Supporting Information

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1 Supporting Information Porous MXene Frameworks Support Pyrite Nanodots towards High-rate Pseudocapacitive Li/Na-Ion Storage Cheng-Feng Du,, Qinghua Liang, Yun Zheng, Yubo Luo, Hui Mao* and Qingyu Yan*. State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi an, Shaanxi , P.R. China School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, , Singapore College of Chemistry and Materials Science, Sichuan Normal University, Chengdu , P.R. China. Corresponding Author * Prof. Hui Mao, rejoice222@163.com. * Prof. Qingyu Yan, alexyan@ntu.edu.sg. S 1

2 Chemicals and Characterization: Titanium powder (99.5%), Aluminum powder (99%), graphite powder (99.9%) were purchased from Alfa Aesar. Lithium fluoride ( 99.0%), ethylene glycol (anhydrous, 99.8%), Urea (99%), Fe(NO 3 ) 3 9H 2 O ( 98%) and sulfur powder ( 99.5%)were purchased from Sigma-Aldrich, hydrochloric acid (technical grade) was purchased from Fisher. All the chemicals were used without further purification. The high purity deionized water was purified using the Milli-Q system (Millipore, Billerica, MA, USA). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance powder diffractometer by using Cu Kα radiation (λ = nm). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) elemental mapping images were acquired on a JEOL JEM-7600F microscope. TEM images were acquired on a JEOL 2010-UHR microscope at 200 kv. Synthesis of Ti 3 C 2 T x MXene: Ti 3 AlC 2 was synthesized following the approach reported by Sun et al. 1 Briefly, the powder of Ti, Al and C were mix with the molar ratio of Ti:Al:C = 3:1.2:2, followed by ball milling for 12 h under Ar atmosphere in a 8000M Mixer/Mill High-Energy Ball Mill. The mixture was then sintering under flowing argon (Ar) in a tube furnace for 2 h at 1350 ºC. The resulting lightly sintered brick was ground with an agate mortar and sieved through a 400 mesh sieve. Exfoliation of Ti 3 C 2 T x MXene: Ti 3 C 2 T x MXene was prepared according to the literature. 2, 3 Briefly, Concentrated HCl was mixed with deionized water (DI water) to prepare a 6 M solution (30 ml total) g LiF was added to this solution. The mixture was stirred for 5 min with a magnetic Teflon stir bar to dissolve the salt. 3 g of Ti 3 AlC 2 powders were carefully added over the course of 10 min to avoid initial overheating of the solution as a result of the exothermic nature of the reactions. The reaction mixture was then held at 40 ºC for 45 h, after which the mixture was washed by DI water addition, centrifugation (3500 rpm 5 min for each cycle), and decanting, until the supernatant reached a ph of approximately 6. The precipitates were then S 2

3 ultra-sonicated in an ice-bath for 8 h. Finally, the supernatant were collected by centrifugation and freeze dried. Synthesis of the Fe(OH) 3 : In a typical synthesis process, a mixture consists of 5 ml DI water, 35 ml ethylene glycol (EG), Fe(NO 3 ) 3 9H 2 O and urea in a molar ratio of 1:4 were added and vigorous stirring for 1 h. The final solution was solvothermally treat at 100 ºC for 24 hours. After which the mixture was several times until the supernatant become clear and then ultra-sonicated in ethanol for 1 h follow by washed and centrifuged in DI water again. The precipitates were then re-dispersed in DI water for further used. Synthesis of the FeS 2 nanodot-decorated MXenes (FeS The FeS 2 nanodot-decorated MXenes was synthesized by a self-assemble and further sulfuration step. Briefly, for FeS the Ti 3 C 2 T x solution was dropwise added into Fe(OH)3 aqueous solution under vigorous stirring and Ar bubbling. After which the mixture was kept stirring for 10 min. After the self-assemble process, the precipitates were collected by centrifugation and freeze dried. The freeze-dried products were then put in a quartz tube located in the downstream region of the furnace, while exceed amount (~0.5 g) of sulfur powders were kept on the upstream in another quartz tube. The distance between two regions is ~10 cm. Subsequently, the freeze-dried products were heated to ~350 C within a heating rate of 3 ºC min -1 under Ar gas, and kept at this temperature for 1 h. Electrode preparation and electrochemical testing: All LIBs and SIBs cells (2025 coin-type) were assembled with the electrodes and freshly scraped Li or Na metal foil as a counter electrode in an Ar-filled glove box with the oxygen and water contend less than 0.1 ppm. The working electrode was fabricated by well mixing 70% active materials, 10% single-wall carbon nanotube, 10% carbon black, and 10% polyvinylidene difluoride with N-methylprolinodone (NMP) as the solvent, followed by a coating on Cu foil and vacuum drying at 100 C S 3

4 for 24 h. 1 M LiPF 6 dissolved in ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1 and 1 M NaClO 4 dissolved in propylene carbonate (PC) with 5% fluorinated ethylene carbonate (FEC) was used as the electrolyte for Li-ion and Na-ion batteries, respectively. Galvanostatic charge-discharge performance of the batteries were tested on a NEWARE battery tester. The cyclic voltammetry (CV) curves were measured on a Solartron Model 1470 electrochemical station. The electrochemical impedance spectroscopy (EIS) measurements were achieved on a Bio-Logic SP-150 potentiostat in a frequency ranging from 0.01 Hz to 100 khz with an AC amplitude of 5 mv. The double-layer capacitance (C dl ) of the FeS and FeS 2 were determined by a simple CV method in a three-electrode system. Briefly, Active materials (5 mg, e.g. FeS or FeS 2 ) was mixed with Nafion (5 wt %, 50 µl) aqueous solution, absolute ethanol (900 µl) and DI water (50 µl), followed by ultrasonication for 60 min. The catalyst dispersion (10 µl) was deposited onto the surface of a glassy carbon (GC) electrode (with a diameter of 3 mm) and dried under ambient condition. The mass loading was calculated to be around mg cm 2. The CV was conducted in saturated NaCl solution with potential window of V (vs. Ag/AgCl) at various scan rates of 2, 5, 10, 20, 30, 40, 50 and 60 mv s 1. Then capacitive current j = j a j c at 0.25 V vs. Ag/AgCl was plotted against various scan rates, while the slope obtained was divided by two to acquire the C dl value. S 4

5 Figure S1. (a) XRD patterns of Ti 3 AlC 2 powder, the isolated Ti 3 C 2 T x nanosheets and the iron hydroxide precursor. (b) The low-magnification SEM image of the freeze-dried Ti 3 C 2 T x nanosheets. (c) The low-magnification SEM image and (d) high-magnification SEM image of the iron hydroxide precursor. After exfoliation, most of the peaks from Ti 3 AlC 2 phase disappears (Figure S1a). A new reinforced peaks of (002) plane at 2θ 6º was observed accompany with a weak peak of (004) plane at 2θ 17º, indicates the successful exfoliation of Ti 3 C 2 T x. 4 Due to the expansion of the interlayer distance, the (002) peak shifts to lower 2θ position after exfoliation. The XRD pattern of iron precursor shows only three weak peaks at 2θ 24.3º, 35.4º and 62.6º (Figure S1a), which correspond to the (021), (110), and (214) planes of Fe 2 O 3 phase (JCPDS No ) and the (440) plane of Fe(OH) 3 phase (JCPDS No ). The formation of Fe 2 O 3 might be caused by the S 5

6 dehydration of iron hydroxide precursor during drying process. 5 Figure S1c-d show the SEM images of the as-synthesized iron hydroxide precursor. As shown, the iron hydroxide precursor consists of nanodots with an average diameter of about 10 nm. S 6

7 Figure S2. (a) The low-magnification SEM image of the iron nanohybrid precursor. (b) The EDX elemental mapping of the iron nanohybrid. S 7

8 Figure S3. (a) XRD patterns of the FeS nanohybrid. (b) The CV curves of FeS nanohybrid for the first five cycles of Na-ion storage at 0.1 mv s 1. (c) The CV curves of FeS 2 for the first five cycles of Li-ion storage at 0.1 mv s 1. (d) Rate capabilities of the FeS nanohybrid and FeS 2 electrodes for Li-ion storage at different currents. Phase purity of the FeS nanohybrid was further confirmed by XRD analysis (Figure S3a). The four weak peaks at 2θ = 33.0º, 37.1º, 47.7º and 56.2º are well matched with the (200), (210), (220) and (311) planes of FeS 2 phase (JCPDS No ). Figure S3b and S3c shows the first five CV curves of FeS nanohybrid for Na-ion storage at 0.1 mv s 1 and FeS 2 for Li-ion storage at 0.1 mv s 1, respectively. Rate capabilities of the FeS nanohybrid and FeS 2 electrodes for Li-ion storage at different currents are shown in Figure S3d. S 8

9 Figure S4. SEM images of the nanohybrid (a) before and (b) after CV cycles. (c) TEM and (d) HRTEM images of the nanohybrid after CV cycles. Figures S4a shows the SEM image of the nanohybrid before the CV cycles. Figures S4b-d show the SEM, TEM and HRTEM images of the nanohybrid after the CV cycles. As shown in Figures S4b, the nanohybrid maintains a similar structure after the CV cycles. The FeS2 nanodots does not show obvious structural changes after the CV scans (Figure S4c-d). The crystal lattice of the FeS2 nanodot can still be observed. S 9

10 Figure S5. The electrochemical performance of the FeS nanohybrid electrode. (a) and (b) the CV curves for Li-ion and Na-ion storage at the scan rate ranging from 0.1 to 2.4 mv s 1, respectively. (c) b-values calculated from CV profiles plotted in (a) and (b). (d) The diffusion and capacitive contributions to the total current at the scan rate of 2.4 mv s 1 for Li-ion storage. As shown in Figure S5a and S5b, CV curves for Li/Na-ion storage with sweep rates range from 0.1 to 2.4 mv s 1 are plotted. When sweep rates increased, the shapes of the CV curves are well preserved, suggesting that both the diffusion and capacitance behaviors are existence. Figure S5c shows b-values calculated from CV profiles plotted in (a) and (b). The diffusion and capacitive contributions to the total current at the scan rate of 2.4 mv s 1 for Li-ion storage are calculated in Figure S5d. S 10

11 Figure S6. Cyclic voltammograms (CV) of (a) FeS and (b) FeS 2 are taken in a potential window ( V vs. Ag/AgCl) at various scan rates of 2, 5, 10, 20, 30, 40, 50 and 60 mv s -1 in saturated NaCl solution. S 11

12 Figure S7. (a) The galvanostatic cycling performance of FeS nanohybrid and pure FeS 2 for Na-ion storage at the current densities of 0.1 A g 1. (b) Rate capabilities of the FeS nanohybrid and FeS 2 electrodes for Na-ion storage at different currents. (c) The galvanostatic cycling performance of FeS nanohybrid at 5 A g 1. S 12

13 (1) Zou, Y.; Sun, Z.; Hashimoto, H.; Tada, S. Low Temperature Synthesis of Single-Phase Ti3alc2 through Reactive Sintering Ti/Al/C Powders. Mat Sci Eng a-struct. 2008, 473 (1-2), (2) Ghidiu, M.; Lukatskaya, M. R.; Zhao, M. Q.; Gogotsi, Y.; Barsoum, M. W. Conductive Two-Dimensional Titanium Carbide 'Clay' with High Volumetric Capacitance. Nature 2014, 516 (7529), (3) Alhabeb, M.; Maleski, K.; Anasori, B.; Lelyukh, P.; Clark, L.; Sin, S.; Gogotsi, Y. Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti 3 C 2 T x Mxene). Chem. Mater. 2017, 29 (18), (4) Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall'Agnese, Y.; Heon, M.; Barsoum, M. W.; Gogotsi, Y. Intercalation and Delamination of Layered Carbides and Carbonitrides. Nat. Commun. 2013, 4, (5) Du, M.; Xu, C.; Sun, J.; Gao, L. Synthesis of α-fe 2 O 3 Nanoparticles from Fe(OH) 3 Sol and Their Composite with Reduced Graphene Oxide for Lithium Ion Batteries. J. Mater. Chem. A 2013, 1 (24), S 13