Supporting Information Hydrogenation Driven Conductive Na 2 Nanoarrays as Robust Binder-Free Anodes for Sodium-Ion Batteries Shidong Fu, Jiangfeng Ni, Yong Xu, Qiao Zhang*, and Liang Li*, College of Physics, Optoelectronics and Energy, Center for Energy Conversion Materials & Physics (CECMP), The Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano and Soft Materials (FUNSOM), The Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China *E-mail: qiaozhang@suda.edu.cn (Q. Z.); lli@suda.edu.cn (L. L.) S.F. and J.N. contributed equally to this work. 1
Experimental Section Synthesis of materials Synthesis of Na 2 nanoarrays: The Na 2 nanoarrays are prepared through an alkaline hydrothermal reaction. Briefly, six pieces of titanium foils (10 10 mm 2 ) polished by sand papers and ultrasonically cleaned in, respectively, acetone, ethanol, and deionized water, were placed into a 50 ml Teflon-lined stainless steel autoclave. The autoclave was filled with 40 ml NaOH aqueous solution (1 M), and then kept at 220 C for 1 4 hours. Upon this hydrothermal process, ordered Na 2 nanoarrays directly grew on the Ti foils. The mass loading of the Na 2 nanoarrays over growth duration of 3 hours is about 0.3±0.02 mg cm 2, as determined by the weight difference before and after removing nanoarray film from Ti substrate. Synthesis of hydrogenated Na 2 (H-Na 2 ) nanoarrays: The Na 2 nanoarrays over growth duration of 3 hours were annealed in mixture atmosphere of Ar/H 2 (95:5 by volume) to generate H-Na 2. The annealing was operated in a tube furnace at 450 C for 2 hours using a ramp rate of 2 C min 1. As a comparison, the Na 2 nanoarrays were annealed at identical thermal conditions except for the atmosphere of air, and the product is denoted as A-Na 2. No appreciable mass changes of nanoarrays were found after thermal annealing in Ar/H 2 or air atmosphere. Characterization of materials The crystalline structure of the nanoarray samples was characterized by X-ray diffraction (XRD, Rigaku D/MAX-2000PC) and Raman spectroscopy (Horiba Jobin Yvon, LabRAM 2
HR800). The shape and morphology of the samples were observed using scanning electron microscopy (SEM, Hitachi SU-8010) and transmission electron microscopy (TEM, FEI Tecnai G2 T20) equipped with an EMSA/MAS energy dispersive spectroscope (EDS). The samples were also investigated by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab 250Xi). The core level binding energy (BE) was corrected with the C1s energy of 284.8 ev. Electrochemical Na-storage evaluation Electrochemical tests were conducted on 2025-type coin cells, which were assembled in an Ar-filled Mikrouna glove box. The Na 2 nanoarray on Ti foil was directly used as the working electrode, with one side of the nanoarray film being removed. Na half-cells and full cells were assembled in an Ar-filled glove box (Mikrouna) using Na metal foil and Na 2/3 (Ni 1/3 Mn 2/3 )O 2 electrode as the counter electrodes, respectively. The Na 2/3 (Ni 1/3 Mn 2/3 )O 2 material was prepared through a sol-gel assisted solid state route. 1 A mixed aqueous solution of sodium, nickel, and manganese and citric acid in a molar ratio of 2: 1: 2: 2 was bathed at 60 C under continuous stirring until most water was evaporated. The resultant gel was firstly decomposed at 400 ºC and finally annealed at 900 C for 12 hours in air to obtain powder materials. The Na 2/3 (Ni 1/3 Mn 2/3 )O 2 electrode is a laminar sheet prepared by spreading 80% Na 2/3 (Ni 1/3 Mn 2/3 )O 2 powder, 10% carbon black, and 10% polyvinylidene fluoride on Al foil. The electrolyte is NaClO 4 (1 M) solution in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume). A 2% (by volume) fluoroethylene carbonate was used as electrolyte additives. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy 3
(EIS) were measured on an Autolab electrochemical workstation. The EIS were recorded with a perturbation potential of 10 mv amplitude over the frequencies from 100 khz to 0.1 Hz. Electrochemical Na-storage tests were performed on a LAND battery test system at room temperature (24±1 C). Figures Figure S1. SEM images of hydrothermally grown Na 2 intermediates at 220 ºC for (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h, revealing a clear structure evolution of Na 2 nanostructure upon hydrothermal duration. Ordered nanotube arrays can be obtained when the duration extends up to 3 h or more. 4
Intensity (a. u.) 002 201 020 Intensity (a.u.) 002 111 020 201 (a) # Na 2 Ti 6 O 13 # 4 h 3 h 2 h 1 h 10 20 30 40 50 60 2 (degree) Ti (b) H-Na 2 Ti Ti Ti A-Na 2 10 20 30 40 50 60 2 (degree) Figure S2. (a) XRD patterns of hydrothermally grown Na 2 intermediates at 220 ºC for various durations. Na 2 Ti 6 O 13 emerges as a minus phase in the product. (b) XRD patterns of A-Na 2 and H-Na 2 nanoarrays. The presence of (002) peak rather than (001) peak reflects structural features. 5
R (%) (a) (b) 20 (c) 6 15 10 5 (Fh (10 3 ) 4 2 F = (1 R) 2 / 2R H-Na 2 H-Na 2 A-Na 2 A-Na 2 0 300 400 500 600 700 800 900 Wavelength (nm) 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 h (ev) Figure S3 (a) Photographs and (b) Ultraviolet-visible diffuse reflectance spectra of H-Na 2 and A-Na 2 nanotube arrays. (c) (Fhυ) 2 as a function of photon energy (hυ) as Na 2 is a direct semiconductor. Here, F is the Kubelka Munk function of the diffuse reflectance R from (b). 2 The bandgaps of 3.49 ev for H-Na 2 and 3.85 ev for A-Na 2 can be estimated from the intercepts of extrapolated straight lines. Ti K Ti K Intensity Ti L Na K O K O% = 56.7 at% H-Ti 2 O% = 60.1 at% A-Ti 2 0 2 4 6 8 Energy (kev) Figure S4. Energy dispersive X-ray spectroscopy of H-Na 2 and A-Na 2. The decreased O element content after hydrogenation suggests formation of oxygen vacancies. 6
Potential vs. Na (V) I ( A) 10 8 H-Na 2 6 4 2 0 A-Na 2 0.00 0.02 0.04 0.06 0.08 0.10 E (V) Figure S5. I E plot. The test was conducted on an Autolab electrochemical workstation using a two-electrode configuration. 3 The electrical conductivities are estimated to be 1.2 10 4 and 1.7 10 7 S cm 1, respectively, for H-Na 2 and A-Na 2. 2.5 Fresh H-Na 2 2.0 Pre-sodiated H-Na 2 1.5 1.0 0.5 CE 84% CE 45% 0.0 0 100 200 300 400 500 600 Capacity (mah g 1 ) Figure S6. Initial galvanostatic curves of fresh and pre-sodiated H-Na 2 electrodes, revealing a drastical increase in the Coulombic efficiency (CE). The presodiation of H-Na 2 was done by direct contact with Na metal in the presence of electrolyte. 7
1000 800 Z img ( 600 400 200 Fresh H-Na 2 Cycled H-Na 2 Fresh A-Na 2 0 0 500 1000 1500 2000 Z real ( Cycled A-Na 2 Figure S7. EIS of the H-Na 2 and A-Na 2 at fresh and cycled stages. Figure S8. SEM image of cycled H-Na 2 shows retention of nanoarray structure. 8
Current (ma) 1.0 0.5 0.0-0.5 1mV s 1 2mV s 1 3mV s 1 4mV s 1 5mV s 1 6mV s 1 7mV s 1 8mV s 1 9mV s 1 10mV s 1-1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Potential (V vs. Na + /Na) Figure S9. CV curves of H-Na 2 at sweep rates from 1 to 10 mv s 1. 9
Potential (V vs. Na + /Na ) Intensity (a.u.) (a) 10 20 30 40 50 60 70 80 2 (Degree) PDF #54-0894 (b) 4.0 3.5 3.0 2.5 1st 2nd 3rd 2.0 0 20 40 60 80 Capacity (mah g 1 ) Figure S10. (a) XRD pattern of Na 2/3 (Ni 1/3 Mn 2/3 )O 2 powder, fully matching the P2-type structure (PDF #54-0894). Inset shows a SEM image of this powder. (b) Galvanostatic profiles of Na 2/3 (Ni 1/3 Mn 2/3 )O 2 composite electrode using metallic Na as counter electrode. References: (1) Wen, Y.; Wang, B.; Zeng, G.; Nogita, K.; Ye, D.; Wang, L. Chem. Asian J. 2015, 10, 661-666. (2) Liao, L.; Zhang, Q.; Su, Z.; Zhao, Z.; Wang, Y.; Li, Y.; Lu, X.; Wei, D.; Feng, G.; Yu, Q. Nat. Nanotechnol. 2014, 9, 69-73. (3) Hu, B.; Mai, L.; Chen, W.; Yang, F. ACS Nano 2009, 3, 478-482. 10