The Effect of Phosphoric Acid on the Positive
Electrode in the Lead-Acid Battery
III. Mechanism
Kathryn R. Bullock*
Globe-Union Incorporated, Milwaukee, Wisconsin 5320I
ABSTRACT
Phosphoric acid a d d e d to b a t t e r y e l e c t r o l y t e modifies the m o r p h o l o g y of
PbO2 corrosion films b y reacting to produce Pbs(PO4)2 as an i n t e r m e d i a t e in
the oxidation of P b to PbO2. Because these modified PbO2 films resist r e d u c tion to PbSO4, b a t t e r y f a i l u r e from f o r m a t i o n of a resistive PbSO4 film at
the positive g r i d / a c t i v e m a t e r i a l interface is forestalled. T h r e e n e w findings
s u p p o r t this mechanism: (i) the identification of Pba(PO4)2 in l e a d c o r r o sion films, (ii) the predictions of a t h e r m o d y n a m i c m o d e l of the Pb/H2SO4/
H3PO4/H~O system, a n d (ii{) t h e m e a s u r e m e n t of sulfation rates of PbO2 films
f o r m e d from Pba (PO4)2 and PbHPO4.
Previous studies of the effect of phosphoric acid
on the positive electrode in the l e a d - a c i d b a t t e r y b y
cyclic v o l t a m m e t r y (1) and b y constant potential
corrosion methods (2) have shown that phosphate
enters the PbO2 corrosion film and modifies its m o r phology. The Pb02 f o r m e d in the presence of 1-13P04
discharges to PbSO4 at a slower r a t e t h a n PbO2
f o r m e d in p u r e H2SO4. These effects occur w i t h as
little as 0.2 weight percent ( w / o ) H3PO4 a d d e d to
the electrolyte.
One m e c h a n i s m for the effect of phosphoric acid
on the PbO2 electrode was proposed b y C a r r and
H a m p s o n (3) who m e a s u r e d the capacitance of PbO2
electrodes in dilute phosphate solutions. They concluded t h a t H3PO4 adsorbs on PbO2 and that d u r i n g
r e d u c t i o n a d s o r b e d PbHPO4 can form. T h e y did not,
however, r e p o r t a n y m e a s u r e m e n t s in m i x t u r e s of
phosphoric and sulfuric acids.
A p o t e n t i a l l y troublesome mechanism is f o r m a t i o n of
soluble P b ( I V ) phosphate complexes. S e v e r a l authors
(4-6) have formed these complexes both chemically
and electrochemically, b u t such complexes a p p a r e n t l y
occur only at HsPO4 concentrations above about 0.8%
(10 g / l ) in 35% H2SO4 (6). Bullock and McClelland
(1) o b s e r v e d a change in the kinetics of the
P b O J P b S O 4 couple at this same H~PO~ concentration,
and much of the l e a d - a c i d b a t t e r y p a t e n t l i t e r a t u r e
r e c o m m e n d s H3PO4 concentrations at or b e l o w this
level (7-9). As a result of the increased solubility of
P b ( I V ) , mossing or shorting can be a p r o b l e m at
higher concentrations.
One w a y that phosphoric acid improves cycle life
is b y p r e v e n t i n g f o r m a t i o n of an insulating PbqO4
l a y e r at the g r i d / a c t i v e m a t e r i a l interface (10-12).
Since our results show t h a t concentrations as low as
0.2 w / o HsPO4 s u b s t a n t i a l l y decrease the r a t e of
PbSO~ formation, a m e c h a n i s m other t h a n P b ( I V )
complex f o r m a t i o n m a y be responsible.
A n a l t e r n a t i v e mechanism, suggested b y the r e sults of x - r a y diffraction analyses of l e a d corrosion
films, is the f o r m a t i o n of Pbs(PO~)2 as an i n t e r m e d i a t e in the oxidation of Pb to PbO2. Microscooic
p a r t i c l e s picked from films f o r m e d at 1.230-1.425V
(vs. Hg/Hgo_SOJ4.5M H~_SO4) at 49~ in 4.5M H~SO4
containing 0-0.2 w / o H~PO4 were a n a l y z e d b y the
McCrone m e t h o d (13). Since these corrosion films
w e r e quite thin (about 1 ~m) and a p p e a r e d homogeneous u n d e r the microscope, the particles w e r e considered to be r e p r e s e n t a t i v e of the corrosion product.
The l e a d ( I I ) phosphate, Pb~(PO4)2 was p r e s e n t in
* Electrochemical Society Active Member.
Key words: additive, battery, corrosion, discharge.
films f o r m e d at 1.230 and 1.275V in the electrolyte
containing 0.2 w / o H3PO4 b u t was not p r e s e n t in films
f o r m e d at 1.315 or 1.425V. Likewise, no Pb~ (PO4)2 was
detected in a film f o r m e d at 1.425V in electrolyte
containing 1.15 w / o H3PO4. Scanning electron m i c r o graphs, electron m i c r o p r o b e analyses, and electrochemical studies of these same corrosion films h a v e
been published p r e v i o u s l y (2).
A n o t h e r observation which supports the Pb3(PO4)2
m e c h a n i s m is that the ~-PbO2 p e a k in the cyclic
v o l t a m m o g r a m s (1) occurs at 1.230V in p u r e electrol y t e b u t shifts to 1.275V in the presence of 0.2 w / o
H3PO~. Such a shift m i g h t result from o x i d a t i o n of
a P b ( I I ) species m o r e stable t h a n PbSO4. F u r t h e r more, a comparison of the coulombs used during grid
corrosion at 1.230V to the w e i g h t of P b in the c o r r o sion l a y e r shows that in p u r e electrolyte n e a r l y all the
Pb is in the W4 state, w h e r e a s in ph0sphated electrolyte a p p r o x i m a t e l y 70% of the P b is in the + 2 state.
In o r d e r to d e t e r m i n e w h e t h e r Pb3(PO4)2 would be
t h e r m o d y n a m i c a l l y stable u n d e r conditions found in
lead corrosion films, the following analysis of the
r e l a t i v e stabilities of the l e a d ( I I ) oxides, sulfates,
and phosphates (shown in Table I) was made.
Thermodynamic Model
Method.--In developing a t h e r m o d y n a m i c model of
corrosion films, the least soluble species is considered
the most stable u n d e r a given set of conditions in
the film. Voss (14) c o m p a r e d the solubilities e x pressed as total soluble l e a d (apb+2 -~- aHPO2--) Of
PbHPO~, Pb3(PO4)2, and PbO from 0 to 14 pH, assuming an a c t i v i t y of 1 for the p r e d o m i n a n t phosp h a t e species in solution at a n y given pH. Bode (15)
c o m u a r e d the total soluble l e a d for PbSO4, PbO.PbSO4,
3PbO.Pb~O~.H20, PbO. and P b (OH) 2 in the p H r a n g e
6-13, assuming a sulfate ion a c t i v i t y of 10 -4. H o w ever, the case of a mi<ed phosohoric acid. sulfuric
acid solvent has not b e e n considered. F u r t h e r m o r e ,
h y d r o g e n and sulfate ion activities decrease in l e a d
Table I. Pb(ll) compoundsconsideredin thermodynamicmodel
1.
2.
3.
4.
5.
6.
7.
8.
PbHPO4
Pb3(POD~
PbSO4
P b O - PbSO~
3 P b O 9 P b S O ~ 9 H~O
4PbO 9 PbSO4
PbO
Pb(OH)~
1848
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Vol. I26, No. 11
1849
LEAD-ACID BATTERY
PbHPO,
+ 2 H + = P b +2 + H 3 P O 4
l o g aPb+-" =
PbHPO~
-
1.8 -- 2 p H
+ 2H~O = HPbO~-
l o g a m , ~ o 2- =
PbHPO4
-
l o g aH3PO~
+ HPOc
39.1 + 3 p H
+ 2H20 = HPbO~-
l o g aiiebo~- =
[6]
-
-
~" + 3 H +
[7]
l o g a ~ l . o ~ -2
+ POc 3 + 4H+
-- 51.0 + 4 p H -
[8]
log ~ v o ( 3
Ph3(POD~
Pb3(POD_- + 4H~ = 3 P b +~ + 2H'zPO;-
[9]
l o g aL.b+'-' = -- 1.7 -- 1.33 p H -- 0.66 l o g aH~eO~Pba(POD.-, + 2H+ = 3 P b +2 + 2HPO,-~
l o g mu, 4~ = -
6.46 -
[I0]
0.66 p H -- 0.66 l o g atxeo-~
Pb3(PO;)._, + 6 H + = 3 P b +~ + 2HsPO~
l o g a l , b+'-' =
-- 0 . 3 1 -- 1.99 p H
/Ill
-- 0 . 6 6 l o g aHaPO4
P b . ~ ( P O ~ ) ~ + 6H..,O = 3 H P b O . ~ - + 2 H P O c -~ + 7 H +
l o g a t t r , , o 2- =
[12]
-- 3 4 . 6 + 2 . 3 3 p H -- 0 . 6 6 l o g a u r o ~ -2
Pb.~(PO4)~ + 6H~O = 3HPbOelog amu,oe- =
-
+ 2POc 3 + 9H +
42.5 + 3 pH
-
[13]
0 . 6 6 l o g a v o ~ -8
Water~Lead System
PbO b
PbO
+ 2 H + = P b +e + H e O
[14]
l o g az,,,+"- = 1 2 . 6 -- 2 p H
PbO
+ H+
+ H~O = HPbO~-
log am, b%- =
[15]
-- 15.5 + p H
PD(OH)2 =
Pb(OH)2
+ 2 H + = P b +-~ + 2 H 2 0
[16]
l O g al.~*'-' = 1 2 . 4 6 -- 2 p H
2
0
4
6
8
IO
12
14
Pb(OH).~
= HPb02
l o g am, bo~- =
+ H+
[17]
-- 1 5 . 6 8 + p H
pH
S u l f a t e ~ W a t e r Syste~n ~
Fig. 1. Predominant ions in mixed H2SO4/H3P04 electrolyte as a
function of pH.
corrosion films f o r m e d in sulfuric acid because the
PbSO4 on the surface is a s e m i p e r m e a b l e m e m b r a n e
(16). Since p h o s p h a t e ion a c t i v i t y p r o b a b l y decreases
as well, it is n e c e s s a r y to consider a wide r a n g e of h y drogen, sulfate, a n d p h o s p h a t e activities beginning
w i t h solution activities a n d decreasing to n e a r l y zero
in the i n t e r i o r of t h e corrosion film.
F i g u r e 1 shows t h e p r e d o m i n a n t ions in solution as
a function of pH. These a r e defined by e q u i l i b r i u m
equations in T a b l e I I (Eq. [1], [2], [3], a n d [18]).
F o r convenience, the n e g a t i v e logs of activities of the
p r e d o m i n a n t p h o s p h a t e a n d sulfate ions in solution at
each p H a r e defined as p P and pS, respectively. F o r
e x a m p l e , p S = - - l o g aHso4- f r o m p i t 0-1.9 a n d - - l o g
aso42- a b o v e p H 1.9.
T a b l e II shows the e q u i l i b r i u m equations used to
calculate t h e total soluble l e a d for eact~ of the species
H S O ~ - = H * + SO~ -'~
[18]
aso4-9
l o g ~ - - - auso~-
-- 1.92 + p H
SulSate/WaterlLead Systm~
PbSO~"
PbSO~ = Pb §
l o g a~.b +~ =
+ SO~ -~
P b S O ~ + H + = P b +s + H S O ~ -
PbSO~ + 2H~O = I { P b O ~ + SO~--~ + 3 H +
l o g a~u.~,o i
=
-- 3 5 . 9 6 + 3 p H
PbO
PbO
9 PbSO,
l o g at.b +-" =
PbO
9 PbSO~
m 0 . 4 5 -- p H
- PbS0:
I
--2.1
-- pit
log anrboa- =
-
4PbO
[21
4PbO 9P b S O ~
[3]
" PbSO~
----- -- 12.0 + p H
l o g aHP~o~- =
atll,O~-r
[24]
-- - - l o g a s o ~ --~
4
[25]
1
-- - - l o g aso~ -~
4
" PbSO~ (tetrabasie)
~
[26]
1
- - pI-I -- - - l o g a s o t -~
5
4PbO
apO4"~
log
~
+ 6 H + = 5 P b +~ + SO~-~ + 4H.~O
8
~
9 H_.O ( t r i b a s i c )
1
3
22.77 + -- pH
2
l o g a r b +-" = 7.68 HPO,--" = H * + P O c
[23]
1
- - l o g aso~ --~
2
3 P b O - P b S O ~ 9 H ~ O + 4 H ~ O = 4 H P b O ~ , - + 6 H + + SO~ -~
aHaPO~
H ~ P O ~ - = H + + H P O , --~
aHPo~ -2
l o g ~ =
-- 7.2 + p H
[22]
+ 4 H ~ + SO~ -~
-
9 PbSO,
2
+ pH
'~
9 H ~ O + 6 H * = 4 P b *~ + SO~ -~ § 4 H ~ O
l o g a~b +-~ = 5 . 2 5 -
aH~Po~o g ~ =
(monobasic)
1
-- - - l o g as0~ -~
2
-- 2 8 . 4 5 + 2 p H
Phosphate/Water System a
[I]
[21]
l o g a s o ~ -~
+ SO~--" + H.~O
+ 3H.-,O = 2 H P b O ~ -
l o g a m , l,o~- =
3PbO
-
9 Pb50~
+ 2H* = 2Pb §
3
H~PO~ = H+ + H~PO~-
[20]
l o g a~.~+c ~ -- 5.87 -- p H -- l o g auso~-
3PbO
Table li.
[19]
-- 7 . 7 9 -- l o g a s o ( -~
5
+ 6H.~O = 5 H P b O ~ -~ + SO~ -~ + 7 H *
[27]
?
1
-- 2 0 . 1 7 + - - p H -- - - l o g a s o ~ --~
5
5
Phosphate~Water/Lead Sysrem ~
PbHPO~
P b H P O ~ = P b --~ + H P O ~ --~
1 0 g cn'~ ~ = -- ii.I -- l o g ex,'zeo~-~
[4]
ebl-IPO~
[5]
+
H ~ =
1ogc~rb ~ =
P b +e +
-- 3.9 -
H.-PO,-
pH
-- 1 o g a H ~ e o ~ -
W. M. Latlmer,
"Oxidation
Potentials,"
2 n d e d . , p. 107, P r e n tice-Hall, Inc., Englewood
Chffs, N.J. {1952).
b E . VOSS, P a p e r
16, P r o c e e d i n g s
of Second
International
Symposium
on Batteries,
Bouruemouth,
England,
O c t . 18-20, 1960.
c p. Delahay, M. Pourbaix,
and P. Van Rysselberghe,
Thzs Journal, 98, 57 (1951).
d H . B o d e , " L e a d - A c i d B a t t e r i e s , " p. 27, J o h n "Wiley & S o n s ,
Inc., N e w Y o r k (1977).
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1850
J. EZectrochem. 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
in Table I as pH, pP, and pS v a r y from 0 to 14. B y
d e t e r m i n i n g w h i c h of the species is least soluble u n d e r
each set of conditions, regions of s t a b i l i t y can be d e fined. In o t h e r words, if sulfate, phosphate, and h y d r o x i d e ions a r e all competing to react w i t h a lead (II)
ion, the regions of s t a b i l i t y define w h i c h r e a c t i o n is
preferred, based on t h e r m o d y n a m i c s alone.
Results.--First, consider the case of the P b ( I I ) /
H2SO4/H.~O system, w i t h o u t a d d i t i o n of HsPO4. Species
3 to 8 in Table I a r e k n o w n to occur in this system.
F i g u r e 2 shows the most stable species in the c o r r o sion film as p H and pS v a r y f r o m 0 to 14. The conditions in the corrosion l a y e r can be a p p r o x i m a t e d b y
d r a w i n g a diagonal f r o m the origin to the point w h e r e
p H and pS ---- 14, since the concentrations of h y d r o g e n
a n d sulfate ion decrease f r o m the s o l u t i o n / c o r r o s i o n
film interface to t h e corrosion f i l m / l e a d interface.
Therefore, the m o d e l predicts t h e corrosion film will
consist of a PbSO4 l a y e r on the outer surface and successive l a y e r s of monobasic l e a d sulfate, tribasic l e a d
sulfate, and lead oxides inside, in a g r e e m e n t w i t h
Ruetschi's results (16). A l t h o u g h the s o l u b i l i t y of
P b O at e v e r y p H is s l i g h t l y less t h a n the s o l u b i l i t y of
P b ( O H ) ~ (see Eq. [14]-[17] in Table I I ) , the values
a r e so close t h a t b o t h species a r e p r o b a b l e w h e n
P b ( O H ) ~ is stable (17). T e t r a b a s i c l e a d sulfate,
4PbO.PbSO4, is not stable w i t h respect to the l e a d
oxides.
Next, consider t h e P b ( I I ) / H 3 P O 4 / H 2 0 system w i t h o u t H2SO4. Species 1, 2, 7, and 8 in Table I can exist
in this system a n d Fig. 3 shows t h e i r regions of s t a b i l i t y as p P and p H v a r y from 0 to 14. A t low v a l u e s
of p H and pP, PbHPO4 is the most stable species, b u t
a s p H a n d p P increase, Pb3 (PO4)2 becomes m o r e stable.
I n h i g h l y a l k a l i n e solutions containin~ low concent r a t i o n s of p h o s p h a t e ion, the l e a d oxides a r e m o r e
stable. P u b l i s h e d e x p e r i m e n t a l results agree w i t h
these predictions. Voss (14) identified PbHPO~ on the
surface of lead electrodes anodized in 1M H3PO4. A w a d
and E l h a d y (18) m e a s u r e d the potential of l e a d in
14
12
PbOIPb(OH)2
I0
3PbO.
8
(D
PbSO 4-
\
6
H20
PbO'PbSO4
4
PbSO 4
2
0
2
4.
6
8
10
12
14
pH
Fig. 2. Most stable Pb(ll) species in sulfuric acid as a function
of pH and p$.
November 1979
14
w
PbO/Pb(OH) 2
12
I0
Pbs(P04) 2
8
Q.
~-
6
4
2
PbHPO 4
0
2
4
6
8
I0
12
14
pH
Fig. 3. Most stable Pb(ll) species in phosphoric acid as a function
of pH and pP.
buffered p h o s p h a t e solutions from p H 1.05 to 13.1 a n d
concluded t h a t Pb behaves as a PbHPO4 electrode in
acid solutions and as Pb3(PO4)2 electrode in basic solutions.
I n the m i x e d solvent system, Pb(II)/H3PO4/H2S04/
H20, t h e conditions a r e defined b y a 3-dimensional
b o x w i t h the axes pH, pP, and PS going f r o m 0 to 14.
The regions of s t a b i l i t y of the compounds listed in
Table I a r e defined b y the solids shown in Fig. 4a-f.
The conditions of s t a b i l i t y for PbHPO4, shown in
Fig. 4a, a r e high concentrations of h y d r o g e n a n d
phosphate. Since low concentrations of HsPO4 a r e
g e n e r a l l y a d d e d to l e a d - a c i d batteries, this compound
w o u l d not be e x p e c t e d to form unless a d s o r p t i o n of
H3PO4 on PbO2 (3) c r e a t e d a localized s t a b i l i t y region.
P b ( O H ) 2 and P b O (Fig. 4b) occur u n d e r h i g h l y
a l k a l i n e conditions, w h i c h a r e found at the g r i d / c o r rosion film interface. T h e y are f a v o r e d b y r e d u c e d
p h o s p h a t e and sulfate ion concentrations. As the sulfate and h y d r o g e n ion activities increase, tribasic lead
sulfate (Fig. 4c) a n d monobasic lead sulfate (Fig. 4d)
become successively m o r e stable.
PbSO4 (Fig. 4e) is stable u n d e r conditions found
at the e l e c t r o l y t e / c o r r o s i o n film interface, high h y d r o gen and sulfate ion activities. E v e n w h e n p h o s p h a t e
is a d d e d to batteries, a s e m i p e r m e a b l e PbSO4 m e m b r a n e can form on the grid and m a i n t a i n the a l k a l i n e
e n v i r o n m e n t in the i n t e r i o r of the corrosion film.
The Pb~(PO4)2 region of s t a b i l i t y (Fig. 4f) is the
most s u r p r i s i n g p r e d i c t i o n of the model. It is stable
u n d e r the widest r a n g e of conditions found in the int e r i o r of the corrosion film. F o r m a t i o n of Pb3 (PO~)2 is
f a v o r e d over PbHPO4 in this environment.
W h e n all six s t a b i l i t y regions a r e combined, the
solid box shown in Fig. 4g is obtained. The left f r o n t
w a l l is the 2-dimensional g r a p h for t h e regions of
s t a b i l i t y at a p H of 14. The right front w a l l is similar
to the 2-dimensional g r a p h for the P b ( I I ) / H 2 S O 4 / H 2 0
system shown in Fig. 2, since the p h o s p h a t e ion conc e n t r a t i o n is v e r y low (10-14). The o n l y difference is
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Vo/. I26, No. 1I
LEAD-ACID BATTERY
1851
)P
%
0
(a)
IP'
(b)
"
.
%.,
I :~/~'?/
..<.
.
0
0
.,+
"."
"+ •
,:
"~
<~
":•
"
Fig. 4. Most stable Pb(ll) species in mixed sulfuric acid/phosphoric acid electrolyte as a function of pH, liP, and /iS. (a) libHP04. (b)
Pb(OH)2/PbO. (c) 3PbO 9PbS04 9 H20. (d) PbO 9PbS04. (e) PbS04. (f) Pb3(POt):~. (g) Complete model.
a s m a l l Pb8 (PO4)2 r e g i o n at v e r y l o w sulfate ion c o n centrations. The top of the box is s i m i l a r to t h e 2dimensionl g r a p h for t h e P b ( I I ) / H s P O 4 / H 2 0 system
shown in Fig. 3, since t h e sulfate ion concentration is
v e r y low (10-14). The o n l y difference is a s m a l l
PbO-PbSO4 region at v e r y low p h o s p h a t e ion concentrations.
A diagonal d r a w n from the top front corner (pP,
pS, p H -- 14) to the b o t t o m back c o r n e r (pP, pH,
pS = 0) m i g h t a p p r o x i m a t e the regions found in the
film b e t w e e n the lead grid and the solution. Such a
diagonal passes t h r o u g h regions for l e a d oxides, t r i basic l e a d sulfate, Pb3(PO4)2, and PbSO4.
The exact a c t i v i t y profiles of phosphate, sulfate, and
h y d r o g e n ions a r e not known. T h e y w o u l d be d e t e r m i n e d b y t h e concentrations of the ions in solution
a n d b y t h e i r diffusion rates in the corrosion film.
Diffusion rates could be affected b y i m p u r i t i e s or
a l l o y i n g components, b y t h e i n t e g r i t y of the PbSO4
film (16), b y t h e a d s o r p t i o n of phosphates on PbOg.
(3), a n d b y the m i c r o s t r u c t u r e of the film. In using
the m o d e l to p r e d i c t t h e regions of s t a b i l i t y of P b ( I I )
sulfates, phosphates, a n d oxides, it is a s s u m e d t h a t
the rates of the c o m p l e x a t i o n reactions are fast r e l a tive to the rates of ionic diffusion in the film.
The l i m i t e d e x p e r i m e n t a l evidence a v a i l a b l e a p p e a r s
to s u p p o r t this assumption. Mahato (19) has used electron m i c r o p r o b e analysis to m e a s u r e the P and S concentrations across lead corrosion films developed b y
v o l t a m m e t r i c cycling in 4.5M H2804 containing 0.2%
I~PO4. In a g r e e m e n t w i t h the predictions of the
model, he found the highest sulfur content ( a b o u t 6%)
at t h e o u t e r surface of t h e corrosion film, t h e highest
P content ( a b o u t 0.6%) in t h e center, a n d no S o r P a t
the g r i d / f i l m interface. His results also show t h a t the
S content in the i n t e r i o r decreases as t h e P content
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1852
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
increases, suggesting a competition b e t w e e n sulfate
and phosphate c o m p l e x a t i o n reactions.
ExperimentalTest of Mechanism
Based on these e x p e r i m e n t a l results and t h e r m o d y n a m i c predictions, f o r m a t i o n of P b j ( P O 4 ) 2 as an
i n t e r m e d i a t e in the corrosion of P b to PbO2 seems
reasonable. F u r t h e r s u p p o r t for this m e c h a n i s m was
o b t a i n e d b y f o r m i n g P b j ( P O 4 ) 2 films on P b electrodes, oxidizing t h e m to PbO2 in H2SO4, and s t u d y ing the r a t e of discharge of the PbO~ to PbSO4.
The t h e r m o d y n a m i c model in Fig. 3, as well as the
d a t a of A w a d and E l h a d y (18) suggest t h a t
P b j ( P O D 2 can be f o r m e d b y anodizing a P b electrode
in a basic p h o s p h a t e solution in the P b ( I I ) potential
region. Therefore, a p u r e P b electrode was etched for
30 sec in an acetic a c i d / h y d r o g e n p e r o x i d e solution,
rinsed once in s a t u r a t e d a m m o n i u m acetate and twice
in distilled water, and corroded at --0.45V vs. a s a t u r a t e d calomel electrode in a s a t u r a t e d NajPO4 solution
(pH ---- 12.8) for 24 h r at room t e m p e r a t u r e . X - r a y
diffraction analysis confirmed t h a t the w h i t e film
w h i c h formed on the P b surface was Pbj(PO4)2.
A p u r e Pb sheet was p r e t r e a t e d for 1 h r in the same
w a y to form a Pb3 (PO4)~ film, was rinsed in deionized
water, and was then anodized in 4.5M H2SO4 at 1.275V
(vs. a Hg/Hg2SO4/4.5M H~SO4 reference electrode) for
1 d a y at 49~ to form a PbO2 film. A control electrode
which had been etched b u t not p r e t r e a t e d was corroded to PbO2 in H_~SO4 u n d e r the same conditions.
X - r a y diffraction analyses of these samples showed
o n l y Pb, fl-PbO2, and ~-PbO2. No Pbj(PO4)2 was
found in the p r e t r e a t e d electrode, p r e s u m a b l y b e cause the Pb.~(PO4)2 was f u r t h e r oxidized to PbO2.
Since a m u c h l a r g e r P b p e a k was found for the cont r o l t h a n for the p r e t r e a t e d electrode, it was concluded t h a t the PbO2 film was m o r e continuous on
the p r e t r e a t e d grid, and this conclusion was s u p p o r t e d
b y scanning electron m i c r o g r a u h s of the surfaces.
M e a s u r e m e n t s of t h e sulfation rates of the PbO2
films w e r e m a d e b y a p r e v i o u s l y d e s c r i b e d m e t h o d
(2). P u r e P b grids w e r e corroded in 4.5M H~_SO4 for
2.5 days at 49~ and then allowed to self-discharge
u n d e r the same conditions in e l e c t r o l y t e s a t u r a t e d
w i t h PbSO4 until t h e i r potentials b e g a n to drop b e l o w
1.1V. T h e y w e r e then rinsed in distilled water, dried,
and analyzed g r a v i m e t r i c a l l y for the weight of sulfate in the corrosion film. Assumin~ t h a t the sulfate
f o r m e d b y the reduction of PbO2 to PbSO4, the weight
of PbO2 o r i g i n a l l y in the corrosion film was calculated
and divided b y the sulfation time to obtain a r a t e for
the discharge reaction. The control sulfated at a r a t e
of 152 m g / h r and the grid which h a d been p r e t r e a t e d
w i t h Pb~(PO4)2 sulfated at a r a t e of 28 m g / h r . P r e viously (2), a sulfation r a t e of 176 m g / h r was r e p o r t e d for a control and 29 m g / h r for a grid corroded
in e l e c t r o l y t e containing 0.2% HjPO4. Thus, p r e t r e a t i n g t h e g r i d w i t h Pba(PO4)2 p r i o r to corroding
to PbO2 gave a sulfation r a t e c o m p a r a b l e to f o r m i n g
PbO2 in e l e c t r o l y t e containing 0.2% H~PO4 and n e a r l y
an o r d e r of m a g n i t u d e less t h a n PbO2 f o r m e d in p u r e
H2SO4.
The effect of p r e t r e a t m e n t w i t h PbHPO4 on s u l f a tion rates was also studied. F i l m s f o r m e d b y c o r r o d ing both in a solution of 0.5M H3PO4 -b 0.5M NaHePO4
and in a solution of 1M I-~PO4 at --0.375V vs. a s a t u r a t e d calomel electrode w e r e identified as PbHPO4
b y x - r a y diffraction. W h e n a p u r e Pb grid was p r e t r e a t e d in the H3PO4/NaH2PO4 solution to form
PbHPO4 and oxidized f u r t h e r in H2SO4 to PbO2, a film
w i t h a sulfation "rate" of 189 m g / h r was obtained.
This r a t e is c o m p a r a b l e to those of the grids corroded
in the absence of a n y phosphate.
Conclusions
A n i m p o r t a n t function of HjPO4 in l e a d - a c i d b a t teries is t h e f o r m a t i o n of Pb3 (PO4)~ as a n i n t e r m e d i a t e
SCIENCE AND TECHNOLOGY
N o v e m b e r I979
in the corrosion of Pb to PbO2. The PbO2 f o r m e d b y
this m e c h a n i s m sulfates v e r y s l o w l y and impedes
f o r m a t i o n of a resistive PbSO4 l a y e r at the g r i d / a c t i v e
m a t e r i a l interface, a p p a r e n t l y because t h e p o r o s i t y
a n d surface a r e a of t h e PbO2 a r e r e d u c e d (2). H o w
P b j ( P O D 2 influences the m o r p h o l o g y of PbO~ is not
y e t understood.
This Pbj(PO~)2 m e c h a n i s m occurs at v e r y low
H~PO4 concentrations. P r e v i o u s l y proposed m e c h a nisms (4-6) involving f o r m a t i o n of soluble P b ( I V )
p h o s p h a t e complexes m a y b e i m p o r t a n t at h i g h e r
p h o s p h a t e concentrations and m a y p r o d u c e mossing or
shorting. The effect of H j P o 4 adsorption on PbO2 (3)
and the possibility of localized PbHPO4 f o r m a t i o n
need f u r t h e r study.
A n o t h e r hypothesis, p r o p o s e d on the basis of cyclic
v o l t a m m e t r i c (20) and x - r a y diffraction (21) studies
of corrosion films, is t h a t HsPO4 increases the ratio of
~- to fl-PbO2. A l t h o u g h this i n t e r p r e t a t i o n deserves
f u r t h e r study, it should at p r e s e n t be v i e w e d w i t h
caution. I n p a r t i c u l a r , T a m u r a et al. (21) use the
~-PbO~ 100% line (111) to show increased ~-PbO2
f o r m a t i o n in the p r e s e n c e of HsPO4 in 10N H2SO4.
This line, at 3.12A, closely coincides w i t h the
Pbj(PO4)~ 100% line (015) at 3.09A (22). Thus,
T a m u r a ' s d a t a could also be used to s u p p o r t the
Pbj(PO4)2 mechanism. In fact a combination of
Pb.~(POD2 a n d /3-PbO2 gives a diffraction p a t t e r n
quite s i m i l a r to a combination of ~- and /~-PbO2, the
m a i n difference being t h a t Pba(PO4)2 has no analog
to t h e a-PbO2 70% line (021) at 2.63A.
Acknowledgments
The a u t h o r is g r a t e f u l to G l o b e - U n i o n I n c o r p o r a t e d
for the s u p p o r t of this r e s e a r c h and to the following
people for their contributions: M. Mueller, who c a r ried out the corrosion and s e l f - d i s c h a r g e e x p e r i m e n t s ;
F. Graetz and D. Mongan, who did the x - r a y diffraction analyses; E. Sensabaugh, who d e t e r m i n e d t h e
l e a d content of the corrosion films; a n d A. O r t i g u e r a
who d i d the scanning electron microscopy. T h e i n t e r est and h e l p f u l discussions of W. T i e d e m a n n and
B. Mahato a r e also appreciated.
M a n u s c r i p t s u b m i t t e d M a r c h 1, 1979; revised m a n u script received M a y 1, 1979. This was P a p e r 53 p r e sented at the Los Angeles, California, Meeting of the
Society, Oct. 14-19, 1979.
A n y discussion of this p a p e r will a p p e a r in a Discussion Section to be p u b l i s h e d in the June 1980
JOURNAL. A l l discussions for the J u n e 1980 Discussion
Section should be s u b m i t t e d b y Feb. 1, 1980.
Publication costs o] this article were assisted by
Globe-Union Incorporated.
REFERENCES
1. K. R. Bullock a n d D. H. McClelland, This Journal,
124, 1478 (1977).
2. K. R. Bullock, ibid., 126, 360 (1979).
3. J. P. C a r t and N. A. Hampson, J. ElectroanaL
Chem. Interracial Electrochem., 28, 65 (1970).
4. V. F. H u b e r and M. S. A. El-tvleligy, Z. Anorg. AlIg.
Chem., 367, 154 (1969).
5. H. Bode and E. Voss, Electrochim. Acta, 6, 11
(1962).
6. R. F. A m l i e and T. A. Berger, J. EIectroanaI. Chem.
Interracial Electrochem., 36, 427 (1972); P r i v a t e
communication.
7. M. Kugel, G e r m a n Pat. 480,149 (1926).
8. D. Evers, U.S. Pat. 3,011,077 (1961).
9. K. Yonetsu et al, J a p a n e s e Pat. 48-5175 (1973).
10. S. Tudor, A. Weisstuch, and S. H. Davang, Electrochem. Technol., 3, 90 (1965).
11. S. Tudor. A. Weisstuch, and S. H. Davang, ibid.,
4, 406 (1966).
12. S. Tudor. A. Weisstuch, and S. H. Davang, ibid., 5,
21 (1967).
13. W. C. McCrone and J. G. Delly. "The P a r t i c l e
Atlas," 2rid ed., VoI. I, pp. 119-129, A n n A r b o r
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Vol. 126, No. 11
LEAD-ACID BATTERY
1853
InterJacial Electrochem., 20, 79 (1969).
19. B. Mahato, This Journal, 126, 365 (1979).
20. W. Visscher, J. Power Sources, 1, 257 (1977).
21. H. Tamura, H. Yoneyama, C. Iwakura, a n d O.
Ikeda, I n t e r n a t i o n a l Lead Zinc Research O r g a n i zation Report on Project LE-254, March 10,
1977.
22. W. F r a n k McClune, Editor, ~'Power Diffraction
File," JCRDS, File No. 11-549 a n d 22-668,
Swarthmore, Penn. (1978).
Science P u b l i s h e r s (1973).
14. E. Voss, P a p e r 16, Proceedings of Second I n t e r n a tional S y m p o s i u m on Batteries, Bournemouth,
England, Oct. 18-20, 1960.
15. H. Bode, "Lead-Acid Batteries," p. 30, John Wiley
& Sons, Inc., New York (1977).
16. P. Ruetschi, This Journal, 120, 331 (1973).
17. P. Delahay, M. Pourbaix, and P. V a n Rysselberghe,
ibid., 98, 57 (1951).
18. S. A. Awad and Z. A. Elhady, J. ElectroanaL Chem.
Performance of LiAI/FeSx Cells with Negative
Electrodes Prepared by a Powder-Metallurgical Method
Reinhard Kn6dler,* Werner Baukal, and Gotthold B~hme
Battelle-Institut e.V., Franklurt am Main, Germany
ABSTRACT
A low cost negative electrode for lithium-metal-sulfide batteries has been
developed. The electrode, which can be used for cells assembled in the discharged state, is an aluminum disk with a high porosity of 50-60%. It can be
prepared by hot-pressing aluminum powder together with NaCI and subsequent leaching with water. The performance of this type of electrode was
tested in half-cell arrangements and in complete cells. No polarization was
caused by this electrode and it showed almost no swelling after cell operation.
This behavior looks promising with regard to a low-cost electrode and cell
fabrication.
kbar. As a c u r r e n t collector, a stainless steel s c r e e n
was embedded i n the powder mixture. After leaching
the disks with water, they had a porosity of 50-60%.
The electrodes used for the cell experiments are 48
m m in diameter and 6-8 m m i n thickness. They have a
capacity of about 12 A-hr, assuming a charge u p to
the composition of LiA1. Figure 1 shows a n SEM
micrograph of such a disk before a n experiment.
The feasibility of this method of p r e p a r a t i o n was
checked by testing the electrodes i n half-cell a r r a n g e ments and in complete cells. I n the case of the h a l f cells the counterelectrode consisted of m e t a l felt
soaked with lithium. The electrodes were p u t into a
LiC1/KC1 melt a n d kept at a distance of about 10 ram.
I n complete cells the positive electrode was prepared
by hot-pressing (500~ 0.5 kbar) a powder m i x t u r e
of Fe, Li~S, and electrolyte (8/66/26 volume percent).
The electrodes were wrapped i n a 300-mesh stainless
steel screen and a ZrO2 felt a n d contained a Mo-screen
as c u r r e n t collector.
The electrodes were put into the melt i n a stainless
steel case. The cells were carefully sealed a n d operated
in open air at 430~ The electrodes were self-supporting and had a distance of about 5-10 ram, so that no
separator was required. The cell capacity was about
10 A - h r (two plateau mode) and was limited b y t h e
positive electrode.
The rechargeable LiA1/FeSx b a t t e r y with a LiC1/KC1
eutectic as electrolyte is a promising candidate for
vehicle traction and load leveling applications (1). If
the complete cells are assembled i n the discharged
state, the negative electrode consists of a n a l u m i n u m
plaque. I n order to provide easy access of the electrolyte to the a l u m i n u m , the electrode has to be porous.
U s u a l l y this is achieved b y pressing a l u m i n u m wires
into the required electrode shape (2). F r o m a n economic point of view, however, it would be desirable
to use a low cost m a t e r i a l for this purpose, e.g., a l u m i n u m powder. This would i n addition allow a d j u s t m e n t of a proper porosity, so that an improved swelling characteristic of the negative electrode could be
expected. Therefore, the purpose of the work described
i n the following was to demonstrate the possibility of
operating cells with negative electrodes prepared from
a l u m i n u m powder, a n d to point out their performance
characteristics.
Experimental
Porous a l u m i n u m disks were prepared b y hot-pressing a l u m i n u m powder together w i t h NaC1 powder.
T a b l e I shows that stable electrodes can be obtained
b y hot-pressing them at 200~ a n d a pressure of 1
* Electrochemical Society Active Member.
Key words: lithium-metal-sulfide battery, anode, powder-electrode, cell impedance.
Table I. Preparation of porousaluminum disks from aluminum powder (grain size < 47 ~m) and a pore-forming substance
Pore-forming
substance
W e i g h t ratio
AI powder:
pore-forming
substance
NH~COs
1:1
NaC1
NaCI
NaCl
Temperature
during
pressing (1 kbar)
"C
Porosity of
disk after
leaching with
water
%
25
60
1:1
25
60
~1:1
200
60
2:1
~00
50
Mechanical
stability
V e r y poor
(no coherence)
Poor
(crumbling)
Rather good
(crumbling only
at the edges)
V e r y good
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