ALKALI METAL MATERIALS

Abstract

There is disclosed a method of making a surface-modified alkali metal material for electrochemical use, the method comprising bringing a barrier agent into frictional contact with an alkali metal substrate to form a tribochemical barrier layer on the substrate. Also disclosed is a surface-modified alkali metal material for electrochemical use, the material comprising an alkali metal substrate bearing a tribochemical barrier layer.

Claims

1. A method of making a surface-modified alkali metal material for electrochemical use, the method comprising bringing an alkali metal substrate into frictional contact with a barrier agent to form a tribochemical barrier layer on the substrate.

2. The method of claim 1, wherein the barrier agent and/or tribochemical barrier layer comprises a material capable of conducting ions derived from the alkali metal substrate.

3. The method of claim 1 or claim 2, wherein the barrier agent is selected from a metal or metal compound, a non-metal or non-metal compound, or combinations thereof.

4. The method of any preceding claim, wherein the barrier agent comprises Li.sub.3N, Si, Zn, Al, C, S, P.sub.2S.sub.5, SiS.sub.2, Li.sub.2S, Li.sub.3PS.sub.4, Li.sub.3PO.sub.4 or combinations thereof.

5. The method of any preceding claim, wherein the barrier agent is particulate, optionally with a particle size in the range of from a fifth or a tenth of the thickness of the substrate.

6. The method of any preceding claim, wherein bringing the barrier agent into frictional contact with the substrate comprises forcing together the barrier agent and the substrate.

7. The method of any preceding claim, wherein the barrier agent and the substrate are forced together with a force in the range of from 0.1 to 1.0 kg/cm.sup.2 substrate.

8. The method of any preceding claim, wherein bringing the barrier agent into frictional contact with the substrate comprises sliding or rubbing between the barrier agent and the substrate, optionally whilst the barrier agent and the substrate are forced together, optionally with a reciprocating motion.

9. The method of claim 8, wherein the reciprocating motion has an amplitude in the range of from 1 to 5 mm and/or a frequency in the range of from 0.1 to 10 Hz.

10. The method of claim 8 or claim 9, wherein the barrier agent and the substrate are slid or rubbed together for a period in the range of from 2 to 10 minutes.

11. The method of any one of claims 9 to 11 wherein the substrate is affixed and an applicator is employed to force the barrier agent against the substrate and optionally to slide or rub the barrier agent along the substrate, optionally whilst continuing to force the barrier agent against the substrate.

12. The method of any preceding claim, wherein bringing the barrier agent into frictional contact with the substrate comprises impinging a stream of pressurised fluid bearing the barrier agent onto the substrate.

13. The method of any preceding claim, comprising removing a passivation layer from the alkali metal substrate.

14. The method of any preceding claim performed in an inert atmosphere optionally consisting essentially of argon and/or comprising nitrogen or dry air.

15. The method of any preceding claim, wherein the frictional contact takes place in the absence of solvents or additives.

16. The method of any preceding claim wherein the alkali metal substrate comprises or consists of lithium metal or a lithium alloy.

17. The method of any preceding claim, wherein the alkali metal substrate comprises or consists of a foil of the alkali metal and/or wherein the alkali metal substrate comprises a polymeric support.

18. The method of any preceding claim, wherein the barrier layer has a thickness in the range of from 0.5 to 10 microns.

19. The method of any preceding claim, wherein the barrier layer covers substantially the entirety of the substrate.

20. The method of any preceding claim, wherein the substrate is sheet-like with opposed faces and the barrier layer is applied to one or both faces.

21. A surface-modified alkali metal material obtainable by any method according to any preceding claim.

22. A surface-modified alkali metal material for electrochemical use, the material comprising an alkali metal substrate bearing a tribochemical barrier layer.

23. The surface-modified alkali metal material of claim 22, wherein the alkali metal substrate and/or tribochemical barrier layer are as defined in any of claims 16 to 20.

24. An electrode, electrode assembly or electrochemical cell comprising a surface-modified alkali metal material according to any one of claims 21 to 23.

25. An electrochemical cell according to claim 24, wherein the cell is a primary or a secondary cell.

Description

[0063] To further illustrate the invention, one or more non-limiting embodiments of the invention will now be described in the following experimental section with reference to the accompanying drawings in which:

[0064] FIG. 1 shows a change in overpotential (E) of electrode processes during cycling of lithium electrodes without a barrier layer (1) and with a barrier layer (2) formed by the treatment with Si powder in a dry air atmosphere, at current density of 0.2 mA/cm.sup.2 and depth of charge-discharge of 1.0 mAh/cm.sup.2;

[0065] FIG. 2 shows a change in overpotential (E) of electrode processes during cycling of lithium electrodes without a barrier layer (1) and with a barrier layer (2) formed by the treatment with Li.sub.3N powder in a nitrogen atmosphere, at current density of 0.2 mA/cm.sup.2 and depth of charge-discharge of 1.0 mAh/cm.sup.2;

[0066] FIG. 3 shows a change in overpotential (E) of electrode processes during cycling of lithium electrodes without a barrier layer (1) and with a barrier layer (2) formed by a mixture of Si and Li.sub.3N powders (2) in a nitrogen atmosphere, at current density of 0.2 mA/cm.sup.2 and a charge-discharge depth of 1.0 mAh/cm.sup.2;

[0067] FIG. 4 shows a change in overpotential (E) of electrode processes during cycling lithium electrodes without a barrier layer (1) and with a barrier layer (2) formed by the treatment of Si powder in an atmosphere of argon, at a current density of 0.2 mA/cm.sup.2 and a charge-discharge depth of 1.0 mAh/cm.sup.2; and

[0068] FIG. 5 shows a change in overpotential (E) of electrode processes when lithium electrodes are cycled without a barrier layer (1) and with a barrier layer formed by the treatment of P2S5 (2) powder in the nitrogen atmosphere, at a current density of 0.2 mA/cm.sup.2 and a charge-discharge depth of 1.0 mAh/cm.sup.2.

EXAMPLE 0

[0069] Lithium foil is treated in a glove-box under argon, or nitrogen, or in a dry room. Metallic lithium foil is positioned on a flat surface of a material neutral to lithium, such as stainless steel. The surface of lithium is prepared by removing any possible contamination from its surface. A simple brush can be used for that purpose. After that a layer of powder material such as Si or Li.sub.3N is applied on the surface in a thin even layer. To initiate a tribochemical reaction a stainless-steel plate is applied and moved across the surface in a reciprocating way. The surface of the stainless-steel plate can have different level of roughness to provide more efficient conditions for tribochemical reaction on the surface of metallic lithium. The friction energy of movement is thus transferred into tribochemical treatment of lithium. The processing time could take from 0.5 to 10 min with the pressure between the stainless-steel plate and lithium foil being in the range from 0.01 to 1 kg/cm.sup.2.

EXAMPLE 1

[0070] Barrier Layer by Tribochemical Treatment with Si Powder—Dry Air Atmosphere

[0071] All work on tribochemical treatment of the surface of lithium foil with silicon was carried out in a glove box in the atmosphere of dry air. The H.sub.2O content was in the range of 20 to 40 ppm. A lithium foil with a thickness of 100 microns was placed on the surface of a stainless steel plate and secured. Then, the surface of metallic lithium was mechanically cleaned by using a stainless steel brush and/or scraper. After removing contaminants from the lithium surface, a uniform layer of silicon powder was applied. The median volume size of the Si particles was estimated to be in the range of from 5 to 15 microns. On the surface of the lithium foil with a layer of silicon powder was laid a stainless steel plate with a rough surface. To carry out the tribochemical reaction between metallic lithium and silicon powder, the rough plate was pressed to lithium foil with a pressure of 0.1-0.2 kg/cm.sup.2 and brought in a reciprocal and progressive movement with amplitude of 1-5 mm and a frequency of 1-10 Hz. The resulting tribochemical reaction (tribochemical treatment of lithium foil) was carried out for 2-3 minutes.

[0072] After tribochemical treatment, the powder of unresponsive silicon was removed from the surface of the lithium foil. After tribochemical treatment, the surface of the lithium foil was dark grey. The thickness of the surface layer was assessed by weight by the difference in the mass of lithium foil before and after the tribochemical treatment. The thickness of the formed barrier layer was 1.5 microns.

[0073] Electrodes of the right size were cut from lithium foil with a barrier layer and they were further pressed through plastic film at a pressure of 100 kg/cm.sup.2.

[0074] Symmetrical lithium cells (Li/electrolyte/Li) were then assembled from the resulting lithium electrodes. Also for comparison we assembled similar symmetrical cells but with lithium electrodes without a barrier layer.

[0075] Two layers of Celgard separator 3501 were used. The electrolyte was a solution 1.0M LiClO4 in sulfolane (SI). The galvanostatic polarization of the cells was carried out at a temperature of 30 C. The voltage range at cathodic and anodic polarization was limited by +/−500 uV with the current density being 0.2 mA/cm2.

[0076] The amount of electricity in cathodic deposition and/or anodic dissolution of lithium was equal to 1.0 mAh/cm2.

[0077] Studies have shown (FIG. 1) that cells with lithium electrodes with barrier layers formed on lithium by tribochemical treatment using silicon in the atmosphere of dry air, demonstrate more stable and prolonged cycling as well as significant reduction in overvoltage compared to cells with lithium electrodes without a barrier layer. This indicates that the tribochemical treatment of lithium foil by silicon produces a barrier layer significantly improves the electrochemical characteristics of the lithium electrode.

EXAMPLE 2

[0078] Barrier Layer by Tribochemical Treatment with Li.sub.3N Powder—Nitrogen Atmosphere

[0079] The formation of the Li.sub.3N barrier layer was carried out in a similar way described in Example 1, except that the treatment of the surface of lithium foil was carried out in an airtight reactor under nitrogen, which was purged by nitrogen gas at a speed of 6 l/min.

[0080] The median volume size of the particles in the Li.sub.3N powder (used instead of the powder of Example 1) was estimated to be in the range of from 5 to 15 microns.

[0081] Studies have shown (FIG. 2) that cells with lithium electrodes with barrier layers formed in tribochemical treatment of lithium foil by lithium nitride in nitrogen atmosphere, demonstrate better stability in cycling and significantly less overvoltage compared to cells with lithium electrodes without a barrier layer. This indicates that the tribochemical treatment of lithium foil lithium by lithium nitride resulted in formation of a barrier layer, significantly improving the electrochemical characteristics of the lithium electrode.

EXAMPLE 3

[0082] Barrier Layer by Tribochemical Treatment with a Mixture of Si and Li.sub.3N Powder—Nitrogen Atmosphere

[0083] A barrier layer of Si—Li.sub.3N was formed in a similar way as described in Example 2.

[0084] The median volume size of the particles in the Si and Li.sub.3N powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.

[0085] Studies have shown (FIG. 3) that cells with lithium electrodes with barrier layers formed in tribochemical treatment of lithium foil with a mixture of silicon and lithium nitride powders under nitrogen atmosphere, demonstrate more stable cycling and lower overpotential compared to cells with lithium electrodes without a barrier layer. This indicates that the tribochemical treatment of lithium foil with a mixture of silicon and lithium nitride powders formed a barrier layer, which significantly improved the electrochemical characteristics of the lithium electrode.

EXAMPLE 4

[0086] Barrier Layer by Tribochemical Treatment with Si Powder—Argon Atmosphere

[0087] The formation of the Si barrier layer was carried out in a similar way as described in Example 2, except that the treatment of the surface of lithium foil was carried out in an airtight reactor under dry argon atmosphere.

[0088] The median volume size of the particles in the Si powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.

[0089] Studies have shown (FIG. 4) that cells with lithium electrodes with barrier layers formed by tribochemical treatment of lithium foil with silicon under argon atmosphere demonstrate more stable cycling and significantly lower overpotential compared to cells with lithium electrodes without a barrier layer. This indicates that the tribochemical treatment of lithium foil with silicon resulted in formation of a barrier layer, significantly improving the electrochemical characteristics of the lithium electrode.

EXAMPLE 5

[0090] Barrier Layer by Tribochemical Treatment with P.sub.2S.sub.5 Powder—Nitrogen Atmosphere

[0091] The formation of the P.sub.2S.sub.5 barrier layer was carried out in a similar way as described in Example 2.

[0092] The median volume size of the P.sub.2S.sub.5 powder (used instead of the particles of Example 2) was estimated to be in the range of from 5 to 15 microns.

[0093] Studies have shown (FIG. 5) that cells with lithium electrodes with barrier layers formed in the tribochemical treatment of lithium foil with phosphorus sulfide in the nitrogen atmosphere, demonstrate more stable cycling and lower overpotential as compared to cells with lithium electrodes without a barrier layer. This indicates that the tribochemical treatment of lithium foil lithium with phosphorus sulfide formed a barrier layer, which significantly improved the electrochemical characteristics of the lithium electrode.