Li-Polymer Enables Product Distinction

Jul 28, 2011 2:54 PM
Tichy, Robin

Sealed Lead Acid (SLA) and NiCd batteries used to be the only options for portable equipment. While these chemistries still have the advantage of lower cost and large operating temperature ranges, their low energy density requires big, heavy packs. Lithium Ion (Li-Ion) battery systems are a good option for lower weight, higher energy density or aggregate voltage, or greater number of duty cycles. Each chemistry has unique characteristics that affect how it performs in a particular portable device. The specific characteristics in terms of voltage, cycles, load current, energy density, charge time, and discharge rates, must be understood in order to specify an appropriate cell for an application.

Li-Ion cells come in three basic form factors: cylindrical, prismatic (rectangular brick shape) and the flat Lithium polymer cells. The most commonly used Li-Ion cell is the cylindrical 18650 cell. Several million cells per month are manufactured. They are used in most notebook computer applications. The 18650 offers the lowest cost per watt hour. 18 refers to the cell diameter in millimeters and 65-zero means that it’s 65 mm long.

Prismatic or brick shaped cells are often cost effective and are available in a myriad of sizes. They also come in a variety of heights ranging from about 4 mm to about 12 mm. The most common size is the 50 mm length and 34mm width foot print.

Lithium polymer cells, sometimes called laminate cells, are available in custom footprint size. They can be very thin or quite large depending on their intended use.

Fig. 1 shows the growth predicted over the next decade for various types of rechargeable cells (including automotive Li-Ion batteries). All of the Li-Ion cells are overtaking the Ni cells in almost every market. The cylindrical and automotive batteries are predicted to have astronomical growth. Lithium polymer batteries, shown in violet, are predicted to become quite common with manufacturers producing 1 billion cells per year by the end of this decade.

Lithium Polymer Differentiation

The primary advantage of Lithium polymer batteries is the variety of form factors available. Manufacturers of Bluetooth devices were the first to recognize the advantage of Lithium polymer batteries. The availability of very thin batteries enabled the Motorola Razr phone to have great market success. Apple created a thin notebook which differentiated it in the highly commoditized notebook market.

Lithium ions move from the negative electrode to the positive electrode during discharge and reversely when charged. The three primary functional components of a Li-Ion battery are the anode, cathode, and electrolyte, for each of which a variety of materials are used. Commercially, the most popular material for the anode is graphite, but some manufacturers use coke. The cathode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), one based on a polyanion (such as lithium iron phosphate), or a spinel (such as manganese).

Li-Ion cylindrical and prismatic cells use a discrete porous polymer membrane – usually polyethylene (PE) which is placed between the electrodes. Once assembled, the cell is backfilled with electrolyte solution.

Lithium polymer uses a PE, polypropylene (PP), or PP/PE separator. Some Lithium polymers use polymer gel containing the electrolyte solution which is coated onto the electrode surface. The structure may then be laminated before packaging.

Construction:

• Li-Ion cylindrical and prismatic material layers are rolled (like a jelly roll).
• Lithium polymer can either be rolled or stacked like a deck of cards.

Packaging:

• Li-Ion cylindrical and prismatic cells are packaged in metal cans.
• Lithium polymer is packaged in a flexible “coffee bag” material.

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