Ionic conductivity of colloidal electrolytes

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Solid State Ionics 167 (2004) 91–97

Ionic conductivity of colloidal electrolytes

Binod Kumar*, S.J. Rodrigues

Metals and Ceramics Division, University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0170, USA

Received 20 May 2003; received in revised form 27 October 2003; accepted 11 November 2003

Abstract

This paper reports an exploratory investigation on the effects of a colloidal phase on the conductivity of lithium-ion liquid electrolytes. The

conductivities of liquid electrolytes comprising of organic solvents, lithium salts and a colloidal phase were characterized. The colloidal phase

consisted of BaTiO3 and Al2O3. The conductivity of a separator material impregnated with liquid and colloidal electrolytes was also

characterized. The low temperature conductivity enhancement in colloidal electrolytes was attributed to a formation of space charge region

between the colloidal phase and liquid electrolyte. The space charge region appears to be essential for the formation of a high conductivity phase.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Colloidal electrolytes; Conductivity; Lithium-ion liquid electrolytes

1. Introduction

1.1. Conductivity of liquids

Conductivity of liquid electrolytes is of profound interest

to chemists and engineers because of their applications in

electrochemical devices such as fuel cells and batteries. The

application of these electrochemical devices continues to

evolve, and it is believed that the evolution of these devices

will require electrolytes with higher conductivity in a wider

temperature range. For example, the performance and ap-

plication of lithium-ion batteries have been limited by two

major technical issues related to the electrolyte. The first is

the rapid decline of current-carrying capacity at low temper-

atures. At temperatures <� 20 jC, the liquid electrolyte

freezes resulting in a major drop in ionic conductivity and

thus the current-carrying capacity. The second problem

stems from high temperature instability and degradation of

the electrolyte, primarily attributed to the decomposition of

lithium hexafluorophosphate, LiPF6.

Above the freezing point, the viscosity and conductivity

of a liquid electrolyte are inversely related; the viscosity

increases, and the conductivity decreases as the temperature

is lowered towards the freezing point. Below the freezing

0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2003.11.024

* Corresponding author. Tel.: +1-937-229-3527; fax: +1-937-229-

3433.

E-mail address: [email protected] (B. Kumar).

point, the liquid electrolytes transform to a solid or solid-

like material resulting in orders of magnitude decline in

conductivity. The influence of temperature on conductivity

of liquid electrolytes around freezing temperature is pro-

found. This drawback of liquid electrolytes has been dealt

with by making use of insulating blankets, heaters and phase

change (to absorb and release heat for maintaining a desired

range of temperature) materials.

The conductivity of nonaqueous electrolytes for lithium

rechargeable batteries has been investigated and reported by

Dudley et al. [1]. They investigated a wide range of solvents

and salts and observed a correlation between the conduc-

tivity and viscosity. The use of these nonaqueous electro-

lytes in lithium-ion cells has been explored by a number of

laboratories. Lithium ion cells using these electrolytes

generally exhibit poor performance at low temperatures.

Smart et al. [2] reported a series of ester solvents as

additives into multicomponent electrolyte formulations.

The low temperature performance of lithium ion cells was

improved by the use of these additives.

1.2. Conductivity of composite solids

If the conductivity transition associated with the freezing

of liquid electrolytes can be reduced or eliminated, the

application range of the electrochemical devices using these

liquid electrolytes can be extended. The colloidal electro-

lytes are potentially effective in achieving this purpose.

Below the freezing point of a colloidal electrolyte, it may

Fig. 1. (a) Low temperature ionic conductivity of ionically conducting

matrix reinforced with insulating particles and (b) temperature dependence

of the conductivity at low and high temperatures [9].

1 Ferro Electronic Materials, 4511 Hyde Park Blvd., P.O. Box 67,

Niagara Falls, NY 14305-0067.2 Al2O3 Nanophase Technologies, Darien, IL 60561.

B. Kumar, S.J. Rodrigues / Solid State Ionics 167 (2004) 91–9792

be treated as a solid composite material. A considerable

number of investigations on electrical conductivity of com-

posite materials have been conducted in the last two

decades, and it would be useful to review these investiga-

tions to develop a basis to explain underlying mechanisms

for conductivity enhancement in colloidal electrolytes.

The history of the composite ionic conductors may be

traced to the work of Liang [3]. In a pioneering work, Liang

[3] reported that lithium iodide doped with 35–45 mol%

aluminum oxide exhibited conductivity on the order of

10� 5 S cm� 1 at 25 jC, about three orders of magnitude

higher than that of the LiI conductivity. However, no

significant amount of aluminum oxide was determined to

be soluble in LiI; thus, the conductivity enhancement could

not be explained by the classical doping mechanism and

creation of Schottky defects such as in the LiI–CaF2system. Subsequently, a number of investigations have

reported enhanced conductivity of silver in the AgI–

Al2O3 system [4], copper in the CuCl–Al2O3 system [5],

fluorine in the PbF2–SiO2 and PbF2–Al2O3 systems [6],

and lithium in polymer–ceramic composite electrolytes [7].

Three review papers [8–10] also document the develop-

mental history and general characteristics of these fast ionic

conductors. Analyses conducted by these review papers

point out that a new conduction mechanism evolves, which

augments the bulk conductivity of single-phase ionic con-

ductors. The new conduction mechanism makes use of

interfacial and/or space charge regions between the two

primary components.

Fig. 1(a) schematically shows the effect of insulating

particle reinforcement on the low temperature conductivity

of the ionically conducting matrix. The ionic conductivity of

the composite gradually increases and reaches a peak at

around 20 wt.% of the insulating doping phase. Further

increases of the dopant decreases the conductivity as it

impedes the transport of charged species. A steady-state

percolation occurs around 20 wt.% of the insulating dopant

phase. The percolation threshold may vary depending upon

the matrix and dopant chemistries, particle sizes, and

processing parameters. The particle size of the dopant has

a major influence on the conductivity, which has been

reported in earlier publications [7–9].

Fig. 1(b) schematically illustrates the particle volume

fraction and temperature dependences of conductivity as

reported by Mikrajuddin et al. [9]. At high temperatures, the

conductivity appears to decrease slightly up to about 0.20

volume fraction, which is followed by a precipitous drop.

However, at low temperatures, conductivity increases up to

about 0.20 volume fraction. After attaining a peak, the

conductivity drops precipitously like the high temperature

curve. The applicability and universality of the temperature

dependence also have been demonstrated and reported for

polymer–ceramic composite electrolytes [7,11].

This paper will present experimental data on conductivity

of colloidal electrolytes and attempt to elucidate the under-

lying mechanism of conductivity enhancement based on the

prior understanding of transport mechanism in solid com-

posite materials.

2. Experimental

Liquid electrolytes comprised of 1:1 solvent blend of

ethylene carbonate (EC) and propylene carbonate (PC) with

a molar concentration of either lithium hexafluorophosphate

(LiPF6) or lithium tetrafluoroborate (LiBF4) were prepared

in a dry box using dried solvents and salts. Colloidal electro-

lytes were prepared by mixing the aforementioned electro-

lytes with dried barium titanate (BaTiO3)1 of an average

particle size of 1 Am and alumina (Al2O3)2 of an average

particle size of 24 nm. The BaTiO3 is a ferroelectric material

B. Kumar, S.J. Rodrigues / Solid State Ionics 167 (2004) 91–97 93

and addition of the material in a liquid electrolyte is expected

to influence the local field and, hence, conductivity.

The conductivity of liquid and colloidal electrolytes was

measured using a two-electrode cell with a cell constant of

0.46 cm. The electrodes were prepared from stainless steel

with a surface area of 0.44 cm2 and the distance between the

electrodes was maintained at 0.96 cm. The electrodes were

inserted through one port in a glass container. The electro-

lyte was poured through a second port, submerging the

electrodes. The glass container with electrodes and electro-

lyte were then sealed and AC measurements for developing

complex impedance plots (zV vs. zU) were conducted. The

conductivity was computed using the real part of impedance

(zV) and the cell constant (0.46 cm).

Conductivity measurements were also conducted using a

microporous separator material. The separator, a copolymer

of tetrafluoroethylene and ethylene, was soaked with liquid

and colloidal electrolytes before conductivity measurement.

The thickness of the separator material was 100 Am. The

conductivity of the separator material soaked with liquid

and colloidal electrolytes was determined using a different

two-electrode cell. The electrodes for this measurement

were made of a stainless steel disc with a surface area of

1.23 cm2. The separator material soaked with an electrolyte

was sandwiched between the two electrodes, and the elec-

trodes were manually tightened to obtain good contacts and

the AC impedance measurement (for obtaining complex

impedance plots) was conducted. The conductivity was

computed using the real part of impedance (zV) and cell

constant.

The AC impedance measurements were carried out using

a Solartron impedance spectrometer model 1287 in the

frequency range of 0.1 Hz and 100 kHz. The cells were

contained in a dry atmosphere glass vessel, which was

Fig. 2. Ionic conductivity of a liquid electrolyte and c

heated in an environmental chamber that allowed tempera-

ture dependence measurements in the � 50 to 50 jC range.

The set temperature was maintained within F 1 jC.

3. Results and discussions

3.1. Liquid and colloidal electrolytes

The Arrhenius plots of conductivity of homogeneous and

colloidal electrolytes containing one molar LiPF6 is pre-

sented in Fig. 2. The conductivity of liquid electrolyte varies

in the range of 1.21�10� 4 to 1.64� 10� 2 S cm� 1 in the

� 50 to 50 jC temperature range. These values are compa-

rable to the conductivity of similar electrolytes as reported

by Ding et al. [12]. An addition of 5 wt.% BaTiO3 to the

electrolyte alters the temperature dependence of conductiv-

ity. The conductivity increases at � 50, � 40 and � 30 jC.As the temperature is raised above � 30 jC, the conduc-

tivity decreases as compared to the liquid electrolyte. The

crossover point lies between � 30 and � 20 jC, and may

have some significance, which will be explained later. If the

conductivity values of the entire temperature range � 50 to

50 jC are considered, it may be inferred that the temperature

dependence of the electrolyte has diminished due to the

presence of BaTiO3 colloidal particles.

There was a slow tendency of the BaTiO3 particles to

settle in the bottom of the cell with time. This necessitated

shaking the cell before each impedance measurement,

particularly at higher temperatures (� 10 to 50 jC).Fig. 3 shows the Arrhenius plots of homogeneous and

colloidal electrolytes. The homogeneous electrolyte con-

sisted of 1 molar LiPF6 in ethylene carbonate and propylene

carbonate (1:1) solvents. The colloidal electrolyte contained

olloidal electrolyte containing 5 wt.% BaTiO3.

Fig. 3. Conductivity of a liquid electrolyte and a colloidal electrolyte containing 10 wt.% BaTiO3.


10 wt.% BaTiO3 and 90 wt.% of the homogeneous liquid

electrolyte. Both the curves exhibit liquid-like behavior in

the � 40 to + 40 jC temperature range. The two conduc-

tivity curves in Fig. 3 can be easily distinguished and

attributed to the higher concentration of BaTiO3. The

crossover point between the two conductivity curves in-

creased to around 20 jC.In a typical lithium rechargeable cell, the electrolyte is

held in a separator material. The separator saturated with an

electrolyte is placed between anode and cathode in a working

cell. The separator in effect is an electrolyte containment

Fig. 4. Conductivity of a separator material impregnated with liquid electrolyte

containing 10 wt.% BaTiO3.

device, which is normally made out of an insulating polymer.

The evaluation of the colloidal electrolytes contained in a

separator material is thus of commercial interest. In view of

this background, Fig. 4 presents the temperature dependence

of conductivity of the separator material impregnated with

three electrolytes, EC/PC (1:1) 1 M LiBF4 liquid electrolyte,

colloidal electrolyte containing 10 wt.% Al2O3 (24 nm) and

colloidal electrolytes containing 10 wt.% BaTiO3 (1 Am).

Both the specimens impregnated with colloidal electrolytes

exhibit higher conductivities. However, the colloidal electro-

lyte containing BaTiO3 exhibits superior conductivities

, colloidal electrolyte containing 10 wt.% Al2O3 and colloidal electrolyte


across the entire temperature range as compared to the

colloidal electrolyte containing Al2O3. The superior conduc-

tivities are attributed to the ferroelectric property of BaTiO3.

The crossover point between liquid and colloidal electrolytes

is absent in this case, which may be attributed to the use of

separator material. The same specimen was characterized 9

days later and the conductivity data are shown in Fig. 5. There

is a slight enhancement in the conductivities of the specimens

impregnated with colloidal electrolytes after the aging.

3.2. Polymer and polymer–ceramic composite electrolytes

In the last two decades, a considerable effort has been

devoted towards the development of polymer and polymer–

ceramic composite electrolytes for lithium rechargeable

batteries. The electrochemical data of a diverse range of

polymer–ceramic composite electrolytes reveal that the

incorporation of a ceramic component in a polymer matrix

leads to enhanced conductivity (specifically at low temper-

atures), increased lithium ion transport number and im-

proved electrode–electrolyte interfacial stability. Detailed

attributes of those polymer–ceramic composite electrolytes

can be found in recent review papers [8,11].

The liquid electrolytes exhibit a behavior very similar to

the polymer electrolytes. For example, the polyethylene

oxide (PEO)/lithium tetrafluoroborate (LiBF4) (8:1) com-

plex possesses a higher conductivity value f 10� 4 S cm� 1

above the melting point (Tm) of PEO, 68 jC. A precipitous

drop in conductivity (similar to liquid electrolytes) begins at

around 68 jC, and at around ambient temperature the

conductivity drops to a very low value [7].

Temperature dependence of conductivity of the PEO/

LiBF4 complex and PEO/LiBF4 (8:1)–MgO (10 wt.%)

Fig. 5. Conductivity of a separator material impregnated with liquid electrolyte, col

10 wt.% BaTiO3. The specimens impregnated with colloidal electrolytes were ag

materials containing micro-and nanosize MgO are shown

in Fig. 6, also reported in a prior publication with greater

details [7]. The lowest conductivity values are associated

with the PEO/LiBF4 (8:1) complex. Near the melting point

of PEO, 68 jC, a precipitous drop in conductivity begins

and, at around ambient temperature, the conductivity drops

to 10� 9 S cm� 1. The specimen containing microsize (f 5

Am) MgO exhibits much improved conductivity as com-

pared to the PEO/LiBF4 complex. At around ambient

temperature, the conductivity is improved by approximately

three orders of magnitude by the incorporation of microsize

MgO in the polymer complex. The highest conductivity

values are associated with the specimen containing nanosize

MgO. The conductivity of this specimen is about four orders

of magnitude higher than the PEO/LiBF4 complex around

the ambient temperature. Furthermore, the temperature de-

pendence of conductivity diminished as the MgO particle

was reduced from micro-to nanosize. At 100 jC, all threespecimens possess similar conductivity values, whereas the

curves diverge as the temperature is lowered to � 40 jC. Itshould also be noted that major benefits in conductivity

enhancement are realized at lower temperatures by the

incorporation of MgO.

A collective evaluation of conductivity data and related

observations on liquid, polymer, and polymer–ceramic

composite electrolytes obtained in our laboratory and

elsewhere leads to a general observation. This observa-

tion, schematically presented in Fig. 7, can be extended to

colloidal electrolytes. At higher temperatures (>Tm), liquid

and colloidal electrolytes possess comparable conductivity.

In some cases, colloidal electrolytes exhibit lower con-

ductivity. As the temperature is lowered and about 10–25

jC above the melting point, Tm, the colloidal electrolytes

loidal electrolyte containing 10% Al2O3 and colloidal electrolyte containing

ed for 9 days.

Fig. 6. Temperature dependence of conductivity of (x) PEO/LiBF4 (8:1); (n) PEO/LiBF4 (8:1)–MgO (10 wt.%), microsize; and (E) PEO/LiBF4 (8:1)–MgO

(10 wt.%), nanosize.


begin to display superior conductivity. With further de-

crease in temperature, the conductivity difference between

liquid and colloidal electrolytes widens and, about 100 jCbelow the Tm, the conductivity can differ by orders of

magnitude.

The structure of electrolytes below the melting temper-

ature is expected to have a significant influence on conduc-

tivity. In the polymer electrolyte literature, the role of

crystalline to amorphous transition is well documented

[7,13]. The amorphous structure of polymer is desired for

higher conductivity. The particle size of the dopants is an

important factor for stabilizing amorphous structure below

Tm [7]. A similar argument can also be applied for formu-

lating high performance colloidal electrolytes.

Fig. 7. A schematic presentation of temperature dependence of conductivity

of liquids, polymers and colloidal electrolytes.

3.3. Solid composite electrolytes

The theory of ionic conductor composites which was

developed in the 1980s [14–16] highlights the importance

of space charge region or phase boundaries for the ionic

conductivity. The boundaries provide a three-dimensional

percolation pathway for the transport of charge carriers.

The phase boundaries are created through the interaction

of two distinct crystalline phases; for example, LiI and

Al2O3. Unlike liquid or polymer electrolytes, the bulk

structure of LiI–Al2O3 remains crystalline, yet the con-

ductivity increases at lower temperature because of the

creation of phase boundaries having defect structure. The

similarity in the temperature dependence of the conductiv-

ity of composite type of materials (LiI–Al2O3, frozen

colloidal electrolytes, polymer–ceramic composites) sug-

gests that the underlying transport mechanism and the

structure of the three-dimensional conduction path may

be similar.

3.4. Space charge region in colloidal electrolytes

A schematic presentation of liquid–colloid interaction

is shown in Fig. 8. After a colloidal particle such as

BaTiO3 or Al2O3 is introduced in the liquid electrolyte,

the electric field associated with the surface charge of the

particle interacts with the structure of the liquid electrolyte

leading to the formation of space charge regions. The

interaction is temperature dependent and it is believed to

be enhanced at lower temperatures. It is proposed that the

volume of the liquid electrolyte, which is influenced by the

colloid is associated with high conductivity, as shown by a

circle around the colloid in Fig. 8(A). As the concentration

Fig. 8. Schematic presentation of space charge region of (A) a single colloid

with surrounding high conductivity region and (B) colloids and a

contiguous network of high conductivity phase.


of the colloid increases, larger volume fraction of the

higher conductivity space charge regions is created. These

eventually overlap and provide a three-dimensional net-

work, as shown in Fig. 8(B). The volume fraction of the

colloidal phase must exceed the threshold to provide

continuity of the high conductivity phase to reach a peak

value of conductivity. The location of crossover points in

the conductivity curves of liquid and colloidal electrolytes

(Figs. 2 and 3) may be related to the concentration of the

colloidal phase.

The conductivity data of colloidal electrolytes presented

in Figs. 2 and 3 appears to suggest that, at higher temper-

atures, liquid-like transport mechanism is the dominant

contributor to the conductivity. As the temperature is low-

ered, the viscosity of the colloidal electrolyte increases and

the liquid-like transport mechanism becomes weaker and the

conduction pathways created through the space charge

regions takes over. The conductivity data presented in Figs.

4 and 5 shows a temperature independent enhancement in

conductivity, which may have been caused by the use of a

separator material.

4. Summary and conclusions

This paper presented and discussed the effects of a

colloidal phase on conductivity of lithium-ion liquid

electrolytes. The colloidal phase consisted of BaTiO3

(f 1 Am) and Al2O3 (24 nm). These colloids influenced

the conductivity of the lithium-ion electrolytes. The sig-

nificant conclusions of this investigation are summarized

as follows.

1. Both BaTiO3 and Al2O3 enhanced low temperature

conductivity of colloidal electrolytes. The additives had a

small but negative effect on the high temperature

conductivity.

2. Due to the presence of these colloidal phases, overall

temperature dependence of conductivity of the lithium-

ion electrolytes diminished.

3. The highest concentration of colloidal phase was 10

wt.%. Because the colloidal phase was not stabilized, it

had a very slow tendency to settle in the bottom of the

container, specifically at higher temperatures.

4. A polymeric separator material was also impregnated

with liquid and colloidal electrolytes. The colloidal

electrolytes again exhibited superior conductivity in the

� 40 to 40 jC temperature range as compared to the

liquid electrolyte.

5. It is proposed that the underlying transport mechanism in

these colloidal electrolytes is similar to those observed in

ceramic–ceramic (LiI–Al2O3) and polymer–ceramic

(PEO/LiBF4–MgO/Al2O3/BaTiO3) electrolytes. A space

charge region between the colloids and liquid electrolyte

leads to the formation of a high conductivity phase

around the colloid. A three-dimensional network of the

high conductivity phase is attributed to the enhancement

in conductivity.

Acknowledgements

The authors gratefully acknowledge financial support

provided by the University of Dayton Research Institute for

conducting this investigation.

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Ionic conductivity of colloidal electrolytes