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f) Sodium/Sulfur Cells

Anode: Molten sodium
Cathode: Molten sulfur
Electrolyte: Al2O3

This type of battery exhibits a high energy density, high efficiency of charge/discharge and is made from inexpensive, non-toxic materials. However, the operating temperature of 300 to 350 °C and the highly corrosive nature of sodium make it suitable only for large-scale non-mobile applications.

The cell is usually made in a tall cylindrical configuration. The entire cell is enclosed by an inert metal container and sealed at the top with an airtight alumina lid. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat.

During the discharge phase, molten metallic sodium at the core acts as the anode, separated by a beta alumina cylinder from a sulfur container made from an inert metal acting as the cathode. The sulfur is absorbed in a carbon sponge. Alumina is a good conductor of sodium ions but a bad conductor of electrons, avoiding self discharge. When sodium gives off an electron, the Na+ ion migrates to the sulfur container. The electron travels through the molten sodium to the contact and through the electric load to the sulfur container. Here, the electron reacts with sulfur to form S-, which then forms sodium polysulfide. As the cell discharges the sodium level drops. During the charging phase the reverse process takes place. Once running, the heat produced by charging and discharging cycles is enough to maintain operating temperatures and no external source is required.

Half-reactions:

2Na -> 2Na+ + 2e-
3S + 2e- -> S32-

Overall Reaction:

2Na + 3S -> Na2S3 2.076 V

Pure sodium presents dangers. Because sodium spontaneously burns on contact with water, the system must be protected from moisture. In modern NaS cells sealing techniques make sodium fires unlikely.

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g) Lithium Ion Cells

Anode: Carbon compound, graphite
Cathode: Lithium oxide
Electrolyte: LiPF6, LiBF4, related compounds

Lithium ion batteries (sometimes abbreviated Li-Ion) are a type of rechargeable battery commonly used in consumer electronics. They are currently one of the most popular types of battery, with one of the best energy-to-weight ratios, no memory effect and a slow loss of charge when not in use. They can be dangerous if mistreated, however, and unless care is taken may have a short lifespan compared to other battery types.

Cathodes consist of a a layered crystal (graphite) into which the lithium is intercalated. Experimental cells have also used lithiated metal oxide such as LiCoO2, NiNi0.3Co0.7O2, LiNiO2, LiV2O5, LiV6O13, LiMn4O9, LiMn2O4, LiNiO0.2CoO2.

A particularly important element for activating Li-ion batteries is the solid electrolyte interface (SEI). Liquid electrolytes in Li-ion batteries consist of solid lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, and organic solvents, such as ether. A liquid electrolyte conducts Li ions, which act as a carrier between the cathode and the anode when a battery passes an electric current through an external circuit. However, solid electrolytes and organic solvents are easily dissolved on anodes during charging, thus preventing battery activation. Nevertheless, when appropriate organic solvents are used for electrolytes, the electrolytes are dissolved and form a solid electrolyte interface at first charge that is electrically insulating and high Li-ion conducting. The interface prevents decomposition of electrolytes after the second charge. For example, ethylene carbonate is dissolved at relatively high voltage, 0.7 V vs. Li, and forms a tight and stable interface. This interface is called an SEI.

The immediate advantage to Lithium battery chemistry is higher charge density. Li ions are small and mobile, but more readily stored than hydrogen. Thus a battery based on Li is smaller than one with hydrogen, such as a NiMH or NiCd, and with fewer volatile gases. Because the ions need fewer storage intermediaries, more battery weight is useable as charge, instead of overhead. Thus, Li batteries are lighter than equivalents in other chemistries- often much lighter.

The Li-ion battery required nearly 20 years of development before it was safe enough to be used on a mass market level. While Li-ion batteries do not suffer from the memory effect, they are not as durable as NiMH or NiCd designs and can be extremely dangerous if mistreated. At a typical 100% charge level (notebook battery, full most of the time) at 25 degrees Celsius, Li-ion batteries irreversibly lose approximately 20% capacity per year from the time they are manufactured, even when unused. (6% at 0 °C, 20% at 25 °C, 35% at 40 °C. When stored at 40% charge level, these figures are reduced to 2%, 4%, 15% at 0, 25 and 40 degrees Celsius respectively.) Every (deep) discharge cycle decreases their capacity. The degradation is sloped such that 100 cycles leave the battery with about 75% to 85% of the original. When used in notebook computers or cellular phones, this rate of deterioration means that after three to five years the battery will have capacities too low to be still usable.

A unique drawback of the Li-ion battery is that its life cycle is dependent upon aging from time of manufacturing (shelf life) regardless if it was charged or not and not on the number of charge/discharge cycles. This drawback is not widely publicized.

As a newer chemistry, with more-advanced applications, Li batteries command a higher price.

One great advantage of Li-ion batteries is their low self-discharge rate of only approximately 5% per month, compared with over 30% per month and 20% per month in nickel metal hydride batteries and nickel cadmium batteries respectively.