Lithium ion (Li-ion) has become the battery chemistry of choice for most medical devices. Its main advantage is its high energy density by weight and volume. It is greater than older chemistries such as nickel cadmium (NiCd), nickel metal hydride (NiMH), and lead-acid (Pb-acid). This has pushed Li-ion batteries into a wide range of medical devices, such as, ventilators, oxygen concentrators, defibrillators, and EMS equipment.

The introduction of lithium iron phosphate (LiFePO4 or LFP) chemistry further extends its use into application such as surgical power tools, portable x-rays, medical carts, and infusion pumps, as well as reliable backup power supplies for critical-care patient monitors.

Iron phosphate background

Although first commercialized by others, it was not until A123 Systems introduced its doped nanophosphate lithium batteries in 2006 that the market for this material really began taking off. Through innovative design and material treatment, A123 was able introduce an iron phosphate cylindrical 2.3Ah cell (26-mm dia. x 65-mm long or 26650) capable of a high discharge rate and long cycle life. These mean that although the battery is rated for, say, 4.6 Ah, it can be discharged at a 20A rate, although for a shorter period than it rated life. And cycle life refers to the number of times a battery can be charged and discharged. For LFP chemistries, it's over 1,000 times. Other chemistries may tolerate only 300 to 400 cycles.

The first customer for LFP was Black&Decker. The company used the batteries in its 36-V industrial grade power tools. The technology now powers a range of products from electric bikes to computer servers.

Since A123's introduction, several companies have introduced cells with iron phosphate cathodes and a wide range of performances. Additional sizes including cells such as the 18650 (18-mm dia. x 65-mm long), 14500 (AA size — 14-mm dia. x 50-mm), and larger format prismatic polymer cells (rectangular 9V-sized batteries) with capacities of 10Ah or more.

The main internal components of a lithium battery are a cathode, anode, separator, and electrolyte. The iron-phosphate cathode has the job of accepting lithium ions and accompanying electrons from the anode during discharge. The ideal cathode material does so reversibly with little resistance and change in its structure as the lithium ions are inserted to ensure high efficiency, good discharge-rate capability, and long cycle life.

Traditionally, lithium battery cathodes have been transition metal oxides that use cobalt (Co), manganese (Mn), or nickel (Ni) based oxides. Recently, manufacturers of conventional lithium batteries have moved to mixed metal oxide cathodes incorporating various mixtures of Co, Mn and Ni, with some materials doped with stabilizers such as aluminum (Al).

An accompanying table reveals each cathode's capacity to hold lithium ions. The most common cathode materials and their nominal specific capacity are measured in milliamp hours per gram of material and typical operating discharge voltages.

The iron phosphate difference

Other differences show up in the performance of LFP batteries when compared with conventional chemistries. The most significant differences include operating voltage, cycle life, safety, and power.

The operating voltage of a lithium battery varies with changes in the cathode. Cells with iron-phosphate cathodes generally charge to a lower voltage than conventional batteries, and discharge at a lower nominal voltage.

For medical-device engineers, LFP presents a challenge because most devices were likely not designed to accommodate it. The discharge range may not be suitable for some devices, and charging can be difficult if not using a “smart” charger. These chargers interact with batteries so that timers and voltage regulators are properly set for the battery chemistry. And then it recharges the battery with a time-varying voltage. However, new applications are easily designed to accommodate conventional transitional-metal oxide and iron-phosphate batteries. With the widespread availability of LFP, designing in this flexibility is becoming more common.

Cycle life refers to the number of charge-discharge cycles in a useful battery life. As a cathode material, LFP is known for its inherent stability and reversibility. The LFP chemistry readily accepts and releases lithium ions with little or no change in its structure. Hence, the cycle life a LFP battery is generally four to five times that of a traditional lithium cell, thanks to the inherent stability of the LFP cathode and lower charge voltage that slows degradation rate of the anode and electrolyte.

The cycle life improvement due to LFP is a plus for medical applications particularly those that may have generated poor customer satisfaction because of less-reliable chemistries, such as Pb-acid and NiMH. These chemistries could not reach end of life predictably, so most hospitals, doctors, and clinics would change out the batteries in their devices based on a calendar-based schedule. LFP lasts longer and has a more gradual and predictable end of life.

Safety improves greatly because of the recent cathode. Some medical-device companies have delayed their transition to lithium batteries because of little experience with them, battery-management requirements are more complex, sterilization issues, and concerns over their safety after recalls dating back to 2006 that involved major cell manufactures. The safety concern is understandable because medical devices are frequently used in life-support and other critical applications, as well as near oxygen and flammable materials where overheating could prove disastrous.

Among the most commonly available cathode materials, LFP is known to be one of the safest. It is thermally stable up to 300C and does not emit oxygen when over charged as do Ni, Co, and Mn based oxides. Oxygen release is often the precursor to thermal runaway in a conventional lithium battery.

Power is the big plus for LFP cells. They offer high power in a small package. Because of the chemistry's stability, LFP batteries also have low impedance growth over time, letting them provide consistent high power throughout the life of the battery. For LFP cells, the increase in delivered capacity can be substantial at high discharge rates.

How cathodes compare

Cathode material	Specific capacity, mAh/g	Nominal cell voltage, V	Characteristics
LiFePO4 (LFP)	140	3.3	Low energy density, but excellent cycle life, safety, and high-rate capability
LiCoO2 (LCO)	160	3.7	Until recently, LCO was the most common cathode material giving the best compromise of capacity, cycle life, and safety.
LiNi0.33Mn0.33Co0.33 (NMC)	180	3.6	This has replaced LCO as the base cathode material of choice in conventional Li-ion cells for it lower cost and improved safety.
LiNi0.8Co0.15Al0.05 (NCA)	185	3.6	Used in some of the next generation highest energy density cells
LiMn2O4 (LMnO)	130	3.9	Low energy density but low cost, good safety and rate capability. It's mixed with NMC and LiNiO2 in high-rate cells

Relative energy densities and characteristics of common cathode materials