Anchored on Carbon Nanotubes as Ultra-Long-Life. Anodes for Reversible Lithium Storage

Supporting Information Monodispersed Carbon-Coated Cubic NiP2 Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage Peili Lou,, Zhonghui Cui,*, Zhiqing Jia,, Jiyang Sun,, Yingbin Tan, and Xiangxin Guo*, State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 China. University of Chinese Academy of Sciences, Beijing 100039 China. *Corresponding author: cuizhonghui@mail.sic.ac.cn (Z. Cui), xxguo@mail.sic.ac.cn (X. Guo) 1

Figure S1. The typical HRTEM image of (a) NiO-CNTs, (b) Ni@C-CNTs and (c) NiP2@C-CNTs. Results and Short Discussion: These typical TEM images show that the particle size of the Ni-based nanocrystals has almost doubled during the reduction process and increases only slightly upon further phosphorization. A thin layer of carbon about 3 nm in thickness are tightly wrapped on the surface of Ni and NiP2 in the Ni@C-CNTs and NiP2@C-CNTs nanocomposites. Figure S2. (a) HRTEM image for NiP2@C-CNTs nanocomposites. (b) EDS line scan of one NiP2@C nanoparticle (green line: P, blue line: Ni and red line: carbon). Results and Short Discussion: These two images clearly show that the NiP2 nanoparticles are wrapped by a thin layer of nanographene-like carbon (~ 3 nm in thickness). 2

Figure S3. The XRD pattern (a), typical TEM (b) and high-resolution TEM (c) image of NiP2-CNTs nanocomposites without carbon coating layer. The XRD pattern (d) and typical SEM (e) and TEM (f) image of NiP2 micro-particles. Results and Short Discussion: To highlight the advances of our strategy, the NiP2 microparticles and the NiP2-CNTs without carbon coating layer were synthesized and investigated. The synthesis process is detailed in the Experimental Section. The as-synthesized NiP2-CNTs have the same phase structure with the NiP2@C-CNTs (Figure S3a) and are of an average particle size about 25 nm (Figure S3b-c). The absence of carbon coating layer is confirmed by the inset of Figure S3c. The NiP2 particles in the NiP2- CNTs composites are prone to merge together forming larger particles as shown in Figure S3b, clearly indicating the important role of the carbon coating layer on constructing the monodispersed NiP2-based nanocomposites. The NiP2 microparticles were prepared by directly heating red phosphorus and metal nickel powder (~100 nm in diameter) under 700 o C for 10 h. As shown in Figure S5d, the as-synthesized NiP2 are of monoclinic structure, which is in consistent with the previous report. 1 The NiP2 microparticles consist of shapeless particles having an average size ranging from 1 to 3 µm (Figure S3e-f). 3

Figure S4. The XRD pattern of the TGA residual of the NiP2@C-CNTs nanocomposites tested under different atmosphere: (a) oxygen and (b) air. Results and Short Discussion: The XRD patterns show that the structures of the TGA residues are highly related to the testing gas atmosphere. Under oxygen flow, pure Ni(PO3)2 formed, facilitating the calculation of NiP2 content (Figure S3a). While under air flow, the Figure S3b clearly show that a mixture of Ni2P and Ni(PO3)2 formed. In this case, the respective amounts of these two products are difficult to know. Calculation of the content of NiP2 in the nanocomposites: Assume the weight of the NiP2@C-CNTs used for TGA test is 100 mg. As shown in Figure S4a and inset of Figure 2f, they transformed into the pure Ni(PO3)2 during test, and resulted a weight increase of 28.03 mg (Figure 2f). The molar mass of Ni(PO3)2 is 216.63 mol/g. Thus the molar of Ni ion is 128.03/216.63 = 0.591 mmol. As all the Ni ion come from the NiP2 (molar mass = 120.64 mol/g), then the weight of the NiP2 should be 0.591 120.64 = 71.298 mg. So the contents of NiP2 in the nanocomposites is 71.298/100 0.713 (i.e., 71.3%) 4

Figure S5. The discharge-charge curves of (a) CNTs and (c) acid-treated CNTs with a current density of ~ 1C. The cycle performance of (b) CNTs and (d) acid-treated CNTs. Results and Short Discussion: As shown in Figure S4a-d, the pristine CNTs and acid-treated CNTs show almost identical discharge-charge curves, except that the acid-treated CNTs deliver a slightly larger reversible capacity than that of the pristine CNTs. This is attributed to that lots of defects and/or pores created by oxidation of acid enable extra lithium intercalation, thus resulting in a larger capacity. 2 Given the content (~28% in maximum, Figure 2f) and reversible capacity (Figure S5c-d) of acid-treated CNTs, its contribution to the capacity of the nanocomposites is very small, which is calculated to be 17.5 mah g -1 at 1C (the contribution of acid-treated CNTs to the reversible capacity of the nanocomposites = 62.5 mah g -1 28% = 17.5 mah g -1 ). 5

Figure S6. The discharge-charge curves of (a) NiP2 micro-particles and (c) NiP2-CNTs with a current density of 1300 ma g -1 (~ 1C). The cycle performance of (b) NiP2 microparticles and (d) NiP2-CNTs. Results and Short Discussion: As the anodes for Li-ion batteries, the NiP2 microparticles deliver a high capacity around 1012.3 mah g -1 upon discharging but a very low capacity of 224.4 mah g -1 on recharging at 1C, resulting in a low Coulombic efficiency of 22.1% (Figure S6a-b). After the initial cycle, the reversible capacity decreased speedily to less than 50 mah g -1 within only 10 cycles, indicating that NiP2 microparticles suffer poor cycle stability. Compared with the NiP2 microparticles, the performance of NiP2-CNTs are improved significantly as the decrease of particle size (~ 25 nm, Figure S5c) and the increase of its conductivity (introduction of CNTs). The NiP2-CNTs deliver a capacity about 900 mah g - 1 on initial discharge and a capacity of 624 mah g -1 on recharge, resulting in a Coulombic efficiency of 69% (Figure S6c). A constant capacity decay is observed in the NiP2-CNTs (Figure S6d), while that is absence for the NiP2@C-CNTs nanocomposites (Figure 3). This sharp contrast highlights the importance 6

of carbon coating layer on preserving the structure stability as well as the advantage of our electrode design concept. Figure S7. The photos of (a) NiP2 microparticles, (b) NiP2-CNTs and (c) NiP2@C-CNTs anode after 300, 500 and 1200 cycles, respectively. Results and Short Discussion: The photo of NiP2 microparticles-based anode clearly shows that almost all of the active materials are peeled off the Cu current collector (Figure S7a). Contrary to this situation, the NiP2-CNTs and the NiP2@C -CNTs remain stable with no obvious detachment even after 500 and 1200 cycles (Figure S7b-c), respectively, indicating that downsizing the particles into nanoscale can significantly alleviate the detachment of active materials upon lithium insertion and extraction. 7

Figure S8. The discharge-charge curves of NiO-CNTs with a current density of 0.5C (a) and 1C (b). The cycle performance of NiO-CNTs at current density of (c) 0.5C and (d) 1C. Results and Short Discussion: As shown in Figure S8a-b, the NiO-CNTs anode shows a stable discharge plateau around 1.2 V during cycling, which is much higher than that of NiP2, and delivers a capacity of 1089 mah g -1 on initial discharge at 0.5C. With increasing the cycle numbers, the reversible capacity of NiO decreases quickly, whether at the current density of 0.5 C or 1 C (Figure S8c-d). This agrees well with previous reports 3 and indicates the importance of carbon coating layer on improving the cycle stability. 8

Figure S9. (a) The typical discharge-charge curves of NiP2@C-CNTs anode at different current densities and (b) the corresponding dq/dv plots. Results and Short Discussion: The discharge-charge curves of the NiP2@C-CNTs anode at different current densities exhibit similar shape (Figure S9a), indicating the same reaction mechanism and stable reaction process. Specifically, the discharge process displays a voltage plateau around 0.73 V, and a charge plateau locates at around 1.14 V, leading to a small overpotential of 0.41 V. As the current density increasing from 200 to 5000 ma g -1, the discharge plateaus decrease from 0.73 to 0.61 V, while the charge plateaus remains almost the same (Figure S9b). Comparing with the reported nickel oxides and sulfides, the NiP2@C-CNTs anodes suffer a smaller overpotential. More impressively, the NiP2@C-CNTs anodes can release almost of the charge capacity below 1.25 V even at a high current density of 5000 ma g -1. This voltage is much smaller than that of the nickel oxides and sulfides (> 1.5 V). 3-6 9

Figure S10. The selected discharge-charge curves of NiP2@C-CNTs anode at a high current density of 5000 ma g -1. Results and Short Discussion: The overlapped discharge-charge profiles during long-term cycling indicates that such designed NiP2@C-CNTs anodes hold excellent structure stability upon repeated lithiation/delithiation. Figure S11. SEM image of pristine NiP2@C-CNTs anode. 10

Figure S12. Plots of Rs, Rint, Rct, and RSEI as a function of discharge states and cycles. Results and Short Discussion: The Rs with little fluctuation reveals the stable cell resistance. The RSEI increases when starting discharge and remains almost the same during cycling. 7 This indicates that stable SEI films have formed during the initial few cycles, which stems from the enhanced structure stability of such designed NiP2@C-CNTs anodes. As shown in Scheme 1, monodispersed carbon coated NiP2 nanoparticles significantly increase the contact areas between each other and thus greatly decrease the average stresses suffered by the carbon coating layer during cycling. This feature guarantees excellent structure integrity of the NiP2@C-CNTs anodes, leading to stable SEI films. The values of Rint increase slightly during cycling, which is caused by the increasing of the new formed interfaces between Ni nanocrystals and Li3P matrices and the gradually decreasing of particle size during conversion reaction. 2 The charge transfer resistance Rct increases gradually during initial discharging and then remains almost the same during cycling. The increment of the Rct is ascribed to the formation of the insulated SEI films. 8 11

Figure S13. Cyclic voltammetry of NiP2@C-CNTs electrode between 0.01 and 3 V (vs Li/Li + ) at a scanning rate of 0.2 mv s -1. Results and Short Discussion: Two reduction peaks at 0.3 and 0.01 V and one oxidation peak at 1.2 V with a shoulder peak at 1.05 V are observed in the initial scan. According to previous reports, the reduction peak at 0.3 V is ascribed to the formation of Ni and Li3P through one-step conversion reaction, which is completely different from its monoclinic counterpart that holds a two-step lithiation process firstly lithiated via an intermediate step forming Li2NiP2. 2,9 And the reduction peak near 0 V is related to the formation of SEI films on the electrode surface. The charge plateau at ~ 1 V illustrates the formation of the final charge product NiP2. After the first scan, the reduction peak moves to 0.65 V, while the oxidation peak remains unchanged. Similar phenomena were observed on other conversion-type electrodes such as oxides, sulfides and nitrides, which is ascribed to the decrease of particle size and the rearrangement of electrode structure during the initial lithiation/delithiation. 12

Figure S14. The relationship between the peak current and the square root of scan rate. Results and Short Discussion: The nonlinear relationship between the two indicates that the lithium storage process of the NiP2@C-CNTs is composed of non-faradaic and Faradaic behavior. 10 Figure S15. (a) the typical galvanostatic charge-discharge profiles and (b) cycle performance of LiFePO4 half cell using Li anode with a current density of 170 ma g -1. Results and Short Discussion: The prepared LiFePO4 cathodes deliver stable capacities around 146 mah g -1 with a high Coulombic efficiency (> 99.5%). 13

Table S1. Comparison of the performance of the NiP2@C-CNTs with that of some previously reported nickel oxide- and sulfide-based anodes. Materials NiO/CNT NiO nanosheet - graphene Porous NiO NiS hollow microspheres NiS-GNS NiS pcw GN Ni3S2 nanobowls rgo NiP2@C- CNTs Cycling stability 73.8% after 50 cycles at 0.05 A g -1 90% after 50 cycles at 0.05 A g -1 91% after 50 cycles at 0.04 A g -1 14% after 50 cycles at 0.05 A g -1 90.8% after 60 cycles at 0.059 A g -1 75% after 100 cycles at 0.12 A g -1 69% after 500 cycles at 0.23 A g -1 ~84% after 1200 cycles at 1.3 A g -1 Rate capability Capacity (mah/g) Operating voltage (V) Discharge plateau (V) 0.05 A g -1 ~ 800 0.05-3 ~ 1.25 3 2.5 A g -1 512 0.001-3 ~1.2 4 Ref. 1.76 A g -1 477 0.01-3 ~ 1.26 11 - - 1-3 ~ 1.26 12 1.18 A g -1 298 0-2.5 ~1.72 and 1.27 13 5.9 A g -1 325 1-3 ~1.72 and 1.45 14 0.46 A g -1 400 0.05-3 ~ 1.22 15 5 A g -1 654.5 after 1500 cycles 0.01-3 ~ 0.73 This work 14

Table S2. Comparison of the performance of the NiP2@C-CNTs with that of some previously reported nickel phosphide-based anodes. Formula Ni:P Ratio [Theoretical Capacity (mah g -1 )] Materials Synthesis method Rate capability and Capacity (mah g -1 ) Cycling stability [cycle number] Operating voltage (V) Ref. NiP2 1:2 [1333] NiP2@C- CNTs NiP2 nanoparticles Reflux method and calcination Thermal decomposition of TOP 1 755 mah g -1 at 5 A g -1-875 [1200] at 1.3 A g -1 654.5 [1500] at 5 A g -1 750 [10] at 0.13 ma cm -1 0.01-3 -0.6-0.9 V (vs Li- In) This work 16 Nanostructured NiP2@C Calcination of the Ni-MOF-74 415 mah g -1 at 1 A g -1 359 [700] at 1 A g -1 0.01-2.5 17 Ni3P-Ni films Electrodeposition - 340 [40] at 0.02 macm 2 0-3 18 Ni3P 3:1 [388] Ordered porous Ni3P film Electrodeposition 243 mah g-1 at 1.9 A g -1 557 [50] at 0.077 A g -1 (0.2 C) 0.02-3 19 Ni3P/Ni/C nanocomposite Intercalation of surfactant and calcination - 635 [200] at 0.1 A g -1 (~ 0.25 C) 0.01-3 20 Ni12P5 12:5 [467.6] Peapod-like Ni12P5@C composites Ni12P5/CNT nanohybrids Hydrothermal method and calcination Thermal decomposition of TOP 1 380 mah g -1 at 3 A g -1 325 mah g -1 at 3 A g -1 600 [100] at 0.1 A g -1 (~ 0.21 C) 665 [100] at 0.1 A g -1 (~ 0.21 C) 0.05-3 21 0.01-3 22 Hierarchical h- Ni2P spheres Thermal decomposition of TOP 1 167 mah g -1 at 1.5 A g -1 365 [50] at 0.27A g -1 (0.5 C) 0.02-3 23 Ni2P nanowires Thermal decomposition of TOP 1 164 mah g -1 at 3.78 A g -1 434 [50] at 0.05 A g -1 (0.1 C) 0-3 24 15

Ni2P 2:1 [543] - Ni5P4 5:4 [771] Porous Ni2P nanosheets Peapod-like Ni2P/C nanoparticles Peapod array of Ni2P graphitized carbon Ni2P/graphene sheets Ni2P@C nanoparticles Ni2P/C nanotube Ni2P@C nanocomposite Sandwiched Ni2P/C Ni-P film Amorphous crystalline Ni- P nanoparticles Ni5P4/C composite Ni5P4@C nanoparticles NiP3 1:3 NiP3 powders [1591] 1 TOP = Trioctylphosphine Organometallic method Hydrothermal method and calcination Hydrothermal method and calcination Solvothermal method Thermal decomposition of TOP 1 Thermal decomposition of TOP 1 Thermal decomposition of TOP 1 Hydrothermal method and calcination Electrodeposition Ionothermal process Wet-chemistry reaction Thermal decomposition of TPP 2 Solid state reaction 244 mah g -1 at 1.08 A g -1 439 mah g -1 at 3 A g -1 420 mah g -1 at 10 A g -1 360 mah g -1 at 0.5 A g -1-310 mah g -1 at 2.7 A g -1 303 mah g -1 at 0.27 A g -1 410 mah g -1 at 5.4 A g -1 287 mah g -1 at 0.542 A g - 1 212 mah g -1 at 0.2 A g -1 357 mah g -1 at 2.3 A g -1 424 mah g -1 at 3.85 A g -1-340 [40] at 0.05 A g -1 (0.1 C) 630 [200] at 0.1 A g -1 (~ 0.18 C) 634 [300] at 0.2 A g -1 (~ 0.36 C) 450 [50] at 0.05 A g -1 (0.1 C) 200 [2] at 0.027A g -1 (C/20) 310 [100] at 2.71 A g -1 (5 C) 435 [50] at 0.05 A g -1 (0.1 C) 625 [200] at 0.108 A g -1 (0.2 C) 399 [50] at 0.05 A g -1 217 [50] at 0.05 A g -1 644 [50] at 0.077 A g -1 (0.1 C) 600 [100] at 0.15 A g -1 (0.2 C) 1082 [60] at 0.159 A g -1 (0.1 C) 0-3 25 0.01-3 26 0.01-3 27 0-3 28 0.01-2 29 0-3 30 0.02-3 31 0.01-3 32 0-3 33 0.02-3 34 0.02-3 35 0.01-2.5 36 0-2.5 37 2 TPP = Triphenylphosphine 16

Table S3. Comparison of the performance of the NiP2@C-CNTs with that of some previously reported non-nickel metal phosphide-based anodes Materials Amorphous FeP2 Fe2P Nanoparticles- GNS nanorod- FeP@C Synthesis method Rate capability and Capacity Solvothermal - Solvothermal Solution reaction 362 mah g -1 at 10 A g -1 80 mah g -1 at 0.6 A g -1 MoP2 Ball milling - MoP@C hybrid Sn4P3 nanoparticles Solvothermal Solvothermal Co2P/graphene Solution method CoP/RGO CoxP nanostructures C@NiCoP Peapods NiP2@C-CNTs Hydrothermal Thermal decomposing method Anion-exchange pathway Reflux method and calcination 386 mah g -1 at 4 A g -1 167 mah g -1 at 1 A g -1 312 mah g -1 at 1.6 A g -1 424 mah g -1 at 10 A g -1 256 mah g -1 at 4.5 A g -1 405 mah g -1 at 10 A g -1 654.5 mah g -1 after 1500 cycles at 5 A g -1 Cycling stability [cycle number] 906 [10] at 0.13 A g -1 560 [200] at 0.1 A g -1 480 [200] at 0.03 A g -1 669 [60] at 0.16 A g -1 1028 [100] at 0.1 A g -1 442 [320] at 0.1 A g -1 798 [250] at 0.1 A g -1 960 [200] at 0.2 A g -1 820 [100] at 0.18 A g -1 670 [350] at 0.2 A g -1 875 [1200] at 1.3 A g -1 Operating voltage (V) Ref. 0.25-2 38 0.01-3 39 0.01-3 40 0-1.5 41 0.01-3 42 0.01-3 43 0.01-3 44 0.005-3 45 0.005-3 46 0.01-3 47 0.01-3 This work References: 1. Boyanov, S.; Bernardi, J.; Bekaert, E.; Ménétrier, M.; Doublet, M. L.; Monconduit, L. P-Redox Mechanism at the Origin of the High Lithium Storage in NiP2-Based Batteries. Chem. Mater. 2009, 21, 298-308. 2. Yang, Z.; Wu, H.; Simard, B. Charge-Discharge Characteristics of Raw Acid-Oxidized Carbon Nanotubes. Electrochem. Comm. 2002, 4, 574-578. 3. Xu, C.; Sun, J.; Gao, L. Large Scale Synthesis of Nickel Oxide/Multiwalled Carbon Nanotube Composites by Direct Thermal Decomposition and Their Lithium Storage Properties. J. Power Sources 2011, 196, 5138-5142. 17

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