Commercialized since 1975, this primary Li battery is the most widely used, as it owns several nice features: high voltage, high energy (both on a gravimetric and a volumetric basis), wide operating temperature range, good power (in some designs), long shelf life, safety and low cost. The Li/MnO2 battery is produced by several manufacturers in coin, cylindrical or prismatic forms (the last one including thin cells) and can be used in a variety of applications. Li foil is used as the anode, heat-treated MnO2 (with a residual H2O content of 1%) as the cathode, and a preferred solution is LiClO4-PC-DME (PC, propylene carbonate; DME, dimethoxyethane). Heat treating of electrochemical grade MnO2 at 350-400C is fundamental, as the non-treated form has a poor performance.
During discharge, Liþ ions are gradually inserted into the channels of the cathode structure, thus giving rise to a so-called solid solution. The formula LixMnO2, 0<x<1, best explains the gradual accommodation of Liþ ions into the host structure.
Li/MnO2 cells for portable and industrial applications are mainly manufactured with the following designs: coin, spirally wound cylindrical, bobbintype cylindrical, 9V prismatic and thin cells. The first two designs are most widely used. The bobbin cells have a laser seal, while the spirally wound may have a crimp or a laser-welded seal. The laser sealing technology ensures a cell life of 10 years at room temperature and a wider temperature range. Indeed, these cells can be operated in the range -40 to 85C, while for those with crimp seals the range is -20 to 60C.
In the bobbin structure, the amount of cathode is maximized to have high capacity and energy. Because of this structure, the cathode surface area is reduced and these cells can only be used at low drains. In the spirally wound structure, a thin layer of the cathode mix is supported on a metal grid; three strips, Li foil, separator and cathode, are tightly wound to form a high surface area structure capable of sustaining high currents.
The cell cans are made of stainless steel and the separator is polypropylene. Bobbin and spirally wound cells are endowed with safety vents, and the latter have a positive temperature coefficient (PTC) device for additional safety. High-power cells can also have relatively high capacities, for example more than 10 Ah in the D size.
The Li/MnO2 system is also available as a prismatic 9V battery of the same shape as Zn-C or alkaline cells. However, their internal construction is quite different. Each of the three cells in series contains stripes of anode, cathode and separator, just as in spirally wound cylindrical cells, bent to completely fill the space. These consumer-replaceable batteries last up to five times longer than alkaline and 10 times longer than Zn-C batteries. Very thin "paper" cells, with thicknesses of 1-2mm are also available.
The Li/MnO2 system is the most widely commercialized among Li batteries. It deserves a more detailed list of applications for its most popular types, that is button and cylindrical cells.
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06272010
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