Lithium-halogen cells

The energy density of liquid cathode lithium cells can be further enhanced to over 500 Wh/kg (1000 Wh/dm3) by the use of halogen additives. BrCl, added to lithium-thionyl chloride cells, boosts the OCV to 3.9 V and prevents the formation of sulphur in the early stages of discharge. D-sized cells are manufactured, Addition of chlorine to lithium-sulphur yl chloride cells increases the energy density and improves the temperature-dependent electrical characteristics. 

Iodine — Iodine, L, is a halogen which occurs naturally mainly as iodide, I- [i]. Iodine (Greek ioeides for colored violet ) is a black solid with a melting point of 113.6 °C which is readily undergoing sublimation to form a violet gas. Iodine occurs in the oxidation states -1,0, +1, +3, +5, +7 and it possesses a rich redox chemistry [ii]. In aqueous solution the formation of I2 from I- occurs with a standard potential of 0.621V vs. SHE and this oxidation process is preceded by the formation of I3 with a standard potential of 0.536 V vs. SHE. For the reaction I2(cryst) + 2e - 21 E = 0.535 V. The I—/I3 redox couple is employed, for example, in solar cells [iii] and in long-lived lithium-iodine battery systems. The oxidation of I2 in organic solvents results formally in I+ intermediates which is a powerful oxidant and useful, for example, in electro-synthetic chemical processes [ii]. 

These lithium/oxychloride batteries are available in hermetically sealed, spirally wound electrode cylindrical configurations, ranging from AA to DD size in capacities up to 30 Ah. These batteries are also available in the AA size containing 0.5 g of Li and in flat diskshaped cells. Figure 14.35 shows a cross section of a typical cell. Table 14.17 lists the different lithium-oxychloride batteries manufactured and their key characteristics. Two types of halogen-additive lithium/oxychloride batteries have been developed, as follows ... 

The simplest example is Lil, which shows not an excellent, but still considerable con-ductivity of 5 x 10 S cm at room temperature, while other lithium halides are almost insulators, probably because of the less polarizable nature of their halogen ions. It is notewortlty that Lil was the first practical solid electrolyte used in a Li/Ij-complex cell in 1972, in which a layer of Lil was formed by contact of the negative and the positive electrodes. The layer was so thin that the resistance was small enough for low current uses, such as in a cardiac pacemaker. 

Catalyst Research Corporation (USA) is a major manufacturer of lithium—iodine batteries used for pacemaker and other applications. The Catalyst Research Series 800 cells use the lithium envelope concept. The cell is constructed with a centrally located cathode current collector and a lithium envelope which surrounds and contains the iodine depolarizer material (Figure 24.14). This depolarizer material is corrosive to the stainless steel case and must be kept irxnn contacting the case for maximum cell life. A second barrier, formed from fluorocarbon plastics, surrounds the lithium envelope, insulates it from the case, and provides a second envelope for the containment of the depolarizer. The corrosive effect of iodine-containing depolarizer on stainless steel is lower than that of depolarizer made with more active halogens, such as bromine or chlorine. 

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