Lithium cells energy content

The interest in ever-higher energy content has caused the development of cells with relatively high voltages to receive much attention in the lithium battery research community in recent years. This has led to the exploration of a number of positive electrode materials that operate at potentials of about 4 V, or even more, positive of the potential of elemental lithium. 

There are also disadvantages. First, litliium batteries are very expensive. Compared on tiie basis of equal energy content, they may cost three to five times more than Leclanch cells. More serious is the matter of safety. For low power applications, these batteries are quite safe, but high power lithium batteries have been known to explode. For example, accidental heating may melt the lithium (m.p. 180.5°C). This can rupture the protective SET layer, leading to a violent reaction between the metal and the solvent, and eventually to explosion. 

If the lifetime of Li-based batteries (the term lithium ion batteries for batteries with polar Li-compounds as negative electrodes is very unfortunate) can be further enhanced, they will be also of importance for electrotraction. The classical battery type used in automobiles, viz, the lead-acid accumulator, is distinctly superior in terms of long-time stability but possesses too low an energy content per unit weight as to drive automobiles. Driving car of sensible size and performance with this alone requires a battery weight on the order of 11, (This problem is not removed by using Ni-Cd accumulators,) Much effort has been undertaken to develop a sodium-sulphur cell. In the Na-S cells ... 

Table 15.3 also shows that certain electrochemical systems, given, the same size are interchangeable, e.g. a lithium cell can replace two carbon-zinc (dry) cells or two silver oxide cells the same goes for three lead-acid cells compared to four alkaline manganese cells. In real life this is only possible to some extent, as certain specific properties of different electrochemical systems regarding their on-load characteristics, their energy content, and special constructive details resist this interchange. Two or three alternatives can always be found and should be evaluated. 

The mass related (gravimetric) energy content, the specific energy (SE) of lithium batteries, is 100 to 500 Wh per kg depending on system and cell type. Preferably portable devices profit from a lithium power supply. For comparison classic lead-acid batteries show a specific energy between 35 and 55 Wh/kg and NiCd batteries, a bit more powerful, from 50 to 70 Wh/kg. The said higher (lithium) values have, however, been only realized by primary systems until now. 

The volumetric energy content, mostly understood as the energy density (ED), goes from 300 to 1300 Wh/L. Lithium batteries therefore require less space than conventional battery systems. Leclanche cells, for example, dehver 165 and alkaline cells 330 Wh/L. 

The first lithium/iodine cardiac pacemaker battery was implanted in 1972. This type of battery proved to be very successful in this field and for other applications, too. The special features of this solid state battery are explained with its technique, which is limited with its extremely high energy density and reliability, especially for low rate applications. This technique is based firstly on the electrode couple of lithium and iodine with its high energy content - the OCV of the lithium/iodine cell is 2.80 V -and secondly on the favorable fact that the product of the cell reaction, the lithium iodide (LiJ), forms very tight and continuous layers between the active material of the electrodes, which are acceptable ionic conductors with negligible electronic... 

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