Shape controlled TiO 2 nanocrystals for Na-ion battery electrodes: the role of different exposed crystal facets on the electrochemical properties

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1 Shape controlled TiO 2 nanocrystals for Na-ion battery electrodes: the role of different exposed crystal facets on the electrochemical properties Gianluca Longoni a, Rosita Lisette Pena Cabrera a, Stefano Polizzi b, Massimiliano D Arienzo a, Claudio Maria Mari a, Yi Cui c, Riccardo Ruffo a,* a Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via Cozzi 55, Milano, Italy b Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca Foscari Venezia and Centro di Microscopia Elettronica G. Stevanato Via Torino 155/b, Venezia-Mestre (Italy) c Materials Science and Engineering Department, Stanford University, 476 Lomita Mall, Stanford, California *Corresponding author address : r.ruffo@unimib.it Supporting Info 1. Materials synthesis In a common synthetic procedure, an homogeneous mixture of oleic acid ( 93 %, Sigma Aldrich), oleylamine ( 98 %, Sigma Aldrich), tetrabutyl orthotitanate(iv) ( 97 %, Fluka) in different proportion, together with 20 ml of absolute ethanol (ACS reagent, Sigma Aldrich), has been placed in a Teflon-lined stainless steel autoclave. Before the autoclave sealing, 70 ml of a mixture made of ethanol and deionized water in proportion 38:1 by weight has been rapidly added to the Teflon becker. The reaction mixture underwent a solvothermal step for 18 hours at 180 or 140 C, depending on synthetic route, under vigorous stirring. The yellowish solid precipitate was then recovered by centrifugation and thoroughly washed several times with ethanol and dried in vacuum overnight. In order to remove as more accurately as possible the capping agents from TiO 2 crystals, a cleaning step procedure was employed. A suspension made of 200 mg of TiO 2 powder and 10 ml of dry hexane (anhydrous, 95 %, Sigma Aldrich), was added to a solution of 150 mg of nytrosil tetrafluoroborate (95 %, Aldrich) in acetonitrile (anhydrous, 99.8 %, Sigma Aldrich). The biphasic mixture was subjected to 10 minutes sonication/10 minutes rest cycles repeatedly, until a visible bleaching of the solid. The product was recovered by centrifugation and washed with DMF (dimethylformamide, 99 %, Sigma Aldrich) first, and then with anhydrous toluene ( 99 %, Sigma Alrich) for three to five times. Lastly the powder was left drying overnight at 60 C in vacuum. The graphene oxide (GO) employed for the crystals wrapping, was prepared using a modified exfoliation Hummer s method described elsewhere[40], starting from graphite flakes. In order to produce a positive electrostatic interaction between TiO 2 crystals and grapheme oxide oxidized groups, anatase nanoparticle surfaces were

2 previously functionalized using (3-aminopropyl)triethoxysilane, APTES ( 98 %, Sigma Aldrich). Functionalization step was performed refluxing for 24 hours a suspension of TiO 2 powder (150 mg) in an APTES 15 mm (0.375 wt %) dry toluene solution. The product was centrifuged, recovered and washed thoroughly with dry toluene several times. Functionalized TiO 2 nanocrystals (50 mg) were eventually suspended in deionized water and a sonicated water dispersion of GO (50 ml of a 0.05 mg ml -1 suspension) was added drop wise under vigorous stirring in order to achieve the desired GO load (5 % by weight) in the final composite. After 3 hours of ulterior mild stirring at room temperature, the final composite was easily recovered by centrifugation, washed with absolute ethanol end, after a complete drying in a vacuum oven at 80 C, subjected to a heat treatment in N 2 flux at 400 C for 2 h in order to obtain a partial reduction of GO to rgo (reduced graphene oxide). The GO wrapping procedure was analogous for the three TiO 2 synthesis performed, and, for convenience, the rgo lettering will be omitted in the following sections. Samples coding used in this work, namely RE, R and NB, have to be thus considered, unless indicated otherwise, as referred to TiO 2 nanocrystals grafted onto reduced graphene oxide sheets. 2. Structural and morphological characterization The as synthesized TiO 2 powders were subjected to XRD analysis using an Expert-PRO diffractometer (PANalytical), employing Cu-Kα radiation at 1.54 Å. Samples were scanned in the 2θ window ranging from 20 to 80 using a 2theta-omega configuration, setting the step size to All the X-ray diffraction analysis taken directly onto electrodes, were carried out using a MiniFlex 600 diffractometer (Rigaku), spanning the θ window at 3 min -1. The radiation utilized was, even in this case, the Cu-Kα radiation (1.54 Å). The morphological features of TiO 2 samples were identified by High Resolution Transmission Electron Microscopy (TEM-HRTEM), by using a Jeol 3010 apparatus operating at 300 kv with a high-resolution pole piece (0.17 nm point-to-point resolution) and equipped with a Gatan slow-scan 794 CCD camera. Samples were prepared by placing 5 L drop of a dilute toluene dispersion of the nanocrystals on a holey carbon film supported on a 3 mm copper grid. Further characterizations were thermal gravimetric analysis and infrared spectroscopy analysis. In the former case TGA-DSC tests were performed in the C window at a heating rate of 10 C min -1, fluxing N 2 inside the furnace chamber at 20 ml min -1. The instrument employed was a TGA/DSC1 STAR e System (MettlerToledo). 3. Electrode preparation and electrochemical tests Electrodes were prepared by blade-casting a thick slurry made by suspending the active material a carbonaceous additive (Carbon Black, Timcal) and a polymeric binder (PVDF, Solvay 6020) in the minimum amount 1-Methyl,2-Pyrrolidone (anhydrous, 99.5 %, Sigma-Aldrich), onto a Cu current collector foil. The relative ratio among the solid slurry components was 80:17:8. The deposition was let dry at 80 C in vacuum overnight and 16 mm diameter discs were punched out after roll pressing the casting type Coin cells were assembled in order to carry out the electrochemical tests of the prepared materials. Inside an argon-filled glove box (MBraun), with a level of oxygen lower than 1

3 ppm, the electrode carrying the active material was piled together with a glass fiber separator wetted with the electrolyte solution and a sodium metal disc. The electrolyte used was a propylencarbonate ( 99 %, Merck) 1 M NaClO 4 (ACS reagent, 98 %, Sigma Aldrich) solution additivated by 2 wt% of fluoroethylene carbonate (99 %, Aldrich). All the chemicals employed for electrochemical testing were used as purchased without further purification. Electrochemical tests, namely Galvanostatic Cycling with a Potential Limitation (GCPL) and Cyclic Voltammetry (CV), were carried out using a VMP3 potentiostat/galvanostat (BioLogic). The cell set-up employed in this phase, used a two electrodes approach, with the TiO 2 deposition as the working electrode, and the sodium metal disk both as the counter and reference electrode. 4. Crystal dimension and facets percentage extension determination Particles lengths and widths have been esteemed and evaluated, a part from XRD diffraction patterns (Table S1), also over a large number of TEM images (FigureS1), for which the results with the corresponding standard deviation, are reported in table S2. Table S1: Crystals dimension calculated from Scherrer s equation along three different direction: [101], [004] and [200]. Together with particles lengths and widths, 2theta angles and full width at half maximum of corresponding peaks are reported. [101] [004] [200] deg. FWHM L [101] deg. FWHM L [004] deg. FWHM L [200] RE R NB

4 Figure S1: Graphic example of the particle size evaluation from TEM pictures for (a) total length along [001] direction, (d) width and (g) (001) face edges length of RE particles; (b) length, (e) major edge and (h) minor edge of R sample particles and (c) length and (f) width of NB particles.

5 Table S2: Measured dimensions of TiO2 crystals. These values has utilized used to calculate the relative percentage exposure of crystalline faces using the equations reported elsewhere. RE R NB Mean Std.dev. Mean Std.dev. Mean Std.dev Middle section Figure S2: TGA analysis of the as prepared TiO2 samples from the solvothermal step (solid lines), and after the cleaning procedure using nytrosil tetrafluoroborate (dashed lines). Black lines refer to RE, while red and blue lines refer to R and NB samples respectively.

6 Figure S3: HRTEM images of (a,d) RE, (b,e) R and (c,f) NB. From lattice fringes in magnified images (d-f), through images processing, the characteristic lattice plane spacings and angles have been identified (red dashed lines and arrows) and numeric values are reported on the images using white lettering and numbering; (g) histogram collecting the percentage of crystal faces exposure for each morphology, the color code represents the energy density of each crystalline face; (h,i) TEM images of TiO2 nanocrystals grafted on reduced graphene oxide, red arrows pointing at the graphene oxide sheets.

7 Empirical equations used for exposed faces calculation. Geometric dimensions, measured from TEM images, are collected in table 2S. For RE, equations used to calculate (001) and (100) exposure, are: %RE S{001}exp = S{001} (S{101} + S{001} + S{100}) = 2a 2 [2a 2 + 8(b + a)(b a) tan(68.9 ) + 4mb ] 100 %RE S{100}exp = S{100} S{101} + S{001} + S{100} = 4mb [2a 2 + 8(b + a)(b a) tan(68.9 ) + 4mb ] 100 Where 68.9 is the angle between {001} and {101} planes and a, b and m are the dimensions derived from crystal models reported in the insets in Table S2 in Supporting Information. For R and NB, where just (001) and (101) have been considered relevant for this work, the useful equations were: %R S{001}exp = S{001} S{101} + S{001} = 2a 2 [2a 2 + 8(b + a)(b a)tan (68.9 )] 100 %NB S{001}exp = S{001} S{101} + S{001} = 2w 2 [4lw + 2w 2 ] 100 where l and w are the length, namely the extension of (101) faces, and the width (sides of (001) faces) of NB crystals if they were depicted as simple parallelepipeds. Figure S4: (a) GCPL tests taken at different C-rate (0.1, 0.2 and 1 C), of pristine RE, R and NB samples, with the specific capacity normalized over the specific surface area (from BET analysis, and m 2 g -1 and for RE, R and NB respectively); (b) GCPL tests, conducted at 1, 0.2 and 0.1 C, of reduced graphene oxide RE, R and NB composites, with specific capacity normalized over the specific surface area of the sample

8 Figure S5: reduced graphene oxide wrapped TiO2 R sample: particles agglomerates are evidenced in the TEM image; red arrows point at graphene oxide sheets.