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Chemistry lessons

No single chemistry or construction method satisfies all applications. But for as long as chemists have ranked metals, lithium has sat seductively at the bottom of the list as the most electronegative of all elements. Battery chemistries involving lithium offer the promise of high-energy density and high cell voltages as well as good low-temperature performance (Table 1). But the very reactivity that makes lithium such a positive choice as a cell component also makes such cells potential bombs.

Battery Chemistries
Primary alkaline Rechargeable alkaline NiCd NiMH Rechargeable Li-ion
Capacity (mAhr) 2000 1400 750 1100 1200
Cycle life 1 25+ 200+ 300+ 500+
Operating voltage (V) 1.5 to 0.8 1.5 to 0.8 1.3 to 1 1.3 to 1 4 to 3.25
Weight (g) 22 22 22 26 38
Continuous output 600 mA 400 mA >5A >4A 2A
Self discharge (% per day) 0.01 0.01 1% 2 to 4 0.2
Retail cost (typ four-cell pack) $3 $5 $10+ $60+ TBA
Disposable Yes Yes No Maybe Yes

The trick is to avoid inherently dangerous metallic lithium. Using only lithium compounds in "primary" (nonrechargeable) lithium cells is easy. But, keeping metallic lithium from plating out when recharging a rechargeable lithium cell is much more difficult. Thus, the rechargeable lithium cells that are becoming available require extremely precise charging (Ref 4) compared to other cells. A typical spec calls for constant-voltage charging at 4.1V [+-]50 mV with charging-current limiting.

thumbnail[Picture 2] These cells go by the puzzling name of "lithium-ion" (Li-ion) cells. Of course, all cells contain ions; in this case, the "ion" in Li-ion signifies that the cell's lithium never occurs in metallic form.

The key to these cells' operation is an obscure chemical mechanism called "intercalation." Inserting guest atoms into a host material with minimal physical alteration of the host structure constitutes intercalation. Intercalation is also reversible: The two major classes of host materials that support intercalation have either a layered-crystal structure or a nonlayered, framework structure.

Li-ion cells use a dual-intercalation process: Intercalation occurs at both the cell's anode and cathode. Take, for example, Sanyo's and Moli's Li-ion cells. The cells' output voltages are nominally 3.6V, allowing a single cell to power 3V logic.

The cells use natural graphite that has a high degree of crystallinity for their anodes. The crystalline graphite increases the absorption capacity of the anode by 150%, compared to earlier anodes that used coke. The electrodes' ability to absorb Li-ions is the key to the rechargeable cells' operation.

A lithium-cobalt oxide, lithium cobaltite (LiCoC2), forms the cathode. Al-though the electrolyte contains some lithium salts, the lithium cobaltite is the principal source of Li-ions in the cell. Both lithium cobaltite and graphite exhibit layered structures.

After assembly, the cells receive an initial charge. Lithium atoms in the cathode dissociate into Li-ions and electrons. As the electrons travel through the external circuit, the ions migrate through the liquid electrolyte in the direction of the anode. The Li-ions and electrons recombine on the surface of the graphite. The reformed lithium atoms intercalate between the graphite's crystal layers.

Intercalation reverses during discharging. Intercalated lithium atoms move from inside the graphite anode toward the anode's surface, where they dissociate into Li-ions and electrons. The ions and electrons travel toward the cathode via the electrolyte and the load, respectively. At the surface of the lithium-cobaltite cathode, the ions and electrons recombine and intercalate between the lithium cobaltite's crystal layers.

The energy density of Sanyo's Li-ion cell (261 Whr/l) is 2.4 times the energy density of the company's general-purpose NiCd cells. Energy density, by weight (114 Whr/kg), is 3.6 times that of current NiCd cells. The Li-ion cell also accepts a "quick" (one-hour) recharge and features a -20C to ~+60C operating range.