Macromolecular Material of Ionic Conduction for Producing Electrolytes or Electrodes

1. Field of the Invention

The present invention relates to a novel ionically conductive macromolecular material used for electrolytes or electrodes. It concerns, in particular, materials which are present in the form of a polyether-salt based network and/or polycondensate.

2. Discussion of the Background

Such ironically conductive macromolecular materials are either not crystalline, or only very slightly crystalline, at ambient temperature. Therefor it is practically impossible to utilize them to produce electrochemical current generators, whether they be primary or secondary.

According to the prior art, solid electrolytes constituted by solid solutions of ionic compounds in polymers are already known. Reference may be made, for example, to European Pat. No. 0 013 199, entitled "Generateurs electrochimiques de production de courant et nouveaux materiaux pour leur fabrication which describes solid solutions of lithium salts in polar aprotic solutions capable of solvating the cation. These solutions constitute solid electrolytes that can be utilized in thin layers in solid generators. Among the polymers cited in this European patent ethylene polyoxides, propylene polyoxides and their copolymers are cited in particular.

In order to improve the mechanical properties of the electrolytes thereby constituted, and their behaviour with respect to crystallization, French patent application published under No. 2 485 274 entitled "Electrolyte solid a base de materiau macromoleculaire a conduction ionique" proposes to utilize as the electrolyte a cross-linked polymer complex material. The cross-linking is carried out with polymers containing hydroxyl functions that can be cross-linked by isocyanates, which leads to urethane networks in which the said functions are capable of reacting with the elements of the anode and/or the cathode and thus to provoke on the one hand a degradation of the polymer which could thus no longer fulfill its role of elastomer binder, and parallely on the other hand the consumption of a portion of the cathode or anode materials through irreversible oxidation.

The products obtained according to this cross-linking method present, due to the large dimension of the knots or meshes of the network initiated by the isocyanates, a high volumic (or massic) percentage of non solvating portion of the cation that can hinder the dissolutions and/or the dissociation of the salt, thereby causing a decrease in conductivity. If it is desired to overcome this poor solvation, it is possible to increase the molecular weight of the functional starting polymer but this generally leads to a decrease in the ionic conductivity of the polyether-salt complex through crystallization at low temperature.

On the contrary, the invention provide a macromolecular material made up of a polyether network salt in which the knots or meshes of the network are small-sized and correspond to cross-linking agents favoring the lowest possible vitreous transition temperature, the said material presenting a large range of redox stability. Furthermore, this material presents the advantage of being virtually free from protons after cross-linking.

In order to do this, the invention foresees the use of metallic or metalloidic derivates to obtain networks containing bonds of the R-O-M type, in which R represents a polyether and M a metal or a metalloid being at least divalent.

Therefore, the present invention concerns a macromolecular material presenting ionic conduction constituted by a salt in solution in a macromolecular material that is constituted by a least two polyether chains connected to each other by an atomic bridge. The bridge comprises at least one atom selected from among silicon, cadmium, boron and titanium). This atom is connected to at least one of the chains by an oxygen atom.

Preferably, the atom is a silicon atom and it is connected to each of the polyether chains through the intermediary of an oxygen atom.

In another preferred example, the said macromolecular material is made up of at least three polyether chains connected between one another by a an atomic bridge and the said bridge comprises at least one silicon atom connected to each of the said three chains by an oxygen atom.

According to another feature of the invention, each of the said chains has a molecular weight of between 250 and 30,000.

To obtain macromolecular materials according to the invention, it is possible to utilize the process in which at least one polyetherglycol, and/or one of its metallic salts, mono or polydispersed, which may be pure or in a mixture with at least one other polyetherglycol, is reacted with at least one cross-linking agent or at least condensation agent. The said polyetherglycol that is caused to react corresponds to the polyether chains mentioned herein-above. The said cross-linking or condensation agent comprises at least one atom of a metal or a metalloid which is at least divalent, chosen from among silicon, cadmium, boron and titanium. The said reaction occurs in solution in the said macromolecular material.

By the expression "mono or polydispersed" it is meant that it is possible to have polyetherglycols or the same molecular weight or of different molecular weights.

Preferably, the process consists, furthermore, in spreading the mixture while the reaction is in progress so as to obtain a thin layer.

According to a first embodiment of this process, the said reaction is carried out with at most a stoichiometric quantity of a halogen or pseudohalogen derivate represented by the formula:

in which:

R' represents an alkyl radical, preferably methyl;

x represents the number of radical R';

X is a halogen or pseudohalogen such as SCN.sup.- ;

M is a metal or metalloid, at least divalent, selected from among silicon, cadmium, boron and titanium;

n is the valency of M with n -x.gtoreq.2

According to a second embodiment, the process consists in carrying out a transetherification through reaction of polyetherglycols on light alkoxides of the said metal or metalloid, represented by the formula R"-O-M in which R" is an alkyl radical of from 1 to 8 carbon atoms and M has the same signification as given herein-above.

According to a third embodiment, the process consists in carrying out a polycondensation on polyetherglycols in the presence of boric acid B(OH).sub.3.

However the invention also concerns composite electrodes produced from macromolecular material described herein-above. These electrodes can be obtained according to embodiments described, it is sufficient that the initial mixture comprises, in the form of powder, the active compound of the said electrode.

The interest of the manufacturing process for macromolecular materials according to the invention for manufacturing electrolytes or electrodes lies essentially in the fact that the reaction can occur at ambient temperature and thus can present a sufficiently slow kinetic rate to permit easy spreading out of the product in a thin layer.

In the case where the reaction is carried out from polyols, the reaction rate is perfectly controllable through the elimination of the hydracid formed. This elimination can be favorized by an increase of the temperature or can occur under vacuum, in the presence of a compound capable of fix the hydracide and which is neutral with respect to the electrochemical chain of the generator.

In the case where the startng mixture contains polyglycol metallic salts, alkaline metal halogenides are formed with precipitate in the reactional medium or can, according to their nature, be put into solution in the polyether formed and thus contribute to the increase of the ionic conduction.

In order to render more apparent the features, objects and advantages of the present invention, the following examples are given by way of non-limitative illustration.

In these examples the conductivities of the materials obtained with the prior art (cross-linking of the tri-isocyanates) are compared with the conductivities of the materials obtained according to the invention.

1. Study of a material obtained from an polyethylene glycol, of MW 3000, into which is put into solution lithium perchlorate LiClO.sub.4 with a O/Li=12 ratio.

Prior art: the polyoxyethyleneglycol (POEG) is cross-linked in the presence of a stoichiometric quantity of aliphatic triisocyanate, in the presence of amine as catalyst and in solution in acetonitrile presenting a dry extract of 70%.

Invention: the reaction is carried out in acetonitrile and in the presence of CH.sub.3 SiCl.sub.3. The product is cross-linked without catalyst in 10 mn at ambient temperature (25.degree. C.).

At the end of the reaction, the product is spread out on a support, the reaction is thus completed by raising the temperature. It is then drawn out so as to obtain a thin film. The CH.sub.3 SiCl.sub.3 was present in a stoichiometric quantity with respect to the initial polyetherglycol. The temperatures are measured as well as the conductivity for which values are obtained, in .OMEGA..sup.- cm.sup.-1 : 10.sup.-6, 10.sup.-5 and 10.sup.-4. The results obtained are tabulated in the following table I.

It will be observed from this table that the temperatures have been decreased by about 24.degree. to 10.degree. C. which means that the conductivity has been increased.

2. Study of the same materials but obtained with a quantity lower than the stoichiometric (10%) of the isocyanate and methyltrichlorosilane (CH.sub.3 SiCl.sub.3).

The results obtained are compiled in table II.

It will be observed in this table that the conductivity has been strongly increased although starting from a lower stoichiometry.

3. The same comparative study was made but starting from a polyetherglycol mixture made of:

80% (by weight) polyethylene glycol

20% (by weight) polypropyleneglycol (PPG)

comprising in solution LiClO.sub.4 with the O/Li=12 ratio carried out after putting into solution in the acetonitrile with a dry extract of 50%.

The results obtained are compiled in table III where the molecular weights are also indicated.