Study finds flooded lead-acid battery performance & cycle life increased by adding dCNT to PAM and NAM; benefit for start-stop
Researchers with Molecular Rebar Design report that the addition of discrete carbon nanotubes (dCNT), which they call Molecular Rebar, to both the positive and negative electrodes (Positive Active Material, PAM and Negative Active Material, NAM) in conventional flooded lead-acid batteries results in: little change to reserve capacity; improved cold cranking, increased charge acceptance, and enhanced overall system efficiency. The company had earlier reported on the impact of dCNT addition primarly to the negative electrode. (Earlier post.)
Life cycle tests show >60% increases when dCNT are incorporated into the negative electrode and up to 500% when incorporated into both electrodes, with water loss per cycle reduced >20% (High-Rate Partial State of Charge, HRPSoC and SBA S010 idling start-stop testing). A paper describing the study and the results is published in an open access paper in the Journal of Power Sources.
In order to meet the demands of modern lead acid battery applications, the technology must provide higher levels of charge acceptance to boost system efficiency and delay common failure mechanisms such as sulfation or dendritic growth. For example, in the modern automobile, advanced systems such as navigation, heating, and air conditioning can increase electrical energy consumption beyond that which the alternator can replenish during normal vehicle operation. The battery system therefor operates at a detrimental energy deficit. In order to maintain batteries at higher states of charge and avoid irreversible sulfation, modern applications demand increased charge acceptance. As a second example, batteries operating in hybrid-electric automobiles as well as some grid storage applications must be able to effectively accept charge in quick, high-current bursts or risk negative electrode passivation. If these challenges can be surmounted, lead acid batteries will remain competitive with other chemistries in the automobile, storage, and telecom markets, amongst others.—Sugumaran et al.
As one approach to address these issues, carbon has been added to the NAM during paste preparation in a variety of forms including carbon black, activated carbon, graphite and carbon nanotubes. Carbon additives have been found to increase the charge acceptance of a battery by more than 200%—but at the cost of paste rheology and paste density. Reductions in paste density directly lead to decreased active material adhesion to the grids, decreased battery capacity which requires higher active material masses to reach specification, and insufficient cold-cranking performance, the authors note.
Further drawbacks include the need for new paste processing machinery, and the potential introduction of gas-evolving impurities into the electrode. If the carbon additive is particularly high in iron residuals remaining from its production, for example, gas evolution and water loss will increase, leading to premature battery failure.
Molecular Rebar’s dCNT are cleaned, functionalized, individualized, multi-walled, carbon nanotubes which are easily incorporated into battery pastes as a concentrated, pourable fluid. The fluid replaces a portion of the water used during the paste mixing process, requiring no alteration to existing industrial production lines.
In this latest study, the researchers incorporated dCNT into the negative electrode or both the negative and positive electrodes and compared the resulting batteries to dCNT-free control batteries across various performance metrics including pasting properties; basic performance (formation, reserve capacity, and cold temperature performance); advanced performance (polarization and triple electrode studies); gassing; cycle life (HRPSoC and SBA models); and battery failure mechanisms.
Of particular note for start-stop/micro-hybrid applications, the team found in HRPSoC testing that the baseline batteries did not complete 4000 cycles before reaching the End of Discharge (EOD) voltage limit (10.38 V). Batteries incorporating dCNT in the negative electrode reached 4000 cycles well before 10.38 V. Moving beyond the parameters of the test, the researchers removed the 4000 cycle limit for the second round of cycles and found that the battery containing dCNT achieved a further 6648 cycles before reaching the EOD limit.
In the more aggressive Japanese SBA S0101 testing, dCNT boosted the performance of simple, flooded automotive battery to performance levels usually only reached by those advanced batteries. Incorporation of dCNT into the negative or negative and positive electrodes allowed a battery to reach 9999 or 35300 cycles, respectively. When compared to the control battery reaching 5883 cycles, this represented a 60% or 500% increase in lifetime.
To explain the observed performance, the authors introduce a new hypothesis: the dCNT/Had Overcharge Reaction Mechanism.
At its core, our hypothesis posits that the surface of dCNT could act as a transient storage medium for adsorbed hydrogen (Had). A quickly-mobilizable supply of high-powered reducing agents, such as these Had, could explain many of our observations regarding the performance of dCNT in the negative electrode of lead acid batteries. In the positive electrode, where hydrogen production is thermodynamically disfavored, dCNT may be fulfilling a different function, potentially one involving mechanical or material strength improvements as opposed to electrochemical enhancements.
… In these ways, transient hydrogen adsorption could explain all of our reported results. Although other CNT may be capable of performing some of these roles, only dCNT are capable of performing them uniformly, and completely, across the entire electrode so that the entire lead matrix feels their effect, even at low dCNT loading concentrations (0.16%). The quality of the dCNT dispersion through the matrix is fundamental to our interpretation of our dCNT/Had Overcharge Reaction Mechanism. The dCNT/Had hypothesis is far from proven, requiring significant research and fine tuning before more general acceptance. A full investigation of the hypothesis, supported with appropriate experimentation and analysis, will be the subject of our next report.—Sugumaran et al.
Nanjan Sugumaran, Paul Everill, Steven W. Swogger, D.P. Dubey (2014) “Lead Acid Battery Performance and Cycle Life Increased Through Addition of Discrete Carbon Nanotubes to Both Electrodes,” Journal of Power Sources, doi: 10.1016/j.jpowsour.2014.12.117