1

Hybrid Polymer Electrolytes for Use in Secondary Lithium

Ion Batteries

Anisha T. Joenathan

G410 Senior Thesis

Spring, 2014

Research Advisor: Lyudmila Bronstein

2

Abstract

In this work, we focused on improving ionic conductivity and transference

numbers by adding ionic liquids (ILs) to polymer electrolytes (PEs) containing organic-

inorganic nanoparticles (OINs) formed in situ within a salt-in-poly(ethylene glycols)

(PEG) material. We also studied the effects of various molecular weight PEGs as bases

for these hybrid polymer electrolytes (HPEs), as well as the effects of different ion

sources on ionic conductivity. Scanning transmission electron microscopy (STEM),

differential scanning calorimetry (DSC), and conductivity measurements were used to

study and characterize the HPEs. The addition of ionic liquids, specifically1-Butyl-3-

methylimidazolium trifluoromethanosulfonate (IL2), resulted in an enhancement of

conductivity by over two orders of magnitude. To study the effect of molecular weight of

PEG, 400, 600, 1000, and 2000 Da PEG samples were used in the syntheses of HPEs.

The results showed that the highest conductivity was achieved using 400 Da PEG. In

another series of experiments, lithium trifluoromethanosulfonate (LiTf), the lithium salt

used regularly, was replaced with lithium bix(oxalate)borate (LiBOB). Conductivity

measurements showed that the samples containing LiBOB display lower ionic

conductivity. On the other hand, replacement of LiTf with NaTf allowed us to achieve

similar conductivity showing promise for inexpensive sodium ion batteries.

3

Introduction

In recent years, lithium ion batteries have gained considerable attention, mostly as

a result of efforts to miniaturize devices, because of their advantageous properties as

compared to traditional batteries. Lithium has the lightest weight, highest voltage, and

greatest energy density of all metals making it the most sensible metal ion source for use

in secondary batteries.1 A typical commercial lithium ion battery system contains a

carbonaceous anode, an organic electrolyte that acts as an ionic path between electrodes

and separates the two electrode materials, and a transition metal oxide cathode.1

However, there are disadvantages of using liquid organic electrolytes; specifically

flammability, due to the reactivity of metallic lithium with electrolyte solution, as well as

their limited battery life, and temperature sensitivity.2, 3 Because of these issues, solid

polymer electrolytes (SPEs) have drawn interest due to their superior safety and utility. In

order for these polymer electrolytes to become viable alternatives to liquid organic

electrolytes however, the challenges that must be overcome include, but are not limited

to, low ionic conductivity, low lithium transference numbers, low capacity and

insufficient power density.4 Polymer electrolytes in their most simple form, are a

polymer coupled with a lithium salt.5, 6 In this research, the issues of low ionic

conductivity and low lithium transference numbers were addressed. Hybrid polymer

electrolytes (HPEs), PEs containing ionic liquids, have been synthesized and studied in

order to determine what additional chemicals components yield more desirable

electrochemical properties. We studied these using two Li salts, specifically lithium

trifluoromethanosulfonate (LiTf) and lithium bis(oxalate)borate (LiBOB), as lithium ion

sources in the synthesis of HPEs for use as a viable alternative energy source.

4

Although lithium ions display many favorable properties, its supplies are limited

and costly.7 Due to the low abundance of elemental lithium in nature, sodium in contrast

has a high availability and it has gained interest as a lower-cost alternative ion source, in

order to produce the most cost-effective ion-batteries. Thus, sodium

trifluoromethanosulfonate (NaTf) was explored as a sodium ion source in these HPEs.

The benefits of using PEs based on poly(ethylene glycol) (PEGs) include low

cost, longer shelf life, non-flammability, longer exploitation life, and its low glass

transition temperature, which allows lithium ions to remain mobile through the

electrolyte at the sub-freezing temperatures.8 Although PEG-based PEs have many

advantageous properties, they tend to undergo crystallization, thereby disturbing cation

transference, and decreasing ionic conductivity. The crystallization of PEGs can be

suppressed through a variety of methods which include cross-linking, polymer grafting,

polymer blending, and the introduction of organic/inorganic fillers.9-11

It is known that in PEG-based SPEs, anions can form ion pairs of different

compositions, which results in lowered conductivity and lower lithium transference

numbers. With respect to these SPEs application in batteries, these aggregations are

particularly disadvantageous, because as a result, more energy, time, and electrochemical

potential would then be required to recharge the battery.12 In previous papers we reported

enhancement of conductivity for SPEs that contained aluminosilicate or silicate organic-

inorganic nanoparticles (OINs), formed in situ within PEG compared to other composite

SPEs. The incorporation of the OINs into the polymetric matrix should result in not only

increased conductivity due to suppression of crystallization, but also increased

transference numbers if anion movement is impeded. These OINs were made from (3-

5

glycidoxypropyl)trimethoxysilane (GLYMO), tetramethyl orthosilicate (TMOS), and

aluminum (tri-sec-butoxide) (AB) in situ allowing fresh OINs/PEG-LiTf interfaces. AB

acts as a catalyst of epoxy group polymerization in GLYMO, allowing for its cross-

linking with terminal hydroxyl groups in PEG and leading to the formation of a network.

In the preceding research, with the incorporation of the OINs in the SPEs, the Li

transference numbers were significantly increased from 0.3-0.5 in pure PEG-LiTf to 0.6-

0.8 with OINs. In order to increase the Li transference numbers further, the introduction

of a stronger Lewis acid was proposed in order to more strongly interact with anions and

thus reduce the migration of anions. 13, 14 It was demonstrated that through the

introduction of triethyl borate (TEB), a Lewis acid, into the OINs, anion-trapping

tricoordinate boron species were formed, further increasing lithium ion transference

numbers and lesser so, ionic conductivity. Scheme 1 displays the SPE based on PEG,

LiTf and TEB-modified OINs and how they are believed to provide a cross-linked

polymeric matrix.

6

Figure 1. Schematic representation of SPE containing boron species.

Next, to increase ionic conductivity and Li ion mobility, ionic liquids were

introduced, which might behave as a medium the lithium ion movement within the

sample. Ionic liquids (ILs) have recently gained attention due to their various

applications, especially with respect to “green” technology. ILs are molten salts that are

7

composed solely from anions and cations (often bulky ones) and melt below

100-150 °C.15 The main advantages of using ILs in batteries are their large

electrochemical windows, nonvolatility, higher conductivity compared to SPEs, large

operation range of temperatures, and non-flammability.16 We explored the addition of

three different ionic liquids 1-Butyl-3-methylimidazolium tetrafluoroborate (IL1), 1-

Butyl-3-methylimidazolium trifluoromethanosulfonate (IL2), and 1-Ethyl-3-

methylimidazolium trifluoromethanosulfonate (IL3), in various amounts to see the effects

on our samples. The structures are displayed in Figure 2.

Figure 2. Chemical structures of ionic liquids incorporated into polymer electrolytes.

To observe the effects of the chain length of the PEG, samples were synthesized

using 400, 600, 1000, and 2000 Da PEG. The difference in chain length was expected to

affect the structure of the cross-linked polymeric matrix, and thus affect the various

properties of the samples, such as ionic conductivity.

IL1: 1-Butyl-3-methylimidazolium

tetrafluoroborate

IL2: 1-Butyl-3-methylimidazolium trifluoromethanosulfonate

IL3: 1-Ethyl-3-methylimidazolium

trifluoromethanosulfonate

8

Experimental

I. Materials

Poly(ethylene glycol) (PEG) with molecular weights of 400, 600, 1000, and 2000

Daltons were obtained from Aldrich and were not further modified before use. Lithium

trifluoremethanosulfonate (LiTf) (96%), lithium bis(oxalato)borate (LiBOB), sodium

trifluoromethanosulfonate (NaTf), tetrahydrofuran (THF) (≥99.9%), aluminum-tri-sec-

butoxide (AB) (97%), (3-glycidyloxypropyl)trimethoxysilane (GLYMO) (≥98%),

tetramethyl orthosilicate (TMOS) (≥99%), triethyl borate (TEB) (99%), 1-Butyl-3-

methylimidazolium tetrafluoroborate (IL1), 1-Butyl-3-methylimidazolium

trifluoromethanosulfonate (IL2), and 1-Ethyl-3-methylimidazolium

trifluoromethanosulfonate (IL3) were all purchased from Aldrich and used without

further purification. Chloroform (100%) was purchased from Mallinckrodt and used as

received.

II. Synthetic Procedures

i. Synthesis of HPEs with 55% OIN and 10% TEB

Synthesis of HPEs based on poly(ethylene glycol) (PEG) and OINs was carried

out by a procedure modified somewhat from that described in ref 17. In a standard

procedure, 0.8 g of PEG (0.091 mmol) in 5 mL of chloroform was mixed with 0.2 g (1.28

mmol) of lithium triflate in 5 mL of THF. After 1 hour stirring at room temperature, the

solution was put aside. The inorganic part of the composite was prepared by a sol-gel

reaction of a mixture of GLYMO with TMOS in a molar ratio of 4:1, with aluminum-tri-

sec-butoxide used as a catalyst. The weighed vial with a stir bar was filled with 2.337 g

(9.88 mmol) of GLYMO, 0.426 g (2.81 mmol) of TMOS, and 0.04 g (0.16 mmol) of AB.

9

Hydrolysis was initiated by addition of 15 % (0.12 mL) of all calculated water containing

HCl (0.01 N solution). After 15 minutes of stirring in an ice bath at 0°C , it was stirred

for 15 more minutes at room temperature. The reaction mixture was then charged with

0.64 mL of 0.01 N HCl solution and stirred for 40 minutes at room temperature. Then

the reaction temperature was raised to 50°C, the vial was opened and stirring was

maintained for 15 minutes. After weighing the vial, the calculated amount of the

precondensed silicate sol was added to the solution containing PEG and Li salt, along

with the calculated amount of IL and stirred for 1 hour at room temperature. The amount

of OIN added was determined by the desired ratio of inorganic component to PEG-LiTf,

and was based on a 55 wt. % OIN content in HPE. The desired amount of IL was

calculated as a weight fraction of the amounts of PEG and OIN in the sample. Afterward,

the reaction solution was placed in two small Teflon dishes on a heater at 70°C and left

overnight for evaporation of solvent and further OIN condensation. The solid film was

treated at 130°C in a vacuum oven for 1 hour to complete condensation. The HPE films

were carefully removed from the dishes and then sealed in a plastic container.

Previously, TEB was incorporated into the OIN through 4 different methods and

it was found that method C yielded a sample with the highest conductivities.12 TEB was

incorporated in amounts of 5, 10, 15, 20, 25, and 30 mol % of OIN. For example, in the 5

mol. % TEB sample, 0.103 g (0.705 mmol) of TEB was added along with 2.504 g

(10.595 mmol) of GLYMO, while in the 10 mol.% TEB sample, 0.206 g (1.41 mmol) of

TEB was added along with 2.337 g (9.89 mmol) of GLYMO, etc.

The various ILs were added to the samples during the final addition of OIN. IL1,

IL2, and IL3 were incorporated in amounts of 10, 20, 30, 40, and 50 wt, % of the total

10

sample weight or until a compatibility threshold was met. For example, in the 10 wt. %

IL1 sample, 0.22 g of IL1 was added, while in the 20% wt. % IL1 sample, 0.44 g of IL1

was added.

3.3. Characterization

Conductivity was measured as follows. The samples were vacuum-dried at room

temperature overnight to minimize the effects of water absorption on measurements.

Using a micrometer, the thickness of the sample polymer film was determined. Gold

contacts of the area of 1.18 × 10-5 ± 0.05 m2 were coated directly onto the polymer films

by using a Polaron E5100 sputter coater. Sputtering of the gold was accomplished in 3

minutes, and during this time the samples were held between -10 and -15°C, as

monitored with a thermocouple. After the gold was sputtered, electrodes were attached,

and each sample was placed into a measurement rig and attached to an LF impedance

analyzer. The oscillation level of the applied pulse was 1 V over a frequency range of 10

Hz to 10 MHz at room temperature. The samples were modeled to a parallel circuit

comprised of a resistor and capacitor. Admittance data was recorded using this circuit

model and subsequently converted to obtain impedance plots.

Differential Scanning Calorimetry (DSC) was performed with a TA Instruments

Q Series calorimeter. Sample masses varied from 5-20 mg and were hermetically sealed

in aluminum pans. The samples were scanned between -80 and +200°C at a scan rate of

10°C/min, using refrigerated cooling systems (RCS) as coolant. Glass transition regions

were then determined using the fictive temperature method.

11

Scanning Transmission Electron Microscopy (STEM) images were obtained at

30 keV. The SPE samples were ground into powder then embedded in epoxy resin and

cut very finely with a diamond knife. Images of the resulting thin sections (ca. 50 nm

thick) were obtained with a Quanta FEG SEM instrument using STEM detector and

analyzed with the ImageJ software. Transmission Electron Micrscopy (TEM) was carried

out at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron

microscope.

Results and Discussion

I. Addition of Triethyl Borate

We began by determining the optimal amount of TEB to be added to the OINs. In

previous research it was determined that at 40 wt.% OINs based on GLYMO and AB, the

best conductivity was achieved, but the solid polymer electrolyte (SPE) was rather weak.

At 75 wt.% of the same OIN, the mechanical properties improved, but the conductivity

significantly dropped.18 At 55% OIN, the highest conductivity was achieved while also

maintaining mechanical properties.

Li ion transference number measurements were conducted on the SPE samples

containing TEB. The lithium ion transference number, T+, was determined from the ratio

of the corresponding ionic mobilities (with assumption of equal densities of anions and

cations) and the values obtained are presented in Table 1.17 For moderate amounts of

added TEB, lithium transference increases significantly as compared to the reference

sample. At 10% TEB the highest conductivity and Li transference numbers were

achieved but the conductivity was mediocre.

12

Li transference numbers were determined by a series of measurements in order to

determine what percent of ionic conductivity is solely lithium ion based.

Table 1. Li transference numbers and ionic conductivity of SPE with boron species17

TEB % T+ (±0.05) Ionic Conductivity (S/cm)

0 0.76 7.12 x 10 -6

5 0.88 8.53 x 10 -6

10 0.89 4.34 x 10 -5

20 0.79 1.64 x 10 -5

These data suggest that the incorporation of boron species into the OINs resulted in a

stronger anion interaction with the boron species, a strong Lewis acid, thus reducing

anion migration. As a result, a majority of the ionic conductivity is based on solely

lithium ion migration.

II. Addition of Ionic Liquids

To further enhance the ionic conductivity of the HPEs, the incorporation of

different ILs into polymer electrolytes was studied based on promising results of

others.19-21 Figure 3 shows the dependence of conductivity on the amount of IL1 added to

the samples containing PEG, LiTf, and 55 wt. % OIN with 10 mol. % TEB.

13

Figure 3. Conductivity of PEs as a function of the IL1 amount in the samples containing

55 wt. % OIN prepared with 10 mol. % TEB.

The addition of 30 wt. % of IL1 resulted in an increase in conductivity by over two orders

of magnitude compared to the samples containing no ionic liquid. However, when added

in amounts greater than 30 wt. %, IL1 appeared as drops on the PE film surface. This is

most likely due to the IL1 saturating the cross-linked PE network and being released from

the structure.

Next, the incorporation of IL2 was studied. Figure 4 displays the dependence of

conductivity on the amount of IL2.

14

Figure 4. Conductivity of PEs as a function of the IL2 amount in samples containing 55 wt. % OIN prepared with 10 mol. % TEB.

The addition of IL2 resulted in an increase in conductivity by over three orders of

magnitude as compared to the samples containing no ionic liquid. Compared to IL1, IL2

was miscible with the PEG-OIN network up to 50 wt. %, after which poor mechanical

properties were and the film became weak. The difference between IL1 and IL2 is the

structure of anions, which are tetrafluoroborate and trifluoromethanosulfonate,

respectively, while cations are identical. The expulsion of IL1 may have been due to

more hydrophobic nature of the tetrafluoroborate in IL1 as compared to

trifluoromethanosulfonate in IL2.15 As a result, rather than being further incorporated like

IL2, IL1 was expelled from the polymer matrix due to the repulsion of the hydrophobic

anion from the hydrophilic matrix.15, 17

Furthermore, the effects of IL3 addition were studied. The difference between IL2

and IL3 is the cation structure. Each of these ionic liquids contain the same anion,

15

however the tail on position 1of the 3-methylimidazolium, is an ethyl group in IL3 and a

butyl group in IL2. However, IL3 prevented cross-linking between silica particles and

PEG so no solidification occurred (no film was formed). This is probably due to the

behavior of ionic liquids to undergo molecular layering at a charged surface.22 As was

shown previously,17 the incorporation of boron species into OINs, resulted in higher Li

transference numbers, meaning that anion interaction with OINs was significantly

strengthened. Also, there are the epoxy groups of GLYMO on the surface of the OINs,

which interact with the hydroxyl groups of the PEG chains, resulting in cross-linking.

The cations of the ionic liquids should therefore be found in the second layer surrounding

the OINs, resulting in the possibility of cations to interact with the epoxy groups of

GLYMO, impeding cross-linking. The general trend in liquid density for ionic liquids

with imidazolium-based cations, is that as the alkyl tail lengthens, the liquid density

decreases.16 Then, the ethyl group on the cation in IL3 should result in a denser layer as

compared to the IL2 with the bulkier butyl group, resulting in the prevention of the epoxy

group cross-linking with the hydroxyl groups of PEG.

DSC measurements were performed on the samples in order to estimate the

mobility of the PEG polymer chains and their possible crystallinity in the HPEs. The

results are displayed in Table 2.

16

Table 2. DSC data and OIN sizes for the HPEs with different amounts of TEB and different amounts of ILs.

Sample % TEB Ionic

Liquid

Tg (C)

NP size (nm)/

Std. Dev. (%)

Conductivity

(S/cm)

GTA-PEG-1 0 --- -43 214/ 53 7.12 x 10 -6

GTA-PEG-2 5 --- -47 203 /35 8.53 x 10 -6

GTA-PEG-3 10 --- -60 36 /58 4.34 x 10 -5

GTA-PEG-4 10 10 wt.%

IL1

No Tg

observed

5.5 /24 5.81 x 10 -4

GTA-PEG-5 10 30 wt.%

IL1

No Tg

observed

5.4 /42 3.22 x 10 -3

GTA-PEG-6 10 30 wt.%

IL2

No Tg

observed

6.9 /36 1.71 x 10 -2

The DSC traces of various PEs are displayed in Figure 5. The DSC trace of the HPE

based on OIN including boron species as well as IL1 is presented in Figure 5c. The DSC

traces of the PEs based on OIN without boron species or ILs and PE based on OIN

including boron species17 (Fig. 5a and b) show glass transitions. However, in the case of

addition of ILs (Fig. 5c), no glass transition is observed, indicating that the glass

transition occurs below the scanned temperature range. This means that the PEG polymer

chains maintain mobility even at extremely low temperatures.

Figure 5. DSC traces of SPEs which contain 55 wt.% OIN without boron species (a), with 10 mol. % TEB (b) and with 10 mol.% TEB and 10 wt.% IL1.

In previous work, it was demonstrated that OIN particle size significantly

influences the electrochemical performance of composite SPEs.17 Thus, in this work, we

again used STEM and TEM to determine sizes of the OIN particles in the native HPE

-100 -50 0 50 100 150 200-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

He

at

Flo

w (

W/g

)

Temperature (°C)

a

17

environment, i.e., without calcination at 500 C in oxidizing media as was performed

earlier. One issue, however, with this method of characterization is the brief exposure of

the HPE sample to water during sectioning for STEM or TEM imaging. As a result of

this brief exposure, PEG swelling and OIN aggregation could occur, thus particle

distribution within an HPE section cannot be determined. This moisture-induced effect

does not alter the size of the OIN particles, allowing for the study of particle sizes and

morphologies. Representative images obtained from the STEM and TEM measurements

are shown in Figure 6. Mean OIN particle sizes and the particle size distributions are

displayed in Table 2.

Figure 6. STEM (A, B, and C) and TEM (D, E, and F) images of the HPE samples with different amounts of TEB added to OINs by method C and ILs: without TEB (A), 5 mol.% TEB (B), 10 mol.% TEB (C), 10 mol.% TEB and 10 wt.% IL1 (D), 10 mol.% TEB and 30 wt.% IL1 (E), and 10 mol.% TEB and 30 wt.% IL2 (F).

In previous research, the size analysis of the HPEs with OIN containing boron

species showed some correlation between the OIN particle size and conductivity; an

18

inverse relationship between particle size and ionic conductivity was observed.17 The

decrease in the particle size, results in increased surface area, thus a higher interaction of

anions with OINs. The average OIN size decreased almost by an order of magnitude with

the incorporation of ILs and the increase of conductivity was also observed.

Although previous research demonstrated that a decrease in the OIN size, results

in the increase in ionic conductivity, the magnitudes of the conductivity increase, which

was observed with the incorporation of ILs, are most likely due to high mobility of Li

ions in the ILs. Table 3 shows conductivities of ILs, organic electrolytes and various

polymer electrolytes as compared to the HPEs described here.15, 23

Table 3. Comparison of conductivities of the samples prepared in this work with the literature data.

Polymer Electrolyte Sample Conductivity (S/cm)

No TEB 7.12 E-06

10 mol.% TEB 4.34 E-05

30 wt.% IL1 3.22 E-03

50 wt. % IL2 3.86E-02

Ionic liquid15 Range from 4.5E-04 to 1.6E-02

Ionic liquid +polymer23 0.18E-03

Ionic liquid + polymer + liquid organic23 0.81E-03

Liquid organic23 1.0E-02

These data show that the HPEs that we have synthesized, have even higher ionic

conductivities than those of any reported samples found in literature. This observation

can be attributed to the ionic liquid being confined in a restricted geometry, resulting in

the alteration of the ionic liquid physical properties, i.e. viscosity.15 In our case,

19

confinement of the ionic liquid can occur within the PE-SiO2 network, which resulted in

decreased viscosity, and thus increased conductivity.

Along with LiTf, we explored another lithium salt, lithium bis(oxalate)borate

(LiBOB), as well as a sodium salt, sodium trifluoromethanosulfonate (NaTf), to study the

influence of the Li salt anion structure and the replacement of Li with Na on

conductivities. The results are displayed in Figure 8.

Figure 8. Comparison of conductivity measurements for HPEs based on different amounts of IL2 and different salts.

The use of LiBOB, resulted in a decrease of conductivity. This decrease in

conductivity may have been due to the bulkier anion and its weaker interactions with the

OINs, thus leading to more ion pairs and lower conductivity. The substitution of lithium

ions for sodium ions, by the use of NaTf, yielded conductivities similar to those of LiTf.

The latter fact opens great opportunities for development of sodium ion batteries with

very high conductivity.

20

We also studied the effects of using different molecular weight PEGs in the

synthesis of our HPEs. The results for using 400, 600, 1000, and 2000 Da PEG, on

conductivity, are displayed in Figure 9.

Figure 9. Comparison of the conductivities of HPEs based on different amounts of IL2

and the PEG molecular weights.

The data presented in Figure 9 show a clear dependence of conductivity on the

length of PEG chains. PEG 400, which is the shortest, exhibited the highest

conductivities, which could be associated with the structure of the HPE network. From

the explanation given earlier for the increase in ionic conductivity of ionic liquids in

confined spaces, the observation with these different length PEG chains reinforces this

statement. Shorter PEGs form networks with smaller cells allowing better IL

confinement, reduced viscosity and higher conductivity.24

21

III. Li Ion Transference

Once the conductivity was significantly enhanced, the measurement of lithium ion

transference numbers was attempted. However, the speed with which the lithium ions

migrate, through the sample, is less than a millisecond, making the determination of a

firm transference numbers difficult. Therefore two different sets of measurements were

performed on samples in order to calculate the transference numbers. The results are

displayed in Figure 5 and Figure 6.

Figure 5. Current discharge with respect to time after bias reversal with Pt electrodes for the HPE with 30 wt.% of IL2.

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E-06 1.00E-04 1.00E-02 1.00E+00 1.00E+02 1.00E+04

Cu

rren

t D

isch

arg

e (

mA

)

Time (s)

-1 V -> 1 V

-2 V -> 2 V

-3 V -> 3 V

-1 V -> 1 V (short times)

-2 V -> 2 V (short times)

-3 V -> 3 V (short times)

22

In Figure 5, the set of measurements displayed are of discharge experiments,

where the sample was placed between two platinum (blocking) electrodes, and the

current discharge after a bias reversal was measured. The observed discharge is much

more rapid with blocking electrodes, since the lithium cations themselves will accumulate

at the interfaces. The peak in the graph signifies the maximum discharge and thus the

maximum movement of the lithium ions. As a consequence, it is difficult to assign a

characteristic time to this process other than to note that it is much faster than the

characteristic time that was previously seen in samples without IL2, which were by the

three orders of the magnitude slower.

Figure 6. Current with respect to time after a bias reversal for the HPE with 30 wt.% of IL2 30% and the HPE without IL2 with lithium contacts.

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

1.20E-01

0 500 1000 1500 2000

Cu

rren

t (m

A)

Time (s)

-1 V -> +1 V

-2 V -> +2 V

-3 V -> +3 V

no IL -1 -> 1 V

23

In Figure 6, a similar set of measurements from further experiments are displayed,

however the sample was placed between two lithium contacts, and the current after a bias

reversal was measured. In contrast, since lithium contacts were used, the accumulation of

ions other than lithium, on the lithium interfaces, blocked the current.

Next, a comparison of the samples with and without IL2, where d is the sample

thickness and tmax is the time of maximum current in the bias reversal experiment is

displayed in Figure 7. The slope on this type of graph would typically represent the

anion mobility but there is not enough data to claim a value here. Primarily this

comparison shows that the mobility of ions other than lithium differs by a much smaller

factor than the enhancement of the conductivity with the ionic liquid..

Figure 7. Comparison of the HPE with 55 wt. % OIN and 10 mol.% TEB with and without IL2.

Figure 7 displays the “d” sample thickness over the “tmax” the time of maximum

current in the bias reversal experiment of two HPE samples, one containing IL2 and the

other without IL2. The slope on this type of graph would typically represent the anion

mobility, however due to the lack of data points, it is difficult to claim a definite value.

0

5E-10

1E-09

1.5E-09

2E-09

2.5E-09

3E-09

0 0.5 1 1.5 2 2.5 3 3.5

d2

/tm

ax

(m2/s

)

Voltage (V)

30 wt. % IL2

no IL2

24

From the few data points displayed, the trend observed shows that the mobility of ions

other than lithium differs by a much smaller factor than the enhancement of the

conductivity with the ionic liquid.

Using the two sets of measurements, a few general statements on the ion mobility

within the HPEs containing IL2 can be made. In previous discharge experiments,

performed on samples without IL2, a peak in the measurements was determined to be

between 0.01 seconds. In the experiments performed on samples containing IL2, the

peaks in the measurements, occur mostly around 0.0001 seconds. These times, refer to

the time it takes for the majority of the lithium to travel between the electrodes. With

respect to the results of the bias reversal experiments, the anion mobility remains fairly

constant between samples not containing and containing IL2, although there are too few

data points to be certain. These two sets of experiments suggest that since the anion

mobility is constant, but the lithium movement has increased almost by two orders of

magnitude, the lithium transference numbers of these HPEs are superior to that of our

best sample without ILs.

Conclusion

For the HPEs, the data obtained from conductivity measurements demonstrates

that the addition of 50 wt. % IL2 to the sample containing 55 wt. % of OIN along with 10

mol.% of TEB yields the best results. The DSC data obtained displays the glass

transition temperature of the samples containing ILs to be below -80C, permitting

lithium ion mobility in these HPEs even further below freezing temperatures than

samples without ILs. STEM and TEM images of the samples show that the OIN size is

25

significantly reduced with the incorporation of ILs, which may be one of the reasons for

such high ionic conductivities. We also studied the influence of the Li salt structure and

the replacement of Li with Na on conductivities, however LiTf produced better ionic

conductivity than LiBOB. This trend is perhaps a result of the bulkier anion of LiBOB,

resulting in weaker interactions with OINs. Interestingly, the conductivity of the samples

containing Na triflate shows comparable ionic conductivities to those with Li triflate.

This opens up possibilities for the development of highly conductive sodium ion

batteries. PEG chain length was then compared between samples, and a dependence on

chain length was observed. It was found that the shortest PEG400 yielded the highest

conductivity. This observation may be associated with the structure of the PEG-SiO2

network, i.e., confinement of ILs within this network. Lastly, Li transference was studied,

and although a definitive Li transference number was not measured, due to the extremely

high Li ion mobility, it was shown through current measurements that Li transference

numbers are significantly increased as compared to the samples without ILs. Further

research is necessary for determining the HPE structure, Li transference numbers, and

synthesizing HPEs based on sodium salts, as well as further enhancing the conductivity.

26

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