A Conductive Additive to Enhance Formation of a Lead/Acid Battery

J. E/ectrochem. Soc., Vol. 139, No. 4, Apr i l 1992 �9 The Electrochemical Society, Inc. L41

1.2

1.0

O CdlSe

" 0.8

o

E 2 ~: 0.4 "6 .9 "~ 0.2

0.0 0 2 4 6 8 12

Sputtering Time (min)

Fig. 2. A representative XPS depth profile of CdSe-ZnSe "superlattice." This thin film was electrosynthesized via a flow technique as described in the text, The XPS data were acquired with monochromatic AIK~ radiation (1486.6 eV) and a 4 keV Ar + ion sputter gun.

(5) yield a modulation period of ca. 10 nm for the particular sample illustrated in Fig. 2.

In summary, we have shown how stoichiometric Group II-Vl compound semiconductor thin films such as CdSe and ZnSe as well as composition-modulated architectures comprising these materials, may be electrosynthesized by using a flow technique. In a broader perspective, it is our opinion that flow strategies are an elegant and versatile alternative to the sta- tionary (beaker) approach for synthetic problems, and this

trend is not unlike that which has been much exploited in re- cent years in analytical (e.g., flow injection analysis) applica- tions (7).

Optical and other characterization of the materials considered in this study is continuing in our laboratory.

A c k n o w l e d g m e n t s This research was supported, in part, by the Texas Higher

Education Coordinating Board (Advanced Technology Pro- gram) and the Research Enhancement Program, The Univer- sity of Texas at Arlington. The authors also thank Vepa Krishna for technical assistance with the assembly of the flow electro- synthesis system.

Manuscript received Nov. 27, 1991.

The University of Texas at Arlington assisted in meeting the publication costs of this article.

REFERENCES 1. Review: K. Rajeshwar, Adv. Mater., 4, 23 (1992). 2. S. D. Lester and B. G. Streetman, Superlattices and Micro-

structures, 2, 33 (1986). 3. V. Krishnan, D. Ham, K. K. Mishra and K. Rajeshwar, This

Journal, 139, 23 (1992). 4. (a) K. K. Mishra and K. Rajeshwar, J. Etectroanal. Chem.,

273, 169 (1989); (b) C. Wei and K. Rajeshwar, Unpublished data (1991).

5. N. Kh. Abrikosov, V. F. Bankina, L V. Paretskaya, L. E. Shel- imova, and E.V. Shudnova, "Semiconducting II-Vl, IV-Vl and V-VI Compounds," A. Tybalewicz, Engl. Translation Editor, Plenum Press, New York (1969).

6. M. Skyllas-Kazacos and B. Miller, This Journal, 127, 869 (1980).

7. B. R. Kowalski, J. Ruzicka, and G. D. Christian, Trends in Anal. Chem., 9, 8 (1990).

A Conductive Additive to Enhance Formation of a Lead/Acid Battery

W e n - H o n g K a o * and Kathryn R. Bul lock *,1

Advanced Battery Research, Johnson Controls Battery Group, Inc., Milwaukee, Wisconsin 53201

ABSTRACT

Barium metaplumbate, a conductive ceramic having the perovskite structure, is relatively stable in sulfuric acid. Addition of this material in positive plates in a lead/acid battery significantly improves formation efficiency. The formation mechanism is changed when the conductive particles are dispersed in the plate. Formation not only proceeds from the grid toward the center of the pellet but also slowly around the conductive particles in the plate. The conductive paths of PbO2 grow and make connection with each other during formation to establish a network which further facilitates the formation.

Before formation, lead/acid battery plates are composed of mixtures of nonconductive or lowly conductive lead (11) compounds. During formation, the material next to the grid is first converted to conductive lead or lead dioxide. The formation then gradually proceeds toward the center of the pellet (1). The formation process is slow and the efficiency is low because of the nonconductive nature of the lead (11) compounds.

To overcome the problem in the formation of a lead/acid battery, particularly the positive plates, conductive additives to the plates have been suggested (2-10). An ideal additive should have good conductivity, stability in the lead/acid environment, and long range order to reduce the amount used. The suggested conductive additives, for example, include carbon (2, 3), doped tin dioxide (4), and lead dioxide (5-9) by either direct addition to the paste (5) or formation from a chemical oxida- tion of the lead (11) compounds with ozone (6, 7), persulfates (8), or peroxides (9).

Barium metaplumbate, a mixed oxide having the perovskite structure, exhibits metallic conductivity at room temperature (11-13) and is reported relatively stable in dilute sulfuric acid

* Elect rochemical Society Act ive Member. 1 Present address: AT&T Bell Laboratories, Mesquite, Texas 75149.

(14). These properties make this compound an attractive additive in a lead/acid battery for formation enhancement. In this paper, the behavior and effects of barium metaplumbate (10) on the formation of positive plates in a lead/acid battery are described.

E x p e r i m e n t a l Barium metaplumbate was synthesized using the process

reported by Weiss (15). An equal-molar homogeneous mixture of reagent grade barium oxide and lead nitrate was heated from room temperature to 750~ in 3 h under a stream of oxygen. The temperature was maintained at 750~ for additional 5 h before cooling down. The material thus synthesized showed a conductivity around 1000 ~-~ �9 cm -~ and was iden- tified as BaPbO3 by x-ray diffraction analysis (XRD). Barium metaplumbate was ground and screened through a 325 mesh sieve before adding to the paste. Barium metaplumbate can also be synthesized from a combination of barium oxides and other oxygen-containing lead compounds under air or oxygen (13, 16-20).

The stability of barium metaplumbate was measured by soaking the material in sulfuric acid of various concentrations for given periods of time. The solid was then separated,

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L42 J. Electrochem. Soc., Vol. 139, No. 4, Apri l 1992 �9 The Electrochemical Society, Inc.

washed, dried, and analyzed using XRD. The concentration of barium metalplumbate after soak was calculated from the XRD pattern based on the "degree of resemblance," a technique reported in the literature (21, 22). The XRD peak of BaPbO3 used for this purpose corresponds to the diffraction from the (110) plane with a d spacing about 3.00 A. The pseudo-first order rate constant for decomposition of barium metaplumbate in sulfuric acid was then determined from the plot of the loga- rithm of inverse concentration of barium metaplumbate versus soak time. The activation energy was determined from the Ar- rhenius plot, i.e., the rate constant versus the reciprocal of absolute temperature.

The mix of positive active material was prepared using orthorhombic lead oxide, glassy fiber, and 5.8M sulfuric acid. The weight ratio of lead oxide to acid was about 9. The content of fiber was less than 1%. The loading of the conductive additive was 10% of the paste weight or less. The mix was then hand pasted onto a lead alloy grid about 6 cm long, 4 cm wide, and 0.2 cm thick. The plates were cured at 50~ with 95% relative

humidity for 24 h. Each positive plate was sandwiched by two conventional negative plates with polyethylene separators im- posed in between. The plates were formed in a flooded cell containing 3.0M sulfuric acid at an 8 h rate to 200% of its theoretical capacity. The process of the formation was moni- tored with a video camera. The first reserve capacity in 4.2M sulfuric acid at a current density about 13.3 mA/cm 2 was then measured. The material utilization was defined to be the ratio of the measured reserve capacity to the theoretical capacity of the plate. Chemical contents in the formed plates were analyzed using the conventional methods.

Resul ts and D iscuss ion Barium metaplumbate can be slowly attacked by dilute sul-

furic acid. The first-order rate constant for decomposition of barium metaplumbate in dilute sulfuric acid was determined to be k = 2.6 x 101~[H +] exp (-EdRT) where E a is 31.6 kcal, R is the gas constant, and T is the absolute temperature. The half- life of this material in sulfuric acid of gravity 1.265 at 20~ is es-

Fig. 1. Cross section of positive o-PbO plates without (A to O) and with (E to H) 10% BaPbO3 after (A, E) 0%, (B) 70%, (F) 125%, (C, G) 144%, (H) 169%, and (D) 200% of theoretical capacity is passed. The grid is on the left hand side of the graphs. The spherical objects are gas bubbles.



J. Electrochem. Soc., Vol. 139, No. 4, April 1992 �9 The Electrochemical Society, Inc. L43

Table I. Characterization of formed o-PbO plates.

Without BaPbO3 With BaPbO3

I. Chemical contents (%): PbO2 43.9 76.0 PbO 16.7 16.5 PbSO4 38.4 4.4

II. First reserve capacity: Material utilization (%) 6.0 43.3

Experimental error: -+2% absolute.

Table II. Effect of BaPb03 loading on material utilization.

BaPbO3 loading (%) Material utilization (%)

0.0 6.0 0.1 8.7 0.5 22.6 1.0 32.7 2.5 37.4 5.0 41.1

10.0 43.3

Experimental error: +-2% absolute.

timated from the rate constant to be 3.4 years, which is compa- rable to the service life of a starting/lighting/ignition (SLI) lead/ acid battery. This material is therefore considered stable in the lead/acid environment.

Orthorhombic lead oxide is difficult to form. We chose this oxide because any formation enhancement by the conductive additive would be more pronounced. The contents of the plates with and without the conductive additive after formation with 200% theoretical input capacity are listed in Table I. The formation efficiency is significantly enhanced by the additive as indicated by the higher PbO2 content. The formed plates were discharged at a current density about 13.3 mA/cm 2, which is equivalent to a 2 h rate, to the 1.75 cutoff voltage. The re- sults, summarized as the material utilization in the first reserve capacity, are also listed in Table I. The material utilization of the plate with BaPbO3 is the same as that of a completely formed conventional positive plate. Without BaPbO3, the material utilization is only 6%.

The formation mechanisms for the plates with and without BaPbO3 appear to be different. As shown in Fig. 1, the formation of a plate without the conductive additive proceeds from the grid toward the center of the pellet (Fig. 1A to 1D), consist- ent with the literature (1). With the conductive particles dispersed in the plate (Fig. 1E), long range conductive paths are apparently established. This would allow formation to proceed not only from the grid but also slowly around the conductive particles inside the plate (Fig. 1F). As the conductive paths of PbO2 grow and make connection with each other (Fig. 1G), a conducting network is formed which further facilitates the formation (Fig. 1H). The plate with the barium metaplumbate additive appears to be completely formed after the passage of about 180% of the theoretical plate capacity.

The effect of loading level of barium metaplumbate in a positive plate on the formation efficiency is shown in Table I1. One can see that significant formation enhancement starts at a loading level as low as 0.5% by weight. The formation enhancement increases with BaPbO3 loading and approaches to a plateau. Beyond about 7% by weight, additional load of BaPbO~ does not further improve the formation efficiency.

The products from decomposition of BaPbO3 in sulfuric acid include BaSO4 and PbO~. Barium sulfate in a positive plate is

known to facilitate nucleation of lead sulfate and to alter the morphology of the plate. We have found (23) that the cycle life of a positive plate would be shortened if the content of BaSO4 becomes greater than 0.3% by weight. We believe, based on the half-life and fraction of barium in the additive, that BaSO4 would not reach the detrimental level during the service life of an SLI lead/acid battery if the loading of barium metaplumbate is limited to about 1% by weight.

Conc lus ion The conductive barium metaplumbate is relatively stable in

dilute sulfuric acid. Addition of this material to a lead/acid positive plate significantly improves the efficiency of formation. The low costs of the raw materials, ease of synthesis, and re- quired low loading level in a plate, e.g., 0.5% by weight, make barium metaplumbate an attractive additive to a lead/acid battery.

A c k n o w l e d g m e n t Assistance of P. Patel in material and plate preparation, J.

Dickson in video tape recording, B. Sunde in image transfer, and the Analytical Service Department of Johnson Controls Battery Group in chemical analyses is gratefully acknowl- edged.

Manuscript submitted Dec. 13, 1991; revised manuscript received Jan. 28, 1992.

Johnson Controls, Inc. assisted in meeting the publication costs of this article.

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A Conductive Additive to Enhance Formation of a Lead/Acid Battery