Galvanic Cells - INTRODUCTION by 11Z4hF

VIEWS: 135 PAGES: 71

									       Galvanic Cells - INTRODUCTION

• Energy sources
• How did the battery business start?
• History of batteries makes history of electric energy

                             Galvanic Cell


 As ELECTROCHEMICAL DEVICE :               As ENERGY SOURCE :
        Electrode reactions               Position on energy market
    Thermodynamics and kinetics                  Power supply
       Properties of Materials             Technology & Economy




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            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             Electrode
   in electrodes                Reactions       Energy = U . Q
  Expressed as Q              Expressed as U

•   Fuel cells
Stream of reagents               Electrode
                                 Reactions      Energy = U . Q
                               Expressed as U



                 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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 Anodic reaction             Ared →Box + n e-                 Potential φox

                                                              Potential φred
Cathodic reaction            Cox + n e- →Dred



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

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    OCV or EMF
                                                 (V)
Daniell       Cu → Cu2+           ZnSO4/CuSO4         1.2     Anode product
                                                              – soluble Zn
Leclanche     MnO2→MnO(OH)        NH4Cl, ZnCl2        1.6     salts
              (→Mn3O4 possible)
Alkali        MnO2→MnO(OH)        KOH                1.55     Anode product
                                                              - ZnO

Zinc-air      O2 → O2- (on        KOH                1.45
              carbon matrix)

Zinc-silver   Ag2O → Ag           KOH                 1.6




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                                       Zn - air
•   A : Zn → Zn2+ (as ZnO) + 4e-
•   C : O2 + 2 H2O + 4e- → 4 OH-                     EMF = 1.65 V

•   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


          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 :
•   Separated elements –                                      -closed in a vessel
                                dry electrodes
                                                           -solid salt to be molten


•   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




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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 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                 Cooling system


    Heat distribution               Electrical                    heaters
                                   networking


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Stationary energy storage Na-S system




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        Lithium – iodine solid electrolyte cell
•   Anode :                          Li → Li+ + 2e
•   Cathode :            nI2∙P2VP + 2e → (n-1)I2P2VP + 2 I- (poly-2-vinylpyridine)

•   Overall :                  2Li + I2 → 2 LiI
• 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)    Anodic and cathodic reactions repeat
related to specified electrodes, run   on both electrodes in charge-
only once                              discharge cycles

Solid metal electrodes (one-way)       Substrates and products stay in
Products may be soluble                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)
discharge
            Concentration of H2SO4 ↓     Concentration of H2SO4 ↓




            PbSO4 → Pb     (reduction)   PbSO4 → PbO2 (oxidation)
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 + e-    Ni(OH)2 + OH-

• Anode Cd

           Cd + 2(OH-)             Cd(OH)2 + 2e-

• Ag-Zn : Ag2O + H2O + 2e        2Ag + 2 OH-
            Zn + 2(OH-)          Zn(OH)2 + 2e



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                         Ni-Cd accumulator




cycle        „masa ujemna”               „masa dodatnia”

             Cd → Cd(OH)2                NiOOH → Ni(OH)2 (reduction)
discharge    (oxidation)




             Cd(OH)2 →     (reduction)   Ni II(OH)2 → Ni IIIOOH (oxidation)
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                         Sintered plate
          NiSO4→Ni(OH)2                          Porous Ni plate
         CdSO4 →Cd(OH)2                   Impregnated with Ni , Cd salts
    Encapsulated in steel/Ni pocket     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 OH-
•   Cathode : 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                          discharge                      overcharge


                               OH-            H2O             H2O         O2
H2O         OH-




 Hads        H2               Hads                            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             A: Mischmetall, La, Ce, Ti            ≈ 300            Mostly used
    (LaNi5)         B: Ni, Co, Mn, Al

    AB2             A: Zr, Ti                             ≈ 400           „Ovonic” alloys
    (TiNi2)         B: Ni, Fe, Cr, V


    • 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
      Standard potential (V)            -3.05        -0.76
      Melting point (oC)                181           419
      Density (kg/m3)                   534          7100
      Elchem. equivalent (Ah/g)         3.86          0.82


                 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
 Charge : mossy, dendritic deposit –                 the structure
        corrosion of fresh Li
        internal shortcutting               First cycle – formation of SEI
                                             (Solis Electrolyte Interface)
   Main application – primary cells       portion of Li used for reaction with
                                                       electrolyte
           Rechargeable –
  attempts with polymer electrolytes      Some transition metal compounds


Capacity: 3.86Ah/g, in accu < 1 Ah/g             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                           Soluble

     Reduction of Me ion to lower oxidation   SO2 + 2e → S2O42-
     state, like MnIVO2 – MnIIIO2             ( in solution, + Lisalt ex. LiAlCl4)
                                              Thionyl chloride:
     Topotactic reaction                      SOCl2 + 4e → S + SO2
                                              Sulfuryl chloride:
     Insertion of Li+ into host structure     SO2Cl2 + 2e → SO2
                                              (solvents for Li salt)
     Some other: V2O5, (CF)n, TiS2
                                              Capacities : ≈ 0.4 Ah/g
     Capacities: 0.31(MnO2), 0.86(CF) 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                                Polymer
•Aprotic                                              Li conduction via
•Protective passivation layer on Li         coordination sites on polymer chains
•Li salts solute and dissociate                    (ex. Poly(ethylenoxide)
•Appropiate physical features:                    Solid foils, processable
 stable non-toxic, nonflammable                     More stable against Li
•Conductivities ≈ 1e-3 S/cm                   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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