Negative electrode for potassium hybrid supercapacitor

Abstract

Negative electrode (200) for a potassium hybrid supercapacitor cell (100), the negative electrode (200) comprising an active layer (1) formed from an active material on a current collector (2), the active layer (1) comprising a first region (3) having a first average thickness (E1) and at least one structuring region (4) having a second average thickness (E2), the second average thickness (E2) being less than the first average thickness (E1) so as to obtain a structured negative electrode (200). Figure 3

Description

Title of the invention: Negative electrode for potassium hybrid supercapacitor

[0001] The present invention relates to the field of potassium hybrid supercapacitors. In particular, the invention relates to a structured negative electrode for such supercapacitors. According to another aspect, the invention relates to a cell for a supercapacitor comprising said negative electrode and according to yet another aspect, the invention relates to a method of manufacturing said negative electrode.

[0002] Supercapacitors (which can also be referred to as supercapacitors) are energy storage devices providing a power density and an energy density intermediate between those obtained for electrochemical batteries and conventional electrolytic capacitors. In addition, they restore energy more quickly than an electrochemical battery.

[0003] Symmetrical supercapacitors are known, the positive and negative electrodes of which are based on activated carbon and the electrolyte is based on tetraethylammonium tetrafluoroborate (TEABF4). These symmetrical supercapacitors have a high power density, high cyclability but a low energy density linked to the average capacity of the supercapacitor (this being of the order of 8 Wh / kg) and to the operating voltage. In order to meet the demand for increased energy density, hybrid supercapacitors have been developed, lying between the symmetrical supercapacitors conventionally used based on activated carbon and batteries. Such capacitors are called asymmetric or hybrid.One of the electrodes, typically the negative electrode, is specifically made from a rechargeable battery material and the other, typically the positive electrode, is formed from activated carbon, the electrolyte located between these two electrodes being typically organic and generally comprising lithium salts in solution and sometimes potassium salts in an organic medium. An example of a so-called potassium hybrid supercapacitor comprises a negative electrode comprising graphite, a positive electrode comprising activated carbon and an electrolyte comprising at least one potassium salt. Charge storage in this type of hybrid supercapacitor occurs at the negative electrode through a redox reaction (faradaic phenomenon), while charge storage at the positive electrode occurs through the formation of an electrochemical double layer (capacitive phenomenon).

[0004] However, the lifespan of these so-called potassium cells and their performance must be improved.

[0005] One of the aims of the present invention is to overcome the aforementioned drawbacks. To this end, the invention provides a negative electrode for a potassium hybrid supercapacitor cell, the negative electrode comprising an active layer formed from an active material on a current collector, the active layer comprising a first region having a first average thickness E1 and at least one structuring region having a second average thickness E2, the second average thickness E2 being less than the first average thickness E1 so as to obtain a structured negative electrode.

[0006] Thanks to the use of a structured negative electrode, containing macroporosities thanks to the presence of the structuring region more or less hollowed out with active material, the access of the electrolyte to the negative electrode is favored and the potassium ions circulate more quickly. This improves the performance of the cell for the same operating current, in particular in fast charging. This means that the current density (mA / cm2) is higher for the structured negative electrode and the repetition of the very fast charge and discharge cycles generates significant heat at the negative electrode. The presence of the structuring region provides very efficient heat dissipation, allowing better overall thermal regulation which makes it possible to achieve higher energy densities and to extend the lifespan of potassium hybrid supercapacitor cells.

[0007] According to one arrangement, the first average thickness El is such that El > 3 x E2 with E2 second average thickness and for example El > 4 x E2.

[0008] According to one possibility, the second average thickness E2 of the at least one structuring region is zero, so that the structuring region is free of active material. This embodiment makes it possible to optimize thermal regulation and makes it possible to broaden the choice of manufacturing solutions for the negative electrode.

[0009] According to one arrangement, the at least one structuring region has a surface in a plane parallel to the plane of the active layer surface, within a range from 5 to 20% of the active layer surface, in particular within a range from 10 to 15%. These proportions are the result of a compromise between the improvement of the obtained performance and an efficient heat dissipation. It is easily understood that when the amount of active material is too reduced, the performance is reduced.

[0010] According to one possibility, the at least one structuring region comprises a cross-section having a shape chosen from a rectangular overall shape or a circular overall shape so as to respectively form a parallelepiped channel or a cylindrical channel extending in the thickness of the active layer. It is understood in the present document that the cross-section of the structuring region is considered in a plane parallel to the plane of the surface of the active layer.

[0011] In other words, the structuring region comprises a surface in a plane parallel to the plane of the surface of the active layer, having a shape chosen from a rectangular overall shape or a circular overall shape.

[0012] According to one arrangement, the surface in a plane parallel to the plane of the surface of the active layer of the at least one structuring region extends between two opposite lateral edges of the active layer, the two opposite lateral edges delimiting in particular at least in part contours of the surface of the active layer. In other words, the structuring region extends over the entirety of a lateral dimension of the active layer, for example over the entire width or the entire length of the active layer.

[0013] According to one possibility, the at least one structuring region comprises a cross-section having a width 1 of between 0.5 and 3 mm, in particular between 1 and 2 mm.

[0014] According to one arrangement, the at least one structuring region comprises a cross-section provided in a central portion of the surface of the active layer, in particular the central portion of the surface of the active layer extends on either side of a central axis arranged between two opposite lateral edges of said active layer, over approximately 50% and for example over 40% of the total surface of the active layer.

[0015] According to another aspect, the invention provides a cell for a potassium hybrid supercapacitor, the cell comprising a negative electrode as previously described, a positive electrode of activated carbon capacitive type and a non-aqueous electrolyte comprising a potassium salt, in particular KPF6, KC1O4, KBF4, potassium bis(trifluoromethanesulfonyl)imide (known by the abbreviation KTFSI), potassium bis(fluorosulfonyl)imide (known by the abbreviation KFSI), potassium bis(oxalato)borate (known by the abbreviation KBOB), KSCN, KSbF6, KAsF6, KA1C14, KSiF6, KSO3CF3 and mixtures thereof, preferably chosen from KC1O4, KBF4, KPF6 and mixtures thereof.

[0016] According to other characteristics, the negative electrode and the cell of the invention comprises one or more of the following optional characteristics considered alone or in combination: - The active layer comprises a surface having the overall shape of a rectangle. - The cross section of the at least one structuring region extends in a direction substantially oriented along the diagonal of the surface of the active layer.

[0017] - The length L of the cross section is identical to the length of the layer active 1 between two opposite side edges of said active layer.

[0018] - The first average thickness El of the first region is included in a range from 27 to 33 micrometers.

[0019] - The negative electrode is of the battery type.

[0020] - The active material of the negative electrode comprises or / is made of graphite, including particulate graphite.

[0021] - The median particle size is 13 micrometers (measured by granulometry D50 laser).

[0022] - The positive electrode is capacitive.

[0023] - The positive electrode comprises or / is made of activated carbon.

[0024] - The electrolyte comprises an organic solvent, in particular acetonitrile.

[0025] - The current collector comprises an aluminum foil.

[0026] - The cell comprises a separator comprising a polypropylene material.

[0027] According to another aspect, the invention also provides a method of manufacturing the negative electrode as previously described, the manufacturing method comprising the following steps: - a) formation of the active layer in an active material on the current collector so that the first region has a first average thickness El, - b) formation of at least one structuring region in the active layer, the at least one structuring region having a second average thickness E2 less than the first average thickness EL

[0028] According to one possibility, the active layer is produced according to step a) by spray coating or by extrusion of the active material. In the latter case, the manufacturing method comprises the use of a pattern at the die outlet.

[0029] According to one arrangement, steps a) and b) are concomitant. The deposition of the active layer is in particular carried out by coating or by spraying using a stencil on the surface of the current collector so as to form the first region and the at least one structuring region.

[0030] According to a variant, step a) consists of depositing the active material over the entire surface of the active layer and step b) is carried out by removing at least a portion of the active material by mechanical action of the screen printing type or by laser irradiation leading to the degradation of at least a portion of the irradiated active material, so as to form the at least one structuring region.

[0031] The structuring of the active layer by laser is in particular carried out by pulsed laser radiation also known by the English terminology pulse laser radiation or any other process compatible with the graphite of the active layer, the thickness of graphite to be degraded as well as the lateral dimensions (or cross section) of the structuring region.

[0032] Other aspects, aims and advantages of the present invention will appear better on reading the following description of an embodiment thereof, given by way of non-limiting example and with reference to the appended drawings. In the remainder of the description, for the sake of simplification, identical, similar or equivalent elements of the different embodiments bear the same numerical references. The figures do not necessarily respect the scale of all the elements represented so as to improve their readability in which:

[0033] [Fig-1] represents the operation of a cell of a potassium hybrid supercapacitor according to the prior art by two three-dimensional schematic views.

[0034] [Fig.2] represents a sectional view of a negative reference electrode and other cross-sectional views of negative electrodes structured according to alternative embodiments of the invention.

[0035] [Fig.3] represents a sectional view of the thicknesses of the first and the region of structuring of the active layer of a negative electrode structured according to an embodiment of the invention.

[0036] [Fig.4] represents a sectional view of a negative electrode structured according to a embodiment of the invention.

[0037] [Fig.5] represents a graph illustrating the evolution of the capacity of a cell of 2 mAh hybrid supercapacitor comprising a negative electrode structured according to an embodiment of the invention during cycles for a charge / discharge current of 200 mA between 0.5 and 3.5 V at room temperature.

[0038] [Fig.6] represents a graph illustrating the evolution of the capacity of a cell of 2 mAh hybrid supercapacitor comprising a negative electrode structured according to an embodiment of the invention during cycles for a charge / discharge current of 400 mA between 0.5 and 3.5 V at room temperature.

[0039] [Fig.7] represents a graph illustrating the evolution of the capacity of a cell of 2 mAh hybrid supercapacitor comprising a negative electrode structured according to an embodiment of the invention during cycles for a charge / discharge current of 800 mA between 0.5 and 3.5 V at room temperature.

[0040] A conventional cell for a potassium hybrid supercapacitor is illustrated in [Fig.l]. It comprises on either side a positive electrode 'ep' comprising a layer of activated carbon on an aluminum current collector 'c' and a negative electrode 'en' comprising a layer of graphite on an aluminum current collector. A polypropylene separator 's' is arranged between the two electrodes to ensure electronic insulation between the electrodes, while allowing easy passage of the ions of the electrolyte, in particular composed of a KPF6 salt in acetonitrile. This type of supercapacitors works on the principle that charge storage at the negative electrode occurs through a redox reaction, while charge storage at the positive electrode occurs through the formation of an electrochemical double layer. During the cell charging period, a capacitive phenomenon takes place around the positive electrode and the PF6 anions agglomerate at the carbon electrode, an electrochemical double layer is formed. Conversely, an intercalation phenomenon of potassium cations K+ takes place between the graphite planes, charge storage at the negative electrode occurs through a redox reaction.

[0041] The cell 100 of the invention differs from that presented above in that the negative electrode 200 of the invention comprises a structured active layer 1 (refer to FIGS. 2 to 4). The active layer 1 is formed from an active material, in particular graphite with a porosity of 40 + / - 2%, formed on the current collector 2. It comprises a first region 3 having a first average thickness E1 (along the z axis) of approximately 30 micrometers and at least one structuring region 4 having a second average thickness E2, the second average thickness E2 being less than the first average thickness E1 (refer to [Fig. 3]).

[0042] According to a possibility not visible in the figures, the second average thickness E2 is zero, the at least one structuring region 4 being completely devoid of active material. As illustrated in Figures 2 B to 2 D and 3, the at least one structuring region 4 is provided in a central portion C of the active layer 1 ([Fig.2] C), the central portion C being defined on either side of a central axis arranged between and substantially parallel to two opposite lateral edges of the surface of the active layer 1 when the latter has the overall shape of a rectangle or square. The central portion C has, for example, a surface area less than 50% of the total surface area of ​​the active layer 1, or even less than or equal to 20% of the total surface area.

[0043] As illustrated in Figures 2 B to 2 D, the surface area of ​​the at least one structuring region 4 measures between 5 and 20% of the total surface area of ​​the active layer 1. Also illustrated in [Fig.2] C, the active layer 1 comprises a plurality of structuring regions 4. The cross-section thereof, in other words its surface area in a plane parallel to the plane of the surface of the active layer 1, can take different shapes, for example rectangular, such as a strip ([Fig.2] B), a circular shape ([Fig.2] D), etc. so as to form a rectangular or cylindrical channel in the thickness of the active layer 1. Also visible in Figures 2 B to 2 D, the at least one structuring region 4 extends between two opposite lateral edges of the active layer 1. The structuring region 4 extends on either side following the y axis in the plane of the active layer 1.According to a variant not illustrated, the structuring region 4 extends on either side following the x axis in the plane of the active layer 1.

[0044] As illustrated in [Fig.2] B, the width 1 on the surface of the active layer 1 is narrow, here it is 2 mm for an active layer 1 surface of 35 mm x 35 mm.

[0045] The method for manufacturing the active layer 1 not shown in the figures comprises the formation of the active layer 1 in an active material on the current collector 2 according to a step a), the active layer 1 comprising the first region 3 having a first average thickness E1 and the formation of the at least one structuring region 4 in the active layer 1 according to a step b).

[0046] The deposition of the active layer 1 is carried out by coating, by spraying, or by extrusion of active material. When the deposition is carried out by coating or spraying, steps a) and b) can be concomitant thanks to the use of a stencil delimiting the contours of the structuring region 4.

[0047] According to a variant, step b) is carried out on the active layer 1 previously deposited by removing active material via a mechanical action of the screen printing type or by laser irradiation leading to the degradation of the irradiated active material facilitating its removal.

[0048] An exemplary embodiment and aging tests are now described with reference to Figures 5 to 7. An active layer 1 made of graphite (with a binder li and a conductive additive a) with a surface area of ​​35 mmx 35 mm deposited by coating on an aluminum current collector 2. A removal of active material is carried out by a mechanical action of the screen printing type to form a vertical strip ([Fig.2] B) which will constitute the structuring region 4 in the center of the electrode. The removal of material is carried out over the entire thickness of the active layer 1 so that the second thickness E2 is zero, the structuring region 4 is devoid of graphic material over a strip width 1 of 2 mm, it represents 10% of the surface area of ​​the electrode (surface area of ​​the active layer 1 comprising the surface area of ​​the first region 3 and the surface area of ​​the structuring region 4).This single-sided negative electrode 200 is associated with a positive electrode of activated carbon using a polypropylene separator. Filling with an electrolyte composed of KPF6 dissolved in acetonitrile with a concentration of 0.8 mol.l1 allows the finalization of the manufacture of the cell 100 for potassium hybrid supercapacitor.

[0049] Aging tests are then carried out with charge / discharge cycles at different charge / discharge rates (100C, 200C and 400C) at ambient temperature and pressure between 0.5 and 3.5 V. The results are presented in Figures 5 to 7 on the cell 100 as previously described and compared to results obtained from conventional cells (without structuring of the negative electrode).

[0050] In particular, [Fig.5] illustrates the evolution of the capacity during cycles for a current of 200mA (100C - charging time approximately 1 minute). As can be seen from the graph, the performance of cell 100 with negative electrode 200 structured are improved compared to those of a conventional cell: The 100 cell containing the structured electrode has a capacity of approximately 1.95 mAh after 5000 cycles against approximately 1.5 mAh for the reference.

[0051] These observations are confirmed with a current of 400mA (200C - [Fig.6]) and a current of 800 mA (400C - [Fig.7]).

[0052] Thus, the present invention makes it possible to improve the performance of the potassium hybrid supercapacitor cells 100 without modifying the required current, by increasing the current density. The lifetime of the potassium hybrid supercapacitor cells 100 is also extended in particular thanks to better thermal regulation of the cell 100 when applying high charge / discharge regimes.

Claims

Claims

1. Negative electrode (200) for a potassium hybrid supercapacitor cell (100), the negative electrode (200) comprising an active layer (1) formed from an active material on a current collector (2), the active layer (1) comprising a first region (3) having a first average thickness (El) and at least one structuring region (4) having a second average thickness (E2), the second average thickness (E2) being less than the first average thickness (El) so as to obtain a structured negative electrode (200).

2. Negative electrode (200) according to claim 1, in which the second average thickness (E2) of the at least one structuring region (4) is zero, so that the structuring region (4) is free of active material.

3. Negative electrode (200) according to claim 1 or 2, wherein the at least one structuring region (4) has a surface in a plane parallel to the plane of the surface of the active layer (1), comprised in a range from 5 to 20% of the surface of the active layer (1), in particular in a range from 10 to 15%.

4. Negative electrode (200) according to one of claims 1 to 3, in which the at least one structuring region (4) comprises a surface in a plane parallel to the plane of the surface of the active layer (1), having a shape chosen from a rectangular overall shape or a circular overall shape, so as to form respectively a parallelepiped channel or a cylindrical channel extending in the thickness of the active layer (1).

5. Negative electrode (200) according to claim 4, wherein the surface in a plane parallel to the plane of the surface of the active layer (1) of the at least one structuring region (4) extends between two opposite lateral edges of the active layer (1).

6. Negative electrode (200) according to one of claims 1 to 5, in which the at least one structuring region (4) has a cross-section with a width (1) of between 0.5 and 3 mm, in particular between 1 and 2 mm.

7. Negative electrode (200) according to one of claims 1 to 6, wherein the at least one structuring region (4) comprises a cross-section provided in a central portion (C) of the surface of the active layer (1).

8. Cell (100) for potassium hybrid supercapacitor comprising a negative electrode (200) according to one of claims 1 to 7 and a positive electrode of carbon capacitive type and a non-aqueous electrolyte comprising a potassium salt.

9. A method of manufacturing the negative electrode (200) according to one of claims 1 to 7, the manufacturing method comprising the following steps: - a) forming the active layer (1) in an active material on the current collector (2), the active layer (1) comprising the first region (3) having a first average thickness (El), - b) forming the at least one structuring region (4) in the active layer (1), the at least one structuring region (4) having a second average thickness (E2) less than the first average thickness (El).

10. A manufacturing method according to claim 9, wherein the active layer (1) is produced by coating or spraying the active material.

11. Manufacturing method according to one of claims 9 or 10, in which step b) is carried out by removing at least part of the active material by mechanical action of the screen printing type or by laser irradiation leading to the degradation of at least part of the irradiated active material.

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