Increase in Positive Active Material Utilization in Lead-Acid BatteriesSimon McAllister, Rubha Ponraj, I. Francis Cheng, Dean B. Edwards

Department of Chemistry, University of Idaho, Moscow, ID 83844-2343Email: ifcheng@uidaho.edu, Tel: 208-885-6387

Background% change in utilization at low rate discharge

(3% and 5% loadings)

-10

-5

0

5

10

15

20-30 30-53 53-74 74-90

Size (µm)

% c

han

ge

in u

tiliz

atio

n

3% loadings

5% loadings

% change in utilization at fast rate discharge (3% and 5% loadings)

-4

-20

24

68

1012

14

20-30 30-53 53-74 74-90

Size (µm)

% c

han

ge

in u

tiliz

atio

n

3% loadings

5% loadins

ExperimentalPreparation of the Positive plate• Lead Strips - Pb and 4-6% Sb with Teflon ring attached. • Paste contains PbO, 0.5% Dynel fibers, additives for total mass of 10 g, 1 ml of 1.4 specific gravity H2SO4 1.2 ml of DI water• Teflon ring is filled with paste• Hydroset for 24 hours to convert Pb0 to PbII

• Dried overnight

ResultsThe lead-acid battery is a highly successful rechargeable electrochemical storage system. Despite its design dating from Plante in 1859 the system can benefit from new approaches. The basic electrode reactions during charge and discharge of a lead-acid battery with 1.3 specific gravity H2SO4 are shown in reactions 1-3 [1,2]

PurposeThe performance of these batteries is limited by the positive plate reaction due to low PbO2 positive active material (PAM) utilization which is around 30% or less at the 1 hour rate. Utilization is the amount of charge we obtain at a certain discharge rate relative to the theoretical charge. Reasons for low utilization include slow diffusion of electrolyte into the interior, and electrical isolation from the buildup of PbSO4 [3,4].

To improve PAM utilization [4-8]:1) Porosity and conductivity of the PAM have to be increased.2) The unreacted material has to be replaced with a filler.

Porosity of PAM is increased by the addition of highly porous additives thereby enhancing the diffusion of acid to the interior of active material and providing fine local reservoirs of acid within porous particles.

Characteristics of Ideal AdditivesLead-acid batteries have a harsh acid and oxidative environment, therefore the following characteristics have to be considered while selecting additives [4]:• Increase utilization• Chemically and oxidatively stable • Good adhesion to active material• Increase PAM utilization without affecting cycle life• Low cost • Light weight

Diatoms are our choice as an additive because of its porosity, abundance and stability towards battery conditions. In this work, we added size sorted diatoms (Melosira) at different loadings and studied their effect on the positive plate performance.

Figure 3 - Teflon ring filled with paste and hydroset. Internal volume of ring is 0.24 ml

Formation and Conditioning

Positive plate

polyethylene separator b/w negative and positive plates

Glass mat with 90% porosity

Figure 5 - Formational cell (cross sectional view)

• Formation step with 1.1 specific gravity H2SO4 to convert Pb0, PbO, and PbSO4 to PbO2

• Theoretical capacity - 0.2241 Ah/g• Fast charge with constant current to obtain 100% theoretical capacity in 24 hrs• Slow charge with constant current (half of fast charge) applied for 12 hrs to reach 125% theoretical.

Conditioning• Acid is changed to 1.3 specific gravity H2SO4 and plates are cycled individually 4 to 5 times in a cylindrical cell.• Discharged at 10 mA g-1

• Charged at fast rate to 125% previous discharge capacity

working electrode (positive plate)

Counter electrode (Pt coil)

reference electrode (Ag/AgCl)

Figure 6 – Picture of cylindrical cell

Performance measurementsFour size fractions, 20-30 µm, 30-53 µm, 53-74 µm and 74-90 µm, at 3 wt% and 5 wt% were tested. 4 plates in each group were pasted.Capacity measurements are taken at a 50 mA cm-2 discharge and a 10 mA cm-2 discharge. Plates are cycled at each rate until the capacity reaches a maximum.

Figure 7 - Performance changes due to the diatoms. Specific capacity is the amount of charge that can be produced for a given mass, in mAh g-1.

% change in specific capacity at fast rate discharge (3% and 5% loadings)

-10-8-6-4-202468

1012

20-30 30-53 53-74 74-90

Size (µm)

% c

han

ge

in s

pec

ific

cap

acit

y

3% loadings

5% loadings

% change in specific capacity at low rate discharge (3% and 5%

loadings)

-12

-10

-8

-6

-4

-2

0

2

4

6

8

20-30 30-53 53-74 74-90

Size (µm)

% c

han

ge

in s

pec

ific

ca

pac

ity

3% loadings

5% loadings

• The addition of 3% diatoms of 53-74 µm exhibited the best performance and they are believed to favor acid diffusion inside the active mass. • The scanning electron micrographs (Figure 8a, b) show the porous structure of diatoms. • The SEM of diatoms recovered from the active material after the performance tests (Figure 8d) proves that diatoms survive in the battery environment.

Conclusion• Diatoms are an inexpensive filler material that increases positive active material utilization by replacing unreacted active material, while maintaining pores for diffusion. • Utilization increases by over 12.7% at a fast rate discharge of 50 mA cm-2 with a corresponding 9.3% increase in specific capacity.

Future work• Duplicate diatoms effect in full positive plate.• Add conductive additives to increase electron flow

References1. H. Bode, Lead-Acid Batteries, translated by R.J. Brodd and K.V. Kordesch, 1997,

page 4.2. S.V. Baker, P.T. Moseley and A.D. Turner, J. Power Sources, 27 (1989) 127.3. P.T.Moseley, J. Power Sources, 64 (1997) 47.4. K.McGregor, J. Power Sources, 59 (1996) 31. 5. H.Dietz, J.Garche, K.Weisner, J. Power Sources, 14 (1985) 305. 6. D.Berndt, Maintenance-Free Batteries, second edition 1997, page 103.7. P.W. Appel, D.B. Edwards, J. Power Sources, 55 (1995) 81.8. D.B. Edwards, V.S. Srikanth, J. Power Sources, 34 (1991) 217.

Acknowledgement Office of Naval Research Award Number:  N00014-04-1-0612, Department of Chemistry, Microelectronics Research and Communications Institute (MRCI), Dr. and Mrs. Renfrew.

negative plate in between separators

Figure 4 - Formation cell (side view)

PbO2 + HSO4- + 3H+ + 2e- discharge

charge PbSO4 + 2H2O Eo = 1.805V (1)

Pb(s) + HSO4- discharge

charge PbSO4 + H+ + 2e-

Positive plate:

Negative Plate:

Eo = -0.340V (2)

Total

PbO2 + Pb(s) + 2H2SO4dischargecharge 2PbSO4 + 2H2O Eo = 2.145V (3)

Figure 2 – Diagram of porous lead dioxide with diffusion of HSO4¯

Figure 8 - Scanning electron micrograph of diatoms of different sizes: a) 20-30 µm b) 53-74 µm c) >90 µmd) Diatoms recovered from active material after the performance tests.

Figure 1 – Schematic of lead-acid battery

Grid

PbO2 PbSO4 layer

HSO4¯e-

HSO4¯ diffusion

enhanced by additives

Electrically isolated PbO2