Nanostructured Electrodes and the Lithium-Air Battery Peter G. Bruce St Andrews Centre for Advanced Materials EaStChem, University of St. Andrews Scotland Science in the 20th century Science in the 21th century Nature of matter: atom sub-atomic Origin of the Universe: Big Bang Molecular Biology: genetics Societal Challenges Food Water Global Warming / Energy Climate change – a challenge for SCIENCE and technology. New science, new ideas, new approaches, new materials. Nanostructured Electrodes Power Density Li-ion relatively low power device (compared with Pb-acid) Li+ LiCoO2 carbon e- Intrinsic limits to Li+ and e- mobility e- BUT LiCoO2 LiCoO2 2 l 2D LiCoO2 LiCoO2 LiCoO2 Li+ diffusion distance time for intercalation Reduce l Li diffusion coefficient micron nano NANOPARTICULATE ELECTRODES Advantages • High surface area in contact with electrolyte • • • • • • • • • Short diffusion distances for Li+ and e- ( t= l2/2D) Change in chemical potential voltage Better accommodation of strain accompanying insertion. Solid state reactions can occur that are impossible in bulk Disadvantages May be more difficult/expensive to synthesize More difficult to fabricate composite electrode Lower volumetric energy density of electrode Surface reactivity Cycling stability TiO2-(B) nanotubes / wires NaOH + TiO2(anatase) + H2O TiO2-(B) nanowires G. Armstrong, A. R. Armstrong, J. Canales and P. G. Bruce Angew. Chem., 43, 2004, 2286 hydrothermal synthesis TiO2-(B) nanotubes / wires TiO2-(B) nanotubes G. Armstrong, A. R. Armstrong, J. Canales and P. G. Bruce Chem. Commun., 2005, 2454 TiO2-(B) Electrode Performance 0.4 0.6 0.8 1.0 Discharge capacity / mAhg + Voltage/ V vs Li /Li 3.0 0.2 Nanowires Nanotubes 2.5 -1 Amount of Li inserted 0.0 2.0 charge 1.5 discharge 1.0 350 C/4, 0.1 mA cm -2 300 250 200 150 nanowires nanotubes 100 50 0 0 50 100 150 200 250 300 350 0 -1 Discharge Capacity / mAhg • 1.6 V vs Li+/ Li - anode • Intercalate Li up to Li0.98TiO2-(B) (330mAhg-1) • Twice that of anatase, Li4Ti5O12 and bulk TiO2-B • >99.9% reversibility A. R. Armstrong, G. Armstrong, J. Canales, R. Garcia and P. G. Bruce Adv. Mater., 17, 862 (2005) 20 40 60 Cycle Number 80 100 350 superior capacity -1 50 mAg (~ C/4) 300 superior capacity retention 250 150 TiO2-B nanoparticles 100 Cathode 200 Electrolyte TiO2-B nanowires Anode Discharge capacity / mAhg -1 Advantage of 1D nanomorphology 50 nanoparticle size same as diameter of nanowires 0 0 20 40 60 80 100 Cycle Number • Micron long wires ensures electron exchange between wires • Nanometre diameter ensures fast Li+ intercalation Mesoporous LiMn2O4 spinel Combining μm and nm dimensions Mn2O4 framework Mn2O4 (MnO2) LiMn2O4 : intercalation Mn2O3 3D Porous Silica e.g. KIT-6 Mn3O4 600oC 5h Dissolve silica template Avoids reaction with template TEM images Mn2O3 Mn3O4 H2/Ar 280oC 1h LiOH 350oC 1h LiMn2O4 Mesostructure preserved throughout ! LiMn2O4 Capacity retention (%) 6 3 Jiao, Bao, Hill, Bruce Angew. Chem. Int. Ed., 47, 9711 (2008) 2 100 100 90 90 Mes o Li Bu lk 1.12 Mn Li 1.88 O 1.1 4 M 2 n 1.8 8O 80 70 60 80 70 Bulk Li1.12Mn1.88O4 Bulk Li1.05Mn1.95O4 60 4 50 Meso Li1.12Mn1.88O4 % Li Discharge Time (min) Cathode Anode ~1μm sized particles BET : 90m2g-1 Electrolyte Li1.12Mn1.88O4 spinel (composition of current interest) 50 Nano Li1.12Mn1.88O4 Increasing Rate 40 0 500 40 1000 1500 2000 2500 3000 3500 Current Density(mA/g) Rate capability Stability at 50 oC and 10 mAhg-1 At high rate, 30C, gravimetric capacity 50% and volumetric 10% higher than bulk. m particles + nm thin walls + 3D network of identical mesoporous channels for electrolyte + crystal structure Lithium-ion Nanobattery TiO2(B)-nanowires LiNi0.5Mn1.5O4 Gel electrolyte Armstrong, Armstrong, Bruce, Reale, Scrosati Adv. Mater., 18, 2597 (2006) 4.7 V 4.00 3.75 3.50 3.25 3.00 2.75 2.50 2.25 2.00 1.75 Discharge Time / mins 360 60 30 20 % of maximum capacity Voltage / V 1.5 V Charge Discharge 0 50 100 150 200 250 300 -1 Specific capacity / mA h g 12 8 100 80 60 40 20 0 0 1 2 3 4 5 Current / C units 6 7 Li-air cell Progress in Rechargeable Lithium Batteries Sony Sony 1991 2000 2005 Future “Beyond Li-ion” Engineering Best Insertion Cathode Possible Energy storage New materials Beyond the Horizon… Cathode major limiting factor LixCoO2 : 140mAhg-1 0.5<x<1 ~ ½Li/Co Graphite LixC6 : 370mAhg-1 0<x<1 ~ 1Li/6C Best intercalation cathode → double capacity to 300mAhg-1! Lithium / Oxygen Battery Schematic Dispense with intercalation cathode use O2 from air! Li2O2 Discharge O2 Li+ Li anode Electrolyte Composite porous cathode Lithium / Oxygen Battery Schematic Dispense with intercalation cathode use O2 from air! Li2O2 Charge O2 Li+ Li anode Electrolyte Composite porous cathode Capacity of Lithium-air Li/LiPF6 in propylene carbonate/porous carbon-MnO2-binder Discharge Capacity / mAh/g 1200 1100 mAhg-1 (carbon+catalyst +binder+O2) 1000 800 600 400 LiCoO2 200 130 mAhg-1 (total mass) 0 1 2 3 4 5 Cycle Number T. Ogasawara, A. Débart, M. Holzapfel, P. Novak & P. G. Bruce, J. Am. Chem. Soc 128(4), 1390 (2006) A. Debart, A. J. Paterson, J. Bao and P. G. Bruce, Angew.Chem.Int. Ed. 47, 4521 (2008). Swagelok Li / O2 Cells O2 filled tube + University of St Andrews Aluminium grid Composite Electrode: Porous Carbon-MnO2 Electrolyte: PC-LiPF6 Lithium Li Li-O2 Cell Porous (super P:EMD:binder) cathode Capacity of Super P / mAhg -1 4.5 Voltage 4.0 3.5 3.0 2.5 2 - 4.1 V 70 mA/g 2000 1500 1000 discharge charge 500 0 0 5 10 15 Cycle number 2.0 0 250 500 750 1000 -1 Overall Cell Capacity / mAh g 2Li+ + 2e- + O2 Li2O2 20 Differential Electrochemical Mass Spectrometry Direct gas exchange between electrolyte and gas phase Li2O2 decomposition on charge In situ DEMS (Differential Electrochemical Mass Spectrometry) Charge of Li2O2 electrode 500 400 300 200 Oxygen evolution (m/z = 32) 100 0 0 250 500 750 1000 1250 Time [min] Oxygen evolution during the decomposition of Li2O2 on charge PTFE Intensity / arbitrary units -11 Ion current [10 A] 600 5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 Cell voltage [V] 700 Powder X-ray Diffraction EMD After charging (100%) EMD EMD Li2O2 Li2O2 Li2O2 EMD Li2O2 EMD PTFE 30 40 50 Li2O2 EMD Before charging 60 2 (FeK1) / degree 70 Charge passed corresponding to Li2O2 decomposition Cell reaction : 2Li + O2 ↔ Li2O2 80 Role of Catalyst Differential Electrochemical Mass Spec – On Charging m/z Relative amounts of gas evolved for Li2O2 Li2O2 / MnO2 - 0.6 16 (from O2, CO & CO2) 0.3 7 28 (CO) 1.6 12 32 (O2) 1.1 100 44 (CO2) 0.6 13 12 (from CO & CO2) • Most abundant process • with MnO2 catalyst is O2 formation (m/z = 32) MnO2 catalyses Li2O2 decomposition Effect of Catalyst Type on Potential 5.0 Fe3O4 CuO NiO -MnO22 Potential / V 4.5 4.0 3.5 3.0 2.5 2.0 0 1000 2000 -1 Capacity / mAhg 3000 Catalyst type influences performance : - overall capacity - charge potential -1 Discharge Capacity / mAhg (carbon) Effect of MnO2 catalyst type -MnO2 Nanowires -MnO2 Nanowires -MnO2 -MnO2 -MnO2 Bulk -MnO2 Bulk 3000 2500 2000 1500 1000 500 0 0 2 4 6 Cycle Number 8 10 Nanowire catalyst • Surface area and crystal structure important for performance • Nanowire α-MnO2 gives highest capacity so far, 3000mAhg-1 A. Débart, A.J. Paterson, J. Bao and P.G. Bruce, Angewandte Chemie, 120, 4597 (2008) α-MnO2 Catalyst 2500 Discharge Capacity / mAhg -1 4.25 Potential / Volts 4.00 3.75 3.50 3.25 3.00 2.75 2.50 Cycle 2 Cycle 3 Cycle 5 2.25 2.00 Limited : 2400mAhg-1 2250 2000 -1 1980mAhg 1750 1500 -1 1430mAhg 1250 1000 -1 1000mAhg 750 500 LiCoO2 250 0 0 500 1000 1500 2000 2500 3000 -1 Capacity / mAhg α-MnO2 catalyst Lower charge potential 0 1 2 3 4 5 6 7 8 9 10 Cycle Number Limit window for cycling avoid deep discharge Negligible capacity fade on cycling, ~2000mAhg-1 (10+ cycles) O2 reduction on glassy carbon electrode in 0.1 M TBAPF6-PC 10 10 0 I / A I / A 0 -10 -20 O2 + e- = O2- -30 1.5 -10 1 V/s 2.0 2.5 3.0 + E / V (vs Li /Li) E0 = 2.32 V k0 = 0.9x10-3 cm/s [O2] = 1.45 mM DO2 = 5.7x10-5 cm2/s 3.5 0.5 V/s -20 0.5 1.0 1.5 2.0 2.5 + E / V (vs Li /Li) 3.0 3.5 • Air cathode: gain in gravimetric > volumetric energy. • Must couple with high capacity anode to realize benefits in a cell. • Li2O2 to Li2O >> capacity. • Better understanding needed for more accurate predictions. Conclusions Li-air Opportunity • High capacity • Low cost Challenges • Charge potential greater than discharge • Cycle life • Electrolyte • O2 selective membrane Acknowledgments • • • • • • • • • A. Debart A.J. Paterson J.L. Bao T. Ogasawara Z. Peng F. Jiao M.K. Shaju A.R. Armstrong G. Armstrong • • • • • M. Holzapfel P. Novak B. Scrosati L. Hardwick S. Freunberger EPSRC