Supporting Information for Synergistically Enhanced Electrochemical Performance of Hierarchical MoS 2 /TiNb 2 O 7 Hetero-Nanostructures as Anode Materials for Li-Ion Batteries De Pham-Cong, Jun Hee Choi, Jeongsik Yun, Aliaksandr S. Bandarenka, Jinwoo Kim, # Paul V. Braun, # Se Young Jeong, Chae Ryong Cho * Department of Nanoenergy Engineering and College of Nanoscience and Nanotechnology, Pusan National University, Busan 46241, Republic of Korea Device & System Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 16676, Republic of Korea Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany # Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Figure S1. SEM images of the TiNb2O7 nanofibers annealed at (a,b) 650 C, (c,d) 750 C, (e,f) 850 C, and (g,h) 950 C with a ramp rate of 3 C min 1 for 4 h in air.
Figure S2. (a) TGA curve of the as-electrospun polymeric TiNb 2 O 7 nanofibers with a ramp rate of 10 C min 1 and (b) XRD patterns of the TiNb 2 O 7 nanofibers as a function of annealing temperature (650, 750, 850, and 950 C).
Figure S3. SEM images and photographs of (a,b) as-electrospun TiNb 2 O 7 nanofibers, (c,d) TiNb 2 O 7 nanofibers annealed in air, (e,f) TNO@MS HRs, and (g,h) MoS 2 nanosheets without TiNb 2 O 7 nanofibers as a backbone. The color of the TNO@MS HRs and MoS 2 nanosheets was changed from white to black after annealing in an Ar/H 2 gas atmosphere.
Figure S4. EDS spectra of the TNO@MS HRs (a) before and (b) after annealing in the Ar/H 2 mixture gas. The atomic percentage ratios of Ti/Nb and Mo/S were approximately 0.5 and matched with TiNb 2 O 7 and MoS 2
Figure S5. XPS spectra of the TiNb 2 O 7 nanofibers. (a) Survey scan and (b-d) high-resolution scan of Ti 2p, Nb 3d, and O 1s chemical bonding states.
Figure S6. XPS spectra of the TNO@MS HRs. (a) Survey scan and (b f) high-resolution scan of Mo 3d, S 2p, Ti 2p, Nb 3d, and O 1s chemical bonding states. XPS was used to investigate the surface chemical bonding states and composition of the TiNb 2 O 7 nanofibers and TNO@MS HRs. Although the Ti, Nb, and O elements in the high-resolution XPS spectra are clearly detected in both samples, Mo and S elements were only detected in the TNO@MS HRs (Figure S5,6). In the survey XPS scan, the C 1s peak at 284.6 ev is attributed to the adventitious carbon element contaminated during sample preparation in air (Figure S5a, S6a). In the TiNb 2 O 7 nanofibers, the binding energies of Ti
2p 3/2 and Ti 2p 1/2 in Figure S5b were 458.7 and 464.4 ev, respectively. Figure S5c presents an XPS spectrum of two Nb 3d peaks, which are assigned to Nb 3d 5/2 (206.9 ev) and Nb 3d 3/2 (209.5 ev). The binding energy of O 1s in Figure S5d was 530.2 ev. Typical high-resolution XPS spectra of the Mo 3d, S 2p, Ti 2p, Nb 3d, and O 1s chemical bonding states for the TNO@MS HRs are presented in Figure S6b-6f, respectively. The three peaks located at 229.4, 232.3, and 236.0 ev in Figure S6b are assigned to Mo 4+ 3d 5/2, Mo 4+ 3d 3/2, and inconspicuous Mo 6+ 3d 3/2, respectively. The peak corresponding to Mo 6+ 3d 3/2 may be attributed to the incomplete reduction process by the hydrothermal method. [S1] The two peaks at 162.1 and 163.3 ev corresponding to S 2p 3/2 and S 2p 1/2 of the S 2p region are deconvoluted, respectively (Figure S6c). In Figure S6d and S6e, the binding energies of Ti 2p 3/2 and Ti 2p 1/2 and Nb 3d 5/2 and Nb 3d 3/2 were 459.2 and 465.3 ev and 207.5 and 210.2 ev, respectively; these peaks are slightly shifted to higher binding energy compared with those of the TiNb 2 O 7 nanofibers. Because of the electron-attracting energy of the MoS 2 nanosheets, the increase of the binding energy for the Ti and Nb ions may be attributed to the electrons around Ti and Nb ions being transferred to the surface of MoS 2 and thus decreasing the electron density of Ti and Nb. [S2] In particular, the extra peaks were observed at higher binding energy than the main Ti peaks (marked as in Figure S6d). These peaks might be attributed to shake-up lines caused by a sudden change in the coulombic potential as the photo-ejected electron passes through the valence band, which is generally expected in paramagnetic states. The outgoing electron interacts with a valence electron and shakes it up to a higher energy level. Therefore, the energy of the core electron is reduced, and a satellite structure appears a few electron-volts above the core level position (on the binding energy scale). This finding may also be considered a screening effect of the MoS 2 layers for the electrons ejected from the TiNb 2 O 7 nanofibers. Consequently, the shift of the chemical bonding states in TNO@MS HRs indicates the presence of electron coupling between the TiNb 2 O 7 nanofibers and MoS 2 nanosheets, which would be beneficial for efficient charge transfer during the charge/discharge process. References: [S1] Al-Mamun, M.; Zhang, H.; Liu, P.; Wang, Y.; Cao, J.; Zhao, H. Directly Hydrothermal Growth of Ultrathin MoS 2 Nanostructured Films as High Performance Counter Electrodes for Dye-Sensitized Solar Cells. RSC Adv. 2014, 4, 21277. [S2] Zhu, B.; Lin, B.; Zhou, Y.; Sun, P.; Yao, Q.; Chen, Y.; Gao, B. Enhanced Photocatalytic H 2 Evolution on ZnS Loaded with Graphene and MoS 2 Nanosheets as Cocatalysts. J. Mater. Chem. A 2014, 2, 3819.
Figure S7 (a,b) TEM images of the TiNb 2 O 7 nanofibers, (c,e) HRTEM images of the TiNb 2 O 7 nanofiber, and (d,f) fast-fourier transform images of the squared-line regions obtained from the SAED patterns shown in the insets.
Figure S8. (a) TEM image of the TiNb 2 O 7 nanofiber and (b d) EDS mapping images corresponding to oxygen (white), titanium (green), and niobium (red) elements.
Figure S9. (a) Cross-sectional and (g) axial bright-field TEM images of the TNO@MS HR. (b f) and (h l) Corresponding elemental EDS mapping images of (a) and (g), respectively.
We have compared the electrochemical performances of TNO@MS HR-based anodes with those of the reported MoS 2 and TiNb 2 O 7 materials of different forms, as summarized in Table S1. Microsphere- or nanoparticle TiNb 2 O 7 has relatively high capacity retention (> 80% after > 500 cycles), but inferior specific charge capacity (< 200 mah g 1 ). On the other hand, exfoliated or hierarchical MoS 2 has relatively large specific charge capacity (> 500 mah g 1 ) with poor capacity retention (~67% after 70 cycles). As we examined, these are attributed to the structural features of TiNb 2 O 7 and MoS 2. The former is advantageous for high electronic conduction with limited capability of Li + ion intercalation. On the other hand, the latter has abundant Li + ion channels, which might rapidly degrade by layer-by-layer dissociation of the MoS 2. In the TNO@MS, TiNb 2 O 7 nanofibers work as a backbone core for high electronic conduction, thin/ independent MoS 2 shells for enhanced Li + ion intercalation, and robust binding between them could maximize the synergistic effect of exhibiting mostly the advantageous aspects of both materials. It should be noted that, even compared to other hybrid type anodes, the TNO@MS HR-based anodes exhibit superior performance, i.e. simultaneously fulfills large charge capacity and high rate/retention capability. Table S1: Comparison of electrochemical performances of TiNb 2 O 7 -, MoS 2 -, and TNO@MS HR-based anodes having different forms. Materials Capacity (mah g 1 ) Retention rate (RR=C f /C i ) Current density (A g 1 ) Voltage range (V vs. Li/Li + ) Cycle number Reference TNO@MS HRs 739 739/872=0.85 1 0.001 3.0 200 This work TiNb 2 O 7 190 190/209=0.91 1.89 1.0 3.0 1000 [S3] microspheres TiNb 2 O 7 115 115/138=0.83 3.87 1.0 3.0 500 [S4] nanoparticles Exfoliated MoS 2 510 510/830=0.61 0.05 0.01 3.0 50 [S5] Hierarchical MoS 2 microspheres 585 585/873=0.67 0.1 0.01 3.0 70 [S6] TiO 2 /C@MoS 2 805 805/795=1.01 0.1 0.01 3.0 100 [S7] Crumpled 680 680/1050=0.65 0.5 0.01 3.0 250 [S8] rgo/mos 2 References: [S3] Park, H.; Wu, H. B.; Song, T.; Lou, X. W.; Paik, U. Adv. Energy Mater. Porosity- Controlled TiNb 2 O 7 Microspheres with Partial Nitridation as A Practical Negative Electrode for High-Power Lithium-Ion Batteries. 2015, 5, 1401945.
[S4] Li, H.; Shen, L.; Pang, G.; Fang, S.; Luo, H.; Yang, K.; Zhang, X. TiNb 2 O 7 Nanoparticles Assembled into Hierarchical Microspheres as High-Rate Capability and Long- Cycle-Life Anode Materials for Lithium Ion Batteries. Nanoscale 2015, 7, 619. [S5] Xiao, J.; Choi, D.; Cosimbescu, L.; Koech, P.; Liu, J.; Lemmon, J. P. Exfoliated MoS 2 Nanocomposite as An Anode Material for Lithium Ion Batteries. Chem. Mater. 2010, 22, 4522. [S6] Ding, S.; Zhang, D.; Chen, J. S.; Lou, X. W. Facile Synthesis of Hierarchical MoS 2 Microspheres Composed of Few-Layered Nanosheets and Their Lithium Storage Properties. Nanoscale 2012, 4, 95. [S7] Chen, B.; Liu, E.; He, F.; Shi, C.; He, C.; Li, J.; Zhao, N. 2D Sandwich-Like Carbon- Coated Ultrathin TiO 2 @Defect-Rich MoS 2 Hybrid Nanosheets: Synergistic-Effect-Promoted Electrochemical Performance for Lithium Ion Batteries. Nano Energy 2016, 26, 541. [S8] Xiong, F.; Cai, Z.; Qu, L.; Zhang, P.; Yuan, Z.; Asare, O. K.; Xu, W.; Lin, C.; Mai, L. Three-Dimensional Crumpled Reduced Graphene Oxide/MoS 2 Nanoflowers: A Stable Anode for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 12625.
Figure S10. CV curves of (a) TiNb 2 O 7 nanofibers and (b) TNO@MS HRs for various scan rates from 0.1 to 2.5 mv s 1 in the voltage range of 0.001 3.0 V and (c) current square root of scan rate relations of TiNb 2 O 7 nanofibers and TNO@MS HRs in the voltage range. The data were obtained from anodic and cathodic current peaks, respectively. The Li ion diffusion coefficient of the samples can be calculated according to the Randles-Sevcik equation: I p = 0.4463 C A (n F) 3/2 (R T) 1/2 (D Li v) 1/2, (S1) where I p is the peak current (A), n is the charge transfer number, F is the Faraday constant, C is the concentration, A is the area of the electrode (cm 2 ), R is the gas constant, T is the absolute temperature (298 K), D Li is the diffusion coefficient (cm 2 s 1 ), and v is the scanning rate (V s 1 ). The relationship between the peak current value and square root of the scan rate exhibits a linear behavior. Improvement of the Li-ion kinetics during the insertion/extraction process is well known as an important factor in achieving better rate capability of the electrode. Table S2: Diffusion coefficients of Li ion calculated from the equation (S1) and Figure S10c. Active materials TNO NFs TNO@MS HRs Anodic D Li (cm 2 s 1 ) 0.90 10 13 1.60 10 12 Cathodic D Li (cm 2 s 1 ) 1.60 10 13 2.44 10 12
TiNb 2 O 7 micro/nanostructure-based anodes have been reported to exhibit excellent rate performances (Table S1): i) nanoparticle form: RR of 83% at 3.87 A g 1 and 500 cycles, and ii) microsphere form: RR of 91% at 1.89 A g 1 and 1000 cycles. The TNO@MS HRbased anodes yield the similar RR of 85% at 1 A g 1 and 200 cycles. The 1st cycle irreversible loss (L irr ), as a strong indicator of the rate performance, was newly measured from charge/discharge curves during the first cycle (Figure S11). The values of L irr for TiNb 2 O 7 nanofiber- and TNO@MS HR-based anodes are 94 and 218 mah g 1, respectively, which corresponds to 23.5 and 21.2% of the first discharge capacity of TiNb 2 O 7 nanofiberand TNO@MS HR-based anodes, respectively. Taking all these account, it suggests that the TNO@MS HR-based anodes provide the rate performance close or comparable to pure TiNb 2 O 7 nanofiber-based anodes. Pure MoS 2 -based anodes are generally known to yield poor rate/retention despite high initial charge capacity (Figure 4a d in the main body). As discussed, the poor rate/retention performances are attributed to the layer-by-layer dissociation of the MoS 2 during the intercalation of Li + ions. This in turn results in decrease in electronic conduction paths, making further incorporation of Li + ions difficult due to the space charge effect. On the other hand, in the TNO@MS HRs where few-atom-thick/independent MoS 2 sheets are strongly bound around the TiNb 2 O 7 nanofiber core, the structural instability of MoS 2 is minimized due to the very thin geometry, and the space charge accumulation rarely occurs due to presence of the TiNb 2 O 7 core as a robust backbone for electronic conduction paths. Figure S11. The first charge/discharge curves of TNO nanofiber-, TNO@MS HR- and MoS 2 - based anodes at current density of 1 A g 1 in the voltage range 0.001 3.0 V.
The lithiation potential (V lith ) of the TNO@MS HR-based anode is high, which is unfavorable in the view point of energy density. As will be estimated below, however, the energy density of the LIBs based on TNO@MS HR-anodes is still competitive with other anodes. As shown in Figure S12a, the V lith values of LTO-, graphite-, and TNO@MS HRanodes are estimated to be ~1.5 [S9], 0.1 [S10], and 1.6 V, respectively. (We estimate the average V lith by attaining midpoint potentials [S11] or the integrated area divided by charge capacity in the charge/discharge curves.) When operated with the cathodes with V lith of ~4.0 V (for example, LiMnO 2 or LiCoO 2 ), the average operation voltages of these anodes are ~2.5, 3.9, and 2.4 V, respectively. With their charge capacity values (LTO (150 mah g 1 ), graphite (350 mah g 1 ) and TNO@MS HRs (850 mah g 1 )), the energy density is calculated to be 375, 1365, and 2040 mwh g 1, as demonstrated in Figure S12b. Furthermore, we believe that the TNO@MS HR-anodes can find visibility for application of high power density LIBs due to its high rate capability. (a) (b) Figure S12. (a) The schematic charge/discharge curves for three different anodes; Li 4 Ti 5 O 12, graphite, and TNO@MS HR-based anodes. (b) Comparison of energy density of LTO, graphite, and TNO@MS HRs, normalized by the energy density of LTO. References [S9] Wang, X.; Xing, L.; Liao, X.; Chen, X.; Huang, W.; Yu,Q.; Xu, M.; Huang, Q.; Li, W. Improving Cyclic Stability of Lithium Cobalt Oxide Based Lithium Ion Battery at High Voltage by Using Trimethylboroxine as An Electrolyte Additive. Electrochimical Acta 2015, 173, 804. [S10] Kim, J.; Kim, J. Y.; Pham-Cong, D.; Jeong, S. Y.; Chang, J.; Choi, J. H.; Braun, P. V.; Cho, C. R. Individually Carbon-Coated and Electrostatic-Force-Derived Graphene-Oxide- Wrapped Lithium Titanium Oxide Nanofibers as Anode Material for Lithium-Ion Batteries.
Electrochimica Acta 2016, 199, 35. [S11] Ni, S.; Zhang, J.; Ma, J.; Yang, X.; Zhang, L.; Li, X.; Zeng, H. Approaching the Theoretical Capacity of Li 3 VO 4 via Electrochemical Reconstruction. Adv. Mater. Interfaces 2015, 3, 1500340.