Tech Briefs

Lithium Batteries for Medical Applications

Recent advances in lithium technology have increased the variety of commercially available batteries.

The element lithium possesses fundamental properties that make it ideal for use as the anode in both primary and rechargeable batteries. Vendors have paired the popular lithium anode with a variety of cathode and electrolyte materials, resulting in the wide choice of different chemistries available today. This article discusses the types of primary lithium batteries commonly used for medical applications and introduces a new type based on recent innovations in materials and manufacturing processes. Information about the basic properties, advantages, and disadvantages are provided for each battery type.

Why Lithium?

altThe popularity of lithium’s use in batteries derives from its fundamental properties, which make it far superior to other metals as an anode material. Lithium’s standard potential is over -3.0 V, exceeding sodium, magnesium, and calcium. Lithium is also the lightest metal with a low density of only 0.54 g/cm3. Indeed, at atomic number 3 (and considered to have formed seconds after the Big Bang), lithium is the first element in the alkali metals group. Additional properties that make lithium ideal for batteries include its low resistivity, high ionization energy, low melting point, and relative abundance.

These and other desirable properties combine to yield an impressive electrochemical equivalence of 3.86 Ah/g, readily exceeding all other contenders. Magnesium, calcium, and sodium achieve electrochemical equivalences of 2.20 Ah/g, 1.34 Ah/g, and 1.16 Ah/g, respectively. The only other metal that comes close to lithium’s electrochemical equivalence is aluminum with a rating of 2.98 Ah/g. But, aluminum’s low standard potential of only -1.7 V presents a significant barrier to its use in batteries.

Comparison of Lithium Battery Types

Although a lithium anode can be coupled with a variety of different cathode and electrolyte materials, the most commonly used for medical applications are Lithium/Manganese Dioxide and Lithium/Polycarbon Monofluoride.

Lithium/Manganese Dioxide (Li/MnO2) batteries are the current market share leader, owing to the combination of low cost and safe operation compared to the other existing primary lithium batteries.

Lithium/Polycarbon Monofluoride (Li/(CF)n) batteries have a similar volumetric energy density and longer shelf life than Manganese Dioxide batteries and are just as safe to operate.

Both Manganese Dioxide and Polycarbon Monofluoride are safe and relatively benign environmentally, but sometimes fail to pack the power demanded by some medical applications. Recent innovations in the formation of carbon fluoride powder hold the potential to eliminate these traditional trade-offs.

Lithium/Carbon Fluoride (Li/CFx) batteries maintain the benefits of high energy and power densities, wide operating temperature range, and long shelf life while employing a solid cathode (with no heavy metals or other toxic materials) to eliminate safety and environmental concerns. In addition, the advanced CFx battery possesses considerably less of the operational problems exhibited by some other batteries, such as passivation.

In terms of increased performance based on higher energy and power densities, CFx offers a competitive price/ performance ratio to the other alternatives, as shown in Table 1. And, over time, with additional advancements and growing economies of scale in manufacturing, Lithium/Carbon Fluoride is expected to become the preferred choice for primary lithium batteries in a growing number of applications.

At present, Li/CFx technology offers the highest theoretical energy density of any available battery chemistry. Today’s commercially available products typically use only 10 percent of their theoretical value, thus providing dramatic potential for future development. A comparison of key features of these three battery chemistries is presented in Table 1.

Perhaps the most significant advancement found in these new Lithium/ Carbon Fluoride batteries is the ability to customize or tune the cathode to meet an application’s specific requirements. By altering how fluorine is introduced into the carbon structure at the atomic level during the manufacturing process, the battery’s fundamental properties can be changed in ways that favor higher energy or power densities The customization process can also balance the battery’s properties in other ways to optimize performance in a particular application.

altAnother major advantage of the advanced CFx battery is its ability to exceed all others in both power density and maximum safe current draw. Laboratory tests have demonstrated up to an eight times improvement in highcurrent 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. (See Figure 1)

Medical Application Requirements Portable medical device applications for primary Li-ion primary batteries include bone-growth stimulators, glucose monitoring systems, and portable automated external defibrillators.

• Bone-Growth Stimulators — ultrasonic osteogenesis and electrical stimulators used to accelerate the healing of fresh fractures and fusions rely on high energy density battery systems that also meet stringent safety requirements.
• Glucose Monitoring Systems — instantly determining the blood-glucose levels of diabetics or hypoglycemia patients is very challenging, requiring batteries that combine long service life with the high energy density and pulse power needed to perform tests quickly, without compromising accuracy.
• Automated External Defibrillators (AEDs) — detection of potentially lifethreatening cardiac arrhythmias, ventricular fibrillation, and ventricular tachycardia require battery systems with extraordinarily high energy density and high pulse currents. Batteries supporting these systems must also meet strict safety standards by avoiding use of toxic materials.

These three applications are representative of those for which the three battery chemistries described in this article are commonly used. As further advances are made in Li/CFx technology, it is clear that LiMnO2 and Li/CFx will grow market share in the future.

This article was written by Joseph Carcone, Vice President of Sales & Marketing at Contour Energy Systems, Azusa, CA. For more information, visit http://info.hotims.com/40439-167.