Thermal Decomposition Mechanism of Formed and Cycled
Lead Dioxide Electrodes and Its Relationship to
Capacity Loss and Battery Failure
S. M. Caulder*
International Lead Zinc Research Organization Research Associate at the Naval Research Laboratory,
Washington, D.C. 20375
and A. C. Simon*
Electrochemistry Branch, Naval Research Laboratory, Washington, D. C. 20375
ABSTRACT
The structural changes accompanying capacity loss in the PbO2/PbSO4
electrode were followed using differential t h e r m a l analysis. The t h e r m a l decomposition mechanism of formed plates was found to differ, depending on the
method of manufacture. All cycled plates, however, gave the same decomposition mechanism after a few cycles. The m a j o r changes i n the DTA curves,
as the positive electrode was cycled to failure, was the gradual disappearance
of the exothermic peak at 200*C and the endothermic peak at 358~ It is
believed that these peaks are associated with an electrochemically active
amorphous form of PbO2. As the electrochemically active PbOs is cycled to
failure it is converted to an electrochemically inactive form of PbO2. This
latter form of PbO2 gives DTA results similar to those obtained on reagent
PbOz. The continual conversion of electrochemically active PbO2 to the electrochemically inactive PbO2 is one of the m a j o r factors that accounts for the
loss i n battery capacity and ultimate failure.
The structure of the lead dioxide electrode after
formation and cycling has been studied by optical (1)
and electron microscopy (2, 3), n e u t r o n and x - r a y
diffraction (4), nuclear magnetic resonance (5), and by
thermal analysis (6). The thermal analysis study was
limited to one type of formed electrode and to an electrode with only a few cycles. Previous investigations
(1-3, 7) have shown that parameters such as grid alloy, oxide blend, and curing process affect the microstructure of formed lead dioxide electrodes and subsequently, the initial part of the life cycle.
The t h e r m a l decomposition of active material obtained from the P b O J P b S O 4 electrode involves the
nucleation of PbOr compounds on the PbO9_ active
material particles. These nucleation sites m a y be the
same as the nucleation sites involved during the electrochemical oxidation and reduction of the P b O J
PbSO~ electrode. No information is available regarding
the PbO2 thermal decomposition mechanism of formed
electrodes with respect to their method of preparation
or microstructure. Information is also lacking as to
w h e t h e r the mechanism is altered along with the structural changes the PbO2/PbSO4 electrode undergoes
during subsequent reduction and reoxidation.
The purpose of this investigation was to determine
whether the method of electrode preparation and subsequent cycling would influence the thermal characteristics of the PbO2 active material and, if so, how
the structural changes of the active material are related to battery life and failure mechanism. The thermal results of this paper were correlated with results
obtained from optical microscopy, mass spectrometry,
x - r a y diffraction, and nuclear magnetic resonance.
Experimental Method
The four series of commercial electrodes used in this
investigation were selected on the basis of their similarity in cycling routines, similarity in Foints at which
oxidized and reduced samples were removed from
9 Electrochemical
Society
Active
Member.
K e y words: differential thermal analysis, lead dioxide electrode.
lead dioxide, electrochemically inactive PbOz. capacity loss. battery
failure.
cycling, and the dissimilarity in appearance of the asformed active material, as observed by optical microscopy. The last criterion was used since it was anticipated that these electrodes would give different
microstructures upon cycling. The plates that were
used were obtained from four different manufacturers,
two in the United States and two overseas. Two of the
series were prepared in the laboratory of the concerned companies, while the others were taken from
production and routinely plant cycled. The active
material of all plates were pasted on P b - S b alloy grids.
The DTA, TGA, NMR, x - r a y diffraction, and mass
sl:ectroscopy samples were obtained by punching out
sections of positive active m a t e r i a l and then grinding
with a mortar and pestle. Additional sample sections
were cut from the electrodes, impregnated with a catalyzed polyester resin, and examined microscopically
(1).
Reference samples of a-PbOz were prepared chemically and electrochemically using the methods of
Angstadt (8) and Bode (9). Samples of reagent fl-PbO2
were obtained from various chemical manufacturers.
The thermal decomposition e q u i p m e n t utilized in
this investigation was a du Pont Model 900 differential
thermal analyzer and a Model 950 thermogravimetric
analyzer. The DTA samples (25 rag) were placed in 4
m m quartz sample holders. No diluent was added to
the samples. A t h e r m a l l y inert reference sample, A12Oa,
was placed in two adjacent quartz holders. ChromelAlumel thermocouples were placed in the center of the
sample and reference materials. Two of the theiTnocouples measured the t e m p e r a t u r e difference (At) b e tween sample and reference, while the third measured
the t e m p e r a t u r e of the heating block. The heating
block assembly was then covered by a t h e r m a l shield.
The entire sample cell assembly was enclosed in a bell
jar. Careful control of parameters such as sample
weight, particle size, sample assembly geometry, and
heating rate was maintained.
Prior to r u n n i n g the PbO2 samples the DTA apparatus was calibrated by m e a s u r i n g the m e l t i n g and
freezing points of pure samples of AgC1, Ag2SO4, and
1546
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Vo/. I2I, No. 12
1547
THERMAL DECOMPOSITION OF ELECTRODES
Pb. A h e a t i n g rate of I5~ rain -1 i n stationary air,
using 4 m m tubes, was used on all subsequent runs.
Isothermal decomposition products were obtained b y
heating PbO2 samples in a l u m i n u m blocks at 225 ~
315 ~ and 355~ for v a r y i n g periods of time.
X - r a y diffraction patterns were obtained using a
Norelco diffractometer equipped with a scintillation
counter. Nickel-filtered copper radiation was used.
The decomposition products were identified b y their
diffraction patterns.
I n the thermogravimetric experiments a sample of
5-25 mg was placed i n a p l a t i n u m weighing boat. The
boat was attached to the balance arm incorporated
w i t h i n the i n s t r u m e n t . The sample was heated at 15~
rain -1 i n a nitrogen atmosphere. A C h r o m e l - A l u m e l
thermocouple approximately 4 m m above the sample
measured the t e m p e r a t u r e of the e n v i r o n m e n t .
SAMPLE: REFERENCE SAMPLES OF (~ AND /~-PbOz
(~) u-PbO 2 (persulfote oxidation)
(~) c~-PbO z (electrochemical deposition - I00 plane}
(E) ~ - P b 0 2 (reagent)
(~ u-PbO 2 (electrochernicu~ deposition)
b- " ~
,o~~
1~7o
34
t
~
4851 j
Thermal Analysis Results
/
;/
]
C
Basis of the problem.--Lead dioxide contains sufficient adsorbed water, as well as lattice w a t e r a n d / o r
h y d r o x y l ions, to provide for a Pb: O ratio of 1: 2. Upon
heating, PbO2 loses 02 and H20 down to a Pb:O ratio
of about 1.80 before experiencing a phase change. Bet w e e n PbOi.90 and Pb304 a n e a r l y continuous series of
nonstoichiometric lead oxides with decreasing oxygen
content have been reported (10-14).
Recent investigations (15-17) on the t h e r m a l properties of the l e a d - o x y g e n system have shown that only
two distinct oxide phases exist b e t w e e n PbO1.9o and
Pb304. These i n t e r m e d i a t e oxides, designated a-PbOx
and /~-PbOx by BystrSm (10), have very similar crystal structures. The crystallographic system to which
these oxides belong is still subject to controversy. The
P b : O ratio over which a-PbOx exists is generally accepted as 1.60-1.51 and that for /~-PbOx as 1.50-1.44.
These oxides are based on a n oxygen deficient metal
fluoride type structure containing both divalent and
t e t r a v a l e n t lead ions. It is the reordering b e t w e e n the
divalent and t e t r a v a l e n t lead ions and the oxygen lattice vacancies that account for these i n t e r m e d i a t e
oxides. This ordering of the lattice with the evolution
of O2 and H20 is accompanied by an enthalpy change,
as shown on the DTA curves. Additional evidence for
this reordering is obtained from x - r a y diffraction patterns which show a shifting of m a j o r lines, accompanied b y intensity changes. T h e r m a l gravimetric
analysis also shows weight plateaus for these i n t e r mediate oxides.
Certain e x p e r i m e n t a l parameters m u s t be carefully
controlled to obtain reproducible t h e r m a l results, since
m a x i m u m peak temperatures are not solely dependent
on the decomposition reaction. Careful control of parameters such as sample weight, particle size, sample
assembly geometry, a n d heating rate are all important.
The effect of heating rate and sample size, for example,
have been summarized in Table I. The DTA results
were obtained with chemically prepared PbO2.
Thermal analysis of reference samples of ~- and 8PbOz.--The positive electrodes used in these experiments contained both a- and /~-PbO2 so that it was
necessary to subject reference samples of these two
allomorphs to t h e r m a l analysis in order to obtain reference DTA curves. The DTA curves, x - r a y analysis,
and mass spectroscopy results, obtained from the reference samples, were used to interpret the DTA results obtained from the positive plate active material.
398~
0
I
I00
I
200
|
300
I
400
T.~
I 529~ 1632~
I
500
600
700
800
Fig. 1. DTA curves of reference samples of a- and ~-Pb02.
Curve o, a - P b 0 2 prepared by ammonium persulfate oxidation of
lead acetate; curve b, a - P h 0 2 prepared by electrochemical deposition (preferred orientation); curve c, General Chemical Corporation reagent p-Pb02; curve d, ~-PbO= prepared by electrochemical
deposition.
The DTA curves of the =-PbO2 prepared by amm o n i u m persulfate oxidation of ammoniacal lead acetate (8) and b y electrochemical oxidation of an alkaline solution of Pb(NO~)2 (9) are given i n Fig. 1. Also
shown is the curve for General Chemicals reagent t~PbO2. The x - r a y analysis of the decomposition reaction intermediates, a-PbOx and ~-PbOz, are s u m m a r ized below in Table II and III. These results are compared with those of Bystrom (10), Weiss (12), and
Angstadt (8).
The chemically prepared a-PbO2 gave a broad exothermic peak at 187~ and a series of endothermic
peaks at 398 ~ 460 ~ 529 ~ and 595~ These peaks were
associated with the following decomposition reactions
187 ~
~ ~-PbOe ordered
398 ~
a-PbO2
~ a-PbO~
460 ~
=-PbOz
~ ~-PbOx + Pb304
529 ~
/9-PbOx -{- Pb304
4 PbO (tetragonal)
a-PbO2[OH] disordered
595 ~
PbO (tetragonal)
~ PbO (orthorhombic)
Angstadt et al. (8) in their thermal studies on aPbO2 prepared b y persulfate oxidation did not observe
a second i n t e r m e d i a t e compound (~-PbOx). However,
they employed a different e x p e r i m e n t a l technique.
G i l l i b r a n d and Halliwell, using DTA, obtained curves
similar in shape to the one shown i n Fig. 1, except
Table I. Effect of heating rate and sample size on the decomposition of lead dioxide (chemical preparation)
HeatinE rate:
Sample size:
T
T
T
T
T
rnax,
max,
max,
max,
max,
1st p e a k ,
2nd peak,
3rd peak,
4th peak,
5th peak,
5~
63.7 m g
~
~
~
~
~
427
465
490
508
597
10~
71.0 m g
15~
62.3 m E
20~
69.0 m E
446
485
510
528
612
452
490
514
533
618
458
497
528
841
634
20~
215.8 rng
452
490
520
542
627
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1548
J. Electrochem. Sac.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
December 1974
Table II. X-ray analysis of ~- and fl-PbO2 decomposition product at Ist endothermic peak (a-PbOx)
Obtained
from:
Electrochemical
Chemical
a - P b 0 2
3.148
2,742
2.721
.
~ - P b 0 2
3.142
-.
.
--
1.920
--
.
---
Weiss
chemical
~-Pb02
Angstadt
chemical
3.147
2.747
2.712
3.137
2.728
2.708
3.148
2.738
2.708
2.244
Angstadt
chemical
1.~6
1.918
1.928
1.913
( x - P b O ~
jS-PbO=
.
1.953
1.933
1.917
1.653
1.646
1.632
--
--
3.149
2.758
2.716
.
--
1.927
1.917
--
fl-PbO=
3.149
2;746
2.713
--
- -
d(A)
Bystrom
chemical
Chemical
a - P b 0 2
--
1.927
1.909
1.654
1.646
1.632
1,928
1.910
1.653
1.645
1.636
--
--
1.647
1.636
1.647
1.635
Table III. X-ray analysis of ~- and fl-PbO2 decomposition product at 2nd endothermic peak (fl-PbOx)
Obtained
from:
Chemical
a-PbO~
Electrochemical
a-PbO2
3.175
3.155
-2.742
2.721
3.170
3.142
--2.725
1.937
1.928
d (A)
--
Chemical
~-PbO2
Bystrom
chemical
~-PUO~
Weiss
chemical
fl-PbO~_
3.170
3.142
-2.737
3.179
3.149
2.750
2.736
3.186
3.147
2.752
2.737
1.952
1.936
1.912
-1.648
1.950
1.938
1.916
1.655
1.641
1.950
1.943
1.917
1.656
1.642
--
1.654
---
1.748
1.649
1.643
1.634
they did not observe an e x o t h e r m i c peak at 187~
Kordes (4), using x - r a y and n e u t r o n diffraction, found
this a-PbO2 compound to be b e t w e e n 40-60% crystalline w h e n compared to a r e f e r e n c e PbO2 standard. His
D T A results on this m a t e r i a l are in excellent a g r e e m e n t w i t h ours. It was concluded from our D T A - T G A
and x - r a y diffraction results that due to the n o n c r y s tallinity of the a-PbO2 it decomposed along two simultaneous reaction paths. It is believed that the noncrystalline m a t e r i a l decomposes to Pb304 w it h o u t going
t h r o u g h the a- ~-PbOx intermediates. Low t e m p e r a t u r e
(225~
~) isothermal studies on this compound as
well as other ~- and fl-PbO2 compounds h a v e shown
that a small amount of Pb304 appears after e x t e n d e d
heating periods, along w i t h the original o r d e r e d PbO2
compound.
Electrochemical oxidation of an alkaline solution of
Pb(NO3)2 produced two products. The initial compound deposited was a hard black form of a-PbO2
w i t h p r e f e r r e d orientation of the (100) planes parallel
to the surface. The D T A curve for this product is
shown in Fig. 1. The second product, w h i c h could be
scraped from the surface of the electrode, was a softer,
less crystalline, b r o w n i s h form of ~-l:'b02 w i t h no
p r e f e r r e d orientation. Its D T A c u r v e is also g iv en in
Fig. 1. The DTA c u r v e (Fig. 1) of a p r e d o m i n a n t l y
black sample gave e n d o t h e r m i c peaks at 105 ~ 343 ~
504 a, 540 ~ and 625 ~ and a small e x o t h e r m i c peak at
210~ These peaks w e r e associated w i t h the following
reactions
105 ~
~-PbO2[OH]-H20
> ~-PbO2 [OH]
--
--
--
--
--
--
Angstadt
No fl-PbOz
observed
ing. X - r a y analysis of samples that had been p r e heated gave identical patterns to those not heated. The
following m ech an i sm was found for the fi-PbO2
465 ~
~-PbO2
+ ~-PbOx + residual ~-PbO2
quenched
510 ~
~-PbOx
> ~ PbOx + Pb304
quenched
632 ~
p-PbOz -}- Pb304
- > PbO tetragonal
+ PbO orthorhombic
O t h er preparations of fl-PbO2 gave results similar to
those obtained by Gillibrand and HalliweI1 (6). In all
cases it was found that ~-PbO2 decomposed before ;~PbO2.
Thermal analysis of formed electrodes.--The DTA
curves obtained from the four f o r m e d electrodes are
shown in Fig. 2. The x - r a y diffraction analysis and
m a x i m u m peak t e m p e r a t u r e s of the q u e n c h e d products
are given in Table IV.
The formed plate from each series gave its ow n
characteristic D T A curve w i t h respect to the n u m b e r
SAMPLE:
FORMED COMMERCIAL
PbO 2 E L E C T R O D E S
0
x
210 ~
a-PbO2 [OH]
> ~-PbO2
190~
343 ~
> ~-PbO=
isothermal
504 ~
--PbO=
> ~-PbOx + Pb304
quenched
540 ~
fl-PbO=
> Pb304
quenched
625 ~
Pb304
> PbO (tetragonal
-}- o r th o r h o m b i c)
~-Pb02
The r eag en t /~-PbO2 gave no e x o t h e r m i c peak in the
vicinity of 200~ thus indicating no structural r e o r d e r -
440 ~
487~
C
o
45
437 ~ 483~
0
I
I00
[
200
[
500
[
400
T, ~
I
500
I~,J-'"615~ [
600
700
800
Fig. 2. DTA curves of four commercially formed PbO2 electrodes
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Vol. 12I, No, 12
THERMAL DECOMPOSITION OF ELECTRODES
1549
Table IV. Decomposition temperature maxima and x-ray analysis for formed PbO2 electrodes
Series
1st p e a k
exothermic
Analysis
2nd peak
endothermie
Analysis
3rd p e a k
endothermie
Analysis
4th peak
endothermie
Analysis
5th peak
endothermic
A
190
a + ~-Pb02
S o m e PbsO~
--
--
485
~-PbO~
526
~-PbO,
PbsO4, P b O R
614
a, ~-PbOs
440
fl-PbOz
Pb804
fl-PbO~
PbsO~
515
530
524
~-PbOz
Pb304
PbsO~
610
~-PbO~
PbsO~
528
PbsO~
618
B
180
C
198
~, /~-PbO~
D
183
,v, ~ - P b O ~
Pb304
~-PbOz
487
439
a-PbOz
479
437
a-PbO~
483
of peaks, m a x i m u m peak t e m p e r a t u r e , and r e l a t i v e
area u n d e r each peak. However, the o v e r - a l l decomposition m e c h a n i s m for t h r e e of the four electrodes
(B, C, D) w e r e the same. Series A deviates from the
general decomposition m e c h a n i s m in that no peak was
observed for an ~-PbOx intermediate.
The e x o t h e r m i c peak exhibited by the f o r m e d active m a t e r i a l b e t w e e n 180~176
was associated w i t h
the r e o r d e r i n g of the ~- and fl-PbO2 and the evolution
of O2, adsorbed and bound water. A portion of this
w a t e r was incorporated in the anion lattice n e t w o r k as
was ev i d en ced by the results obtained by heating samples in conjunction w i t h a mass spectrometer. The mass
spectra t ak en u n d e r isothermal conditions at various
t e m p e r a t u r e s and at 10 -6 m m pressure showed that as
t he t e m p e r a t u r e increased from 27 ~ t o 180~ t h r ee
w a t e r peak m a x i m a occurred. The r e o r d e r i n g of the
PbO2 lattice was e v id e n c e d by comparing the x - r a y
diffraction patterns of the original samples w i t h those
obtained before and after the e x o t h e r m i c peak. The
original sample did not show the high 28 angle Ka
doublets for/~-PbO2. A f t e r heating to 180~176
these
doublets w e r e resolved, in addition to a shifting of the
low 2~ angle values. A f e w additional lines also appeared for both a- and fl-PbO2. The mass spectra of the
active m a t e r i a l effluent also showed peaks for CO2,
occluded air (02 and N2), and organic compounds.
These organics w e r e due to leaching out of organic
ma t eri al s from the separators and the m i g r a t i o n of
these e x p a n d e r products from the negative to the positive electrode.
X - r a y studies on the active m a t e r i a l held isotherm a l l y at 225 ~ for periods of time ranging from 2 hr to
several weeks g a v e x - r a y patterns for o r d e r e d a- and
fl-PbO2. T h e r m o g r a v i m e t r i c e x p e r i m e n t s over the t e m p e r a t u r e range 100~176
showed a v e r y small w e i g h t
loss which could be a t t r i b u t e d to absorbed w a t e r since
mass s p e c t r o m e t r y showed that e v e r y sample contained some. A f t e r prolonged heating, duration of up
to several months, some of the samples gave lines for
Pb304. However, t h r e e of the four electrodes had contained residual Pb304 in the f o r m e d electrode. Series
D, which contained no Pb304 in the original oxide
blend, also showed several Pb304 diffraction lines.
Continued heating of the electrode active m a t e r i a l
showed t h a t each sample b e g a n to decompose en d o t h e r m i c a l l y at 290~ w i t h the active m a t e r i a l f r o m
series B, C, and D giving peak m a x i m a at 440 ~ 434 ~
and 437~
respectively. No t e m p e r a t u r e m a x i m a in
this range was observed for series A. X - r a y analysis
of q u e n c h e d and i s o t h e r m a l l y held samples of series
B, C, and D gave patterns for a-PbOz. X - r a y analysis
Analysis
PbO
Tetragonal
615
PbO
Tetragonal
PbO
Tetragonal
PbO
Tetragonal
chemically p r e p a r e d ~- and fl-PbO2 w h i l e series A
and C gave m u c h smaller peaks thus indicating that
simultaneous decomposition reactions w e r e occurring.
This was confirmed by x - r a y diffraction analysis of
samples quenched from the 480 ~ e n d o t h e r m i c peak.
The final e n d o t h e r m i c peak b e t w e e n 610~176
was
due to the decomposition of Pb~O4 to PbO tetragonal.
Thermal analysis of cycled lead dioxide e~ectrodes.-T h e r m a l analysis of the four samples of active m a terial gave identical D T A curves. A r e p r e s e n t a t i v e
D TA curve is sh o w n in Fig. 3. The D T A curves obtained from these cycled electrodes w e r e unlike any
of t h e curves obtained from the active m a t e r i a l of
the f o r m e d electrodes. The m a i n differences b e t w e e n
the f o r m e d and cycled electrodes : w e r e that t he r e
was a general decrease in area u n d e r the first exot h er m i c peak at 200~ the occurrence of an e n d o t h e r mic peak at 358~
and the appearance of one large
en d o t h er m i c peak at 465~ w h i ch replaced the two
endothermic peaks observed w i t h three of the formed
electrodes (B, C, and D) b e t w e e n 434~176
The D TA curves of the oxidized m a t e r i a l from
cycled electrodes consisted of five peaks. A n exot h er m i c peak at a p p r o x i m a t e l y 200~ and four e ndothermic peaks at 358 ~ 465 ~ 536 ~ and 617~ These
peaks w e r e associated with the following t h e r m a l decomposition reactions. This decomposition mechanism
applies only to n o n e q u i l i b r i u m runs w i t h samples
r e m o v e d at each peak m a x i m u m . All of the cycled
samples contained adsorbed water.
a-PbO2(OH) + fl-PbO2(OH)
+ amorphous PbO2 (OH)
200 ~
> a-PbO2 (ordered)
~-PbO2 (ordered) + amorphous PbO2
[1]
a-PbO2 +/~-PbO2 + amorphous PbO2
358 ~
> ~-PbO2 + ~-PbO2
[2]
quenched
SAMPLE:
OXIDIZED P b O 2 / P b S O 4 ELECTRODE
WITH iO0 S.A.E. CYCLES
200 ~
~ ~ o ' ~
130~
~oo
290~
572 ~
358 ~
524 ~
of quenched and isothermally held samples of series
A t ak en b e t w e e n 375~176
gave x - r a y patterns for
~-PbOz. Isothermal runs on a n u m b e r of A series
samples held at 325 ~ for v a r y i n g periods of time
gave a f ew samples whose x - r a y patterns w e r e close
to a-PbOx.
The third e n d o t h e r m i c peak m a x i m u m was obs e r v e d b e t w e e n 480~176
for all electrode samples.
X - r a y analysis showed the m a j o r product to be ~-PbO~.
The f o u r t h e n d o t h e r m i c peak m a x i m a w e r e bet w e e n 515~176
and w e r e associated with the decomposition of fl-PbOx to Pb304. The area u n d e r this
peak v a r i e d considerably. Series B and D gave peaks
w h i ch w e r e m o r e characteristic of pure samples of
617~
0
UJ
I
I00
T,~
I
I
I
I
I
J
200
300
400
500
600
700
(corrected for chrornel alurnel thermocouples)
800
Fig. 3. A representative DTA curve of a PbO2 electrode cycled
100 SAE cycles.
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1550
J. E l e c t r o c h e m . Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
465 ~
=-PbO2 + ~-PbO2 ~ / ~ - P b O =
+ small a m o u n t (a-PbOx, P-PbO=,Pb304)
~-PbO=
536 ~
....
[3]
> Pb304
-}-small amount P b O tetragonal
Pb304
617 ~
> PbO tetragonal
[4]
[5]
The exothermic peak at 200 ~ was the result of
reordering of the PbO2 lattice with the evolution of
H20 and 02. This reaction was also observed i n each
of the formed electrodes. Continued heating of the
active material produced an endothermic peak at
358~
X - r a y diffraction patterns of quenched samples showed lines for ordered ~-PbO2 and /~-PbO2.
TGA showed a slight weight loss over the 290~176
t e m p e r a t u r e range. A r e r u n of a sample which had
previously been r u n to 358 ~ gave no endothermic peak.
X - r a y analysis of this sample showed lines for /~-PbO2
and a-PbOx. The material in this sample responsible
for the endothermic peak at 358 ~ must therefore be
associated with a-PbO2 or a possible amorphous form
of PbO2. Evidence strongly favors the existence of
such a n amorphous form since samples containing
no a-PbO~ still gave a n endothermic peak at 358~
Microscopic studies of the original active material
show two distinct forms of PbO~ (1). The p r e d o m i n ant form, approximately 85-95% of the total, occurs
at brown, brownish-orange, or bright orange, individual crystals of small size and translucent appearance.
The other form occurs in dense black or slate gray
masses, of glassy appearance and with no definite
form, and does not appear to be translucent. Both
forms appear unreactive to a m m o n i u m acetate and
appear in electrodes which show only ~-PbO2 lines,
as well as those showing both a- and ~-PbO2 thus
indicating that the m a t e r i a l is not a-PbO2. Microscopic e x a m i n a t i o n of samples that were heated to
the 358 ~ peak, and then quenched, showed that most
of the black material had been replaced by a red
material. Samples of this red material gave the m a j o r
lines for Pb304.
The black form of c~-PbO2, which was prepared
electrochemically (Fig. 1) gave an endothermic peak
of similar m a g n i t u d e at 345 ~ whereas all of the/~-PbO2
preparations studied gave much larger e n t h a l p y
changes at around 400 ~ or higher. DTA curves r u n on
discharged electrodes gave a 358 ~ endothermic peak of
approximately the same m a g n i t u d e as that observed
in the charged plate. These samples had not experienced large capacity losses at this time. The other
peaks associated with PbO2 decomposition had diminished in area. Microscopical studies showed that
the black material was still present in a discharged
electrode. This material evidently does not undergo
oxidation or reduction to an appreciable extent after
the first few cycles. The most logical explanation
of the above facts appears to be that the black PbO~
is an amorphous form or contains an amorphous form
of PbO2 that is relatively inactive.
While as yet no evidence has been obtained that
would directly link the two cases of unreactive material, recent pulsed nuclear magnetic resonance s t u d ies (5) have revealed the presence of a hydrogen
species in the electrochemically prepared PbO2 structure which appears to be related to the loss of capacity
of the lead acid battery. This hydrogen species has
not been found in chemically prepared /~-PbO2, and
the latter is k n o w n to be unsatisfactory as a b a t t e r y
electrode. Upon heating the electrochemically prepared PbO2 to 150~ this hydrogen species disappears.
This t e m p e r a t u r e roughly corresponds to the exothermic peak observed at 180 ~ on the differential
thermal analyzer. This peak was not observed w h e n
the chemically prepared ~-PbO2 was heated.
D e c e m b e r 1974
The endothermic peak at 463~ was duer to the
decomposition of a-PbO~ and residual PbO2 to ~-PbOz.
Also occurring simultaneously was the decomposition
of ~-PbOx and fl-PbOx to Pb304.
The endothermic peak at 535~ was due to the decomposition of ;~-PbO~ (from fl-PbO2) to Pb~O4 and
the decomposition of Pb304 to PbO tetragonal (from
a-PbO~). As the a m o u n t of =-PbO2 increased in the
original sample the peak at 535~ decreased. This
would be expected if this peak was due to the decomposition of ~-PbO2 to Pb304.
The endothermic peak at 618~ was due to the
decomposition of Pb~O4 to tetragonal PbO.
The a'bove analysis was based on cycled samples
taken from electrodes which had showed only a slight
loss in its initial capacity. As the electrodes were
cycled to failure the m a j o r changes in the DTA curves
were the gradual disappearance of the exothermic
peak at 200~ and the endothermic peak at 358~
The other endothermic peaks were only reduced
slightly indicating that the plate still possessed a high
PbO2 content. This PbO2 was electrochemically i n active. X - r a y analysis also showed that the electrode
had a high PbO2 content and a low PbSO4 content
at failure. The DTA curve obtained from this electrochemically inactive PbO2 resembled the D T A curves
obtained from reagent F,bO~.
Conclusions
1. From the evidence that has been obtained it is
concluded that an amorphous form of PbO2 exists
in positive plate active material, i n addition to aand ~-PbO2. This material appears to possess thermal
properties similar to ~-PbO2.
2. It was concluded from thermal, mass spectroscopy,
NMR, and microscopy results that as the PbO2/PbSO4
electrode was cycled the electrochemically active PbO2,
which m a y be an amorphous compound, u n d e r w e n t
a structural reordering with the loss of a hydrogen
species. This structural reordering lead to an electrochemically inactive PbO2 compound which gave NMR,
DTA, and mass spectroscopy results similar to those
obtained on reagent PbO2. The continual conversion
of electrochemically active PbO2 to the electrochemically inactive form is one of the m a j o r factors that
causes b a t t e r y capacity loss and ultimate failure.
3. The thermal decomposition mechanism of formed
plates has been found to differ, depending on the
method of manufacture. However, after a n u m b e r of
cycles all plates gave the same t h e r m a l decomposition
mechanism.
4. It was also concluded that similar DTA, NMR,
and mass spectroscopy results are obtained when
electrochemically active PbO2 is cycled to failure or
w h e n it is heated isothermally at 225~
Acknowledgment
We wish to t h a n k the Naval Research Laboratory
for the support of this j o i n t investigation and for
permission to publish this work. Appreciation is also
expressed to the I n t e r n a t i o n a l Lead-Zinc Research
Organization, Incorporated, which has assisted i n this
and other investigations b y m a i n t a i n i n g the position
of Research Associate at the Naval Research Laboratory. Appreciation is also expressed to Mr. Michael
McDowell for r u n n i n g the mass spectroscopy samples.
Manuscript submitted April 22, 1974; revised m a n u script received J u l y 3, 1974.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1975 JOURNAL.
All discussions for the J u n e 1975 Discussion Section
should be submitted by Feb. 1, 1975.
The publication costs of this article have been assisted by the International Lead Zinc Research Organization, Incorporated.
REFERENCES
1. A. C. Simon and S. M. Caulder, This Journal, 118,
659 (1971).
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Vol. 121, No. 12
T H E R M A L D E C O M P O S I T I O N OF E L E C T R O D E S
J. B u r b a n k and E. J. Ritchie, ibid., 116, 125 (1969).
E. J. Ritchie and J. B u r b a n k , ibid., 117, 299 (1970).
D. Kordes, Chem. Ing. Tech., 38, 638 (1966).
S. M. Caulder, J. S. Murday, and A. C. Simon, This
Journal,, 120, 1515 (1973).
6. M. I. G i l l i b r a n d and B. Halliwell, "Power Sources,"
D. H. Collins, Editor, p. 179, P e r g a m o n Press,
New York (1966).
7. A. C. Simon, S. M. Caulder, and E. J. Ritchie, This
Journal, 117, 1264 (1970).
8. R. T. Angstadt, C. J. Venuto, and P. Rfietschi, ibid.,
109, 177 (1962).
2.
3.
4.
5.
1551
9. H. Bode and E. Voss, Z. Electrochem., 60, 1053
(1956).
10. A. BystrSm, Arkiv. Kemi, Mineralog. GeoL, 20A,
No. 11 (1945).
11. T. Katz and R. Faivre, Ann. Chim., 5, 5 (1950).
12. R. Weiss, Thesis, Nancy, France (1959).
13. G. Butler and J. L. Copp, J. Chem. Soc., 1956, 725.
14. J. S. A n d e r s o n and M. Sterns, J. Inorg. Nucl. Chem.,
11, 272 (1959).
15. W. B. White and R. Roy, J. Am. Ceram. Soc., 47,
242 (1964).
16. D. Fogue, P. Fouilloux, P. Bussiere, D. Weigel, and
M. Prettre, J. Chim. Phys., 62, 1088 (1965).
17. E. M. Otto, This Journal, 113, 525 (1966).
Effect of Gaseous Pretreatment on Oxidation of Iron
A. W. Swanson*'1 and H. H. Uhlig*
Department of Metallurgy and Materials Science, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
ABSTRACT
Gaseous pretreatment of pure iron surfaces m a y either increase
sequent thin film oxidation rate or decrease it, depending on the
effect is less than for copper. Traces of impurities including carbon
plain diminished faceting of iron, and also lower oxidation rates
pressure O2.
I n previous papers (1, 2) it was shown that gaseous
p r e t r e a t m e n t of single-crystal and polycrystalline Cu
has a large effect on subsequent thin film oxidation
rates in oxygen. Surface facets are formed, the orientation and oxidation rate of which v a r y with the
gas used for pretreatment. It was found that the pret r e a t m e n t of single-crystal Cu, w h a t e v e r the crystal
face, in hydrogen at 350~176
favors formation
of the slowly oxidizing (111) face; on the other hand
p r e t r e a t m e n t i n nitrogen favors formation of the much
more rapidly oxidizing (100) face. Both pretreated
and u n t r e a t e d copper surfaces follow two-stage logarithmic oxidation kinetics w h e n oxidized at 175 ~
225~ in 1 atm O2.
T a m m a n n and KSster (3) first observed that oxidation of Fe at low temperatures (thin oxide films)
follows the direct logarithmic equation. Similar behavior for Fe i n air at 300~ was reported b y L u s t m a n and Mehl (4),-in air up to 200~ b y V e r n o n et al.
(5), in air up to 300~ by Davies et al. (6), and in
1.3 X 10 -2 Torr 02 up to 200~ by G r a h a m et al.
(7). R u n k and Kim (8) observed two-stage logarithmic behavior for 0.2, 0.4, and 0.8% C steels in 100
Torr 02 at 200~176
similar behavior was reported
by Needham et al. (9) for zone-refined Fe in 4 •
10 -6 Torr O2 up to 350~ I n some of these investigations, a wide variety of oxide thicknesses were reported u n d e r otherwise comparable conditions b u t
various metal surface t r e a t m e n t s were used prior to
oxidation. It was the purpose of the present investigation to determine to what extent Fe similar to
Cu oxidizes in the thin film region at rates d e p e n d ent on the type of gaseous pretreatment.
Experimental
Two kinds of iron were used. The first was Armco
iron sheet 0.0037 in. (0.0094 cm) thick decarburized
in wet H2 to 0.0013% C (by chemical analysis). The
second was Battelle zone-refined iron containing
<0.0005% C, with other impurities consistently low,
* Electrochemical
Society A c t i v e M e m b e r .
1 P r e s e n t a d d r e s s : R a y t h e o n C o m p a n y , R e s e a r c h Division, Walt h a m , M a s s a c h u s e t t s 02154.
K e y w o r d s : iron oxidation, effect of s u r f a c e p r e t r e a t m c n t , surface
carbon films, thin film oxidation, Initial oxidation.
the subgas. The
m a y exi n low-
cold rolled to 0.015 in. (0.038 cm), a n d s u b s e q u e n t l y
annealed in argon at 800~ for 1 hr.
Oxidation and p r e t r e a t m e n t tests were r u n in a
3 cm ID fused silica tube inserted into a horizontal
tube furnace m a i n t a i n e d automatically at •176
A
0.5 cm wall stainless steel tube s u r r o u n d i n g the silica
tube insured a more' u n i f o r m furnace temperature.
The furnace could be slid along r u n n e r s b r i n g i n g
it into or out of the specimen zone. Gases were led
through an i n n e r sn-lall diameter silica t u b e to the
rear of the larger silica tube, t h e r e b y preheating
the gas before it impinged on the specimen.
Dried nitrogen, hydrogen, and argon were purified
b y passing over Cu chips at 400~
the argon was
additionally purified over Ti chips at 800~ All the
foregoing gases were then dried through a trap immersed in solid CO2 and acetone. Oxygen was dried
using CaCI~. Specific m i x t u r e s of CO-CO2 were obtained by m o n i t o r i n g the flow of the individual gases
through flow meters. Specific H20-H2 m i x t u r e s were
obtained by b u b b l i n g H2 t h r o u g h distilled H20 m a i n tained at a prescribed temperature.
The 3 • 10 cm iron sheet specimens b e n t into a
semicircle were pickled for 2 m i n at room t e m p e r a t u r e
i n 10 volume per cent (v/o) conc HNOs, rinsed four
times i n distilled water, a n d then dried b y i m m e r s ing in acetone, followed b y benzene i n a nitrogen
atmosphere. It was suspected that carbon tended to
migrate to the metal Surface during heating of the
specimens (as detected b y reduced oxidation rates
of the iron), requiring a f u r t h e r oxidation-reduction
t r e a t m e n t of the surface in order to remove it. Holm
(10) earlier observed that carbon m i g r a t i n g to the
surface of iron reduced the thickness of a previously
formed oxide film; Uhlig et aI. (11) noted the same
effect with nickel. Sewell (12) found similarly that
the oxidation r a t e of nickel is affected b y only a
few parts per m i l l i o n of carbon m i g r a t i n g to the
surface. Blickwede (13) showed t h r o u g h surface analysis of commercial steels that m a r k e d surface enr i c h m e n t in carbon occurs after a n n e a l i n g i n N2-H2
mixtures.
To avoid such effects with the present specimens,
both Armco and zone-refined iron were subjected to
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