This invention relates to the electrodeposition of lithium and baths therefor.

For a variety of applications, in particular in battery technology, there is a demand for uniform, good quality lithium or lithium coated materials. Use of electrodeposition techniques to produce such materials have hitherto not proved to be a practical or an economical proposition because of the general difficulties in finding suitable feed stock substances for an electrodeposition bath. The most widely available lithium salts such as lithium chloride or lithium carbonate are very insoluble in suitable solvents and the salts which are more soluble tend to be very expensive.

It has been found that a bath comprising a solution of lithium ions, for example lithium chloride, tends to deteriorate somewhat and provide poor quality deposits after a relatively short bath life and also that it is not possible to arrest this deterioration by regeneration of the lithium ion concentration.

SUBSTITUTESHEET

Disclosure of the Invention

The invention is concerned with the provision of suitable lithium electrodeposition processes and baths which can in general be operated relatively inexpensively and at comparatively low temperatures.

According to one aspect, the invention consists in a process for the electrodeposition of lithium which comprises plating onto a primary cathode in a bath of lithium ions in solution with an organic solvent, and using a supplementary cathode which removes or renders harmless by-products formed in the process which might be detrimental to the process or the deposit.

It has been found that a supplementary cathode of aluminium is especially useful. The supplementary cathode may comprise pure aluminium, an aluminium alloy or may be a substrate having an aluminium or an aluminium alloy surface.

The supplementary cathode may be used continuously in the electrodeposition process or alternatively it may be used occasionally either together with the primary cathode or in the absence of the

SUBSTITUTESHEET

primary cathode, for example after a particular individual primary cathode has been successfully plated. In any event, the supplementary cathode may be part of the same cell as the primary cathode, i.e. using a common anode, or may be in a separate cell through which the electrolyte is circulated.

The anode or anodes are inert, insoluble anodes composed of platinum or other suitable material.

Some lithium deposition may take place on the aluminium supplementary cathode. The deposited metal can be recovered by causing it to be used as an anode from time to time.

Lithium chloride may be used to provide the lithium ions in solution with the organic solvent, and preferably, the lithium chloride is used in combination with a further source of lithium ions, for example lithium hexafluoroarsenate. It has been found that a mixed bath of lithium chloride and lithium hexafluoroarsenate provides lithium deposits of enhanced quality. Furthermore, regeneration of the bath, and hence improved bath life, may be effected solely or mainly by means of

SUBSTITUTESHEET

the addition of lithium chloride which in general is one of the least expensive forms of lithium salt commercially available.

The organic solvent preferably comprises dimethyl sulphoxide, used either alone or in combination with one or more other solvents present as minor constituents.

According to another aspect, the present invention consists in a process for the electrodeposition of lithium onto a cathode in a bath comprising a solution of lithium ions in dimethyl sulphoxide. This process may employ either an inert, insoluble anode, or an active, soluble anode, for example a lithium anode, in which case, the anode will be a source of lithium ions and additional replenishment of the bath will not generally be necessary. The lithium ions are preferably provided by lithium chloride or lithium chloride in combination with lithium hexafluoroarsenate.

A general benefit of the processes of the invention is that they may generally be operated at relatively low temperatures. Typically a temperature of 25°C has been found useful.

SUBSTITUTESHEET

In one example of the electrodeposition process according to the invention, a plating bath comprises a concentration of between 0.1 and 2 molar, typically 0.6 molar, of lithium chloride, and a concentration of between 0.08 and 0.5 molar, typically 0.1 molar, of lithium hexafluoroarsenate in solution with dimethyl sulphoxide. The bath has a temperature of 25°C. An article to be plated with lithium is immersed in the bath and a voltage applied between it as the cathode, and a platinum anode so as to cause lithium to be deposited on the article. The current density is preferably maintained in the range from 2 to 20 mA/cm ² . A supplementary cathode of pure aluminium is occasionally immersed in the bath when the plating process is inoperative, for example, between the plating of individual articles, and a voltage is applied between it and the anode so as to remove by-products of the plating process by deposition on the supplementary cathode. The supplementary cathode is removed before the plating process is resumed. The bath can be regenerated by the addition of lithium chloride as necessary.

SUBSTITUTESHEET

In a second example of a process according to the invention, a plating bath comprises the same solution of lithium chloride and lithium hexafluoroarsenate in dimethyl sulphoxide as described in the first example, and the temperature of the bath and current density used are the same. However, the solution is circulated from the one bath containing the primary cathode to a second bath containing the supplementary cathode in which the by-products are neutralised, the solution then being re-circulated back to the first bath. Each bath is provided with a separate platinum anode, and voltages are applied between the cathodes and anodes as required for simultaneous continuous or intermittent plating and neutralisation of the by-products.

In a third example of a process according to the invention, a plating bath comprises the same solution of lithium chloride and lithium hexafluoroarsenate in dimethyl sulphoxide as described in the first example above. The bath has a temperature of 25°C. An article to be plated with lithium is immersed in the bath and a voltage applied between it as the cathode, and an anode so as to cause lithium to be deposited on

SUBSTITUTESHEET

the article. The current density is preferably maintained in the range from 2 to 20 mA/cm ² . The anode may be a platinum anode or a lithium anode. Lithium ions are replenished in the bath by the addition of lithium chloride as necessary, although this may not be necessary if a lithium anode is employed.

SUBSTITUTESHEET