Overcharge protection

The performance of a battery is often designed to be limited by one electrode ia order to achieve special performance characteristics, such as overcharge protection and safety. The coulombic efficiency of the active mass is of particular iaterest ia battery design and performance. 

In the overcharge tests we carried out, there was no fire or explosion. The cell impedance increased suddenly in every test. This was due to the oxidation of the electrolyte with a low charging current, or to the separator melting with a high charging current. In practical applications, an electronic device should be used to provide overcharge protection and ensure complete safety. 

Table 11. Cathode Surface Layer Additives Overcharge Protection...

The effect of these ferrocene-based additives on overcharge protection is shown in Figure 44, where AA cells based on lithium, LhMn02, and electrolytes with or without additives were overcharged. In the absence of these redox shuttles (A), the cell voltage continues to rise, indicating the occurrence of major irreversible decompositions within the cell whereas the presence of shuttle agents (B—E) locks the cell potential in the vicinity of their redox potentials... 

Other less prominent types of additives, also intended for overcharge protection, were termed shutdown additives in the battery industry based on their tendency at high potentials to release gas, which in turn would activate a current interrupter device (CID), or to polymerize and block the ion passage in the electrolyte. The former included such... 

The use of electroactive polymers for overcharge protection has been recently reported for lithium-ion batteries.The electroactive polymer incorporated into a battery s separator is an attractive new option for overcharge protection. Thomas et al. developed a mathematical model to explain how electroactive polymers such as polythiophene can be used to provide overcharge protection for lithium-ion... 

Fig. 43. Mechanism of overcharge protection (redox additive). After [586],...

The immobilization of the electrolyte is achieved by the addition of 4 to 7 % of a gelling agent (e g., sodium carboxymethyl cellulose) or by soaldng-up of a porous separator system or by the capillary action of the powdered-zinc anode (compressed type). Zinc oxide addition has an effect of corrosion inhibition, paste-forming and overcharge protection. 

Nonaqueous electrolyte solutions enable the lithium-ion cells to attain high voltage up to about 4 V, however, no overcharge protection mechanism is included in principle, as the aqueous electrolyte solutions in rechargeable cells such as Pb-acid, Ni-Cd, and Ni-MH have overcharge protection mechanisms, which are the electrolysis of H O to and their recombination to H O. Present lithium-ion... 

A lot of metal complexes and aromatic compounds have been proposed as an overcharge protection agent however, their redox potentials were often too low for 4 V class lithium-ion cells. Successful examples are given in Fig. 4.17, where their onset oxidation potentials are given (the values lack consistency due to different measurement conditions). It is clear from these examples that 7i-electron conjugated systems are essential. 

Fig. 4.17 Examples of redox shuttle-type overcharge protection agents and their oxidation potentials...

We have found that the alkylbenzene derivatives with no hydrogen atom in benzyl positions can act as a mediator to decompose carbonate solvents, as postulated in Fig. 4.20, because they showed a reversible redox reaction and increased the amount of evolved CO without losing themselves. A terf-butylbenzene and 1,3-di-tert-butylbenzene showed lower OCVs compared with toluene, ethylbenzene, and cumene after the overcharge test at 60°C, as shown in Fig. 4.21, which is a good indication as an overcharge protection agent. 

Z. Chen, J. Liu, A. N. Jansen, G. GirishKumar, B. Casteel, K. Amine, Electrochem. Solid-State Lett. 2010, 13, A39-A42. Lithium borate cluster seilts as redox shuttles for overcharge protection of lithium-ion cells. 

In 2003, Shima and Ue from Mitsubishi Chanical Corporation and Yamaki from Kyushu University presented the mechanism behind the overcharge prevention. According to the description in the relevant reference, the aromatic compounds without hydrogen at the benzylic position (e.g., t rt-butylbenzene) evolve mainly carbon dioxide (CO2) gas, which is generated by the indirect decomposition of carbonate solvents. The CO2 gas evolution reaction using a redox mediator is expected as a new overcharge protection method [3]. 

---