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 Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). 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 Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). 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 Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). 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. Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). 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). Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). 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 Downloaded on 2016-04-08 to IP 130.203.136.75 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).