Transcript Mechanisms of ionic transport in ionically conducting polymers

Mechanisms of ionic transport in
ionically conducting polymers
Summary
• 2. Mechanisms of ionic transport in ionically
conducting polymers
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2.1. Interaction between polymer and salt
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2.1.1.Ion solvatation by the polymer
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2.1.2.Hard - soft acid -base principle
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2.1.3.Anions
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2.1.4.Complex formation
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2.2. Transport theory and models
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2.2.1.Arrhenius model
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2.2.2.VTF and WLF models
Liquid-solid electrolytes
diferences
• The essential feature that distinguishes a polymer
electrolyte from low-molecular weight solvent-based
systems is that net ionic motion in polymer electrolytes
takes place without long-range displacement of the
solvent.
• Gel electrolytes (i.e. polymers containing a lowmolecular-weight fraction) and polyelectrolytes again
rely on an incorporated low-molecular-weight solvent
medium to assist ionic transport.
• In a polymer electrolytes no low-molecular-weight solvent
is present and ion transport relies on local relaxation
process in the polymer chain which may provide liquidlike
degrees of freedom, giving the polymer properties similar
to those of molecular liquid.
• The macroscopic properties that are similar to those of a
solid are the result of chain entanglements and possibly
crosslink.
• Ion transport in polymer electrolytes is considered to
take place by a combination of ion motion coupled to the
local motion of polymer segments and inter- and
intrapolymer transition between ion coordinating sites
Intra polymer
180º bond rotation at CO bond A-B
Inter
Fiona M. Gray “Solid Polymer Electrolytes, Fundamentals and
technological applications”
VCH Publishers, 1991.
Interaction between polymer and salt
Ion solvatation by the polymer
• Salt dissolve in a solvent only if the associated energy
and entropy changes produce an overall reduction in
free energy of the system
• In polymer exist polar groups and it can be expected that
polymers behave as solvents and dissolve salts to form
stable ion-polymer complexes
• It is possible when the interaction between the ionic
species and the coordinating groups on the polymer
chain compensate for the loss of salt lattice energy
• Polymers are macromolecules
– Deformation of the polymer structure and destruction of lattice
structure – gain of entropy
– Localized ordering of the polymer host by the anions – decrease
of entropy
– The distance of the apart of the coordinating groups and the
polymer´s ability to adopt conformations that allow multiple interand intramolecular coordination are important
• No polymer electrolytes: poly(methylen oxide) [(CH2O)n]
nor poly(trimethylene oxide) [(CH2CH2CH2O)n]
– Rigidity of the chain
– Inability to adopt low-energy conformations to maximize
polymer-cation coordination
– PPO – difficult due to the steric hindrance of the CH3 groups
Solubility of salts in PEO
Hard - soft acid - base principle
• Principle suggested by Pearson as a means of accounting for and
predicting the stability of complexes formed between Lewis acids
and bases
– Small acid and bases (termed HARD) are
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SOFT acid or bases are
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highly electronegative,
of low polarizability
Hard to oxidize
They tend to hold their electrons tightly
Low electronegativity
Tend to be large
Highly polarizable
Easy to oxidize
They hold their valance electronce loosely
Preference for complexes between hard acids and bases or soft acid and bases
PEO can be considered as a hard base
R.G. Pearson J.Am.Chem.Soc. 85 (1963) 3533
R.G. Pearson J.Chem.Ed. 45 (1968) 581, 643
Classification
of hard and
soft acids
and bases
PEO complexes
• The data suggest that Mg2+ (hard) would be expected to
form very stable complexes with PEO, whereas Hg2+
(soft) would show only a weak interaction
• Complexes with both cations are readily formed
transference number measurements showed that:
– Mg2+ ions are immobile
– Hg2+ ions are mobile in PEO
• Relationship between complex formation and the
consequential effects on cation mobility
Anions
• In polar solvents as water or methanol, hydrogen
bonding is important for specific anion solvatation,
whereas aprotic liquids (no hydrogen bonding) and
solvating polymers have negligible anion stabilization
energies
• Differences in the general solvatation energies of anions
do occur as the dielectric constant of the solvents varies.
• On passing from a polar, protic medium trough to a less
polar one, most anions are destabilized, the
destabilization being greatest when the charge density
and basicity of the ions are low:
F->>Cl->Br->I- ~SCN->ClO4- ~CF3SO3->BF4- ~AsF6Most suitable choices of anions for aprotic
Low-dielectric-constant dipolar polymer-based SPE
Anions
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Large anions
With delocalized charge
Weak bases
Posses low ion-dipol stabilization energies
Have low lattice energies i.e. little
tendency to form tight ion pairs (I- or ClO4-)
Anions
• Formation of polymer electrolytes is
controlled by the cation solvatation energy in
opposition to the salt lattice energy
• i.e. strongly solvated ions such a Li+ can be
complexated by PEO, even when the counter ion is
relatively small, like Cl- and there is associated high
energy lattice.
• The larger I- anion is required for the heavier, less
solvated K+ ion
Complex formation
• In the case of PE (polymer electrolytes)
complexes are formed when the polymer
host interact with the salt to form a new
polymeric system
• At high enough temperatures or in
systems where crystallization is prevented,
the ions are solvated by the polymer to
form a homogeneous polymer-salt solution
Complex salt-PEO
• With high-molecular-weight linear PEO the
system crystallizes to form spherulites of well
defined stoichiometries
• These “crystalline complexes” are often
recognized by their melting points which can be
well in excess of 100oC
• The amorphous regions within the spherulites of
complexes material can be of a very different
stoichiometry and it is somewhat inappropriate
to refer to the entire system as a “complex”
Modes of solvatation of the Li+ by
oxygen atoms from PEO
P.G.Bruce et al. Solid State Ionics 78 (1995) 191
Li+
coordenation
and
CF3SO3groups
P.G.Bruce et al. Solid State
Ionics 78 (1995) 191
Anions and conductivity
It is generally accepted that anions are mobile and
in some systems net cation mobility is vanishingly
small (disappear)
Anions assist in cation transport
- by formation of ion pairs
- triple ions
- higher aggregates
With the assistance of polymeric chain segmental
motion, the ionic cluster may itself move or it may
act as transient center for the mobile species.
Aggregates formation
For higher salt
concentration Ion pairs
and aggregates can be
formed
These species are less
mobile and can promote
a crosslink
a) via cation
b) via triple ion
Host polymer
To act as a successful polymer host, a polymer or the active part
of a copolymer should generally have a minimum of three
essential characteristics:
1)
Atoms or groups of atoms with sufficient electron donor
power to form coordination bonds with cations
2)
Low barriers to bond rotation so that segmental motion of
the polymer chain can take place readily
3)
A suitable distance between coordinating centers because
the formation of multiple intrapolymer ion bonds appear to
be important
Polyethers
• polyethers [-(CH2)nO-]
• Differences in physical properties
Differences are due to the molecular conformation and crystalline
structure (not chemical)
Poly
oxymethylene
and PEO
PEO crystalline
structure is more
open than PMO
Glass transition
temp.
For high Mw, Tgs are
of -65 to -60oC
PEO
• Among many polymers studied no one seems
to be so good as PEO – more studied up to
now as host polymer
• Large variety of salts can be dissolved in
PEO, but more interesting are with small
atoms of Li or Na, which give SPEs for
industrial application
• Two aspects in particular govern the
magnitude of the conductivity
– The degree of crystallinity
– The salt concentration
– Nature of salt and polymer also are important
• Interpretation of electrical conductivity difficult –
more of systems studied comprise more than
one phase
• Hysteresis in the conductivity is very common on
thermal cycling because of slow crystallization
kinetics – phase changes
• Transformations requiring the redistribution of
salt between phases
• As the temperature is raised the crystalline phase
progressively dissolves in the amorpouse phase, thus
increasing in the concentration of charge carries
• Simultaneously the polymer dynamics are affected by
the reduction in the amounth of crystalline material and
• an increase in the transient crosslink density due to the
increase in the salt concentration in the conducting
amorphous phase
Influence of salt concentration
• Ionic mobility is closely correlated to the
relaxation modes of the polymer host
– This can be observed through the increase
in the Tg of polymeric systems as salt
concentration is increased
Due to the intermolecular and
intramolecular interractions crosslink
Crosslink
Reduction of conductivity
for high salt concentration
• Stiffening of the polymeric matrix
• Reduced availability of
coordenating sites
• Strong ion-ion interractions in the
system of low permitivity (PEO) –
cooperative migration of several
ions
σ(T )  n i q iμ i
i
Low salt concentration
• The mobility of ions is relatively unaffected by
concentration
• Transient crosslink density is low and therefore
the conductivity will be controlled by the number
of charge carries
• As the charge carries increase, ion pairs and
mobile higher aggregates are predicted to form
• May form higher and less mobile clusters
• May also act as transient crosslink species
Conductivity
σ(T )  n i q iμ i
i
Where:
ni – number of charge carriers (i)
qi - charge of each one
i - mobility
Conductivity – impedance
measurements
Z"
Mass transport

Low frequency
High frequency
Z
Z'
σ
l
'
ZA
(S/cm)
Z’- real part and Z” – imaginary part
Conductivity model
Cation moving via
intra-polymer
coordenation
Cation moving via
inter-polymer
coordenation
Measurements data
Typical
impedance
diagrams
for
homogene
ous
PPOLiClO4
A.C. response
εε 0 A
Cb 
l
for A=1cm2 and l=1cm Cb is a
dielectric constant value of polymer
Generally of 5-20
(o permittivity of vacuum 8,85*10-14
F/cm)
1
1
1


Ce Ce1 Ce2
Ionic conductivity models
Arrhenius
Arrhenius
Ea- activation energy
- linear fitting for
semi-cristaline and
amorphous systems
σ  σ0exp(-Ea /kT )
Log  = log o + (-Ea / 2,303 RT)
R – ideal gas constant =8,31441 Jmol-1K-1
Vogel-Tamman-Fulcher (VTF)
Vogel-TammanFulcher (VTF)
For the amorphous
systems
  σ0exp[B/k(T T0 )]
Where B- constant, 0 - constant of T-1/2 and
T0 temperature at which configurational entropy is zero (generally
close to Tg)
-Correlation within conductivity and visco-elastic properties of SPE.
Free volume
•
A small amount of unfilled volume is associated with the end of a polymer
chain. This volume is called the free volume and is schematically
represented in the diagram below.
• For a given mass of polymer the amount of free volume will depend
on the number of chain ends, hence the number of chains and
hence the degree of polymerisation.
Williams – Landel - Ferry (WLF)
Equivalent to VLF
viscoelastic properties
also relaxation process
(aT-deslocation factor)
 C1 (T  T0 )
logaT 
(C2  T  T0 )
log aT 
σ  σ 010
 C1 (T  T0 ) 


 C 2  T  T0 
 17,44(T  Tg )
51,6 T  Tg
C1 and C2 temperature
dependent constants and To –
WLF temperature reference
VTF and
Arrhenius
Condutivity as a function of
salt concentration