Optimizing lithium battery performance from a tailor-made processing

of the positive composite electrode

Eric Ligneel *, Bernard Lestriez, Olivier Richard, Dominique Guyomard

Institut des Materiaux Jean Rouxel, 2 rue de la Houssiniere B.P. 32229, 44322 Nantes Cedex 3, France

Abstract

We studied the effect of the dispersion and/or morphology within the positive composite electrode on the lithium battery performance. Model

electrodes have been prepared in which the surrounding of the same AM (Li1.1V3O8) is changed by playing with plasticization of the polymeric

binder (B) (PMMA) by ethylene carbonate (EC) and propylene carbonate (PC). We have also varied the concentration, and the nature of the

solvent used for the electrode elaboration (THF and CHCl3). The cycled capacity varies between 100 and 250 mA h/g (C/5 rate, 3.3–2 V)

depending on these parameters. The performance was shown to depend on the quality of AM, B, and carbon black (CB) dispersion within the

electrode. The quality of the AM dispersion depends on the solvent concentration that affects the mixture viscosity. Formation of AM

agglomerates results from high viscosity, while gradient concentrations appear due to sedimentation in low viscosity suspensions. Optimal

distribution and subsequent electrode performance result from optimal solvent concentration. Better CB dispersion was obtained in the case of

favourable interactions between the solvent and the polymer. This resulted in a higher electronic conductivity, a higher porosity, and improved

electrochemical performance. Finally, the addition of EC–PC also favours the CB dispersion and further increases the cycling capacity.

q 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; A. Organic compounds; C. Electron microscopy; D. Electrical conductivity; D. Electrochemical properties

1. Introduction

Most studies in the field of electrochemical devices deal

with the search for new formulations and structures of

electrochemically active material or with the optimization of

already known compositions. But generally the active material

(AM) cannot function by itself as an electrochemical device.

For example, composite electrodes for electrochemical energy

storage systems, like lithium batteries, are mixtures of the AM

and an electronic conducting agent, often carbon black (CB),

embedded in a non-electroactive polymeric binder (B). Such a

complex medium must provide efficient transport of electrons

and ions from the current collector/electrode and electro-

lyte/electrode interfaces, respectively, to the grain surface of

the AM. It seems quite obvious that the organization of the CB

and B dispersion and/or morphology within the composite

electrode should have an influence on the electrode perform-

ance. However, such an issue is a research area that has never

been studied yet.

0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jpcs.2006.01.093

* Corresponding author.

E-mail address: eric.ligneel@cnrs-imn.fr (E. Ligneel).

Recent results from D. Guy et al. showed however, that the

electrochemical performance of the AM (the composite

electrode in fact) depends on the non-electroactive components

of the composite electrode [1,2]. As a matter of fact, the cycled

capacity of lithium trivanadate (Li1.1V3O8) based composite

electrodes could vary between 100 and 280 mA h/g depending

on the composition of the polymeric binder. Thus, the

optimisation of material performance in an electrochemical

device requires also the optimisation of the composite

electrode as a whole, and not only that of the intrinsic active

material.

The goal of the present work is to study the effect of CB and

B dispersion and/or morphology within the composite

electrode on the electrode behaviour and performance. Various

composite electrodes have been prepared in which the

surrounding of the same AM (Li1.1V3O8) is changed by

varying the processing conditions and by pre-plasticizing the

poly(methylmethacrylate) (PMMA) binder. The electrodes

were realised by the solvent casting technique and the

parameters studied were: (i) the solvent concentration that

controls the viscosity of the initial suspension; and (ii) the type

of solvent, tetrahydrofuran (THF) or trichloromethane

(CHCl3), which may change the interactions between the

compounds. CHCl3 is known to be a better solvent of PMMA

than THF [3]. These processing parameters should modify

Journal of Physics and Chemistry of Solids 67 (2006) 1275–1280

www.elsevier.com/locate/jpcs

Fig. 1. Evolution of the discharge capacity with the THF concentration

expressed in milliliter per milligram of dried material.

E. Ligneel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1275–12801276

the characteristics of the electrode such as its microstructure

and electrical conductivity, which in turn should modify the

electrochemical performance.

2. Experimental

A home-synthesized Li1.1V3O8 (at 580 8C [4]) was used as

the active material, and carbon black (Super-P, noted CB,

ERACHEM), as a conductive agent. The binders were pure

PMMA (MwZ996,000 g/mol, Aldrich) or plasticized PMMA.

A mixture of ethylene carbonate (EC)/propylene carbonate

(PC) (1:1w/w) was used as the plasticizer. Battery-grade

chemicals such as EC (Aldrich), PC (Aldrich), THF (purityO99.9%, SDS), CHCl3 (purityO99.95%, SDS), and lithium

bistrifluorosulfonimide (LiTFSI, 3M) were all used as

received.

The composite positive electrodes were prepared by

following a classical solvent route where THF or CHCl3 was

used as the dispersing medium. The Li1.1V3O8 and CB powders

were dispersed in the binder solution to form a suspension

using either a magnetic stirrer for 12 h or a high-speed mixer

(MICCRA D-8, ART) at 15,000 rpm for 10 min. The

suspension was then spread on aluminum disks (1 cm2) for

electrochemical measurements or on adapted supports. These

composite films were dried at room temperature for 2 h to

evaporate the solvent, dried further under vacuum at 50 8C for

1 h. For electrochemical measurements they were then

transferred under dry argon atmosphere in a glove box

(H2O!1 ppm) for battery assembly. The dry composite

electrode was constituted of 73%w/w of Li1.1V3O8, 8%w/w

of CB and 19%w/w of PMMA. When the binder was

plasticized, the liquid additive represents 20%w/w of the dry

matter.

Dispersion of the CB particles in the suspensions was

analysed by low angle laser light scattering (laser granulo-

metry, BECKMAN COULTER LS 230). Transmission

electron micrographs (TEM) were obtained in HF2000-FEG

transmission electron microscope. A drop of the suspension

was placed on a microscope grid and was allowed to dry prior

to the measurement. Solvent evaporation rate during the drying

step was detected with gravimetric analysis, through monitor-

ing weight percentage loss as a function of time. Scanning

electron microscopy (SEM) imaging was performed on gold–

palladium sputtered samples using a JEOL JSM 6400F

apparatus. Composite electrodes were cleaved under liquid

nitrogen and fixed vertically on a support adapted for the SEM.

Two-electrode Swageloke test cells [5], using the

composite positive electrodes, a porous paper soaked with

the electrolyte as the separator and metallic lithium as the

negative electrode were assembled in the glove box. A mixture

of EC/PC (1/1, w/w) containing 1 M LiTFSI was used as liquid

electrolyte. All voltages given in the text are reported vs. LiC/

Li. Cell cycling was performed at 20 8C, monitored by a

VMPe system in galvanostatic mode. The voltage range used

was 3.3–2 V. We used a standard galvanostatic procedure

corresponding to the insertion of one lithium ion in 2.5 h during

the discharge and one lithium in 5 h during the charge.

3. Results and discussion

3.1. Effect of the solvent quantity

Electrochemical performance of composite electrodes

prepared with varying amounts of THF is shown in Fig. 1.

The solvent concentration is expressed in milliliter per

milligram of dried matter. In this part, the dispersion tool

used was the magnetic stirrer and the binder was plasticized

PMMA. There exists an optimal concentration for which the

discharge capacity of the electrode is the largest. This

phenomenon was observed for the two kinds of solvent that

were used (THF and CHCl3).

SEM observations of the composite electrodes were done

below the optimal solvent concentration, at the optimum and

above the optimum. In the case of the optimal solvent

concentration, the Li1.1V3O8 grains appear homogeneously

distributed in a 3D network of PMMA/CB, Fig. 2a. Contrarily,

below the optimum, the composite electrode microstructure

was not homogeneous, and many aggregates of Li1.1V3O8

grains could be detected, Fig. 2b. Such microstructure can

readily explain the poor performance of this composite

electrode. Lithium insertion occurs simultaneously to the

vanadium reduction according to the electrochemical equation:

Li1:1V3O8 CxLiCCxeK/Li1:1 CxV3O8

As most insertion compound, Li1.1V3O8 has low intrinsic

electronic conductivity. The Li1.1V3O8 grains within the

agglomerates are not in direct contact with the CB network

and cannot be reached by electrons. Part of the AM does not

participate, therefore, to the electrochemical reaction. When

the solvent concentration was less than the optimal one,

rheological measurements (not included here) showed that the

suspension was very viscous. We can deduce, thus, that the

inefficient dispersion could be due to the fairly low mechanical

energy available for overpowering the viscosity forces with the

magnetic stirrer used. Above the optimal solvent concentration,

the composite electrode microstructure was not homogeneous

either. A polymer skin at the surface of the electrode could be

Fig. 2. SEM observations of composite electrodes elaborated with various THF

concentrations: (a) 0.006 ml/mg, (b) 0.001 ml/mg, (c) 0.020 ml/mg.

z z

(a) (b)Real compositeelectrode

Model compositeelectrode

Fig. 3. Modelling of the real composite electrode with a gradient composition

with a model multilayered composite electrode. Richer polymer regions appear

darker.

Fig. 4. Discharge capacity of electrodes for various polymer concentrations.

E. Ligneel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1275–1280 1277

observed, Fig. 2c. The existence of a polymer rich layer at the

surface implies moreover depletion of this constituent in the

bulk. We suppose that this gradient concentration could result

from a sedimentation phenomenon above the optimal solvent

concentration in low viscosity suspensions.

Theoretically, sedimentation can be evaluated thanks to the

relation established by Stokes [6]. For spherical grains, the

sedimentation rate, Us, is given by

Us Zg

18ha

ðrgKrAÞd2 (1)

where d is the diameter of the grains, rA is the suspension

density, rg is the grain density, and ha is the suspension

viscosity. Similar equations were derived for the more

complicated shapes of grain like the Li1.1V3O8 sticks or the

CB fractal aggregates [7]. They predict for the sedimentation

rate the same type of dependence with respect to the grains

diameter and density, and to the suspension viscosity and

density. We can notice the importance of the dilution that

determines the viscosity of the suspension. Moreover, the

density and the diameter of the AM and CB grains are

important parameters. When their density and their size

increase, the sedimentation rate increases. We used Eq. (1)

to estimate the polymer skin thickness, assuming a sedimen-

tation phenomenon occurred in the suspensions studied here.

We coupled the measurements of the solvent evaporation

rate, and measurements of the viscosity as a function of

the solvent concentration. We were able to predict the

evolution of the viscosity during the drying step and to

calculate the sedimentation speed. This calculation gives a

polymer skin thickness of 5 mm, in agreement with SEM

observations. Therefore, sedimentation appears as a plausible

mechanism to explain our experimental results at the high

dilution rates.

In order to understand better the role of the gradient

concentration on the electrochemical performances, we used a

multilayer model with step-like variation of the polymer

concentration (Fig. 3). We prepared several homogeneous

Fig. 5. Low angle laser light scattering (laser granulometry) measurement of the

CB particles dispersion size into THF/PMMA and CHCl3/PMMA solutions.

E. Ligneel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1275–12801278

electrodes in which the Li1.1V3O8 to CB ratio was kept at the

constant value of 73%w/w/8%w/w, and the polymer concen-

tration was varied between 5 and 40%w/w. The electro-

chemical performance was determined, and results are reported

in Fig. 4. The discharge capacity is the highest at 19%w/w of

PMMA, which corresponds to the nominal composition. Thus,

it clearly shows that the formation of a polymer gradient

concentration due to sedimentation, which creates in the whole

electrode a departure of the polymer concentration from the

nominal composition, results in a decrease of the electro-

chemical performance.

As a summary, the initial volatile solvent concentration has

a strong impact on the distribution of the electrode constituents.

For a concentration below the optimal one, the mechanical

energy available for mixing could be insufficient to overcome

viscosity forces to reach a good dispersion of the constituents

Fig. 6. TEM micrographs of the various kinds of CB agglomerates:

(a) tenuously touching aggregates, (b) close packing of aggregates.

in the bulk of the electrode. Above the optimal concentration,

sedimentation of the AM and CB compounds in the low

viscosity suspensions can create gradient concentration. In

these two cases, the electrochemical performances are

degraded. Therefore, the viscosity of the suspension and the

type of mixing tool are important parameters to consider for

realizing the best composite electrode. Moreover, these

parameters must be systematically adjusted since the grain

size and density vary from one AM to another.

3.2. Effect of the solvent nature

In this part, a high-speed mixer was used to homogenise the

suspensions, the binder was pure PMMA, and we selected the

optimal solvent concentration. The effect of the solvent nature

on the CB dispersion was analysed by low angle laser light

scattering (laser granulometry). Measurements were effected

on CB suspensions in solutions of the polymer in both solvents;

THF and CHCl3. CB particle size ranges from 10 to 20 mm to

less than 1 mm, Fig. 5. This was confirmed by TEM

observations, Fig. 6. A much better dispersion of the CB is

obtained in the PMMA/CHCl3 solution than in the PMMA/

Fig. 7. Composite electrodes elaborated with (a) THF and (b) CHCl3.

Table 1

Results of discharge capacity, porosity and electronic conductivity for various

electrodes elaborated with THF and CHCl3

Solvent used Discharge capacity

(mA h/g)

Porosity

(%v/v)

Electronic

conductivity (S/m)

THF 163 40 25

CHCl3 183 60 50

E. Ligneel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1275–1280 1279

THF one. Smaller CB aggregates are produced during the

mixing step in the PMMA/CHCl3 solution.

SEM observations of the corresponding composite electro-

des showed major differences between them, Fig. 7. The

electrode elaborated with THF appeared to be denser with

compact regions of CB coated by the polymer. In agreement

with these visual examinations, porosity of the electrode

elaborated with THF was lower (Table 1). The electronic

conductivity of this electrode was much lower too (Table 1).

The discharge capacity of the electrode elaborated with

CHCl3 (183 mA h/g) is higher than the discharge capacity of

the electrode elaborated with THF (163 mA h/g) (Table 1).

This better performance could be due to two parameters:

highest ionic and/or electronic conductivities. The ionic

conductivity of a composite electrode is generally related to

the overall porosity [8]. However, because the imposed current

is rather low the differences observed between the two types of

electrodes can readily be attributed to their different electronic

conductivities. The discharge capacity is controlled by

electronic transport within the composite and a higher

electronic conductivity improves the electrochemical perform-

ances [9–12].

We believe that a better dis-agglomeration of the initial CB

powder in PMMA/CHCl3 than in PMMA/THF solution, due to

better PMMA/CHCl3 interactions, could explain these results.

CB consists of elementary particles fused together in various

forms of aggregation [13], called primary aggregates, which

are the smallest dispersible units. In composite materials, they

further give larger secondary structures, often-denoted

Fig. 8. Schematic drawing for the interpretation of the solvent effec

agglomerates, from the close packing of aggregates in a

compact state, like pellets in the initial carbon black powder, to

the network-like structure of tenuously touching primary

aggregates. On the other hand, it is well known that acid–

base interactions contribute to polymer solubility in common

solvents [14]. CHCl3 is acidic and is a very good solvent of

PMMA that is a basic polymer. On the contrary, THF that is

basic only interacts with PMMA via dispersive interactions,

and is a poor solvent of this polymer [15]. In polymer–filler–

solvent systems, the poor polymer–solvent interactions drive

polymer adsorption on the filler surface [16]. We believe that

large amount of PMMA adsorbed onto CB from THF prevents

CB dis-agglomeration by creating shells of entangled polymer

chains [17]. Finer CB dispersion in CHCl3/PMMA solution

results in thinner insulating polymer barriers between the CB

aggregates, after the volatile solvent removal. So the inter-

particle resistance between the CB agglomerates decreases,

which leads to a higher electronic conductivity of the CB

network. The finer CB dispersion can also form more numerous

electrical contacts between the AM grain surface and the CB

network. Both phenomena (higher electronic conductivity and

more numerous contacts) are favourable to an improvement of

the insertion kinetics in the Li1.1V3O8 grains.

As a summary, the nature of the solvent used in the

composite electrode processing has an effect on the electro-

chemical response. Better electrochemical performance was

obtained in the case of favourable interactions between the

solvent and the polymer. Analysis of the literature on polymer–

CB suspensions shows that favourable polymer–solvent

interactions is a driving force for low adsorption of polymer

chains on the surface of CB particles, which leads to an

efficient CB dispersion within the composite electrode (Fig. 8).

3.3. Effect of the plasticizer

Comparison between the electrochemical performance

obtained for composite electrode prepared with the EC–PC

t on the construction of the composite electrodes (see the text).

E. Ligneel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1275–12801280

plasticized PMMA (Fig. 1) and with pure PMMA (Table 1)

shows a 40% performance increase when the plasticizer is

added. We showed the same performance improvement for

Li1.1V3O8 and LiFePO4-based composite electrodes with other

binders as poly(ethylene oxide) (PEO), or a mixed blend of

PEO and a copolymer of vinylidene fluoride with hexafluor-

opropylene (PVdF-HFP) [1,18]. This suggests that the

plasticizer effect is intrinsic, independent to the kind of

polymer or AM. We found that its presence during the mixing

step seems to improve the CB dispersion [19]. A more detailed

study will be addressed in a forthcoming paper.

4. Conclusion

The electrochemical performance of a composite electrode

depends on two types of parameters. Those linked to the

characteristics of the active material, and those linked to the

composite electrode composition and processing conditions.

Here, we showed that the viscosity, the mechanical energy of

the dispersion tool used, the nature of the volatile solvent, and

the binder plasticization have strong impacts on the electro-

chemical performance of an electrochemical device.

Acknowledgements

The authors wish to thank P. Moreau for performing the

TEM observations.

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