Lithium battery basics

Sol Jacobs
Tadiran Batteries
Port Washington, N.Y.
www.tadiranbat.com

Currently used in Tadiran PulsesPlus batteries, hybrid lithium cells have proven themselves in remote monitoring applications, such as utility meters that require up to 20 years of service life under harsh environmental conditions.

Lithium batteries are becoming the battery of choice for today's feature-rich and power-hungry electronic devices. Of all battery chemistries, lithium is the lightest nongaseous metal, offering unique benefits because its intrinsic negative potential exceeds that of all metals. Batteries based on lithium chemistries have the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of any type.

Lithium cells use a nonaqueous electrolyte and have a relatively high internal impedance, with nominal output voltages between 2.7 and 3.6 V. The absence of water along with the chemical and physical stability of the materials gives lithium batteries an extended operating temperature range. At least one lithium-based system operates at temperatures as low as -55 and as high as +150°C.

Several lithium-battery chemistries are in use. They are polycarbon monofluoride lithium, manganese dioxide lithium (LiMnO2), lithium thionyl chloride (LiSOCl2), sulfur dioxide lithium (LiSO2), and lithium iodine (LiI2).

Polycarbon monofluoride cells have an output of 2.8 V and moderately high energy density. Cylindrical types are manufactured with a spiral-shaped cathode and crimped elastomer seals. Though generally safe, under extreme conditions the elastomer seals can fail before the case does, letting the relatively reactive cell constituents escape.

Manganese-dioxide lithium cells are comparable to poly cells in terms of construction, energy density, safety, and output voltage, but typically have about half the service life. They are well suited to applications having relatively high continuous-current or pulse-current requirements because the internal impedance is somewhat lower than for other types.

Lithium-iodine cells use only solid constituents. Plus, the separator in a lithium-iodine cell can heal itself if it cracks. The major drawback to lithium-iodine is its high internal impedance, which limits its use to low-drain applications.

Sulfur dioxide-lithium cells, used almost exclusively in military/aerospace applications, have lower energy density than manganese dioxide-lithium or carbon monofluoride lithium cells. Their service life and energy density are less than half that of lithium thionyl chloride cells.

Lithium thionyl chloride cells have the highest energy density of all lithium types. Service life is typically 15 to 20 years. These cells are best suited for applications having low continuous-current and moderate pulse-current requirements. Their long service life and low self-discharge rate make them ideally suited for use in harsh environments.

Lithium thionyl chloride cells are constructed two ways. One is spirally wound and the other uses a bobbin construction. There are also hybrid systems such as the PulsesPlus battery, which combines bobbin LiSOCl2 construction with hybrid layer capacitors.

Virtually all lithium batteries use the spirally wound construction, with the exception of bobbin-type LiSOCl2. Spirally wound lithium batteries tend to deliver high current, with less capacity, and much higher self-discharge, compared with bobbin LiSOCl2 cells.

Bobbin LiSOCl2 cells are particularly well suited for low-current applications because of their high-energy density, low self-discharge rate, and potential operating life of 15 to 20 years. They feature an operating temperature range of -55 to 150°C, high capacity, small size, and an ability to withstand broad fluctuations in pressure, temperature, and shock.

High current-pulse "hybrid" systems

The past decade has seen a dramatic rise in applications involving high-current pulses, including automotive, GPS tracking, remote monitoring, utility meters, and oceanographic devices. High-current pulse applications typically require low continuous current (or no continuous current) coupled with high pulse currents of up to several amperes for a period of seconds to almost 20 min.

One solution is to combine lithium thionyl chloride bobbin-type construction with a hybrid-layer capacitor (HLC). Currently used in Tadiran PulsesPlus batteries, hybrid lithium cells have proven themselves in remote monitoring applications, including utility meters that require up to 20 years of service life under harsh environmental conditions.

Lithium-battery characteristics
Characteristic LiSOCl2 w/
hybrid-layer capacitor
LiSOCl2
bobbin
LiSOCl2
jelly roll
LiSO2 LiMnO2
Energy density (Wh/l) 1,420 1,420 800 410 650
Power High Low High High Moderate
Voltage 3.6 - 3.9 V 3.6 V 3.0 to 3.6 V 2.0 to 3.0 V 2.0 to 3.0 V
Pulse amplitude High Small Moderate High Moderate
Passivation Low High Moderate Fair Moderate
Performance at elevated temperature Excellent Fair Moderate Moderate Fair
Performance at low temperatures Excellent Fair Excellent Excellent Poor
Operating life Excellent Excellent Moderate Moderate Fair
Self-discharge rate Low Low Moderate Moderate Moderate
Operating temperature -55 to 100°C -55 to 150°C -55 to 85°C -55 to 60°C 0 to 60°C