Galvanic Cells - INTRODUCTION
• Energy sources
• How did the battery business start?
• History of batteries makes history of electric energy
Galvanic Cell
As ELECTROCHEMICAL DEVICE :
Electrode reactions
Thermodynamics and kinetics
Properties of Materials
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As ENERGY SOURCE :
Position on energy market
Power supply
Technology & Economy
1
Electrical power generation
• Fuel – combustion – heat effect – mechanical energy –
generating electricity
CHEMICAL ENERGY
indirectly into ELECTRICAL
• Renewable energy source ( wind, water, geothermal) –
transformation of work to electric energy
• Galvanic, fuel, fotovoltaic cells
CHEMICAL ENERGY
directly into ELECTRICAL
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DIFFERENT CELLS
• galvanic cells – primary and secondary
Chemical substances
in electrodes
Expressed as Q
•
Electrode
Reactions
Expressed as U
Energy = U . Q
Fuel cells
Stream of reagents
ISOLATED
Electrode
Reactions
Expressed as U
Energy = U . Q
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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Anodic reaction
Ared →Box + n e-
Cathodic reaction
Cox + n e- →Dred
Overall cell reaction
A+B=C+D
Potential φox
Potential φred
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
: Cu = Cu2+ + 2e
Less negative to the right
φ = - 0.76 V
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
B/
inert electrodes
metalls, graphite, semiconductors
Current collector, not a redox reagent
Charge and mass transport – on the surface
C/ multi-function, multi-component electrodes
electroactive component (often insulator)
electronically conducting matrix
other additives with special functions
Charge and mass transport – on triple-contact sites
Redox active
Cond. matrix
electrolyte
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Various types of batteries
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Specyfic energy - Energy density
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Typical battery application
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Zn/MnO2 Cells
•
Leclanche type – electrolytes lightly acidic or neutral:
anodic reaction – product: Zn salts soluble in the electrolyte
( NH4Cl, NH4OH, ZnCl2 → complexes of Zn with OH- and Cl-
•
Alkaline – electrolyte: concentrated KOH:
anodic reaction – product: solid ZnO – the composition ot
the electrolyte does not change
•
Different anodic mechanism → different yields of the cells :
in alkaline cells the maximum current density is higher
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Zn electrode and redox cycling
•
Solid Zn anode : Zn – 2e-→ Zn2+ in solution + 2e- → Zn as powder, needles
(→ due to specyiic features of electrocrystallization of metals)
Volumen of anode ↑
electrical contact within the anode ↓
•
Powder Zn anode : Zn – 2e- → ZnO ( in OH-solution) + 2e- → Zn as powder
discharge (work)
charge
•
Zn metallurgical foil - 100% material as energy
•
complex structure (Zn + conducting matrix + glue) - part of electrode
„useless” as energy source
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MnO2 cathode
• MnIVO2 + H2O ↔ MnIIIO(OH) + OH(other compounds of MnIII possible)
• OH- ion takes part in the anodic reaction – formation of Zn
complexes
• At higher load (high current density) possible limitation of anode
kinetics due to low concentration of comlexing ions
• Valid for Leclanche type ( Zn complex salts soluble in the electrolyte)
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Cells with Zn anode
Cell name
Cathode
Electrolyte
Daniell
Cu → Cu2+
ZnSO4/CuSO4
1.2
Leclanche
MnO2→MnO(OH)
(→Mn3O4 possible)
NH4Cl, ZnCl2
1.6
Alkali
MnO2→MnO(OH)
KOH
1.55
Zinc-air
O2 → O2- (on
carbon matrix)
KOH
1.45
Zinc-silver
Ag2O → Ag
KOH
1.6
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OCV or EMF
(V)
Anode product
– soluble Zn
salts
Anode product
- ZnO
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Zn - air
•
•
A : Zn → Zn2+ (as ZnO) + 4eC : O2 + 2 H2O + 4e- → 4 OH-
•
•
•
•
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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EMF = 1.65 V
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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
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
Apply special conditions of discharge
Reserve cells
one-time discharge
•
Eliminate water from cells
non-aqueous solutions
synthesis in inert atmosphere
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Reserve (activated) cells
inactive electrolyte :
-closed in a vessel
-solid salt to be molten
•
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
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Reserve cells - examples
•
Mg + 2 H2O
Mg anode reactions
Mg(OH)2 + 2H+ + 2e
(Mg covered with MgO
layer, proton recombinates
with OH from cathode space)
Mg(OH)2 + H2
Mg open to water,
no contribution to current
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 3520C
Combination with bromides
Thermal dissociation KCl = K+ + Cl-, high
conductivities, simple reaction mechanism
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Thermal batteries – applications
•
Pyrotechnic heat source – squib, burned serves as inter-cell conductor
•
Insulation – ceramic, glass, polymers – depends on time of discharge
(salt must be kept molten !)
•
Voltages – single OCV : 1.6 V (Li/FeS2) , to ca. 3 V (Ca/K2Cr2O7)
•
Activated life-time : minutes, in special constructions hours
•
Energy density : 2 – 35 Wh/kg
•
•
High currents possible
Applications – mainly military
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Solid electrolyte cell Na-S
Anode
Na → Na+
Cathode
xS → Sx2- , x 3~5
Overall
2Na + xS → Na2Sx
OCV = 2.07 V
Temperature 310 – 350oC
sulphur Tmelt = 118, boil= 444oC
β-alumina Na2O∙11Al2O3 , conducts
Na ions σ300 C ca 0.5-0.1 S/cm
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Solid electrolyte cell Na-S
•
•
•
• Can be used as rechargeable cell
Applications : stationary energy storage, motive power
Working with high-temperature cells:
warm-up on start
keep warm at intervals in operation
manage excessive heat during operation (ohmic and
reaction)
Construction of stacks : electrical and heat management
Insulated enclosure
Heat distribution
Cooling system
Electrical
networking
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heaters
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Stationary energy storage Na-S system
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Lithium – iodine solid electrolyte cell
•
•
Anode :
Cathode :
•
Overall :
• LiI
•
•
Li → Li+ + 2e
nI2∙P2VP + 2e → (n-1)I2P2VP + 2 I- (poly-2-vinylpyridine)
2Li + I2 → 2 LiI
thin layer on contact between Li and cathode, ionically conducting
OCV ca 2.8 V
Discharge rates 1 – 2 μA/cm2 (very low)
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Primary and secondary cells - basic
PRIMARY
SECUNDARY
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 chargedischarge 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 :
Valve-Regulated-Lead-Acid
sealed Ni-Cd
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Lead-acid accumulator
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cycle
„negative mass”
„positive mass”
Pb → PbSO4 (oxidation)
PbO2 → PbSO4 (reduction)
Concentration of H2SO4 ↓
Concentration of H2SO4 ↓
PbSO4 → Pb
PbSO4 → PbO2 (oxidation)
discharge
(reduction)
charge
Concentration of H2SO4 ↑
Concentration of H2SO4 ↑
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Phenomena in discharge cycle
• CH2SO4
• PbSO4 – insulator ( ca. 1010 Ώcm)
• Vmol PbSO4 > Vmol Pb, PbO2
worse porosity
diffusion of the electrolyte into the structure impaired
R int
What happens with:
current density at U = const ?
Voltage at I = const. ?
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Alkaline accumulators
• Ni –Cd , Ni – Fe, Ni – MeH ( 1.2V)
Ag – Zn ( 1.5V)
Ni – Zn (1.6V)
• Cathode Ni
NiIII OOH + H20 + eNi(OH)2 + OH-
• Anode Cd
Cd + 2(OH-)
• Ag-Zn : Ag2O + H2O + 2e
Zn + 2(OH-)
Cd(OH)2 + 2e2Ag + 2 OHZn(OH)2 + 2e
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Ni-Cd accumulator
cycle
„masa ujemna”
„masa dodatnia”
NiOOH → Ni(OH)2 (reduction)
discharge
Cd → Cd(OH)2
(oxidation)
Cd(OH)2 →
Ni II(OH)2 → Ni IIIOOH (oxidation)
(reduction)
charge
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(further electrolysis after charging effects in evolution of O2)
((further electrolysis after charging effects in evolution of H2)
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Oxygen and hydrogen formation in cells
• Reactions possible in water solution
• Equilibrium potentials : E (H+/H2) = 0V
, E (OH-/O2) = 0.4 V
• BUT – overpotentials due to phenomena at gas-solid electrode
phase boundary make true potentials higher
• For different metals the hydrogen evolution potential grows from:
Pt - Ni - Ag - Zn - Cd - Pb (and compounds)
• Still, at the end of charge/discharge cycle co-evolution of gases in
cells occurs
• In effect: overpressure inside the cell, - H2 i O2
• „oxygen recombination” – electrodes not equivalent in charge,
ex. QCd > QNi
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Basic secondary cells
Ni-Cd
•Pocket electrode
construction of electrodes
•Sintered plates
Pb acid
•Pasted plates
•Tubular positive plates
•Plante’ design
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Technology of electrode masses in Ni-Cd
•
Electrodes prepared in discharged state : Ni(OH)2 and Cd(OH)2 as
Compresed powder
NiSO4→Ni(OH)2
CdSO4 →Cd(OH)2
Encapsulated in steel/Ni pocket
•
•
•
•
•
Sintered plate
Porous Ni plate
Impregnated with Ni , Cd salts
Transformed to hydroxides „in situ”
Additives: graphite ,”-” mass – Fe+ Ni (→ Cd crystallization)
Formation of plates : several charge-discharge cycles
Assembly and hermetic closure
Separators – ionic conductivity and oxygen diffusion (thickness ca0.2 mm)
For O2 recombination higher capacity of „-” mass (Cd) – fully charged Ni
mass – O2evolution – diffusion – Cd oxidised to CdO, no possiblity of H2
formation
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Nickel/Metal Hydride
•
•
•
•
Anode : 2 NiO(OH) + 2 H2O + 2e → Ni(OH)2 + 2 OHCathode : H2 + 2 (OH-) → 2 H2O + 2e
Hydrogen stored as hydride in metallic phase,
Capacity of metal hydride electrode c. 0.4 Ah/g -- comparable with Cd and
Ni sintered plates 0.3-0.5 Ah/g
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Scheme for reaction mechanism at Me electrode
charge
H2O
Hads
overcharge
discharge
OH-
OH-
H2
Hads
H2O
H2O
O2
Hads
Me-H
Reversibility of electrode reaction, catalytic for H adsorption
and H-O2 recombination
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Hydrogen absorbing alloys
•
•
A – metal forming stable hydrides
B – weak hydrides, catalyst, resistance to corrosion, control Hads pressure
Class (basis) Components
Storage Ah/kg
Remarks
AB5
(LaNi5)
A: Mischmetall, La, Ce, Ti
B: Ni, Co, Mn, Al
≈ 300
Mostly used
AB2
(TiNi2)
A: Zr, Ti
B: Ni, Fe, Cr, V
≈ 400
„Ovonic” alloys
• Nickel - catalyst for H2 dissociation,, regulator for Zr, Ti, V hydride formation,
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Some details on production of alloys
•
Ni mass – traditional, new technologies for MeH electrode powder
•
Ovonic alloy – example : main components : Zr-Ti-V-Ni + Cr, Mn, Co, Fe...
•
Preparative technics: electric arc or inductive oven, Ar atmosphere
•
Production of powder : hydrogenation of cast alloy (volume expansion =
crushing of a piece), followed by mechanical pulverisation
•
Sintered plates : MeH powder + Ni, Ni(CO)5 + resin →
pressing and sintering under vacuum
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Lithium cells
Atomic mass
LITHIUM
ZINC
-3.05
-0.76
Melting point (oC)
181
419
Density (kg/m3)
534
7100
Elchem. equivalent (Ah/g)
3.86
0.82
Standard potential (V)
Anodic reaction : Li = Li+ + 1e
•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?
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Anode
Metallic Li (foil)
Intercalation : Li – Li+ in matrix
Stable passivation layer on discharge
Carbon materials : coke, graphite etc.
6 – 12 C atoms take 1 lithium atom into
the structure
Charge : mossy, dendritic deposit –
corrosion of fresh Li
internal shortcutting
Main application – primary cells
Rechargeable –
attempts with polymer electrolytes
First cycle – formation of SEI
(Solis Electrolyte Interface)
portion of Li used for reaction with
electrolyte
Some transition metal compounds
Capacity: 3.86Ah/g, in accu < 1 Ah/g
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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
Soluble
SO2 + 2e → S2O42( in solution, + Lisalt ex. LiAlCl4)
Thionyl chloride:
SOCl2 + 4e → S + SO2
Sulfuryl chloride:
SO2Cl2 + 2e → SO2
(solvents for Li salt)
Some other: V2O5, (CF)n, TiS2
Capacities: 0.31(MnO2), 0.86(CF) Ah/g
Capacities : ≈ 0.4 Ah/g
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Carbon layers in regular graphite
Layered structure of LiCoO2
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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, processable
More stable against Li
Conductivities : 1e-7 –1e-4 S/cm
Gel
2 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
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General discharge profile - elements
•
Discharge of a galvanic cell
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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 products
2. Step-wise – change in reaction mechanism and potential
3. 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
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