WO2021123732A1

WO2021123732A1 - A negative electrode and method of treating a negative electrode working surface

Info

Publication number: WO2021123732A1
Application number: PCT/GB2020/053117
Authority: WO
Inventors: Alex MADSEN; Ian Hunt
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2019-12-16
Filing date: 2020-12-04
Publication date: 2021-06-24
Anticipated expiration: 2022-06-16
Also published as: GB201918469D0; CN114830371A; GB2590395B; GB2590395A

Abstract

A method of, and apparatus for, treating a working surface (14) of a negative electrode (10) for an alkali metal-ion cell, the negative electrode comprising a negative electrode material. The method comprising: providing a negative electrode having a working surface, the working surface having a native solid electrolyte interface (SEI) layer (16) on the negative electrode material; removing the native SEI layer to expose an un-passivated surface of the underlying negative electrode material; and simultaneously chemically treating the un¬ passivated surface of the underlying negative electrode material to create an engineered SEI layer (20).

Description

A negative electrode and method of treating a negative electrode working surface

The present invention relates to a method of treating a working surface of a negative electrode, to apparatus for treating a working surface of a negative electrode, to a negative electrode, and a cell comprising the negative electrode. More specifically the present invention relates to a pre-treated negative electrode for use in an alkali metal-ion cell.

Introduction

First cycle irreversible capacity loss is a well-known phenomenon observed in alkali metal ion cells, such as lithium and sodium ion cells. The mechanism and extent of the capacity loss can vary depending on the material used for either the negative electrode or cathode. For negative electrodes in particular, one reason for the capacity loss is the formation of a solid electrolyte interface (SEI), which occurs at a low voltage. The SEI forms on the negative electrode due to decomposition of the electrolyte components present on the negative electrode working surface. These components react with the sodium or lithium ions at the surface to form surface layer compounds containing sodium or lithium ions.

An SEI layer can also form when a negative electrode surface is exposed to the atmosphere, particularly if the negative electrode material is a highly-reactive material such as lithium or sodium. As such, almost immediately after fabrication of a negative electrode, a ‘native’ SEI layer, or ‘native passivation layer’ will form.

Although the formation of the SEI can stabilize the negative electrode performance, and hence the presence of the SEI may be desirable, the capacity loss associated with it heavily affects the capacity and the cycle life of the battery. Thus, there is a need to provide a stable negative electrode which demonstrates reduced first cycle irreversible capacity loss and improved performance characteristics, such as improved stability and reduced interfacial resistance to charge transfer, during the life-time of the battery.

Summary of invention

Against this background the invention resides in a method of treating a working surface of a negative electrode for an alkali metal-ion cell, the negative electrode comprising a negative electrode material and the method comprising: providing a negative electrode having a working surface, the working surface having a native solid electrolyte interface (SEI) layer on the negative electrode material; removing the native SEI layer to expose an un-passivated surface of the underlying negative electrode material; and chemically treating the un- passivated surface of the underlying negative electrode material to create an engineered SEI layer.

The engineered SEI layer that is produced on the underlying un-passivated surface of the negative electrode is different from the native SEI layer that is removed from the working surface. The native SEI layer is formed spontaneously and in an uncontrolled manner, and hence is of unpredictable composition and thickness. By contrast, the engineered SEI layer is of a known composition and thickness. Thus, the properties of the engineered SEI layer, such as its resistance and thermal/chemical stability, can be controlled and tuned to particular requirements to optimise performance of the negative electrode, and hence a cell into which the negative electrode is incorporated.

The step of removing the native SEI layer may comprise mechanically removing the native SEI layer.

The step of chemically treating the un-passivated surface of the underlying negative electrode material may comprise exposing the negative electrode to a reagent to create the engineered SEI layer. The reagent may, for example, be a non-aqueous alkali metal ion electrolyte.

The step of exposing the negative electrode to the reagent may comprise arranging the negative electrode in a bath of the reagent. In this case the step of removing the native SEI layer may be carried out while the negative electrode is arranged in the bath.

The negative electrode may be an elongate strip. In this case, the step of exposing the negative electrode to the reagent may comprise passing the negative electrode from a first reel, through the reagant, and to a second reel. Where the reagent is arranged in a bath, this may comprise passing the strip through the bath. The step of removing the native SEI layer may be carried out as the negative electrode passes through the reagent and/or the bath.

The invention also extends to a negative electrode for an alkali metal-ion cell, the negative electrode comprising a negative electrode material and a working surface, the working surface comprising a mechanically-polished surface of the negative electrode material having an engineered SEI layer formed thereon.

The negative electrode material may comprise lithium, sodium, calcium or magnesium.

In the method or the negative electrode described above, when the negative electrode comprises lithium, the engineered SEI layer may comprise lithium fluoride and/or lithium carbonate. The invention extends further to apparatus for treating a working surface of a negative electrode for use in alkali metal-ion cell. The negative electrode comprises a negative electrode material and the apparatus comprises removal means for removing a native SEI layer from the working surface to expose the underlying negative electrode material, and treatment means for chemically treating the underlying negative electrode material at the working surface to create an engineered SEI layer.

The removal means may comprise mechanical removal means for mechanically removing a native SEI layer. The mechanical removal means may comprise a movable polishing means, such as a rotatable grinding wheel or a rotatable or movable polishing brush.

The treatment means may comprise a reagent bath configured to receive a reagent and a negative electrode, to submerge the negative electrode in the reagent. In this case, the removal means may be arranged to remove the native SEI layer from the working surface while the negative electrode is arranged in the reagent bath.

The negative electrode may comprise an elongate strip. In this case, the apparatus may comprise first and second reels for supporting the elongate strip and configured to rotate to cause the elongate strip to be passed between the reels. The reagent bath may be arranged between the reels, such that the elongate strip passes through the reagent bath as it passes between the reels.

The invention also extends to an alkali metal-ion cell comprising the negative electrode described above, or a negative electrode made according to the method described above.

It will be appreciated that features of any of the above aspects or embodiments may be used alone, or in appropriate combination, with the other aspects or embodiments also.

Description of the Figures

In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying Figures, in which:

Figure 1 is a schematic view of a negative electrode with a native SEI layer;

Figure 2 is a schematic view of the negative electrode of Figure 1 with the native SEI layer removed to reveal an un-passivated surface of the underlying negative electrode material; Figure 3 is a schematic view of the negative electrode of Figure 2 with an engineered SEI layer that has been created on the surface of the underlying negative electrode material;

Figure 4 is a schematic view of apparatus for treating the negative electrode of Figure 1 by removing the native SEI layer to reveal then un-passivated surface of the underlying negative electrode material and treating the un-passivated surface to create the engineered SEI layer;

Figure 5 is a schematic view of another apparatus for treating the negative electrode of Figure 1 via a reel-to-reel process by removing the native SEI layer to reveal then un-passivated surface of the underlying negative electrode material and treating the un-passivated surface to create the engineered SEI layer; and

Figure 6 is a close up of a negative electrode undergoing treatment in the apparatus of Figure 5.

Description of specific embodiments of the invention

Figure 1 illustrates a negative electrode 10 for use in alkali metal-ion cell such as a lithium-ion or sodium-ion battery. The negative electrode 10 has a body 12 made of a negative electrode material, which may be any material capable of generating alkali metal ions, such as lithium, sodium, calcium or magnesium metal. In the forgoing example, the negative electrode material is lithium, and the negative electrode body 12 defines an elongate strip.

The negative electrode 10 comprises a working surface 14 at which, in the assembled cell, reactions take place. At the working surface 14, prior to cell construction/assembly, the lithium negative electrode material spontaneously reacts with materials in the surrounding atmosphere, for example oxygen, nitrogen and moisture in air, to form an SEI layer, made up of lithium compounds, such as lithium oxide and lithium nitrate. Because this SEI layer occurs naturally and spontaneously when the lithium negative electrode is exposed to the atmosphere, it is referred to here as a ‘native’ SEI layer 16.

According to a method of treating the negative electrode, the native SEI layer 16 is removed from the negative electrode 10 to expose an un-passivated surface 18 or ‘pristine’ surface of the underlying negative electrode material of the body 12, as shown in Figure 2. The native SEI layer 16 may be removed by mechanical means, for example by abrading the native SEI layer 16. In the particular examples illustrated, the native SEI layer 16 is removed using a grinding wheel, though other abrasive removal means may be used. For example, the native SEI layer 16 may be removed with a movable or rotatable polishing brush. The brush may comprise bristles for abrading the surface, for example plastic bristles. Non-mechanical means of removing the native SEI layer 16 are also envisaged, such as laser ablation or chemical means. A combination of mechanical and chemical means is also envisaged.

Also according to the method, the un-passivated surface 18 of the underlying negative electrode material of the body 12 is treated to create an engineered SEI layer 20, shown in Figure 3. In this particular example, the engineered SEI layer 20 is formed by exposing the un-passivated surface 18 to a chosen reagent under chosen conditions.

The resulting engineered SEI layer 20 is different from the native SEI layer 14. In particular, unlike the native SEI layer 16 which is formed spontaneously and in an uncontrolled manner, and hence is of unpredictable composition and thickness, the engineered SEI layer 20 is of a known composition and thickness. Thus, while the exact material and properties of the native SEI layer 14 are uncontrolled and unpredictable, the properties of the engineered SEI layer 20, such as its resistance and thermal/chemical stability, can be controlled and tuned to particular requirements to optimise performance of the negative electrode 10 and hence a cell into which the negative electrode 10 is incorporated.

In one particular example, the engineered SEI layer 20 comprises lithium fluoride and lithium carbonate containing compounds. In this case, the reagent is a non-aqueous lithium-ion electrolyte material, for example a LiPF₆ salt dissolved in one or more carbonate solvents such as vinylene carbonate (VC) ethylene carbonate (EC) orfluoroethylene carbonate (FEC). When the un-passivated surface 18 is exposed to the reagent, lithium at the un-passivated surface 18 reacts with the reagent to form the lithium fluoride and lithium carbonate containing compounds of the engineered SEI layer.

The un-passivated surface 18 may be treated by any suitable method that allows the reagent to come into contact with the un-passivated surface 18. For example, where the reagent is a liquid reagent, the un-passivated surface 18 may be treated by spraying or washing the body 12 of the negative electrode 10 with the reagent, or by submerging the body 12 of the negative electrode 10 in the reagent. A washing treatment may involve a sponge that has been soaked in the reagent. For example, the sponge may be provided as a sponge roller, and for a continuous treatment process may be continuously fed with reagent. It is also envisaged that the reagent may be a gas, such that the treatment is a gaseous treatment.

Figure 4 shows an example of apparatus 30 in use in treating the working surface 14 of the negative electrode 10 according to the method described above. The apparatus 30 comprises removal means for removing the native SEI layer from the working surface 14, which in this case takes the form of a polishing or grinding wheel 32.

The wheel 32 comprises a polishing or grinding surface 36, and is configured to rotate about a central axis 38 while that grinding surface 26 is in contact with the working surface 14, so that the grinding surface 26 abrades the native SEI layer to remove it.

The apparatus 30 also comprises treatment means for chemically treating the underlying negative electrode material at the working surface 14, which in this case takes the form of a bath 34 that acts as a reservoir for holding the reagent 22. In this way, the negative electrode 10 can be submerged in the reagent 22 in the bath 34 to expose the un-passivated surface of the negative electrode 10 to the reagent 22.

In this example, the grinding wheel 32 is arranged to make contact with the negative electrode 10 while the negative electrode is arranged in the bath 24. The grinding wheel 32 is therefore arranged at least partially in the bath 34, and hence is at least partially submerged in the reagent 22 when the reagent 22 is in the bath 34. The apparatus is arranged to allow for relative movement between the grinding wheel 32 and the negative electrode 10, so that the grinding wheel 32 is able to abrade the working surface 14 across the entire area of the negative electrode 10. For example, the grinding wheel may be arranged for movement generally in the plane of the negative electrode 10: for example left and right as shown in Figure 4. Alternatively, the negative electrode 10 may be arranged for movement in the plane of the negative electrode 10.

Another apparatus for treating a negative electrode 10 is shown in Figures 5 and 6. This apparatus is particularly suited for treating negative electrodes 10 that take the form of long strips, and for treating such negative electrodes as a part of a continuous process. The process may include other steps before or after the treatment process shown. For example, the apparatus of Figure 5 may form a treatment station 40 that forms part of a larger apparatus. The larger apparatus may comprise a fabrication station upstream of the treatment station 40 of Figure 5, in which the negative electrode is fabricated, and/or the larger apparatus may comprise additional treatment stations upstream or downstream of the treatment station 40.

Similar to the apparatus 30 of Figure 4, the treatment station 40 of Figure 5 comprises removal means in the form of a polishing or grinding wheel 42 and treatment means in the form of a bath 44 that acts as a reservoir for holding the reagent 22. In addition, the treatment station 40 comprises transportation means for transporting the negative electrode through the treatment station 40 in a process direction P. In this example, the transportation means comprises first and second reels 46a, 46b configured to support the negative electrode 10 in a reel-to-reel process. As the reels 46a, 46b rotate, the negative electrode 10 is passed from the first reel 46a to the second reel 46b in the processing direction P to move the negative electrode 10 through the treatment station.

The bath 44 is arranged between the reel 46a, 46b downstream of the first reel 46a and upstream of the second reel 46b, so that the negative electrode is passed through the bath 44 as it passes between the reels 46a, 46b.

As with the apparatus 30 of Figure 4, in the apparatus 40 of Figure 5, the wheel 42 is arranged partially in the bath 44, so as to remove the native SEI layer from the negative electrode 10 as the negative electrode passes through the bath 44.

As is shown in detail in Figure 6, the arrangement of the treatment station 40 is such that a while a first or upstream part of the negative electrode 10 is undergoing a first stage of the treatment process, a second or downstream part of the negative electrode 10 is simultaneously undergoing a second stage of the treatment process.

More specifically, a first part of the negative electrode 10 that is immediately beneath the grinding wheel 42 is undergoing a first stage of the treatment in which the native SEI 16 is removed from the working surface 14 to expose the un-passivated surface 18 of the underlying negative electrode material. Simultaneously, a second part of the negative electrode 10 immediately downstream from the grinding wheel 42 is undergoing a second stage of the treatment in which the un-passivated surface 18 of the underlying negative electrode material is exposed to the reagent 22 in the bath 44 to create the engineered SEI layer 20. This process is carried out continuously as the negative electrode 10 passes through the bath 44, such that every part of the negative electrode undergoes the first and second parts of the process in quick succession, providing simple and efficient means of removing the native SEI layer 16 and subsequently immediately creating the engineered SEI layer 20.

Claims

1. A method of treating a working surface of a negative electrode for an alkali metal-ion cell, the negative electrode comprising a negative electrode material, the method comprising: providing a negative electrode having a working surface, the working surface having a native solid electrolyte interface (SEI) layer on the negative electrode material; removing the native SEI layer to expose an un-passivated surface of the underlying negative electrode material; and chemically treating the un-passivated surface of the underlying negative electrode material to create an engineered SEI layer.

2. The method of Claim 1, wherein the step of removing the native SEI layer comprises mechanically removing the native SEI layer.

3. The method of any preceding claim, wherein the step of chemically treating the un- passivated surface of the underlying negative electrode material comprises exposing the negative electrode to a reagent to create the engineered SEI layer.

4. The method of Claim 3, wherein the reagent is a non-aqueous alkali metal ion electrolyte.

5. The method of Claim 3 or Claim 4, wherein the step of exposing the negative electrode to the reagent comprises arranging the negative electrode in a bath of the reagent, and wherein the step of removing the native SEI layer is carried out while the negative electrode is arranged in the bath.

6. The method of Claim 5, wherein the negative electrode is an elongate strip, the step of arranging the negative electrode in the bath comprises passing the negative electrode from a first reel, through the bath, and to a second reel, and wherein the step of removing the native SEI layer is carried out as the negative electrode passes through the bath.

7. A negative electrode for use in an alkali metal-ion cell, the negative electrode comprising a negative electrode material and a working surface, the working surface comprising a mechanically-polished surface of the negative electrode material having an engineered SEI layer formed thereon.

8. The method of any of Claims 1 to 6, or the negative electrode of Claim 7, wherein the negative electrode material comprises lithium, sodium, calcium or magnesium.

9. The method of any of Claims 1 to 6 or Claim 8, or the negative electrode of Claim 7 or Claim 8, wherein the negative electrode comprises lithium and the engineered SEI layer comprises lithium fluoride and/or lithium carbonate.

10. Apparatus for treating a working surface of a negative electrode for an alkali metal-ion cell, the negative electrode comprising a negative electrode material and the apparatus comprising: removal means for removing a native SEI layer from the working surface to expose the underlying negative electrode material; and treatment means for chemically treating the underlying negative electrode material at the working surface to create an engineered SEI layer.

11. The apparatus of Claim 10, wherein the removal means comprises mechanical removal means for mechanically removing a native SEI layer.

12. The apparatus of Claim 11, wherein the mechanical removal means comprises a movable polishing means, such as a rotatable grinding wheel or a rotatable or movable polishing brush.

13. The apparatus of any of Claims 10 to 13, wherein the treatment means comprises a reagent bath, configured to receive a reagent and a negative electrode to submerge the negative electrode in the reagent.

14. The apparatus of Claim 13, wherein the removal means is arranged to remove the native SEI layer from the working surface while the negative electrode is arranged in the reagent bath.

15. The apparatus of any of Claims 10 to 14, wherein the negative electrode comprises an elongate strip, wherein the apparatus comprises first and second reels for supporting the elongate strip and configured to rotate to cause the elongate strip to be passed between the reels, and wherein the reagent bath is arranged between the reels, such that the elongate strip passes through the reagent bath as it passes between the reels.

16. An alkali metal-ion cell comprising the negative electrode of any of Claims 7 to 9, or a negative electrode made according to the method of any of Claims 1 to 6.