Rutile-TiO2

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Facile Synthesis of Carbon-coated Spinel Li4Ti5O12/RutileTiO2 Composites as An Improved Anode Material in Full Lithium-Ion Batteries with LiFePO4@N-doped Carbon Cathode Ping Wang, Geng Zhang, Jian Cheng, Ya You, Yong-Ke Li, Cong Ding, Jiang-Jiang Gu, Xin-Sheng Zheng, Chao-Feng Zhang, and Fei-Fei Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15982 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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ACS Applied Materials & Interfaces

Facile Synthesis of Carbon-coated Spinel Li4Ti5O12/Rutile-TiO2 Composites as An Improved Anode Material in Full Lithium-Ion Batteries with LiFePO4@N-Doped Carbon Cathode Ping Wang,†‡ Geng Zhang,†‡ Jian Cheng,‡ Ya You,& Yong-Ke Li,‡ Cong Ding,‡ Jiang-Jiang Gu,‡ Xin-Sheng Zheng,‡ Chao-Feng Zhang*§ and Fei-Fei Cao*‡ ‡

College of Science, Huazhong Agricultural University, Wuhan, Hubei, 430070, People’s Republic of

China. §

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui, 230009,

People’s Republic of China. &

Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, United

States

KEYWORDS: Li4Ti5O12; rutile-TiO2; carbon coating; anode; full lithium-ion battery ABSTRACT: The spinel Li4Ti5O12/rutile-TiO2@carbon (LTO-RTO@C) composites were fabricated via hydrothermal method combined with calcination treatment employing glucose as carbon source. The carbon coating layer and the in situ formed rutile-TiO2 can effectively enhance the electric conductivity and provide quick Li+ diffusion pathways for Li4Ti5O12. When used as anode material for lithium-ion batteries, the rate capability and cycling stability of LTO-RTO@C composites were improved in comparison with pure Li4Ti5O12 or Li4Ti5O12/rutile-TiO2. Moreover, the potential of approximately 1.8 V rechargeable full lithium-ion batteries has been achieved by utilizing an LTO-RTO@C anode and a LiFePO4@N-doped carbon cathode.

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1. Introduction Lithium-ion batteries (LIBs) play a predominant role in the development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their many distinct merits, such as high capacity, high energy/power, high efficiency, long cycling life, low-cost, and environmental compatibility.1-3 Currently, graphite and carbon-based anodes are widely used for commercial LIBs.4-8 However, the organic electrolyte will be decomposed at a low voltage on the surface of carboneous materials, resulting in thermally unstable solid electrolyte interphase (SEI) film.4,5 Additionally, dendritic lithium is easily formed on the anode surface at high rates, resulting in serious safety concerns.4-6 Therefore, it is necessary to search alternative anode materials with high performance and outstanding safety properties for LIBs. In recent years, considerable attention has been given to Li4Ti5O12 as a potential alternative anode material for LIBs because of its inherent safety and chemical compatibility with the organic electrolyte.9-12 Compared to the carbon-based or metal-based anode materials, Li4Ti5O12 has the following advantages: i) higher Li+ insertion voltage (ca. 1.5 V vs Li+/Li) than commercial graphite anode (close to 0 V vs Li+/Li), which can alleviate the SEI layer formation (usually happen below 1.0 V vs Li+/Li) as well as lithium dendrites on the anode surface;4,7,10 ii) as a “zero-strain” material, spinel Li4Ti5O12 has excellent reversibility and structural/thermodynamic stability towards Li+ insertion/extraction, which gives rise to high electrochemical performance and good cycle durability.4,1011 Despite of above advantages, Li4Ti5O12 has poor electronic conductivity (10-13 S m-1) and sluggish Li+ diffusion ability (10-9-10-13 cm2 s-1), resulting in low capacities at high current rates, which limits its application in high power appliances like EVs and HEVs.11-13 To solve these problems, many effective strategies have been proposed and executed, such as building nanostructures, aliovalent ion doping, and surface coating with conductive layer (CNT, GO, carbon).4,13-15 Among them, the carbon coating is one of the most effective and facile way on improving the electric conductivity and Li+ diffusion ability simultaneously.2,11,16 2


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However, it is worth noting that high-content carbon introduction will reduce the tap density and increase the risk of SEI formation, in addition, when the voltage is higher than or equal to 1.0 V (vs Li+/Li), the capacity contributed by the carbon matrix is negligible. Therefore, the carbon content of the hybrid Li4Ti5O12-based electrode material should be controlled with relatively low content. Additionally, it is well known that rutile-TiO2 has a voltage plateau at ~1.7 V (vs Li+/Li) for Li+ insertion/extraction13,17 During the initial discharge process, the transformation from rutile-TiO2 to LixTiO2 inserted by Li+, and the presence of Ti3+ are able to enhance the entire electric conductivity.13,17 Besides, rutile-TiO2 has a higher Li-ion diffusion coefficient (10-6 cm2 s-1) than pure Li4Ti5O12.17 Hence, modification of rutile-TiO2 together with low-content carbon coating can be expected to improve the electrochemical performance of Li4Ti5O12.4,13,16,18 Much more attention has been paid on olivine-type LiFePO4 as a promising candidate cathode material for the next generation LIBs due to the superior thermal stability and good cycling performance in recent years.19,20 Even more noteworthy is that the LiFePO4 is composed of Fe and PO4 moieties, which is safer and cheaper compared with commercial LiCoO2.19,20 However, its wide application in practice is limited by its poor kinetic properties.19 Our group recently fabricated high rate LiFePO4@nitrogen-doped carbon (referred as LFP@NC) nanocomposites by using polybenzoxazine as a novel nitrogen/carbon source.19 A very flat Li insertion potential in LiFePO4 is located at approximately 3.4 V vs Li+/Li,19-23 while the value for Li4Ti5O12 is ~1.5 V (vs Li+/Li).10,11 Thus, a new rechargeable full lithium-ion battery with an output voltage of 1.8 V is desirable by assembling a battery with a Li4Ti5O12-based anode and a LiFePO4-based cathode. In this work, we developed a facile hydrothermal method combined with annealing treatment to fabricate spinel Li4Ti5O12/rutile-TiO2@carbon composites (referred as LTO-RTO@C). These LTO-RTO@C composites could deliver a specific capacity of 156.7 mA h g-1 at 0.1 C and 79.6 mA h g-1 with a high output of 1.45 V at 10.0 C, which is higher than that of LTO-RTO (62.2 mA 3



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h g-1) and Li4Ti5O12 (20.4 mA h g-1). Additionally, they can maintain a high reversible capacity of 121.6 mA h g-1 at 2.0 C after 200 cycles. The improved electrochemical performance of LTO-RTO@C composites could be attributed to the synergetic effect of in situ formed rutile-TiO2 modification together with carbon coating. Furthermore, this as-prepared LTO-RTO@C as an anode can be used for a new 1.8 V unique rechargeable full lithium-ion battery with our previous reported LFP@NC cathode. 2. Experimental section 2.1 Materials LiOH·H2O, tetrabutyl titanate (TBT), ethanol, glucose (C6H12O6), and Li2CO3 were bought from Sinopharm Chemical Reagent Beijing (China) Co., Ltd., which are analytical grade and direct used without any further refinement. 2.2 Preparation of Li4Ti5O12/rutile-TiO2 (LTO-RTO) Precursors Firstly, 0.2 g (5.0 mM) of LiOH·H2O was thoroughly dissolved in 37.5 mL ethanol. Then, 2.1 mL (6.0 mM) of TBT was slowly titrated into the above solution under magnetic stirring and further stirred for 12 h in a sealed vessel at ambient temperature. Subsequently, 37.5 mL deionized water was slowly added into the vessel and keeping vigorous stirring for another 90 s. After that, a jelly-like mixture was obtained and transferred to Teflon lined autoclave (100 mL) and heated 180 oC for 36 h in a convection oven. After then, the obtained LTO-RTO precursors were collected by centrifugation, repeatedly washed with ethanol for several times, and finally dried in the convection oven at 80 oC for 12 h. Similarly, the Li4Ti5O12 precursor was synthesized by similar procedures except for changing the mass of LiOH·H2O to 0.3 g (6.8 mM) and TBT to 2.6 mL (7.5 mM), respectively. 2.3 Synthesis of LTO-RTO@C Composites Typically, 80.0 mg of C6H12O6 was completely dissolved in deionized water (30.0 mL). Then 300.0 mg LTO-RTO precursor was added into the above solution under stirring for another 4


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30.0 min. Next, the obtained suspension was quickly transferred to a 50.0 mL Teflon lined autoclave, which was placed in an air-circulating oven at 180 oC for 12 h. After cooling down to ambient temperature naturally, the precipitates were collected by centrifugation and thoroughly washed with deionized water for several times, then dried in the convection oven at 80 oC. Finally, the dried LTO-RTO@C precursor was annealed in a tube furnace under Ar atmosphere at 600 oC for 6 h with a heating rate of 5 oC min-1, after that, LTO-RTO@C composites were obtained. For the preparation of Li4Ti5O12 and LTO-RTO material, the corresponding precursor was annealed via same method. 2.4 Structure and Morphology Characterizations The morphology of products was characterized by scanning electron microscope (SEM, JEOL 6701F), transmission electron microscope (TEM, Hitachi H-7650), and high-resolution transmission electron microscope (HRTEM, JEOL 2100F). The crystalline structure of products was studied by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer with filtered Cu Kα radiation (λ=1.54056 Å) operated at 40 kV and 40 mA. The thermogravimetric analysis (TGA) of the LTO-RTO@C composite was carried out by a thermogravimetric analyzer (SHIMADZU; DTG-60) under air atmosphere from room temperature to 900 °C. The Raman spectra of products were obtained by using a DXR Microscope (Thermo Scientific) with a laser wavelength of 532 nm. 2.5 Electrochemical Evaluations Electrochemical measurement was carried on using coin-type cell (CR 2032) assembled with lithium foil or LFP@NC (counter electrode), a glass fiber (GF/D, Whatman) membrane separator, and working electrode in argon filled glove-box in which the oxygen and water content were both below 0.1 ppm. The working electrodes were obtained by casting a slurry combining the active material (80 wt%) with super-P acetylene black (10 wt%), and poly (vinyl difluoride) (10 wt%) was pasted on a Cu foil (Li4Ti5O12, LTO-RTO, and LTO-RTO@C) or Al foil (LFP@NC) with an 5



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areal loading of 1.8-2.0 mg/cm2. The electrolyte was supplied by Tianjin Jinniu Power Sources Material Co. Ltd., which consisted of a solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1, in wt %). In full cell system, the loading mass of LTO-RTO@C is slightly higher than LFP@NC, and the capacity was calculated based on the mass of LTO-RTO@C. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were performed on a CHI 760E electrochemical workstation (CH Instruments Inc., Shanghai). The galvanostatic discharge/charge was tested by using a LAND CT2001A battery test system (Wuhan Jinnuo Electronics Co., Ltd., China). 3. Results and Discussion

The synthetic route for LTO-RTO@C is illustrated in Figure 1. The Li4Ti5O12/rutile-TiO2 precursor was first synthesized by a hydrothermal process using LiOH·H2O and tetrabutyl titanate as starting materials with a Li/Ti molar ratio at 5:6. After that, a carbon precursor layer was introduced to LTO-RTO precursor by pyrolysis of glucose as the carbon source under the subsequent hydrothermal process. Finally, the LTO-RTO@C composite with hierarchical and interconnected architecture was obtained through calcination at 600 oC under Ar atmosphere. The crystalline structure of the product was studied by XRD technique. As shown in Figure 2a, there are nine intense diffraction peaks appearing at 18.4°, 35.6°, 43.2°, 47.4°, 57.2°, 62.8°, 66.1°, 74.3°, and 79.3°, which can be indexed to the (111), (311), (400), (331), (333), (440), _

(531), (533), and (444) planes of the spinel Li4Ti5O12 (JCPDS No. 49-0207, space group Fd3 m(227), a = 8.36 Å), respectively, indicating that the Li4Ti5O12 with both high purity and crystallinity was prepared at a nominal Li/Ti molar ratio of 68:75. When the Li/Ti molar ratio was decreased to 5:6, two weak but distinct peaks emerged at 27.4o and 56.6o of LTO@RTO composites except for the peaks from spinel phase Li4Ti5O12, which can be assigned to the (110) and (220) planes of rutile TiO2 (JCPDS No. 21-1276). This result demonstrated that a small amount of rutile-TiO2 was in situ generated during the Li4Ti5O12 formation process at a relatively 6


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low Li/Ti ratio. After coating with carbon, the LTO-RTO@C composites showed similar XRD pattern with that of LTO-RTO, indicating the carbon coating process does not affect the crystallinity of Li4Ti5O12 and rutile-TiO2. Additionally, no significant graphite peaks observed in LTO-RTO@C composites could be attributed to the low degree of graphitization or the less content.23 To better understand the structure of LTO-RTO@C composites, Raman spectroscopy was employed. As displayed in Figure 2b, all three samples (Li4Ti5O12, LTO-RTO, and LTO-RTO@C composites) present five Raman peaks at 234, 262, 341, 430, and 681 cm-1, which are in good agreement with the typical Raman features of spinel Li4Ti5O12 (3F2g + Eg + A1g): i) the frequency peaks (F2g) at 234 cm-1, 262 cm-1, and 341 cm-1 originate from the vibration mode of Li-O bands; ii) the band at 430 cm-1 is assigned to the vibrations of Li-O ionic bands located in LiO4 tetrahedral (Eg); iii) the peak at 681 cm-1 (A1g) with a shoulder at 756 cm-1 is attributed to the stretching vibrational mode of Ti-O bands in the TiO6 octahedral.18,24-26 However, no obvious rutile-TiO2 Raman modes can be found in the spectra of LTO-RTO and LTO-RTO@C composites which might be due to the low content of rutile-TiO2 as well as the overlapping of some peaks ascribed from the Li4Ti5O12.25-28 In the Raman spectrum of LTO-RTO@C composites, the appearance of two broad bands at 1349.2 cm-1 and 1603.7 cm-1 confirms the presence of carbon material in the composite because these two peaks come from the amorphous carbonic disordered graphite (D-band) and crystalline graphite carbon or highly ordered carbon (G-band), respectively.27,29 It indicates that the carbon was successfully introduced into LTO-RTO via carbonization of glucose. The ID/IG value of LTO-RTO@C (0.999) further suggests that the carbon in this composite existed in the form of disordered structure, which is consistent with the above XRD result. The SEM images of LTO-RTO@C (Figure 3a, b) show that the composite exists in the form of irregular microparticles. The TEM image (Figure 3c) further confirms that LTO-RTO@C composite is composed of many nanosheets stacking upon each other, and forming a hierarchical 7



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and interconnected architecture, which is favourable for accelerating the kinetics of lithium intercalation into the LTO-RTO host structure and providing abundant pathways for Li+ diffusion, thus improving the electrochemical performance.3,16,29,30 In addition, it seems that the carbon coating does not change the original morphology in comparison with that of Li4Ti5O12 and LTO-RTO (Figure S1). As shown in the HRTEM image (Figure 3d), the clearly visible lattice fringe with a d-spacing of 0.48 nm corresponds to the (111) plane of spinel Li4Ti5O12 with high crystallinity.13,16,18 However, no obvious lattice fringe corresponding to graphite is found owing to its amorphous nature. The carbon content in LTO-RTO@C composites measured by TGA is 4.0 wt % (Figure S2). Thus, this low content or low degree of graphitization of carbon may explain why no diffraction peaks of carbon are found in the XRD pattern. Additionally, the lattice fringe belonged to rutile-TiO2 can’t be observed in the HRTEM image either, which may result from the low content.13,18 To evaluate the potential application of LTO-RTO@C composites as the anode materials for LIBs, we tested its electrochemical performance toward Li+ insertion/extraction in half cells with lithium foil as the counter electrode. Only one pair of oxidation/reduction peak at about 1.5/1.6 V is detected in the CV curve of LTO-RTO@C composites electrode (Figure S3), which corresponds to the discharge/charge plateaus of spinel Li4Ti5O12, further confirming a small quantity of rutile-TiO2 in these composites. The galvanostatic discharge-charge curves of Li4Ti5O12, LTO-RTO, and LTO-RTO@C cycling at each C-rate for 10th cycle is depicted in Figure 4a and Figure S4. It can be seen that all samples at 0.1 C (completing the charge or discharge process in 10 h) have a very flat voltage plateau around 1.5 V vs Li+/Li, which can be attributed to the typical characteristic of Li+ insertion/extraction process in Li4Ti5O12.31-33 At 5.0 C, the relatively flat voltage plateau is still retained for cells with LTO-RTO and LTO-RTO@C, whereas pure Li4Ti5O12 electrode exhibits a short and sloping curve, which may be caused by the serious electrode polarization arising from its poor electronic conductivity and sluggish Li-ion 8


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diffusion ability.34,35 The potential difference between charge and discharge plateaus can reflect the polarization degree of the electrode.36 As shown in Figure S5, the gap values for all samples at low rates are close with each other, while the difference enlarges significantly with the increase of charge/discharge C-rates. The gap value grows remarkably from 13 mV to 560 mV with the C-rate from 0.1 C to 10.0 C for the Li4Ti5O12 electrode. In contrast, with the help of rutile-TiO2, the gap values of LTO-RTO decrease to 10.1 mV and 260 mV for 0.1 C and 10.0 C, respectively. Meanwhile, after further coating with carbon, the initial gap value still maintains 10.0 mV at 0.1 C. However, when the rate increases to 10.0 C, the gap value of LTO-RTO@C electrode only exhibits a minor increase (54 mV). It indicates that the Li+ insertion/extraction kinetic of Li4Ti5O12 is significantly improved by the modification of in situ formed rutile-TiO2 together with carbon coating.25,37 At 0.1 C, a specific capacity of Li4Ti5O12 is only delivered by 73.4 mA h g-1; after the rutile-TiO2 is introduced, the value is remarkably increased to 140.1 mA h g-1; with a further carbon coating, a specific capacity of 156.7 mA h g-1 is achieved (Figure 4b). When the charge/discharge rate is increased to 5.0 C, LTO-RTO@C composite can still maintain a reversible specific capacity of 100.1 mA h g-1 (67.8% of its initial specific capacity), which is higher than those of the LTO-RTO and pure Li4Ti5O12 samples (83.9 and 36.8 mA h g-1). Even at 10.0 C, the specific capacity of the LTO-RTO@C electrode still retains at 79.6 mA h g-1, while only 62.2 mA h g-1 and 20.4 mA h g-1 for LTO-RTO and Li4Ti5O12, respectively. When the charge-discharge rate is lowered to 0.1 C again, the specific capacities of the three electrodes are nearly recovered. The above results demonstrate that LTO-RTO@C electrode exhibits the highest specific capacity and best rate performance. It can be found that the in situ formed rutile-TiO2 obviously increases the specific capacity of the composite despite its low content, and improves Li+ insertion/extraction kinetics. This improved electrochemical performance may result from two reasons: i) the carbon coating enhances the electric conductivity; ii) the hierarchical and interconnected architecture could alleviate the local destruction of structures caused by a large 9



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number of Li+ flow.37-42 In addition to the high rate performance, there are a superior cycling performance for LTO-RTO@C electrode. After 200 charge/discharge cycles at 2.0 C, the specific capacity for LTO-RTO@C electrode is still as high as 121.6 mA h g-1 without obvious fading, in contrast, those of Li4Ti5O12 and LTO-RTO are only 45.3 and 90.5 mA h g-1, respectively (Figure 4c). The stable cycling properties imply that the carbon coating serves as a bridge among LTO-RTO particles and makes it easier to fasten the transport of electron into Li4Ti5O12.16 To further study the effect of carbon coating layer and in situ formed rutile-TiO2 on the improvement of the conductivity and ion diffusion for Li4Ti5O12, EIS measurements were analysed (Figure S6). Obviously, the diameter of semicircle in high frequency region and the slope straight line in low frequency range for LTO-RTO@C electrode are much smaller and higher than that of Li4Ti5O12 and LTO-RTO electrodes. This result further indicated that the carbon coating layer and the in situ formed rutile-TiO2 can not only enhance electrical conductivity but also improve Li+ diffusion process.33 An elementary diagram of a 1.8 V full lithium-ion battery can be designed with an LTO-RTO@C anode and a LFP@NC cathode in Figure 5a. As show in the Figure 5b, all galvanostatic discharge curves of as-assembled full lithium-ion cell at different rates have a very flat voltage plateau around 1.8 V implying an ideal battery system. The plateau specific capacity of LTO-RTO@C//LFP@NC full cell at 0.1 C and 0.2 C were 100.1 and 81.2 mA h g-1, respectively, which is 83.3% and 81.1% of its total capacity (Figure 5b). When the discharge rate is increased to 1.0 C, the plateau specific capacity can still maintain 77.8% of its total capacity. Additionally, the LTO-RTO@C//LFP@NC full cell shows stable capacity retention and excellent charge/discharge efficiency (Figure 5c). It demonstrates that the potential application of LTO-RTO@C as an anode in future lithium-ion batteries with LFP@NC as a cathode. 4. Conclusion

In summary, LTO-RTO@C composite was prepared via a facile, low-cost, and large-scale synthesis method. The as-prepared LTO-RTO@C composite electrode presented a high reversible 10


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specific capacity, excellent rate performance, and cycling performance when used as anode materials for LIBs, which results from the hierarchical and interconnected architecture, carbon coating, and rutile-TiO2 modification. Furthermore, a new high rate rechargeable full lithium-ion battery with an output voltage of 1.8 V is achieved by assembling a battery with an LTO-RTO@C anode and a LFP@NC cathode.

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ASSOCIATED CONTENT Supporting Information (Figure S1-S6). This material is available free of charge via the Internet at http://pubs.acs.org.

The detailed preparation method of LFP@NC nanocomposites and additional characterizations

including

TEM

images,

TGA

curves,

CV

curves,

galvanostatic charge-discharge voltage profiles, the polarization of ∆E versus rate plots and EIS results. AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (C.Z.). * E-mail: [email protected] (F.C.). Author Contributions †P.W. and G.Z. contributed equally to this work.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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This work was supported by the Specialized Research Fund for the Doctoral Program

of

Higher

Education

Science and Technology Project

(20130146120013), for Young Experts

Wuhan

Chenguang

(2015070404010192),

National Natural Science Foundation of China (NSFC 21303064\21603080) and

Fundamental

Research

Funds

for

the

Central

Universities

(2662015PY163\2662015QC046). Financial support provided by the Hefei University of Technology (HFUT) (No. 407-037171) is also acknowledged. REFERENCES (1) Jeong, G.; Kim, Y. U.; Kim, H.; Kim, Y. J.; Sohn, H. J. Prospective Materials and Applications for Li Secondary Batteries. Energy Environ. Sci. 2011, 4, 1986-2002. (2) Cao, F. F.; Guo, Y. G.; Wan, L. J. Better Lithium-Ion Batteries with Nanocable-Like Electrode Materials. Energy Environ. Sci. 2011, 4, 1634-1642. (3) Xin, S.; Guo, Y. G.; Wan, L. J. Nanocarbon Networks for Advanced Rechargeable Lithium Batteries. Acc. Chem. Res. 2012, 45, 1759-1769. (4) Yi, T. F.; Fang, Z. K.; Xie, Y.; Zhu, Y. R.; Yang, S. Y. Rapid Charge–Discharge Property of Li4Ti5O12–TiO2 Nanosheet and Nanotube Composites as Anode Material for Power Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 20205-20213. (5) Yang, J.; Zhou, X. Y.; Zou, Y. L.; Tang, J. J. A Hierarchical Porous Carbon Material for High Power, Lithium Ion Batteries. Electrochim. Acta 2011, 56, 8576-8581.

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(6) Tan, L. P.; Lu, Z.; Tan, H. T.; Zhu, J.; Rui, X.; Yan, Q.; Hng, H. H., Germanium Nanowires-Based Carbon Composite as Anodes for Lithium-Ion Batteries. J. Power Sources 2012, 206, 253-258. (7) Yang, X.; Wei, C.; Zhang, G., Activated Carbon Aerogels with Developed Mesoporosity as High-Rate Anodes in Lithium-Lon Batteries. J. Mater. Sci. 2016, 51, 5565-5571. (8) Yi, J.; Li, X. P.; Hu, S. J.; Li, W. S.; Zhou, L.; Xu, M. Q.; Lei, J. F.; Hao, L. S. Preparation of Hierarchical Porous Carbon and Its Rate Performance as Anode of Lithium Ion Battery. J. Power Sources 2011, 196, 6670-6675. (9) Liu, X. Y.; Huang, T.; Yu, A. S. A New, High Energy Rechargeable Lithium Ion Battery

with

a

Surface-Treated

Li1.2Mn0.54Ni0.13Co0.13O2

Cathode

and

a

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Figure 1 Schematic illustration of the fabrication process for LTO-RTO@C composites.

Figure 2 (a) XRD patterns and (b) Raman spectra of LTO-RTO@C composites, LTO-RTO and Li4Ti5O12.

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Figure 3 (a, b) SEM, (c) TEM, and (d) HRTEM images of LTO-RTO@C composites.

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Figure 4 Electrochemical performance of Li4Ti5O12, LTO-RTO, and LTO-RTO@C composites in the voltage range of 1.0-3.0 V vs Li+/Li. (a) Galvanostatic charge-discharge voltage profiles of LTO-RTO@C composites. (b) The rate performance and (c) cycling performance of Li4Ti5O12, LTO-RTO, and LTO-RTO@C composites.

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Figure 5 (a) Schematic illustration of an LTO-RTO@C//LFP@NC lithium ion battery device. (b) Galvanostatic charge-discharge voltage profiles and (c) rate performance of an LTO-RTO@C//LFP@NC lithium ion cell, the voltage range is 1.0-3.0 V vs Li+/Li.

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TOC figure:

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Rutile-TiO2

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