Voltage delay

The swelling of the cathode (CFx) influences the discharge through the formation of a LiF precipitate (23). A film of LiCl is formed on the Li anode by its reaction with the depolarizer SOCI2 this causes the voltage delay during discharge (24). 

The presence of passivating films reduces the cell voltage below the anticipated thermodynamic value calculated assuming a simple metal/ metal ion process at the anode. More important, however, is the fact that the films are responsible for a time-lag between the point at which a current drain is initiated and the point at which the cell reaches its operating voltage. An example of this voltage delay is shown in Fig. 3.23 where... 

The reaction of lithium with the electrolyte to form a surface film significantly modifies its behaviour. On the one hand, the film confers chemical stability and useful shelf life on the system. On the other, it is responsible for greatly depressed exchange currents and the consequent phenomenon of voltage delay, as discussed in Chapter 3 in connection with magnesium aqueous batteries. It is convenient to discuss separately film formation with insoluble and with liquid and soluble cathode systems. 

The formation of passivating films on lithium in contact with liquid or soluble cathodic reagents is a prerequisite for the construction of a practical cell. The film acts in the same way as a separator, preventing further direct chemical reaction of lithium and the cathodic reagent. However, film formation involving the action of S02, SOCl2, etc. on lithium is considerably more complex and may produce much more severe voltage delay characteristics than in the case of insoluble cathodes described above. 

In practical terms, the twin objectives of protecting the lithium from corrosion while avoiding unacceptable levels of voltage delay can be considered to have been met, However, the detailed mechanisms of film formation and disruption are still matters of some controversy. In particular, the interaction of thin films formed rapidly on lithium surfaces exposed to the atmosphere with the thicker films formed by subsequent reaction with the cathodic reagent is not well understood,... 

The (CF ), cathode is stable in contact with organic electrolyte systems, and the cells have a satisfactory shelf life. In common with other lithium power sources, polycarbon fluoride-based cells may suffer from voltage delay, but only under severe discharge conditions. 

In 1998, Gan and Takeuchi identified a key role of anode surface him composition in SVO cell performance [64], The addition of carbon dioxide synthons such as dibenzyl carbonate and benzyl succinimidyl carbonate was found to reduce resistance build-up and alleviate voltage delay in silver vanadium oxide cells. 

Bergman, G.M. and E.S. Takeuchi. 1989. Voltage delay and complex impedance characteristics of a high-rate lithium/silver-vanadium oxide multiplate battery. J. Power Sources. 26 365-367. 

Figure 18.15 Suppression of the voltage delay at the beginning of the discharge of a lithium/thionylchloride cell effect of an additive (GTE). 

For military applications also activateable batteries were developed whose electrolyte during shelf-life is separated from the electrode stack and pushed into the cell within seconds only just before use of the battery. Of course the shelf-life of such batteries is still longer than that of active batteries of the thionylchloride type with their capacity loss of 10% during 10 years of storage. But for military purposes the reliability of the improved system and the avoidance of the initial voltage delay make the activateable technology more attractive than the reduction of selfdischarge. 

The type of response shown in Fig. 3.15a is also typical of batteries that have developed a protective or passivating film on an electrode, the voltage recovering as the film is broken during the discharge (see Sec. 3.2.12 on Voltage Delay). The specific characteristics, how-... 

Expensive, but Not independent Pressurized Voltage delay Available in small Higher cost than For very low... 

Good capacity retention, even under Delayed action (voltage delay)... 

---