Polybis (2-(2-methoxy ethoxy ) ethoxy ) phosphazene – [630871]

Poly[bis (2-(2-methoxy ethoxy ) ethoxy ) phosphazene] –
based electrolytes for lithium -ion batteries

Subject : Inorganic Polymers

Supervising teacher , Student: [anonimizat]. Prof. PhD. Denisa FICAI Cătălina -Diana UȘURELU

BUCHAREST
2020

POLITEHNICA UNIVERSITY OF BUCHAREST
FACULTY OF APPLIED CHEMISTRY AND MATERIALS SCIENCE

TABLE OF CONTENTS

1. The components of lithium -ion batter ies ………………………….. ………………………….. .. 1
2. The working principle of a lithium -ion battery ………………………….. ……………………. 2
3. Solid electrolytes for lithium -ion batteries. ………………………….. ………………………… 4
4. Polyphosphazenes as potential electrolytes in lithium -ion batteries ……………………. 7
5. The preparation of MEEP -based electrolytes. ………………………….. …………………… 10
6. The mechanism of ionic conductio n in MEEP -based electrolytes ……………………. 12
7. The properties of MEEP -based electrolytes………………………………………..1 3
8. Bibliography………………………………………… ……………………………..2 3

1
1. The components of lithium -ion batte ries
Lithium -ion batteries are a type of secondary (rechargeable) batteries commonly used as
key components in portable electronics, power tools and hybrid/full electric vehicles [1].
Generally, a lithium -ion battery consists of four main components: a negative electrode
(the anode), a positive electrode ( the cathode), an electrolyte and a separator.
In a lithium -ion battery, the anode (the negative electrode) is the electrode where the
oxidation reaction takes place during the discharge process [2]. The materials that are usually used
in the manufacturing of the anode are: metallic lithium, graphitic carbon, hard carbon, synthetic
graphite, lithium titanate ( Li2TiO 3), tin-based alloys and silicon -based materials [3]. At the
opposite end , the cathode (the positive electrode) is the electrode where the reduction reaction
takes place during the dis charge cycle of the lithium -ion battery [2]. The materials which are
typically used for fabricating the lithium -ion batter ies’ cathode are: a lithium manganese oxide, a
lithium cobalt oxide, iron (II) sulfide (FeS 2), vanadium pentoxide (V 2O5), a lithium nickel cobalt
manganese oxide, lithium iron phosphate (LiFePO 4) or an electr ically conducting polymer [3].
The electrolyte is the medium that enables the back -and-forth movement of the lithium ions
between the cathode and the anode during the charge -discharge cycles of the batter ies [4]. Usually ,
lithium -ion batteries use liquid electrolytes which consist of a lithium salt, such as : lithium
hexafluorophosphate (LiPF 6), lithium perchlorate ( LiClO 4), lithium triflouromethanesulfonate
(LiCF 3SO 3), lithium hexafluoroarsenate ( LiAsF 6) etc. dissolved in an aprotic organic solvent . The
most widely used solvent s consist of a mixture of alkyl carbonate s an example being the ethylene
carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate
(EMC) mixture [5].
The separator is a thin, porous membrane placed between the anode and the cathode which
allows the lithium ions to move freely from one side to the other during the charge -discharge cycles
of the battery but prevents the movement of the electrons through t he electrolyte, forcing them to
traverse an external circuit . Another role of the separator is to prevent the physical contact of the
two electrodes thereby avoiding any possible electrical short -circuits or the self-discharge of the
battery . The separators can be fabricated from a variety of materials, such as : microporous polymer
films (e.g. polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE)), nonwoven
fabrics (commonly made from polymer fibers), ceramic s or naturally abundant materials (e.g.
cellulose, rubber, wood) [6].

2
2. The working principle of a lithium -ion battery
A lithium -ion battery is a system which stores the chemical energy generated from the
redox reactions that take place at the electrodes and convert s it into electrical energy [7]. Therefore ,
it is right to state that the working principle of the lithium -ion batteries is build on the reversible
redox -reaction s which occur s between the anode material (the negative electrode, the „reductant”)
and the cathode material (the positive electrode, the „oxidant”).
In a lithium -ion battery, the anode and cathode are physically separated but connected
electrically through an external electrical circuit and ionically through the electrolyte. As
illustrated in Fig. 1, when the battery gets connected to a power source i .e. during charge, the
positive side of the power source will attract and remove electrons from the cathode material
(commonly, a lithium oxide). Consequently, these electrons will flow through the external circuit
and reach the anode material (commonly, a gr aphite layer) while the lithium cations formed at the
cathode will be attracted by the negative electrode and will flow trough the electrolyte towards the
anode . Once the lithium cations get to the anode, they will insert between the layers of the graphite
(anode material ), process known as „intercalation”. When all the lithium cations get stored at the
anode, the battery is fully charged. As soon as the power source is removed and the battery gets
connected to a load (an electrical power consuming device) i.e. during discharge, the lithium ions
will strive to go back to their stable state and reintegrate into the cathode material. Due to this
tendency, the lithium ions will migrate from the anode through the electrolyte back to the cathode ,
process called “deintercalation ”, while the electrons accumulated at the anode will flow through
the load back to the cathode, thus producing an electrical current throughout the load.

Fig. 1 Lithium -ions batteries charge and discharge mechanisms
Image source: https://liveapi.authorcafe.com/preview.php?assetid=57970

3
The reaction scheme below shows the redox reactions that occur during the charge –
discharge cycle of a lithium -ion battery w hich uses graphite as the anode material and lithium
cobalt oxide (LiCoO 2) as the cathode material [8]:
LiCoO2Charge
DischargeLi1-xCoO2 + xLi++ xe-Cathode (positive electrode):
Anode (negative electrode): C + xLi++ xe-Charge
DischargeLixC
LiCoO2 + C Li1-xCoO2 + LixC Battery as a whole:Charge
Discharge

4
3. Solid electrolytes for lithium -ion batteries
Nowadays, most of the lithium -ion batteries available on the market are still using liquid
electrolytes because of their high ionic conductivity [9]. However, since lithium readily reacts with
water and other aqueous electrolytes, only the aprotic organic electrolytes such as the above –
mentioned alkyl carbonate mixture s can be used in the manufacture of rechargeable lithium -ion
batteries [10]. Anyway, this type of electrolytes presents several disadvantages. Firstly, the organic
aprotic electrolytes have high volatility and flammability and pose important safety issues as they
may leak and cause the battery to explode or catch fire. Secondly, in the case of lithium -ion
batteries which use liquid electrolyte s and metallic lithium as anode , it was observed that, the
lithium ions transported by electrolyte tend to accumulate unevenly at the surface of the lithium
metal anode forming the so-called “dendrites ”. Dendrites are tiny, rigid , tree-like lithium
formations which appear on the surface of the lithium metal anode , expand, and pierce the
separator of the lithium -ion battery causing a short circuit in the cell. Also, the liquid electrolytes
may corrode or react with the surface of the electrode s affecting the efficiency and the lifespan o f
cells and, if the battery over -heats due to overvoltage or short circuit conditions , there is a risk that
the liquid electrolyte will catch fire or explode [11].
Given the need of safer and more reliable electrolytes, researchers have turned their
attention to the development of solid electrolytes. Compared to liquid electrolytes solid electrolytes
have a significant number of advantages:
 They are not flammable, the y can not leak accidentally and some of them can suppress the
lithium dendrite formation thus being much safer than liquid electrolytes;
 Solid electrolytes have higher thermal stability and wider operating temperatures ranging
from -50°C to 200°C where organic liquid electrolytes may fail due to freezing, boiling, or
decomposition [12].
 In the case of carbonaceous anodes, which allow the intercalation of lithium ions within
the carbon layers during charging, the specific capacity of a battery ( i.e. the maximum
amount of charge a battery can store relative to its mass ) is limited, having values of about
372 mA∙h/g. This is because the maximum amount of lithium that can be intercalated
within the graphite struc ture is 1 lithium atom per 6 carbon atoms [13]. Therefore, a better
choice as anode material is believed to be the metallic lithium , which has a very high

5
theoretical specific capacity (3860 mA∙h/g), a low density (0.59 g/cm3) and the lowest
negative electrochemical potential ( -3.040 V vs. the standard hydrogen electrode) [14].
However the use of metalic lithium as anode in lithium -ion batteries is obstructed by the
fact that, in liquid electrolytes, spiky structures called „dendrites ”, which were mentioned
above , tend to form on its surface [15]. Some solid electrolytes and especially those that
have no defects in their structure can hinder the growth of dendrites and prevent their
expansion towards the cathode . Therefore , employing the lithium metal as anode is
possible in the case of lithium -ion batteries with solid electrolytes [16]. As a result, the
batteries which use solid electrolytes will have a higher capacity than those using liquid
electrolytes, which is another significant advantage.
 If a solid electrolyte is used, the existence of a separator in the lithium ion battery is no
longer necessary [6].
 As there is no longer a risk of electrolyte leakage, the use of thick, hard battery cases is not
required anymore .
 Solid electrolyte batteries can be designed in different shapes and sizes and are usually
thinner than liquid electrolyte batteries [17].
Two types of materials are mainly used for the manufacture of solid electrolytes: inorganic
ceramics and polymers. Regarding their characteristics and behavior, some differences have been
reported in the literature.
Firstly, c eramics are hard and brittle and have higher modulus of elasticity, which makes
them more suitable for rigid battery manufacturing . In contrast, the polymeric electrolytes are soft
and have a lower modulus of elasticity, which makes them more adequate for flexible battery
designs.
Secondly , polymers are easier to process than ceramics, so the production cost of a battery
that uses a solid polymer electrolyte will be lower . However, the ceramic s have the advantage that
they present a greater stability at high temperatures and other aggressive environments [18].
Another aspect worth mentioning is the interfacial resistance that appears at the contact
surface between the electrolyte and the cathode which ca n lead to an increase in the internal
resistance of the battery and result in a loss of energy and power density. It was proved that using
a polymer electrolyte which is quite flexible and moldable ensure s a lower interface resistance ,

6
while the rigid inorganic ceramic electrolytes will have a larger interfacial resistance due to
insufficient contact with the electrode [19].
An important difference between the two types of solid electrolytes also arises from the
way in which the ionic conduction is achieved . In ceramics , the i onic conduction consists in the
movement of ions from one site to another via point defects which are vacancies in the crystal
lattice. At room temperature very little ion mov ement takes place, since the atoms are in relatively
low energy states, so a high temperature is needed for ion ic conduction to take place . Therefore,
ceramic solid electrolytes are more adequate for high -temperature applications [18]. The polymer
electrolytes are obtained only by dissolving a metal salt in a high molecular weight polymer host.
In the solid polymer electrolytes, the metal salts are solvated with the help of the polar groups
existing in the polymer . These groups contain lone pairs of electrons that coordinate to the metal
cation by Coulombic interaction, causing the dissociation of lithium salt [19]. It is believed that in
polymers, ionic conduction occurs only in the amorphous phase , above the glass transition
temperature (T g), where polymer chain motion creates a dynamic, disordered environment that
plays an essential role in facilitating ion transport [20]. The movement of Li+ cations takes place
by transferring the Li+ ions from one coordination point to another along the same polymer chain
or by jumping from one chain to another [19].
Intensively studied as promising solid polymer electrolytes for lithium -ion batteries have
been the materials based on poly(ethylene oxide) (PEO) , whose structure is shown in Fig. 2 . This
is due to the lone pairs of electrons existing at the oxygen atoms in the repeating units of the
polymer , which can coordinate the Li+ ions. However, the PEO based electrolytes usually show
low ionic conductivities of only 10-8-10-4 S∙cm-1 at temperatures below 60°C , which are not
sufficient for practical application. This is due to the significant degree of crystallinity of PEO
which restrain s the easy movement of the lithium ions along the polymer chains . The fact that at
room temperature, the chain segments of the PEO don’t move freely, but align on certain
direction s, giving rise to rigid crystalline formations, mak es the transport of lithium ions from one
coordination site to another difficult to accomplish [21].

Fig. 2 The structure of poly(ethylene oxide ) PEO
CH2CH2 OH O H
n

7
4. Polyphosphazenes as potential electrolytes in lithium -ion batteries
As mentioned above, in most cases, the low conductivity of polymeric electrolytes is
determined by the tendency of their chains to align and form ordered regions called crystalline
formations. These highly ordered regions restrict the movement of ions. It has been determined
that, in polymers, ionic conduction is limited only to the amorphous phase, at temperatures above
the glass transition temperature, when the chain segments acquire mobility and are able to reorient,
these phenomena leading to an increas e in the free volume of the polymer. The free volume of the
polymer represents the void space between the polymer chains in which ions can hop freely. Under
these conditions, interesting as potential solid electrolytes turned out to be the polyphosphazenes ,
especially poly[bis (2-(2-methoxy ethoxy ) ethoxy)phosphazene] [20].
Polyphosphazenes are inorganic polymers with a backbone consisting of alternating
phosphorus and nitrogen atoms and with two organic or inorganic side groups bonded to each
phosphorus atom. These two groups attached to the phosphorous atoms are extremely significant
for the final properties of these polymers, as they can be easily replaced by an uncountable number
of nucleophiles, resulting in hundreds of different polyphosphazenes with different properties [22].
Besides their chemical versatility, another aspect that makes polyphosphatases so interesting is the
fact that the polyphosphazenes backbone is endowed with a high degree of torsional freedom and
high chain flexibility [23]. This high chain flexibility is reflec ted in the case of some
polyphosphazenes whose glass transition temperatures are as low as –100°C [24]. The glass
transition temperature s (Tg) as well as the mobility of the chains are also determined by the nature
of the side groups attached to the phosphorous atoms. If the side groups are bulky or promote the
formation of ionic interactions or hydrogen bonds between the polymer chains, then the respective
polyphosphazenes will be more rigid and the glass transition temperature will be higher. It has
been shown , that the substituent groups have the ability to tune the glass transition temperature of
the polyphosphazene over the range of -100°C (as in the case of alkoxy side groups) to 200°C or
higher (as in the case of polyphosphazenes with adamantylamino substituent groups) [23]. An
additional advantage is that polyphosphazenes have a flame retardant effect due to the phosphorus –
nitrogen polymer backbone. As a result, the use of polyphosphatene -based electrolytes will
improve the safety of lithium ion batteries becuase the risk for the battery to catch fire when
overcharging or overheating occurs , will no longer exist [25].

8
In this paper I chose to talk about solid polymer electrolytes based on poly[bis (2-(2-
methoxy ethoxy ) ethoxy)phosphazene] (MEEP) .
MEEP is an amorphous linear polyphosphazene, with a low glass transition temperature
(Tg) of -83°C [26] which bears methoxy—ethyoxy—ethoxy side groups linked to the phosphorous
atoms, as shown in Fig. 3. The role of the two substituent groups attached to the phosphorus atoms
is twofold: firstly , they impart to the macromolecule s the ability to coordinate metal cations thus
making the MEEP an ionic conductor [27]; secondly, they are short and unable to self -align into a
crystalline structure, so they manage to ensure a very low glass transition temperature to the
polyphosphazene, as well as a high flexibility to the polymer chains [28].

P N
O CH2CH2O CH2CH2O CH3O CH2CH2 O CH2CH2 O CH3

n

Due to its high segemental mobility and low crystallinity, it has been found that after
complexation with various lithium salts, MEEP exhibits a ionic conductivity 2 -3 orders of
magnitude higher than the corresponding PEO salt complexes at moderate temperatures (25 -60°C)
[29].
The presence of the dissolved salts in the polymer electrolytes is mandatory as they fulfill
two major roles: firstly, they introduce ionic charge carriers in polymers thus considerably
increasing their ionic conductivity; secondly, when added in the right amount, the salts suppress
the crystallization of the polymers [30].
However, it has been shown that simple salts such as lithium chloride (LiCl), are not able
to provide high ionic conductivity , due to the ir reduced tendency to dissociate . On the contrary ,
the bulkier the anion of the lithium salt, the easier the dissociation will be and, as a consequence ,
the conductivity of the polymer -salt complexes will increase. Therefore, the lithium salt with
bulky anions, which have a low basicity due to the fact that they can stabilize themselves by the
delocalization of the negative charge on several centers are preferred .
Popular lithium salts frequently used in solid polymer electrolytes are: lithium perchlorate
(LiClO 4), lithium tetrafluoroborate (LiBF 4), lithium triflate ( LiCF₃SO₃), lithium bis(oxalato)borate
(LiBOB), lithium hexafluorophosphate (LiPF 6), lithium bis(trifluoromethanesulfonyl)imide Figure. 3 The structure of MEEP

9
(LITFSI) and lithium difluoro(oxalato)borate (LIDFOB). Their structures are illustrated in Fig. 4
[21].
ClO
O-Li+
OO BF
FFF-
Li+SO
OO-Li+F
F
FBO O
O OO
OO
O
P–F
FF F
F F-Li+
Li+N–
S SO
OO
OCF3F3CLi+
BO
OO
OF
F-Li+LiClO4 LiBF4LiCF₃SO₃ LiBOB
LiPF6LITFSI LIDFOB

Fig. 4 The structures of preferred salts used in obtaining solid polymeric electrolytes

10
5. The preparation of MEEP -based electrolytes
In 2011, Yang M. et al. reported a simple process for obtaining MEEP -based electrolytes
which involved the following steps: the synthesis of polydichlorophosphazene, the nucleophilic
substitution of polydichlorophosphazene with the sodium salt of 2 -(2-methoxy ethoxy) ethanol
(also called “ sodium (methoxyethoxy)ethoxide ”) and the complexation of MEEP with a metal salt.
The three steps were described as follows:
 Polydich lorophosphazene synthesis: The synthesis of polydichlorophosphazene
involved the ring opening polymerization of hexachlorocyclotriphosphazene which was conducted
in sealed reaction bottles, under vacuum, at temperatures between 230 -270°C, for 5 -25 hours [9].
The synthesis of polydichlorophosphazene can be illustrated by the equation (1):
P
N N
P P
NCl Cl
Cl
ClCl
Cl230-270°C
vacuumP N Cl
Cln(1)

 Poly[bis (2-(2-methoxy ethoxy ) ethoxy )-phosphazene] ( MEEP) synthesis : MEEP was
prepared by reacting a solution of polydichlorophosphazene in tetrahydrofuran (THF) with sodium
(methoxyethoxy)ethoxide , at 65°C, under stirring and in an inert nitrogen atmosphere, for 48 hours
as in the equation (2).
P N Cl
Cln+ CH3O CH2CH2O CH2CH2O-Na+2n65°C
THF
P N
O CH2CH2O CH2CH2O CH3O CH2CH2 O CH2CH2 O CH3

n(2) +2nNaCl

After the reaction was completed, the obtained mixture was concentrated by evaporating
the THF under vacuum, when a brown product was formed. The brown product was then put into
freshly distilled petroleum ether and a brown precipitate formed. Further, the mixture was
subjected to filtration and the MEEP thus separated was dried under vacuum, at 80°C, for 8 hours
[9]. The obtained MEEP was a sticky elastomer, with high viscosity [31].

11
 Preparation of solid MEEP electrolytes: An amount of lithium salt and the corresponding
amount of MEEP were dissolved separately in THF under reflux (≈60 -70°C) until two
homogeneous solutions were obtained. The salt solution thus obtained was poured quickly into the
MEEP solution, under continu ous stirring. The resulting mixture was then heated to reflux (≈60 –
70°C) and reacted 1 -2 hours under stirring. Then, the resulted composition was subjected to rotary
evaporation for the THF removal, when a viscous product was obtained. In the end, the pro duct
was dried under vacuum at 80°C, for 4 hours [9]. Compared to the pure MEEP, the resulting
complexes were less sticky and had a higher viscosity [31].
The complexation of a lithium salt (Li+X-) with MEEP is illustrated in equation (3).
P+N
O CH2CH2O CH2CH2 O CH3O CH2CH2 O CH2CH2 O CH3

n+Li+X-P N
O CH2CH2O CH2CH2 O CH3O CH2CH2 O CH2CH2 O CH3

Li+n
X-(3)

12
6. The mechanism of ionic conduction in MEEP -based electrolytes
The mechanism of ionic conduction in MEEP -based electrolytes is found ed on the
polyphosphazene ability to complex (dissolve) the Li+ cations and can be described as follows:
once added to the polymer matrix, the lithium salts will dissociate because of the electron -donor
oxygen atoms existing at the side groups of the MEEP which coordinatively bind the lithium ions.
The fact that MEEP has 6 such atoms per repeating unit , gives the polymer the power to easily
solvat e and coordinat e the Li+ cations because the dissociation of the lithium salt is favored . Also,
the large number of etheric oxygen atoms that coordinate to the lithium ions makes possible the
dissolution of a larger amount of lithium salt in the polymer, which will lead to a higher ionic
conductivity . At temperatures above the glass transition temperature (Tg) of the MEEP (i.e. even
at room temperature ), the polymer chains will undergo constant segmental motion, which leads to
the appearance of free volumes in the polymer. The generate d free volume will ensure the empty
space necessary for the easy movement of ions in the material . So, under the influence of an electric
field, the lithium ions will be able to migrate from one coordination site to another in the same
polymer chain or to hop from a polymer chain to a neighboring polymer chain and generate ionic
conduction in the polymeric material [32]. The mechanism of Li+ ions conduction in MEEP -based
electrolytes is illustrated in the Fig. 5.

Fig. 5 The mechanism of Li+ ions conduction in MEEP -based electrolytes
Image source: https://apps.dtic.mil/dtic/tr/fulltext/u2/a279696.pdf

13
7. The properties of MEEP -based electrolytes
In 1986, P. M. Blonsky et al. prepared MEEP -lithium triflate ( LiCF₃SO₃) electrolytes by
varying the amount of lithium salt dissolved in MEEP and measured the glass transition
temperature (Tg) of these complexes , as well as their electrical conductivity (σ), at different
temperatures . Thus, they determined to what extent the amount of salt dissolved in the polymer
affects the glass transition temperature and the electrical conductivity of the MEEP -based
electrolytes and also how the ionic conductivity of the complexes varies when increasing the
operating temperature. Following the measurements performed, they obtained the following
results :

From the data presented in Table 1 , it can be concluded that by increasing the amount of
salt in the electrolyte, the glass transition temperature (T g) of the resulted complexes will shift to
higher values [29]. As expected, pure MEEP showed the lowest glass transition temperature (Tg)
value due to the high rotational freedom of its chains . By adding lithium triflate ( LiCF₃SO₃) in the
MEEP matrix , MEEP -LiCF₃SO₃ complexes will be obtained and a gradual immobilization of the
polymer chains will occur, due to the crystalline nature of the lithium salt. This will lead to a
restriction in the segmental motion of the polymer, an increase in its mechanical rigidity and an
increase in th e glass transition temperature of the resulted electrolyte [33].
Regarding the electrical conductivity, at 30°C , the MEEP -LiCF₃SO₃ complexes exhibit an
electrical conductivity in the range of 1.2∙10-5-2.7∙10-5 [29]. This is three orders of magnitude
higher than in the case of PEO -LiCF₃SO₃ electrolyt es which , at room temperatur e, present a LiCF₃SO₃:MEEP
repeating unit
(mole ratio) Tg
(°C) σ30°
(S∙cm-1) σ55°
(S∙cm-1) σ70°
(S∙cm-1) σ90°
(S∙cm-1)
0 -83.5 8.1∙10-8 1.6∙10-7 1.9∙10-7 2.1∙10-7
0.125 -69.4 2.2∙10-5 5.7∙10-5 8.5∙10-5 1.3∙10-4
0.167 -65.7 2.2∙10-5 6.2∙10-5 8.7∙10-5 1.5∙10-4
0.250 -62.4 2.7∙10-5 7.5∙10-5 1.2∙10-4 2.2∙10-4
0.500 -58.9 1.2∙10-5 5.7∙10-5 1.0∙10-4 1.9∙10-4 Table 1 Glass transition temperatures and ionic conductivities of MEEP – LiCF₃SO₃
complexes at different temperatures [29]

14
conductivity in the order of 10-8 S∙cm-1. The superior electrical conductivity is attributed to the
capacity of methoxy—ethyoxy—ethoxy side groups to complex the lithium cations and also to the
high chain mobility of the polyphosphazene which allows the rapid migration of the lithium cations
from one coordination site to another [33]. Another thing worth mentioning is that unlike PEO,
MEEP presents at each of its repetitive unit s not one, but six oxygen atoms which can coordina te
the lithium cations . This encourages the dissociation of the lithium salts and justifies the higher
electrical conductivity of MEEP -based electrolytes [34].
By analyzing the data in the Table 1, it can also be seen that up to a certain optimum value,
the electrical conductivity of the MEEP -based electrolytes increases with the amount of salt
dissolved in the polymer and then decreases . Although an increase in the concentration of lithium
salt would naturally lead to an increase in the number of charge carriers in the electrolyte , above a
certain v alue of the concentration of the LiCF₃SO₃ in MEEP, the restriction of the free movement
of the polymer chains by the lithium salt becomes significant and the flexibility as well as the
electrical conductivity of the polymer decrease considerably [33].
Another trend easily observable in the table is that the conductivity of MEEP -salt
electrolytes increases with increasing the operating temperature. The increase in ionic conductivity
with the increase in temperature is because as temperature rises, the MEEP -salt complexes will
acquire more energy. Receiving more energy, the segmental motion of polymer chains increases
while the strength of the association between the polymer and the lithium ions decreases such that
the ions will be able to migrate more easily through the electrolyte from one coordination site to
another [35].
Majo r drawbacks of MEEP -lithium salt complexes are their poor mechanical properties
[36] and poor dimensional stability which obstruct their casting as free standing, stable thin films
electrolytes in solid state lithium -ion batteries. At room temperature and above, the MEEP -lithium
salt complexes are viscous and sticky fluids which have the tendency to flow under pressure [37].
In addition, although high , their ionic conductivity at room temperature is not sufficient for their
use in practice [38].
One method to increase the dimensional stability of the MEEP -based electrolytes is by
crosslinking the MEEP. MEEP can be crosslinked by expos ure to 1-5 MRads of gamma -radiation
or by ultraviolet irradiation. By irradiation with gamma or UV rays, free radicals appear at the side
groups of MEEP . These free radicals will stabilize themselves by establishing covalent bonds with

15
other free radicals formed at the side groups of the polymer neighboring chains and thus the
polymer will crosslink . Crosslinking causes the MEEP to turn into a nonflowing rubbery
elastomer. It is recommended that t he crosslinking be done after the dissolution of the lithium salt
in the MEEP, so that the MEEP -lithium salt complexes can maintain a fairly good value of
conductivity. Because only few cross -links per polymer chain are enough to prevent the electrolyte
flow, the segmental motions of the rest of monomer units will not be affected by the cross -link
sites [34].
One way to improve the conductivity of MEEP -based electrolytes is to combine the MEEP
with liquid electrolytes such as propylene carbonate, ethyl ene carbonate and others when gel
electrolytes are obtained . To prevent the leakage of the liquid component , crosslinking of the
polymer matrix is mandatory. When cross -linked, the liquid component gets incorporated into the
polymer matrix so that its leakage is prevented and an electrolyte that possesses both high electrical
conductivity and good dimensional stability is forme d [38].
In 2015, S. Jankowsky et al. prepared MEEP -based gel electrolytes by swelling MEEP ,
which was previously crosslinked using UV radiation , with various amounts of a liquid electrolyte s
consisting of lithium bis(oxalato)borate (LiBOB) in a mixture of ethylene carbonate (EC) /
dimethyl carbonate (DMC) (1:1 by weight) and, respectively, lithium (hexafluoro)phosphate
(LiPF 6) in a mixture of ethylene carbonate (EC) /diethylene carbonate (DEC) (3:7 by weight) [39].
a) The ionic conductivity of MEEP -based gel electrolytes
The ionic conductivity of these gel electrolytes at room temperatures was expected to be
higher than that of MEEP -based electrolytes because they combine the mechanism of Li+ ions
conduction through a liquid electrolyte with the mechanism of Li+ ions conduction through solid
polymers. Therefore , ionic conduction may occur just like in the case of polymer solid electrolytes ,
when the oxygen atoms existing at the side groups of the polymer coordinate at the Li+ cations and
transfer these cations from one coordination point to another placed on a different chain segment,
during the reorientation movement of the polymer chains . There is also the possibility that ionic
conduction may occur as in the liquid electrolytes when the Li+ ions get surrounded by
coordinative solvent molecules and migrate through the liquid via diffusion . In gel -type
electrolytes, both mechanisms are possible, but how the two mechanisms complement each other,
and which mechanism dominates is not yet known. One of the theories assumes that the liquid
component (the mixture of organic carbonates) plays only a minor role in the movement of Li+

16
ions and works mainly as a plasticizer for the polymer matrix . Like any plasticizer, the liquid
component will penetrate the macromolecular chains of the polymer, where they will act as
lubricants, favoring the mutual sliding of the chain segments. This will increase the free volume
of the polymer and lower the glass transition temperature (Tg), facilitating the movement of ions
from one coordination site to another. The second theory assumes that the liquid component f orms
miniature “channels” or “tunnels” in the polymer matrix whereby the lithium ions surrounded by
the solvent molecules move freely by diffusion . This second theory of ionic conduction is
illustrated in Fig. 6.

Table 2 shows the compositions of the prepared MEEP -based gel electrolytes and their
ionic conductivity at 30 °C. MEEP (wt.-%) represents the mass percentage of MEEP in the gel Sample MEEP
(wt.-%) LiBOB
(wt.-%) EC/DMC (1:1)
(wt.-%) (N=PR 2) units:Li σ(30°C)
(mS∙cm-1)
GPE 1 36.3 14.7 49.0 1.7:1 2.31
GPE 2 50.0 5.1 44.9 6.7:1 2.68
Sample MEEP
(wt.-%) LiPF 6
(wt.-%) EC/DEC (3:7)
(wt.-%) (N=PR 2) units:Li σ(30°C)
(mS∙cm -1)
GPE 3 48.0 15.4 36.6 1.7:1 1.10
GPE 4 60.0 3.6 36.4 8.7:1 1.58
Fig. 6 Li+ ions transport in the polymeric gel electrolyte network [39]
Table 2 Compositions of the prepared gel polymer electrolytes, (R2P=N) units:Li mole
ratios and the ionic conductivity σ at 30 °C [39]

17
electrolyte ; LiBOB (wt.-%) and LiPF6 (wt.-%) represent the mass percentage s of LiBOB and,
respectively , LiPF 6 in the gel electrolyte ; EC/DMC (1:1) (wt.-%) and EC/DEC (3:7) (wt.-%) are
the mass percentage s of ethylene carbonate/ dimethyl carbonate (1:1 by weight) and, respectively,
ethylene carbonate/diethylene carbonate (3:7 by weigh t) mixtures in the gel electrolyte ; (N=PR 2)
units:Li is the mole ratio of MEEP repeating units to lithium cations ; σ (30°C) is the ionic
conductivity of the electrolyte at 30 °C [39].
As shown in Table 2 , at 30°C, the ionic conductivity of MEEP -based gel electrolytes is
two orders of magnitude higher than the ionic conductivity of MEEP -LiCF₃SO₃ complexes
obtained in the previously mentioned study, which at 30°C had a conductivity on the order of 10-
5 S∙cm-1 [29]. In the case of MEEP -based gel electrolytes , the ionic conductivity of all samples at
30°C is higher than 1 mS·cm-1 when LiPF6 is used as salt (GPE 3 . GPE 4) and higher than 2
mS·cm-1 when LiBOB is used as salt (GPE 1 , GPE 2) [39]. These values are sufficient for the use
of these electrolytes in practice [41].
Another thing easy to notice is that although the mass percentage of salt in the electrolyte
increase d, the ionic conductivity of the electrolyte did not increase but decreased. This is becaus e
the optimal lithium salt content is likely to have been already exceeded in the case of GPE 2 and
GPE 3. As mentioned earlier, too high a content of lithium salt can lead to decreased mobility of
polymer chains and decreased conductivity [33].
b) The thermal stability of MEEP -based gel electrolytes
S. Jankowsky et al. went further and determined the glass transition temperature and the
thermal stability of the of the prepared MEEP -based gel electrolytes .

Fig. 7 Thermal properties of GPE 1: a) DSC heating curves
b) Thermogravimetric analysis [39]

18
Fig. 7 a) illustrates the DSC heating curves of GPE 1 . This sample contains , as a liquid
component , LiBOB dissolved in a mixture of EC/ DMC (1:1 by weight) . Analyzing the DSC curve,
we can conclude that the negative peak at -8°C corresponds to the EC recrystallization , while t he
positive peak at 19°C corresponds to the EC melting. The glass transition temperature of the
polymer network (Tg) is -64°C [39]. This is higher than that of the pure MEEP due to the
crosslinking of the polymer, but also to the dissolution of the lithium salt, both lead ing to a certain
restriction in the movement of the polymer chains [38]. However, because the glass transition
temperature still has a negative value , at room temperature, the mobility of the chain segments will
be high enough to allow a good ionic conductivity .
According to the thermogravimetric analysis shown in Fig. 7 b) the temperature at which
the thermal decomposition of the GPE electrolyte begins is 183°C . From this temperatur e on, a
continuous decrease in the mass of the sample can be observed which can be attributed to the
depolymerization and other thermal degradation processes of the polymer matrix . The small
decrease in the mass of the sample that can be observed at a temperature of about 130°C is due to
the evaporation of dimethyl carbonate (boiling point 90°C). The small shoulder in the DTGA at
216°C is attributed to the thermal degradation of ethy lene carbonate. This temperature is slightly
lower than the temperature at which pure ethylene carbonate degradation begins [39] i.e. 263°C
[42] probably due to the presence of the lithium salt [39].

Fig. 8 Thermal properties of GPE 3: a) DSC heating curves
b) thermogravimetric analysis [39]

19
Fig. 8 a) illustrates the DSC heating curves of the GPE 3. This sample contains , as liquid
component , LiPF 6 dissolved in a mixture of EC/ D EC (3:7 by weight). The glass transition
temperature of the polymer network (Tg) is -70°C, which means that at room temperature, the
chain segments will have sufficient re-orientational freedom for the conductivity to take place . A
certain peak corresponding to the melting of ethylene carbonate can also be observed , but the
recrystallization of ethylene carbonate is suppressed in the presence of diethyl carbonate.
According to the thermogravimetric analysis shown in Fig. 8 b), the temperature at which
the thermal degradation of the electrolyte begins is 73°C . This is due to the thermal instability of
LiPF 6, which begins to decompose at ≈70°C releasing hydrofluoric acid which in turn can cause
the onset of other degradation reactions . The thermal degradation of ethylene carbonate is observed
at ≈230°C .
As a result, the thermal stability of GPE 3, which contains LIPF6 as salt is much lower
compared to that of GPE 1 which contains Li BOB as salt.
c) The feasibility of MEEP -based gel electrolytes
In order to estimate the lifetime and also the feasibility of a long-term use of the prepared
MEEP -based gel electrolytes in combination with lithium metal anodes, S. Jankowsky et al. went
further and performed plating -stripping tests on batteries containing GEP 2 and GEP 4 as
electrolytes and metallic lithium as anode [39].
As previously mentioned, the use of lithium metal as anode in lithium -ions batteries is
preferred because of its high specific capacity compared to the graphite anodes which are usually
employed in this type of batteries . However, the use of lithium metal as anode material was avoided
due to several inconveniences, such as: deposition of dendrites on the surface of the anode, infinite
changes of volume during the charge -discharge cycles and side reactions that occur at the
electrolyte -liquid interface [43].
An anode degradation process which occurs frequently in lithium -ion batteries is lithium
plating. Lithium plating is a side reaction which happens at the interface between the anode and
the electrolyte, especially during the battery charging and consists in the fact that the Li+ cations
that are transported by the electrolyte, snatch an electron from the surface of the metallic anode
leading to the formation of lithium metal which will deposit on the surface of the anode . This leads
to the destruction of the solid electrolyte interphase (SEI) [44]. The solid electrolyte interphase
(SEI) is a layer formed on the surface of the anode in the first few charge -discharge cycles of the

20
battery and is composed of products resulting from the decomposition of the electrolyte as a result
of its reactions with the anode . The SEI provides a passivation layer on the anode surface, which
prevents further electrolyte decomposition and ensures a long battery life. The SEI is conductive
to lithium ions, but isolative to electrons . Because SEI allows Li+ ions transport , lithium plating
will still occur at the interface between the lithium metal and the primary SEI to form metallic
lithium [45]. Part of the plated lithium will be consumed irreversibly because it will react with the
electrolyte in order to form a new SEI layer. But most of the plated lithium is reversible . During
discharging , oxidation of the plated lithium atoms deposited on the surface of the anode takes place
and lithium ions will form , which will pass back into the electrolyte. This process is known as
lithium stripping [46]. The stripping will lead to the breakage of the SEI layer so the creation of a
new SEI layer is necessary . This will lead to a new irreversible consumption of lithium metal and
electrolyte , which will damage the battery [47]. During the charg e-discharge cycle, the repeated
plating and stripping processes that occur at the surface of the anode will lead to anode corrosion,
appearance of several gaps in the the SEI film , severe anode polarization and a capacity loss [43].
Because the measurements showed that GPE 2 and GPE 4 have the highest ionic
conductivities, the plating -stripping experiments were performed on these two samples. The tests
were performed at an electric current density j = 0.3 mA·cm-2 and, respectively , j = 0. 4 mA·cm-2
and at 60 ° C

As shown in Fig. 8 a), when using GPE 2 which contains LiBOB as electrolyte, the battery
can withstand 500 cycles of plating -stripping without a decreasing in the battery capacity .
Therefore, the MEEP -based gel electrolyte can ensure the long -term transport of Li+ ions without Fig. 8 Plating / stripping of GPE 2 with j = 0.3 mA·cm-2 at 60°C. a) Charge /
discharge profile b) The corresponding overpotential versus time profile [39]

21
giving side reactions with the metallic lithium anode . According to Fig. 8 b), an anode
overpopotential of ≈0.05 V was maintained in the first ≈340 hours of battery charging -discharge,
probably due to the side reactions of the lithium metal anode with the electrolyte, but after 340
hours, no anode polarization was observed , a sign that a stable SEI has formed at the electrolyte –
anode interface and no irreversible consumption of lithium metal occur. As a proof , it can be seen
that after ≈340 h , the overpotential remains at a fairly constant value and does not increase up to
1000 hours of plating -stripping cycles .

As can be seen in Fig. 9 a), when using GPE 4 which contains LiPF 6, as electrolyte , the
battery will have a shorter lifetime than in the case of LiBOB containing gel electrolyte . As a
consequence, the battery will withstand fewer charge -discharge cycles . After approximately 220
cycles, the battery capacity begins to decrease . According to Fig. 9 b), there is an intense increase
in the overpotential value after ≈200 hours of plating -stripping, which is indicating a degradation
of the interface between lithium metal and gel polymer electrolyte. This difference in battery life
may be due to the formation of a weaker SEI, but also to the thermal instability of LiPF 6 which at
the temperature at which plating -strpping tests were perform ed (at 60°C ) undergoes thermal
degradation processes .
In conclusion , S. Jankowsky et al. obtain ed LiBOB containing gel electrolyte s based on
MEEP which are dimensionally stable, have a conductivity greater than 2 mS∙cm-1 at 30 °C, are
thermally stable up to ≈180°C and, in combination , with a metallic lithium anod e, can withstand
up to 500 charge -discharge cycle s.

Fig. 9 Plating / stripping of GPE 4 with j = 0. 4 mA·cm-2 at 60°C. a) Charge / discharge
profile b) The corresponding overpotential versus time profile [39]

22
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