METHOD OF FORMING A METALLIC LITHIUM COATING

Abstract

A method of forming a lithium coating on a substrate, the method comprising: melting a solid lithium target to form a molten lithium target; agitating the molten lithium target; vaporising at least part of the agitated molten lithium target to form a vaporised material; and condensing the vaporised material on a substrate to form a lithium coating.

Claims

1. A method of forming a lithium coating on a substrate, the method comprising: melting a solid lithium target to form a molten lithium target; agitating the molten lithium target; vaporising at least part of the agitated molten lithium target to form vaporised material; and condensing the vaporised material on a substrate to form a lithium coating.

2. A method according to claim 1, wherein the lithium target comprises a passivation layer that is at least partly dispersed within the lithium target by the agitation

3. A method according to any preceding claim, wherein vaporising at least part of the agitated lithium target comprises bombardment of the lithium target with energetic particles.

4. A method according to any preceding claim, wherein vaporising at least part of the agitated lithium target comprises magnetron sputtering.

5. A method according to claim 4, wherein a magnetron is used to melt the lithium target.

6. A method according to any preceding claim, wherein agitating the molten lithium target comprises creating a magnetohydrodynamic effect in the lithium target.

7. A method according to claim 6, wherein a magnetron is used to create the magnetohydrodynamic effect in the lithium target.

8. A method according to claim 7, wherein the power of magnetron discharge from the magnetron is selected such that the magnetohydrodynamic effect causes a passivation layer on the surface to be broken and removed from an area on the target surface.

9. A method according to any of claims 7 to 8, wherein the lithium target is positioned horizontally.

10. A method according to any preceding claim, wherein the method takes place in a chamber containing the lithium target, the substrate and, optionally, a magnetron.

11. A method according to any preceding claim, wherein in a first time period a first power regime is applied to the lithium target to melt the lithium target and initiate agitation of the molten lithium target and in a second time period a second power regime is applied to the lithium target to initiate increased vaporisation of at least part of the agitated lithium target.

12. A method according to claim 11 wherein the voltage in the first power regime is lower than the voltage in the second power regime.

13. A method according to any of claims 11 and 12, wherein the current density in the first power regime is lower than in the second power regime.

14. A method according to any of claims 11 to 13, wherein in a preliminary time period, a preliminary power regime is applied to melt the lithium target.

15. A method according to claim 14, wherein the voltage and/or current density in the preliminary power regime is less than in the first power regime.

16. A method according to any of claims 11 to 13, wherein the first power regime is selected to both melt the lithium target and to initiate agitation of the liquid lithium in the lithium target.

17. A method according to any preceding claim, wherein the current density of the lithium target is in the range of 10-1000 mA/cm.sup.2.

18. A method according to any preceding claims, wherein agitating the lithium target comprises using a magnetron to create a magnetohydrodynamic effect in the lithium target and wherein the operation regime of the magnetron is selected such that the magnetohydrodynamic effect causes the lithium of the target to move at a rotational speed of 1-100 rotations per second or a linear speed of 0.1-10 meters/second.

19. A method according to any preceding claims, wherein during agitation of the liquid lithium in the lithium target, the temperature of the lithium target is in the range of 180 C. to 1000 C., in particular in the range of from 180 C. to 500 C. or in the range of from 500 C. to 1000 C.

20. A method according to any preceding claim, wherein the method takes place in a chamber containing working gas and wherein the pressure of the working gas is within the range of 10.sup.3 to 10.sup.2 mbar.

21. A method according to claim 20, wherein the working gas is selected from argon, neon, helium, a mix of inert gases and a mix of inert gases with other gases.

22. A method according to any preceding claim, wherein melting, agitation, and/or vaporisation of the lithium target is carried out in a pulsed mode.

23. A method according claim 22, wherein the frequency of the pulse mode is between 1000 Hz and 100000 Hz

24. A method according to any of claim 22 or 23, wherein the duty cycle of the pulse mode is 0.4 to 1.

25. A method according to any preceding claim, wherein the substrate comprises a porous material.

26. A method according to any preceding claim, wherein the substrate comprises a non-porous material.

27. A method according to any preceding claim, wherein the method comprises a method for forming a lithium coated electrode for an electrochemical cell.

28. A lithium coated substrate formed by the method according to any of claims 1-27.

29. A lithium coated electrode for an electrochemical cell formed by the method according to any of claims 1-27.

30. A method of forming a metallic lithium coating on a substrate substantially as herein described with reference to the accompanying illustrative drawings.

31. A lithium coated substrate formed by a method of forming a metallic lithium coating substantially as herein described with reference to the accompanying illustrative drawings.

Description

[0074] The present invention will now be further described with reference to the following non-limiting examples and the accompanying illustrative drawings, of which:

[0075] FIG. 1 is a schematic illustration of a magnetron chamber; and

[0076] FIG. 2 is a flow diagram showing a method of forming a lithium coating in accordance with an embodiment of the invention.

[0077] FIG. 1 shows a simplified vacuum chamber suitable for coating an electrode substrate in accordance with an embodiment of the invention. The chamber 10 contains a magnetron 12, lithium target 14 and electrode substrate 16. The magnetron 12 and lithium target 14 (acting as the cathode) are connected to a power supply unit 18. A controller 20 controls the voltage and current of the power supply unit 18 and enables the power to be supplied to the magnetron.

[0078] An embodiment of a method according to the invention is shown in FIG. 2. A substrate and a lithium target are loaded into the chamber 22 and the chamber is partially evacuated 24. The chamber is then filled with working gas at a low pressure 26, typically an inert gas such as argon. The current and voltage applied to the magnetron are set to produce conditions under which the surface film on the lithium target is etched 28. The current and voltage settings are next adjusted to melt the lithium and create conditions for the magnetohydrodynamic effect (MHD) 30 which results in the onset of slow rotation of molten lithium; the rotation speed being defined by current density on the target surface. As a result of this rotation, the surface of the lithium target gets mechanically cleaned, thus continuously removing the surface coating from the sputtering area. The current and voltage are then set to a sputtering regime or mode to sputter the lithium onto the substrate 32.

EXAMPLE 1

[0079] A rectangular sample of non-woven polypropylene of 6 cm4 cm was placed at the height of 6 cm over a horizontally positioned magnetron with lithium target. The magnetron, non-woven sample and lithium target were set up in a magnetron chamber and pumped down to 10.sup.4 mmHg (10.sup.4 mbar). The magnetron chamber was further filled with dry argon such that the pressure in the chamber reached 510.sup.3 mbar. A power supply unit was used to control current and voltage to the magnetron.

[0080] A purpose built controller was used to provide pulse power with regulated frequency in the range 1-50 kHz and a duty cycle of about 0.5.

[0081] The initial magnetron discharge was at constant current densities on the target of 20 mA/cm.sup.2 and a voltage amplitude of about 200V. Under these conditions, etching of the lithium target was initiated.

[0082] The voltage was then increased to 260V, whilst keeping the current at the same level. Under these conditions the lithium target melted.

[0083] The voltage was then increased to 300V, to initiate agitation of the liquid lithium by the magnetohydrodynamic effect.

[0084] To achieve deposition by sputtering in a sputtering mode, the current density on the target was increased to 40 mA/cm2. The voltage was gradually increased to 320V. Under these conditions, deposition rates of 2 m/min (or 0.4 mAh/min) were observed.

[0085] Sputtering was continued for 5 minutes, following which a metallic lithium coating had been deposited on the sample with thickness 10 m.

EXAMPLE 2

[0086] The sputtering of metallic lithium target was carried out on a copper foil substrate. The pressure in the vacuum chamber was similar to that of Example 1 with the distance from the target to the sample being 4 cm.

[0087] The power supply to the magnetron discharge was set to pulse at a frequency of 20 kHz and duty cycle about 0.5.

[0088] The voltage was initially set at 400V to melt the lithium target with current density of 50 mA/cm2.

[0089] During transition from liquid phase to the sputtering regime the voltage amplitude was changing from 400 to 460V. During this transition, agitation of the liquid lithium was observed.

[0090] In sputtering mode, the current density on the target was 50 mA/cm2 and the voltage was 460V. During sputtering, which lasted for 3 minutes, the surface of the copper substrate was coated by lithium with a thickness of 6 m.

EXAMPLE 3

[0091] The sputtering of metallic lithium target was carried out on a copper foil substrate. The working pressure in the chamber was set in the following way: the system was pumped down to the working pressure of 510.sup.3 mbar, then it was purged with argon at the same pressure for 30 min. This was followed by using the same method as described in Example 2.

[0092] During sputtering, which lasted 3 minutes, the copper substrate was coated by lithium with a thickness of 6 m.

EXAMPLE 4

[0093] The sputtering of metallic lithium was carried out using a method similar to Example 3, with the exception that the substrate material was a 34 cm piece of non-woven polypropylene.

[0094] The current discharge was stabilised at 20 mA/cm.sup.2 for the steps of melting the lithium target, initiating agitation in the lithium target and sputtering mode.

[0095] During the sputtering time of 1.5 minutes, the polypropylene substrate was coated with lithium to a thickness of 0.6 m.

EXAMPLE 5

[0096] In a comparative example, the conditions of example 1 were repeated but limiting the voltage the current density on the lithium target to prevent the magnetohydrodynamic effect.

[0097] A rectangular sample of non-woven polypropylene of 6 cm4 cm was placed at the height of 6 cm over a horizontally positioned magnetron with lithium target. The magnetron, non-woven sample and lithium target were set up in a magnetron chamber and pumped down to 10.sup.4 mmHg (10.sup.4 mbar). The magnetron chamber was further filled with dry argon such that the pressure in the chamber reached 510.sup.3 mbar. A power supply unit was used to control current and voltage to the magnetron.

[0098] A purpose built controller was used to provide pulse power with regulated frequency in the range 1-50 kHz and a duty cycle of about 0.5.

[0099] The initial magnetron discharge was at constant current densities on the target of 20 mA/cm.sup.2 and a voltage amplitude of about 200V. Under these conditions, etching of the lithium target was initiated.

[0100] The voltage was then increased to 260V, whilst keeping the current at the same level. Under these conditions the lithium target melted.

[0101] Under these conditions, no magnetohydrodynamic effect was observed. Some etching and evaporation of the lithium target was observed but no coating was deposited on the substrate surface.