As the initial implementation phase of its MicrocellTM foam grid technology, Firefly’s 3D cell architecture involves replacing the conventional lead metal-based negative plate with a foam electrode. These products are configured in such a way as to be easily incorporated into existing lead acid manufacturing processes possessed by all existing lead acid manufacturers.
Because of this relatively seamless integration into established manufacturing techniques, it is Firefly’s intention to manufacture the pasted foam negative electrodes and furnish them to existing lead acid manufacturing partners who will incorporate them into finished battery products. To all outward appearances, these batteries will be indistinguishable from currently available products. Furthermore, they will have similar charge / discharge performance and other fundamental properties. Due to the use of an electrolyte compatible with conventional lead acid cell designs, the open circuit voltages and recharge float voltages correspond to those of conventional lead acid batteries. This condition permits the use of conventional lead acid chargers with Firefly Energy’s 3D batteries.
3D Technology Performance Attributes
The real quest for performance improvements in lead acid batteries is all about surface area. Although selective, incremental enhancements have led to some gains over the last several decades, the overwhelming restriction to lead acid battery efficiency has always been the lack of interface area between the active chemistry and the electrodes. With chemistry capable of delivering approximately 170 Watt Hours per Kilogram (Wh/kg), why are today’s batteries only averaging around 30-50 Wh/kg? The answer to this question lies in the century-old paradigm that lead electrodes are necessary in lead acid batteries. Unfortunately, lead corrodes, so increasing surface area increases corrosion and decreases life.
Firefly’s technology does increase surface area, tremendously. This has the obvious benefit of increasing the interface area between the active chemistry and the electrode, yielding better and faster utilization of the chemistry. Beyond that, fast charge and discharge rates are now achievable, and a higher percentage of active material is accessible, so efficiency goes up. In fact, utilization efficiencies can potentially rise over 90%.
Utilization and Spatial Efficiency
Traditional lead flat-plate and spiral-wound electrodes can be thought of as “two-dimensional” in terms of reactivity and “one-dimensional” in terms of electrolyte diffusion. In a conventional battery, roughly only one-half of the active materials are available for reaction to produce energy, due to the plate structure where diffusion must take place through previously-discharged material in order to sustain the energy-producing electrochemical reactions.
The Firefly Energy architecture goes well beyond the traditional lead acid construction. Firefly’s three-dimensional MicrocellTM composite plate will result in a significant increase in active material utilization levels. The key lies in the basic construction of the Firefly composite plate. Lead plates have a linear structure that requires electrolytic diffusion over relatively large distances. Firefly’s diffusion paths are much shorter, which means that under very high current loads, the effect of electrolyte diffusion would not be significant unless a full discharge was carried out in ~5 seconds or less. In addition, if the proper balance of active materials and electrolyte is achieved in the Firefly design, utilization levels well in excess of the practical limit of ~67% should be achieved due to the dispersed nature of lead sulfate (PbSO4) buildup.
Thus, lead sulfate buildup is not as likely to “shut down” the discharge reaction by choking off electrolyte diffusion. Moreover, since electrolyte diffusion paths in the Firefly electrodes are on the order of microns rather than millimeters (– a potential improvement of 2-3 orders of magnitude) – this change in electrode design should result in large increases in active-material utilizations and high-rate discharge capacities, as well as sharp reductions in recharge times.
Low and High Temperature Advantages
Though economical in many applications, lead-acid batteries have a relatively low specific-energy and, similar to competitive current batteries, are severely affected by cold temperatures. This effect, or increase in internal resistance, is due to the “slowing down” of the battery’s chemical-reaction and ion-diffusion rates. As a “rule of thumb”, reaction rates are cut in half for each 10ºC drop in temperature. “Cold cranking” is a discharge which needs a high current, and reaction-rates are critical to sizing a battery. A high current implies a lot of active material conversion in a short time, and this is related to the amount of electrode surface area covered with active-material that is available for conversion. Therefore, a starter battery needs a lot of surface area (meaning a large number of lead plates).
Sizing a lead-acid battery for starting applications at -18ºF, for example, requires an approximate 200% size increase over room temperature operation. Because of an acknowledged corrosion rate for the positive lead grids in lead-acid batteries, attempts to increase cold temperature starting power by increasing electrode surface area without “sizing up” the overall battery, results in short warm temperature life.
Firefly’s 3D (and 3D2) products have outstanding discharge performance at low ambient temperatures relative to commercial flooded lead-acid and VRLA batteries. This is due to the extremely high electrochemically-available surface area of the MicrocellTM foam coated with sponge lead. The Firefly 3D negative, with hundreds or thousands of tiny cells, each with its own complement of sponge lead and electrolyte, is ideal for discharge (and charge) conditions where electrolyte diffusion is limited by surface area, distance or temperature. Diffusion rates at low temperatures are reduced in a 3D cell just as they are in conventional commercial products, but the distances traveled to react with the sponge lead are much smaller. This enhanced electrolyte supply also results in higher, flatter voltage-time curves on discharge, which means higher energy outputs when combined with the lower current densities that accrue from the high foam electrochemical surface area.
As the temperature is lowered, it takes more power to start the engine (due mainly to increased oil viscosity) at the same time that the available power from the battery drops to only 40% of what can be provided at ambient when the car is started at –18oC. By comparison, a Firefly 3D battery will provide almost 70% of its ambient-temperature power at –18oC. This means that Firefly’s 3D engine-start battery could be smaller to have the same cold-crank amps, or it would be more powerful and last longer if its size were comparable to a commercial product.
The Firefly composite plate technology is distinctly different from traditional batteries, and the net result is Firefly’s battery does not need to be “sized up” for cold weather performance.
The optimum operating temperature for a Lead-acid battery is 25°C (77°F). As a rule of thumb, every 8-10°C (14-185°F) rise in temperature will cut the battery life in half. This is a simple calculation based on field observations and on the increased chemical activity at higher temperatures. Lead grids corrode in the acidic electrolyte in the presence of lead dioxide, the positive plate’s active material.
Firefly batteries have superior performance in terms of thermal management. The heat-transfer characteristics of the MicrocellTM foam are even better than metals such as aluminum and copper, and approach that of diamond.
Even though the Firefly 3D design utilizes a standard lead grid positive plate, the Firefly negative foam plate operates much cooler, and generates a “calming” influence to reduce the temperature of the lead grid positive.
The thermal response patterns for these materials used mean that the heat transfer performance of Firefly’s carbon-graphite foam-based battery technology is outstanding. Thus, batteries made with MicrocellTM foam electrodes will transfer heat out of the battery rapidly as it is generated by the electrochemical reactions taking place, thus making thermal runaway less likely, and enabling overall “cool” battery operation compared to conventional lead-acid batteries. The fact that heat is generated more uniformly and dissipated rapidly translates to longer life in many applications.
Dramatic Cycle Life Improvements
A full discharge of today’s lead acid battery causes extra strain, and each cycle robs the battery of a small amount of capacity. In lead-acid batteries, deeper discharges convert larger amounts of charged active-material into lead sulfate. Lead sulfate has a significantly larger volume (about 37% more) than the charged material, and this volume change stresses the electrode structures. This expansion induces mechanical forces that deform the grid, and ultimately result in the lead grid “disappearing” into the paste.
The resulting expansion and deformation of the plates also causes active material to separate from the electrodes with a commensurate loss of performance. Additionally, over time, sulfate crystals can grow together, resulting in large lead sulfate crystals that are difficult or impossible to convert back into the charged state. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger lead acid battery and shallower discharges are typically recommended. Depending on the depth of discharge and operating temperature, the sealed lead-acid battery provides 200 to 300 discharge/charge cycles. Short cycle life also results from grid corrosion of the positive electrode, which undergoes extensive oxidative stress during extended recharge conditions. These changes are exacerbated at higher operating temperatures.
In contrast, Firefly’s composite plate technology provides a design which fully accommodates the volume changes of the active material during charge and recharge. Within each Firefly plate is contained a full compliment of active materials, electrolyte, and volume which will allow complete discharge without causing physical stress on the plate itself. This results in an electrode plate which does not undergo volume change during deep discharges. Firefly’s electrode material is not reactive in the lead-acid chemistry and so does not corrode. This is in part due to a natural stability of the base material, but is also due to the formation process used which maximizes exposure of the most chemically resistive surfaces and minimizes exposure of chemically less-stable surfaces.
The growth of large sulfate crystals is also restricted, resulting in a low incidence of crystals which are too large to recharge. The strong resistance of Firefly’s electrode material to corrosion also severely reduces the deleterious effects of long recharges. Because of the removal of grid corrosion as a life-limiting factor, the Firefly approach offers significant improvements over conventional lead-acid technologies in both float and deep-cycle applications.
Cycling in irregular applications such as partial-state-of-charge (PSoC) regimes used in hybrid vehicles and photovoltaic energy storage are also well suited to 3D technology. This is because the conditions of partial or heavy sulfation of the negative plate – a process that can render present-generation lead acid products unrecoverable – are easily reversed in 3D products, even after long periods of storage. Sulfation reversal is achieved because the nature of the lead sulfate deposits in 3D cells is fundamentally different from those in traditional lead acid cells. In the latter, lead sulfate is deposited on the surfaces of the plates in dense layers of relatively large crystals, somewhat remote from the lead grid members. Because the sponge lead active material in a 3D cell is deposited on the walls of the many small pores in thin layers, and the high surface areas in the foams result in relatively low current densities, the lead sulfate deposits are comprised of small, porous crystal structures (on the order of 3-10 microns, much smaller than in commercial products) that are easily dissolved on the subsequent recharge. Moreover, these very small crystal sizes grow only slowly over time. A final factor that facilitates recharge is the proximity of the composite foam (as well as residual sponge lead) that can act as efficient current-carrying paths during recharge for the small, local deposits of lead sulfate crystals. This resistance to the effects of sulfation make Firefly 3D batteries ideal for seasonal applications where devices and their associated batteries (electric lawn mowers, boats, RVs, motorcycles, etc.) may go unused for months on end, often in a partially or fully discharged state. Conventional batteries are difficult or impossible to recover from these conditions, and are often replaced far short of their potential life span. With 3D products this problem is greatly reduced.
The low self-discharge rate and easy recovery from sulfation also mean that 3D batteries are not subject to the distribution chain and inventory time constraints of conventional lead acid products. 3D batteries can be subjected to much longer periods of inactivity without damaging effects. Conventional lead acid batteries are limited to storage times of 3 – 6 months at most before requiring a recharge – often with great logistical difficulty and expense.
Float and cycle lifetimes for 3D batteries have yet to be fully determined, but it is anticipated that they will be superior to those of comparable lead acid products due largely to the superior thermal conductivity levels of the composite foam relative to conventional lead electrodes, in combination with lower cell impedances and negative plate current densities.
A final life characteristic of the 3D cells is that, because of their use of light weight carbon-graphite foam, their low mass makes them highly resistant to vibration. 3D cells subjected to vibration testing at Caterpillar’s Technology Center have exceeded Caterpillar’s stringent specifications by a wide margin, at which point they still had not failed. Clearly, foam robustness under the abusive conditions found in commercial, off-road, military, and many other applications will not be an issue for Firefly products.
3D Performance Summary
In summary, then, the 3D cell architecture results in numerous attributes:
Instantaneous Power (2 hours and faster run-time rates)
Fast recharge capability
Continuous power through discharge process
Recovery to full capacity after off-season storage
Excellent Cold temperature capacity utilization
High temperature resiliency
Recovery to full capacity after discharge
Excellent inherent resistance to vibration