acid batteries

DETERMINATION OF OPTIMAL FORMATION CONDITIONS FOR TUBULAR POSITIVE ELECTRODES LEAD/ACID BATTERIES M. T. LIN, Y. Y. WANG

Department

of Chemical

OF

and C. C. WAN

Engineering, National Tsing Hua University, Hsinchu, Taiwan, R.O.C.

(Received 3 September 1985; in

revisedfirm 30 October 1985)

Almtrmt-The optimum conditions of formation for tubular lead/acid battery were studied by experimental design method. Electrode capacity was found to be maximum at a H,SO, concentration of 1.06 sp. gr., a charge amount of 150% theoretical capacity, a charge time of 36 h at a current density of 0.54 A dme2, and an electrolyte temperature of 50°C. The polymorphism and the composition of various lead compounds in tubular positive electrode were analysed with X-ray diffraction method after soaking, curing, formation discharging and recharging. Finally, the binding energy of Pb. S and 0 elements of tubular positive electrode was also analysed with X-ray photoelectron spectroscopy method.

4 h so that the acid absorption reached about 170 mg H2S04 per g of oxide. The resultant electrode was then cured at 25°C for 5 days. The cured tubular positive electrodes were placed between two pasted flat-plate negative electrodes of similar size with separator in between. The capacity of the negative plates was considerably larger than that of the positive electrode so that the latter controlled the cell capacity. Various conditions of formation were chosen. The concentration of HISOb electrolyte was varied from

INTRQDUCIION Shedding of the positive active material is one of the major drawbacks 6f the traditional pasted-plate lead/acid battery. To alleviate this problem, tubular lead/acid battery was designed. A fabric gauntlet was used to cover the positive electrode and thus lengthen the cycle life of the battery. There have been a number of studies on the preparation of tubular lead/acid battery[l-51. However, little published information is available on the processes of formation of tubular electrodes. Our laboratory[6] has attempted to determine the optimum formation condition by considering four factors: electrolyte concentration, charge amount, charge time (i.e. current density) and temperature. In this work, we improve our results by a statistical approach. Furthermore, qualitative and quantitative analysis of the composition of lead compounds during soaking, curing, formation, discharging and charging with Xray diffraction (XRD) and X-ray photoelectron spectroscopy (ESCA) methods are carried out.

EXPERIMENTAL 1. Tubular electrode preparation

Lead oxide powders (33.9 % free lead) were packed into a plate which consisted of seven parallel, round tubes with one end sealed. A schematic diagram of the tubular positive electrode is shown in Fig. 1. The tube material is of glassfibre braided with the following specifications: length 35 mm, inside diameter 8 mm and pitch 9.7 mm. The spine alloy is a typical antimonial lead with Sb content at 3.5%. Lead oxide powders were filled in dry form into the tubes by vibration method by means of a filling machine designed by Hadi Co. of West Germany. The filled density was measured to be 2.79 g cmwJ. After packing, the other end of the tube was sealed and the electrode was soaked in 1.40 sp. gr. H2SOa at 30°C for 565

Fig. 1. Schematic diagram of the tubular positive electrode.

M. T. LIN, Y. Y. WANG AND C. C. WAN

566

1.04 to 1.08 sp. gr., the charging from 150 % to 350 % over the theoretical capacity, the charging duration from 24 to 48 h and the temperature from 30 to 50°C. After formation, 1.28 sp.gr. H2S04 was used as electrolyte and the cell was charged for 12 h. The cell then discharged at constant current until the voltage fell to 1.70 V. After discharging, the cell was recharged with a charge factor of 120 y/,. 2. X-ray

difiaction

RESULTS 1. Optimization

analysis

photoelectron

A Perkin-Elmer

spectroscopy

PHI

DISCUSSION

offormation

conditions

Electrolyte concentration, charge amount, charge time (i.e. current density) and electrolyte temperature were considered to be four major factors that affect the performance at formation procedure. A statistical approach was adopted to design the experiments for the determination of optimal conditions. In particular, an orthogonal

The composition and polymorphism of the various lead compounds in the positive electrode was determined by XRD method using a cobalt target, a 4” min-’ scanning rate, and a 28 range of l&60”. 3. X-ray

AND

array Lz7 (313) pattern[7]

was applied.

The levels of individual factor were selected and summarized in Table 1. Based on theory, with four experimental factors, there are only three terms of interaction between factors to be determined. We chose: (i) charge amount and charge time, (ii) charge amount and temperature and (iii) charge time and temperature. The results are shown in Table 2. From Table 2, it can be seen that the optimum formation condition is the following: H,SO, concen-

analysis

590 AM ESCA

was used to analyse the binding energy of Pb, S and 0 elements in the positive electrode.

Table 1. The levels of factors Factor charge amount of theoretical capacity ( %) A

charge time (h) B

1

150

24

1.04

30

3 2

350 250

48 36

1.08 1.06

z

Level

electrolyte concentration (SP. 8r. H,SO,) C

electrolyte temperature (“C) D

Table 2. The results of experiment from the pattern Lz7 (3”) Column

1

2

3

4

5

6

7

8

9

A

B

A

A

C

A

A

B

D

;

:

;I

;I

2:

1 1 1 2 2 2 3 3 3 2

1 1 1 2 2 2 3 3 3 3 3 3 1 1 1 2 2 2 2 2 2 3 3 3 1 1 1

1 2 3 1 2 3 1 2 3 2 3 1 2 3 1 2 3 1 3

1 2 3 1 2 3 1 2 3 3 1 2 3 1 2 3 1

1 2 3 2 3 1 3 1 2 1 2 3 2 3

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1.5 16 17 18 19 20 21 22 23 24 25 26 27

150 150 150 150 150 150 150 150 150 250 250 250 250 250 250 250 250 250 350 350 350 350 350 350 350 350 350

24 24 24 z: 36 48 48 48 24 24 24 36 36 36 48 48 48 24 24 24 36 36 36 48 48 48

10

11

12

13

Random

; 3 3 3 1 1 1 3 3 3 1 1 1 2 2 2

1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 1.08 1.04 1.06 I .08 1.04 1.06 1.08

: 3 1 2 3 1 2

: 3 1 2 3 1 2 3 1

: 1 2 1 2 3 2 3 1 3 1 2

order

B

expeZ’men t (

; 30 40

1 2

z 50 30 50 30 40 40 50 30 50 30 40 30 40 50 50 30 40 30 40 50 40 50 30

: 3 1 3 1 2 3 1 2 1 2

Result

AiTkg -‘)

:

1 2

1 2

6 25

71.02 81.97

3 :

3 :

3 :

2 3 1 1 2 3 3 1

2 3 : 3 1 1 2

2 3 1 3 1 2 2 3

22 15 23 18 19 14 7 26 16 21 2:

131.71 73.38 143.24 69.64 46.52 59.52 83.18 67.12 57.59 82.48 65.90 67.87

1 3 :

2 3 1

3 1 2 3

1 2 2 3

If : 10

118.22 65.28 60.85 99.91 71.00

3 1 3 1 2

2 3 3 1 2

1 2 2 3 1

3 1 1 2 3

17 11 12 2 4

73.12 75.99 82.44 75.86 50.05

: 3

: 1

1 :

3 :

245 27

55.81 92.48 117.83

Optimal formation

tration is 1.06 sp. gr., charge amount is 150 y0 theoretical capacity, charge time is 36 h (i.e. current density 0.54 Adm-‘) and electrolyte temperature is 50°C. The resultant discharge capacity is 143.2 Ah kg- ‘. This result is far better than that obtained by varying a single factor at a time which shows a maximal value at 92.4 Ah kg- r. From Table 2, we can obtain the impact of those factors on formation process. Let Xi be the discharge capacity of the ith experiment, then

y xi = 2140.0 i-l and the average capacity is x = “x1 27

= 79.3

s,=z(x,-x)z

then

S,=

(3)

> 2 = 169611.6

ZZXf -CF

(4)

= 15141.8.

(5)

Let Ak be the sum of the discharge capacity in

Table 3. Analysis of variable factors Factor

Freedom

A B C D AxB

AxD

sT

FJ

Xi

5

Xi

(7)

F X,. i=1

(8)

i=l

Aa =

308.3 309.5 631.3 50.0 649.1

0.382 0.383 0.782 0.062 0.803

541.4 807.8

0.670

t 26

2165.6 4846.9 15141.8

733.7

(9) S,, S,, S,, S, x B, S, x D and S,,, obtained. Let s,=sT-s,-s,-sc-sI,-ss,~a-s,,o-ss,,D.

can be similarly

(10) All the results were summarized in Table 3. It was observed that as far as the individual factor concerns, the electrolyte concentration is the most important factor during formation. In other words, electrode capacity is most sensitive to the change of the electrolyte concentration during formation. If we consider the impact of combined factors, the charge amount and the temperature is the most critical during formation. analysis

The phase composition of tubular electrodes can be

616.6 619.1 1261.6 100.0 2596.2

2534.8

and

5 i=l

AZ =

2. X-ray dijjbction

mS*

2 2 2 2 4

4

BxD &

S

AI=

1,2

(2)

’

and the correction factor CF is C jiXi 27

experiments where level of factor A was k, k = 3 then

(1)

Let S, be the variance of the discharge capacities then

CF =

567

conditions of lead/acid batteries

0.908

l mS = S/freedom. t F, = mS/S,.

determined using XRD method developed by Hill[8]. The results are summarized in Table 4 and Figs 2-4. Figure 2 shows that the main component of the positive electrode after soaking is PbSO4 (84.6 wt %). After curing, almost all the remaining PbO has been transformed into PbS04 and basic lead sulphate. Figure 3 shows that after formation, the content of PbOl (a- + #l-PbO,) in the interior (close to the grid of the positive electrode) is more than that in the exterior (close to the tube). In addition, this is also true for /I-PbOz. It is probably because the local current

Table 4. QuantitativeXRD analysisof the crystallinephases of the tubular positiveelectrode (in wt %) Process After soakiig After curing After formation (interior) After formation (exterior) After discharging After 1st recharging (interior) After 1st recharging (exterior)

PbS04

PbsO*

84.6 85.3 35.6

1.6 1.3

PbO*PbSO, 6.9 7.6 12.9

a-ZPbO.PbSO. 5.4 5.8

a-PbO + &PbO

a-PbO*

B-Pm,

1.5 5.9

45.6

73.5

7.8

2.8

16.0

82.1 28.5

9.4 11.2

4.2 4.9

4.3 55.6

59.8

14.7

25

23.0

M. T. LIN, Y. Y. WANG AND C. C. WAN

568

Fig. 2. X-ray diffraction

Fig. 3.

X-ray diffraction

pattern of the tubular positive electrode,

pattern

of the tubular positive (b) interior.

density is higher in the interior. Consequently, more PbOz was formed and the content of &PbOp is also higher[9]. This is different from the results of traditional pasted-plate lead/acid battery. According to Hill’s report[8], the content of a-Pb02 in the interior could reach 47 wt % after formation in a pasted-plate electrode. This is because H2S04 diffuses into the

(a) after soaking

electrode

and (b) after curing.

after formation.

(a) exterior

and

interior of plate with difficulty. Consequently, more aPbOs was formed[9]. On the contrary, the effect of current density on formation is more significant than that of electrolyte diffusion in tubular lead/acid battery which has a much thicker electrode. Figure 4 shows that the content of &PbO, increases in both interior and exterior after discharging/charging

Optimal formation conditions of lead/acid batteries

(b)____

interior

569

I r,

!l

Fig. 4. X-ray diirsction pattern of the tubular positive electrode after recharging, (a) exterior and (b) interior.

cycle. This is because some PbS04 is continuously transformed into /%-PbOr during discharging/charging cycle. 3. X-ray photoelectron

spectroscopy

analysis

Figures 5-7 show the ESCA spectrum of the positive electrode after curing, formation and discharging. The binding energy Pb, S and 0 elements were obtained and summarized in Table 5. Figure 5 shows that the binding energy of Pb (4f,,,) and Pb (4f,,,) after

(al-after .(b)_-ICI----after

formation was lowest. Thus the Pb4* (PbO,) produced after formation has lower binding energy than those Pb*+ (PbSO*) produced after curing and discharging. Note that the binding energy Pbs + (PbSOI) after curing and discharging were identical. This implies that the electronic structure of Pb2+ in PbS04 does not change during electrochemical reaction. Figure 6 shows that the binding energy of 0 (1s) after formation is also lower than those after curing and discharging. Note also that the width of 0 (1s)

oueinq

after

formation discharging

Fig 5. X-ray photoelectronspectrumof Pb, (a) after curing, (b) atIer formation and (c) after discharging.

M. T. LIN, Y. Y. WANG AND C. C. WAN

570

I la) ----after

curing

-(b) -----after formation CC) ----after discharging

/1 i i

Fig. 6. X-ray photoelectron spectrum of 0. (a) after curing, (b) after formation and (c) after discharging.

Binding

energy/e”

Fig. 7. X-ray photoelectron spectrum of S, (a) after curing, (b) after formation and (c) after discharging.

Table 5. Experimental binding energy (eV) of various elements

Sampk After curing After formation After discharginp.

PWf,,,)

PWf,,,)

138.8 136.6 139.0

143.8 141.4 143.8

Ws)

g (2P)

532.0 529.0 532.2

168.4 0 168.4

All values are referred to the binding energy of carbon in

graphite = 284.3 eV.

peak was broadened after formation. Similar phenomenum was observed by Kim et aL[lO] when they analysed the absorption of Hz0 on PbOl with ESCA method. Hence, it is probable that when PbOz was

electrochemically formed, Hz0 would be incorporated into the lattice of PbOl. Figure 7 shows that there is no obvious peak of S (2~) after formation. This implies that the main

Optimal formation

conditions

of the positive electrode was PbOl. Furthermore, the binding energy of S (2~) remained unchanged after curing and discharging. This shows that theelectronic structure of S in ?bsO, does not change during electrochemical reaction.

component

of Lead/acid batteries

571

3. With ESCA analysis, we found that the binding energy of Pb and 0 in positive electrode decreases and S peak is absent after formation.

REFERENCES CONCLUSIONS

1. E. Sundberg,

Several conclusions can be reached for tubular lead/acid battery in our study. 1. We found that, by experimental design, the optimum

formation

conditions

were

1.06 sp.gr.

HtSOb, charge amount of 150% theoretical capacity, 36 h charge time (i.e. 0.54 A dm-” current density) and 50°C electrolyte temperature. 2. We found that, by XRD analysis,PbSO., is the main component of a tubular positive electrode during soaking and curing. There are also a small quantity of PbO and basic lead sulphate. The content of PbOz formed in the interior was also found far greater than that in the exterior after formation. Most of the PbOz was in B-structure.

Pergamon

Proc. Second Int. Conj Press, Oxford, (1967).

2. E. Voss, J. Power Sources 7, 343 (1982).

Lead, p. 227.

3. T. Rogatchev. G. Papazov and D. Pavlov, J. Power Sources 10, 291 (1983). 4. A. S. M. Lindholm, J. Power Sources 10, 71 (1983). 5. J. M. Stevenson and A. T. Kuhn, J. Power Sources 8,385 (1982). 6. H. W. Yane. Y. Y. Wana and C. C. Wan. J. Power Sources 7 15,45 (198j). Taguchi Kenichi, The Design of Experiments. Marnzen, . Tokyo (1975). 8. R. J. Hill, J. Power Sources 9, 55 (1983). Q D. Pavlov, G. Papazov and V. Iliev, J. electrochem. Sot. -. 119, 8 (1972). C/tern. 10. K. S. Kim, T. J. O’Leary and N. Winograd, Adyt. 45, 2214 (1973).