CARBON CONDUCTIVE ADDITIVES FOR ELECTRODES IN ELECTROCHEMICAL ENERGY STORAGE DEVICES

CARBON CONDUCTIVE ADDITIVES FOR ELECTRODES IN ELECTROCHEMICAL ENERGY STORAGE DEVICES 15.03.2013 Flavio F. C. Mornaghini, Dario Cericola, Pirmin Ulmann, Thomas Hucke and Michael E. Spahr

CARBON CONDUCTIVE ADDITIVES FOR ELECTRODES IN ELECTROCHEMICAL ENERGY STORAGE DEVICES Introduction on carbon conductive additives Conductive additives in symmetric supercapacitors Current collector coating in Li-ion positive electrodes

Conductive carbon black structure Carbon black aggregates of primary particles Carbon black structure: Total accessible void volume per unit weight of carbon black Intra-aggregate space Interstices between the aggregates 200 nm Primary particle porosity Transmission electron microscopy Absorption stiffness (oil absorption) 37 ml/5g 32 ml/5g 3

Conductive carbon black structure Primary particle size 10 nm Carbon black structure: Total accessible void volume per unit weight of carbon black Intra-aggregate space Interstices between the aggregates Primary particle porosity Absorption stiffness (oil absorption) Transmission electron microscopy 37 ml/5g 32 ml/5g 4

Effect of conductive additives on the volume resistivity of activated carbon conductive additive mixtures Powder conductivity method F piston mold h sample R Resistivity [Ω cm] (@ 4.5 kn cm 2 ) 0.16 0.14 0.12 0.10 0.08 Super C45 Ensaco 350P KS 6L 0 5 10 15 20 Additive amount [wt.%] Carbon black (Super C45, Ensaco 350P) and graphite (KS 6L) conductive additives reduce the volume resistivity of activated carbon mixtures. 6

Effect of conductive additives on the press density of activated carbon conductive additive mixtures Density [g cm -3 ] (@ 4.5 kn cm -2 ) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.92 0.73 1.64 0.66 Super C45 Ensaco 350P KS 6L activated carbon Density [g cm -3 ] (@ 4.5 kn cm -2 ) 0.80 0.75 0.70 0.65 0.60 Super C45 Ensaco 350P KS 6L 0 5 10 15 20 Additive amount [wt.%] The press density of activated carbon conductive additive mixtures increases with increasing additive amount. Graphite typically improves the compressibility of powder mixtures. Carbon black aggregates might fill the voids between activated carbon particles. 7

Expected impact of conductive additives on the specific surface area of activated carbon electrodes BET surface area [m 2 g -1 ] 2500 2000 1500 1000 500 0 45 770 20 1950 Super C45 Ensaco 350P KS 6L activated carbon Fraction of retained BET [%] (calculated) 100 95 90 85 80 Super C45 Ensaco 350P KS 6L 0 5 10 15 20 Additive amount [wt.%] Low surface area conductive additives are expected to lower the surface area of activated carbon electrodes. The surface area of Ensaco 350P is comparable to the one of the activated carbon. 8

Effect of conductive additives on slurry processing Solvent needed for dispersion [wt.%] 0.90 0.85 0.80 0.75 0.70 Super C45 Ensaco 350P KS 6L 0 5 10 15 20 Solvent: H 2 O:alcohol 1:1 (wt.) Additive amount [wt.%] Activated carbon mixtures containing carbon black (Super C45, Ensaco 350P) conductive additives require large amounts of solvent the be processed. The effect is negligible when small amounts of low surface area carbon black (e.g. 5 wt.% of Super C45) are used. 9

Electrode preparation, cell assembly and testing Electrode composition: 10 wt.% binder (PTFE) 90 wt.% carbon material activated carbon:conductive additive => 95:5 (wt.) rolled and pressed freestanding electrodes (no current collector) electrode area: 1.13 cm 2, thickness ~ 200 µm loading (C): ~ 24 mg cm -2, density (C) ~ 0.6 g cm -3 Cell assembly: 2 identical electrodes, symmetric configuration 1 mol dm -3 tetraethylammonium tetrafluoroborate in acetonitrile Testing: impedance spectroscopy, cyclic voltammetry CV with increasing sweep rate for rate capability evaluation 95 wt.% activated carbon 5 wt.% Ensaco 350P 95 wt.% activated carbon 5 wt.% KS 6L 100 µm 100 µm 10

Effect of conductive additives on the impedance spectra of symmetric supercapacitors -Im (Z) [Ohm] 8 7 6 5 4 3 2 10 mhz 100 khz 5 wt.% SC45, 95 wt.% activated carbon 5 wt.% E350P, 95 wt.% activated carbon 5 wt.% KS 6L, 95 wt.% activated carbon pure activated carbon ESR [Ohm] EDR [Ohm] 5 wt.% E350P 0.8 1 5 wt.% SC45 0.8 1 5 wt.% KS 6L 1.1 1.4 pure AC 1.2 1.3 1 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Re (Z) [Ohm] The addition of carbon black (Super C45, Ensaco 350P) results in lower impedance compared to a cell made with pure activated carbon electrodes. Graphite (KS 6L) has a minor impact on the impedance. 11

Effect of conductive additives on the capacitance of symmetric supercapacitors 40 5 wt.% E350P, 95 wt.% activated carbon 40 pure activated carbon C carbon [F g -1 ] 20 0-20 1 mv s -1 100 mv s -1 C carbon [F g -1 ] 20 0-20 1 mv s -1 100 mv s -1-40 0 1 2 3-40 0 1 2 3 U [V] U [V] The addition of Ensaco 350P does not affect the gravimetric capacitance at low sweep rate and improves it at high rates. 12

Effect of conductive additives on the rate capability of symmetric supercapacitors C grav @ 2V C grav @ 2V C grav @ 2V C vol @ 2V C vol @ 2V C vol @ 2V 1 mv s -1 10 mv s -1 100 mv s -1 1 mv s -1 10 mv s -1 100 mv s -1 5 wt.% E350P 31.1 27.8 25.8 18.1 16.2 15.0 5 wt.% SC45 30.5 28.5 26.7 16.5 15.4 14.4 5 wt.% KS 6L 29.0 25.1 23.3 17.8 15.4 14.3 pure AC 32.0 28.1 23.0 19.7 17.3 14.2 At low sweep rate the gravimetric capacitance of the cells is dominated by the total surface area of the electrodes, while at high rates other parameters, like the pore structure and resistivity of the electrode, seem to play a more important role. This transition takes place at higher rates when the volumetric capacitance is considered. The polar surface of Super C45 may improve the wettability of the electrodes. 13

C-NERGY TM Li-Quid 101: ready-to-use water based dispersion for current collector coating Ready-to-use water based dispersion of very fine carbon powder, may be applied by coating or printing. 15

Effect of corrent collector coating and conductive additives on the adhesion of NMC positive electrodes Peeling force [g cm -1 ] 90 80 70 60 50 40 30 20 10 2 wt.% with Li-Quid 101 without Li-Quid 101 5 wt.% Al current collector with/without Li-Quid 101 coating PVDF binder (5 wt.%) T-peel test (180 ) sample dimensions: 3.5 x 7.15 cm 0 SC65:KS6L (1:1) SC65 SC45 SC65:KS6L (1:1) SC65 SC45 The adhesion decreases when large amounts of conductive additive are used. Thank to its lower surface area, graphite helps preventing adhesion loss. Adhesion is enhanced by a thin layer of Li-Quid 101 primer. 16

Specific charge [mah g -1 ] Effect of corrent collector coating on the performance of NMC positive electrodes in Li half-cells 170 160 150 140 a b c a CC-CV 3 4.3 V vs Li/Li +, 1 M LiPF 6 in EC:EMC (1:3 v/v), 2 wt.% SC65 a: 60 ma g -1 b: 1 C c: 5 C with Li-Quid 101 without Li-Quid 101 130 0 20 40 60 80 100 Cycle number (z) charging U vs Li/Li + [V] 4.4 4.2 4.0 3.8 without Li-Quid 101, 60 ma g -1 (100 th Z) 3.6 with Li-Quid 101, 60 ma g -1 (100 th Z) without Li-Quid 101, 60 ma g -1 (5 th Z) with Li-Quid 101, 60 ma g -1 (10 th Z) 3.4 0 20 40 60 80 100 120 140 160 Charge [mah g -1 ] The thin C-NERGY TM Li-Quid 101 layer helps preventing the increase of electrode polarisation during cycling; consequently the cycling stability improves significantly. 17

Effect of corrent collector coating on the performance of NMC positive electrodes in Li half-cells Specific charge [mah g -1 ] 170 160 150 140 a b c a CC-CV 3 4.3 V vs Li/Li +, 1 M LiPF 6 in EC:EMC (1:3 v/v), 2 wt.% SC65 a: 60 ma g -1 b: 1 C c: 5 C with Li-Quid 101 without Li-Quid 101 130 0 20 40 60 80 100 Cycle number (z) charging U vs Li/Li + [V] 4.4 4.2 4.0 3.8 without Li-Quid 101, 1C (10 th Z) 3.6 with Li-Quid 101, 1C (10 th Z) without Li-Quid 101, 60 ma g -1 (5 th Z) with Li-Quid 101, 60 ma g -1 (5 th Z) 3.4 0 20 40 60 80 100 120 140 160 Charge [mah g -1 ] The thin C-NERGY TM Li-Quid 101 layer helps preventing the increase of electrode polarisation at high rates, resulting in improved galvanostatic charging up to 1 C rate in NMC electrodes containing a low amount of conductive additive. 18

Conclusions Conductive additives improve the conductivity of porous electrodes, but can also be used to tailor their porous structure; the high rate performance of symmetric supercapacitors can be improved by the addition of carbon black or graphite conductive additives. C-NERGY TM Li-Quid 101 improves the adhesion and the electrical contact between the metal foil and the electrode mass; this leads to lower internal electrical resistance, improves performance in the high current drain regime and extends cell life. The amount and type of conductive additive play an important role in the adhesion of Li-ion positive electrodes. 19

Acknowledgments TIMCAL technicians: Elena Bonfiglioli, Antonio Leone, Francesco Matarise, Salvatore Stallone We gratefully acknowledge Daniel Weingarth and Dr. Rüdiger Kötz (Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland) for their help with the experiments on supercapacitors and for fruitful discussions. 20

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