Carbon nanotube engineering company Molecular Rebar Design has developed Molecular Rebar Lead Negative, a new additive for lead acid batteries comprising discrete carbon nanotubes (dCNT) which uniformly disperse within battery pastes during mixing.
In an open access paper published in the Journal of Power Sources, a Molecular Rebar team reports that NS40ZL 12V automotive lead-acid batteries containing dCNT showed enhanced charge acceptance of more than 200%, reserve capacity, and cold-cranking performance; decreased risk of polarization; and no detrimental changes to paste properties, when compared to dCNT-free controls. The study focused on the dCNT as Negative Active Material (NAM) additives only, but early-stage research is underway to test their functionality as a Positive Active Material (PAM) additive as well.
NS40ZL (38B20L) batteries are defined by the Japanese Industrial Standards Committee (JISC, Standard #D 5301 2006) as 12 V, B-type, automotive batteries rated with 45 min reserve capacity, 35 Ah C20 rate, and 280 s cold-crank performance.
As the uses and expectations for lead acid batteries evolve, so too do their modes of failure. Traditionally, the majority of failures were related to grid corrosion and softening in the Positive Active Material (PAM) however, in contemporary applications, these have been replaced by sulfation in the Negative Active Material (NAM), acid stratification, and short-circuit producing dendrite formation. These “contemporary applications” are defined as any in which the battery experiences: frequent, short, high-current surges during charge and discharge (ex. micro-hybrid vehicles); minimal, but consistent overvoltage charging (ex. solar/photovoltaic cells); deficit charging caused by the increased energy demands of modern electronics (ex. automotive power); or, two to three deep cycles per day (ex. motive power). Recent studies suggest that such failure mechanisms can be averted by increasing the battery’s charge acceptance, often through addition of excess carbon.
… Although the concept that carbon enhances charge acceptance is universally acknowledged, controversy exists regarding which allotrope produces the best effect, and in what concentration. … The mechanism by which carbon increases charge acceptance is also disputed.
… The ideal carbon additive would be uniformly distributed throughout the paste and capable of boosting charge acceptance at concentrations low enough to avoid the decreases in paste density, cold-cranking performance, and reserve capacity experienced with current NAM additives.—Swogger et al.
Molecular Rebar Design has a proprietary process that disentangles and functionalizes stock carbon nanotubes, which usually appear as “fuzzy balls” of highly entangled nanotubes that do not disperse uniformly within the material. Molecular Rebar Lead Negative (MRLead-), is provided as a pourable, aqueous fluid comprising surfactant-stabilized, discrete, carbon nanotubes (dCNT). The highly dispersed nature of the fluid enables additive uniformity throughout the NAM to ensure the dCNT benefits are felt plate-wide.
Molecular Rebar functionalization includes addition of carboxylic acid groups to the surface of the tubes and an increase in the number of open ends. The process also provides additional cleaning such that their residual catalyst content (Fe, Al, etc.) drops ~80% from 5% by weight carbon to less than 1%, resulting in near pristine dCNT.
In the study, the Molecular Rebar team compared the performance of the dCNT-augmented batteries with standard versions. All battery data presented in the study was an average of five concurrently tested batteries. Prior to testing, all batteries were fully charged using a constant voltage of 14.8 V and a limiting current of 8 A for 20 h. Batteries were allowed to cool for 4 hours prior to use.
Among their findings were:
The addition of dCNT made no significant changes to rheology, density, or penetration; i.e., incorporation of dCNT will require no changes to existing paste mixing, pasting, or manufacturing processes.
dCNT change the microstructure of the material, producing a more ordered geography than CNT-free plates that may increase plate porosity.
dCNT decrease the voltages required during formation and delays the onset of overvoltage-related side reactions such as gassing to increase the efficiency of the process. The change in plate microstructure for dCNT-containing batteries is likely one of the contributors to formation improvement but other metrics, such as polarization or charge acceptance, could be playing a role, the team suggested.
A marginal improvement of ~1-2% is observed in the reserve capacity of batteries containing dCNT when compared to CNT-free controls. In other words, while other studies have shown that carbon additives tend to negatively impact reserve capacity, dCNT have a non-negative effect on it.
dCNTs have a positive effect on two cold-cranking metrics: duration and voltage.Inclusion of dCNT into battery plates produces a ~6-10% increase in the length of time taken to decrease the battery voltage to 6 V at 270 A in a 18 C environment across the three cycles.
dCNT batteries also produce a marginal (~2%) increase in the voltage produced after a 30 s discharge at 270 A in a 18 C environment. These data indicate that batteries containing dCNT perform comparably, or slightly better than, CNT-free batteries in cold conditions. This is, again, counter to the expectation that additional carbon has a negative outcome in cold-cranking tests.
Batteries containing dCNT allow for ~15% more charge to be absorbed and ~3% more charge to be used during tests than control batteries.
Batteries containing dCNT are capable of dealing more effectively with high current and overvoltage. By limiting overvoltage situations, the batteries better resist polarization-inducing side reactions.
Batteries built with dCNT channel larger currents to accept more charge than control batteries at 0 ˚C.
Polarization studies showed that dCNT incorporation allows batteries to channel up to ~200% more current at the same voltage, or produce ~21% less overvoltage at the same current, than control batteries.
Steven W. Swogger, Paul Everill, D.P. Dubey, Nanjan Sugumaran (2014) “Discrete carbon nanotubes increase lead acid battery charge acceptance and performance,” Journal of Power Sources, Volume 261, 1 September 2014, Pages 55-63, ISSN 0378-7753, http://dx.doi.org/10.1016/j.jpowsour.2014.03.049