WUT - MESC - Galvanic Cells II 1 Galvanic Cell Galvanic Cells - INTRODUCTION • Energy sources • How did the battery business start? • History of batteries makes history of electric energy As ELECTROCHEMICAL DEVICE : Electrode reactions Thermodynamics and kinetics Properties of Materials As ENERGY SOURCE : Position on energy market Power supply Technology & Economy
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WUT - MESC - Galvanic Cells II 1 Galvanic Cell Galvanic Cells - INTRODUCTION Energy sources How did the battery business start? History of batteries makes.
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WUT - MESC - Galvanic Cells II 1
Galvanic Cell
Galvanic Cells - INTRODUCTION
• Energy sources• How did the battery business start?• History of batteries makes history of electric energy
As ELECTROCHEMICAL DEVICE :Electrode reactions
Thermodynamics and kineticsProperties of Materials
• Renewable energy source ( wind, water, geothermal) – transformation of work to electric energy
• Galvanic, fuel, fotovoltaic cells
CHEMICAL ENERGY directly into ELECTRICAL
WUT - MESC - Galvanic Cells II 3
DIFFERENT CELLS
• galvanic cells – primary and secondary
Chemical substances in electrodes
Expressed as Q
Electrode Reactions
Expressed as UEnergy = U . Q
• Fuel cells
Electrode Reactions
Expressed as U
Stream of reagents
Energy = U . Q
ISOLATED PORTABLE/TRANSPORTABLE
INDEPENDENT FORM ELECTROENERGETICAL NETWORK
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Some milestones in history
1780 L. Galvani – „animal electricity”1800 A. Volta – pile (battery of zinc and silver discs, separated by cloth wet with salty solution)
1866 G. Leclanche – zinc – MnO2 cathode battery
1859 G. Plante’ – lead acid accu made of Pb plates,1881 – Faury et al – pasted plates instead of solid Pb
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Transformation from isolated current sources to electrical network
• Electromagnetic induction – discovered by Faraday about 1840• Electromechanical generator – Siemens about 1857• T. A . Edison : electric bulb 1879, lighting system in NY, Ni-Fe
accumulator • DC contra AC – Edison contra Westinghouse, first big power plant
in America – Niagara Falls – advantages of supplying energy with AC
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Electrical circuits with batteries
• Management of voltage and current – connecting the batteries
• Ohm’s law in simple DC circuit : external resistance (load),internal resistance( ohmic drop on battery components), polarisation resistance (ohmic drop on reaction)
E = I ( Rinter + Rpol + Rload)
• Energy and power
Energy = Q ∙U = I ∙ t ∙ U = (m / k) ∙U
Power = energy produced/consumed in time unit
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Electrode potential
• φ= φo + RT/nF ln ( aMe / aMe(n+) )
• Standard potential at unit activity of particles - φo
• + deviation from standard due to non-unit activity (concentration)• Can not be measured directly
Electrode reaction• Transport of charge or charge and mass over phase boundary electrode – electrolyte• Phases : electrode = fragment of condensed phase electronically conductive
electrolyte = ionically conducting „space”
Observed effects of electrode reaction :• Change of oxidation grade of an atom in a molecule / ion in solution• Accompanying changes : creation / decomposition of a phase
changes in phase structures
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Ared →Box + n e-Anodic reaction
Cathodic reaction Cox + n e- →Dred
Potential φox
Potential φred
Overall cell reaction A + B = C + D With E = Δ φ
Electromotoric force E comes from change in free enthaply of the overall reaction,
Also combining the ΔG with electrical equivalent of energy E = -ΔG /nF
And defining Eo = ΔG o/nF for standard conditions we get Nernst equation :
E = Eo – RT / nF ln K
where K – equilibrium constant of reaction ABCD
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Signs + / - in cells - convention
More negative potential on left side : Zn = Zn2+ + 2e φ = - 0.76 V
Less negative to the right : Cu = Cu2+ + 2e φ = 0.34 V
formal scheme for the cell
External connection / Zn / Zn SO4 aq // CuSO4 aq / Cu / external connection
Sign - // sign +
But .....
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Structure and functions of electrodes
A/ metallic reactive electrodes (deposition-dissolution, formation of compounds on the surface)
Reagent and current collector(two-in-one) Charge and mass transport – on the surface
• Cathode reaction on inert catalytic electrode ( graphite + catalyst + binder)• Oxygen supply forced by underpressure in cathode space • Slow kinetics of oxygen electrode – main limitation for current value
• Parasitic processes : Zn + O2
OH- + CO2
loss of water
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Electric vehicles• „zero-emission” buses and vans on tests in USA and Germany
• Repleceable anodic casette of Zn with KOH (gelled)
• Ca. 200 Wh/kg and 90 W/kg at 80% d.o.c.
• Supercapacitor in hybrid system to boost accelaration
• External regeneration of anodes
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Zn/MnO2 cells
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Zn/MnO2 cells
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How to get „more” from a single cell?
• Redox potential for Me – Men+ couples
• Apply special conditions of discharge
• Eliminate water from cells
Zn-Zn2+ -0.76 V O2-OH- 0.4 V
Mg-Mg2+ -2.36 V Ag+-Ag 0.8 V
Na-Na2+ -2.92 V MnO2-MnO(OH) app. 0.74 V
Li-Li+ -3.05 V F2 – 2F- 2.87 V
non-aqueous solutions
synthesis in inert atmosphere
Reserve cells
one-time discharge
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Reserve (activated) cells
• Separated elements –
• Signal to make contact electrolyte – electrodes : closing the circuit inside the cell
• Activation on signal (decision) or by event (water flow, emergency)
• No or poor activity if energy demand intermittent
• Very long storage time (no parasitic reactions and self-discharge)
• Energy supply – short time, but high current densities
dry electrodes
inactive electrolyte :
-closed in a vessel
-solid salt to be molten
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Reserve cells - examples
• Mg anode reactions Mg + 2 H2O
Mg(OH)2 + 2H+ + 2e Mg(OH)2 + H2
(Mg covered with MgO Mg open to water,
layer, proton recombinates no contribution to current
with OH from cathode space) drawned from the cell
• Both reactions take place, H2 evolution wastes part of electrode, but
• Gas bubbling → intensive stirring → quick transport → high current
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Reserve cells – examples cont.
• Cathodes in Mg cells :
• 2 AgCl + 2e → Ag + 2 Cl-
• 2 CuCl + 2e → Cu + 2 Cl-
• other simple salts : PbCl2 , CuSCN, Cu2I2
• Overall reaction : Mg + PbCl2 = MgCl2 + Pb
• Electrolytes : sea-water, simple salts specific for best cathode rate
• construction: composite cathodes, mechanical separation of electrodes, soakable separators for electrolyte
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Water and gas activated batteries - applications
•Air-sea rescue systems
•Sono and other buoys
•Lifeboat equipment
•Diverse signals and alarms
•Oceanographic and meteo eq.
•And many others, including military
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Molten salts and thermal batteries
Main parts of a thermal battery
Anodes : Li alloys : Li(20)Al, Li(40)Si (melt higher than Li – 181 and 600/7090C resp.)
Cathodes : Ca, K, Pb chromates, Cu, Fe, Co sulfides, V2O5, WO3
Electrolyte: molten LiCl-KCl eutectic 3520CCombination with bromides
Irreversible use of electrodes Recovery of electrodes – by supplying electrical energy we restore electrode oxidation state and structure
Anodic and cathodic process (redox) related to specified electrodes, run only once
Anodic and cathodic reactions repeat on both electrodes in charge-discharge cycles
Solid metal electrodes (one-way)
Products may be soluble
Substrates and products stay in electrode phase
Redox reaction „all-solid state”
Minimalizing changes in electrode structure and shape
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Secondary cells - basic
• Energy density from < 20 (Pb) , 35 (NiCd), 75 (NiMeH) to 150 Wh/kg (Li-ion)
• Cycling life 220-700 (Pb) 500 – 2000 (Ni-Cd)
• Voltage 2 V (Pb) 1.2 V (Ni-Cd)
• Flat discharge profiles
• Poor charge retention (shelf life of Ni-Cd – fully discharged, Pb must be kept charged because of sulfation of plates)
• Vented constructions – evolution of H2 / O2
• Tight closure of cells – oxygen recombination ( at end of charge oxygen developing in anodic process diffuses to cathode and oxidates surplus of cathode material – no overpressure :
• For O2 recombination higher capacity of „-” mass (Cd) – fully charged Ni mass – O2evolution – diffusion – Cd oxidised to CdO, no possiblity of H2 formation
Compresed powderNiSO4→Ni(OH)2
CdSO4 →Cd(OH)2
Encapsulated in steel/Ni pocket
Sintered platePorous Ni plate
Impregnated with Ni , Cd saltsTransformed to hydroxides „in situ”
•Reactivity of metallic lithium: reduces most substances (even Teflon®)•Stable passivation – key to electrode stability•What shall we do with excess lithium? •Transport and consume in cathode reaction•Why not leave lithium cations in the electrolyte?
Anodic reaction : Li = Li+ + 1e
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Anode
Metallic Li (foil) Intercalation : Li – Li+ in matrix
Stable passivation layer on discharge
Charge : mossy, dendritic deposit – corrosion of fresh Li
internal shortcutting
Main application – primary cells
Rechargeable –
attempts with polymer electrolytes
Capacity: 3.86Ah/g, in accu < 1 Ah/g
Carbon materials : coke, graphite etc.
6 – 12 C atoms take 1 lithium atom into the structure
First cycle – formation of SEI
(Solis Electrolyte Interface)
portion of Li used for reaction with electrolyte
Some transition metal compounds
Capacity: 0.372 Ah/g
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Irreversible loss of capacity on first cycle, electrode : artificial graphite
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Cathodes• Redox potentials in 0 – 1 V range - OCV of Li cells from 3 to 4 V
Solid: MexOy
Reduction of Me ion to lower oxidation state, like MnIVO2 – MnIIIO2
Topotactic reaction
Insertion of Li+ into host structure
Some other: V2O5, (CF)n, TiS2
Capacities: 0.31(MnO2), 0.86(CF) Ah/g
Soluble
SO2 + 2e → S2O42-
( in solution, + Lisalt ex. LiAlCl4)
Thionyl chloride:
SOCl2 + 4e → S + SO2
Sulfuryl chloride:
SO2Cl2 + 2e → SO2
(solvents for Li salt)
Capacities : ≈ 0.4 Ah/g
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Layered structure of LiCoO2
Carbon layers in regular graphite
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Electrolytes
Conductivity, Li+ transference number
Electrochemical and thermal stability
Liquid organic•Aprotic•Protective passivation layer on Li•Li salts solute and dissociate•Appropiate physical features: stable non-toxic, nonflammable •Conductivities ≈ 1e-3 S/cm
Polymer Li conduction via
coordination sites on polymer chains(ex. Poly(ethylenoxide)Solid foils, processableMore stable against Li
Conductivities : 1e-7 –1e-4 S/cm
Gel2 in 1 : polymer matrix immobilizing liquid electrolyte
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Solution Ionic conductivity (20oC) S/cm
1M H2SO4 10-1
Nafion® foil (H+) 10-2
1M LiBF4 in acetonitrile 10-3
PEO-LiClO4 complex 10-6
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Step-wise intercalation of Li into graphite, observed as voltage plateaux
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Parameters and definitions
• EMF or OCV• nominal voltage (accepted as typical for a certain battery)• End (cut-off) voltage • Theoretical capacity : comes from amount of active materials• Rated capacity • Energy density (Watthour/l) and specyfic energy (Watthour/kg) :
theoretical E = Q × EMF, practical E = Q×ΔU• Power density• Shelf life
WUT - MESC - Galvanic Cells II 65
General discharge profile - elements
• Discharge of a galvanic cell
WUT - MESC - Galvanic Cells II 66
C - rate
• Charge / discharge current of a battery, given as
I (amper) = Cn (amperhours) . M (multiply or fraction of C)
!!! Traditional convention, but units are uncorrect!!!
However, most producers and studies use this measure !!!
• Ex. For a 250 mAh rated battery (declaration of producer) :
1C – rate = 250 mA
0.1C –rate = 25 mA and so on
• We can compare batteries at equal C-rates or study discharge for a given battery at different C-rates
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Discharge profiles
1. Flat – minimal change in reactants and products2. Step-wise – change in reaction mechanism and potential3. Sloping - composition, internal R ... Change continouosly
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Continuous and intermittent discharge
Possibilty for partial recovery of voltage during pause
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Discharge
• Discharge mode – constant current / resistance / power
(time to reach cut-off U may differ)• Electrode design = f (type of service)• Max. quantity of active material = max. energy supply• Max. electrode surface = high discharge rate (current, power) • Possibility of partial restoration of voltage – stand-by intervals