Sn4+ Ion Decorated Highly Conductive Ti3C2 MXene: Promising

Feb 2, 2016 - Two-Dimensional Ti 3 C 2 T X /CMK-5 nanocomposite as high ..... capacitance for integrated co-cathode energy conversion/storage devices...
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Sn4+ Ion Decorated Highly Conductive Ti3C2 MXene: Promising Lithium-Ion Anodes with Enhanced Volumetric Capacity and Cyclic Performance Jianmin Luo, Xinyong Tao,* Jun Zhang, Yang Xia, Hui Huang, Liyuan Zhang, Yongping Gan, Chu Liang, and Wenkui Zhang* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China S Supporting Information *

ABSTRACT: Two-dimensional transition metal carbide materials called MXenes show potential application for energy storage due to their remarkable electrical conductivity and low Li+ diffusion barrier. However, the lower capacity of MXene anodes limits their further application in lithium-ion batteries. Herein, with inspiration from the unique metal ion uptake behavior of highly conductive Ti3C2 MXene, we overcome this impediment by fabricating Sn4+ ion decorated Ti3C2 nanocomposites (PVP-Sn(IV)@ Ti3C2) via a facile polyvinylpyrrolidone (PVP)-assisted liquid-phase immersion process. An amorphous Sn(IV) nanocomplex, about 6−7 nm in lateral size, has been homogeneously anchored on the surface of alk-Ti3C2 matrix by ion-exchange and electrostatic interactions. In addition, XRD and TEM results demonstrate the successful insertion of Sn4+ into the interlamination of an alkalization-intercalated Ti3C2 (alk-Ti3C2) matrix. Due to the possible “pillar effect” of Sn between layers of alk-Ti3C2 and the synergistic effect between the alk-Ti3C2 matrix and Sn, the nanocomposites exhibit a superior reversible volumetric capacity of 1375 mAh cm−3 (635 mAh g−1) at 216.5 mA cm−3 (100 mA g−1), which is significantly higher than that of a graphite electrode (550 mAh cm−3), and show excellent cycling stability after 50 cycles. Even at a high current density of 6495 mA cm−3 (3 A g−1), these nanocomposites retain a stable specific capacity of 504.5 mAh cm−3 (233 mAh g−1). These results demonstrate that PVP-Sn(IV)@Ti3C2 nanocomposites offer fascinating potential for high-performance lithium-ion batteries. KEYWORDS: MXene, Ti3C2, lithium-ion battery, nanocomposites

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is an early transition metal, A is a group IIIA or IVA element (i.e., group 13 or 14), X is C and/or N, and n = 1, 2, or 3.7 When Ti3AlC2 was etched by HF, the lost Al atoms will be replaced by O, OH, and/or F atoms, forming Ti3C2(OH)xOyFz for the Ti3C2 (a kind of MXene material) parent phase. These materials have been applied in many fields, such as hydrogen storage,8,9 photocatalysis,10 and sewage purification.11,12 Since they have high volumetric capacitance, the main application field of these materials is supercapacitors, which has attracted the attentions of researchers from all over the world. A number of researches have reported that MXenes can also be used as electrode materials for LIBs.13−18 Mashtaliar et al.14 reported a capacity of 410 mAh g−1 for a Ti3C2 MXene “paper” anode at

he depletion of fossil-fuel resources and the pollution of the environment have made the development of advanced renewable energy technologies a global imperative. Lithium-ion batteries (LIBs), with their high capacity and cycling stability, already dominate the portable electronics market, and they are considered the most promising candidate for the future of energy storage. Two-dimensional (2D) materials such as graphene,1 sulfides, 2 nitrides, 3 oxide, 4 and metal dichalcogenides (TMDs)5 have also aroused considerable interest because of their unique physical and chemical properties when used as LIB electrode materials. Recently, first reported by Gogotsi et al. in 2011, a new group of early transition metal carbides/ carbonitrides labeled MXenes have been added to this class of 2D materials.6 They are produced by etching of the A layers in HF solution for a certain time from the MAX phases, their name being derived from their composition, Mn+1AXn, where M © XXXX American Chemical Society

Received: November 20, 2015 Accepted: February 2, 2016

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ACS Nano 1C rate without adding any binder. This Ti3C2 MXene “paper” was fabricated by intercalating Ti3C2 with dimethyl sulfoxide, bath sonication for 6 h, and filtering the Ti3C2 colloidal solution. Density functional theory (DFT) calculations by Tang et al.19 predict that the Li storage capability of Ti3C2 can reach 320 mAh g−1. However, once the surface of Ti3C2 was terminated with −F or −OH groups after HF etching, Li transport would be blocked and thus result in the decreased storage capacity of Li; the theoretical specific capacity rapidly decreased to 67 mAh g−1 for Ti3C2(OH)2 or 130 mAh g−1 for Ti3C2F2.19 Sun et al. demonstrated that the Ti3C2 anode materials exhibited a capacity of 123.6 mAh g−1 at 1C rate.20 Although Ti3C2 MXene has good electrical conductivity6 and a low Li+ diffusion barrier,19 the capacity of Ti3C2 anodes is not high enough, which limits its application as lithium electrode materials. Recently, Gogotsi et al.21 confirmed that −F and −OH groups on the surface of alkalization-intercalated Ti3C2 (alkTi3C2) would result in a negatively charged surface of the alkTi3C2. Therefore, some metal cations will be successfully absorbed onto the alk-Ti3C2 by electrostatic interactions when the alk-Ti3C2 is immersed in a solution containing metal cations. Peng et al.11 confirmed that Ti3C2 after alkalization intercalation presented efficient Pb(II) uptake performance, and the sorption behavior of Pb(II) was related to the ionchange sites of hydroxyl groups in Ti3C2-activated Ti sites. Inspired by the unique metal ion uptake behavior of alkTi3C2, we attempted to fabricate PVP-Sn(IV)@Ti3C2 nanocomposites via a facile polyvinylpyrrolidone (PVP)-assisted liquid-phase immersion process. The Sn(IV)@Ti3C2 nanocomposites have four distinguishing features: (i) Alk-Ti3C2 acts as the 2D network to enhance the electronic conductivity and shorten the Li+ diffusion pathway22,23 and provides part of the capacity for the whole composite. (ii) Sn4+ can be successfully anchored onto the alk-Ti3C2 matrix in the form of an amorphous Sn(IV) complex by ion-change interaction and electrostatic interactions, which will effectively confine the occurrence of Sn(IV) detached from the matrix. The possible “pillar effect” of Sn between layers of alk-Ti3C2 and the synergistic effect between the alk-Ti3C2 matrix and Sn endow the nanocomposite with outstanding electrochemical properties. (iii) With the surfactant, the adsorbed Sn(IV) can be uniformly distributed in the alk-Ti3C2 matrix. The nanosized Sn(IV) aggregates will efficiently accommodate strain induced by volume change during the electrochemical reaction processes. As anode materials, the synthesized Sn(IV)@Ti3C2 nanocomposites display extraordinary electrochemical performance with superior reversible volumetric capacity, high Coulombic efficiency, and excellent cycling stability and rate performance.

Figure 1. Schematic illustration of the fabrication process of PVPSn(IV)@Ti3C2 nanocomposites.

surface and enter the interlamination of Ti3C2, constituting PVP-Sn(IV)@Ti3C2 nanocomposites. The structure and morphology before and after HF exfoliation were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The raw material Ti3AlC2 powders contain small amounts of TiC as secondary phases, which are estimated to be 98 wt % purity) were purchased from Shanghai Kennametal-Sintec Co., Ltd. The Ti3AlC2 powder (3 g) was immersed in 40% HF solutions (30 mL) at room temperature by stirring for 24 h to exfoliate the Al layers. Then the resulting powder was rinsed by deionized water several times to remove the residual HF and impurities until the pH of solution reached 5−6. After that the obtained wet sediments (Ti3C2) were desiccated at 80 °C for 12 h. Alkalization of Ti3C2. After desiccation, Ti3C2 was treated using 1 M LiOH solution for 24 h to increase the interlayer spacing. Then the alkalized Ti3C2 (alk-Ti3C2) was rinsed by deionized water several times to remove the remaining alkali until the pH of the solution reached 8− 9. The obtained wet sediments (alk-Ti3C2) were desiccated at 80 °C for 12 h. Preparation of PVP-Sn(IV)@Ti3C2 Nanocomposites. Alk-Ti3C2 was then immersed in a 1 M SnCl4 solution at room temperature; at the same time 0.1 g of PVP was dissolved into the solution and stirred for 24 h. Finally, PVP-Sn(IV)@Ti3C2 nanocomposites were obtained after the centrifugation of the mixed solution followed by drying at 80 °C for 24 h. Material Characterization. XRD was conducted with an X’Pert Pro diffractometer using Cu Kα radiation (λ = 0.154 18 nm) to characterize the phase purity and crystalline structure of the sample. SEM, used to observe the morphology of the samples, was conducted with a Hitachi S4700. A TEM (FEI, Tecnai G2 F30) equipped with an EDS detector, was used to observe the microstructure of the sample and investigate the element distribution. FT-IR was performed by an infrared spectrophotometer (Nicolet 6700) using the KBr pellet method. XPS analysis was conducted using an Al Ka monochromatic X-ray source (1486.6 eV, Axis Ultra DLD, Kratos). XRF was conducted using an Arl Advant’X IntelliPowerTM 4200. Elemental analysis was also carried out by ICP-MS (PerkinElmer, Elan DRC-e). Electrochemical Measurement. The electrochemical performance of materials (Ti3C2, Sn(IV)@Ti3C2, PVP-Sn(IV)@Ti3C2) was evaluated using coin-type test cells (CR2025) with lithium metal working as both the counter and reference electrode. The working electrode was composed of active materials, acetylene black, and a polyvinylidene fluoride binder in a weight ratio of 70:15:15. The electrolyte was 1 M LiPF6 in ethylene carbonate and dimethyl

structural development of the samples after 50 cycles. It was found that PVP-Sn(IV)@Ti3C2 could maintain its layered structure after long-term cycling (Figure 6). The layer spacing along the c-axis is about 1.345 nm (Figure 6b), which is larger than the layer spacing along the c-axis (1.276 nm) of PVPSn(IV)@Ti3C2 before cycling. The increment of layer spacing, 0.069 nm, is mainly due to the volume expansion during the interlamination of the alk-Ti3C2 matrix, which may be caused by the intercalation of Li+ and alloying effect between Li+ and Sn. This expansion value of 0.069 nm is much larger than that of Ti3C2 (0.043 nm, Figure S11), further confirming the successful insertion of Sn4+ in the interlamination of the alkTi3C2 matrix. Additionally, the more clearly cross-sectional STEM image of PVP-Sn(IV)@Ti3C2 nanocomposites, corresponding elemental mapping of Ti, C, and Sn, and line-scan elemental mapping after 50 cycles (Figure S12) are supplied to jointly confirm the successful insertion of Sn4+ in the interlamination of the alk-Ti3C2 matrix. The corresponding results are consistent with the results in Figure S4 before cycling, where the Sn signal can maintain a homogeneous distribution and remains invariant on the surface and in the interlamination of Ti3C2. The HRTEM image of PVP-Sn(IV)@ Ti3C2 after cycling (Figure 6C) clearly shows that the d-spacing is measured to be 0.210 nm, which belongs to the (220) planes of the tetragonal Sn phase (JCPDS card no. 65-5224).29 The crystalline Sn formed on surface of the alk-Ti3C2 matrix after cycling results from the electrochemical transformation of the superficial Sn(IV) nanocomplex with lithium. The EDS mapping in Figure 6d confirms that Sn is still homogeneously distributed on the surface and in the interlamination of Ti3C2, suggesting the stable and robust structure of the PVP-Sn(IV)@ Ti3C2 nanocomposite. G

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ACS Nano carbonate (1:1 vol %). The cell assembly was conducted in a glovebox under an argon atmosphere with a Celgard membrane as the separator. The galvanostatic charge−discharge tests were performed on a Neware battery test system between 0.01 and 3.0 V at room temperature. CV measurement was conducted with a CHI660D electrochemical workstation at a scan rate of 0.1 mV s−1 within 0−3 V. EIS measurements were tested by a CHI660D electrochemical workstation with the frequency ranging from 0.01 to 100 kHz. GITT experiments were also performed on a Neware battery test system by discharging/ charging the cells for 1 h at a current density of 32.5 mA g−1. The relaxation period was set as 1 h.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07333. XRD patterns of Ti3C2 samples via different HF immersion times, XRD patterns of Ti3C2 immersed in different 1 M solutions for 24 h, XRD patterns of samples after Sn loading, cross-sectional STEM image of PVP-Sn(IV)@Ti3C2 nanocomposites before and after 50 cycles, and corresponding line-scan STEM-EDS analysis as well as the elemental mapping of Ti, C, and Sn, XPS spectra of alk-Ti3C2 before and after Sn(IV) nanocomplex loading, cycling performance of Ti3C2, alkTi3C2, and PVP@alk-Ti3C2, TEM and HRTEM images of Ti3C2, CV curves of PVP-Sn(IV)@Ti3C2 nanocomposites with sweep rates ranging from 0.1 to 2 mV s−1, GITT curves of alk-Ti3C2 and PVP-Sn(IV)@Ti3C2, discharge−charge curves of PVP-Sn(IV)@Ti3C2 electrodes at different current densities, XRF elements measurement of PVP-Sn(IV)@Ti3C2, and comparison of electrochemical performance of various Sn-based anode materials (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (X. Tao): [email protected]. *E-mail (W. Zhang): [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Professor Yury Gogotsi, Drexel University, for providing the structural information about Ti3C2. The authors acknowledge financial support by the National Natural Science Foundation of China (Grant Nos. 51002138 and 51572240), the Natural Science Foundation of Zhejiang Province (Grant Nos. LQ14E020005, LR13E020002, LY13E020010, and LY15B030003), Scientific Research Foundation of Zhejiang Provincial Education Department (Grant No. Y201432424), and Ford Motor Company. REFERENCES (1) Geim, A.; Novoselov, K. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (3) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (4) Ma, R.; Sasaki, T. Nanosheets of Oxides and Hydroxides: Ultimate 2D Charge-Bearing Functional Crystallites. Adv. Mater. 2010, 22, 5082−5104. H

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