Polymer overcharging

A compromise is to add some gelled electrolyte. Commercial cells use a porous polyethylene or polypropylene separator filled with a polymer and gel filling with a liquid electrolyte. They offer improved safety with more resistant to overcharge and less chance for electrolyte leakage. 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

Following Adachi et al., aromatic compounds with similar functionalities were proposed for polymer electrolytes as redox shuttle additives, which included bipyridyl and biphenyl carbonates and di-fluoroanisoles. All these additives could protect the cathode from overcharging in the vicinity of 4.1... 

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... 

The conductive polymers were also tested at levels of 13 wt.% in a positive plate which contained PbS04, a-Pb02, and p-Pb02 [22]. The optimum concentration was found to be lwt.%. At 2 3 wt.% additive, the discharge capacity was increased by about 30% and the specific surface-area from 3-4 to 5-6m g. Cycle-life declined at additive concentrations above 5wt.% due to mechanical instability of the electrode. Polypyrrole and polythiophene oxidized during overcharge, but polyaniline remained stable. 

Polyaniline, polypyrrole, polyparaphenylene and polyacetylene (doped with SO , HSOj), added to the positive paste as powders or fibres, enhance the formation process and increase the capacity of the cells. The amount of polymers added to the paste should be within the range 0.8—2.0 wt%. Higher polymer loads impair the mechanical stability of PAM and hence the life of the battery is shortened dramatically. Polymers disintegrate on overcharge. Polyaniline has proved to be most stable on battery overcharge. 

In 1997, NEC Moli Energy discovered that pyrrole (114) and N-methylpyrrole (95) can be used as additives in small quantities [118]. According to the description, When the battery is overcharged, monomer additives begin to polymerize once the polymerization voltage is reached. Eventually, a sufficient amount of conductive polymer is created and a conductive frame is formed between the electrodes, thus shortening the battery internally and discharging it [118]. 

An assessment of the relative diffusion rates of ionic and molecular species in the PAN-based electrolyte may be made from the diffusion coefficients calculated for ferrocene from cyclic voltammograms. Some data are presented in Table 3.8. The ratio of diffusion coefficients of ferrocene in the PAN-based polymer electrolyte and PC/LiC104 liquid electrolyte at room temperature is the same as that obtained for the conductivity of LiC104 in these electrolytic media. It may be noted here that the ferro-cene/ferrocenium couple has been shown [36] to be useful for the overcharge protection of secondary Li batteries. 

To overcome the poor mechanical properties of polymer- and gel-polymer-type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF, PVdF-HFP, and other gelling agents, have been developed as an electrolyte material for lithium batteries [156-166]. Gel-coated and/or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown at a particular temperature allows safe deactivation of the cell under overcharge conditions. 

Hydrogel electrolytes which include water-absorbable cross-linked polymers can show high electrical conductivity like liquid electrolytes. Thus they are clearly promising materials for all-solid-state rechargeable alkaline batteries. The rechargeable alkaline batteries have a peculiar safety mechanism in overcharging and overdischarging. For example, in Ni-MH batteries. 

An attractive feature of C/LiMn204 polymer Li-ion cells is their ability to sustain abuse. Safety and abuse tests passed by C/LiMn204 polymer Li-ion cells are listed in Table 35.25. In addition, C/LiMn204 polymer Li-ion cells can sustain nail penetration in the fully charged state or the overcharged state without explosion or Are. 

Shutdown additives usually undergo an irreversible oxidative polymerization to form a non-conductive polymer layer that electrically blocks the current flow to protect the cell from being overcharged. The disadvantage of this strategy is that once the shutdown operation is activated, the cell is permanently disabled and the module that connects the damaged cell in series is consequently non-operational. For this reason, this type of overcharge protection additive received litde interest. Classic examples of such additives are primarily aromatic compounds such as biphenyl [149], cyclohexylbenzene [56, 158], and xylene [44, 170]. 

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