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Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Pseudocapacitive Behaviors of Li2FeTiO4/C Hybrid Porous Nanotubes for Novel Lithium-Ion Battery Anodes with Superior Performances Yakun Tang,† Lang Liu,*,† Hongyang Zhao,† Yue Zhang,† Ling Bing Kong,‡ Shasha Gao,† Xiaohui Li,† Lei Wang,† and Dianzeng Jia† †

Key Laboratory of Energy Materials Chemistry, Ministry of Education, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China ‡ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore S Supporting Information *

ABSTRACT: Hybrid nanotubes of cation disordered rock salt structured Li2FeTiO4 nanoparticles embedded in porous CNTs were developed. Such unique hybrids with continuous 3D electron transportation paths and isolated small particles have been shown to be an ideal architecture that brought out enhanced electrochemical performances. Meanwhile, they exhibited improved extrinsic capacitive characteristics. In addition, we demonstrate a successful example to use cathode active material as anode for lithium-ion batteries (LIBs). More importantly, our hybrids had much superior electrochemical performances than most of the reported Li4Ti5O12-based nanocomposites. Therefore, it is concluded that Li2FeTiO4 can be a prospective anode material for LIBs. KEYWORDS: Li2FeTiO4, anodes, lithium-ion batteries, pseudocapacitive, hybrid porous CNTs

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transportation could be impeded by Fe2+ and Ti4+.17 Moreover, Li2FeTiO4 is usually present in the form of bulk. As is wellknown, reducing particle size is a valid method to promote the transportation of Li+, specifically ionic conductivity of cation disorder materials can be increased by manipulating their particle size. As a consequence, development of unique nanostructures has been an efficient strategy to improve Li+ diffusion property of electrode materials. Recently, it was demonstrated that hybrid porous carbon nanotubes, with active nanoparticles to be embedded, not only offered numerous storage sites of interfacial Li+, which can bring facile Li+ intercalation and deintercalation, but also provided a continuous 3D electron transportation path owing to the enhanced ionic conductivity and mitigated particle aggregation and growth during the cycling.18,19 Here, we report a unique architecture in which Li2FeTiO4 nanoparticles (10−30 nm) are embedded in porous carbon nanotubes (CNTs). As far as I know, this is the first study on electrochemical properties of Li2FeTiO4 as an anode material for LIBs. The formation process of the porous CNTs with embedded Li2FeTiO4 nanoparticles is schematically illustrated in Scheme 1. First, sulfonated polymer nanotubes (SPNTs)20 were immersed in tetrabutyltitanate (TBT) solution for saturated absorption of TBT. Then, an ethanol solution containing

ne urgent task of rechargeable LIBs is to increase the cycle life and energy/power density for the applications in renewable energy systems.1,2 In this respect, anode material is an essential subassembly that determines the performance of LIBs.3,4 However, the current commercial graphite anode materials cannot meet these stringent requirements, due to their safety issue, low energy density and poor rate performance.5 As a result, transition metal oxides (TMO) have been intensively exploited as alternative anode materials, due to the widespread availability and environmental benignity.6,7 Unfortunately, most of the TMO showed poor cycling performance, because of their pulverization and huge volume changes during the charge−discharge cycling.8,9 Therefore, it is still a challenge to search novel anode candidates for LIBs. Utilization of cathode materials as anode has been found considered to be a very attractive research topic in recent years.10 For example, Liu et al. prepared carbon coated Li2MnSiO4, which exhibited a capacity of 150 mAh g−1 at 400 mA g−1.11 Xu et al. reported a Li2FeSiO4 electrode with a capacity of 500 mAh g−1 at 50 mA g−1.12 Similarly, other conventional cathode materials have also been used as anodes.13,14 Therefore, cathode materials could be used as prospective anode materials for LIBs, although more systematic studies should be conducted. Li2FeTiO4 is a typical cathode material of LIBs.15 However, the discharge capacity of Li2FeTiO4 is far below its theoretical capacity.16 Li2FeTiO4 has a cubic cation disordered rock salt structure (with the Fm3̅m space group), which implys that Li+ © XXXX American Chemical Society

Received: March 18, 2018 Accepted: June 1, 2018

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DOI: 10.1021/acsami.8b04418 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration Showing Steps to Synthesize the Hybrids

LiNO3 and Fe(NO3)3·9H2O was added to the mixture and stirred at 60 °C. After centrifugation, the precursor was wiped with tissue paper. Finally, the precursor was further calcined in N2 to obtain porous CNTs with embedded Li2 FeTiO 4 nanoparticles (10−30 nm), denoted as LFT. In Figure S1a, all diffraction peaks of LFT can be indexed as typical rock salt structure (ICSD#183562), well consistent with the literature data.17 Figure S1b shows TGA curve of the LFT. The total weight loss is about 31.7 wt % after 500 °C. Because the oxidation of Fe2+ to Fe3+ can increase the weight by 4.4 wt %, the carbon content is 34.6 wt %. Meanwhile, it is found that the N2 adsorption−desorption isotherm of LFT has a type IV characteristic (Figure S1c). Additionally, the LFT has a specific BET surface area of as high as 198.3 m2 g−1, including micropores (0.6−2 nm) and mesopores (2−10 nm) (Figure S 1d). The micropores are favorite in promoting electronic conduction,21 while the mesopores in iso-oriented nanocrystals are vital for Li+ storage, thus leading to LIBs with high performances. In Figure 1a, the LFT revealing the 1D structure with diameters of 100−120 nm. There are several small nanoparticles on the surface of the nanotubes, resulting in a rough surface. As shown in Figure1b, the nanoparticles with diameters of 10−30 nm are well distributed within the CNT backbones. A small number of the nanoparticles are just attached on the surface. The Li2FeTiO4 nanoparticles have no heavy agglomeration. The formation of the unique structure can be attributed to the fact that the gel was filtrated into walls of the SPNTs and the highly cross-linked copolymer network had space confinement effect in the process of pyrolysis. Such isolated nanoparticles and the 3D carbon matrix would effectively improve the Li+ transportation and ionic conductivity for electrode applications. To further explore the structure of the LFT, HRTEM study was carried out, as presented in Figure 1c. The (111) plane of rock salt is observed, which shows an indistinct lattice fringe (0.241 nm space). The carbon matrix consists of disordered graphene layers covering the Li2FeTiO4 nanoparticles to form the hybrid structure. In addition, EDS mappings of the LFT showed strong signals of C, Fe, Ti and O within the hybrids, as seen in Figure 1d. The original SEM image selected for the EDS mapping is shown in Figure S2. After the SPNTs were calcined in N2 at 700 °C for 8 h, the tube structure almost disappeared because of the excessive melting, as observed in Figure 1e, f. This result further confirms that the LFT contains Li2FeTiO4 nanoparticles embedded in the porous CNTs. At the same time, the Li2FeTiO4 nanoparticles embedded in the porous CNTs could improve mechanical integrity of the tube skeleton.

Figure 1. (a) SEM, (b) TEM, (c) HRTEM images, and (d) EDS mappings of the LFT. (e) SEM and (f) TEM images of the carbon materials from SPNTs.

Figure 2a shows galvanostatic charge−discharge (GDC) profiles of the LFT recorded over a voltage range of 3−0.01 V at 0.2 A g−1. There are two discharge−charge plateaus at approximate 0.7 and 1.4 V, which are similar to the reported results of FeO anode.22 After the first discharge, the subsequent GDC curves showed a similar profile. The first discharge capacities of LFT is 632.3 mAh g−1, while charge capacities of it is 408.6 mAh g−1. It is inevitable to form an SEI layer during the charge and discharge process, which result in the large irreversible capacity loss. Cyclic voltammogram (CV) curves of LFT were recorded at a scan rate of 0.1 mV s−1 (Figure 2b). In first cathodic scan, a strong reduction peak at approximate 0.68 V is the initial reduction of Fe2+ to Fe0.12 Figure 3 shows the ex-situ XPS of LFT and LFT based electrode discharged to 0 V. Prior to the XPS measurements, the LFT and cycled electrode were exposed in air. In both cases, the spectra of Ti are almost the same, with Ti 2p binding energies at about 458.4 eV for Ti 2p3/2 and 464.2 eV for Ti 2p1/2, as illustrated in Figure 3a, c, which are typical for Ti4+ with an octahedral coordination. Fe 2p XPS of the LFT and LFT electrode discharged to 0 V are shown in Figure 3b, d. For LFT, the Fe 2p3/2 peak at 709.8 eV with a satellite peak at 715.9 eV and the Fe 2p1/2 peak at 723.2 eV with a satellite peak at 729.3 eV could be assigned to Fe2+ in Li2FeTiO4. The Fe 2p3/2 peak at 712.6 eV with a satellite peak at 719.7 eV and the Fe 2p1/2 peak at 726.0 eV with a satellite peak at 732.8 eV indicate the presence of Fe3+ impurity, due to the oxidation of the materials in air. After discharging to 0 V, in the high-resolution XPS of Fe 2p3/2, Gaussian fitting of the peaks indicates that a small amount of Fe0 can be observed, as identified by the peak at 707.2 eV. The peak at 709.3 eV with a satellite peak at 715.5 eV implies that Fe2+ are partially remained after discharging to 0 V. And, the Fe 2p3/2 peak at 712.9 eV indicates the presence B

DOI: 10.1021/acsami.8b04418 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) GDC profiles at 0.2 A g−1, (b) CVs at a scan rate of 0.1 mV s−1, (c) cycle performances at 0.2 A g−1 (d), rate performance, and (e) cyclic performance at 2 A g−1 of LFT.

Figure 3. XPS spectra of the (a, b) LFT electrode and (c, d) LFT electrode discharged to 0 V.

in first cycle, can be ascribed to decomposition of the solvent. In addition, a distinct peak is observed at low voltage range near 0 V, which can be ascribed to the electrochemical reaction of

of Fe3+ impurity. In first anodic scan, there is a broad peak at about 1.4 V, corresponding to the oxidation of iron.23,24 The weak cathodic/anodic peak pair, at approximate 1.2 and 0.6 V C

DOI: 10.1021/acsami.8b04418 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) CV curves of the LFT electrode at different scan rates. (b) Relationship between peak current of logarithm anodic and logarithm scan rate. (c) Capacitive contribution and diffusion contribution of the LFT electrode at 2.0 mV s−1. (d) Normalized contribution ratio of capacitive capacities at different scan rates.

carbon matrix.25 After the second scan, the CV curves become similar, implying excellent reversibility of the LFT electrode. Furthermore, the ex-situ XRD of the phase transformation of the pure Li2FeTiO4 at 3 and 0.01 V were measured to further clarify the electrochemical processes (Figure S3). There was no significant changes in the observed peak position, indicating that the rock salt structure remains well during the charge and discharge processes. Meanwhile, the enlarged fraction of Figure S3a, the (200) diffraction peak shifted, indicating that the structural rearrangement during the electrochemical reaction process (Figure S3b), which is attributed to the local octahedral deformation around the Fe2+. Cycle curve of the LFT electrode at 0.2 A g−1 is showed in Figure 2c, exhibiting excellent cyclic stability. Meanwhile, the LFT electrode exhibits a high capacity of 356.1 mAh g−1 after 200 cycles, which confirms that the as-synthesized hybrid with the embedded Li2FeTiO4 nanoparticles shows superior electrochemical performances than most of the reported Li4Ti5O12based electrode, as listed in Table S1. Meanwhile, the electrochemical performance of LFT surpasses those of both pure carbon material from SPNTs and pure Li2FeTiO4 (Figure S4). The improved electrochemical performance of the LFT electrode can be readily due to the unique structure, as discussed above. Because the CNTs embedded with the nanoparticles, particle aggregation is effectively prevented, whereas conductivity of the CNTs is not affected. At the same time, more unhindered ion channels are present for Li+ transportation, while the diffusion distance of Li+ is shortened. Moreover, the porous CNTs acted as a robust skeleton to support the electroactive Li2FeTiO4 during the electrochemical reactions, which can effectively prevent the collapse of the hybrid structures, thus alleviating the structure/volume change of the nanocomposite. Moreover, because it is in the near surface region of active materials that lithium storage mainly occurs, the smaller the particle size, the larger the surface area and thus the higher the capacity will be. It is also believed that the 3D continuous nonwoven fabric like networks of the hybrid

nanotube further enhances the transports of electrolyte ion, which could have also contributed to the enhanced capacity. Figure 2d shows rate performance of LFT, where the charge−discharge rates were tested from 0.5 A g−1 to 8 A g−1. Specifically, the last reversible capacities were 264.1, 206, 151.2, 118.3, and 103.6 mAh g−1 at 0.5, 1, 3, 6, and 8 A g−1, respectively. Meanwhile, the LFT electrode exhibits a capacity of 226.1 mAh g−1 after 1500 cycles at 2 A g−1 (nearly 100% Coulombic efficiency) (Figure 2e). In addition, the cycled LFT electrode was analyzed by using TEM. After cycling, the original textural properties of the hybrid nanotubes were well retained without obvious fracture, while the nanoparticles were also present inside the carbon matrix (Figure S5). Electrochemical impedance spectroscopy (EIS) of LFT is shown in Figure S6. The equivalent circuit of EIS shown in the inset in Figure S6. Obviously, the charge transfer resistance of LFT is sufficiently low, which ensured fast electron transport and enhanced conductivity during the Li+ insertion/extraction reactions. The superior rate capability of LFT should have its kinetic origin. Figure 4a shows CVs of LFT at different scan rates. The curve shape is well reserved as the scan rate is increased from 0.1 to 5 mV s−1. According to the equation i = avb, we can examined the degree of capacitive effect. Where a and b are constants, i is measured current and v is scan rate.26 The value of b is determined from the slope of the log i versus log v plot (Figure 4b). For a diffusion-controlled process, b is close to 0.5, whereas b is close to 1.0 for a surface capacitance dominated process.27 Hence, a high value of b (0.84) suggests that the LFT is mainly governed by capacitive kinetics. Figure 4c shows typical voltage profile of the capacitive current (blue region). The LFT has a capacitive contribution of 66%. With increasing scan rate, the capacitive contribution is expectedly increased (Figure 4d). This is not surprising, because capacitive contribution is usually accompanied by small particles with high surface area and high porosity. D

DOI: 10.1021/acsami.8b04418 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

(8) Wang, J.; Shen, L. F.; Nie, P.; Xu, G. Y.; Ding, B.; Fang, S.; Dou, H.; Zhang, X. G. Synthesis of Hydrogenated TiO2−ReducedGraphene Oxide Nanocomposites and Their Application in High Rate Lithium Ion Batteries. J. Mater. Chem. A 2014, 2, 9150−9155. (9) Zhong, Y.; Ma, Y. F.; Guo, Q. B.; Liu, J. Q.; Wang, Y. D.; Yang, M.; Xia, H. Controllable Synthesis of TiO2@Fe2O3 Core-Shell Nanotube Arrays with Double-Wall Coating as Superb Lithium-Ion Battery Anodes. Sci. Rep. 2017, 7, 40927−40935. (10) Chen, N.; Yao, Y.; Wang, D. X.; Wei, Y. J.; Bie, X. F.; Wang, C. Z.; Chen, G.; Du, F. LiFe(MoO4)2 as a Novel Anode Material for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 10661− 10666. (11) Liu, S. S.; Song, L. J.; Yu, B. J.; Wang, C. Y.; Li, M. W. Comparative Study of the Cathode and Anode Performance of Li2MnSiO4 for Lithium-Ion Batteries. Electrochim. Acta 2016, 188, 145−152. (12) Xu, Y.; Li, Y. J.; Liu, S. Q.; Li, H. L.; Liu, Y. N. Nanoparticle Li2FeSiO4 as Anode Material for Lithium-Ion Batteries. J. Power Sources 2012, 220, 103−107. (13) Chen, N.; Wang, C. Z.; Hu, F.; Bie, X. F.; Wei, Y. J.; Chen, G.; Du, F. Brannerite-Type Vanadium-Molybdenum Oxide LiVMoO6 as a Promising Anode Material for Lithium-Ion Batteries with High Capacity and Rate Capability. ACS Appl. Mater. Interfaces 2015, 7, 16117−16123. (14) Guo, X. W.; Fang, X. P.; Mao, Y.; Wang, Z. X.; Wu, F.; Chen, L. Q. Capacitive Energy Storage on Fe/Li3PO4 Grain Boundaries. J. Phys. Chem. C 2011, 115, 3803−3808. (15) Sebastian, L.; Gopalakrishnan, J. Li2MTiO4 (M=Mn, Fe, Co, Ni): New Cation-Disordered Rocksalt Oxides Exhibiting Oxidative Deintercalation of Lithium. Synthesis of an Ordered Li2NiTiO4. J. Solid State Chem. 2003, 172, 171−177. (16) Yang, M.; Zhao, X. Y.; Yao, C.; Kong, Y.; Ma, L. Q.; Shen, X. D. Nanostructured Cation Disordered Li2FeTiO4/Graphene Composite as High Capacity Cathode for Lithium-Ion Batteries. Mater. Technol. 2016, 31, 537−543. (17) Kuezma, M.; Dominko, R.; Hanzel, D.; Kodre, A.; Arcon, I.; Meden, A.; Gaberscek, M. Detailed In Situ Investigation of the Electrochemical Processes in Li2FeTiO4 Cathodes. J. Electrochem. Soc. 2009, 156, 809−816. (18) Tang, Y. K.; Liu, L.; Zhao, H. Y.; Gao, S. S.; Lv, Y.; Kong, L. B.; Ma, J. H.; Jia, D. Z. Hybrid Porous Bamboo-like CNTs Embedding Ultrasmall LiCrTiO4 Nanoparticles as High Rate and Long Life Anode Materials for Lithium Ion Batteries. Chem. Commun. 2017, 53, 1033− 1036. (19) Xia, G. L.; Zhang, L. J.; Fang, F.; Sun, D. L.; Guo, Z. P.; Liu, H. K.; Yu, X. B. General Synthesis of Transition Metal Oxide Ultrafine Nanoparticles Embedded in Hierarchically Porous Carbon Nanofibers as Advanced Electrodes for Lithium Storage. Adv. Funct. Mater. 2016, 26, 6188−6196. (20) Tang, Y. K.; Liu, L.; Zhao, H. Y.; Kong, L. B.; Guo, Z. P.; Gao, S. S.; Che, Y. Y.; Wang, L.; Jia, D. Z. Rational Design of Hybrid Porous Nanotubes with Robust Structure of Ultrafine Li4Ti5O12 Nanoparticles Embedded in Bamboo-Like CNTs for Superior Lithium Ion Storage. J. Mater. Chem. A 2018, 6, 3342−3349. (21) Tang, Y. K.; Liu, L.; Wang, X. C.; Zhou, H. J.; Jia, D. Z. HighYield Bamboo-like Porous Carbon Nanotubes with High-Rate Capability as Anodes for Lithium-Ion Batteries. RSC Adv. 2014, 4, 44852−44857. (22) Petnikota, S.; Marka, S. K.; Banerjee, A.; Reddy, M. V.; Srikanth, V. V. S. S.; Chowdari, B. V. R. Graphenothermal Reduction Synthesis of ‘Exfoliated Graphene Oxide/Iron (II) Oxide’ Composite for Anode Application in Lithium Ion Batteries. J. Power Sources 2015, 293, 253− 263. (23) Zhu, X.; Song, X. Y.; Ma, X. L.; Ning, G. Q. Enhanced Electrode Performance of Fe2O3 Nanoparticle-Decorated Nanomesh Graphene as Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 7189−7197. (24) Ji, L. W.; Toprakci, O.; Alcoutlabi, M.; Yao, Y. F.; Li, Y.; Zhang, S.; Guo, B. K.; Lin, Z.; Zhang, X. W. α-Fe2O3 Nanoparticle-Loaded

Such a porous hybrid nanotube architecture not only prevented serious agglomeration and growth of the Li2FeTiO4 nanoparticles during cycling, but also provided a 3D conductive carbon matrix, with numerous interfacial Li+ storage sites. As a result, the transportation of Li+ and electrons was significantly enhanced, whereas the contact area for reaction between the Li2FeTiO4 particles and the electrolyte was enlarged, so that the electrode would have high electrical conductivity. In addition, the fast Li+ storage in the hybrid was governed by capacitive effect. The extraordinarily performances, together with the costeffectiveness, abundance, and environmental benignity of the raw materials, make our Li2FeTiO4-based nanohybrid to be a promising LIB anode material.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04418. Details on materials and methods and additional EIS, cycle performance, XRD, TGA curve, N2 adsorption− desorption isotherm and pore size distribution, SEM and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Lang Liu: 0000-0002-2390-8738 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51672235), the Graduate Research Innovation Project of Xinjiang (XJGRI2016002) and the Program for Changjiang Scholars and Innovative Research Team in the University of Ministry of Education of China (IRT1081).

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REFERENCES

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DOI: 10.1021/acsami.8b04418 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX