Black Diamond Structures
In lead-acid battery cycling tests, addition of discrete carbon nanotubes (dCNT) to Positive Active Material (PAM) extends life. Despite this observation, dCNT are undetectable in PAM following formation. This paradox led us to examine aspects of the positive electrode that are established prior to or during formation and the role dCNT play in their evolution. One such aspect is the corrosion layer, the protective boundary between current collector and PAM, which provides lasting benefits to the positive plate without the persistent need for any components that may have assisted in its development.
Here, we detail physical and electrical differences identified in dCNT-modified positive plates and confirm the absence of dCNT following formation via scanning electron microscopy. When compared to control samples, we find the corrosion layer constructed in dCNT-modified positive electrodes to be thinner and more uniform following formation and after high-temperature cycling in a variety of grid alloys. Mechanistic theories are also discussed.
We present evidence that dCNT alter the structure of the corrosion layer in a dose-responsive fashion, thus providing an original hypothesis for the observed dCNT-based enhancements for corrosion-intensive cycling.
The conversion of clusters of multi-wall carbon nanotubes into individual, discrete nanotubes called MOLECULAR REBAR® for new applications
Carbon Nanotubes (CNT’s) have been extensively researched and promoted since the early 1990’s with Richard Smalley of Rice University as a leading proponent. Their intrinsic properties of very high specific stiffness and strength, excellent electron conductivity and the nanoscale geometry were fully expected to revolutionize various industries from electronics to advanced materials. This theoretical performance expectation justified globally intense research interest and expense (estimated to be $35 billion over the last 20+ years). Unfortunately, the promise and the commercial delivery did not match, and today, while CNT’s have a variety of uses, they are not as widely used as originally predicted. The purpose of this article is to illustrate with practical examples that carbon nanotubes, if made into a discrete and dispersible form, can reach their full theoretical properties and commercial potential.
The key technical issue is that carbon nanotubes, as produced in commercial processes such as a fluidized bed reactor, are entangled in micron sized, or larger, clumps that are difficult to separate while maintaining the desired length of the discrete nanotubes. As entangled micron sized clumps of materials, the nanotubes do not have the aspect ratio nor the nanoscale to deliver the theoretical properties needed to produce valued performance.
Contemporary applications are changing the failure mechanisms of lead acid batteries. Sulfation at the negative electrode, acid stratification, and dendrite formation now precede positive electrode failures such as grid corrosion and active material shedding. To attenuate these failures, carbon has been explored as a negative electrode additive to increase charge acceptance, eliminate sulfation, and extend cycle life. Frequently, however, carbon incorporation decreases paste density and hinders manufacturability.
Discrete carbon nanotubes (dCNT), also known as MOLECULAR REBAR®, are lead acid battery additives which can be stably incorporated into either electrode to increase charge acceptance and cycle life with no change to paste density and without impeding the manufacturing process. Here, full-scale automotive batteries containing dCNT in the negative electrode or both negative and positive electrodes are compared to control batteries. dCNT batteries show little change to Reserve Capacity, improved Cold Cranking, increased charge acceptance, and enhanced overall system efficiency. Life cycle tests show >60% increases when dCNT are incorporated into the negative electrode (HRPSoC/ SBA) and up to 500% when incorporated into both electrodes (SBA), with water loss per cycle reduced >20%. Failure modes of cycled batteries are discussed and a hypothesis of dCNT action is introduced: the dCNT/Had Overcharge Reaction Mechanism.
Performance demands placed upon lead acid batteries have outgrown the technology’s ability to deliver. These demands, typically leading to Negative Active Material (NAM) failure, include: short, high-current surges; prolonged, minimal, overvoltage charging; repeated, Ah deficit charging; and frequent deep discharges. Research shows these failure mechanisms are attenuated by inclusion of carbon allotropes into the NAM. Addition of significant quantities of carbon, however, produces detrimental changes in paste rheology, leading to lowered industrial throughput. Additionally, capacity, cold-cranking performance, and other battery metrics are negatively affected at high carbon loads.
Presented here is MOLECULAR REBAR® Lead Negative, a new battery additive comprising discrete carbon nanotubes (dCNT) which uniformly disperse within battery pastes during mixing. NS40ZL batteries containing dCNT show enhanced charge acceptance, reserve capacity, and cold-cranking performance, decreased risk of polarization, and no detrimental changes to paste properties, when compared to dCNTfree controls. This work focuses on the dCNT as NAM additives only, but early-stage research is underway to test their functionality as a PAM additive.
Batteries infused with Molecular Rebar Lead Negative address the needs of modern lead acid battery applications, produce none of the detrimental side effects associated with carbon additives, and require no change to existing production lines.