WO1993014246A1 - Method for producing an intermetallic lithium compound - Google Patents

Abstract

A lithium halide solution is electrolyzed in an organic solvent between an insoluble anode and a cathode made of a material which is capable of forming an intermetallic compound with lithium. The used electrolyte is regenerated by means of the halogen produced at the anode as the halide source, and a low-cost lithium compound such as Li2CO3 or lithium waste as the lithium ion source.

Description

 PROCESS FOR PRODUCING AN INTERMETALLIC COMPOUND

LITHIUM

The present invention relates to a process for producing an intermetallic lithium compound, according to which a solution of a lithium halide in an organic solvent is electrolysed between an anode and a cathode, the cathode being made of a material capable of forming a compound. intermetallic with lithium, thereby producing an intermetallic lithium compound at the cathode.

Such a method is known from document JA-A-63218158. This document relates essentially to an electric cell with organic electrolyte, the cathode of which is made of an Al-Li alloy and the anode of organic matter, and it mentions the possibility of manufacturing the Li-Al alloy for the cathode electrochemically, in particular by electrolysis of a solution of a Li compound such as Lil, LiClO

LiAsFg, LiBF4 and LiHF2 in an aprotic solvent such as propylene carbonate between an aluminum cathode and a lithium anode.

This known method has the disadvantage of using a very expensive material, namely a lithium anode, to supply the electrolyte with lithium ions during the electrolysis.

The object of the present invention is to provide a process as defined above, which avoids the drawback of the known process and which therefore makes it possible to produce lithium intermetallic compounds industrially in a particularly economical manner.

To this end, according to the invention

- An insoluble anode is used as the anode, thereby producing a halogen at the anode;

- Is used as organic solvent a solvent resistant to halogenation under electrolysis conditions;

- The solution is electrolyzed until its lithium halide content is significantly reduced, thereby producing a spent electrolyte;

- the used electrolyte is regenerated using as halogenide the halogen produced at the anode and as source of Uthium ions either a lithium compound chosen from the group comprising L_2Cθ3, LiOH.I ^ O and L_2θ, or possibly oxidized lithium waste;

- And the electrolyte thus regenerated is recycled. By the expression "insoluble anode" is meant here an anode made of a material which does not react chemically with the electrolyte and which does not oxidize under the effect of the electrolysis current.

The use of a halogenation-resistant solvent under the conditions of electrolysis or, in other words, a solvent which, charged with a given quantity of lithium halide, does not react appreciably with the halogen produced at the anode is essential for the economy of the process, which is based on the reuse of this halogen. Needless to say, the solvent must also have a higher electrochemical decomposition voltage than that of lithium halide. It is obvious that a person skilled in the art is capable of finding a solvent having such properties experimentally, when he knows that the electrolysis can be carried out at room temperature and on solutions having a concentration of lithium halide. of the order of 1 M. Electrolyzing until a used electrolyte is obtained, as is also the case in the electrolytic production (electrowinning) of base metals such as zinc and copper, is also essential for the economy of the process, as does the choice of the source of lithium ions. It goes without saying that, when using the aforementioned sources to regenerate the electrolyte, materials which hinder electrolysis, such as for example water, are introduced into the electrolyte, these materials must be separated from the electrolyte before recycling it.

It should be noted here that the document JA-A-63218158, discussed above, describes two examples relating to the preparation of Al-Li compounds and that these two examples relate only to the electrolysis of an organic solution of L1CIO4. This document therefore militates in favor of the use of LiClθ4- However, the Applicant has found that the electrolysis of an organic solution of LiClθ4 between an aluminum cathode and an insoluble anode has serious drawbacks: the current efficiency is low (43%), when the electrolysis is carried out without a diaphragm, and there is formation at the anode of a particularly corrosive product which for example attacks very quickly a glassy carbon anode. By using a lithium halide according to the process of the present invention, current efficiencies greater than 90% can be obtained in non-compartmentalized cells, i.e. in cells without diaphragm, and there is no risk whatsoever to see the anodes corrode disastrously, when they are made of glassy carbon. It should also be noted that it is already known to prepare a concentrated aqueous solution of Lil by adding H2S and an aqueous solution of Lil containing I dissolved in an aqueous solution or suspension of L_2CÛ3 or LiOH (Chemical Abstracts , vol. 111, no 22, November 27, 1989, Columbus, Ohio, US, abstract no 198031 and Database WPAT, Week 8929, Derwent Publications Ltd., London, GB, AN 89-210857, both of which relate to JP-A-01148708).

In short, JA-A-63218158 teaches, insofar as the contents of its examples can be disregarded, electrolysis in an organic medium of a lithium halide, electrolysis which does not produce halogen and which does not require a supply of lithium halide since the anode is made of lithium. As for the aforementioned document JP-A-01148708, it relates to the preparation in an aqueous medium of a lithium halide, preparation which requires a contribution of the corresponding halogen.

In the process of the invention, lithium bromide or iodide is advantageously used as the lithium halogen and as the organic solvent a solvent which, when it contains the lithium halide in solution, dissolves the halogen produced at the anode. The solvent advantageously contains at least one compound chosen from the group comprising esters, preferably cyclic esters, and nitroalkanes such as nitromethane (CH3NO2).

Cyclic esters, which are very suitable, are the alkylene carbonates, more particularly propylene carbonate and ethylene carbonate, as well as gamma-butyrolactone.

The solvent can consist of said compound alone, for example propylene carbonate. It is however desirable that the solvent also contains at least one ether, preferably an ether chosen from the group comprising furan, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxalane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether ^* and tetraethylene glycol dimethyl ether. Indeed, the addition of an ether lowers the viscosity of the solvent, delays its degradation and can increase its conductivity.

A solvent, which is particularly suitable, consists of 40-80% by volume, preferably 50%, of alkylene carbonate and 20-60% by volume, preferably 50%, of ether. Thus the combination of propylene carbonate (40-80%) and tetrahydrofuran (20-60%) allows the electrodeposition of lithium at 25 ° C with excellent current yields for a voltage significantly lower than that required by the propylene carbonate alone. Indeed, maintaining a current density close to 250 A / m ² requires only about 5 volts whereas with propylene carbonate alone it takes at least 5.8 to 6 volts for the same spacing of the electrodes of 8 mm . The above-mentioned esters and ethers have the following formulas: - Propylene carbonate

- ethylene carbonate

• gamma-butyrolactone

• furan

• tetrahydrofuran

XX

- 2-me-thyltetrahydrofiιra_ιne

- tetrahydropyran

XX

-1,3-dioxalane

- ethylene glycol dimethyl ether CH3OCH2CH2OCH3

- diethylene glycol dimethyl ether CH ₃ (OCH ₂ CH2) 2θCH ₃ - tetraethylene glycol dimethyl ether CK ^ OCP ^ CH ^ OCI ^

The concentration of the lithium halide in the solvent is advantageously 0.7-1.7M.

It is desirable that the solution to be electrolyzed already contains 0.35-0.85 gram atom / l of halogen, which will be produced at the anode, because the conductivity of the electrolyte increases with its halogen content.

It is obvious that an anode made of a material which resists halogenation under the conditions of electrolysis will be used, for example a graphite, vitreous carbon, tantalum or niobium anode. Composite anodes can also be used, for example an anode consisting of a metal plate which is covered with a thin layer of inert conductive plastic.

The cathode can be made of any material forming intermetallic compounds with lithium, such as As, Sb, Bi, In, Sn and Pb. It is preferably made of aluminum or aluminum-based alloy such as for example types 5005 and 5052 from the firm SA Sidal (Industrial Aluminum Co.), because this produces an Al-Li master alloy which can be used in particular to manufacture aerospace alloys of Al at 2-3% Li.

It is particularly advantageous to carry out the electrolysis at a current density such that the rate of deposition of lithium at the cathode is at most equal to the speed at which the deposited lithium is absorbed by the cathode, this latter speed being determined by the rate of diffusion of lithium through the cathode. In fact, when the deposition rate exceeds the absorption rate, the degradation of the solvent becomes substantial and the current yield decreases appreciably. The increased degradation of a solvent containing an alkylene carbonate is manifested by the formation on the cathode of a deposit consisting of lithium, lithium carbonate and polymer. The rate of absorption of lithium by the cathode can be maintained at a desired level by regularly renewing the cathode, for example by using a ribbon-shaped cathode and a pair of anodes and by continuously scrolling the cathode under tension between the anodes. When the cathode is in aluminum, it is desirable to operate so that the average lithium content of the cathode remains less than 15%, preferably 7-10%, by weight, this in order to maintain optimal conditions to the diffusion of lithium in the cathode. It should be noted that homogeneous alloys are not obtained. This is how the outer layers of the cathode titrate 18-19% of lithium, whatever the average lithium content of the cathode.

It is obvious that a person skilled in the art is capable of determining experimentally the optimal current density to be imposed in each particular case, that is to say the ideal compromise between a maximum production speed and a minimum degradation speed. , as it is also capable of determining the optimal thickness of the cathode.

The electrolysis can be carried out at room temperature, but it can also be carried out at higher temperatures, of course provided that it remains below the boiling point of the electrolyte. This is how tests were carried out with electrolytes of the propylene carbonate-tetrahydrofuran type at around 42 ° C., at a current density of 400, 500 and 600 A / m ² and with current yields greater than 90 %. Similar tests were carried out with electrolytes of the propylene carbonate-ethylene glycol dimethyl ether type at 60-70 ° C.

When using a solvent which dissolves the halogen (Ha °) produced at the anode, it is particularly desirable to ensure that the atomic ratio between the lithium in solution and all of the halogen in solution (Ha ° + Ha ~) remains at all times greater than

1: 3. Otherwise, there is a risk that the halogen produced at the anode will escape from the solution and / or react with the solvent. It is therefore particularly recommended to regenerate the electrolyte before said ratio is reached.

Still in the case where a solvent is used which dissolves the halogen produced at the anode, the spent electrolyte can be regenerated by carrying out the following operations: a) the source of the above-mentioned lithium ions is suspended in the spent electrolyte, the quantity in suspension being in excess compared to the quantity of halogen which will be transformed into halide in the following stage (b); b) the suspension is treated with a reducing agent capable of transforming the halogen into hydrogen halide, thereby enriching the suspension in lithium halide; c) separating the filterable solids from the suspension resulting from (b), thereby producing an electrolyte enriched in lithium halide and optionally containing water; and d) the electrolyte is dried, when it contains water.

It is important to use the reducing agent in the presence of the source of lithium ions; otherwise, there is a risk of halogenating the solvent, in particular when the latter contains an ether.

For the same reason, the source of lithium ions must be in excess relative to the quantity of halogen which it is desired to transform into halide. Said excess preferably amounts to at least 1%.

It is advantageous to use a quantity of reducing agent such that the regenerated electrolyte still contains sufficient halogen so that the electrolysis can be resumed or continued with an electrolyte at 0.35-0.85 atom-gram / 1 halogen.

It is obvious that the electrolyte resulting from step (c) will be free of water only when non-oxidized lithium waste has been used as a source of lithium ions.

It is particularly advantageous to carry out electrolysis continuously, for this purpose using an electrolyte reserve. The electrolyte is circulated continuously through an electrolysis cell having an inlet and an outlet for the electrolyte: the flow of electrolyte entering the cell comes from the reserve and the flow leaving the cell goes partially to the reserve and partially for regeneration; the regenerated electrolyte also joins the reserve. The ratio between the reserve and the quantity of electrolyte circulating in the cell is such that the composition of the reserve remains substantially constant as a function of time.

In a particular embodiment of the process of the invention, electrolysis is carried out continuously, using as a reserve a solution of 1.4 mol / l of LiBr and 30 g 1 of bromine in a solvent consisting of 50% by volume of propylene carbonate (PC) and 50% by volume of tetrahydrofuran (THF). The cathode consists of a 1mm thick aluminum strip, which is scrolled at a speed of 63 m / h between two glassy carbon anodes 2 cm apart from the aluminum strip.

The electrolysis is carried out at room temperature and protected from air humidity in a closed cell free from diaphragms, this cell being provided with an inlet and an outlet for the electrolyte as well as d 'an input and an output for the cathode tape. The speed of circulation of the electrolyte through the cell is such that the duration of stay of the electrolyte in the cell is 50 seconds. A current density of 500 A / m ^{2 is} imposed, thus transforming the alu-unium into an Al-Li alloy, having an average Li content of 7%.

The flow of electrolyte leaving the cell has a LiBr content of 1.15 mol / l. On this flow we practice continuously a 10% bleeding, the rest being directed towards the reserve. The bleeding is regenerated as follows: r is suspended therein 22 g l of L-2CO3 and bubbled through this suspension 6.3 1 of H2S per liter of suspension, thus producing LiBr according to the reaction

Br ₂ + Li ₂ C0 ₃ + H ₂ S -> S ° + 2 LiBr + H ₂ O + CO ₂ The suspension is filtered to remove excess L_2C03 and the S ° formed and a solution containing 1.7 is thus obtained mole / 1 of LiBr, 3 g 1 of colloidal S °, 5 g / 1 of bromine and 0.5% of H2O. Water is removed from this solution by passing it through a molecular sieve to obtain a regenerated electrolyte, which is added to the reserve. The solution can also be dried by pervaporation through a permselective membrane. By leaving 5 g / l of bromine in the regenerated electrolyte, the bromine concentration in the reserve is maintained at 30 g 1.

The colloidal sulfur present in the regenerated electrolyte does not interfere in any way, no trace of sulfur being detected in the cathode. The degradation of the electrolyte is less than 1 kg per kg of lithium produced.

The Al-Li alloy produced can be ground and incorporated into conventional lithium-free aluminum alloys by the powder metallurgy technique. The alloy can also be destroyed by vacuum distillation of Uthium, the lithium vapor then being condensed in pure form.

Example 1

This example relates to a series of electrolysis tests. In all the tests, electrolysis is carried out without diaphragms, at room temperature, protected from air humidity, with a pair of glassy carbon anodes of type V25 (trade name of the company Le Carbone Lorraine ) and with an aluminum cathode one millimeter thick in a fixed position, the anodes being 1.8 cm apart from the cathode. The electrolyte is a roughly molar solution of LiBr in a solvent consisting of half and half of PC and THF in tests I-IV and half and half of PC and diethylene glycol dimethyl ether in tests V-VII.

Stirring of the electrolyte is ensured by circulating it in the electrolysis cell, except in test II where one operates with 200 ml of electrolyte against 3 liters in the other tests. The other parameters and the results are listed in the table below.

Tests π m IV VI VII

a) Initial conductivity 4.8 3.4 3.4 3.4 8.5 of the electolyte, in mS / cm b) Form of the cathode:

P = full and G≈grid c) Cathode surface, 172 40 172 232 172

d) Current density, 400 120 450 375 400 in A / m ²

e) Duration of electrolysis, 45,380 ^* 30 20 30 31 in minutes f) Final cathode composition

% by weight of Li 6.92 11.54 5.31 2.49 3.9 3.9 5.34 4.45% by weight of Al 87.4 78.43 91.9 97.1 96.1 94 94, 45 g) Current efficiency, 96.6 90 95 93 90.8 94.8 89.8 in% h) Degradation of the elec¬ <0.5 <0.6 <0.5 <0.5 <0 , 5 <0.5 <0.5 trolyte, in kg / kg Li

Example2

This example describes the regeneration of a used electrolyte.

The spent electrolyte contains 7.9 g / l of Li (1.14 M) and 27.4 g / l of bromine (Br °).

25.5 g of L-2CO3 are put in suspension in two others of the electrolyte and then they are introduced by bubbling 3.8 l of H2S in 50 minutes.

After bubbling the hydrogen sulfide, the electrolyte is filtered to remove excess.

L_2Cθ3 and the elemental sulfur (S °) formed. Solid matter consists of 74% of

L_2Cθ3 and 17.2% of S ° representing 52% of the total S ° produced during the reduction step. The filtered electrolyte contains 9.32 g / l of Li (1.34 M), 1.28 g / l of colloidal S °, 12.4 g / l of residual Br ° and 0.32% of water. This water is removed by passing the electrolyte through molecular sieves, after which the electrolyte is regenerated.

Claims

1. Method for producing a lithium intermetallic compound, according to which a solution of a lithium halide in an organic solvent is electrolysed between an anode and a cathode, the cathode being made of a material capable of forming an intermetallic compound with lithium, thereby producing a lithium intermetallic compound at the cathode, this method being characterized in that an insoluble anode is used as the anode, thereby producing halogen at the anode; an organic solvent which is resistant to halogenation under the conditions of electrolysis is used; the solution is electrolyzed until its lithium halide content is significantly reduced, thereby producing a spent electrolyte; - the used electrolyte is regenerated using as halogenide the halogen produced at the anode and as lithium ion source either a lithium compound chosen from the group comprising L_2C03, LiOH.H2θ and L_2θ , or possibly oxidized lithium waste; and the regenerated electrolyte is recycled. 2. Method according to claim 1, characterized in that an organic solvent is used as a solvent which, when it contains lithium halide in solution, dissolves the halogen produced at the anode. 3. Method according to claim 1 or 2, characterized in that lithium lithium bromide or iodide is used as the lithium halide. 4. Method according to any one of claims 1-3, characterized in that an organic solvent is used which contains at least one compound chosen from the group comprising esters and nitroalkanes. 5. Method according to claim 4, characterized in that the esters are cyclic esters. 6. Method according to claim 5, characterized in that the esters are alkylene carbonates. 7. Method according to claim 6, characterized in that the solvent contains propylene carbonate and / or ethylene carbonate. 8. Method according to claim 5, characterized in that the solvent contains gamma-butyrolactone. 9. Method according to claim 4, characterized in that the solvent contains nitromethane. 10. Method according to any one of claims 4-9, characterized in that the solvent also contains at least one ether. 11. Method according to claim 10, characterized in that the ether is chosen from the group comprising furan, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether. 12. Method according to any one of claims 4-11, characterized in that the solvent consists of 40-80% by volume of alkylene carbonate and 20-60% by volume of ether. 13. Method according to claim 12, characterized in that the solvent consists of 50% by volume of alkylene carbonate and 50% by volume of ether. 14. The method of claim 13, characterized in that the solvent consists of 50% by volume of propylene carbonate and 50% by volume of tetrahydrofuran and / or diethylene glycol dimethyl ether. 15. Method according to any one of claims 1-14, characterized in that the concentration of lithium halide in the solvent is 0.7-1.7 M. 16. Method according to any one of claims 2-15, characterized in that the solution to be electrolyzed already contains 0.35-0.85 gram atom / 1 of halogen, which will be produced at the anode during l 'electrolysis. 17. Method according to any one of claims 1-16, characterized in that a graphite, vitreous carbon, tantalum or niobium anode is used, or a composite anode having a conductive plastic skin. 18. Method according to any one of claims 1-17, characterized in that an aluminum cathode or an aluminum-based alloy is used. 19. Method according to any one of claims 1-18, characterized in that electrolysis at a current density such that the rate of deposition of lithium at the cathode is less than or substantially equal to the speed at which the lithium deposited is absorbed by the cathode. 20. The method of claim 19, characterized in that the rate of absorption of hthium by the cathode is maintained at a desired level by regularly renewing the cathode. 21. The method of claim 20, characterized in that a ribbon-shaped cathode and a pair of anodes are used and the cathode is passed continuously between the anodes. 22. Method according to any one of claims 19-21, characterized in that an aluminum cathode is used and care is taken that the average lithium content of the cathode remains below 15% and preferably 10 % in weight. 23. The method of claim 22, characterized in that the average content of hthium in the cathode remains less than 7% by weight. 24. Method according to any one of claims 2-27, characterized in that one ensures that the atomic ratio between the lithium in solution and the totality of the halogen in solution (Ha ° + Ha ") remains at any time greater than 1: 3. 25. Method according to any one of claims 2-24, characterized in that the spent electrolyte is regenerated by carrying out the following operations: (a) the above-mentioned lithium ion source is suspended in the spent electrolyte, the quantity suspended being in excess relative to the quantity of halogen, which will be transformed into halide in the following stage (b ); (b) the suspension is treated with a reducing agent capable of transforming the halogen into hydrogen halide, thereby enriching the suspension in lithium halide; (c) separating the filterable solids from the suspension resulting from (b), thereby producing an electrolyte enriched in lithium halide and optionally containing water; and (d) the electrolyte is dried, when it contains water. 26. The method of claim 25, characterized in that said excess amounts to at least 1%. 27. The method of claim 25 or 26, characterized in that the amount of reducing agent used is such that the regenerated electrolyte still contains sufficient halogen so that the electrolysis can be resumed or continued with an electrolyte to 0.35-0.85 gram atom / 1 of halogen. 28. Method according to any one of claims 25-27, characterized in that H2S is used as a reducing agent. 29. Method according to any one of claims 25-28, characterized in that the electrolyte is dried by passing it over a molecular sieve or by pervaporation through a permselective membrane. 30. Method according to any one of the preceding claims, characterized in that the solution is electrolysed at room temperature. 31. A method according to any one of claims 1-29, characterized in that the solution is electrolyzed at a temperature between room temperature and the boiling point of the electrolyte.