WO2015103314A2

WO2015103314A2 - Method and apparatus for improving charge acceptance of lead-acid batteries

Info

Publication number: WO2015103314A2
Application number: PCT/US2014/072846
Authority: WO
Inventors: Srinivasan Venkatesan; Fabio Albano; Subhash Dhar; William Koetting; Susmitha GOPU; Erik W. ANDERSON; Lin Higley; Kevin DAHLBERG
Original assignee: Energy Power Systems LLC
Priority date: 2013-12-31
Filing date: 2014-12-30
Publication date: 2015-07-09
Also published as: WO2015103314A3

Abstract

A lead-acid battery is disclosed. The lead-acid battery includes a positive electrode comprising positive active material and barium acetate and a negative electrode comprising negative active material.

Description

METHOD AND APPARATUS FOR IMPROVING CHARGE ACCEPTANCE

OF LEAD-ACID BATTERIES

DESCRIPTION OF THE INVENTION

Related Applications

[001 ] This application is a continuation in part of and claims the benefit of priority to U.S. Application No. 14/145,640, filed December 31 , 2013, which is a continuation in part of and claims the benefit of priority to U.S. Application No.

13/768, 192, filed February 15, 2013, which is a continuation in part of U.S.

Application No. 13/588,623, filed August 17, 2012. This application is a continuation in part of and claims the benefit or priority to U.S. Application No. 13/842,777, filed March 15, 2013, which is a continuation in part of U.S. Application No. 13/475,484, filed May 18, 2012. This application incorporates the disclosure of all of the applications identified above, the entire disclosure of U.S. Application No.

13/350,505, filed January 13, 2012, the entire disclosure of U.S. Application No. 13/843,953, filed March 15, 2013, and the entire disclosure of U.S. Application No. 13/350,686, filed January 13, 2012.

Field of the Invention

[002] This disclosure is related to lead-acid batteries in general and carbon additives for improving charge acceptance of lead-acid batteries in particular.

Background of the Invention

[003] Conventional batteries for vehicle applications include flooded Starting- Lighting-Ignition (SLI) batteries and Absorbed Glass Mat (AGM) batteries. A conventional flooded SLI battery is filled with liquid electrolyte in the cell

compartments and may require maintenance to ensure proper performance of the batteries. A conventional AGM battery includes porous micro-fiber glass separators that absorb the electrolyte, and does not need maintenance.

[004] In micro-hybrid electric vehicles (HEVs), a battery experiences charge-discharge cycles that are typically very shallow (<10% depth-of-discharge, DOD). Yet, over time, the accumulated capacity turnover can be substantial. Under these conditions, a conventional flooded SLI battery can withstand an accumulated capacity turnover of about 150 times its nominal capacity. A conventional AGM battery can withstand about 450 capacity turnovers. In both cases, long rest times and insufficient recharge periods result in irreversible sulfation. The dominant failure mode of lead-acid batteries in micro-hybrid applications is sulfation, which causes cyclic capacity fade due to reduced charge acceptance.

[005] Regenerative breaking (REGEN) is an almost universal feature in hybrid-electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs). The electric drive is operated as a generator during deceleration to recharge the battery, hence, the battery is operated at partial state-of-charge (PSOC) to provide significant charge acceptance during REGEN.

[006] For current micro-hybrid vehicles to increase their impact on fuel economy from the current 5-8% to 15-18% improvement, the battery needs to be able to combine the stop/start and REGEN functions more efficiently. Thus, there is a need for a battery that can withstand prolonged operation at PSOC and produce energy throughputs required by HEV and PHEV vehicles. Preferably, batteries for these applications should be able to withstand 8-10% swings in charge/discharge capacity around 50-70% SOC for at least 60,000 cycles with an approximate 4,800 to 6,000 capacity turnovers before experiencing any decay in charge acceptance.

[007] Full recharge or overcharge cycles (reset cycles) are commonly used in the industry to mitigate sulfation issues at PSOC. Micro-hybrid duty cycles, however, offer limited time slots for battery recharging, which are very often interrupted by new discharge periods before full recharge is attained. Moreover, charging times are limited by the passenger driving cycles where the average duration of an urban trip is 30 minutes with a large number of stop/start operations and idle modes. Hence, the battery can rarely achieve a full charge under real-world operating conditions. Another issue with overcharging the batteries to mitigate sulfation is that it promotes hydrogen evolution from the negative plates, causing the batteries to dry out. Thus, there is a need for a more effective solution other than modifying the charging regimen of known batteries.

[008] Figure 1 illustrates the processes taking place at the negative plate of a lead-acid battery during charge and discharge processes. During discharge, lead sulfate crystals form within the active mass and continue to grow with each partial cycle. During a full charge, the sulfate is reconverted into active mass, i.e. , spongy lead (Pb) at the negative electrode and highly porous Pb0₂ at the positive electrode. However, there is a size limit to which the sulfate crystals can grow. When the lead sulfate crystals grow to a threshold larger than the pore size, they restrict access to the sulfuric acid, making the process of sulfation irreversible and resulting in permanent loss of capacity and power. Even when the sizes of the crystals are smaller than this threshold, the diffusion rate of the sulfate ions may not keep up with the discharge rate at high current. Hence, keeping the size of lead sulfate crystals small is desirable for improving the fundamental mechanisms of lead-acid batteries.

[009] "Sulfation" refers to a result of grain size growth of lead sulfate, which is deposited on battery plates during discharge of a lead-acid battery. Normally, the lead sulfate deposit is so fine-grained that, during recharge, it easily reverts back to sulfuric acid, lead, and lead dioxide - the components of a lead acid battery that produce electricity. This process is called "soft sulfation."

[010] When "hard sulfation" occurs, however, the grains of lead sulfate become too large to react effectively during recharge. According, some lead sulfate remains on battery plates at the end of a charging cycle. As a result, the lead-acid battery gradually loses its capacity, energy, and power, due to the passivating lead sulfate.

[01 1 ] The mechanism for lead sulfate crystal growth depends on the thermodynamic driving force that forms large grains from small grains. However, the rate of transport of lead ions through the acid depends on the solubility of lead in the acid which is in the low ppm range for a fully charged battery.

[012] Preventing sulfation from causing battery failure is difficult because there are many ways sulfation can occur, but only a few methods to control its formation. Two conventional techniques in the battery industry attempt to control sulfation. The first conventional technique is to use an extended charge, known as an equalization charge to slow-down the rate of sulfation by periodically "pushing" a[L the cells to a full charge so that most of the fine-grained lead sulfate discharge product is removed before it has a chance to grow into large, "hard sulfate" grains.

[013] The second conventional technique to reduce the rate of battery sulfation is to add sodium sulfate to the acid. Sodium sulfate reduces lead solubility in acid because the higher concentration of sulfate (from the relatively high sulfuric acid concentration and the addition of sodium sulfate) drives down the lead ion solubility based on the common ion effect as an outcome of the solubility product restriction. While proven to be helpful, these conventional techniques are not reliable.

SUMMARY OF THE INVENTION

[014] In accordance with the invention, an electrode and a lead-acid battery including the same are disclosed.

[015] According to an embodiment, an electrode comprises active material comprising lead and a carbon additive configured to increase a charge input of the lead-acid battery by at least 17%, relative to a negative electrode without the carbon additive.

[016] According to another embodiment, an electrode comprises active material comprising lead and a carbon additive of at least 1 % in weight of the active material.

[017] According to another embodiment, a lead-acid battery comprises a positive electrode and a negative electrode. The negative electrode further comprises active material comprising lead and a carbon additive of at least 1 % of the active material in weight.

[018] Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

[019] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[020] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[021 ] Figure 1 illustrates chemical processes at a negative electrode during charge and discharge of a lead-acid battery. [022] Figure 2 illustrates graphical representations of forms and

characteristics of carbon additives.

[023] Figure 3 is a table summarizing characteristics of different

embodiments of carbon additives.

[024] Figure 4 illustrates cyclic voltammograms of different embodiments of carbon additives.

[025] Figure 5 illustrates comparisons of characteristics of different embodiments of carbon additives.

[026] Figure 6 illustrates the dynamic charge acceptances of different embodiments of lead-acid batteries with different depths of discharge at 25 ^°C.

[027] Figure 7 illustrates the dynamic charge acceptances of different embodiments of lead-acid batteries with different depths of discharge at 41 ^°C.

[028] Figure 8 illustrates the dynamic charge acceptances of different embodiments of lead-acid batteries with carbon additive under different compression levels.

[029] Figure 9 illustrates the dynamic charge acceptances of different embodiments of lead-acid batteries with second acid fill.

[030] Figure 10 illustrates the dynamic charge acceptances of different embodiments tested according to the SBA protocol.

[031 ] Figure 1 1 illustrates an embodiment of a cell for a lead-acid battery.

[032] Figure 12 illustrates changes of charge currents of different

embodiments of lead-acid batteries;

[033] Figure 13 illustrates changes of charge inputs, charge currents, and C rates of different embodiments of lead-acid batteries.

[034] Figure 14 illustrates changes of charge inputs of different embodiments of lead-acid batteries.

[035] Figure 15 illustrates changes of charge currents of different

embodiments of lead-acid batteries.

[036] Figure 6 illustrates changes of charge resistances and discharge resistances of different embodiments of lead-acid batteries.

[037] Figure 17 illustrates changes of average charge resistances and average discharge resistances of different embodiments of lead-acid batteries.

[038] Figure 18 illustrates changes of charge acceptances of different embodiments of lead-acid batteries as functions of state of charge. [039] Figure 19 illustrates changes of charge acceptance of different embodiments of lead-acid batteries as functions of capacity turnover.

[040] Figure 20 illustrates a top view of a cross-wire structure based on carbon material, according to an embodiment.

[041 ] Figure 21 illustrates an angled view of the cross-wire structure of Figure 20.

[042] Figure 22 illustrates a top view of a negative and a positive electrodes having carbon material, according to an embodiment.

[043] Figure 23 illustrates an angled view of the negative and positive electrodes of Figure 22.

[044] Figure 24A illustrates the charge resistance of batteries made according to exemplary embodiments of this disclosure.

[045] Figure 24B illustrates the discharge resistance of batteries made according to exemplary embodiments of this disclosure.

[046] Figures 25A and 25B illustrate the molecular structures of the organic poisons additives according to exemplary embodiments of this disclosure.

[047] Figures 26-29 illustrate cyclic voltammetry diagrams comparing the performance of the disclosed embodiments.

[048] Figure 30 illustrates different embodiments of acetate additive that may be used in a battery.

DESCRIPTION OF THE EMBODIMENTS

[049] Reference will now be made in detail to the exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[050] Three factors, among others, may affect charge acceptance of a lead- acid battery, i.e., the effective amount of charge being accepted by a lead-acid battery during charge. The first factor is related to the age of a battery, i.e., the number of capacity turnovers that the battery has experienced. As the battery is cycled over time, its capacity may fade, and the effective amount of energy input may decline. The second factor is related to the resistance of the battery. Sulfates may grow and accumulate over the cycle life of the battery, increasing the internal resistance of the electrodes and limiting the effective amount of charge that can be accepted by the battery. The third factor is related to the charging protocol. If the voltage limits are setup in a manner that they are reached at an early stage, then the effective amount of charge input may be reduced accordingly.

[051 ] Dynamic charge acceptance (DCA) can be defined as a ratio between the average amount of current input during a duty cycle l_reCu (A) and the nominal capacity of the battery Cn (Ah):

/ I(t)dt

DCA = ^ =— .

Cn C_n

[052] The amount of charge input may vary according the cycle life and the cycle protocol of the battery. The value is normalized by the capacity in order to make it comparable among batteries of different sizes and so that different charging protocols can be directly compared.

[053] The lead-acid battery disclosed herein may be suitable for vehicle applications. The DCA of the lead-acid battery may be high enough to match any vehicle alternator output, which corresponds to input current pulses of about 10 s at 120-240 A, i.e. , a value greater than or equal to about 2 A/Ah.

[054] According to an embodiment, carbon may be used in a lead-acid battery to reduce sulfation and improve dynamic charge acceptance at a partial state of charge (PSOC). Preferred forms of carbon may include, e.g., carbon blacks, activated carbons, graphitic carbons. These carbon may also be in the form of graphene, carbon nanotubes, fullerenes, double walled carbon nanotubes, carbon fibers, carbon felt, meso-carbon microbeads (MCMB), carbon cones, carbon needles, carbon platelets, carbon nano-belts, carbon nano-wires, or another suitable formulation.

[055] These carbon forms may have different inherent properties, including particle size distribution, aggregates sizes and shapes, specific surface area, electrical conductivity, porosity, surface functionality, and impurities. These properties may improve the charge acceptance of lead-acid batteries. Figure 2 illustrates schematic representations of these properties of carbon materials relevant to improvement on DCA of lead-acid batteries. According to various embodiments of the present disclosure, carbon or combinations of carbons are used in lead-acid batteries to improve DCA. The mechanisms by which carbon forms may affect charge acceptance in lead-acid batteries are also determined. [056] In some embodiments, three forms of carbon are respectively added to the electrodes to improve the performance of lead-acid batteries at high rate (i.e., peak power) operation in partial state of charge (HRPSOC). Examples of these carbon forms may include:

[057] Expanded graphites made by Timcal at Strada Industriale, CH-6743 Bodio, Switzerland, and Superior Graphite at 10 South Riverside Plaza, Suite 1470, Chicago, IL, USA;

[058] Carbon blacks made by Cabot at 157 Concord Road, Billerica, MA 01821 , USA, and Kuraray at Ote Center Building, 1-1 -3, Otemachi, Chiyoda-ku, Tokyo 100-81 15, Japan; and

[059] Activated carbons made by EnerG2 at 100 NE Northlake Way, Seattle, WA 98105, USA.

[060] One of ordinary skill in the art will recognize that carbon additives other than those listed above may also be used in lead-acid batteries consistent with this disclosure.

[061 ] Although these carbon additives are all based on carbon as far as composition and chemistry are concerned, they operate differently in a red-ox environment, such as those in lead-acid batteries. Table 1 in Figure 3 includes a list of the carbon additives used in the various embodiments in this disclosure and their properties.

[062] To test the certain embodiments of carbon additives, small discs of certain of the carbon powders disclosed above were prepared using a binder and compacted under isostatic compression using a mold. Cyclic voltammetry was then performed on the small discs to identify the onset of hydrogen evolution and the relative kinetics of its evolution in the various materials.

[063] The overpotential for hydrogen evolution on carbon is lower than that on lead and therefore hydrogen evolution is favored when carbon is used in a negative electrode. Thus adding carbon to lead acid chemistry requires special care in managing the hydrogen gas evolution at the negative plate. Figures 4a-4c illustrate results from the voltammetry tests conducted to pre-screen suitable carbon compounds, including carbon black (Figure 4a), activated carbon (Figure 4b), and graphite (Figure 4c), according to an embodiment, in order to limit hydrogen evolution due to overpotential favorable conditions. [064] As shown in in Figures 4a-4c, the onset of hydrogen evolution occurs at the interception between the current curves and the X axis. The higher the potential voltage (i.e., less negative or closer to zero Volts) at which the sample curve intersects the zero current line, the more effective the additive. In some embodiments, a small hydride formation peak may occur before the intercept, but the hydrogen evolution regions are a primary concern in these embodiments. The slope of the current rise indicates the rate of hydrogen evolution. Thus, a higher slope corresponds to a faster kinetics of hydrogen evolution.

[065] According to Figures 4a-4c, it can also be seen that the type of carbon may influence the rate of hydrogen evolution. A carbon material having a higher surface area provides a lower operating current density. Hence it may be desirable to select a carbon material with a high surface area and low hydrogen over-potential. Figures 5a-5c illustrate comparisons of the surface areas, potentials of hydrogen evaluation, and rates of hydrogen evolution associated with the carbon materials used in the various embodiments of this disclosure.

[066] While all carbon additives tested are capable of increasing the electrical conductivity of the active mass by percolation phenomena, it has been reported that their effectiveness dies out quickly above a quantity of 2-4% wt of the active material. Without wishing to be bound by theory, the present inventors believe that the reason for this may be a function of the type of carbon rather than the conductivity of the carbon being lower than that of the metal itself. Capacitance is believed to play a major role in limiting carbon's effectiveness. It is also possible that excessive amounts of carbon can make the hydrogen evolution reaction dominant, thus, limiting the charge acceptance of the active material. The carbon contribution to electrical conductivity may be important when the level of sulfates in the active material increases above a certain threshold that negatively affects the power performance. This phenomenon is commonly seen in the HRPSOC regimens associated with hybrid vehicles applications.

[067] According to an embodiment, high surface-area carbons, e.g., carbon blacks with a surface area of over 1 ,500 m²/g, may provide extra nucleation sites for sulfate crystals, thereby restricting their growth and limiting their size during

HRPSOC. These carbon materials also exercise a steric effect that limits the growth of large sulfate crystals by making unfavorable the thermodynamics of their growth. They also contribute to the capacitance of the negative active mass. Thus, it is desirable to use a material with a relatively higher surface area and a relatively lower content of contaminant.

[068] According to an embodiment, the onset of hydrogen evolution may be considered a marker for better kinetics. And the slope of hydrogen evolution may be considered a reinforcing parameter. Based on these parameters, the inventors of this disclosure have identified the PbX 51 carbon ("PbX51 " hereinafter) listed in Table 1 as an exemplary material for improving charge acceptance of lead-acid batteries. Other carbon material may also be used without being limited to those disclosed here. For example, carbon materials produced by Sid Richardson Carbon and Energy Company may also be used to provide similar improvements described here.

[069] The inventors of this disclosure believe that carbon may improve charge acceptance of lead acid batteries for the following reasons:

[070] Carbon gives higher conductivity at PSOC;

[071 ] Carbon increases the capacitance of the negative electrode;

[072] Carbon provides protective coating on the lead sulfate crystals thus preventing them from growing into large crystals; and

[073] Carbon nucleates smaller lead sulfate crystal growth;

[074] The inventors also believe that the lead sulfate reduction provided by carbon is chemically driven and not just an electrochemical process. The reducing agent here is the "nascent hydrogen" or atomic hydrogen at the surface of the carbon. This atomic hydrogen production is the first step in the electrochemical water discharge reaction, presented by formulas 1 and 2 below.

[075] (1 ) 2C + H₂0 + 2e 2C... H +(OH)-

[076] (2) 2C... .H + PbS0₄ «"» 2C + Pb + H₂S0₄ + 2e

[077] Overpotential is defined as an added applied potential that enables a reaction to occur. In the context of the present disclosure, relatively higher hydrogen overpotential on lead requires greater potential be used to get the hydrogen to evolve. On the other hand, relatively lower hydrogen overpotential provides more efficiency and allows the hydrogen evolution to take place at lower potential. Thus, the lower the hydrogen overpotential (i.e., less negative or closer to zero Volt), easier it is for the water discharge reaction to take place and the easier it will be for the reduction of lead sulfate. Thus the carbon materials that show a less negative hydrogen overpotential have a better lead sulfate reduction rate. [078] According to an embodiment, two methods may be used to validate and verify this hypothesis:

[079] 1 . A platinized carbon electrode may be used instead of pure carbon. Since hydrogen evolution is expected to be highly favored on Pt substrates, the charge acceptance may also be improved; or

[080] 2, Adding a hydrogen evolution poison to the electrolyte, i.e., an "electrode poison ion," which once adsorbed at the surface of the carbon prevents the atomic hydrogen from recombining. Formulas 3 and 4 below represent the chemical process without the electrode poison and the chemical process with the poison, respectively.

[081 ] (3) M H + M H -> 2M + H₂ ( without electrode poison)

[082] (4) M H + (Poison) M... .H - M H + M H

[083] As a result, the coverage of atomic hydrogen at the surface may increase, along with the dwell time and with it the rate of sulfate reduction.

[084] High surface-area carbons having particle sizes of 0-20 nm, when used in the electrodes, may enhance DCA of lead-acid batteries. In one

embodiment, the surface area of the carbon additive may be at least about 750 m²/g. In a further embodiment, the surface area of the carbon additive may be at least about 1 ,500 m²/g. Further enhancements may also be achieved by creating a mixture or a matrix of particle size distributions including a combination of small and large particle sizes.

[085] In a further embodiment, the DCA of negative active materials may be improved by optimizing the particle size distribution by combining large particles with small particles having high surface area. In a further or alternative embodiment, the carbon content may be at least 1 % by weight of the negative active material of the electrode. In a further or alternative embodiment, the carbon content may be greater than 3% of the negative active material by weight. In a further embodiment, the carbon content may be increased to up to 20-25% of the negative active material by weight to enhance the capacitance performance. In a still further or alternative embodiment, the carbon content may be less than 30% of the negative active material by weight. In a still further or alternative embodiment, carbon structures that are compatible with hydrogen evolution (e.g., PbX51 discussed above) may be used in the negative active materials. [086] Table 2 lists embodiments of this disclosure tested for DCA performance at 25 ^°C under different testing protocols.

TABLE 2

[087] Figure 6 illustrates the testing results showing the DCA performance of the embodiments listed in Table 2. As shown in Figure 6, DCA values are highly dependent on the state of charge of a battery and the testing regimen.

[088] Table 3 lists additional embodiments of this disclosure tested for DCA performance at 41 ^°C under different testing protocols.

TABLE 3

[089] As shown in Figure 7, DCA values are relatively higher at 41 °C compared with those at 25 °C, in Figure 6.

[090] Table 4 lists additional embodiments of this disclosure tested for DCA performance under different compression levels relative to a free standing stack of electrodes and separators.

TABLE 4

[091 ] As shown in Figure 8, DCA values are relatively higher at higher compression levels. [092] Table 5 lists additional embodiments of the disclosure tested for DCA performance with relatively higher acid fills, in which a battery was refilled with acid after all the electrode pores were made available by the completion of the formation processes.

TABLE 5

[093] As shown in Figure 9, DCA values are relatively higher with second acid fill compared with batteries without second acid fill.

[094] Table 6 lists additional embodiments of the disclosure tested for DCA performance under the SBA cycling protocol, which is a standard developed by the Battery Association of Japan to determine the cycle life of lead acid batteries for use in vehicles with idling stop-start systems. The SBA cycling protocol is defined in the Battery Association of Japan Standard, SBA S 0101 :2006, which is incorporated by reference in its entirety.

TABLE 6

[095] Figure 10 shows that DCA values are higher with 3.0 NAM, a carbon- black enhanced negative active material. [096] According to the above-disclosed embodiments, DCAs of embodiment #4 is much higher than DCAs of the other embodiments analyzed above. The above-disclosed tests show that the DCA of lead-acid battery is affected by the testing protocol. In addition, DCA increases when SOC becomes higher, when temperature becomes higher, when secondary acid fill is performed, or when compression amount is increased. This is because, under these conditions, the cell resistance generally decreases.

[097] According to an embodiment, tests were conducted to compare the charge acceptance performance of a lead-acid battery with a conventional negative active material (NAM) having a conventional form of carbon (e.g. , graphite) and a lead-acid battery with a negative active material (NAM) having the Pbx51 carbon additive identified above. For ease of references, "NAM 0.0" hereinafter refers to the conventional lead-acid battery with the conventional negative active material, and "NAM 51 " hereinafter refers to the lead-acid battery with the negative active material having the Pbx51 carbon additive. As described above, Pbx51 is a carbon black with high surface area, less negative hydrogen over-potential, and low rate of hydrogen evolution. Except for the Pbx51 , other components in the NAM 0.0 battery and the NAM 51 battery are the same.

[098] The NAM 0.0 battery and the NAM 51 battery each include 5 cells, although less or more cells may also be included. Figure 1 1 illustrates the structure of a cell 1100 used in both NAM 0.0 and NAM 51 . Each cell 1100 includes a first unit 1102 and a second unit 1104. First unit 1102 includes a plurality of separators 1106 separating a Pb foil 1 108, a positive end plate 1110, and a negative bipole plate 1112. Negative bipole plate 1112 is disposed on a spacer 1114 through one of separators 1106. Second unit 1104 also includes a plurality of separators 1116 separating a Pb foil 1118, a positive bipole plate 1120, and a negative plate 1122. Pb foil 1118 is disposed on a spacer 1124 through one of separators 1116. Positive bipole plate 1120 of second unit 1104 is electrically coupled to negative bipole plate 1112 and Pb foil 1108 of first unit 1102. Positive end plate 1110 of first unit 1102 provides a positive terminal for connecting with other cells or circuits. Negative end plate 1122 and Pb foil 1118 of second unit are coupled together to provide a negative terminal for connecting with other cells or circuits. Pb foil 1 108 and negative bipole plate 1112 of first unit 1102 are coupled together and electronically connected with positive bipole plate 1120 of second unit 1104. [099] In an embodiment, the cells for NAM 0.0 and NAM 51 are formed and treated according to a standard protocol including 335% formation and four C/2 conditioning cycles. After the conditioning cycles, nominal capacity of these cells is about 2 Ah at a C/10 rate.

[0100] Each battery was subject to the following test method including:

[0101 ] Step 1 : charging at C/2 rate up to 4.9 V;

[0102] Step 2: charging at C/10 rate up to 4.9 V or 105% of Nominal

Capacity;

[0103] Step 3: discharging at 1 C rate to a specific state of charge (e.g., 20%, 40%, 60%, and 80%);

[0104] Step 4: resting for 30 minutes; and

[0105] Step 5: charging at 4.95 V for 10 minutes.

[0106] The analysis metrics include the maximum current and the charge input, which may be measured during Step 5 above. Figures 12-15 illustrate the test results showing comparisons between the NAM 0.0 battery (i.e. , the control) and the NAM 51 battery. Figure 12(a)-(c) show the change of charge current (A) as a function of time (s) for SOC of 20%, 40%, 60%, and 80%, respectively, during the 10-minute charge time in Step 5 above. The charge current of NAM 51 remains greater than the charge current of NAM 0.0 until the batteries are almost fully charged.

[0107] Figure 13(a) illustrates comparisons of charge input (Ah) of NAM 0.0 and NAM 51 for different SOCs during the 10-minute charge time in Step 5 above. Figure 13(b) illustrates comparisons of charge input (in percentage) of NAM 0.0 and NAM 51 for different SOCs during the 10-minute charge time in Step 5 above.

Figure 13(c) illustrates comparisons of maximum charge current of NAM 0.0 and NAM 51 for different SOCs during the 10-minute charge time in Step 5 above.

Figure 13(d) illustrates comparisons of charge C rate for different SOCs during the 10-minute charge time in Step 5 above. As shown in Figures 13(a)-13(d), compared with NAM 0.0, NAM 51 may achieve greater charge input (in both Ah and

percentage), greater maximum charge current, and greater C rate.

[0108] Figures 14(a)-(e) illustrate the comparisons of charge input (mAh) for various SOCs and various charge intervals. The charge input is measured after the batteries are charged for 1 second (Figure 14(a)), 5 seconds (Figure 14(b)), 10 seconds (Figure 14(c)), 30 seconds (Figure 14(d)), and 60 seconds (Figure 14(e)). As shown in these figures, charge input is greater for NAM 51 than NAM 0.0 in all charge intervals and all SOCs during the charge in Step 5.

[0109] Figures 15(a)-(e) illustrate the comparisons of charge current (A) for NAM 0.0 and NAM 51 at various times during the charge in Step 5. The charge current is measured at 1 second (Figures 15(a)), 5 seconds (Figures 15(b)), 10 seconds (Figures 15(c)), 30 seconds (Figures 15(d)), and 60 seconds (Figures 15(e)) after charge starts in Step 5. As shown in these figures, charge current for NAM 51 is greater than NAM 0.0 at all times.

[01 10] Table 7 below summarizes the comparisons between NAM 0.0 and NMA 51 illustrated in Figures 12-15 discussed above.

TABLE 7

[01 1 1] As Table 7 above shows, the NAM 5.1 battery achieved consistently higher charge acceptance than the NAM 0.0 battery. In particular, at 20% state of charge (SOC), the NAM 51 battery received charge input 20%-35% greater than the NAM 0.0 battery. Charge current in the NAM 51 battery is 30%-60% greater than the NAM 0.0 battery. At 40% state of charge, the NAM 51 battery received charge input 17%-24% greater than the NAM 0.0 battery. Charge current in the NAM 51 battery is 55%-80% greater than the NAM 0.0 battery. At 60% state of charge, the NAM 51 battery received charge input 35%-41 % greater than the NAM 0.0 battery. Charge current in the NAM 51 battery is 105%-131 % greater than the NAM 0.0 battery. At 80% state of charge, the NAM 51 battery received charge input 30%- 33% greater than the NAM 0.0 battery. Charge current in the NAM 51 battery is 102%-137% greater than the NAM 0.0 battery.

[01 12] In an embodiment, the hybrid pulse-power capability (HPPC) test is conducted on the NAM 0.0 and NAM 51 batteries to compare charge and discharge resistances at various depths of discharge (DODs) including, for example, 20% DOD, 40% DOD, 60% DOD, and 80% DOD. The HPPC test includes the following steps:

[0 13] Step 1 : discharging at 1 C rate (nominal);

[01 14] Step 2: charging at 1 C rate (nominal) followed by constant voltage roll off;

[01 15] Step 3: resting for 1 hours;

[01 16] Step 4: discharging by 10% depth of discharge;

[0 7] Step 5: resting for 1 hour after discharging;

[01 18] Step 6: discharging pulse at 5C rate for 10 seconds or voltage below 1.2 V/cell;

[01 19] Step 7: resting for 40 seconds;

[0120] Step 8: charging pulse at 5C rate for 10 seconds or voltage lid of 1.66 V/cell; and

[0121] Step 9: repeating Steps 4-7 for 9 times.

[0122] Discharge and charge resistances were obtained based on the voltage AV and current ΔΙ values measured during above Step 5 and Step 8, respectively, according to the following formula:

R = ΔΧ//ΔΙ.

[0123] Figures 16 and 17 illustrate comparisons between the NAM 0.0 and NAM 51 batteries based on the HPPC test results. According to the HPPC test results, the NAM 51 battery has better performance than the NAM 0.0 battery. As shown in Figure 16(a), the NAM 51 battery has lower discharge resistance than the NAM 0.0 battery at all DODs. According to Figure 17, the average discharge resistance of the NAM 51 battery is 4-40% lower than the NAM 0.0 battery, depending on the specific depth of charge. As shown in Figure 16(b), the NAM 51 battery has lower charge resistance than the NAM 0.0 battery at all DODs.

According to Figure 17, the average charge resistance of the NAM 51 battery is 40- 47% lower than the NAM 0.0 battery, depending on the specific depth of charge. As shown in Figure 16(c), the NAM 0.0 battery could not sustain the 5C charge pulse for more than 0.5 second at any depth of discharge. The NAM 51 battery, on the other hand, could sustain the 5C charge pulse for 10 seconds at depth of charge greater than 40%.

[0124] According to an embodiment, the charge acceptance of a battery may be determined for all state of charge according to the HPPC testing protocol. As shown in Figure 18, the NAM 51 may achieve greater charge acceptance than a conventional AGM battery within the entire range of SOC.

[0125] According to an embodiment, the charge acceptance of a battery may also be determined according to the SBA testing protocol for various numbers of capacity turnovers. As shown in Figure 19, the charge acceptance of the

conventional AGM battery quickly declined to a significantly low value after about 1000 cycles. The charge acceptance of NAM 51 , however, remained a substantially constant high value for over 6000 cycles and only declined slightly above 6000 cycles.

[0126] It will be apparent to persons of ordinary skill that variations and modifications may be made in the use of carbon to improve charge acceptance without departing from the scope of the appended claims or their equivalents. The present inventors do not intend to restrict the invention to the particular carbon blacks, activated carbons, or graphitic carbons described above. Rather, it is intended that these variations and modifications in the form of the carbon used be considered part of the invention, provided they come within the scope of the appended claims and their equivalents.

[0127] For example, Figures 20 and 21 illustrate a cross-wire structure 2000 including carbon material disclosed above, according to an embodiment. Cross-wire structure 2000 includes a set of lead wires 2002 and a set of carbon wire 2004. Lead wires 2002 may be arranged in a parallel fashion. Carbon wires 2004 may also be arranged in a parallel fashion, crossing lead wires 2002. Carbon wires 2004 and lead wires 2002 may then be woven with each other to form cross-wire structure 2000. Carbon wires 2004 may include high capacitance carbon felt wire, carbon tape, or composite carbon. Cross-wire structure 2000 may be incorporated in a negative electrode and/or a positive electrode of a lead-acid battery. Carbon wires 2004 may provide enhanced charge acceptance performance of the lead-acid battery consistent with the embodiments disclosed above. [0128] Figures 22 and 24 illustrate a lead-acid cell 2200 having a positive electrode 2202 and a negative electrode 2204, according to an embodiment.

Negative electrode 2204 may include pasting paper having carbon material disclosed above. The carbon in the pasting paper may be made from carbon felt, carbon tape, or any other carbon material treated for enhancing capacitance.

Positive electrode 2202 may include glass pasting paper or carbon-based pasting paper similar to that in negative electrode 2204. The carbon-based pasting paper may provide enhanced charge acceptance performance of the lead-acid battery consistent with the embodiments disclosed above.

[0129] According to a further embodiment, negative electrode 2204 includes a carbon fiber needle milled PAN fiber veil. The PAN veil is pretreated with a plasma arc to activate the carbon material with a high surface area for increased dynamic charge acceptance. Strips of surface activated carbon veil are rolled onto both sides of a pasted bipolar plate to form the negative electrode. This embodiment provides a direct replacement for non-active AGM and the glass fiber pasting paper in the conventional lead-acid batteries, while providing enhanced charge acceptance performance. Alternatively, this embodiment may be used as an under layer or as a carrier for more fragile PAN SACV applications and provide enhanced charge acceptance performance.

[0130] According to another embodiment, the positive electrode and/or negative electrode may each include a substrate or grid coated with carbon particle paste. The carbon particle paste may include carbon material disclosed above. The carbon-coated substrate may provide enhanced charge acceptance performance of the lead-acid battery consistent with the embodiments disclosed above.

[0131 ] According to a still further embodiment, an acetate additive may be added to the acid solution of the electrolyte in a lead-acid battery to further improve its charge acceptance performance. For example, an acetate ion containing compound, such as R-COOH, where R stands for an inorganic or organic ion with positive charge, selected from group 1 or group 2 elements, such as hydrogen, alkaline, alkaline earth, organic groups including methyl, ethyl, propyl, butyl, or the like, may be added to the electrolyte. According to an embodiment, acetate ions derived from the added salt or acetic acid may react with the passive lead sulfate film on the surface of the positive or negative electrode, thereby removing lead sulfate formed on the surface of the electrode due to sulfation. Accordingly, the lead sulfate is dissolved in a controlled fashion, forming lead acetate, which is more soluble than lead sulfate.

[0132] For example, the acetate additive may include sodium acetate. When a "sulfated" electrode is exposed to a sodium acetate solution, the following reaction takes place:

PbS0₄ + 2CH₃COONa -» Pb(CH₃COO)₂ + Na₂S0₄

[0133] Since lead acetate solubility is higher than that of lead sulfate, all or portions of the lead sulfate from the electrode surface may be dissolved during charging and discharging cycles, thus reducing the charge resistance and improving the charge acceptance of the battery. As a result, the passivating lead sulfate may be effectively removed from the surface of the electrode plates, thus restoring the electrode surface for normal operation. By virtue of the presence of acetate ions in the electrolyte solution, further buildup of lead sulfate may be prevented during charging and discharging.

[0134] According to an embodiment, the lead sulfate has greater solubility in sodium acetate than in sulfuric acid. The desirable region for sulfate solubility is between 2-7 mg/L. This corresponds to a sulfuric acid concentration inside the battery between 1 .080 and 1.280 g/cm3. The lead sulfate that is dissolved exists as lead acetate in the electrolyte solution. Lead dioxide and lead formation at the respective electrodes still occur during charging when lead acetate is present in the solution.

[0135] In one embodiment, magnesium acetate, calcium acetate, ammonium acetate, potassium acetate, sodium acetate, or, simply, acetic acid with hydrogen positive ion may be added to the electrolyte to achieve similar effects to provide positive ions. In another embodiment, acetate, tartrate, oxalate, formate, or other carboxylate groups may be used in the electrolyte to provide negative ions.

[0136] Concentration of the additives and the operational procedures may also be optimized. Figure 30 shows the concentrations of various embodiments of the acetate additive in weight percentage and corresponding molar concentration. For example, the acetate additive may form about 1 % to about 5% of the electrolyte by weight or mass (i.e., mass concentration). Alternatively, the concentration of the acetate additive in moles (i.e., molar concentration of the acetate additive) may be less than 3% of the electrolyte. One of ordinary skill in the art would recognize that the mass concentration and the molar concentration disclosed above may be modified according to the species of the acetate added to the electrolyte. One of ordinary skill the art would also recognize that the mass concentration may be measured based on weight the total weight of the electrolyte including all of the additives or the weight of the electrolyte without the additives. For example, in one embodiment, 1 % of the additive by weight may refer to 1 gram of additive added to 100 grams of electrolyte, resulting in a total of 101 grams of electrolyte solution. Alternatively, 1 % of the additive by weight may refer to 1 gram of additive added to 99 grams of electrolyte, resulting in a total of 100 grams of electrolyte solution.

[0137] Embodiments of the present disclosure may improve overall cycle life of lead-acid batteries and revive battery cells showing sulfation related failures. By adding the additive early enough in during the operation of a battery, sulfation tendencies can be prevented, thereby improving charge acceptance. The disclosed additive may be effective in all types of lead-acid batteries including SLI batteries for automotive applications.

Barium Acetate Additive to Improve Charge Acceptance

[0138] According to an embodiment, the charge acceptance of the lead-acid battery may be further enhanced by adding acetate to the active material used to form the substrate. This acetate may be added to the NAM paste in addition to other additives, such as the carbon additive discussed above, or added without the carbon additive. The acetate additive may be any percentage of the total additive mixture added to the active material. According to an embodiment, the acetate additive may be about 5% of the total additive mixture by weight. According to an embodiment, the acetate additive may be about .5% of the total active material by weight.

[0139] According to an embodiment, the acetate may be barium acetate. The barium acetate additive is added to the active material of the electrode (i.e. , the positive active mass (PAM) or the negative active mass (NAM)) during the mixing operation. When the barium acetate additive is added to the active material mass, the following formula represents the chemical reaction during the charging and discharging operation of the lead-acid battery:

Ba(CH3COO)2 + H2S04 <r -> BaS04 + 2 CH3COOH

[0140] The barium acetate additive may cause the battery electrode, to operate more efficiently, compared with those without the barium acetate additive. In particular, the barium acetate additive may better promote formation of fine crystals of lead sulfate in the substrate, than traditional lead-acid battery with lead sulfate or barium sulfate, which is added to the paste mix to act as nuclei for forming small lead sulfate. According to an embodiment, the barium acetate additive may be added to the NAM paste to promote the formation of needle-like, nano-size crystals of barium sulfate insitu. These needles like nano crystals are isomorphous with lead sulfate and promote the growth of nano-size lead sulfate crystals that is necessary for the efficient operation of the lead-acid battery.

[0141 ] In addition to forming the nano crystals of barium sulfate, the barium acetate also releases negative acetate ions during the operation. These negative acetate ions prevent excessive lead sulfate to be deposited on the surface of the electrode, thereby providing better lead sulfate reduction rate as described above in connection with the carbon additive. Accordingly, the barium acetate may further enhance the utilization efficiency of the active materials and enhance the de-sulfation mechanism.

[0142] Figures 24A and 24B illustrate, respectively, charge resistances and discharge resistances of batteries made according to various exemplary

embodiments disclosed herein. The charge resistances and discharge resistances shown in Figures 24A and 24B are obtained based on the Hybrid Pulse Power Characterization (HPPC) test described above. The charge resistance and the discharge resistance of each battery are tested at various state of charge (SoC) between nearly 0% SoC and 90% SoC.

[0143] In Figures 24A and 24B, the control battery ("the control") represents an embodiment of a battery without any of the additives disclosed above. The HRx2 battery ("the HRx2") represents an embodiment of a battery similar to the control battery and is charged at a higher rate that is twice the rate used to charge the control battery. The PAM 3.5 battery ("the PAM 3.5") represents a battery including a special formulated PAM made with extra fibers and extra tetrabasic lead sulfate formation conditions and a NAM without any additives disclosed above. The NAM 5.1 battery ("the NAM 5.1 ") represents a battery including a NAM made with a 1 .6% carbon additive disclosed above and a PAM without any additives disclosed above. The NAM 5.1 + 5%BA battery ("the NAM 5.1 + 5%BA") represents a NAM 5.1 battery having additionally a 5% barium acetate. The P-3.5 & N-5.1_5%BA battery ("the P- 3.5 & N-5.1_5%BA") represents a battery including a PAM made with extra fibers and extra tetrabasic lead sulfate formation conditions, a NAM made with a 1 .6% carbon additive disclosed above, and the 5% barium acetate. [0144] As shown in Figures 23A and 23B, in general, batteries having the carbon additive have lower charge resistance and lower discharge resistance than the control. Batteries (i.e. , the NAM 5.1 , the NAM 5.1 +5%BA, and the P-3.5 & N- 5.1_5%BA) having carbon additive in the negative active mass have lower charge resistance than other batteries (i.e. , the control, the HRx2, and the PAM 3.5). In addition, batteries (i.e., the NAM 5.1 +5%BA, and the P-3.5 & N-5.1_5%BA) having barium acetate have lower charge resistances and lower discharge resistances than those without the barium acetate additive. According to Figures 24A and 24B, the barium acetate additive may provide improved charge acceptance performance when added to the active material of a lead-acid battery.

[0145] In other embodiments, the barium ion in the above-disclosed embodiments may be replaced or added in addition with other ions with positive charge, such as sodium ions, calcium ions, strontium ions, magnesium ions, and the like, to provide similar effects. Among the negative ions, the acetate may be replaced with citrates, tartrates, propionates, oxalates, and the like, without departure from the principle of this disclosure.

[0146] According to further embodiments, the charge acceptance of a lead- acid battery may be further improved by additional additive, such as organic additives or poisons. Organic additives or poisons with desired properties may be added to the PAM paste, the NAM paste, or the electrolyte to suppress hydrogen evolution and more effectively aid the lead sulfate reduction, thereby enhancing charge acceptance of the lead-acid battery.

[0147] For example, 4 hydroxy methoxy benzaldehyde, methoxy

benzaldehyde, or the like may be used as the organic additive and added to the electrolyte. Figures 25A and 25B illustrate the molecular structures of the exemplary organic additive studied by the inventors. Other organic additive with similar properties may also be used without being limited to those disclosed here. In one embodiment, about 100 ppm of the organic additive may be added to achieve the desired improvement.

[0148] Figures 25-29 illustrates cyclic voltammograms showing performance comparisons between lead-acid batteries with the organic additives disclosed here and those without the organic additives. In these cyclic voltammograms, when a curve crosses the zero voltage line, hydrogen evolution occurs within the corresponding battery. The slopes of a curve represent the rate of the hydrogen evolution, where a greater slope corresponds to a greater rate of hydrogen evolution.

[0149] Figure 25 shows the performance comparison between a control battery ("PBX-51 control") having a carbon additive and no organic additives and an exemplary battery ("2-H-4-M") with the same carbon additive and an organic additive, 2-Hydroxy-4-methoxybenzaldehyde. According to the cyclic

voltammograms in Figure 25, compared with the PBX-51 control battery, the 2-H-4- M battery has hydrogen evolution at a relatively earlier point. However, the rate of hydrogen evolution in the 2-H-4-M battery is lower than the PBX-51 control battery.

[0150] Similarly, Figure 26 shows the performance comparison between the PBX-51 control battery and an exemplary battery ("2Methoxy") with the same carbon additive and an organic additive, 2Methoxy. According to the cyclic voltammograms in Figure 26, compared with the PBX-51 control battery, the 2Methoxy battery has hydrogen evolution at a relatively earlier point. However, the rate of hydrogen evolution in the 2Methoxy battery is lower than the PBX-51 control battery.

[0151 ] Figures 28 and 29 show performance comparison between the PBX-51 control battery and another control battery ("SC-159 control"), which has a different carbon additive and no organic additives. These figures show that the two control batteries have similar starting points and rates of hydrogen evolution.

[0152] The cyclic voltammograms in Figures 26-29 show that the organic additives disclosed here allow hydrogen evolution to occur more uniformly throughout the entire operational range of a battery. As a result, the organic additives allow better management of hydrogen evolution, reduce bubbling during the operation of a battery due to hydrogen evolution, and improve battery

performance at the high rate of discharge.

[0153] This may indicate that the product of reaction is actually the additive organic (i.e., the poison) that works rather than the starting materials. The poisons may postpone hydrogen evolution while initiating favorable hydrogen onset. The mechanism of reaction is based on the fact that the first step of reaction in the cathode reaction is the atomic hydrogen formation represented as follows:

H20 + e -» 2[H] + [OH-]

[0154] When two hydrogen atoms recombine, hydrogen evolution occurs. The organic molecules chemisorb onto the substrate of the electrode, thus preventing two hydrogen atoms from coming together and preventing gas evolution. This process increases the residence time of atomic hydrogen so as to providing more time to react with the lead sulfate, thereby leading to lead sulfate reduction.

[0155] The atomic hydrogen produced above, if prevented from

recombining to itself to generate molecular hydrogen gas, may then react with the lead sulfate, thereby reducing the lead sulfate to lead metal according to the following formulation:

2[H] + PbS04 - Pb + H2S04

[0156] Since atomic hydrogen is consumed in this reaction, hydrogen gas evolution is prevented or reduced. Thus, the poisons operate by keeping the atomic hydrogens apart according to a Chemisorption process. In particular, the poisons adsorbs onto the surface of the electrode more strongly, thereby providing enough "residence time" for the atomic hydrogen to react with other species available nearby.

[0157]

[0158] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. For example, the additives disclosed above may be introduced to the negative active material or the positive active material to improve the charge acceptance and to achieve comparable results as discussed above. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

WHAT IS CLAIMED IS:

1 . A lead-acid battery, comprising:

a positive electrode comprising positive active material and barium acetate; and

a negative electrode comprising negative active material and barium acetate

2. The lead-acid battery of claim 1 , wherein the barium acetate is about 1 .5% by weight of the positive active material of the positive electrode.

3. The lead-acid battery of claim 1 , further comprising an electrolyte having an organic electrode poison.

4. The lead-acid battery of claim 3, wherein the organic electrode poison is 4 hydroxy methoxy benzaldehyde.

5. The lead-acid battery of claim 3, wherein the organic electrode poison is 2 methoxy benzaldehyde.

6. The lead-acid battery of claim 1 , wherein the negative electrode further comprises a carbon additive.

7. A lead-acid battery, comprising:

a positive electrode comprising positive active material;

a negative electrode comprising negative active material; and

an electrolyte comprising an organic electrode poison.

8. The lead-acid battery of claim 7, wherein the organic electrode poison is 4 hydroxy methoxy benzaldehyde.

9. The lead-acid battery of claim 7, wherein the organic poison is 2 methoxy benzaldehyde.

10. The lead-acid battery of claim 7, wherein the positive active material further comprises barium acetate.

1 1. The lead-acid battery of claim 10, wherein the barium acetate is about 1 .5% by weight of the positive active material of the positive electrode.

12. The lead-acid battery of claim 7, wherein the negative electrode further comprises a carbon additive.

13. The lead-acid battery of claim 12, wherein the carbon additive has an average particle size less than 50 nm in diameter measured according to ASTM standard.

14. A lead-acid battery, comprising:

a positive electrode comprising positive active material; and

a negative electrode comprising negative active material and a carbon additive, wherein the carbon additive is at least 1 % by weight of the negative active material.

15. The lead-acid battery of claim 14, wherein the carbon additive has an average particle size less than 50 nm in diameter measured according to ASTM standard.

16. The lead-acid battery of claim 14, wherein the positive electrode further comprises barium acetate.

17. The lead-acid battery of claim 16, wherein the barium acetate is about 1 .5% by weight of the positive active material of the positive electrode.

18. The lead-acid battery of claim 16, wherein the barium acetate is about 5% by weight of total additive added to the positive active material.

19. The lead-acid battery of claim 14, further comprising an electrolyte having an organic additive.

20. The lead-acid battery of claim 19, wherein the organic additive include at least one of 4 hydroxy methoxy benzaldehyde or methoxy benzaldehyde.