Additive Puts New Life In Lead Acid

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www.batteriesinternational.com October 2002 - Features

Additive puts new life in lead acid Tony Ferreira of Hollingsworth and Vose describes the science and performance of a new paste additive.

The evolution of lead acid battery technology has been driven by the introduction of new materials and new manufacturing processes and machinery. The awareness that purity played such a key role in the performance of the battery directed the early battery technicians to specify ever-purer raw materials. This trend has culminated in the ultra pure requirements for the manufacture of today's highly performing stationary batteries (Ref.1). In tandem with the trend for increasing raw material purity, there were great efforts to make more efficient electrodes, elements and other battery components. The move towards improved battery components was paralleled with ever improving manufacturing technologies. Another major strategy used by battery technologists to obtain better battery performance and also to improve the process of making these devices, has been to use appropriate additives, particularly active material additives. The role of these additives has been reviewed and it is generally accepted that modern batteries could not function effectively without their extensive use. (Ref.2). In this article we shall introduce a new material additive and briefly review its impact on the battery making process, as well as highlight some key performance advantages. This new inorganic additive has a noticeable impact on key processes of battery making, namely the mixing of the pastes that make up the electrode and also the process of making the battery plates. The impact on these process steps is largely due to the very hydrophilic nature of the new active material additive. The surface chemical composition of the inorganic active material additive, as well as its geometry and interaction with the surrounding active material structures, help us understand its impact on battery performance. This impact is mostly seen on the better charge acceptance and high rate discharge characteristics of the battery. The impact is universal and it cuts across manufacturing technologies and battery designs. The use of this new inorganic additive holds much promise for the next round of battery development. Table 1: Description of NewInorganic Active Material Additive Chemical Composition: Borosilicate Chemical Grade Glass Surface Area: >0.3 m2/g Density: Average Diameter Range: Length to Diameter Ratio: Addition Level:

2.4 to 2.6 g/cc 0.25 to 10 microns >5:1 1% to 3% of Oxide Weight

DESCRIPTION AND USAGE The new additive is a modified glass microfiber, that is designed and manufactured exclusively for lead acid battery applications. The major characteristics of this new active material additive are summarized in Table 1. It is available in industrial quantities and is shipped in pre-weighed plastic bags. It can be added directly to a paste mixer in much the same manner that expanders are added in most lead acid battery manufacturing plants. Normal precautions are

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recommended when handling fine particulate, (light facial mask, gloves and appropriate garments). As one can glean from the above description, this new additive is totally compatible with the electrochemistry of the lead acid battery. Chemical grade borosilicate glass is extensively used in the lead acid battery in the form of very absorbent separators that act both as separators and acid reservoirs in one important segment of the valve regulated type of lead acid battery (Ref.3). IMPACT ON PROCESS Preparing a paste batch with the inclusion of this new additive offers interesting possibilities and challenges to the battery technologist. As seen in Table 1, the surface of this additive is borosilicate glass. It is well known that water has a zero contact angle with glass surfaces. This means that there is a very high affinity between the additive surface and water. Battery paste preparation proceeds by two possible routes: "Dry Mixing - the oxide and additives are blended dry prior to being wetted by water and sulfuric acid. "Wet Mixing - water floods the mixer before the addition of the dry ingredients. The acid is added to the wet oxide mass. The rheology of paste mixing is complex and has been extensively studied (Ref. 4). In general terms, the amount of water added to the dry oxide will largely determine the density (cube weight) of the final paste. The other large contribution to paste density is the addition of sulfuric acid. Sulfuric acid reacts with the oxide to create lead sulfate. Thus, sulfuric acid impacts paste density by bulking it. Table 2: Charge Acceptance results of SLI batteries (flooded) and VRLA) Group: SLI Gr. 31 Flooded Deep Cycle SLI Gr.34 VRLA-Spiral Wound SAEJ537 (Spec. 22 5A) SAE J537 (Spec.24A) EN50432 (Spec. 9.6A) Std+/39.2A 36.8 A 8.8 A Std+/HV- 44.8 A (+14)* 43.4 A (+18%) * 11.2 A (+27%)* HV+/Std- n/a 50 A (36%)* 12.3 A (+40%)* HV+/42.5 A (+8%)* 50 A (+36%)* 10.6 A (+20%)* * % Difference from control (STD +/-) Table 3: Cold Cranking results of flooded GR 31 batteries. Average of six batteries per group. Group: Std+/Std+/HV-

750 Amp Discharge Volt 30 s Time to 7.2 v 8.10 v 46 s (112%) (153%) (99%) 8.37 v 66 s

850 Amp Discharge Volt 30 s Time to 7.2 v 7.16 v 33 s (110%) 7.78 v 48 s

When the paste mix includes the new additive, there will be competition for the water being added. The presence of the modified glass microfiber impacts the total amount of interstitial water. This means that the battery paste can be made with a greater amount of water in it and yet retain the required paste density (cube weight). Once this modified paste is used to paste battery plates, one gets pasted plates with a slightly higher amount of water and with an additive that is more hydrophilic than the particles of oxide. The physical model for the interaction of oxide particles and modified glass microfibers can be visualized as a blending of spheres and rods. The free volume of the packing of spheres is dramatically modified by the presence of rods (Ref. 5). When plates containing the modified glass microfiber additive are being made, more of them are produced. These additional plates are a result of the presence of the additive is four times less dense than lead, and the additional water used to prepare the modified batch. Another valuable contribution from the presence of the new additive is the impact the extra moisture has on the speed of the oxidation reaction that occurs in the first step of curing.

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Beyond the impact on moisture, the presence of this new additive is transparent in the remaining manufacturing steps. When the additive is present in amounts in excess of 0.5% of oxide weight, it imparts the same mechanical reinforcement that one encounters with the addition of much larger organic reinforcing fibers. To summarize, the greatest impact of this new additive to the process of making lead acid batteries is found in the critical steps of paste preparation and plate pasting. This new active material additive gives the battery technologist a new tool: the ability to better control the moisture content of paste mixes, plate pasting and curing. Thus, Moisture Control Management is the single greatest contribution of this new additive to battery manufacturing. This represents a step forward in the evolution of the process of lead acid battery manufacturing. Table 4: Cold Cranking results of VRLA Spiral Wound Deep Cycle GR 34 batteries. Average results of two batteries per group Group:

Std+/HV+/StdStd+/HVHV+/-

SAE J537 Cold Cranking @ 800A 10 s Volt= 7.5 s Volt 10 s Time to 7.2 v 7.93 v 23 s (105%) (77%) (102%) 8.12 v 31 s (108%) (103%) (113%) 8.01 v 27 s (107%) (90%) (106%) 8.18 v 31 s 7.94 v (109%) (103%) (106%)

EN50342 Cold Cranking @ 800A/480A C (Ah)= 9.6Ah Volt 10 s 7.65 v (89%) 8.15 v (110%) 8.00v (96%) 9.7 Ah (101%)

C (Ah) to 6.0 v 8.5 Ah 10.6 Ah 9.2 Ah

Table 5: Initial Performance values for Gr U1 (35 Ah) 12 v monoblock used in UPS applications. Average of two batteries of each group. Group

HV +/Std+/HVHV +/StdStd +/-

1 min 171.4 A (2.9 Ah) 6.2 Ah 6.5 Ah 5.7 Ah 5.3 Ah

5 min 102.3 A (8.5 Ah) 12.7 Ah 13.4 Ah 12.4 Ah 11.3 Ah

2 hr 11.7 A (23.7 Ah) 28.2 Ah 28.7 Ah 27.5 Ah 26.4 Ah

5 hr 5.5 A (27.5 Ah) 31.7 Ah 32.4 Ah 31.3 Ah 31.1 Ah

20 hr 1.8 A (35 Ah) 35.1 Ah 35.5 Ah 34.7 Ah 35.1 Ah

IMPACT ON SLI BATTERY PERFORMANCE Since over 60% of all lead acid batteries are destined for the SLI or automotive application, it is natural that this application should come under scrutiny in the present evaluation study. Thus, both flooded and VRLA battery designs were selected to participate in the study. The flooded battery type used was a Gr 31 having a Cold Cranking rating of 750 Amps and a Reserve Capacity of 180 minutes. The batteries tested in this study were made by a well-known manufacturer in the US and are good representations of the present technology being used to produce flooded automotive batteries. The automotive VRLA battery selected was the high performance spiral wound design manufactured in Europe. This battery has been modified to take on deep cycle functions. It was this latter variation that was used in this study. The spiral wound battery has a Cold Cranking rate of 750 Amps and a Reserve Capacity of 95 minutes. ENHANCED SLI BATTERY CHARGE ACCEPTANCE Charge acceptance was tested under two regimes: the SAE J537 and the EN50432 European Standard. Improvements are significantly higher with the VRLA battery testing. The improvement in charge acceptance is a key performance enhancement, particularly when this is measured to be in excess of 25%. This means that the battery can be brought back from a discharged state significantly faster. Charge acceptance improvements are significantly higher with the VRLA battery testing. Here, the modified plate

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batteries are shown as HV+, when the positives have been modified by the addition of the new additive. HVdenotes a negative plate modification and HV+/- denotes modification to both plates. The key observation of these charge acceptance results is that the positive modified group in the VRLA spiral design batteries gains a significant advantage from the new additive. This is due to its peculiar design as a deep cycle SLI battery. Overall, when both electrodes are modified, significant improvements occur. IMPROVED SLI BATTERY COLD CRANKING Cold cranking is perhaps the most important performance characteristic for SLI batteries. This is a particularly critical requirement in intemperate climates. The impact on cold cranking by adding the new additive to active materials is very significant and is reproducible in both flooded and VRLA battery types. It is an enhancement that is tied to the design of the battery. Therefore, the addition of the new additive will have a greater impact over the performance of the limiting electrode. Cold cranking test results are tabulated in Table 3 for flooded Gr 31 batteries. It is clear from these results that this type of battery greatly benefited from a modification of the negative plates. The improvement in discharge time to 7.2 volts is quite significant (over 50%) when the battery is cranked at its nominal value (750 A), and remains equally significant when the battery is cranked at the higher current of 850 Amps. Basically, when the negative plates of this type of battery are modified, the battery easily exceeds the increased cold cranking rate. The cranking results of the deep cycle version of the spiral wound Gr 34 battery show that the modified positive plates gained the most from the addition of the new inorganic additive. Here, the gains are in the range of 20% for the discharge time to 7.2 volts and higher (over 30%) when one examines the cold cranking capacity requirement of the EN50342 specification. These results will be explained below in terms of the structural changes to the active materials induced by the addition of the new additive. Table 6: BET test results of plates removed from SLI-VRLA gr 34 batteries Plates Std+ HV + Std HV 2.23 2.58 Unformed 1.66 2.34 2 2 m /g m2/g m2/g m /g % Difference + 40% + 15% Formed 3.83 4.48 0.55 1.20 2 2 2 m /g m /g m /g m2/g % Difference + 17% + 118% IMPACT ON STATIONARY VRLA (AGM) BATTERY PERFORMANCE Another important step in evaluating the impact of the new additive on the performance of the lead acid battery was to introduce it in Stationary types. Given the vast array of such batteries, two representative applications were selected: the Uninterrupted Power Supply (UPS) and the Telecom battery. Both of these applications call for VRLA batteries using the AGM (Absorbent Glass Mat) technology of electrolyte immobilization. In the present evaluation, two characteristics are examined: initial performance and deep cycling of the UPS batteries. The initial performance of UPS batteries shows that the high rates were improved by the addition of the modified glass microfibers to the plates, particularly the negative plates. ACTIVE MATERIAL STRUCTURAL CHANGES The performance chara-cteristics reported in the previous section are due to the structural changes induced to the active materials of the batteries under investigation, by the addition of the new modified glass microfiber additive. The investigation tools that are used to look into these structural changes are very widely employed in this field and, even though additional analytical methods were employed, for the sake of brevity only the most pertinent results are reported here. Also, given the scope of this work, only selected active materials were tested to explain a specific effect. In this manner, BET (Specific Surface Area) analysis of formed and unformed plates is reported here. The results of this analysis help to explain the impressive high rate results obtained with SLI batteries. The increased activity of the negative plates is also easier to understand when one examines the results of the X-R Diffraction analysis of cycled negative plates. The Scanning Electron Microscopy (SEM) technique is used to gain a

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better insight into the changes induced to the active material structures. BET MEASUREMENTS The Specific Surface Area of active materials is greatly impacted by the raw materials employed, as well as by the methods of manufacturing the electrodes (Ref.4). Thus, BET testing, measuring the specific surface area of active materials, accurately predicts many of the key characteristics of the battery. High rate discharges, in particular, are highly dependent on high specific surface area values. Table 6 shows the values of BET measurements done on unformed and formed plates removed from Gr 34 spiral wound SLI-VRLA batteries. From that table, one can deduce that the impact of the addition of the new additive to the formed negative active material is quite considerable (118% gain in specific surface area). The gain in the formed positive is less significant (17%). However, it should be mentioned that the gain in the BET values of the unformed positive is much greater than the formed positive (about 40%). This result may be a consequence of the particular requirements of the deep cycle version of this advanced SLI battery design and the radical differences in the pore structure of the positive and negative plates. X-RAY DIFFRACTION ANALYSIS This analytical technique gives us a glimpse into the chemical composition and the crystallography of electrode surfaces. With this in mind, following a reserve capacity discharge, one battery from each of the four test groups of Gr 31 flooded SLI batteries was torn down. The plates from the 3rd cell were removed and the negative plates were washed free of acid and air-dried. The plates were then analyzed for their X-R Diffraction spectra. In Fig. 3 (overleaf) one sees the spectrum of standard negative plates superimposed on the corresponding XRD spectrum of modified negative plates. The picture that emerges is that the lead oxide peaks are more intense on the modified plates and there is a decrease of the pure lead lines in these modified plate spectra. This is an indication that these plates oxidized more readily than the standard plates when exposed to the oxygen in the atmosphere. Because these modified plates contain hydrophilic glass microfiber in their structure and since water catalyses the oxidation of lead, this finding is not unexpected. It also predicts that modified negative plates would be capable of having more sites for a more vigorous internal oxygen cycle in VRLA batteries. SCANNING ELECTRON MICROSCOPY (SEM) The often-quoted expression that 'a picture is worth a thousand words' is very appropriate when one is with Scanning Electron Microphotographs. The inner structure of materials reveals itself under power magnifications of 5,000X and 10,000X. The SEM shown in Fig 4 illustrates the direct comparison at a 2,500X magnification between standard organic fibers and the new modified glass microfibers. The difference in diameters is at least an order of magnitude between the two materials. Another interesting observation is the relative amount of each. The larger organic fibers are few and far between. However, the much more numerous smaller inorganic fibers are every where and form supporting networks for the active material. Fig 5 provides a closer look (5,000X and 10,000X) at the interface between the special inorganic microfibers and the surrounding active material. It is obvious that around each microfiber one finds a micro gap that will fill up with electrolyte when the electrode is wetted. The very surface of the glass microfiber is equally covered in active material particulate. This intimate contact between the modified glass microfibers, and the possibility that they provide micro gaps that allow electrolyte to have easy access to the active material during intense utilization periods, explain the enhanced performance of the limiting electrodes during high rate discharges. The SEM is also a powerful tool to analyze the underlying structures of the negative active material. Using well-known techniques (Ref.5), the NAM of cycled Gr 31 plates that had been subjected to the XRD analysis were also treated to reveal their skeleton structure. Fig. 6 A & B show the difference between the skeleton structure of standard and modified negative plates. The modified negative NAM appears to be much more open with a very large surface area crystal formation. The three analytical techniques used, illustrate the structural changes that are induced by the presence of modified glass microfibers in active material with the inner specific surface area as measured by BET greatly increased. XRD shows that the surface of negative plates is easier to oxidize, explaining the higher capacity to enhance the oxygen internal cycle. SEM views of the active materials also provide a glimpse of how these special microfibers interact with the active materials, create a network of micro gaps through which the electrolyte has easier access to

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the active material. Since all electrochemical reactions rely on fast ion transport, and given the observation that the new microfiber additive impacts the active material structure to facilitate electrolyte penetration, this new additive can be known as an ION FLOW ENHANCING ADDITIVE. (1)F. Fleming, 'Float Life Verification of a VRLA Battery Utilizing a High Purity Electrochemical System', Intelec 1999, Section 3-3 (2)A. Ferreira, J. Power Sources, 95 (2001) 255-263 (3)G. Zguris, J. Power Sources, 67 (1997) 307-313 (4)H. Bode, Lead-Acid Batteries, John Wiley, New York, 1977 (5)D. Pavlov, J. Power Sources, 7 (1981/82) 153-164battery science

All material subject to strictly enforced copyright laws. © 2005 Euromoney Institutional Investor PLC.

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