AEROSIL® fumed silica in gel lead acid batteries...AEROSIL® fumed silica is a synthetic silica produced by a ﬂ ame hydrolysis process developed by Evonik and, like all synthetic

TECHNICAL INFORMATION 1412

AEROSIL® fumed silica in gel lead acid batteries

L+L 19-01-155 vND TI 1412 AEROSIL fumed silica in gel lead acid batteries, US.indd ––– 05. Juni 2019; 12:45 Uhr

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1. Introduction

Lead acid batteries are among the most widespread types of batteries worldwide. For many years, they have been used mainly as starter batteries in cars and trucks, but in the last couple of decades application fi elds have increased, and with them the requirements on the battery (Fig. 1). A number of innovative improvements have therefore been commercialized, some of which use silica to achieve the required performance.

In addition to the so-called fl ooded lead acid batteries (LAB) there are batteries where the electrolyte has been immobi-lized. One obvious advantage of electrolyte immobilization is increased safety: The electrolyte cannot leak out even if the battery is damaged. However apart from the safety aspect, electrolyte immobilization provides strong advantages in bat-tery performance and life time under both normal and extreme conditions.

The following chapters will focus on batteries where the electrolyte is immobilized in a gel.

Contents1. Introduction 2

2. Summary: benefits of AEROSIL® fumed silica in gel lead acid batteries 3

3. Silica 43.1 AEROSIL® fumed silica 4

3.2 Surface chemistry 5

3.3 Thixotropy 6

4. The gelling process 74.1 Gelling characteristics 8

4.1.1 Silica surface area 84.1.2 Silica loading 94.1.3 Silica densifi cation 94.1.4 Absorbed moisture 104.1.5 Acid concentration 104.1.6 Dispersing conditions (time and shear force) 114.1.7 Temperature 134.1.8 Conclusion / Summary 13

4.2 Electrochemical performance 14

5. Silica sol versus AEROSIL® fumed silica 145.1 Diff erence in gelling behavior 14

5.2 Diff erence in performance during battery operation 16

6. Incorporation of AEROSIL® fumed silica in the production process of gel batteries 176.1 Addition of AEROSIL® fumed silica to sulfuric acid 17

6.2 Dispersion of AEROSIL® fumed silica in sulfuric acid 17

6.3 Filling of AEROSIL® fumed silica / sulfuric acid gel into the battery 18

7. Literature 20

8. Glossary 21

L+L 19-01-155 vND TI 1412 AEROSIL fumed silica in gel lead acid batteries, US.indd ––– 05. Juni 2019; 12:45 Uhr2L+L 19-01-155 vND TI 1412 AEROSIL fumed silica in gel lead acid batteries, US.indd ––– 05. Juni 2019; 12:45 Uhr2

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2. Summary: benefits of AEROSIL® fumed silica in gel lead acid batteries

Today’s applications use various types of lead acid batteries.Today’s applications use various types of lead acid batteries.Today’s applications use various types of lead acid batteries.Today’s applications use various types of lead acid batteries.Today’s applications use various types of lead acid batteries.

Lead Acid Battery(LAB)

VRLA (Valve regulated lead acid battery)

flooded

Immobilized electrolyteOxygen recombination cycle

Sulfuric acid not fixed

GelSulfuric acid trapped

in a silica gel

AGMSulfuric acid trapped

in an absorbing glass mat

SLI(Starting, lighting, ignition)

EFB(Enhanced flooded batteries)

Figure 1: Classification of types of lead acid battery

The following sections give a deeper insight into silica (sec-tion 3), gelling (section 4.1), and electrochemical performance (section 4.2). This chapter provides an overview of the most important benefi ts of gel batteries formulated with AEROSIL® fumed silica.

In fl ooded cells a vertical acid gradient is formed during deep cycling. The more concentrated sulfuric acid settles at the bot-tom of the battery due to its higher density, while the acid at the top is less concentrated. This eff ect is well known as acid strati-fi cation (Fig. 2).

Figure 2Electrolyte stratification according to [1] in a 350 Ah PzS cell

top ofbattery

bottomof

battery 0 1 2 3 4 5 6 7 8 9 10

Gel AGM flooded

dens

ity H

2SO

4/g/

ml

cycles

1,15

1,17

1,19

1,21

1,23

1,25

1,27

1,29

This acid gradient gives rise to a gradient in the voltage V [2], as can easily be seen from a simplifi ed version of Nernst equation:

V0 ~ acid density + 0.84 [V]

As a result, acid stratifi cation causes non-uniform utilization of the active material and this can enhance an irreversible forma-tion of PbSO4, which leads to sulfation.

In addition the solubility of PbSO4 is dependent on acid concen-tration and decreases at high concentrations [3], which enhanc-es the sulfation eff ect.

Acid stratifi cation, in short, is a major problem of fl ooded bat-teries to be overcome because this triggers further processes that cause premature battery failure and reduce battery service life.

The best measure for avoiding acid stratifi cation and achieving uniform acid concentration over the entire height of the battery is gelling [1].

Fixation of the sulfuric acid in a gel formed by SiO2 particles prevents stratifi cation of the acid.

The thixotropic behavior of AEROSIL® fumed silica (see section 3.3) is one of its most important properties in regard to gel bat-teries. The gel structure is maintained throughout the life of the battery and no loss in gel strength is observed. [4]

Diff erent types of AEROSIL® fumed silica (see section 3.1) can be used to adjust the gel properties for specifi c needs (see sec-tion 4.1).

Due to their higher purity, gels made from AEROSIL® fumed silica show lower overpotential compared with those made from less pure silica sols. A lower overpotential reduces water loss and therefore the risk of dryout. This results in a longer service life (see section 4.2).

To summarize, the most important feature of gel batteries for-mulated with AEROSIL® fumed silica is a longer cycle life.

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3. Silica

3.1 AEROSIL® fumed silica

AEROSIL® fumed silica is a synthetic silica produced by a fl ame hydrolysis process developed by Evonik and, like all synthetic silicas produced by Evonik, is x-ray amorphous [5].

AEROSIL® fumed silica is characterized by a 3-dimensional net-work with a fi rm, chemically bonded SiO2 backbone (Fig. 3).

Figure 3Aggregate structure of AEROSIL®

500 nm

a) TEM of AEROSIL® 200 showing the aggregate structure

20 nm

b) HR-TEM of AEROSIL® 200 showing the merging zones of the primary structures

AEROSIL® fumed silica is available with diff erent surface areas ranging from 50 – 380 m²/g and densifi cation varying from 40 – 130 g / l (Fig. 4).

AEROSIL® 200 V AEROSIL® 200

Figure 4AEROSIL® 200 V vs. AEROSIL® 200; both cylinders contain the same weight.


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3.2 Surface chemistry

Basically, two diff erent surface groups can be distinguished on the AEROSIL® fumed silica surface: Silanol groups and siloxane groups (Fig. 5). The silanol groups are responsible for the acidic character of AEROSIL® fumed silica and reactions with water or acid occur mainly at those sites.

SiSi

Si Si Si

Vicinal and bridged

Siloxane groupsGeminal

Isolated

Si

O

O OO

O

HH

H H H

O

Si

Si Si

Si

Si

Figure 5 a:SiO2 surface groups: silanol groups (different binding modes) and siloxane group

Figure 5 b:Hydrogen bridge linkage between two idealized AEROSIL® 200 particles in an enlargement true to scale

The forces holding AEROSIL® particles together depend on the type of hydrogen bonding between the particles. The bond strength of hydrogen bridges, at 4 – 40 kJ / mol, lies between that of covalent bonds (e.g., about 360 kJ / mol for a C-H bond) and van der Waals forces (0.5 – 5 kJ / mol) [5].

As shown in Fig. 6 the density of the silanol groups on the silica particle surface is to a fi rst approximation independent of the specifi c surface area [5].

Figure 6Total concentration of silanol groups in hydrophilic AEROSIL® fumed silica measured by the LiAlH4 method [5]

3

2.5

2

1.5

1

0.5

0

1,5

1,0

0,5

0,0

0 100 200 300 400

Spec. surface area /m²/g

Sila

nol g

roup

den

sity/

SiO

H/n

m²

Sila

nol g

roup

con

cent

ratio

n/m

mol

/g

6

3.3 Thixotropy

Thixotropy is the time-dependent, reversible viscosity reduction of pseudo plastic liquids when shear is applied, and the return to higher viscosity on standing [6].

Visc

osity

time

mechanical shearing

stabilization + restructuring stabilization + restructuring

mechanical shearing

Figure 7: Schematic of thixotropic behavior of AEROSIL® fumed silica

The weak hydrogen bonds that interconnect the AEROSIL® particles can easily be broken by mechanical treatment and the 3-dimensional network is restored on standing (Fig. 7).

Thixotropic behavior is important for the fi lling process of the silica gel into the battery. Gentle stirring can increase the gel time.

Due to the thixotropic nature of gels formed by AEROSIL® fumed silica, the gel reconstitutes a� er the fi rst charging process (section 5.1).

Thixotropy is a characteristic of fumed silica. Colloidal silicas like silica sol do not show this behavior (see section 5.1).


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4. The gelling process

Gelling is a process where particles are linked together, forming a 3-dimensional branched structure throughout the liquid, held inside the network by capillary forces. This process is accompa-nied by an increase in viscosity.

When silica is dispersed in water it acts as a weak acid with a negative surface charge. If the pH is further reduced by acid the silica reaches at pH 2 the point of zero charge. Further decrease of the pH then leads to protonation and hydrolysis of the silica surface and fi nally to the formation of a 3-dimensional network (Fig. 8 / 9).

water

Powder or pH ~ 2

pH < 2

Si Si

Si

Si

SiSi

Si

SiO

OO

O

OO

O

OO O O

OSiSi Si

Si Si SiSi Si

Si

H

H

H

HH

H

H

H

O

OO

OOO O O

O

O

OO

O

O

O O

Si Si

Si

Si

SiSi

Si

SiO

OO

O

OO

O

OO O O

OSiSi Si

Si Si SiSi Si

Si

O

OO

OOO O O

HH

H

H

HH

H

H

O O

O

O

OO

O

O

Si Si

Si

Si

SiSi

Si

SiO

OO

O

OO

O

OO O O

OSiSi Si

Si Si SiSi Si

Si

O

OO

OOO O O

HH H

HH

HH

H

HH

H

H H

H H

O O

O

O

OO

O

O

H

Figure 8: Schematic of the surface reaction of silica in an acidic medium

Figure 9: Three-dimensional structure of AEROSIL®

500 nm

a) gel made from 5,7 % AEROSIL® 200 and 37 % sulfuric acid b) Schematic of the array of SiO2 tetrahedrons in AEROSIL® fumed silica based on a model of Evens and King [7]

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4.1 Gelling characteristics

The gelling process of AEROSIL® fumed silica with sulfuric acid is characterized by the gel time and the gel hardness, which is measured as the penetration depth of a small metal ball into the silica / sulfuric acid mixture. As the viscosity of the gel increases with time the ball is slowed down and the penetration depth decreases [8, 9] (Fig. 10).

Figure 10: Gel time and gel hardness

Time

Visc

osity

Gelling starts

Gel time Is the time between mixing SiO2 with sulfuric acid and the beginning of gel formation (no stirring!)

indication for battery fi lling time

Gel hardness Gives information on rigidity of the gel

The gel time describes the kinetics of the gelling process and is an important parameter for processability of the gel because it determines operational fl exibility during battery fi lling. A long gel time means that the viscosity increases slowly, giving enough time for the fi lling process.

The gel hardness is determined as the penetration depth of a metal ball into the gel a� er 24 hours and gives information on the stiff ness and rigidity of the gel in the battery. A low penetra-tion depth indicates a high gel hardness. In the following text the term “penetration depth” will be used to describe gel hardness.

Both gel time and penetration depth are infl uenced by various parameters, of which the particular fumed silica used is only one.

The following section examines this in detail.

Silica

• Surface area of silica (4.1.1)• Silica loading (4.1.2)• Silica densifi cation (4.1.3)• Absorbed moisture (4.1.4)

Incorporation conditions

• Acid concentration (4.1.5)• Dispersing conditions (4.1.6)• Temperature (4.1.7)

The laboratory tests were done under defi ned and reproducible conditions [10]. The results are shown in the following graphs, plotted as %.

4.1.1 Silica surface area

The silica surface area has considerable infl uence on the gelling properties. Higher surface areas reduce gel time and give lower penetration depth, indicating higher gel hardness for the same silica concentration [4].

Figure 11Influence of surface area on gel time and penetration depth (all 5.7 % in 37% H2SO4)

AEROSIL® 150 AEROSIL® 200 AEROSIL® 300

gel time penetration depth

%

140

120

100

80

60

40

20

0

Among the diff erent surface groups the isolated silanol groups are the most reactive, while the reactivity of the siloxanes depends on the bond angle (strained siloxane bridges are more easily opened).


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The gelling behavior depends on the distribution of the surface groups. AEROSIL® grades with high surface areas (e.g., AEROSIL® 300) and hence a larger amount of free silanol groups on the surface (Fig. 5) feature shorter gel times (Fig. 11).

4.1.2 Silica loading

Increasing the silica content within the sulfuric acid results in reduction of the gel time [4, 8, 9]. The eff ect is more pronounced for densifi ed AEROSIL® grades such as AEROSIL® 200 V, shown on the right in Fig. 12.

Figure 12 a (AEROSIL® 200 )Influence of AEROSIL® fumed silica content on gel time and penetration depth

4,5 5,1 5,7

gel t

ime/

pene

trat

ion

dept

h/% 250

200

150

100


AEROSIL® content

Figure 12 b (AEROSIL® 200 V )Influence of AEROSIL® fumed silica content on gel time and penetration depth

4,5 5,1 5,7

gel t

ime/

pene

trat

ion

dept

h/% 500

400

300

200

100

AEROSIL® content


4.1.3 Silica densification

AEROSIL® fumed silica is available as a fl uff y or densifi ed pow-der (Fig 4).

The use of densifi ed AEROSIL® fumed silica leads to lower viscosity of the dispersion and longer gel times (Fig. 13 a). This gives more time for battery fi lling, and the lower viscosity allows easy fi lling of batteries, especially where electrode plate distances are narrow. Furthermore, the gel hardness increases further on standing (Fig. 13b).

Figure 13 aComparison of gel time and penetration depth of AEROSIL® 200 and AEROSIL® 200 V


AEROSIL® 200 AEROSIL® 200 V

gel t

ime/

pene

trat

ion

dept

h/%

140

120

100

80

60

40

20

0

Figure 13 b Increase of gel hardness of AEROSIL® 200 and AEROSIL® 200 V on standing

0 5 10 15 20 25 30 35 40 45


pene

trat

ion

dept

h/%

time/days

500450400350300250200150100

500

10

4.1.4 Absorbed moisture

Due to the hydrophilic nature of the surface of AEROSIL® fumed silica water is absorbed on the surface when it is exposed to humidity [11].

This results in an increase in the mass and passivation of the reactive free silanol groups.

Figure 14a (AEROSIL® 200)Change of surface silanol groups on exposure to moisture, as determined by IR [12]

start climate chamber: a�er climate chamber:5 days at 35°C / oven for

83 % rel. Humidity 24 hrs at 110°C

SiOH isol moisture

rel.

amou

nt/%

500

400

300

200

100

0

SiOH bridged SiO2 • H2O

Figure 14b (AEROSIL® 200V)Change of surface silanol groups on exposure to moisture, as determined by IR [12]

start climate chamber: a�er climat chamber:5 days at 35°C / oven for

83 % rel. Humidity 24 hrs at 110°C

rel.

amou

nt/%

700

600

500

400

300

200

100

0

SiOH isol moistureSiOH bridged SiO2 • H2O

Initially, AEROSIL® 200 and AEROSIL® 200 V show a similar distribution of surface groups (Fig. 14) [13]. On exposure to humidity (climate chamber) the number of free silanol groups is reduced while the number of bridged silanol groups increases, as does the amount of weakly bonded water. The latter increas-es more signifi cantly for AEROSIL® 200 V, which can be attrib-uted to the more pronounced capillary condensation of water inside the densifi ed agglomerates [14].

The absorbed moisture can be removed by drying, as seen in Fig. 14.

The absorbed moisture infl uences gel time and gel hardness. With increasing moisture the gel time increases and the gel hardness decreases. (Fig. 15)

Figure 15 Influence of moisture on gel time (_____) and penetration depth (----)

0,05 1,05 2,05 3,05 4,05 5,05 6,05


gel t

ime

/ pe

netr

atio

n de

pth/

%moisture content/%

220

200

180

160

140

120

100

4.1.5 Acid concentration

Gel time as well as gel hardness show strong dependence on sulfuric acid concentration. Lower acid concentration results in a so� , less viscous gel [4] (Fig. 16).

Figure 16 Influence of acid concentration on the gel time and penetration depth of AEROSIL® 200 (normalized with values for 37 % sulfuric acid)

20 25 30 35 40


gel t

ime/

%

pene

trat

ion

dept

h/%

conc. H2SO4/%

3500

3000

2500

2000

1500

1000

500

0

450,0400,0350,0300,0250,0200,0150,0100,050,00,0


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4.1.6 Dispersing conditions (time and shear force)

The dispersion of AEROSIL® fumed silica in sulfuric acid is the major step in gel production and, along with the silica properties discussed above, determines the quality of the gel.

The shear energy applied, which is the product of agitation time and speed, leads to deagglomeration of the AEROSIL® fumed silica agglomerates, which are separated into smaller segments (aggregates) (Fig. 17).

The fi nal aggregate size is an important parameter for the for-mation kinetics of the gel [14]. Smaller aggregates need more time ( gel time) to form the 3-dimensional web but aggre-gates that are too large give an unstable web ( gel hardness) due to slight inhomogeneity in the silica distribution.

large agglomerates

low gel strengthtoo low

too highsmall agglomerates

high gel strength

Shear energy

Figure 17: Influence of shear energy on aggregate size and gel time / gel strength

This is also true for dispersing AEROSIL® fumed silica in sulfuric acid.

AEROSIL® 200 shows a steady increase in gel time with increase in agitation time and agitation speed. The impact of time slows down a� er a certain level of deagglomera-tion is reached. The gel hardness is less infl uenced by agi-tation speed than agitation time. If the agitation time is too short the agglomerate size remains too large, resulting in a so� er gel ( low penetration depth (section 4.1)).

12

60 80 100 120 140 160

variation of time/5000 rpmpenetration depth

rota

tion

spee

d/rp

m

16

14

12

10

8

6

4

2

0

12.000

11.000

10.000

9.000

8.000

7.000

6.000

5.000

4.000

disp

ersin

g tim

e/m

in

penetration depth/%

AEROSIL® 200 V

60 70 80 90 100 110 120

variation of time/5000 rpmvariation of rotation speed/time = 10 min

rota

tion

spee

d/rp

m

16

14

12

10

8

6

4

2

0

12.000

11.000

10.000

9.000

8.000

7.000

6.000

5.000

4.000

disp

ersin

g tim

e/m

in

penetration depth/%

AEROSIL® 200

50 75 100 125 150 175

rota

tion

spee

d/rp

m

16

14

12

10

8

6

4

2

0

12.000

11.000

10.000

9.000

8.000

7.000

6.000

5.000

4.000

disp

ersin

g tim

e/m

in

gel time/%

variation of time/5000 rpmvariation of rotation speed/time = 10 min

AEROSIL® 200 V

50 60 70 80 90 100 110

variation of time/5000 rpm

AEROSIL® 200

variation of rotation speed/time = 10 min

rota

tion

spee

d/rp

m

16

14

12

10

8

6

4

2

0

12.000

11.000

10.000

9.000

8.000

7.000

6.000

5.000

4.000

disp

ersin

g tim

e/m

in

gel time/%

Figure 18: Influence of dispersion conditions (agitation time and speed) on gel time and penetration depth (5000 rpm / 10 min are set as 100 % reference)

The densifi ed AEROSIL® 200 V needs more energy for deag-glomeration. The eff ect of agitation speed on gel time is more pronounced than that of agitation time.

The diff erence in gel time and gel hardness between AEROSIL® 200 and AEROSIL® 200 V described in section 4.1.3 is still maintained.

It can be concluded that the dispersing energy has to be cor-rectly applied for ideal formation of an electrolyte gel within the battery (Fig. 18).

The agitation conditions infl uence electrical performance as well as gel characteristics [15]. This is attributed to the absorption of the electrolyte in the three dimensional network.


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4.1.7 Temperature

Temperature is an important parameter during gel preparation and should be carefully controlled. The shear energy needed for incorporation of AEROSIL® into sulfuric acid generates heat, which is absorbed by the liquid. The temperature increase is the same for AEROSIL® 200 and AEROSIL® 200 V.

Figure 19 a Influence of dispersion time on temperature using a rotor stator at a constant rotational speed of 5000 rpm

Figure 19 b Influence of rpm on temperature using a rotor stator for 10 min

20 20,5 21 21,5 22 22,5 23 23,5 24

dispersing time/min

disp

ersin

g tim

e/m

in

temperature/°C

16

14

12

10

8

6

4

2

0 20 25 30 35 40 45

rotation speed/10 min

rota

tion

spee

d/10

min

temperature/°C

12.000

10.000

8.000

6.000

4.000

2.000

0

5,7 % AEROSIL® 200 sulfuric acid (d = 1,28 g / ml) 5,7 % AEROSIL® 200 sulfuric acid (d = 1,28 g / ml)

Fig. 19 a shows that the temperature increase is moderate at 5000 rpm. However, the temperature increase can be much higher at very high rotor speeds (Fig. 19 b). (A Polytron® PT-6100 rotor stator system with PT-DA 6045 / 6 dispersing aggregate was used and a water bath for cooling.)

Higher temperatures resulting from the dispersing step can lead to short gel times. Cooling is recommended to increase gel time (Fig. 20).

Figure 20 Influence of temperature on gel time (AEROSIL® 200 V)

10 15 20 25 30 35

gel t

ime/

%

temperature/°C

300

250

200

150

100

50

0

4.1.8 Conclusion / Summary

Table 1Summary of influences on gel time and gel hardness

Silica

4.1.1 Surface area of silica ▲ surface area ▼ gel time ▲ gel hardness

4.1.2 Silica loading ▲ silica loading ▼ gel time ▲ gel hardness

4.1.3 Silica densification ▲ densification ▲ gel time ▼ gel hardness

4.1.4 Absorbed moisture ▲ moisture content ▲ gel time ▼ gel hardness

Incorporation conditions

4.1.5 Acid concentration ▲ acid concentration ▼ gel time ▲ gel hardness

4.1.7 Temperature ▲ temperature ▼ gel time

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4.2 Electrochemical performance

AEROSIL® fumed silica also improves the electrochemical performance of the battery, especially under a deep discharge regime.

Gel batteries show an increase in capacity during the initial cycles.

The pathways for the oxygen recombination cycle have to be formed fi rst. Gas diff usion rates are much higher in gases than in liquids [16].

Without cracks the oxygen recombination cycle does not oper-ate. This will cause water loss followed by shrinkage of the electrolyte gel and consequent crack formation, which off ers a pathway for the oxygen and therefore starts the recombination cycle [17].

Due to the large surplus of electrolyte in gel batteries this water loss is not critical for battery operation.

5. Silica sol versus AEROSIL® fumed silica

5.1 Difference in gelling behavior

The reaction of sodium silicate (waterglass) with sulfuric acid results in the formation of single particles (e.g., silica sols) or aggregates (e.g., precipitated silica, silica gels) depending on the reaction conditions. The surfaces of these particles consist of highly reactive silanol groups.

AEROSIL® particles by contrast are formed in a high tempera-ture process and have fewer silanol groups on the surface.

On contact with acid both silica sol and AEROSIL® fumed silica form a gel (Fig. 21).

gelsilica sol

fumed silica

Sodium silicate

+sulfuric acid

SiCl4

+H2 / O2

Nucleation

Growth & aggregation

gelation

H2SO4

H2SO4

Nucleation

Growth

Figure 21: From raw material to gel: schematic of particle synthesis and gel formation


15

As illustrated in Fig. 22 the silica network of a gel made from AEROSIL® fumed silica is denser than that made from a silica sol. This results from the fractal structure of the AEROSIL® particles.

Figure 22: Difference in gel structure between a gel made from AEROSIL® and silica sol

500 nm 500 nm

a) gel made from 5.7 % AEROSIL® 200 and 37 % sulfuric acid a) gel made from silica sol and 37 % sulfuric acid

Like all gels, both of the present gels show syneresis (Fig. 23). This eff ect is more pronounced for gels made from silica sol, due to their reactive surface. Si

Si

Si

SiSi

Si

Si

SiO

OO

O

OO

OO

O

OO

O

OH

OH OH

OH

Si

SiSiSi

OOO

O O

OH

OHOH OOO

OO O

Si

SiSiSi

OOO

O O

OH

OH

-H2O -H2O

OH OOO

OO O

Si

Si

Si

SiSi

Si

Si

Si

SiO O

O

OO

O

OO

OO

O

OO

O

OH

OH

OH

OH

O

Si

Si

Si

SiSi

Si

Si

Si

SiO

OO

O

OO

OO

O

OO

O

OH

OH

OH

OH

O

Si

SiSiSi

OOO

O O

OHOH

OHOH OOO

OO O

Si

Si

SiSi

Si

SiO

OO

OO O

OSi

Si

SiSi

Si

Si

SiO

OO

O

OO

O OO

Si

SiSiSi

OOO

OOO

OO O

Si

SiSiSi

OOO

OOO

OO O

Si

SiSiSi

OOO

OOO

OO O

Si

Si

Si

SiSi

Si

Si

SiO

OO

O

OO

OO

O

OO

O

Si

Si

Si

Si

Si

Si

Si Si

Si

Si

Si

Si Si

Si

Figure 23: Schematic of the progress of syneresis [18]

But nonetheless a more signifi cant diff erence is noticed during formation of the battery. Charging puts stress on the gel due to the change in volume of the electrodes and the evolution of gas (because the oxygen recombination cycle has not started yet). The eff ect is comparable to that of agitation and as a conse-quence the gel structure is damaged.

In case of AEROSIL® fumed silica the three-dimensional web is restored on standing (thixotropy, section 3.3).

16

web structure is destroyed by forces

restructuring

syneresis

reformation of the web

silica sol

AEROSIL® fumed silica

Figure 24: Schematic of the different behavior of gels made from AEROSIL® fumed silica and silica sol after agitation

Gels made from a silica sol behave diff erently. The particles do not revert to the web structure but rather form compact aggre-gates (Fig. 24) [8, 18], which cannot hold the acid as effi ciently. As a result gels made from a silica sol release signifi cantly more liquid than those made from AEROSIL® fumed silica.

The risk of acid leakage from gel batteries made from silica sol is therefore much higher and operating the battery in a horizontal position can cause release of acid followed by corrosion of the surroundings.

5.2 Difference in performance during battery operation

The diff erences in performance of gels made from AEROSIL® fumed silica and silica sol are a result of the intrinsic diff erences described above.

Cycle life, the most important feature of a battery, is signifi -cantly longer when the battery is formulated with AEROSIL® fumed silica rather than with a silica sol, especially under deep discharge conditions [4].

Water loss in regulated batteries is critical since it can dry out the battery, resulting in premature capacity loss (shorter cycle life). Gel batteries made with silica sols show higher water loss than those formulated with AEROSIL® fumed silica; this water loss has been attributed to a higher impurity level [4].

Cycling performance is dramatically infl uenced by the type of silica used: cycle times for gel batteries prepared with a silica sol are much shorter than for those made with AEROSIL® fumed silica under the same cycling regime [4].

Analysis of cycled plates has shown that when a silica sol is used the specifi c surface area of the negative active mass is sig-nifi cantly higher than in a battery formulated with AEROSIL® fumed silica. Further analysis of the plate reveals that silica sol particles have penetrated into the plate, and this was seen as one of the reasons for the early failure of gel batteries [4].


17

Summary: comparison of gels made with AEROSIL® fumed silica and silica sol

Table 3: Comparison of gels made from AEROSIL ® fumed silica and silica sol

Gel made with AEROSIL® fumed silica Gel made with silica sol

Physical appearance of silica Powder Liquid sol

Incorporation into sulfuric acid Dispersion needed Mixing sufficient

Sulfuric acid Concentration as needed for battery Concentration has to be adjusted (water compensation)

Purity High Low

Presence of unwanted impurities (e.g., Fe) Low High

Thixotropic gel Yes No

Shrinkage of gel acid squeezed out Only initially Over entire lifetime of battery

Mounting of battery In all positions Risk of leakage in horizontal orientation due to shrinkage of gel

Water loss Low, only initially (until oxygen recombination cycle has started)

Higher, over entire lifetime of battery (due to higher impurity level)

Cycle life High Low

6. Incorporation of AEROSIL® fumed silica in the production process of gel batteries

Industrial production can be split into several independent steps.

6.1 Addition of AEROSIL® fumed silica to sulfuric acid

It is advantageous to use a stirrer that sucks in the silica. (Fig. 25) This avoids dust and improves and speeds up the wetting process.

Using precooled sulfuric acid can help increase the gel time.

6.2 Dispersion of AEROSIL® fumed silica in sulfuric acid

A� er wetting of the silica is complete the dispersion step is started. A rotor stator device represents state-of-the-art tech-nique (Fig. 26). The energy input required depends on the batch size and the planning should therefore be done by a skilled engineer.

Cooling is recommended at this stage of the process.

Figure 25: Commercially available systemsContinuous rotor stator mixer showing the inlets for AEROSIL® fumed silica (A)and the liquid (B), and the outlet for the product (C). This system is practically dust-free.

A

B

C

18

6.3 Filling of AEROSIL® fumed silica / sulfuric acid gel into the battery

The fi lling of the batteries is done from a stirred tank to avoid gelling during the fi lling process. Special fi lling equipment is available on the market. Stirring is also recom-mended at this stage of the process to prolong pot life.

Fig. 27 shows an industrial production line with modules for all 3 steps.

Figure 26 a: Schematic of an industrial set-up, from dosing of AEROSIL® fumed silica to gel filling (courtesy Kustan)


19

Sulfuric acid

AEROSIL®fumed silica

formation of the gel inside the battery

Figure 26 b: Schematic of process flow

20

7. Literature

1 Gelled-electrolyte batteries for EV; J. Power Sources 40 1992 47

2 D. Pavlov, G. Petkova, T. RogachevD. Pavlov, V. Naidenov, S. RuesvkiV. Danel, V. PlichonV. Danel, V. Plichon

J. Power Sources J. Power Sources Electrochim. ActaElectrochim. Acta

1751612828

2008200619831983

586658781785

3 V. Danel, V. Plichon Electrochim. Acta 27 1982 771

4 J. C. Hernandez, M. L. Soria, et al. J. Power Sources 162 2006 851

5 Evonik Industries AG; Resource Efficiency GmbH, Business Line Silica; Technical Overview “ AEROSIL® – fumed silica”

6 H. A. Barnes J. Non-Newtonian Fluid Mech. 70 1997 1

7 Solubility of lead sulfate in solutions of sulfuric acid J. Research Nat. Bureau of standards 22 1939 55

8 T. Tantichanakul, O. Chailapakul, N. Tantavichet J. Power Sources 196 2011 8764

9 T. Tantichanakul, O. Chailapakul, N. Tantavichet Journal of Industrial and Engineering Chemistry

19 2013 2085

10 Standard dispersing conditions: 5.7 % AEROSIL® fumed silica; 37.4 % (= 1.28 g / ml) H2SO4). Rotor stator: Polytron® PT-6100 with PT-DA 6045 / 6deviations are marked in the text

11 B. Morel, L. Autissier, D. Autissier, et al. Powder Technology 190 2009 225

12 Moisture determined by weight loss upon drying; silanol groups are determined by IR as powder

13 Siloxane groups are identical for AEROSIL® 200 / AEROSIL® 200 V; determined by 29Si-NMR; unpublished results

14 F. Kramm, H. Niepraschk International Telecommunications Energy Conference

1999 6-2

15 M. Gentcen, K. B. Dönmez, Y. Sahin, et al. J. Solid State Electrochem. 18 2004 2469

16 T. Kaskiala Minerals Engineering 15 2002 853

17 J. Kwasnik, J. D. Milewski, T. Pukacka, B. Szczesniak J. Power Sources 42 1993 165

18 S. Wilhelm, M. Kind Polymer 6 2014 2896


21

8. Glossary

A

Acid stratification Is used to describe the acid gradient formed within the electrolyte when a LAB is cycled. During charging a lead acid battery, high density acid is produced in the plates. This acid leaves the plates and drops as a result of gravitation to the lower part of the cell while lower density acid concentrates at the top of the cell.

Active material Compounds which react in the electrodes during charge / discharge: lead dioxide on the positive plate and spongy lead on the negative.

AGM (Absorbent Glass Mat)

Mat of microglass fibers which is used as separator in AGM lead acid batteries

AGM-battery One type of VRLA batteries. The acid is fixed / hold within the fiber net of the AGM.

Agglomerate Loose collections of primary particles and / or aggregates that can be split up during dispersion (as defined in DIN 53 206).

Aggregate As defined in DIN 53 206, a merged group of particles positioned next to one another in one plane, with a surface smaller than the total surface of the primary particles.

Anode Electrode on which oxidation occurs; releases electrons on discharge (opposite to cathode)During charge, the anode is positive; reverse on discharge.

B

BET See specific surface area

C

Capacity The capacity of a battery is the amount of electrical energy it can deliver at a constant current over a fixed period of time to the cut-off-voltage. Unit: ampere-hours /Ah

Cathode Electrode on which reduction occurs; electrons are absorbed on discharge (opposite to anode).During discharge, the cathode is positive; reverse on charge.

Cell A cell is the smallest unit of a battery consisting of a positive electrode / separator / negative electrode and electrolyte

Charge to replenish a battery by applying current from an external power source (opposite of discharge).2PbSO4 + 2 H2O → PbO2 + Pb + 2H2SO4

C-rate The unit to describe charge and discharge times of batteries.Example: 1 C – the battery is charged / discharged within 1 hour. 2 C – the battery is charged / discharged within ½ hour. C / 2 – the battery is charged / discharged within 2 hour.

Corrosion Destructive attack of metal parts by acid from the electrolyte; example: iron starts to form rust as a corrosion product after contact with sulphuric acid.

Cycle One discharge plus one recharge equals one cycle.

Cycle life Number of cycles a battery can deliver before the end criteria (voltage) is reached.

D

Deep discharge Term “deep discharge” is used that when left theoretical capacity is below 40 % (DOD = 60).

Density Specific gravity; for sulphuric acid the density depends on the concentration; example 1.28 g / l = 37.36%; 1,25g / l = 33.82%.

Depth of discharge (DOD)

Value to express the state of discharge; example DOD = 100 % = full discharge

Disagglomeration / de-agglomeration

Generally mechanical process involving aggregate groups with relatively loose attachment being broken up (for example by dispersion)

Discharge The battery delivers current; Oopposite of chargePbO2 + Pb + 2 H2SO4 → 2PbSO4 + 2H2O

Dispersion From the Latin dispersio (scatter). As defined in DIN EN ISO 862:1995-10, a system (a disperse system) consisting of multiple phases, with one continuous phase (the dispersing agent or dispersant) and at least one fiely distributed phase (the dispersed phase). Forms or energy that can be used to form a dispersion include chemical, electrochemical, electrical and mechanical energy.

Dispersing Using dispersion machines to evenly distribute powdered substances in liquids

Dissolver Dissolvers are used to disperse substances, most commonly in liquids. The dissolver provides the energy required for → disagglomeration.

DOD Abbreviation for “depth of discharge”

22

E

EFB Enhanced flooded battery

Electrical Resistance The electrical resistance of a material / battery describes the difficulty of electrons to pass through it. For a good battery the electrical resistance should be low.Unit: Ohm [Ω]

Electrolyte Medium that serves to conduct the ions; in case of LAB the electrolyte is sulphuric acid.in contrast to other battery types e.g. Lithium ion batteries the electrolyte in LAB takes part in the chemical reaction H2SO4 →←→←→ H→ H→ 2O.

Electromotive force Force that makes electrons move to produce an electric current.

F

Flame hydrolysis(fumed)

Evaporable substance such a silicon tetrachloride reacting with gaseous water, formed in the oxyhydrogen flame.

G

Gassing Giving off oxygen gas at positive plates and hydrogen at negatives plate.Gassing occurs when the overpotential is reduced or the oxygen recombination cycle in VRLA is not operating.

Geminal Substituents attached to the same atom

H

Hydrogen bond Bond that forms between a hydrogen atom covalently bonded to an electronegative element (proton donator) and a lone electron pair of another electronegative atom (proton donator).

Hydrolysis Cleaving a chemical compound after reacting with water

L

LAB Lead acid battery

N

Negative plate Battery electrode delivering the electrons during the electrochemical reaction. The negative plate has a lower electrical potential than the positive.Example: Pb + HSO4

– → PbSO– → PbSO–4 + H+ + 2e– -0.36 V– -0.36 V–

Nernst equation Ecell: Electromotive force = cell potentialE°cell: Standard cell potential

Ecell = E°cell + RT lnQrzFR: Universal gas constant

R = 8.314 472(15) J K−1 mol−1

T: Absolute temperatureF: Faraday constant

F = 9.648 533 99(24)×104 C mol−1

z: Number of electrons transferreda: Chemical activityQr: Reaction quotient

O

OPzV Type of VRLA, gel based, often stationary usage

P

Precipitated silica Synthetic silica created by the reaction of sodium silicate with sulfuric acid. Different to silica sol precipitated silica are isolated at the end of the reaction sequence as solids.

Primary particle As defined in DIN 53206, the smallest particle (individual particle) that a powdered solid can consist of such a particle can be recognized as an individual unit under electron microscope.

Proton acceptor Compound that can accept protons (H+ ions). Opposite of a proton donator

Proton donator Compound that can donate protons (H+ ions). Opposite of a proton acceptor

Positive plate Battery electrode that accepts the electrons during the electrochemical reactionThe positive plate has a higher electrical potential than the negative.Example: PbO2 + 3 H+ + HSO4

– + 2e– + 2e– – → PbSO– → PbSO–4 + 2H2O +1.68 V

R

Reaction quotient The reaction quotient (Qr) is a function of the activities or concentrations of the chemical species involved in a chemical reaction.

Rheology The study of flow. A branch of physics concerned with describing, explaining, and measuring the phenomena that occur as bodies deform in flow.

Rotor stator device Type of dispersion machine used to mix a pigment or a filler material. Consists of an outer stationary ring and an inner concentric rotating ring. Both rings have openings (holes or slits) that the liquid has to pass through under a high shear load.


23

S

Sealed lead acid battery (SLA)

See VRLA

Separator Divider between the positive and negative plates of an element which allows the flow of ions to pass through it.

Silanol group Surface-resident groups on the silica surface with the formula Si-OH

Silica Collective term for the compounds that share the chemical formula SiO2. Silica can either be fumed silica, such as AEROSIL®, precipitated silica, or silica sol. The different forms of silica differ in their physical / chemical properties, such as the size of the specific surface area, the size of the particles, the loss on drying, or the loss on ignition.

Silica sol Synthetic silica created by the reaction of sodium silicate with sulfuric acid. Different to precipitated silica the created small particles (5 – 100 nm) are stabilized in the liquid.

Siloxane groups Si-O-Si units produced by the condensation of silanol groups

SLI Stands for Starting, Lighting, Ignition.

Specific surface area As defined in DIN 66131, the surface area of a solid in relation to its mass, measured in m²/g. It is generally measured on the basis of the BET method (Brunauer, Emmet, Teller in Journal of American Chemical Society 60 (1938) p 309).

Sulfation Formation of large, stable PbSO4 crystals on the negative plate that are difficult to be converted into active material and therefore inhibit current flow.

Syneresis Syneresis is the squeeze out of liquid from a gel.

T

TEM Transmission electron microscope

Thixotropy Extend to which a liquid´s viscosity decreases in accordance with the shear intensity and shear duration and returns to its original state when the shear stress has been removed.

V

Valve In case of LAB the valve regulates the gas pressure inside the battery. It automatically releases the formed gas when a certain internal pressure has built up. It operates only in one direction.

VRLA = Valve Regulated Lead Acid Battery

Maintenance-free lead acid batteries which are sealed, containing immobilized sulfuric acid as electrolyte. Examples: gel batteries, AGM batteries.

van der Waals forces Intermolecular forces that occur as weak bonding forces between inert atoms and saturated molecules, mainly in real gases but also in liquids and in solids.

Vicinal Substituents attached to adjacent atoms.

Viscosity DIN 13342 defines viscosity as substance´s ability to absorb a shear stress that is dependent on the shear rate through shear deformation.

W

Water loss During battery operation, hydrogen and oxygen gas can be formed by decomposition of water. If internal pressure builds up the gases can escape through the valve. Repeated venting can result in a battery dry out.

TI-1

412-

EN-0

2-20

19/0

5-L+

L

Europe / Middle-East / Africa / Latin AmericaEvonik Resource Effi ciency GmbHBusiness Line SilicaRodenbacher Chaussee 463457 Hanau-WolfgangGermany +49 6181 59-12532 +49 6181 [email protected]

North America

Evonik CorporationBusiness Line Silica299 Jeff erson RoadParsippany, NJ 07054-0677USA +1 800 233-8052 +1 973 929-8502ask-si-na� [email protected]

Asia / Pacifi c

Evonik (SEA) Pte.Ltd.Business Line Silica3 International Business Park#07-18 Nordic European CentreSingapore 609927 +65 6809-6877 +65 [email protected]

This information and any recommendations, technical or otherwise, are presented in good faith and believed to be correct as of the date prepared. Recipients of this information and recommendations must make their own determination as to its suitability for their purposes. In no event shall Evonik assume liability for damages or losses of any kind or nature that result from the use of or reliance upon this information and recommenda-tions. EVONIK EXPRESSLY DISCLAIMS ANY REPRESENTATIONS AND WARRAN-TIES OF ANY KIND, WHETHER EXPRESS OR IMPLIED, AS TO THE ACCURACY, COMPLETENESS, NON-INFRINGEMENT, MERCHANTABILITY AND / OR FITNESS FOR A PARTICULAR PURPOSE (EVEN IF EVONIK IS AWARE OF SUCH PURPOSE) WITH RESPECT TO ANY INFORMATION AND RECOM-MENDATIONS PROVIDED. Reference to any trade names used by other companies is neither a recommendation nor an endorse-ment of the corresponding product, and does not imply that similar products could not be used. Evonik reserves the right to make any changes to the information and / or recom-mendations at any time, without prior or subsequent notice.

AEROSIL® is a registered trademark of Evonik Industries or one of its subsidiaries.

L+L 19-01-155 vND TI 1412 AEROSIL fumed silica in gel lead acid batteries, US.indd ––– 05. Juni 2019; 12:45 Uhr

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AEROSIL® fumed silica in gel lead acid batteries...AEROSIL® fumed silica is a synthetic silica produced by a ﬂ ame hydrolysis process developed by Evonik and, like all synthetic