Methods for manufacturing an ultracapacitor with improved energy density

Abstract

In order to increase the energy density of ultracapacitors, the invention proposes a method for producing a slurry to manufacture an electrode of an ultracapacitor, the method involving dispensing a first binder material and a carbide derived carbon (CDC) particulate material into a mixing vessel; optionally the mixing vessel may include at least one additive material; mixing the materials with a dispersion medium to obtain a dispersion that has a base solid content; mixing a second binder into the dispersion; and diluting the dispersion to a target solid content to obtain the slurry. The slurry is coated onto a current collector, formed into an electrode by a calendering process, and multiple electrodes and a separator are assembled into an electrode assembly. The electrode assembly is wetted with an electrolyte and encapsulated within a housing to form the ultracapacitor.

Description

[0001] METHODS FOR MANUFACTURING AN ULTRACAPACITOR WITH IMPROVED ENERGY DENSITY

[0002] The invention relates to a method for producing a slurry that is used in the manufacture of an ultracapacitor. The invention further relates to methods for manufacturing an electrode and an ultracapacitor.

[0003] The terms used herein for macropores, mesopores, and micropores are based on the classification adopted by IUPAC. According to this classification, macropores have a pore diameter greater than 50 nm, mesopores have a pore diameter between 2 nm and 50 nm, and micropores have a pore diameter of less than 2 nm. Furthermore, as used herein, the terms microporous, mesoporous, or macroporous indicate that the corresponding type of pores is present in the material. It does not exclude the presence of other types of pores, e.g., a microporous material is not limited to having only micropores, but may also include meso- and / or macropores.

[0004] The particle sizes as used herein are measured by laser diffraction in a liquid medium. Auxiliary materials such as surfactants can be used. The evaluation of the measurement is carried out according to Mie and / or Fraunhofer. In a typical manner, the fraction X of particles with a size smaller than S is referred to as DX with size S, e.g., D90 30 pm means that 90 % of particles have a size smaller than 30 pm. This terminology is not limited to particle sizes and can be used for other size distributions.

[0005] Specific surface area, e.g., surface area per unit weight, refers to values obtained by nitrogen physisorption in conjunction with the Brunauer-Emmet-Teller (BET) and Rouquerol evaluation methods, as described in the IUPAC Technical Report: "Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)" by Thommes et al., Pure Appl. Chem. 2015; 87(9-10): 1051-1069. This disclosure is incorporated herein by reference. For the sake of brevity the specific surface area so determined is also referred to as BET nitrogen surface area or BET surface area.

[0006] Energy storage cells as described herein may be classified into three varieties: batteries, capacitors, and ultracapacitors (sometimes also called supercapacitors). The term “battery” as used herein designates an energy storage cell that stores electrical energy exclusively by electrochemical redox reaction. While this typically also includes primary batteries that can only be discharged, the term “battery” as used herein exclusively designates a secondary battery, e.g., a battery that may be charged and discharged.

[0007] The term “capacitor” as used herein designates an energy storage cell that stores electrical energy electrostatically. The term “ultracapacitor” as used herein designates a special kind of capacitor and may be further distinguished into a double-layer capacitor (DLC), a pseudocapacitor, and a hybrid capacitor. The DLC stores energy electrostatically using a Helmholtz double layer. The pseudocapacitor stores electrical energy electrochemically by Faradaic electron charge-transfer such as intercalation or electrosorption. The hybrid capacitor uses both mechanisms of the DLC and the pseudocapacitor.

[0008] DE 10 2022 100 863 A1 and EP 4 202 962 A1 disclose methods for manufacturing ultracapacitors. The ultracapacitors include an electrode assembly coated with a microporous carbon material as the active material.

[0009] EP 4 304 982 A1 discloses a method for manufacturing a microporous carbide-derived carbon material that can be used as active material in an electrode assembly.

[0010] WO 2023 / 117 170 A2, WO 2023 / 117 490 A1 , WO 2023 / 117 491 A1 , and WO 2023 / 117 492 A2 disclose electrode material compositions and energy storage cells with fast charge and discharge capabilities.

[0011] Ultracapacitors are becoming more and more important in various applications due to the growing electrification of many areas of life. Ultracapacitors can be easily charged and are able to provide large peak powers for a short amount of time. Thus, ultracapacitors are generally desired, where high power demands need to be met that cannot be met by batteries. The high peak power capability typically comes with the trade-off of a much smaller energy density of the ultracapacitor (typically 10 Wh / dm3) compared to batteries (e.g., lithium ion batteries with typically 650 Wh / dm3).

[0012] It is the object of the invention to improve the energy density of ultracapacitors. The object is achieved by the subject-matter of the independent claims. Preferred embodiments are subject-matter of the dependent claims.

[0013] The invention provides a method for producing a slurry for manufacturing an electrode of an ultracapacitor, the method comprising: a) dispensing a first binder material, at least one additive material, and a carbide derived carbon (CDC) particulate material into a mixing vessel; b) mixing the materials with a dispersion medium, e.g., water, to obtain a dispersion that has a base solid content; c) mixing a second binder into the dispersion; and d) diluting the dispersion to a target solid content to obtain the slurry.

[0014] Preferably, the first binder material includes or consists of carboxymethyl cellulose (CMC) or salts thereof, preferably sodium CMC.

[0015] Preferably, the additive material includes a carbon-based conductive additive. Preferably, the carbon-based conductive additive is chosen from a group consisting of carbon black, carbon nanotubes, graphene, and mixtures thereof.

[0016] Preferably, the CDC particulate material has a skeletal density of at least 2.2 g / cm3, preferably of at least 2.5 g / cm3, preferably of at least 2.9 g / cm3, as determined by means of helium pycnometry at constant volume.

[0017] Preferably, the components of the slurry have the following dry proportions (% dry): first binder: 1 % dry to 5 % dry; carbon-based conductive additive: 0 % dry to 10 % dry;

[0018] CDC particulate material: 85 % dry to 98 % dry; and second binder: 1 % dry to 5 % dry, wherein the dry proportion indicates the proportion of solid component of the respective component relative to all solid components in the slurry, wherein the dry proportions of the components are chosen such that the slurry consists of 100 % dry. Preferably, in step b) the dispersion medium is added in an amount such that the dispersion has a base solid content of 40 wt% to 50 wt% with respect to the total weight of the dispersion.

[0019] Preferably, the second binder includes or consists of a latex binder, preferably polyacrylic acid (PAA) or styrene butadiene rubber (SBR).

[0020] Preferably, in step d) the dispersion is diluted to a target solid content of 38 wt% to 43 wt% with respect to the total weight of the dispersion.

[0021] The invention provides a method for manufacturing an electrode for an ultracapacitor, the method comprising: a) performing a previously described method to obtain a slurry; b) providing a current collector; c) coating at least one side of the current collector with the slurry to obtain a coating layer; d) compacting the coating layer to obtain the electrode.

[0022] Preferably, the method comprises a step of e) cutting the electrode to size.

[0023] Preferably, in step b) the current collector is provided with a functional surface that increases wettability.

[0024] Preferably, in step c) the at least one side is coated with a coating loading of 30 g / m2to 85 g / m2, more preferable of 40 g / m2to 85 g / m2.

[0025] Preferably, in step d) compacting involves a contact force of 2.5 kN / mm to 4.5 kN / mm and / or applying heat to obtain a temperature during compacting of 70 °C to 110 °C. The contact force is determined by dividing the applied force measured in kN with the width of the electrode measured in mm. In order to calculate the pressure, the contact area of the calendering rollers on the electrode is required. The contact area is dependent on the used calendering machine (e.g., calendering rollers diameter).

[0026] The invention provides a method for manufacturing an ultracapacitor, the method comprising: a) manufacturing an electrode according to a previously described method; b) rolling the electrode together with a separator and another electrode to obtain an electrode assembly; c) encapsulating the electrode assembly together with an electrolyte and electrically contacting the electrode assembly to a positive terminal and a negative terminal, respectively.

[0027] Preferably, step b) involves cutting the electrode obtained in step a) to size.

[0028] An idea is to manufacture high-density carbon-based electrodes for ultracapacitors. It is believed that the increased density is enabled by using a carbide-derived carbon (CDC) material and / or the manufacturing process. The CDC exhibits a comparatively large skeletal density, ideally above 2.9 g / cm3that is significantly greater than the skeletal density of conventional activated carbon that is based on organic precursors such as coconuts.

[0029] The ideas described herein allow for an almost 25 % increase in electrode density compared to conventional activated carbon materials. The electrode density is determined after compacting and by excluding the weight of the current collector. The electrode density achievable with CDC is up to 0.75 g / cm3compared to 0.60 g / cm3that can be achieved with conventional activated carbon material.

[0030] The increase in electrode density enables an increased energy content for the ultracapacitor at the same given volume of the energy storage cell. Consequently, energy density is improved.

[0031] Electrodes for ultracapacitors typically involve a current collector and a coating layer comprising the active material and possibly auxiliary components, such as binders and / or carbon additives.

[0032] The electrode is generally manufactured by making a slurry, where the active material and auxiliary components are homogeneously dispersed in a dispersion medium (e.g., water) to a specific solid content (e.g., the total mass of solid components divided by the sum of total mass of solid component and mass solvent used to achieve homogeneous dispersion during slurry mixing).

[0033] The slurry may then be coated on both sides of the current collector to achieve the target electrode loading (e.g., dry weight of all components per unit area - expressed in g / m2). The dispersion medium may be subsequently removed, e.g., by passing the coated current collector through a drying chamber. The electrode may be compacted to the target density, e.g., by a calendering process. The current collector can be aluminium foil with typical thicknesses in the range of 8 pm to 30 pm. The surface of the current collector may be treated to increase wettability of the current collector from slurry during coating. This also enables an increased adhesion of the coating once dried. Common surface treatment may involve chemical etching or coating a thin (0.5 pm to 2 pm thick) conductive carbon coating on the bare current collector surface, prior to coating with the slurry.

[0034] As will be made apparent by this disclosure, the density of the electrode coating can be adjusted by changing the type of active porous material used in the coated layer. Using CDC, a slurry with higher solid content than when using coconut carbon-based active material can be obtained. This allows for increased densities of the coated layer after coating and drying, thereby, also increasing the overall final calendered electrode density. The electrode densities described (typically 0.60 g / cm3for coconut carbon and about 0.75 g / cm3for CDC) can be reached regardless of electrode loading (e.g., up to 61 g / m2for coconut carbon and up to 82.5 g / m2for CDC).

[0035] The electrode density is determined by the following method.

[0036] The current collector is cut using a punch of defined area (A), e.g., 2 cm x 2 cm to obtain samples. In total, two samples are collected randomly over the width of the current collector. In case the current collector is coated, the samples are collected such that any side having a coating is completely coated. If, for example, the coating is only on one side of the current collector, then the sample is cut to have a completely coated side and a completely uncoated side. If, for example, the coating is on both sides of the current collector, then the sample is cute to have two completely coated sides.

[0037] The average thickness Ti of the samples is determined by thickness gauge that is capable of applying a predefined amount of pressure on the sample. The samples are measured at different points.

[0038] The weighing step is performed in a glove box with a protective atmosphere of dry air. The sample is dried at 120 °C for 10 min to exclude atmospheric humidity from weight. The average mass Mi of the samples is determined with an analytical scale having a resolution of 0.00001 g. After coating and calendering the current collector, another two samples are cut as previously described and measured in average thickness T2 and average weight M2 as previously described.

[0039] The electrode density is defined as the mass of the active material divided by its volume according to the following equation:

[0040] Slurry preparation - Example 1

[0041] Carbide-derived carbon (CDC) particulate material as obtainable by the method disclosed in EP 4 304 982 A1 is provided. The CDC particulate material and a first binder, namely sodium carboxymethyl cellulose (Na-CMC), are dispensed into a mixing vessel. The first binder has a dry proportion of 2 % dry and the CDC particulate material has a dry proportion of 96 % dry.

[0042] A dispersion medium, e.g., water is added until a base solid content of 40 wt% with respect to the total weight of the dispersion is achieved. High intensity mixing of the dispersion is performed.

[0043] After high intensity mixing, a second binder, namely styrene butadiene rubber (SBR), is added with a dry proportion of 2 % dry and low energy stirring of the dispersion is performed.

[0044] Before, during, or after low energy stirring, more dispersion medium is added to dilute the dispersion to a target solid content of 38 wt% with respect to the total weight of the dispersion.

[0045] Slurry preparation - Example 2

[0046] This slurry preparation is similar to Example 1 , but further involves varying the dry proportion of the CDC particulate material to 90 % dry, 92 % dry, 94 % dry, and 98 % dry. The sum of the dry proportions of the first and second binder are adapted to 10 % dry, 8 % dry, 6 % dry, and 2 % dry, respectively, wherein each binder has at least 1 % dry and at most 5 % dry. The remaining process is unchanged.

[0047] Slurry preparation - Example 3

[0048] This slurry preparation is similar to Example 1 , but further involves adding a carbon-based conductive additive, namely carbon black, with a dry proportion of 1 % dry. The dry proportion of the CDC particulate matter is reduced to 95 % dry. The remaining process is unchanged.

[0049] Slurry preparation - Example 4

[0050] This slurry preparation is similar to Example 3, but involves varying the dry proportion of the carbon based conductive additive to 3 % dry, 5 % dry, 8 % dry, and 10 % dry. The dry proportion of the CDC particulate matter is changed to 93 % dry, 91 % dry, 88 % dry, and 86 % dry with each binder remaining at 2 % dry. The remaining process is unchanged.

[0051] Slurry preparation - Example 5

[0052] This slurry preparation is similar to Example 3 and 4, but involves partially or completely replacing the carbon black with any of graphene, carbon nanotubes, or mixtures thereof. The remaining process is unchanged.

[0053] Electrode manufacture - Process 1

[0054] A current collector is provided in the form of an aluminium foil. The current collector is treated on both sides to improve wettability, e.g., by etching or by a thin carbon coating. One of the previously described slurries is selected and coated with a coating loading of 45 g / m2on each side. The coated current collector is dried. Subsequently, the coated current collector is fed into a calender that applies a contact force of at least 2.5 kN / mm up to 4.5 kN / mm, preferably 3.0 kN / mm to 4.0 kN / mm and a temperature of 75 °C to 100 °C. With these parameters a consistent thickness of the active material on each side of the current collector can be achieved reaching from 59 pm to 61 pm. The electrode density after calendering is about 0.75 g / cm3.

[0055] Electrode manufacture - Process 2

[0056] The electrode manufacture of Process 1 is repeated with the coating loading chosen to be 50 g / m2, 75 g / m2, and 82.5 g / m2The remaining process is unchanged. The electrode density after calendering is about 0.74 g / cm3to 0.75 g / cm3.

[0057] Ultracapacitor manufacture - Example 1

[0058] Two electrodes according to Process 1 or Process 2 are provided and rolled into a cylinder or rectangular shape together with a separator therebetween to form an electrode assembly. The electrode assembly is encapsulated in a housing and wetted with a suitable electrolyte. The electrode assembly gets electrically connected to a positive and a negative terminal to form the ultracapacitor.

[0059] Activated carbon from organic precursors - Comparative Example

[0060] The previously described processes are performed with the CDC particulate material being replaced with activated carbon material from organic precursors, in this case from coconut. The parameters are chosen to be as similar as possible to ensure a meaningful comparison. In particular, the slurry recipe was the same, such that the compared samples are made from the same recipe. It could not be avoided to reduce the contact force of the calender to a range from 1.5 kN / mm to 3.5 kN / mm, as at higher contact forces surface defects, such as surface cracking, delamination, or wrinkles, started to emerge. For these experiments the maximum coating loading was limited to 65 g / m2, although experiments show that up to 80 g / m2are possible without defects. Table 1 lists the experimental results for the Comparative Example: Table 2 lists the experimental results for the Process 1 and Process 2 samples:

[0061] The electrode density after calendering is determined after compacting and by excluding the weight of the current collector. A comparison between the different approaches shows that the typical electrode density according to the comparative example is around 0.6 g / cm3, whereas the electrode density can be increased to about 0.75 g / cm2with the ideas disclosed herein. Thus, there is about a 25 % increase in electrode density, which in turn allows a significant increase of the energy content of ultracapacitors manufactured according to the invention. As the size of the ultracapacitors can be kept the same, the energy density of the ultracapacitors can significantly increase.

Claims

Claims1. A method for producing a slurry for manufacturing an electrode of an ultracapacitor, the method comprising: a) dispensing a first binder material and a carbide derived carbon (CDC) particulate material, and optionally at least one additive material, into a mixing vessel; b) mixing the materials with a dispersion medium to obtain a dispersion that has a base solid content; c) mixing a second binder into the dispersion; and d) diluting the dispersion to a target solid content to obtain the slurry.

2. The method of claim 1 , wherein the first binder material includes or consists of carboxymethyl cellulose (CMC) or salts thereof, preferably sodium CMC.

3. The method of any of the preceding claims, wherein the additive material includes a carbon-based conductive additive, wherein the carbon-based conductive additive is preferably chosen from a group consisting of carbon black, carbon nanotubes, graphene, and mixtures thereof.

4. The method of any of the preceding claims, wherein the CDC particulate material has a skeletal density of at least 2.2 g / cm3, preferably of at least 2.25 g / cm3, preferably of at least 2.3 g / cm3, preferably more than 2.3 g / cm3, as determinedly means of helium pycnometry at constant volume.

5. The method of any of the preceding claims, wherein the components of the slurry have the following dry proportions (% dry): first binder: 1 % dry to 5 % dry; carbon based conductive additive: 0 % dry to 10 % dry;CDC particulate material: 85 % dry to 98 % dry; and second binder: 1 % dry to 5 % dry, wherein the dry proportion indicates the proportion of solid component of the respective component relative to all solid components in the slurry, wherein the dry proportions of the components are chosen such that the slurry consists of 100 % dry.

6. The method of any of the preceding claims, wherein in step b) the dispersion medium is added in an amount such that the dispersion has a base solid content of 40 wt% to 50 wt% with respect to the total weight of the dispersion.

7. The method of any of the preceding claims, wherein the second binder includes or consists of a latex binder, preferably polyacrylic acid (PAA) or styrene butadiene rubber (SBR).

8. The method of any of the preceding claims, wherein in step d) the dispersion is diluted to a target solid content of 38 wt% to 43 wt% with respect to the total weight of the dispersion.

9. A method for manufacturing an electrode for an ultracapacitor, the method comprising: a) performing a method according to any of the preceding claims to obtain a slurry; b) providing a current collector; c) coating at least one side of the current collector with the slurry to obtain a coating layer; d) compacting the coating layer to obtain the electrode.

10. The method of claim 9, wherein an electrode density obtained after compacting is 0.7 g / cm3to 0.8 g / cm3, preferably 0.74 g / cm3to 0.76 g / cm3, more preferably 0.745 g / cm3to 0.755 g / cm3, wherein the electrode density is the density of the compacted coating layer.11 . The method of claim 10, wherein in step b) the current collector is provided with a functional surface that increases wettability.

12. The method of claim 10 or 11 , wherein in step c) the at least one side is coated with a coating loading of 40 g / m2to 85 g / m213. The method of any of the claims 10 to 12, wherein in step d) compacting involves a contact force of 2.5 kN / mm to 4.5 kN / mm and / or applying heat to obtain a temperature during compacting of 70 °C to 110 °C.

14. A method for manufacturing an ultracapacitor, the method comprising: a) manufacturing an electrode according to a method of any of the claims 10 to 13; b) rolling the electrode together with a separator and another electrode to obtain an electrode assembly; c) encapsulating the electrode assembly together with an electrolyte and electrically contacting the electrode assembly to an positive terminal and a negative terminal, respectively.

15. A slurry obtainable by the method of any of the claims 1 to 9, or an electrode obtainable by the method of any of the claims 10 to 13, or an ultracapacitor obtainable by the method of claim 14.

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