fizyka 3 michaŁ marzantowicz - warsaw university of ...adam.mech.pw.edu.pl/~marzan/cells.pdf ·...

FIZYKA 3 MICHAŁ MARZANTOWICZ

Energy conversion: capacitors

Separation of charge requires work. This energy can be then released in an external circuit.


Energy conversion: fuel cells

Fuel cells directly convert energy released during electrochemical „burning” of fuel into electrical energy.


Energy conversion: galvanic cells

In galvanic cells, the energy released during chemical reactions at the electrodes is converted into electrical energy. These reactions can be irreversible (primary cells) or reversible – secondary/rechargeable cells.


History of cells: „Baghdad battery”

„Battery from Bagdad” –250 b.c.

Iron Fe

Copper Cu

Vinegard

It has not been confirmed, that this artifact served as a battery .models were used to electroplate gold and silver.


History of cells

Luigi Galvani (1791): „animal” electricity


History of cells

Animal electricity: from frogsto Frankenstein and Matrix


History of cells

Alessandro Volta verified Galvani’s experiments in 1800.The flow of current is caused not by frogs, but by two different metals used as electrodes, and immersed in an electrolyte.

☺

The Volta pile – zinc and copper or silver plates, immersed in saline. High initial voltage, but problems with stability.


Kinds of half-cells

metal/insoluble saltmetal/metal ion

redoxgaseous


Electrochemical cell

Electrolytic cell (electrolizer) – theflow of electrons is forced by externalsource. Reduction occurs at cathodeand oxidation at the anode.

Galvanic cell – the reaction isspontaneous. The electrons are deposited at the anode (oxidation) which becomes negative, and are taken from the cathode(reduction) which becomes positive.


Daniell cell

Anode (-) Cathode (+)

oxidation reduction


Galvanic series


Standard potential

Cell for estimation of standard potentials: hydrogen half-cell and studied half-cell.


Galvanic coating


Electrochemical cells


Parameters of the galvanic cell

The basic parameters are electromotive force (SEM) and internal resitance.

The open voltage is higher than thevoltage under electric load – thedifference is caused by internal resitance. Linking in series increases voltage, but also resitance.

The cell should have lower resistance, than the load resistance – otherwise, more power will be lost in the cell than used in the load.


Leclanché cell – zinc and carbon

1 – positive contact (+) 2 – graphite rod 3 – zinc enclosure4 – manganese oxide (IV) 5 - wet ammonium chloride (electrolyte) 6 – negative contact (-)

Addition of zinc chloride to the electrolyte allows to obtain electromotive force of about 1.5V

☺ Cheap in productionDischarge increases internal resistanceElectrolyte leakages, degradation upon storage


Alkaline cell

SEM is close to that of acidic cells (above 1.5V)

☺ The capacity is about 3 times greater (3000 mAh for AA type)☺ Prolonged usage

The capacity depends on current.

Electrolyte reacts with aluminum

cathode: 2 MnO2 + H2O + 2 e– → Mn2O3 + 2 OH–

anode: Zn + 2 OH– → Zn(OH)2 + 2 e–

total: 2 MnO2 + H2O + Zn → Mn2O3 + Zn(OH)2


Nickel – cadmium/metal hydride cell

Rechargeable NiCd cell:- alkaline nickel oxide NiOOH- metallic cadmium

Voltage: about 1.2V. Resistant to inconvenient working conditions, durable(up to 20 years). Withstand about 1000 charge cycles, have „memory effect”.

Rechargeable NiMH cell:- alkaline nickel oxide NiOOH- metal alloys (e.g. vanadium, titanium, nickel, chrome, cobalt, iron) with porous structure which can release hydrogen upon discharge, andbond hydrogen during charging.Cathode: NiO(OH) + H2O + e− → Ni(OH)2 + OH−

Anode: MH + OH− → M + H2O + e−

Most hybrid cars use NiHM cell. Only new models use Li-ion cells.


Lead-acid cells

Anode - oxidation

Cathode- reduction


Li-ion cells


Li-ion cells

„gęstość” energii

Pot

encj

ał w

odn

iesi

eniu

do

litu


The assembly of Li-ion cells


Polymer electrolytes in Li-cells

- Flexible, obtained as foils - Light (high energy density/kg)- Safe (if used properly)


Polymer electrolytes

- Degradation due to ageing, phase transitions, chemical reactions- Sensitive to low temperature- Conduct both types of ions- Safe and stable electrolytes have lower conductivity


Modification of polymer electrolytes

The properties of traditional electrolytes, of „salt in polymer” type can be improved by:• branching of polymer

• increasing the amount of salt

• immobilizing the anions

• addition of solvent

• addition of fillers


Polymer in salt

The polymer preventscrystallization of salt.

Ion transport includes ion-ioninteractions.

☺ Conductivity is less dependent on temperature (ion-ion interactions are not „frozen” )☺ high cation transference numbers

Unstable, salt may separate from polymer matrix and crystallize.


Polyelectrolytes

For typical electrolytes, 50% charge is transported by anions.It is not convenient for application in cells.

USABC limits: 10-4 S/cm for polyelectrolytes 10-3 S/cm for traditional electrolytes

Anion can be „built” into the polymerchain.The most significant problem isdissociation – release of cations.Ionic conductivity is lower, than that of traditional electrolytes.


Polyelectrolytes

The anions can release and trap cations.

J.F. Snyder et al., J. Electrochem. Soc. 2003;150:A1090-A1094

K. Sinha, J. Maranas, Macromolecules 47 (2014) 2178


Gel electrolytes

Obtained by addition of polar molecules of organic solvent with low molecular weight.

Ionic conductivity increases, but safety andchemical stability decrease..

A. Manuel Stephan, European Polymer Journal 42 (2006) 21–42


Electrolytes with ionic liquids

Ionic liquid – salt which remains amorphous below 100oC.

Polymer acts as a „sponge” which provides mechanicalproperties and supports ion transport.


Electrolytes with fillers

+ Increase of conductivity+ Improvement of mechanical properties+ Better electrochemical stability+ Increased amount of amorphous phase

- Act as crystallization seeds- Different density causes sedimentation- May block transport of ions


Li-ion batteries: cathode materials


Cathode materials

Structures:- Tunnels- Layers or step-layers- 3D framework

Electrode materials have mixed conductivity:- ionic conductivity allows intercalation and deintercalation- electronic conductivity allows exchange of electrons


Nanocrystallization

Nanocrystalline materials have big surface area.Nanopores allow efficient exchange of ions with electrolyte. Conductivity of nanocrystalline material increases even by a million times!


Li-ion batteries: anode materials

Graphite is the most frequently usedmaterial for anode in Li-ion cells. Lithium can be also intercalated insilica or titanium oxides.


Na-ion batteries

Shrinking resources of lithium can result in high prices, making the production non-profitable.

Some technologies and materials used in Li-ion cells can be applied to Na-ion cells.


Na-ion batteries


Practical demands


Applications of Li-ion cells

Good ratio of electrical energy to cell mass and volume. Over 2000 charge cycles, no memory effect.Possibility to charge a battery under loadLow self-dicharge (5% monthly in comparison to 20% for NiCd)

Independently from charge and discharge cycles, ageing occurs. Cells degrade faster druring prolonged storage if they are totally discharged, or fully charged.Lithium is highly reactive – problems with disposal of batteries.

fizyka 3 michaŁ marzantowicz - warsaw university of ...adam.mech.pw.edu.pl/~marzan/cells.pdf ·...

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