CA1138035A

CA1138035A - Salt composition usable as a fused electrolyte in accumulators

Info

Publication number: CA1138035A
Application number: CA000350429A
Authority: CA
Inventors: Frederic Breda; Pierre Jonville; Angelo Bonomi; Jack Ambert
Original assignee: PROGE
Current assignee: PROGE
Priority date: 1979-04-26
Filing date: 1980-04-23
Publication date: 1982-12-21
Also published as: DE3016495C2; IT8021654A0; US4370394A; DE3016495A1; IT1148799B; FR2455367A1; FR2455367B1; GB2052135B; GB2052135A; US4287270A; JPS55144665A; CH642777A5

Abstract

ABSTRACT
An alkali salt composition free from hydroxides, melting at a temperature in the region of or greater than 180°, and usable in its molten state as a negative electrolyte for accumulators comprising an alkali metal negative electrode and an alkali alumina separator, contains an alkali metal amide, at least one alkali halide, and an alkali hydride. It may also contain aluminium amide.

Description

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230P~9902 'SALT COMPOSITION USABLE AS A

This invention relates to electrochemical energy sources, and in particular to accumulators or secondary cells with an alkali met~l anode operating in a molten alkali salt electrolyte, and comprising a separator in the ~orm of an alkali alumina wall between the anode and cathode compartments.
The term "alkali alumina" used herein em~races ~ and ~" alkali aluminates or mixtures of these aluminates. ~ alkali alumina can be represented by the formula ME20 . 11 al203, and ~"
alkali alumina can be represented by the formula ME~O . 5 A1203, where ME represents one or more alkali metals ? for example Na~ K or LiO These aluminas can al50 contain one or more additives~
More particularly, the invention relates to an alkali salt composition free from hydroxides, which melts at a temperature in the region o~ or exceeding 180 and is usable in its molten state as an anode electrolyte, and in particular contains an alkali amide and at least one alkali halide.

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In accumulators which use an alkali metal in its molten sta-te as the anode, if this metal is in direct contact with the alkali alumina wall, it produces a gradual degrada-tion of the wall as the charging and discharge cycles progress. During recharging, atoms of the alkali metal, for example sodium~ are ~ormed by electrolytic reduction o~ the sodium ions a-t the interface of the molten metal with the alkali alumina.
Sodium dendrites can therefore form in the pores o~
the latter in the vicinity of its surface, and these can gradually lead to the formation of microfissures, which can extend right through the thickness of the separator and finally create short-circuits between the anode and cathode compartments. These defects have been remedied by interposing a liquid electrolyte between the actual anode and the separator~ In this case 5 the reduction of the sodium ions during charging takes place at the molten sodium-interposed el~ctrolyte interface, and the sodium atoms which are ~ormed are immediately absorbed by the anode, with the result that no deposi-t of sodium metal can come into con-tact with the alkali alumina.
Such an interposed electrolyte is described in French Patent No~ 2142695 (C.G.E.), which uses mixtures of NaOH with NaBr or NaI. Likewise, Belgian ;

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Paten-t No. 861,315 (B~D.C.) describes -the use of interposed salts constituted by alkali tri or tetra-alkyl aluminates or borates, certain mixtures of which melt at temperatures less than 100C. Furthermore, U. S. Patent 3,472,745 (North American Rockwell) describes electrolytes based on alkali amides or hydroxides which contain an alkali cyanide or iodide in order to lower the melting point of the mixture;
such mixtures melt below 180C~
It has now been discovered on the one hand that mo]ten salt mixtures based on aluminates and borates containing organic substituents lack stability when in contact with molten sodium at around 180C, and on the other hand that the presence of alkali hydroxides in an interposed electrolyte is not advisable when the accumulator operates at a temperature less than about 300C. This is because the sodium reacts with the caustic soda in accordance with the reaction NaGH +
2Na _~ NaH + Na20, the sodium oxide which is formed crystallising in the mixture below 300C and making the electrolyte unsuitable for normal operation.
Hydroxîdes have therefore had to be removed from molten alkali salt mixtures operating at a tempera-ture of the order of 180 to 200C, and it has been found that electrolytes based on alkali ami.des, in particular 1 3~
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NaNH2, and containing an alkali halide such as NaC1, NaBr, NaI and their mixtures, are very suitable for this temperature range. It has however been found that in such an electrolyte, the alkali amide can decompose in the presence of the molten alkali metal to liberate nitrogen, the presence of which is undesirable because the pressure which it creates can lead to fracture of the anode compartment.
- An object of the present invention is to provide a fused-salt electrolyte which is stable, ~ree ~rom hydroxides, and does not liberate nitrogen in use.
According to the invention there i5 provided an alkali salt composition free from hydroxide, usable in its molten state as an anode electrolyte ~or accumulators with an alkali metal negative electrode and with an alkali alumina separator, the said composi-tion containing an alkali amide, at least one alkali halide 9 and an alkali hydride.
The presence o~ an alkali hydride opposes the above-mentioned formation of nitrogen, by displacing the reaction equilibrium towards the left in the following equation (in the case of sodium): ' NaNH2 + Na ~ 2NaH + ~N2 (1) The alkali metal used is preferably sodium, and the electrolyte according to the invention is preferably ,, .

based on NaNH2 and a sodium halide. The quantity of NaH preferably lies between 2 and 20~. By way of example, a mixture con-talning NaNH2 and 10% by weight of NaH melts at 190C, and this mel-ting point can be further considerably lowered (to abou-t 180C or even lower) by adding 5-20% by weight of NaI or NaBr, or

2-10% of NaCl. For comparison, NaNH2 by itself melts a-t about 200C.
More than one of the aforesaid halides can be used simultaneously, their total being preferably less than 25-30% by weigh-t of the composition. It should be noted that the properties o~ -the present electrolyte can be further improved by adding aluminium amide (or more simply powdered aluminium) in a quantity o~ 5 to 15%
by weight of the total composition. In this respect, it has been found tha-t the presence of Al(NH2)3 can strongly slow down the tendency o~ the sodium amide to attack the alumina of the separator in accordance with the reaction 6NaNH2 + A1~03~==~2Al(NH2)3 + 3Na20 ( ) by displacing its equilibrium towards the left.
Furthermore, the direct addition o~ powdered aluminium to the mixture rapidly provides the required amide in accordance with the reaction Al + 3NaNH2- ~ 3Na + Al(~2)3

3 ~j The use of such an interposed electrolyte enables an accumulator of very long life to be obtained.
The invention will be further described by way of example with reference to the accompanying drawing, in which:-Figure 1 is a diagrammatic sec-tion through a sodium-nickel accumulator or secondary cell in which an anode electrolyte according to the i~vention is used, and Figure 2 is a diagrammatic representation.... of a modification of the anode compartment of such an accumulator.
The accumulator 1 illustrated comprises a ~irst outer glass casing 11 enclosing the positive compartment 2, and a second casing inside the first Por the negative compartment 3. This second casing is composed of a glass capsule 12, and, joined to it, an alkali alumina tube 13 containing sodium 14. A metal conducting wire 15, buried in the sodium and joined to the top of the two casings by way o~ glass-metal seals 16, 17 respectively, constitutes the (-) pole of the accumulator. The cathode compartment comprises a positive electrode 18, for example a mixture of powdered nickel and NiC12, immersed in a liquid electrolyte 19, for example Na(AlC14) 9 the positive pole being represented by a conducting wire 20, the coiled end of which is buried in -the cathode mass 18.
In the negative compartmen-t 3, the accumulator l comprises a molten interposed salt 21 having the composition described hereinafter. It should be noted that this interposed sal-t has a coefficient of wettability for the alkali alumina which is much higher than that of the molten sodium for said alkali alumina. Consequently, for inter-facial tension reasons, the sodium tends to collect a-t the centre of the negative compartment 3, whereas the interposed salt tends to form a continuous film between the sodium and alkali alumina9 which is precisely the behaviour required to enable the sodium to be isolated from the material of the separator.
In a further embodiment (see Fig. 2), a porous mass 22 impreganated with the interposed salt 21 is disposed between the sodium and ~he separator wall in order to create a more rigid mechanical barrier between these fv~ctional elements. The porovs mass used can be fibre or ~abric of an insulating ceramic material (preferably non--siliceous), such as zircon7 magnesia, alumina or a porous inert ceramic such as A120~.
Specific examples of cells as shown in the drawing will now be given~

3~i EXAMPLE 1.

70 parts by weight of NaNH2, 20 parts of NaI and lO parts of NaH were mixed intimately under -the protection of nitrogen. A ~uantity o~ thi~
mixture and of metal sodium was placed in an alkali alumina tube 13 joined in a sealed manner at its top to an open "Pyrexl' (Trade Mark) tube 12, the quantities o~ the components being chosen such that after melting, -the level of the electrolyte N2l slightly exceeded the level of the sodium (Nl4).
The whole assembly was heated under inert gas until melting took place, a molybd~num rod 15 was inserted through the top of the tube so that its lower end was immersed into the molten sodium, and the top of the tube was closed by a ~lame, the conducting rod being simultaneously sealed at 16 as shown in the dra~ing.
The negative tube was then surrounded by the positive electrode 1~3 and the entire assembly was inserted, under the protection of an inert gas, into a second capsule ll, open at its top and containing the molten cathode electrolyte (Na(AlC14)) l9. The capsule ll was then closed with a ~lame so as to seal the negative and positive conductors 15 and 20 into its top, as shown at 17 and 23 in the drawin~.

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., 33~ïi An accumulator was thus obtained in which the anode comprised 7 g of sodium, corresponding to a capacity of 6 Ah in aocordance with the overall reaction:
2Na ~ NiC12~ 2NaCl ~ Ni ~4) This accumulator was operated at 185C, and was subjected to 122 successive charging cycles (lA,2 hours) and discharge cycles (lA,2 hours).
After these 122 cycles, no appreciable degradation of the accumulator elements was notedO The present accumulator thus proved to have a much greater stability than comparable known accumulators using the same type of solid electrolyte, which show signs of degradation after : 15 only 45 cycles.

A mixture comprising 70 parts of NaNH2, 15 ~ parts of NaI, 5 parts of NaH and I0 parts o~ powdered : aluminium was prepared, and was used as the interposed anode salt in a secondary cell which in other respects was as set out in Example 1. This cell contained 7 g o~ anode sodium, and its theoretical capacity was 6 Ah, It was subjected to a series of 131 charging and discharge cycles under the ~ollowing conditions:

,. .

Charging lA, 2 hours; discharge lA, 2 hours, :
After 131 cycles, no appreciable degradation of the accumulator elements was noted.
The invention is not limited to the embodiments 5 heretofore described, starting from which other methods a-nd other embodimen^ts can be conceived without departing from tne scope of the invention, Thus ~or example, accumulators according to the invention could each comprise several negative and/or positive compartments~

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. An alkali salt composition free from hydroxide, usable in its molten state as an anode electrolyte for accumulators with an alkali metal negative electrode and with an alkali alumina separator, the said composition containing an alkali amide, at least one alkali halide, and an alkali hydride.

2. A composition as claimed in claim 1, for use with a sodium negative electrode, and in which the metal in the alkali salt is sodium.

3. A composition as claimed in claim 2, comprising NaNH2, 5 to 20% by weight of a sodium halide chosen from NaI and NaBr, and 2 to 20% by weight of NaH.

4. A composition as claimed in claim 3, further containing 5 to 15% by weight of (NH2)3Al with respect to the total composition.

5. A process for preparing a hydroxide-free alkali salt composition for use as a molten anode electrolyte in a secondary cell having an alkali metal anode and an alkali alumina separator, which method comprises:
preparing a mixture of alkali metal amide, hydride, and halide;
and adding aluminium to the mixture;
whereby the aluminium reacts with the alkali metal amide to form aluminium amide and alkali metal.

6. A process as in claim 5 in which the alkali metal is sodium.

7. A secondary electrochemical cell having an alkali metal negative electrode, a positive electrode, an alkali alumina separator between said electrodes, and between the separator and the negative electrode a molten-salt electrolyte composition comprising an alkali amide, at least one alkali halide, and an alkali hydride.

8. A cell as in claim 7 in which the alkali metal of the negative electrode and the alkali metal in the electrolyte composition is sodium.

9. A cell as in claim 8 in which the electrolyte composition contains NaNH2, 5 to 20% by weight of a sodium halide chosen from NaI and NaBr, and 2 to 15%
by weight of NaH.

10. A cell as in claim 9 in which the electrolyte composition further contains 5-15% by weight of (NH2)3Al, relative to the total composition.