Anhydrous organic electrolyte.

p0001The invention relates to an anhydrous organic electrolytes for galvanic cells having a negative light alloy electrode containing at least one organic solvent with an activated by a neighboring group -0-, = CO or N = CH bond.

p0002Solvent after given the generic definition can be found as an electrolyte component, especially in high-energy cells lithium based very often. Here, their selection is made on the one hand from the point of utmost insensitivity to the strong reaction electrode material, on the other hand with regard to a good solvation to inorganic salts. The group of ether meets these requirements to a high degree. Ethers are also known as aprotic solvents, because they are not capable of delivering protons and therefore can react difficulties with alkali metals. Your behavior is consistent with a Lewis base.

p0003As examples of employed in lithium cells ether may be mentioned here: the aliphatic monoethers dimethyl ether, diethyl ether, diisopropyl ether or n-Buthyläther and from aliphatic polyethers diethylene glycol dimethyl ether (DME), Äthylenglycoldiäthyläther or Tetraäthylenglycoldimethyläther and finally cyclic ethers such as tetrahydrofuran, 1,4-dioxane and tetrahydropyran ,

p0004for example, other electrolyte solvent, to extending the application of the invention are the butyrolactone and acetonitrile.

p0005In practice, usually mixtures are used of several solvents, especially when one is forced to approximately between the capacity of the electrolytic liquid for the conductive salt and the vapor pressure behavior to include an optimal compromise or set a favorable viscosity, which is why, for example, a PC / LiCl0₄ electrolytes with DME diluted.

p0006The loosened CH bond at the here in question organic liquids makes them susceptible to the absorption of oxygen. Especially pronounced is the tendency to oxidize to nonvolatile peroxide in the airwaves. The explosion hazard associated has recently proven to be major constraint in dealing with ether-containing high-energy cells.

p0007Ether peroxides usually form from hydroperoxide, which is produced in a first stage by splitting a CH bond in addition to the oxygen bridge of the ether molecule and O₂ intercalation and subsequently falls into a little stable alkyl peroxide radical and an alcohol. About a radical chain mechanism to unite it, the alkyl peroxide radicals corresponding peroxide polymers. From peroxide ethers therefore both hydroperoxide monomers and Alkylidenperoxid polymers can be isolated together.

p0008Relevant experience with cambered lithium cells both to the test and during operation have shown that oxidation processes of the type described evident also occur in the cells, despite hermetic seal against the outside air.

p0009Minor amounts of oxygen can be used in the cell by the decomposition of oxygen-containing compounds such as LiCl0₄, Cr0_x, Mn0₂ or be formed from H₂0 residues of not fully dewatered solvent by electrolysis. Also is an entrainment of peroxide which is formed already at the ether-purification by distillation, it is possible in the cell. To trigger an explosion of the peroxide is then needed only an accidental power supply, which can occur, for example by a short circuit with the following overheating of the cell or by exposure to light.

p0010Previously known measures for explosion protection targeted, with no special relationship to the present electrochemical system, it from an imminent crowning the cell by premature power interruption, for example by means of a fusible conductor portion, either completely avoid or mechanical measures the volatility of an explosion and thus the destructive power to soften. The latter can be achieved by valve constructions in which a deformable ball is mounted sealingly in an adapted opening in the region of the cell cover, wherein a maximum permissible internal pressure is exceeded, however irreversibly deformed and an opening gap releases eg.

p0011The invention has for its object to provide measures with which directly from an electrolyte outgoing according to the preamble initially formulated in explosion of lithium batteries can be curbed.

p0012The object is achieved in that the electrolyte is a stabilizer substance is added which prevents the formation of polymeric peroxides or decomposed existing peroxides.

p0013Particularly effective agent against chemical changes in the solvent in question are substituted phenols, preferably alkyl-substituted phenols, out of the following group: 2,6-di-tert-butyl-4-methylphenol, 2,4,6, tri-tert-butyl-phenol, 2,2'-methylenebis (4-methyl-6-tert.butyl-phenol, 2,2'-isobutylidene-bis- (4,6-dimethyl-phenol), 4,4'-butylidene-bis (2-tert-butyl-5-methylphenol), 4,4'-thiobis (2 -tert-bis- (2-tert-butyl-5-methyl-phenol).

p0014It also includes polynuclear phenols which are sterically hindered, and mixtures of hindered phenols styrenated.

p0015In addition to said phenol derivatives also shows a quinoline derivative, the 2, 2, 4-trimethyl-1, 2, - dihydroquinoline, in polymerized form, a positive effect in terms of an explosion postponement.

p0016Most of these products can be accommodated also be regarded as methyl phenol derivatives. Through further substituents but all of these phenols are more or less steric hindrance, which means that they are more stable than non-substituted phenols and can less easily than those of being oxidized. This in turn implies, which is also important in the context of the invention, a low reactivity with respect to the lithium electrode.

p0017Overall, said substituted phenols can be summarized under the label "Phenolic antioxidants". Here, however, nothing has been said about the specific mechanism by which they engage in the reaction process of the cell and avert the risk of Ätherperoxidexplosion in the one case or another.

p0018Another, very suitable for the stabilization of the electrolyte substance group comprising low-sulfur or phosphorus organic compounds that are easier to oxidize and thus less stable than the substituted phenols. But their action as an antioxidant is therefore all the more plausible. Inventive representatives from this group are the 4,4'-thiobis (2-tert-butyl-5-methyl-phenol), trimethyl phosphite, triphenyl phosphite and tris-nonylphenyl phosphite.

p0019Analogously form the phosphorus-containing agent is to be a group that can be overwritten with "phosphidic antioxidants". The thio compound is 4,4'-thiobis (2-tert-butyl-5-methylphenol) is an example of the invention both the first group of substituted phenols as well as for the above-mentioned group of the S and P connections.

p0020Another stabilizer substance of the invention is finally the Tetracyanoäthylen.

p0021The application concentration of stabilizers according to the invention should be in the range of 10 mg to 10 g / kg solvent, preferably at 100 mg / kg of solvent (= 100 ppm).

p0022In various test series fresh lithium cells of nominal capacity were 1 Ah with the electrolyte system Diätylenglycoldimethyläther / propylene / LiClO₄ and with Cr O_x as Depolarisatormasse after addition of 2,6-di-tertiary-butyl-4-methyl phenol to the electrolyte in an amount of 100 mg / 1000 g solvent with a current of 100 mA (approximately 10stündig) overweight. It was found that an overload over 18 hour period at a charging voltage of 40 volts is feasible without any complaints, whereas the same cells without this addition after 30 min. had to be taken because of crown from the experiment.

p0023When on 24 volts or 12 volts degraded charging voltages fresh lithium cells showed the overcharge test up to 144 hours of continuous no failures due to explosions, whereas isolated those cells which had been previously discharged to 30%. Their share in the corresponding test batches was only 10%. At a charging voltage of 6 volts, which is common in practice no risk of explosion was observed. These findings indicate that, compared with non-stabilized cells, an increase in charging time by more than a factor of 10 is possible.

p0024Furthermore, in the cells provided with stabilizer no change in the electrolyte conductivity at room temperature and no reduced capacity utilization was found.

p0025Based on these results and other experimental findings with other application concentrations of the additive stabilizer substances of the invention are generally characterized by the operation specified in the characterizing part of claim 1, which ensures that even outside of normal operating conditions, there is no acute risk of explosion for the high-energy cells.

p0026The chemistry of their mode of action here is of minor importance. It may however be assumed that under the stabilizer substances of the invention they exist, are the radicals assets, which are more stable than those mentioned in the introduction, resulting from ether hydroperoxides alkyl peroxide radicals. This is certainly in the substituted phenols, such as the 2,6-di-tert-butyl-4-methylphenol, the case. By now their more stable radicals in the radical chain of Ätherperoxidpolymerisation, turn, they cause their immediate termination because of their stability. The explosive Peroxidpolymer not be adopted. The stabilizer substance acts as a radical inhibitor or "scavenger".

p0027The stabilizer substance can cause but by their very presence, a decomposition of the hydroperoxide formed primarily or otherwise present peroxide. The further reaction to peroxide polymer is thus blocked from the outset and eliminates the risk of explosion. The stabilizer substance functions without changing itself, as a decomposition catalyst. In this sense should mainly affect the phosphitic antioxidants, while the 4,4'-thiobis (2-tert-butyl-5-methyl-phenol) because of its dual nature as methyl phenol derivative on the one hand and as a low-thio compound other hand, a dual role both as a radical scavengers such as peroxide decomposers belongs.