for Magnesium-Ion Batteries

Transcription

1 [Supporting Information] Cointercalation of Mg 2+ Ions into Graphite for Magnesium-Ion Batteries Dong Min Kim, Sung Chul Jung, Seongmin Ha, Youngjin Kim, Yuwon Park, Ji Heon Ryu ǂ, Young Kyu Han*,, Kyu Tae Lee*, School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1, Gwanak ro, Gwanak gu, Seoul 08826, Republic of Korea Department of Physics, Pukyong National University, Busan 48513, Republic of Korea ǂ Graduate School of Knowledge based Technology and Energy, Korea Polytechnic University, Gyeonggi, 15073, Republic of Korea Department of Energy and Materials Engineering, Dongguk University Seoul, Seoul 04620, Republic of Korea Corresponding Author (Kyu Tae Lee); (Young-Kyu Han)

2 Experimental section Material characterization XRD patterns of the powders and electrodes were obtained using a Bruker D2 PHASER with Cu Kα radiation (λ= Å) operated in the 2θ range of HR-TEM and EDS mapping images were collected using a scanning transmission electron microscope (HR-TEM, STEM, JEOL JEM-2100F). The FT IR analysis was conducted with Varian 670. Electrochemical characterization Natural graphite powders were mixed with polyvinylidene fluoride (PVdF) at a weight ratio of 8:2. The slurry was cast onto a current collector (Al foil), and electrodes were dried at 120 C in a vacuum oven overnight. The average loading amount of active materials was about 1.0~1.8 mg cm 2. Galvanostatic experiments were performed at a current density of 2 ma g -1 using a WBCS 3000 (WonATech, Korea) and a TOYO (TOSCAT 3100, Japan) at 30 C. The electrochemical performance was evaluated using 2032 coin cells with magnesium metal (Goodfellow) or activated carbon (AC) counter electrode. The AC electrode was composed of activated carbon (MSP 20), carbon black (Super P) and binder (polytetrafluoroethylene: PTFE) at a weight ratio of 8:1:1. The potential of the AC quasi reference electrode was calibrated to V vs. Mg/Mg 2+. For the GITT experiments, a specific current of 2 ma g -1 was applied for 10 h, followed by rest for 10 h. DFT calculations The DFT calculations were performed using the Vienna ab initio simulation package (VASP). 1 We employed the projector augmented wave (PAW) method 2 and the van der Waals density functional (vdw-df) 3 wherein the revised Perdew Burke Ernzerhof (revpbe) exchange functional 4 was adopted. The electronic wave functions were expanded on a plane wave basis set of 520 ev. We treated 2p 6 3s 2 for Mg, 2s 2 2p 2 for C, 2s 2 2p 4 for O, and 1s for H as the valence electron configurations. The graphite was simulated by a hexagonal supercell including one graphene sheet consisting of 32 C atoms. A k-point mesh was used for Brillouin zone integrations. Similar calculation methods were successfully used in our previous studies of graphite intercalation compounds. 5-6 The DFT calculations of free

3 Mg 2+ -solvent complexes were performed using the Gaussian 09 program package. 7 B3LYP functional 8-9 and standard 6-31G(d) basis sets were used. The Calculation details for evaluating diffusivity We calculated the co-diffusion barrier of Mg 2+ -DEGDME complexes in graphite by using the nudged elastic band (NEB) method. In the co-diffusion process, one Mg 2+ -DEGDME complex in the double-layer structure moves parallel to the graphene surface. The calculated diffusion barrier (0.21 ev) was used to evaluate the diffusivity D, which is defined as D = a 2 ʋ exp( E b /k B T), where a is the hopping distance, ʋ is the attempt frequency, E b is the diffusion barrier, k B is the Boltzmann constant, and T is the temperature. We also examined the diffusivity of Li + ions in graphite by applying the same procedure for comparison with the Mg 2+ -DEGDME diffusivity. The obtained diffusion parameters are summarized in Table S3. We note that the typical value (ʋ = s 1 ) as the attempt frequency was used for the Li + diffusion in graphite. We used a different ʋ value of s 1 for the Mg 2+ -DEGDME codiffusion because the use of this value leads to a consistency with the diffusivities obtained from ab initio molecular dynamics (AIMD) simulations. We independently carried out AIMD simulations of [Mg-DEGDME]C 16 and LiC 6 for 10 ps at T = 600 K. The mean square displacements (MSDs) of the O and Li atoms in [Mg-DEGDME]C 16 and LiC 6, respectively, were calculated to determine their D values using the Einstein relation r 2 (t) = 6Dt where r 2 is MSD (Figure. S4). The calculated D value of the O atoms in [Mg-DEGDME]C 16 was found to be 10.1 times higher than that of the Li atoms in LiC 6. We assume that this diffusivity difference in AIMD simulations is similar to that in NEB calculations. Therefore, we selected the ʋ values of and s 1 for [Mg-DEGDME]C 16 and LiC 6, respectively, to make the NEB diffusivity difference between [Mg-DEGDME]C 16 and LiC 6 similar to that observed in AIMD simulations at T = 600 K.

4 Figure S1. Structures of Mg 2+ -EC, Mg 2+ -DEC, Mg 2+ -DME, and Mg 2+ -DEGDME complexes. Orange, white, gray, and red balls represent Mg, H, C, and O atoms, respectively. Figure S2. Voltage profiles of the Mg-Mg symmetric cell with 0.3 M Mg(TFSI) 2 in DME/DEGDME (1:1) electrolyte at a current density of 2.8 ma cm -2

5 Figure S3. HR-TEM images of graphite electrodes (a) before and (b) after the intercalation of Mg 2+ ions (discharge). (c) The magnified HR-TEM image of (b) and the corresponding EDS mapping images: (d) Mg, (e) C and (f) Mg and C

6 Figure S4. MSDs of the O and Li atoms in [Mg-DEGDME]C 16 and LiC 6, respectively, during AIMD simulations for 10 ps at T = 600 K. Table S1. Intercalant gallery height of the Mg 2+ -DEGDME complexes for various stage numbers Intercalant gallery height (Å ) Stage Stage Stage Stage

7 Table S2. Theoretical intercalant gallery height obtained from the DFT calculations Intercalant gallery height (Å ) Mg 2+ -DEGDME Mg 2+ -DEGDME Mg 2+ -2DEGDME Mg 2+ -DEGDME + DME 9.95 Table S3. Diffusion barriers (E b ), diffusion parameters, and diffusivities (D) at T = 300 K for Mg 2+ -DEGDME co-intercated graphite and Li + intercalated. composition E b (ev) a (Å ) ʋ (s 1 ) D (cm 2 s 1 ) [Mg-DEGDME]C LiC

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