ions and electrons. To understand the contribution of K-ions on electrochemical performance, EIS testing was performed at 10mHz to 1MHz frequency range and 5mV perturbation. In Figure 6, a semicircle is observed in high-middle frequency range, while in low frequency range, a sloped line appears. The semicircle reflects the charge transfer resistance, which is assigned to the charge transfer process during electrochemical reaction. The sloped line represents Warburg impedance, which 35

is associated with the Li+ diffusion of activated material. As shown in Figure 6, in high-middle frequency range, the semicircular diameter of 0.1K-LTO/graphene is much smaller than that of 0K-LTO/graphene and 0.2K-LTO/graphene. That is, 0.1K-LTO/graphene exhibited a lower charge transfer resistance. In the low frequency range, 0.1K-LTO/graphene had a greater slope than the other two, which indicated that the diffusion rate of Li+ ion 0.1K-LTO/graphene is faster. The EIS results were well consistent with the rate performance above.

Figure 6. EIS spectra of 0K-LTO/G, 0.1K-LTO/G, 0.2K-LTO/G. In order to prove the universality of the MAX phase on the preparation of LTO/graphene, Ti3SiC2 was also used as the raw material. The prepared sample from Ti3SiC2 also exhibited


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excellent electrochemical performance. The XRD pattern (Figure 7a) of the product using Ti3SiC2, 0.1 mol L-1 KOH and 1 mol L-1 LiOH as raw materials was mostly indexed to spinel Li4Ti5O12 phase.3,23 And the diffraction peaks of the as-sintered Li4Ti5O12 were sharp and intense, indicating its highly crystalline nature. The initial discharging-charging capacity of this sample was 151.6 mAh g-1 and 152.5 mAh g-1, respectively. And the efficiency of the first-round coulomb was 100.6%. Until the end of the 600 cycle, discharging capacity still maintains 131.7 mAh g-1. What’s more, the coulombic efficiency was maintained around 100% during the 600cycle long-term cycling.

Figure 7. a) XRD pattern and b) Long-term cycling performance of Li4Ti5O12/Graphene hybrid prepared by using Ti3SiC2 as raw material. In summary, herein we reported for the first time using MAX phase compounds as raw materials to in situ prepare LTO/graphene hybrids by hydrothermal route. Meanwhile, the excellent electrochemical performance of the sample prepared by Ti2AlC and Ti3SiC2 strongly proved the universality of the MAX phase on the preparation of LTO/graphene. The electrochemical performance of LTO/graphene is modified by controlling the KOH


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concentration. Adding more K-ions during the synthesis can lead to more mesoporous in the hybrids. The mesoporous LTO/graphene hybrids that were in situ synthesized provided interconnected porous structures, which ensured the accessibility of electrode to liquid electrolyte and provided fast Li-ion diffusion pathways. Moreover, the in situ generated graphene formed a continuous conductive network, facilitating the electron transportation. The porous hierarchical structure of 0.1K-LTO/graphene showed promise as a potential electrode material for Li-ion batteries, with a excellent cycle performance and superior rate capability. The special structure rendered this material with a high capacity of 151 mAh g-1 at 0.5 C, a favorable highrate capability of more than 110 mAh g-1 at 50 C. Attractively, a capacity of 145 mAh g-1 is retained without apparent capacity decay, even after 600 cycles at 1 C.

EXPERIMETAL METHODS Materials All materials or chemicals used in this experiment were analytical grade: LiOH (AR, Shanghai Aibi Co., Ltd), KOH (AR, Sinopharm Chemical Reagent ), Ti (Hebei Shinergy Photovoltaic Technology Co., Ltd), Al (Hebei Shinergy Photovoltaic Technology Co. Ltd), C (Shanghai Shanpu Co., Ltd) and Anhydrous ethanol (AR, Sinopharm Chemical Reagent Co., Ltd). Synthesis of titanium aluminum carbide (Ti2AlC) The Ti, Al and C powder were weighed at a molar ratio of 2: 1: 1 and placed in a polytetrafluoroethylene jar filled with argon gas. The jar was placed on a planetary ball mill and milled at a speed of 300 r/min for 3 hours. After ball milling, the mixture was placed in a


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graphite crucible, and then transferred to the SHS-3 type self-propagating synthesis device, with tungsten wire as the ignition wire. Then the mixture was ignited and combusted under argon atmosphere. After cooling, the as-received Ti2AlC bulk material was then ball milled on the planetary ball mill at a speed of 300 r/min for 3 hours to prepare finer powders. The X-ray diffraction (XRD) result of Ti2AlC was shown in Figure S1. According to the XRD pattern, the sample consisted mainly of Ti2AlC, with a small amount of TiC. Synthesis of LTO/graphene hybrids Ti2AlC (0.5 g) was added to a total of 50 ml KOH (0/0.1/0.2 mol L-1) and LiOH (1mol L-1) aqueous solution under magnetic stirring. The solution was sonicated for 30 min to get a homogeneous suspension. Then the suspension was poured into a Teflon-lined stainless steel autoclave and maintained at 240 °C for 48 h. Then the precipitates were redispersed and washed with distilled water until pH ≈ 7. After sonication, the suspension was collected, and then dried at 60 °C for 3 h to obtain Li4Ti5O12/graphene precursors. The Li4Ti5O12/grpahene precursors were placed in a tube furnace and calcined at 500 °C for 8 h with a heating rate of 10 °C min−1 in Ar. The as-received products were named as LTO/G, 0.1K-LTO/G and 0.2K-LTO/G according to the concentration of KOH (0, 0.1 and 0.2 mol/L), respectively. Materials characterization The as-received LTO/graphene hybrids were analyzed by X-ray diffraction (XRD, XRD7000) with Cu Kα radiation, scanning electron microscopy (SEM, FEI Quanta 200FEG), and transmission electron microscopy (TEM, JEOL JEM-2100F).The specific surface area and pore size distribution have been recorded on an ASAP 2020 surface area and porosity analysis instrument. The carbon content of the hybrid material was measured by thermogravimetric


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analysis (TGA, METTLER TOLEDO TGA1). Electrochemical measurements The Li4Ti5O12/graphene, PVDF, acetylene black was weighed at a mass ratio of 8: 1: 1 and grounded in an agate mortar for 30 minutes until the slurry was mixed very well. Then the slurry was coated on the surface of the round copper sheet. After drying in a vacuum oven, the copper sheets were used as working electrodes. The R2032 coin-type cells were assemblied in an Arfilled glove box. Lithium foils were used as the counter and reference electrodes, and a polypropylene film (Celgard 2400) was used as a separator. The electrolyte employed was 1 M LiPF6 dissolved in a 1:1 by volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Galvanostatic cycling was performed on a NEWWARE BST-5V20mA high-precision battery detection system. Cyclic voltammetry(CV) and electrochemical impedance spectroscopy (EIS) tests were performed at the Zennium electrochemical workstation. In this work, unless otherwise specified, all impedance measurements were carried out after three cycles of the prepared electrodes.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT J. Xu thanks the National Natural Science Foundation of China (No. 21671167), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP: PPZY2015A025) and Starting fund for returned overseas Chinese scholars of Ministry of Education of China. W. Yao thanks the National Natural Science Foundation of China (No. 51602277), the Natural Science Foundation of Jiangsu Province (No. BK20140473). ASSOCIATED CONTENT Supporting Information Available: XRD patterns of Ti2AlC and hydrothermal product of Ti3AlC2, SEM images, EDS spectra and TEM images of 0.1K-LTO/G after 2h hydrothermal, SEM images of 0K-LTO/G and b) 0.2K-LTO/G, Raman spectra of 0K-LTO/C and 0.1K-LTO/C, TG curve of 0.1K-LTO/G, EDS spectra of 0.1K-LTO/G, STEM image of 0.1K-LTO/C and elemental mapping images of Ti, O and C, CV curves of 0.2K-LTO/C, table about the comparison are provided.

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TOC GRAPHICS The mesoporous LTO particles are loaded on the graphene, wherein the lithium ions shuttle in the mesopores and the electrons are transported on the graphene.


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Mesoporous Hierarchical Structure of Li - ACS Publications

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