GB2069989A

GB2069989A - Making a Compound/Substrate Composite Structure

Info

Publication number: GB2069989A
Application number: GB8104272A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1980-02-27
Filing date: 1981-02-11
Publication date: 1981-09-03
Also published as: DE3107574A1; JPS56143677A

Abstract

A method of making a composite structure comprising a substrate banded to a compound one of whose constituent elements is mobile within the compound, eg lithium, the method comprising taking a substrate material containing at least one element of the compound, causing the element to emerge at a surface of the substrate material, and applying a medium containing the other element(s) of the compound to said surface under conditions promoting formation of the compound. The compound may be LiN3, Li8AlN3, Li8ON2 or Li9N2Cl3.

Description

SPECIFICATION Making a Compound/Substrate Composite Structure This invention relates to making a composite structure comprising a compound bonded to a substrate, and extends to electrical devices such as a solid-state cell including such a composite structure.

A solid-state cell depends on good mechanical and electrical contact between the electrolyte and each electrode, on the mechanical reliability of the electrolyte, and on the conductivity of the electrolyte. These requirements have hitherto been regarding as conflicting, since they appear to demand an electrolyte which is resilient; tough or rigid; and thin.

The present invention provides a method of making a composite structure comprising a substrate bonded to a compound one of whose constituent elements is mobile within the compound (for example moving upon application of an electric field), the method comprising taking a substrate material containing at least one element of the compound, causing the element to emerge at a surface of the substrate material, and applying a medium containing the other element(s) of the compound to said surface under conditions promoting formation of the compound.

The composite structure may be a triple-layer structure, in which (after the foregoing steps) the element in uncombined form is concentrated at the external surface of the compound by depleting the substrate material. In certain instances this triple-layer structure will constitute a solid-state cell, but even the two-layer composite may find use as (possibly part of) an electrical device.

The compound may be a lithium compound such as lithium nitride (Li3N) or Li3AlN2 or Li80N2 or Li9N2C13 or a mixture of such compounds. The substrate may contain lithium, and the applied medium (in the case of lithium nitride) may be nitrogen gas or other nitriding fluid.

The substrate material, as already implied, must be such that the lithium is mobile in it, the lithium in practice being caused to emerge at the surface of the substrate material because of a free-energy driving force associated with the formation of the lithium compound where the lithium emerges. Suitable substrate materials where this is possible include solid solution electrodes such as molybdenum sulphide (Mo3S4) containing lithium (i.e. LixM03S4), or LiXNiPS3, Li,,Fe0Cl, Li#Fe2S3, LiXNiO2 or LiXTiS2. These solid solution electrodes can absorb or exude at least some lithium with negligible change in volume.

For simplicity, the invention will be discussed in relation to the pair LixMo3S4 substrate and Li3N compound, but the invention is not limited to this pair.

As already stated, the element must be mobile within the compound and this is met in the case of Li and Li3N, as Li3N conducts Li well when an electric field is applied or when, in more general terms, there is a lithium-activity-gradient to provide for Li movement. Using this property, fresh lithium can continuously migrate from the Li #Mo3S4, through any Li3N which has already formed on the surface of the LixMO3S4, to the exposed surface of the structure, there to be nitrided in situ to form further coherent Li3N.This occurs spontaneously if there is a sufficient lithium-activity difference between the LixMo3S4 and the exposed surface of the Li 3N Thus, the following composite structure, with a good interface, may be formed: Li3N!Li#Mo3S4. This structure may find application in electrical devices.

The emergence of further lithium to form further lithium nitride comes eventually to a halt because of space charge progressively builds up, negative at the LixMo3S4~Li3N interface and positive at the exposed LiSN surface, becoming eventually sufficient (as the LisN becomes thicker) to prevent continued lithium transport through the Li3N under the influence of the difference in lithium activity between Li3N (at the free surface) and LixMo3S4. (Depletion of lithium from the LixMo3S4 is not in practice a limiting factor.) The space charge can be counteracted by applying the nitriding fluid in negatively charged form, such as plasma (N3-ions) or as low-pressure nitrogen gas with an electron beam or as nitride ions generated by microwave discharge, and in this way lithium nitride growth can be caused to continue.

The lithium activity in the LixMo3S4 may be insufficient in the first place for migration and nitriding to occur spontaneously, and in this case the activity there can be enhanced by applying to the LixMo3S4, at its face opposite the face to be nitrided, a reservoir of lithium which is pumped into the LixMO3S4 by (for example) applying to the latter an electric potential negative with respect to the reservoir.

When the Li3N has been grown to the desired thickness as set forth above, the optional triplelayer structure may be achieved by discontinuing the-nitriding and applying a negatively charged electrode which will accept lithium to the exposed surface of the LIST. The electrode may be inert (such as nickel sponge) or may itself be a solid solution electrode such as LiXAi or LixVS2. The triple layer structure thus may be represented as Li,,lLi,NILi,Mo,S, or in effect Liy.x!Li3NiLixM03S4 LiiLi3NiMo3S4.

As long as the lithium on the left-hand side is not in the same form as on the right-hand side (e.g. not both Li#ZrS2), this is a solid-state cell reversible with respect to lithium, and the above, example has hitherto been assembled by pressing together discs of Li, Li3N and M 0354. (When x=0, the cell is fully charged.) Since, if possible, the Li3N disc should be only about 10m thick, this would be a mechanically troublesome component, but both mechanical properties and interface properties are improved by using the present invention.

The invention will now be described by way of example with reference to the accompanying drawing, which shows (schematically) successive stages in the method according to the invention.

As a substrate material, a 1 cm2 disc, 2-3 mm thick, of Lix Mo3S4 was taken (step (a)). x was initially 3. As step (b), carefully purified nitrogen gas at 1 atmosphere and 20000 was applied to the disc surface, these conditions being suitable for the nitrogen to react with the lithium, which then spontaneously energes, to form lithium nitride. The lithium nitride is well bonded to the substrate material as regards electrical conductivity, it is in the form of a coherent film, and is rigid because of the rigidity of the underlying LixMo3S4 (a framework solid solution substance undergoing little size change as x varies).Because a space charge builds up as shown schematically in Figure 1(c), the lithium nitride does not grow beyond 10-7 to 1 0-8m in thickness.

Lithium nitride formation is continued as step (c) by counteracting the space charge by applying the nitrogen at low pressure with electrons (there are also other ways), and lithium nitride conducts lithium so well that the lithium can continue to be nitrided at the growing surface of the composite structure, until the lithium nitride is as-thick as required, for example 10-8 to 1 0-4m.

This composite structure is according to the invention.

An option, shown as Figure 1(d), is suitable if it is necessary to boost the activity of lithium in the substrate. This necessity would arise say if x < 2, in which case the lithium activity would be insufficient for lithium to migrate spontaneously to the surface for nitriding.

In this option, a lithium reservoir such as LiAl is pressed, by way of lithium ion conductor such as polyethylene oxide incorporating LiONS, onto the bottom of the substrate LixMo3S4. An electric field is applied, with the substrate more negative than the reservoir. Lithium ion tends to enter the substrate from the reservoir, thus increasing the lithium activity in the substrate to above the lithium activity in lithium nitride, so that the free energy change favours lithium nitride formation.

Upon applying nitrogen or other nitriding fluid as in Figure 1 (b) therefore, lithium nitride duly forms to make the composite structure according to the invention. The procedure of step (c) may of course be carried out with the Figure 1 (d) option.

Alternatively, butyl lithium dissolved in an appropriate solvent such as hexane could serve as the reservoir. When this substance is brought into contact with the substrate LixMo3S4 additional lithium will be spontaneously injected into the substrate and the activity of lithium consequently increased as required.

If desired, the method is continued at step (e), where a porous nickel grid is placed on top of the lithium nitride, all nitrogen gas (indeed all gas) is removed, and an electric field is applied. The result, step (f), is to form a lithium-rich layer in the nickel at the expense of the substrate material. In the drawing, all the lithium is shown as removed from the substrate material, but this it not necessary. In the case of LixTiS2 as the substrate, it is desirable to stay within the range x=0.4 to 0.9, since the material changes its size more noticeably when x changes outside that range.

Instead of a nickel grid, in which the lithium is as active as massive metallic lithium, one could use the intermetallic material LiAl (in which the proportion of Li is variable). LiAl has the slight disadvantage of having a lithium activity about 0.5V less than pure lithium but has the advantages of good discharge characteristics especially at high temperature, and of avoiding the handling of pure lithium, which would be liquid at high temperature.

The resulting triple-layer structure is a cell as already described.

Claims

1. A method of making a composite structure comprising a substrate bonded to a compound one of whose constituent elements is mobile within the compound, the method comprising taking a substrate material containing at least one element of the compound, causing the element to emerge at a surface of the substrate material, and applying a medium containing the other element(s) of the compound to said surface under conditions promoting formation of the compound.

2. A method according to Claim 1, further comprising thereafter concentrating the emergent element in its uncombined form at the external surface of the compound by depleting the substrate material, whereby the resultant composite structure is a triple-layer structure substrate-compound-element.

3. A method according to Claim 1 or 2, wherein the emergent element is lithium.

4. A method according to Claim 3, wherein the compound is LisN or Li3AlN2 or Li80N2 or Li9N2Cl3.

5. A method according to Claim 4, wherein the compound is Ui3N and the applied medium is a nitriding fluid such as nitrogen gas.

6. A method according to Claim 3, 4 or 5, wherein the substrate material is a solid solution electrode containing lithium.

7. A method according to Claim 6, wherein the substrate material is Mo3S4 or NiPS3 or FeOCI or Fe2S3 or NiO or TiS2, in each case containing lithium.

8. A method according to Claim 1 substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

9. A composite structure made by a method according to any preceding claim.

10. An electrical device comprising a composite structure according to Claim 9.

11. A solid-state cell comprising a triple-layer composite structure according to Claim 9 when appendant to Claim 8 or (directly or indirectly) to Claim 2.