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1 In the format provided by the authors and unedited. DOI: /NNANO Ultrafast lithium diffusion in bilayer graphene Matthias Kühne, Federico Paolucci, Jelena Popovic, Pavel M. Ostrovsky, Joachim Maier & Jurgen H. Smet Contents 1 Characterization of the solid polymer electrolyte 1 2 Charge transfer 3 3 Weak localization 3 4 Numerical solution to Fick s second law 4 5 Li diffusion 5 1 Characterization of the solid polymer electrolyte The ionic conductivity of the poly(ethylene glycol) methyl ether methacrylate and bisphenol A ehtoxylate dimethacrylate (PEGMA, Mn = 500/BEMA, Mn = 1700) based solid polymer electrolytes prepared with different w/w ratios and different content of lithium bis(trifluoromethane) sulfonimide salt (LiTFSI) was measured upon UV curing by impedance spectroscopy in a selfmade conductivity cell with stainless steel electrodes for a frequency range of 1 105Hz at ambient temperature. The resulting impedance spectra (Fig. S1) correspond to the bulk resistance of the polymer electrolytes yielding ionic conductivities ranging from σ = S/cm (samples NATURE NANOTECHNOLOGY 1

2 3 and 4) to σ = S/cm (sample 7). Based on the observed homogeneity and mechanical properties of the prepared materials, sample 5 (0.35 M LiTFSI in PEGMA/BEMA w/w 3:7 with 2-4 wt% photoinitiator) with σ = S/cm was selected as a suitable electrolyte for gate-controlled intercalation of our bilayer graphene devices. Besides ionic conductivity, the most important parameters for the lithium polymer electrolyte performance in a metallic lithium cell are the lithium transference number 1 and the interfacial stability. The lithium transference number was measured by galvanostatic polarization in a selfmade Li electrolyte Li cell similar as in Ref. 2 (Fig. S2). Upon applying a constant current I, the time dependent voltage change was recorded until the steady state was reached (Fig. S2a). When the electrolyte does not react with lithium, i.e., the electrolyte resistance R el remains constant, the lithium transference number is determined from t pol + = IR tot,0 IR SEI,0 U IR SEI,, where U is the steady state voltage. R SEI,0 and R SEI, are resistances of the solid electrolyte interphase (SEI) before and after the polarization. The first semicircle before and after polarization in Fig. S2b indicate a change to the electrolyte conductivity itself, which has to be taken into account ( R el = R el, R el,0 ). Hence, the transference number becomes t pol + = IR tot,0 IR SEI,0 U I(R SEI, + R el ) =0.40. This Li-ion transference number being < 0.5 can be expected for the given class of electrolyte material. Its finite value clearly demonstrates that the polymer electrolyte shows Li-ion conductivity. From Fig. S2b it can be concluded that the reaction of 0.35 M LiTFSI in PEGMA/BEMA w/w 3:7 with 2-4 wt% photoinitiator electrolyte with lithium both on the interface (SEI, second semicircle) and in the bulk (first semicircle) is not completed even after a 3 days aging process. NATURE NANOTECHNOLOGY 2

3 While the SEI is observed to slightly grow with time, it is the bulk of the electrolyte which shows a more severe decrease in ionic conductivity. We attribute this behaviour to the instability of the polymer electrolyte with respect to lithium M LiTFSI in PEGMA/BEMA w/w 3:7 with 2-4 wt% photoinitiator nontheless proves a functional Li-ion conducting electrolyte whose mechanical properties make it particularly suitable for our Li-intercalation experiments. 2 Charge transfer There are contradictory reports concerning the extent of transferred charge in bulk Li y C 6 (with 0 y 1), 3 5 which is possibly dependent on the intercalant concentration. In the case of C 6 Li 2y C 6, recent ab initio calculations favor a Li density-independent charge transfer on the order of 0.88 e per Li atom. 6, 7 This is supported by experimental evidence for the full ionization of Li in C 6 LiC 6 on SiC. 8 Assuming a complete charge transfer, the induced electron density in C 6 LiC 6 should amount to n cm 2, in agreement with a reported calculation of cm Weak localization Using only the first two terms of Eq. 1 in the main text, we analyse the temperature dependence of weak localization in our samples at different states of lithiation. In Fig. S3 we show the extracted length scales as a function of temperature for a lithiated sample with n Li = cm 2. In addition we plot the transport mean free path l tr. Exemplary magnetoconductivity traces for this sample are shown in Fig. 2b of the main text. As can be seen in Fig. S3, while both l i and l tr show no clear temperature dependence, l φ drops with increasing temperature as dephasing mechanisms NATURE NANOTECHNOLOGY 3

4 gain efficiency, thereby suppressing interference effects. This finding being general to all our samples and at all states of lithiation, we consider l i (respectively τ i ) a temperature-independent parameter. 4 Numerical solution to Fick s second law We look for the numerical solution of Fick s second law in one dimension (Eq. 2 in the main text) with the following boundary conditions n Li (x, t) t n Li (0,t)=n meas Li (0,t) = D δ 2 n Li (x, t) x 2, (1) n Li (x, t) x =0. (2) x=l The first boundary condition sets the lithium density at x =0equal to the measured one at all times t. The second boundary condition imposes zero flux at the ionically blocking electrode at x = L at all times t. We replace the derivatives in time and space by their approximative finite-difference formulations n Li t xi,t j = nj+1 i n j i, t 2 n Li x 2 xi,t j = nj i+1 2nj i + nj i 1 ( x) 2. (3) Here we make the transition from continuous variables x and t to discrete ones x i and t j, and we write n j i = n Li(x i,t j ), t = t j+1 t j and x = x i+1 x i. Combining (1) and (3), we obtain n j+1 i which gives a solution stable in time provided = n j i + Dδ t nj i+1 2nj i + nj i 1 ( x) 2, (4) D δ t ( x) (5) 4 NATURE NANOTECHNOLOGY 4

5 As shown in Fig. S4, we divide the region of interest of the sample of length L into N x = L/ x+1 discrete steps in x direction. In our case (device shown in Fig. 4 in the main text) L = 80 µm and x =1µm yielding N x = 81 steps. We discretize the time frame of interest into N t steps depending on the choice of D δ respecting the stability condition (5). We start with an initial density profile n 0 i =0for all i =0...(N x 1). Taking into account the boundary conditions (2), we first set n 1 0 = n meas Li (0,t 1 ), then calculate (4) for i =1...(N x 2) and finally set n 1 i=n x 1 = n1 i=n x 2. We then proceed with t j for j =2...(N t 1). Depending on the choice of D δ, t needs to be chosen such that (5) is fulfilled. Hence, n j 0 is extracted from the measured n Li (0,t) by interpolation, if needed. The solution provides us with both Li density profiles across the sample at a given time t j, as well as the time-dependent Li density evolutions at a specific position x i. 5 Li diffusion In Fig. S5, the time-dependence of the Li density n Li (x, t) extracted from Hall measurements of the bilayer graphene device illustrated in Fig. 4 of the main text are displayed for the 1 st,3 rd and 4 th lithiation. Again the measured data (coloured, solid lines) corresponds to simultaneous measurements taken at the same four positions x outside the electrolyte and spaced apart by 20 µm as described in the main text. For the 3rd lithiation, good agreement with a numerical solution to Fick s second law (1) can be obtained for D δ cm 2 /s. For the 4th lithiation, this value even seems to underestimate the fast Li diffusion measured in the experiment. As discussed in the main text, Li diffusion is NATURE NANOTECHNOLOGY 5

6 slower during the very first lithiation (top panel in Fig. S5). Here, n meas Li (x = 40 µm,t) (green solid line) also shows unexpected behaviour which may be attributed to the sudden loosening of a restraining condition on local Li accumulation at t 230 s (such as residues located on top of the bilayer or possibly a wrinkle, locally blocking Li atoms from entering a certain region). After the 1 st lithiation, intercalate diffusion seems to be more reproducible with a tendency to become more facile at higher lithiation cycle numbers. Fig. S6 compares the lithium in-plane diffusion coefficient obtained here on freely exposed bilayer graphene with experimental diffusion coefficient values reported for graphite Data reported as a function of known or estimated values of lithium content y in Li y C 6 appear in the left graph, while data reported as a function of electrode potential vs. Li/Li + appear in the right graph. Experimental values for which neither y nor the electrode potential is explicitly reported as well as D δ for NaCl in water are given in the central graph. Data stem from electrochemical experiments near room temperature, which display a wide spread over seven orders of magnitude. 1. Evans, J., Vincent, C. A. & Bruce, P. G. Electrochemical measurement of transference numbers in polymer electrolytes. Polymer 28, 2324 (1987). 2. Popovic, J., Hasegawa, G., Moudrakovski, I. & Maier, J. Infiltrated porous oxide monoliths as high lithium transference number electrolytes. J. Mater. Chem. A 4, 7135 (2016). 3. Dresselhaus, M. S. & Dresselhaus, G. Intercalation compounds of graphite. Adv. Phys. 30, 139 (1981). 6 NATURE NANOTECHNOLOGY 6

7 4. Holzwarth, N. A. W. Electronic band structure of graphite intercalation compounds. In Graphite Intercalation Compounds II Vol. 18 of Springer Series in Materials Science (eds. Zabel, H. & Solin, S. A.) 7 (Springer-Verlag, 1992). 5. Winter, M., Moeller, K. C. & Besenhard, J. O. Carbonaceous and graphitic anodes. In Lithium Batteries: Science and Technology Edn. (eds. Nazri, G.-A. & Pistoia, G.) 145 (Springer US, Boston, MA, USA, 2003). 6. Guzman, D. M., Alyahyaei, H. M. & Jishi, R. A. Superconductivity in graphene-lithium. 2D Mater. 1, (2014). 7. Shirodkar, S. N. & Kaxiras, E. Li intercalation at graphene/hexagonal boron nitride interfaces. Phys. Rev. B 93, (2016). 8. Sugawara, K., Kanetani, K., Sato, T. & Takahashi, T. Fabrication of Li-intercalated bilayer graphene. AIP Adv. 1, (2011). 9. Kaloni, T. P., Cheng, Y. C., Upadhyay Kahaly, M. & Schwingenschlögl, U. Charge carrier density in Li-intercalated graphene. Chem. Phys. Lett. 534, 29 (2012). 10. Yazami, R. & Touzain, P. A reversible graphite-lithium negative electrode for electrochemical generators. J. Power Sources 9, 365 (1983). 11. Takami, N., Satoh, A., Hara, M. & Ohsaki, T. Structural and kinetic characterization of lithium intercalation into carbon anodes for secondary lithium batteries. J. Electrochem. Soc. 142, 371 (1995). 7 NATURE NANOTECHNOLOGY 7

8 12. Levi, M. D. & Aurbach, D. Diffusion coefficients of lithium ions during intercalation into graphite derived from the simultaneous measurements and modeling of electrochemical impedance and potentiostatic intermittent titration characteristics of thin graphite electrodes. J. Phys. Chem. B 101, 4641 (1997). 13. Funabiki, A. et al. Impedance study on the electrochemical lithium intercalation into natural graphite powder. J. Electrochem. Soc. 145, 172 (1998). 14. Yu, P., Popov, B. N., Ritter, J. A. & White, R. E. Determination of the lithium ion diffusion coefficient in graphite. J. Electrochem. Soc. 146, 8 (1999). 15. Piao, T., Park, S., Doh, C. & Moon, S. Intercalation of lithium ions into graphite electrodes studied by AC impedance measurements. J. Electrochem. Soc. 146, 2794 (1999). 16. Ong, T. S. & Yang, H. Lithium intercalation into mechanically milled natural graphite: Electrochemical and kinetic characterization. J. Electrochem. Soc. 149, A1 (2002). 17. Yang, H., Bang, H. J. & Prakash, J. Evaluation of electrochemical interface area and lithium diffusion coefficient for a composite graphite anode. J. Electrochem. Soc. 151, A1247 (2004). 18. Levi, M. D., Markevich, E. & Aurbach, D. The effect of slow interfacial kinetics on the chronoamperometric response of composite lithiated graphite electrodes and on the calculation of the chemical diffusion coefficient of Li ions in graphite. J. Phys. Chem. B 109, 7420 (2005). 19. Persson, K. et al. Lithium diffusion in graphitic carbon. J. Phys. Chem. Lett. 1, 1176 (2010). 20. Cussler, E. L. Diffusion: Mass transfer in Fluid Systems (Cambridge Univ. Press, 1997). 8 NATURE NANOTECHNOLOGY 8

9 Figure S1: Characterization of eight different LiTFSI in PEGMA/BEMA solidified polymer electrolyte samples with 2-4 wt% photoinitiator at ambient temperature. (a) The composition of the studied samples. (b) Impedance spectroscopy of all samples. 9 NATURE NANOTECHNOLOGY 9

10 Figure S2: (a) Galvanostatic polarization of 0.35 M LiTFSI in PEGMA/BEMA w/w 3:7 with 2-4 wt% photoinitiator, (b) Impedance spectra before polarization (black), after polarization (red) and 3 days after polarization (blue). All measurements are performed in a Li electrolyte Li cell at room temperature. 10 NATURE NANOTECHNOLOGY 10

11 Figure S3: Temperature dependence of the phase relaxation length l φ (circles), the intervalley scattering length l i (squares) and the mean free path l tr (triangles) for sample #1 in its most lithiated state. Different colours correspond to measurements on three different positions along the bilayer. NATURE NANOTECHNOLOGY 11

12 Figure S4: Schematic grid used for our numerical calculation of Fick s second law. 12 NATURE NANOTECHNOLOGY 12

13 Figure S5: Li diffusion in the bilayer graphene device shown in Fig. 4 of the main text: 1st, 3rd & 4th lithiation. The yellow, green and red dashed lines are numerical fits to data (solid lines) measured at x = 20 µm, 40 µm and 60 µm, respectively. In the upper panel, n meas Li (x = 40 µm,t) shows an unexpected time dependence, presumably due to a restraint on local Li accumulation suddenly loosening at t 230 s. n Li (x = 60 µm,t) calculated for D δ = cm 2 /s is plotted as a solid grey line. NATURE NANOTECHNOLOGY 13

14 Figure S6: Room-temperature chemical diffusion coefficient D δ of Li in graphite from different electrochemical experiments, reported as a function of composition y in Li y C 6 or C 6 Li 2y C 6 (left panel) or as a function of electrode potential vs. Li/Li + (right panel). In case neither is explicitly stated in the reference, data is shown in the middle panel together with D δ for NaCl in water from Ref. 20 (green dashed line). See the respective references for details. D δ and Dδ represent Li diffusion mainly perpendicular and parallel to the carbon sheets in bulk graphite, respectively. Values for D δ of Li in bilayer graphene extracted from our experiment are shown as red squares. 14 NATURE NANOTECHNOLOGY 14