Electrolytes overcharge protection

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. 

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 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... 

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. 

These AA-size batteries with lithium metal anodes were introduced into the consumer market, with claims of safe performance under abusive electrical conditions due to an intrinsic shut-down mechanism involving polymerization of the electrolyte which protects the cell under short circuit and overcharge (shutdown at 4.0... 

The redox shuttle [13-15, 17] is an electrolyte additive that can be reversibly oxidized/reduced at a characteristic potential and provides an intrinsic overcharge protection for lithium-ion batteries that neither increases the complexity and weight of control circuitry nor permanently disables the cell when activated. The redox shuttle molecule (S) has its defined redox potential, at which it can be oxidized on the positive electrode and form a radical cation (S ) (see Equation 1). 

Feng JK, Cao YL, Ai XP, Yang HX (2008) Tri-(4-methoxyphenyl) phosphate a new electrolyte additive with both fire-retardancy and overcharge protection for Li-ion batteries. Electrochim Acta 53 8265-8268. doi 10.1016/j.electacta.2008.05.024... 

X 10 S cm". This approach is seemingly espedally useful for battery electrolytes, because the transference number of the lithium ion is increased recently, Weng et al. [519] reported an improved synthesis of tetrafluoro-catechol [105] that is a starting material for the fluorinated boronate ester, 2-(pentafluorophenyl)-tetrafluoro-l,3,2-benzodioxaborole (PFPTFBB). PEPTEBB acts as an anion receptor. In addition, it is an effective redox shuttle for overcharge protection of lithium-ion batteries. Conceptually, this approach is similar to the use of lithium salts with large anions or the immobilization of anions at polymer backbones. 

Dilute solution theory is not often used in the treatment of lithium batteries, because most electrolytic solutions used in lithium batteries exhibit concentrated behavior. However, dilute solution theory becomes useful for cases such as the examination of side reactions such as redox shuttles for overcharge protection, because concentrated solution theory becomes more complicated when there are more than three species (anion, cation, and solvent) in solution. 

The charge state of the cell must be maintained in operation to have a cell voltage of 0.9 to 1.2 V [6]. Overcharging the cell is to be avoided due to electrolytic decomposition of water and evolution of gas. The cell voltage should therefore not exceed 1.4 V. Cathodic protection stations should be operated so that the cell voltage lies in the desired range. 

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