IE48721B1 - Secondary electrochemical generator employing a non-aqueous electrolyte - Google Patents

Abstract

1. Secondary electrochemical generator employing a non-aqueous electrolyte, characterized in that its positive active material is chosen out of the group formed by iron hexacyanoferrate, by the compounds derived therefrom by at least partial replacement of the iron by other elements of the groups 4A, 5A, 6A, 7A and 8 of the periodic table and which have at least two stable oxidation states other than zero, and mixtures of these compounds.

Description

The invention relates to a reversible positive active material for a secondary electrochemical generator employing a non-aqueous electrolyte.

A few years ago, there appeared on the market a new 5 family of electrochemical generators having the pecularity of using non-aqueous liquid electrolytes at ordinary temperature. Such electrolytes make it possible to use negative active materials constituted by very energetic light metals, such as lithium. These metals are difficult to use with aqueous electrolytes because of their chemical reactivity with water. These electrochemical generators therefore have substantially higher energy-to-mass ratios than aqueous electrochemical generators and their power is greater than that of electrochemical generators with solid electrolytes.

Further, the operation temperature of these cells is not restricted.

However, only primary generators with non-aqueous electrolyte have been produced up till now. Indeed, when attempting to make secondary generators, manufacturers have come up against two difficulties : - the light metal which dissolves in the form of salt during discharge must be redeposited evenly during charging; and - the need to use a positive active material which 25 reacts reversibly in a non-agueous electrolytic medium.

The invention aims to solve this second difficulty.

The invention provides a secondary electrochemical generator employing a non-agueous electrolyte characterized in that its positive active material is selected from the group formed by iron hexacyanoferrates; the compounds which are derived therefrom by at least partial replacement of the iron by other elements in groups 4A, 5A, 6A, 7A and 8 of the periodic table and which have at least two stable oxidation states other than zero; or a mixture of such compounds and/or iron hexacyanoferrates.

The groups of the periodic tahle of elements are designated in accordance with S 1.2 of the 1970 regulations for the nomenclature of inorganic chemistry as established by the International Union of Pure and Applied Chemistry.

Iron hexacyanoferrates are crystallized compounds which include iron cations and complex anions formed by a central atom of iron surrounded by six CN- cyanide groups.

The cations include two or three positive charges, according to whether the iron contained is divalent or trivalent. Likewise, the anions Fe(CN)g have four or three negative charges.

Iron hexacyanoferrates are obtained in particular by reaction of soluble hexacyanoferrates, e.g. alkaline ferrocyanides or alkaline ferricyanides with salts of iron (II) or (III). Besides the ions indicated hereinabove these hexacyanoferrates may contain alkaline cations together with crystallization water.

A possible explanation of the usefulness of iron hexacyanoferrates which are practically insoluble in organic solvents, as reversible positive active materials for electric storage cells with non-aqueous electrolytes relies on the fact that the iron in these compounds is capable of being reduced from state (III) to state (II) without destroying their structure, with probable insertion of cations which come from the electrolyte (Li+, for example) to re-establish electrical neutrality of the crystal as in one of the following reactions: Fecr3+ + Li+ + e- + (Fe2+Li+)cr te''CN)6]cr + Li+ + e+ (Li+[Fe (CN) J4) cr where the index cr indicates substances belonging to the hexacyanoferrate crystal. The reverse reaction is then possible during charging.

There also exist compounds derived from iron hexacyanoferrates by at least partial replacement of iron by other elements in groups 4A,5A,6A,7A and 8 of the periodic table which elements, like iron, have at least two stable oxidation states other than the zero state. The replacement can totally or partially affect either the cations or the 48731 central atom of the anions or both simultaneously. These derived compounds can be used in the same way as iron hexacyanoferrates as reversible active materials, as can mixtures of at least two iron hexacyanoferrates and/or derived compounds. in particular, iron can be replaced by manganese, cobalt and/or nickel.

The invention will be better understood from the description hereinbelcw of embodiments thereof given with reference to the accompanying drawings in which : - figure 1 shows the first discharge curves of three active materials in accordance with the invention; - figures 2a to 2d show successive discharge and recharge curves for one of these active materials (corresponding to curve A of figure 1); - figures 3a to 3e show successive discharge and recharge curves of the same active material, discharge and recharging being recorded in different cycling conditions; - figures 4a and 4c show successive discharge and recharge curves of a second active material (corresponding to curve B in figure 1); and - figures 5a to 5d show successive discharge and recharge curves of the third active material (corresponding to curve C in figure 1).

In all the preceding graphs, the capacity related to the 25 mass of active material (C,C^,CB,CC)in Ah kg is plotted along the x-axis and the potential of the electrode with respect to lithium (E,EA,Eg,Ec) in volts is plotted along-the Y-axis.

Tests giving the following results were made in the 30 following conditions: Iron hexacyanoferrates were prepared by mixing at ambient temperature an aqueous solution of potassium hexcyanoferrate (II) (i.e, ferrocyanide) or hexacyanoferrate (III) (ferricyanide) with an aqueous solution of ferrous or ferric chloride, as the case may be. The precipitates obtained after washing were dried down to a constant weight, either at 80°C at atmospheric pressure or at ambient temperature in a vacuum.

The active materials were mixed with graphite in equal parts by weight, with 0,1 to 1% gelatine as a binder? about 10 mg of each mixture was laid on a platinum support in a layer 0.1 mm thick over an area of 1 cm .

The electrodes thus constituted were cycled in conjunction with negative lithium electrodes in an electrolyte constituted by a molar solution of lithium perchlorate in a mixture of 80% dimethoxyethane and 20% propylene carbonate by volume at a current density of 1 mA/cm .

EXAMPLE 1 Iron hexacyanoferrate was prepared from potassium ferricyanide and ferrous chloride, in a proportion of 3 moles of ferricyanide to 4 moles of ferrous chloride. The reaction and the formula of the product obtained, such as described in technical literature, are as follows : The theoretical capacity per mass unit of the product assuming that the above formula is true and that all the iron (III) is reduced to iron (II), (i.e. 3 faradays per mole) is 89.3 Ah/kg.

Curve A in figure 1 is the first discharge curve of an electrode including said active material. The discharge capacity per unit mass of active material is plotted along the X-axis and the potential of the electrode with respect to lithium is plotted along the Y-axis. The electrochemical efficiency is about 87% for an end voltage of 2 volts. The generally level portion of the discharge curve in the vicinity of 3 volts is relatively inclined; this tends to confirm the discharge mechanism in the homogenous phase.

Figures 2a to 2d show successive discharge and charge curves of an electrode which includes the same active material, discharge being continued up to an end voltage of 2.5V and charging being continued up to four volts. The four figures relate respectively to the 1st, 10th, 50th, 300th discharge-recharge cycles. It is seen that charging and discharging other than on the first occasion occur oh two generally level portions of curve, the upper portion possibly corresponding to oxidation and to reduction of iron in sites other than those where there were atoms of iron (III) in the initial product. The electrochemical efficiency ' drops from 94% at the 1st cycle to 49% at the 300th cycle for the 2.5V end voltage.

Figures 3a to 3e show that the active material can be cycled on the lower portion only of graph, by limiting the charging voltage to 3,5V. These five figures represent respec lively the discharge-recharge curves for the 1st, 10th, 51th, 100th and 150th cycles. The electrochemical efficiency drops from 94% at the first cycle to 34% at the 150th cycle.

EXAMPLE 2 An iron hexacyanoferrate was prepared using 3 moles of potassium ferrocyanide for 4 moles of ferric chloride. The reaction given in the technical literature is as follows: 3K4Fe (CN) 6 + 4Fe’ci3 + 10H20 * 12KC1 + Fe4 ’ ^Fe (CN) 6j3,10H20 The product obtained in these conditions is usually called Prussian blue, but the meaning of the expression is not precisely defined; some authors refer thus to a compound whose formula is given above, while others consider that Prussian blue is a mixture of different substances. However, whatever the exact nature of the product obtained by the present preparation, its mass related to that of the initial reagents is compatible with the formula set forth. The corresponding theoretical capacity per unit mass (for 4 faradays per mole) is 103.0 Ah/kg.

The curve B of figure 1 is a first discharge curve of an electrode which includes said active material, drawn in the same way as the curve A. The electrochemical efficiency is 55% for an end voltage of 2 volts.

Figures 4a to 4c show successive discharge and recharge curves of electrodes which include the same above active material, produced in the same conditions as those of the curves in figures 2a to 2d, They relate respectively to the 1st, 5th and 10th discharge-recharge cycles. Efficiency drops rapidly, from 41% to 24%. Discharge and charging are effected on a single level portion of curve in the range of potentials examined.

EXAMPLE 3 Iron hexacyanoferrate was prepared from equimolecular quantities of potassium ferricyanide and of ferrous chloride: KjFe'(CN)6 + FeCl2 + H20 + 2KC1 + KFe'Fe(CN)g , H20 Such a substance is sometimes called Turnbull's blue. Its theoretical efficiency (1 faraday per mole) is 82.3 Ah/kg.

Curve C in figure 1 is a first discharge curve of an electrode which contains said active material. The discharge potential and the slope of the curve are comparable to those of the electrodes of the preceding examples. Electrochemical efficiency reaches 87%.

Figures 5a to 5d show successive discharge and charge curves of an electrode which contains said same active material. The four figures relate respectively to the 1st, 10th, 50th and 250th discharge-recharge cycles. Efficiency 487 21 observed increases at the beginning of cycling; 94% at the first cycle, to 107% at the 10th, then drops until it reaches 53% at the 250th cycle. Obtaining an efficiency of more than 100% is not surprising, since the theoretical capacity adopted takes into consideration the state of oxidation of the initial substance - a state which can be exceeded after recharging by passing iron atoms which were initially in state (II) to state (III). The discharges and recharges are effected on two portions of curve situated substantially at the same levels as those of the curves 2a to 2d.

The examples which have just been described show that it is possible to cycle electrodes which contain iron hexacyanoferrates as their active material. Such substances can be used as active materials in secondary electrochemieal generators which contain non-aqueous electrolyte and in particular in those which have lithium negative electrodes.

The invention is not limited to the active materials mentioned in the examples, nor to the modes of preparation described therefor. In particular, an active material in accordance with the invention can be obtained by reaction of a ferricyanide and/or a ferrocyanide with iron (II) and/or (III) salts in proportions other than those described, and the preparation can make use simultaneously of a ferricyanide and of a ferrocyanide as well as of an iron (II) salt and an iron (III) salt.

Further, the invention is not limited to the particular reactions, chemical formulae or oxidoreduction mechanisms described for the active materials; which descriptions have merely been put forward as working hypotheses.

Claims (7)

1. CLAIMS:1. A secondary electrochemical generator employing a nonaqueous electrolyte characterized in that its positive active material is selected from the group formed by : iron hexacyanoferrates; the compounds which are derived therefrom by at least partial replacement of the iron by other elements in groups 4A, 5A, 6A, 7A and 8 of the periodic table and which have at least two stable oxidation states other than zero? and mixtures thereof.

2. A secondary electrochemical generator according to claim 1, wherein said other elements are included in the group formed by manganese, cobalt and nickel.

3. A secondary electrochemical generator according to claim 1, wherein the active positive material consists of the product obtained by reaction of an alkaline ferricyanide with an iron (II) salt at a molar ratio of 3:4.

4. A secondary electrochemical generator according to claim 1, wherein the active positive material consists of the product obtained by reaction of an alkaline ferricyanide with an iron (III) salt at a molar ratio of 3:4.

5. A secondary electrochemical generator according to claim 1, wherein the active positive material consists of the product obtained by reaction of an alkaline ferricyanide with an iron (II) salt in equimolecular quantities.

6. A secondary electrochemical generator according to claim 1, wherein the active positive material consists of the product obtained by reaction of an alkaline ferricyanide and/or an alkaline ferrocyanide with an iron (II) salt and/ or an iron (III) salt.

7. A secondary electrochemical generator according feo claim 1 substantially as herein described with particular reference to any one of the examples.