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 by creating a magnetohydrodynamic effect in 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. The method according to claim 1, wherein the molten lithium target comprises a passivation layer that is at least partly dispersed by the agitating.

3. The method according to claim 1, wherein vaporising at least part of the agitated molten lithium target comprises bombardment of the molten lithium target with energetic particles.

4. The method according to claim 1, wherein vaporising at least part of the agitated molten lithium target comprises magnetron sputtering.

5. The method according to claim 1, wherein the method takes place in a chamber containing the solid lithium target, the substrate and, optionally, a magnetron.

6. The method according to claim 1, wherein in a first time period a first power regime is applied to the solid lithium target to melt the solid lithium target and initiate agitating of the molten lithium target and in a second time period a second power regime is applied to the molten lithium target to initiate increased vaporisation of at least part of the agitated molten lithium target.

7. The method according to claim 6, wherein a current density in the first power regime is lower than in the second power regime.

8. The method according to claim 7, wherein in a preliminary time period, a preliminary power regime is applied to further melt the solid lithium target, wherein the current density in the preliminary power regime is lower than in the first power regime.

9. The method according to claim 1, wherein a current density of the solid lithium target is in the range of 10-1000 mA/cm.sup.2.

10. The method according to claim 1, wherein the method takes place in a chamber containing working gas and wherein a pressure of the working gas is within the range of 10.sup.−3 to 10.sup.−2 mbar.

11. The method according to claim 10, wherein the working gas is selected from argon, neon, helium, a mix of inert gases and a mix of inert gases with other gases.

12. The method according to claim 1, wherein melting, agitation, and vaporisation of the lithium target is carried out by a magnetron operating in a pulsed mode.

13. The method according to claim 12, wherein a frequency of the pulsed mode is between 1000 Hz and 100000 Hz and wherein a duty cycle of the pulsed mode is 0.4 to 1.

14. The method according claim 1, wherein the substrate comprises a porous material.

15. The method according to claim 1, wherein the substrate comprises a non-porous material.

16. The method according to claim 1, wherein the method comprises a method for forming a lithium coated electrode for an electrochemical cell.

17. A method of forming a lithium coating on a substrate, the method comprising: melting a solid lithium target comprising a passivation layer to form a molten lithium target; agitating the molten lithium target by creating a magnetohydrodynamic effect therein with a magnetron to at least partly disperse the passivation layer within the molten lithium target; vaporising at least part of the agitated molten lithium target to form vaporised material by magnetron sputtering with the magnetron; and condensing the vaporised material on a substrate to form a lithium coating.

18. A method of forming a lithium coating on a substrate, the method comprising: melting a solid lithium target comprising a passivation layer to form a molten lithium target; agitating the molten lithium target by creating a magnetohydrodynamic effect therein with a magnetron operating in a pulsed mode to at least partly disperse the passivation layer within the molten lithium target; vaporising at least part of the agitated molten lithium target to form vaporised material by magnetron sputtering with the magnetron in a pulsed mode; and condensing the vaporised material on a substrate to form a lithium coating.

Description

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

(2) FIG. 1 is a schematic illustration of a magnetron chamber; and

(3) FIG. 2 is a flow diagram showing a method of forming a lithium coating in accordance with an embodiment of the invention.

(4) 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.

(5) 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

(6) A rectangular sample of non-woven polypropylene of 6 cm×4 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 5×10.sup.−3 mbar. A power supply unit was used to control current and voltage to the magnetron.

(7) 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.

(8) 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.

(9) The voltage was then increased to −260V, whilst keeping the current at the same level. Under these conditions the lithium target melted.

(10) The voltage was then increased to −300V, to initiate agitation of the liquid lithium by the magnetohydrodynamic effect.

(11) 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 μm/min (or 0.4 mAh/min) were observed.

(12) Sputtering was continued for 5 minutes, following which a metallic lithium coating had been deposited on the sample with thickness 10 μm.

EXAMPLE 2

(13) 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.

(14) The power supply to the magnetron discharge was set to pulse at a frequency of 20 kHz and duty cycle about 0.5.

(15) The voltage was initially set at 400V to melt the lithium target with current density of −50 mA/cm2.

(16) 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.

(17) 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 μm.

EXAMPLE 3

(18) 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 5×10.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.

(19) During sputtering, which lasted 3 minutes, the copper substrate was coated by lithium with a thickness of 6 μm.

EXAMPLE 4

(20) The sputtering of metallic lithium was carried out using a method similar to Example 3, with the exception that the substrate material was a 3×4 cm piece of non-woven polypropylene.

(21) 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.

(22) During the sputtering time of 1.5 minutes, the polypropylene substrate was coated with lithium to a thickness of 0.6 μm.

EXAMPLE 5

(23) 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.

(24) A rectangular sample of non-woven polypropylene of 6 cm×4 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 5×10.sup.−3 mbar. A power supply unit was used to control current and voltage to the magnetron.

(25) 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.

(26) 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.

(27) The voltage was then increased to −260V, whilst keeping the current at the same level. Under these conditions the lithium target melted.

(28) 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.