Electrochemical power sources for electric vehicles

Electrochemical power sources for electric vehicles

J. Electroanal. Chem., 1 1 8 ( 1 9 8 1 ) 1 5 7 - - 1 6 5 Elsevier Sequoia S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s ELEC...

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J. Electroanal. Chem., 1 1 8 ( 1 9 8 1 ) 1 5 7 - - 1 6 5 Elsevier Sequoia S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s

ELECTROCHEMICAL

157

POWER SOURCES FOR ELECTRIC VEHICLES

D.B. ~ T T H E W S School of Physical Sciences and The Institute of Energy Studies, University of South Australia,

The Flinders

Bedford Park, South Australia.

ABSTRACT Recent work on high energy density and mechanically on testing E.V. batteries lead dioxide in various

is described.

Cyclic voltammetry

sulfate electrolytes

acid battery which is capable of very rapid mechanical acid batteries under electric vehicle conditions for regenerative

batteries

and

studies of lead and

have led to improved discharge

ities whilst work on slurry electrodes has demonstrated

efficiencies

rechargeable

the feasibility recharging.

capac-

of a lead-

Tests of lead-

have shown high charge and energy

braking and that the circulation

of the electrolyte

does not affect battery performance.

INTRODUCTION The viability of on-the-road

electric vehicles

depends on many factors both tech-

Two such factors are range and recharging time.

nical and social.

as "How far can I go?"

Questions

"Will I have enough fuel for a second trip downtown?"

"How long will I be immobilised in the mind of the prospective

such and

if I run out of fuel?" are likely to be prominent buyer.

At the moment E.V.'s would score badly in comparison with petrol or L.P.G. vehicles

on all of these counts.

be partially refueled rapidly, electrical

The typical

power demands for fast charging would increase to undeliverable

with the number of E.V.'s on the road. which is sufficient

to cope with the occasional This paper describes time.

density battery,

and the levels

The typical E.V. has a range of up to 80km,

for the majority of trips for city and suburban dwellers but may

still cause anxiety because of the possibility

recharging

lead-acid battery powered E.V. can

but not as rapidly as its petrol counterpart

of being stranded or not being able

second trip.

some recent efforts to overcome the problems of range and

These efforts include work on the development of a battery that can be rapidly recharged,

test system.

0022-0728/81/0000--0000/$02.50, © 1981, Elsevier Sequoia S.A.

of a high energy

and of a E.V. battery

158

DEVELOPMENT OF A HIGH ENERGY DENSITY BATTERY In theory, the energy density of the lead-acid battery is inferior to just about any other battery, real or proposed [i].

Nevertheless in practice, because of

factors such as cost, safety, cycle life and self-discharge rate, the Pb-acid battery is still used in the great majority of E.V.'s [2], and considerable effort is being expended to increase their energy density at high rates of discharge. The Negative Plate The discharge reaction of the negative plate of a lead-acid battery is =

Pb(s) + SO 4 (aq) ÷ PbSO4(s ) + 2e-

Since hydrogen ions do not participate in this reaction, then the behaviour of lead in sodium sulfate and ammonium sulfate was studied.

In order to prevent the forma-

tion of basic lead sulfates of the type PbSO4.nPbO the pH was kept below 4 with sulfuric acid.

Details of the electrode preparation, apparatus and procedure are

given elsewhere

[3].

The electrodes were high purity, mechanically polished lead -1

discs and the technique used to study them was cyclic voltammetry at 10mV min

The tests yielded data on current, potential and charge as a function of the number of charge-discharge cycles.

Prior to testing, the electrodes were cathodically

pretreated ~n 8itu in order to remove any oxidation products on the lead surface. After electrochemical testing the surface morphology was investigated by scanning electron microscopy. Figure 1 gives results for some of the electrolytes tested and shows the

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7.2

6.0

%

/..8



.,c3.6 × 2.4

xxxxXX

xxXX~ ~ . . . . . .

x

e

A~A~,&&AAAAAAA&.A/ _ e ~ I A A & & e x x

1.2

I

8

I

16

I

2~.

I

32

I

40

/.,8

Cycle number

Fig. i. The dependence of discharge capacity on cycle number for lead in 4.3M H2SO 4 (~); pH 4 (NH4)2SO 4 at 4.0M (AAA), 2.0M (xxx) and 0.SM (-.').

159 superiority

of the pH 2, 0.5 mol dm -3 ammonium sulfate solution.

Fig. 2. Scanning electron micrograph discharge of lead in 4.3M H2SO 4. S.E.M. studies,

of lead sulfate crystals produced by the

such as that shown in figure 2 demonstrated

that the discharge

reaction is terminated by coverage of the lead with well developed crystals and that the larger the crystals

lead sulfate

then the higher the charge delivered

during discharge. Further studies were carried out to elucidate

the mechanism of crystallization

of lead sulfate on lead electrodes

[4].

high purity, mechanically

lead discs which were pre-conditioned

hydrogen

evolution.

dm -3 solutions

polished

The electrodes

used in this study were

The Tafel slope for the lead dissolution

of sulfuric acid, ammonium sulfate

(pH 4.0) and sodium sulfate

(pH 4.0) was found to 26 ± 1 mV which is consistent with a two electron, transfer controlled reaction. o.c.v,

charge

This Tafel region extended to some 80mV from the

and was followed by nucleation

Linear potential

by vigorous

reaction in 0.5 mol

scan measurements

103 mV s -I yielded a linear dependence

and growth of lead sulfate. [4] at sweep rates v in the range 10 -3 to

of log I on log v with a slope of 0.63 ± P 0.01 for lead in 0.5 mol dm -3 sulfuric acid.

160

Potential step measurements

[4] in 0.5 mol dm -3 sulfuric acid produced current

transients of the type shown in figure 3.

At small times t the current was found

I Time

Fig. 3. Current-time transients recorded after potential steps to -920 mV (top curve), -925 mV, and -930 mV (bottom curve) versus mercury sulfate electrode in 0.SM H2SO 4 with full ohmic compensation. to be proportional to t 3 and the maxima were characterised by

d log im/dE = (31 ± 3mV)-1

d log tm/dE = (21 ± 3mY) A theoretical result

-1

s t u d y o f t h i s system [5] d e m o n s t r a t e d t h a t t h e above d a t a i s t h e

o f h e t e r o g e n e o u s p r o g r e s s i v e n u c l e a t i o n and t h r e e d i m e n s i o n a l growth o f

lead s u l f a t e the crystal

crystals

w i t h t h e r a t e d e t e r m i n i n g s t e p i n v o l v i n g t h e s o l i d p h a s e or

s u r f a c e but n o t t h e s o l u t i o n .

Growth o f l e a d s u l f a t e

the surface leads eventually to self-inhibition

of crystal

crystals

over

growth.

The P o s i t i v e P l a t e E l e c t r o d e s p r e p a r e d by p r e s s i n g l e a d d i o x i d e powder i n t o p o l i s h e d l e a d d i s c s were a n o d i c a l l y p r e - t r e a t e d in various s u l f a t e

i n 8g~u and t h e n c h a r g e - d i s c h a r g e c y c l e d a t 30 mV min

electrolytes

[6].

e l e c t r o d e s were s t u d i e d by S.E.M. these results and s u l f u r i c

After the electrochemical testing Some o f t h e r e s u l t s

-1

the

a r e shown i n f i g u r e 4;

showed t h a t a s o l u t i o n c o n t a i n i n g ammonium s u l f a t e

(0.5 mol dm-3)

a c i d (1.0 mol dm-3) g i v e s more c h a r g e t h a n c o n c e n t r a t e d s u l f u r i c

acid,

161

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I

I

I

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2.75

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>., 2.25

xllX xXOA4A X tA X A ~A Ae

u 2.00

3

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1.75 t



1.50 1.25 ×

1.00 ×

0.75



5

10

15

Cycle

number

20

25

30

Fig. 4. The dependence of discharge capacity on cycle number for a lead dioxide powder electrode in 4.3M HgSO~ (,----), 1.0M H~S04 (xxx), 1.0M H2SO 4 + 0.5M (NH4)oSO 4 (AAA), 1.0M H2SO 4 + 0.SM (NH4J2SO 4 + 0.1M ZnS04-(..-). and the addition of zinc sulfate (0.1 mol dm -3) produces even higher charges.

The

discharge reaction is

PbO 2 + 4H3 O+ + S04 = + 2e- ÷ PbSO 4 + 6H20

The relation between the large lead sulfate crystals and the much smaller lead dioxide particles is illustrated by the SEM in figure 5.

The charge delivered

by lead dioxide is related to the porosity of the lead sulfate deposit, as observed by S.E.M.

DEVELOPMENT OF A RAPIDLY RECHARGEABLE BATTERY An acceptable E.V. battery should be rechargeable in minutes rather than hours. It is not feasible to do this electrically, what is required is a mechanical recharging procedure similar to that employed in petrol vehicles. The slurry electrode battery would use such a procedure.

In this battery the

active material is in the form of a slurry which when discharged may be drained and replaced by a fresh batch of slurry.

A slurry battery using the same active

materials as a lead-acid battery was constructed and tested by constant current discharge.

Some discharge curves are given in figure 6.

A slurry electrode

cell delivered 0.50 A for 124 minutes at a current density of 60A m -2 (collector electrode) and at an average voltage of 1.75V.

162

Fig. S. Scanning electron micrograph of the surface of a lead dioxide powder electrode after discharging on the 30th cycle in 1.0M H2SO 4.

2.0

1,5

'E 1.0

A POSITIVE :- 12.7g PbO2-1OOMESH NEGATIVE:-

t) 0'5

12g

"~A

Pb - lO0MESH

~B

B POSITIVE ~- 12g PbOz NEGATIVE:-

21g

Pb

Time / hr

Fig. 6. Discharge curves for a pumped slurry cell containing ISSg of 4.3M H SO . Cell A. contained 12,7g PbO 2 and 12g Pb. Cell B. contained 12g PbO 2 and 2~g ~b.

163

TESTING OF E.V. BATTERIES A System has been developed to test batteries under E.V. service conditions. The test pattern is variable within wide limits of current and time and a typical test pattern including regenerative baking is shown in figure 7.

The test

pattern is repeatedly applied to the test battery with each cycle of the test pattern corresponding to a given amount of charge delivered by the battery. Cycling through the test pattern is continued until the battery voltage reaches a pre-determined limit, whereupon the battery is automatically switched over to charging.

Charging is first carried out at constant current and then, when the

battery voltage begins to rise abruptly, charging is carried out at constant voltage.

At the end of charging the battery is automatically discharged again.

Charge-discharge cycling is continued until the capacity delivered drops below a pre-determined value.

Results obtained under such accelerated test conditions

are shown in figure 8. Experiments carried out to determine the effects of regenerative braking showed that regenerative braking is nearly 98% charge efficient and 88% energy efficient. The energy losses occurred early in the discharge when, because the battery was almost fully charged,

charge acceptance was manifested by large voltage excursions.

Tests carried out on lead-acid batteries modified to permit rapid circulation of the electrode with a total electrolyte volume about double that of the original volume showed that little movement of electrolyte between plates could be produced in this way and there was no effect of circulation on capacity at either high acid concentration (4.3 mol dm -3) or at low acid concentrations

(2 and 1 mol dm-3).

t 8O ~o <

"E

0

D

I

4O

,

DisL

80

6

do

Fig. 7. E.V. test pattern

L_J I

30

50 60 Time ISec

70

80

164 60 -

ANIKA

3

,,.

50 ~- ~o ~ 3o

B

o 20

lo

0

I

I

20

40

I

I

60 Trip

I

80 100 number

I

I

120

1~0

160

Fig. 8. Capacity as a function of number of charge-discharge trips for three leadacid batteries: A, a commercial battery; B, a different make to A; ANIKA 3, a hand pasted battery.

CONCLUSIONS Tests on lead and lead dioxide electrodes indicate the possibility of a high energy density battery using acidic ammonium sulfate electrolyte provided that sufficient protons can be supplied for lead dioxide reduction e.g., by electrolyte or slurry circulation and provided internal resistance can be kept low during discharge e.g., by addition of inert supporting electrolyte. Our understanding of the mechanism of lead sulfate crystallization has not yet advanced to the stage where we know how various electrolytes influence nucleation, crystal growth and crystal dissolution;

this is a serious shortcoming which prevents

a rapid optimization of the electrolyte composition. Tests are now in progress on small-scale versions of batteries made from commercial battery plates using the same cyclic voltammetry techniques as that used for studying the individual electrodes

[3].

Initial results confirm the validity of

this method of evaluating electrode materials. Studies on a lead-acid slurry battery have demonstrated the feasibility of the slurry battery principle.

It remains to optimise this system with respect to

choice of active materials, collector materials, cell design, separator material, particle size, flow rate etc. before the technical and economic feasibility can be demonstrated.

A positive result in this work can be expected to have a dramatic

effect on public acceptance of electric vehicles.

165

ACKNOWLEDGMENTS It is a pleasure to acknowledge support for this work by the South Australian Departments of Transport and of Mines & Energy, and by the Commonwealth Department of National Development & Energy.

Assistance from Lucas Industries and South

Australian Battery Manufacturers is also gratefully acknowledged.

REFERENCES 1 2 3 4 5 6

D.A.J. Rand, d. Power S o u r c e s , 4(1979) 101-143. K.V. Kordesch (Ed.) B a t t e r i e s Vol. 2, Marcel Dekker, 1977, Ch. 2. D.B. Matthews and R. Garrad, Aust. J. Chem., accepted for publication, 1980. S. Fletcher and D.B. Matthews, J. Appl. Electrochem., accepted for publication, 1980. S. Fletcher and D.B. Matthews, J. Appl. Electrochem., accepted for publication, 1980. D.B. Matthews, M.A. Habib and S.P.S. Badwal, Aust. J. Chem., submitted for publication, 1980.