There are a number of advantages to having a battery with a higher energy density—for the soldiers on the ground and the unmanned aerial vehicles overhead during any mission. Higher energy density lets them extend missions, be more agile and go further into the field. It’s important to understand how energy density—both gravimetric and volumetric—plays a role in such systems and how existing primary lithium batteries stack up in different military applications. Primary batteries do and will continue to play a vital role in actual missions. The exclusive focus on primary batteries here is not meant to diminish the growing importance of rechargeable batteries; rather it is to limit the scope of the topic to one that can be covered in sufficient depth. 

The burden on today’s soldier to carry an increasing amount of high-tech equipment, such as advanced soldier systems, next-generation radios and imaging and sensing systems, is great and growing. The Future Force Warrior will be asked to bear an even greater burden. The batteries needed to power all of this equipment already constitute too high a percentage of the total weight. And because the batteries must last for an entire mission, soldiers often need to carry spares (or a charging system when rechargeable batteries are used). 

A similar situation exists for a small or micro unmanned aircraft system (UAV) (Figure 1) where the battery needs to power the motor, controls, radio and imaging equipment. As with the soldier, the battery must last the full duration of the mission. The allowable weight for the battery typically limits today’s mission duration to between 30 minutes and two hours. 

Lighter Is Better

The solution to these limitations is a lighter battery. By doubling the energy density, the weight of the battery pack needed for a mission of any given duration can be cut in half, which would allow the soldier or UAV to carry other equipment or systems. Alternatively, the same size and weight in battery pack(s) with double the energy density could double any mission’s duration. This would be particularly valuable in remote reconnaissance and surveillance, and target acquisition missions, where being aloft longer may make the difference between success and failure. 

Doubling the energy density is, of course, far easier said than done. Commercial off-the-shelf batteries that employ a variety of different technologies are readily available. The two types that are most popular in military applications today are Lithium/Sulfur Dioxide (Li/SO2) and Lithium/Manganese Dioxide (Li/MnO2). While these batteries have similar energy densities of 200-250 watt-hours/kilogram (Wh/kg), volumetric energy densities are 350-450 watt-hours/liter (Wh/l) and 500-650 Wh/l, respectively. Note that because energy densities vary with the different form factors used for different applications, the ranges used here depict typical values for cells in applications requiring moderate to high rates of discharge. 

When Weight Is an Issue

In applications where the weight is a significant design consideration, Sulfur Dioxide and Manganese Dioxide batteries have similar gravimetric energy density. Despite these similarities, Manganese Dioxide batteries are increasingly preferred, owing to their enhanced safety over pressurized Sulfur Dioxide batteries. In applications where the space available for the battery is limited, however, Manganese Dioxide batteries have a greater advantage with their 40% improvement in volumetric energy density.  

In a BA-5X90/U battery pack, for example, Manganese Dioxide’s increased volumetric energy density delivers about 11.5 amp-hours (Ah) of service compared to about 7.5 Ah with Sulfur Dioxide. But the problem remains: the 11.5 Ah battery in this example is significantly heavier than the 7.5 Ah battery because Manganese Dioxide’s gravimetric energy density is relatively the same as Sulfur Dioxide. 

The military has looked at other lithium-based systems and are investigating a Lithium/Polycarbon Monofluoride (Li/(CF)n)system as a potential next-generation solution. While the military likes the potential advantages in gravimetric and volumetric energy density (350 Wh/kg and 700 Wh/l), there are some significant disadvantages. First, these systems typically have a low power density, so changes need to be made to satisfy device power requirements. Additionally, low temperature performance of these Polycarbon Monofluoride systems is poor and show start-up related issues at these low temperatures. Finally, the costs of these systems are relatively high and therefore do not satisfy the cost per unit of energy for these other systems.

Advanced Lithium/Carbon Fluoride Batteries 

In the never-ending quest for a better battery, a new technology has emerged as an off-shoot of the Lithium Polycarbon Monofluoride systems: Lithium/Carbon Fluoride (Li/CFX). Carbon Fluoride not only has higher gravimetric and volumetric energy densities of >700 Wh/kg and 700-1000 Wh/l, respectively, it also shows promise in satisfying the demand for ever-increasing improvements in price/performance, shelf life, service life, durability, safety and environmental impact. 

This advanced Carbon Fluoride battery maintains the benefits of high energy and power densities, wide operating temperature range and long shelf life found in Sulfur Dioxide batteries, while employing a solid cathode (with no heavy metals or other toxic materials) to eliminate the safety and environmental concerns. In addition, the advanced CFx battery possesses none of the operational problems exhibited by some other batteries, such as passivation. 

The biggest advantage of Carbon Fluoride technology is the significant improvement in both gravimetric and volumetric energy densities. Because this technology is new, it is only now being commercialized. But as shown in Figure 2, the gravimetric and volumetric energy density improvements may be even greater in some configurations than the conservative estimates provided above. 

Extending UAV Mission Life

In the BA-5X90/U battery pack example cited above, a Carbon Fluoride version of this popular battery should be able to deliver 16 Ah of operation—perhaps even more—while weighing less than the Sulfur Dioxide version. This greater than 50 percent improvement in service life could extend a UAV mission by more than an hour, and eliminate the need for soldiers to carry spares on a three-day mission. 

An additional major advantage of the advanced Carbon Fluoride battery is its ability to exceed all others in both power density and maximum safe current draw. Laboratory tests (Figure 3) have demonstrated up to an eight times improvement in high-current applications, and a nearly two-times improvement in low-current applications. This makes the advanced CFx battery particularly well suited for applications that require high sustained or pulse currents. 

Like the other lithium-based primary batteries, Carbon Fluoride batteries can be packaged in a variety of form factors, including coin, cell, film or prismatic. This enables CFx batteries to accommodate both standard sizes and customized packs, which may combine cells in series and/or parallel to satisfy specific needs for operation in the typical military range of 6-24 volts. 

Other Important Considerations 

How does the Carbon Fluoride battery stack up against other battery types in other respects? Figure 4 provides a comparison summary for the four types covered here, as well as for Lithium/Thionyl Chloride (Li/SOCl2). The use of only solid materials and a nontoxic electrolyte makes Carbon Fluoride batteries far safer than Sulfur Dioxide batteries, especially in those applications that draw a high, sustained current where Sulfur Dioxide batteries might overheat and fail. Solid materials eliminate the need for pressurized cans that can vent or leak corrosive or noxious gases, making Carbon Fluoride batteries safe even when mishandled or damaged, or when subjected to a short circuit condition. This is obviously a particular concern for the soldier, but even the batteries used in weapons and unmanned vehicles must still be handled during transport and replacement. 

Temperature Concerns

Operating temperature range is not a factor for the soldier, but can be for weapon and surveillance systems. And here, too, Carbon Fluoride has made improvements over both Manganese Dioxide and Sulfur Dioxide. Indeed, the operating temperature range of Carbon Fluoride batteries far exceeds the requirements of today’s military applications. 

With its higher gravimetric and volumetric energy densities, Carbon Fluoride batteries will provide a longer service life than both Manganese Dioxide and Sulfur Dioxide batteries. Just as significantly, Carbon Fluoride batteries also afford a longer shelf life—up to 50 percent longer than either Manganese Dioxide or Sulfur Dioxide batteries. 

Is Lithium/Carbon Fluoride a better battery? Whether for the soldier on the ground or the unmanned aerial vehicle overhead, the higher energy density and other advantages do make CFx a much better battery. Does it cost more? Yes, at least until production ramps up and economies of scale kick in. But considering its higher energy density, Carbon Fluoride already enjoys a significant price/performance advantage today.  

Contour Energy Systems
Azusa, CA.
(626) 610-0660.
[www.contourenergy.com].