Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries Supplementary information Polymer characterization. The composition of the A-BCEs has been determined using 1 H NMR. Knowing the molecular weight (M n ) of the starting PEO, the experimental M n of the PSTFSILi block was calculated from 1 H NMR spectra of the ABCPs, based on the integration of the aromatic protons (δ = 6-8 ppm) of PSTFSILi blocks normalized by this of ethylene protons (δ = 3.4-4 ppm) of PEO. The table 1 in the main text lists the synthetized A-BCEs together with their molecular weight, composition and equivalent molar ratio [EO]/[Li]. The resulting water-soluble triblock polymers were analyzed by aqueous size exclusion chromatography (SEC) operated at 30 C. The chromatographic device was equipped with a Waters 515 liquid chromatograph pump, a differential refractive index detector (Waters model 410), a MCX guard column, and two MCX (1000 and 100000 Å) columns in series (PSS). The mobile phase was composed of 30 wt % acetonitrile and 70 wt % H 2 O containing 0.05 wt% of NaHPO 4 at a flow rate of 1 ml min -1. The evaluation of M n values of triblock copolymers were derived from a conventional calibration based on the narrow-distribution poly(sodium 4-styrenesulfonate) standards from PSS. The aqueous SEC chromatograms (Figure 1) show a shift to smaller retention volumes with increasing molar mass of triblock copolymer and are all unimodal indicating a controlled polymerization. PEO-macroalkoxyamine Triblock_9.5wt% PSTFSILi Triblock_21.4wt% PSTFSILi Triblock_43wt% PSTFSILi RI Response 8 10 12 14 16 18 20 Retention Volume (ml) Figure 1: Aqueous size exclusion chromatograms of PEO- macroalkoxyamine (macroinitiator) together with the triblock copolymers with different PSTFSILi block lengths. NATURE MATERIALS www.nature.com/naturematerials 1
Thermodynamic properties. Thermal analyses were performed on a DSC 2920 (TA Instruments) using a heat/cool/heat cycle from -70 C to +100 C. The heating and the cooling rates were 5 C.min -1. Sample thermal history can modify the results of the DSC experiment. We have optimized our procedure to put the samples in the same thermal state before recording the different parameters. The first cycle was used to obtain samples with the same thermal history; afterwards the second and next cycles are very similar. The thermograms of different triblock copolymers having various ratio of P(STFSILi)/PEO are shown in Figure 2. An endothermic peak, corresponding to PEO crystallites melting is obtained in the first cases with low P(STFSILi)%. There is a sharp decrease of both the melting temperature and enthalpy of melting when the proportion of P(STFSILi) increases. This is likely due to,, beyond PEO confinement in block copolymers, to the dissociation of the anionic groups resulting in a higher concentration of solvated Li + in the PEO chains reducing crystallinity. The data corresponding to DSC curves are summarized in Table 2. 0,0 Heat flow (W/g peo ) -0,4-0,8-1,2 0,0-0,5-60 -40-20 0 % wt STFSILi 10% 21% 31% 43% -1,6-60 -40-20 0 20 40 60 80 Temperature ( C) Figure 2: DSC thermograms for 4 samples of copolymer P(STFSILi)-PEO-P(STFSILi) with different wt% P(STFSILi). Samples with 31 and 42.9 wt% of P(STFSILi) exhibited a distinct glass transition at -25 C and did not show any melting peak. This is a significant glass transition temperature (T g ) increase compared to pure PEO. Such upward displacement is characteristic of the reduction of PEO segmental motions, which can be correlated to strong interactions between -CH 2 -CH 2 - O- functions and lithium cations, coming from anion group dissociation. This behaviour is similar to that of homo PEO directly laden with TFSILi salt. Both PEO melting temperature 2 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION (T m ) and T g evolution are therefore in agreement and can be attributed to a TFSILi anion group dissociation. Wt% Tm Hf (Triblock) Hf (PEO) Cristallinity Tg C P(STFSILi) C J.g -1 J.g -1 %* 9.5 54.3 115.4 127.5 59.4-21.4 51.3 79.4 100.1 46.6-30.8 40.5 48.2 69.6 32.4-25.1 42.9 - - - - -25.8 Table 1: Thermodynamic data for the different copolymers P(STFSILi)-PEO-P(STFSILi): Melting temperature T m (onset), melting enthalpy H f, degree of crystallinity and glass transition temperature T g. *The reference melting enthalpy of 100% crystalline PEO is taken as 214.6 J/g -1. Ionic conductivity and transport number. Symmetrical lithium/bce/lithium cells were assembled in the glove box trough a lamination process then sealed in a hermetic coffee bag to carry out impedance spectroscopy measurements as a function of temperature in the range 20 to 90 C in a Voetch oven. The conductivity and transport number were measured by impedance spectroscopy using a Solartron frequency analyzer 1260. The frequency range was 10 7 to 10-3 Hz and the signal amplitude was 20 mv. A characteristic impedance spectrum is given in figure 3 in Nyquist coordinates. 1 Beaumont, R., et al. Heat capacities of propylene oxide and some polymers of ethylene and propylene oxides. Polymer 7, 401-416 (1966) NATURE MATERIALS www.nature.com/naturematerials 3
-200 Electrolyte Li/aBCP Interface Diffusion Z''/ohms -100 10 6 Hz 4 10 3 Hz 10-3 Hz 0 Re Rint RD t + =R e/(r e+r D) 100 0 100 200 300 400 Z'/ohms Figure 3: Characteristic impedance spectrum obtained at 90 C on a lithium/a-bce/lithium cell using the A-BCE with 30% P(STFSILi). The physical processes involved as a function of the explored frequencies are also identified. R e, R in t and R D stand for the resistances of A- BCE electrolyte, resistance of Li/A-BCE interface and resistance of diffusion respectively. Finally, the calculation of the transport number according to Macdonald 2 is also given. The transport number determination is a very complex and old problem. For the impedance method, we use the equation proposed by Macdonald 2 assuming a dilute electrolyte. Pollard and Compte 3 proposed to take the activity in place of concentration to account for possible deviations from the dilute electrolyte approach. However, a discussion of this effect in a later paper by J. Ross MacDonald 4 stated that the dilute approximation is good for most practical situation, even at high concentration. As a matter of proof, we measured 5 the impedance diffusion of a high Mn PEO laden by LiTFSI at EO/Li=30, and the transport number that we calculated with the MacDonald approximation is 0,15 in very good agreement with the result obtained using pfg-nmr for a high Mn PEO by Orädd and al. 6. Pfg-NMR enables to directly measure the displacement over a given time so as it is not subject to the 2 Macdonald, J. R. Binary electrolyte small-signal frequency response. Electroanal. Chem. Int. Electrochem. 53, 1-55 (1974). 3 Paulard, R., Comte, T. Determination of transport propertiesfor solide electrolytes from the impedance of thin layer. J. Electrochem. Soc. 136, 3734-3748 (1989). 4 Franceschetti, D. R., Macdonald, J ; R., Buck, R. P. Interpretation of Finite-length-Warburg-type impedances in supported and unsupported electrochemical cells with kinetically reversible electrodes. J. Electrochem. Soc. 138, 1368-1371 (1991). 5 Bouchet, R., Lascaud, S., Rosso, M. An Electrochemical Impedance Spectroscopic study of the anode Li/POE- LiTFSI of a lithium polymer battery. J. Electrochem. Soc. 150, A1385 (2003) 6 Orädd, G., Edman, L., Ferry, A. Diffusion: a comparison between liquid and solid polymer LiTFSI electrolytes. Solid State Ionics 152, (2002) 131-136. 4 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION notion of activity or concentration. This comparison can be found in a very recent paper of D. Devaux and al. 7. Mechanical analysis. Stress-strain tests at 60 C on an A-BCE P(STFSILi)-PEO-P(STFSILi) with 31 wt% of P(STFSILi) compared with a neutral triblock copolymer Polystyrene-PEO- Polystyrene (PS-PEO-PS, 25 wt% of PS) laden with LiTFSI at [EO]/[Li]=30 are shown in Figure 4. Compared to results at 40 C, the tensile stress of the whole polymers decreases of course but the conclusions are not changed. 0,6 0,5 PSTFSILi-PEO-PSTFSILi Stress / MPa 0,4 0,3 0,2 0,1 PS-PEO-PS - EO:Li=30 0 PEO 100K - EO:Li=30 0 100 200 300 400 500 Strain / % Figure 4 : Strain-Stress curves obtained at 60 C compared for the different polymer electrolytes starting from the PEO to the PS-POE-PS and finally to PSTFSILi-PEO- PSTFSILi. Thermal analysis. Thermogravimetric measurements were carried out under argon atmosphere (60 ml.min -1 ) with a TG Q500 (TA-Instrument apparatus) at a heating rate of 10 C min -1 from room temperature to 500 C. Around 20 mg of sample were weighed in an alumina crucible. A small piece of composite cathode was taken from a battery in a charged state to be tested. The results are plotted in figure 5. The curve for the A-BCP is smooth until 350 C were the complete decomposition starts. Similar results are obtained on the composite electrode with 31% weigh loss above 400 C corresponding to the amount of A-BCP used in the formulation. The presence of LiFePO 4 active material does not change the thermal stability of the A-BCP which is a promise of safety. 7 Devaux, D., Bouchet, R., Glé, D., Denoyel, R. Mechanism of ion transport in PEO/LiTFSI complexes: effect of temperature, molecular weight and end groups. Solid State Ionics 227, 119-127 (2012), NATURE MATERIALS www.nature.com/naturematerials 5
120 100 Weight % 80 60 40 20 cathode A-BCE 0 0 100 200 300 400 500 600 Temperature / C Figure 5: Characteristic weight loss as a function of temperature for the 30 wt% of P(STFSILi) A-BCP alone and of the composite cathode with LiFePO 4. 6 NATURE MATERIALS www.nature.com/naturematerials