WO2021206651A1

WO2021206651A1 - An anode electrode comprising graphene for use in energy storage units and production method thereof

Info

Publication number: WO2021206651A1
Application number: PCT/TR2020/051343
Authority: WO
Inventors: Edip BAYRAM; Zehra Kuru; Cağdaş KIZIL
Original assignee: Yueksel Tohum Tarim Sanayi Ve Tic AS
Current assignee: Yueksel Tohum Tarim Sanayi Ve Tic AS
Priority date: 2020-04-06
Filing date: 2020-12-21
Publication date: 2021-10-14
Anticipated expiration: 2022-10-06

Abstract

The invention relates to an electrode coating unit (1) comprising an anode electrode (1.1), a cathode electrode (1.2), a solution (1.3) comprising a predetermined amount of charged graphene particles, and an electrode which has mobility due to an electrical field formed as a result of a potential difference applied to both the anode electrode (1.1) and the cathode electrode (1.2), has the capacity thereof increased by coating the graphene particles (1.4) on a positively-charged copper foil due to the negative charge of the cathode electrode ( 1.2) and by reducing the same, and is to be used as an anode in the Li-ion batteries.

Description

AN ANODE ELECTRODE COMPRISING GRAPHENE FOR USE IN ENERGY

STORAGE UNITS AND PRODUCTION METHOD THEREOF

Technical Field

The invention relates to a coating unit comprising an anode electrode with graphene for use in rechargeable energy storage units, and to a method of producing said anode electrode.

Prior Art

An increase in energy requirement and mobility with the developing technology has led to a rapid increase in interest in electrochemical energy storage systems such as Li-ion. A rechargeable Li- ion battery generally consists of a cathode, an anode, an electrolyte, and a separator. While the Li- ion battery is discharged, the Li + ions released by oxidation at the anode move along the electrolyte towards the cathode and are reduced at the cathode. An exact opposite situation to that process takes place during the battery charging process. The electroactive materials of the anode and the cathode should be coated on the current collectors in order to collect the electrons released during the charge/discharge cycle of the battery. Mostly, a thin aluminum at the cathode and a copper foil at the anode are used as a current collector in commercial Li-ion batteries in the state of the art. The separator consists of an electrically insulating material and avoids short circuit between the cathode and the anode while allowing the transition of the Li⁺ ions during the charging/discharging process.

A metallic lithium was used as an anode electrode due to a high lithium storage capacity in the first Li-ion batteries produced in the state of the art. However, the cycle life of a battery is decreased rapidly as the insulating dendritic structures are formed on the surface of the metallic lithium during charging/discharging process, and this eventually causes a short circuit between the cathode and the anode. Further, a highly exothermic reaction of the metallic lithium with the components of the air has also caused safety issues in these batteries.

Silicon (Si), which is used in the batteries in the state of the art, is known to have the highest capacity following the metallic lithium as an anode material. On the other hand, a volume change of up to 400% during charging/discharging process in silicon-based anodes causes the silicon anode to break apart, the battery to swell, and the life of the battery to decrease. Tin (Sn), which is used in the batteries in the state of the art, may be used in Li-ion batteries as an anode with a high capacity (980 mAh/g) which may form an alloy with lithium. However, the fact that tin-based anode production has some difficulties and that tin has a high charging potential due to a low electrical conductivity thereof causes electrolyte and electrodes to break down, the cycle life to shorten, and the costs to increase.

Graphite is a material which is commonly used in commercial Li-ion batteries in the state of the art as an anode, is available naturally, and has a relatively low cost. Graphite is a carbon allotrope formed by tight stacking of planar graphene plates. It is preferable as it has a high electrical conductivity, long cycle life, and a low cost, although it has a low anode capacity (372 mAh/g) as compared to the other anode materials.

On the other hand, graphene is planar carbon plates forming graphite and theoretically has an anode capacity of 744 mAh/g. Because of this characteristic thereof, graphene has become prominent as the most likely anode material to be used instead of graphite in the state of the art. In the production of graphene on an economical and industrial scale, Hummers or modified Hummers, which are the method of producing graphene from graphite, are used, and the resulting product is called graphene oxide (GO). Therefore, functional groups containing great quantities of oxygen such as carbonyl, carboxylic and lactonic, which reduce the quality of graphene, are formed in the graphene plates produced, and consequently, this causes low capacity and cycle life in case that it is used in Li-ion batteries as an anode. This is eliminated by implementing various chemical and/or thermal reduction processes before or after coating graphene on the copper foil in the casting and spray methods. In the state of the art, methods need to be developed, which do not include additional reduction steps and thus, are more advantageous.

In the state of the art, casting, spray coating, dip coating, and spin coating methods are used for coating the anode active materials on the current collectors. Among these methods, the most widely used method is the casting method due to its suitability for industrial mass production. In this method, a mixture having an appropriate fluidity of anode active material, a binding polymer providing a mechanical strength, and a conductivity enhancing material is cast on the copper foil and pressed with rollers until it reaches a certain thickness. The liquid mixture formation and pressing processes which are applied during the coating of graphene on the copper foil using the casting method cause the graphene layers to approach each other and to form graphite, and consequently the anode capacity decreases.

On the other hand, the coating thickness is usually adjusted using a hot press in the spray coating method, which is another method. This causes the graphene plates to be compressed so as to form graphite, and consequently the anode capacity decreases. Spray coating method is also a disadvantageous method as it causes clogging of fine spray needles during the mass production.

On the other hand, a polymer binder of 2% to 10% should be used in order to provide mechanical strength both in casting and spray coating methods. These, on one hand, increase the mass of the anode, but on the other hand, decrease the electrical conductivity and cause the anode capacity to decrease. In these methods in the state of the art, this disadvantage should be eliminated without a polymer binder.

Brief Description of the Invention:

The object of the invention is to achieve an electrode coating unit comprising graphene for use in rechargeable energy storage units, and a method of producing said anode electrode.

The capacity of the anode electrode produced using the method of the invention is greater than the capacity of the anode electrode produced using the methods in the state of the art.

The graphene concentration in the solvent used in the mixture prepared by the invention is low in order to prevent a graphitic structure from being formed by aggregating the graphene plates, and is 0.2 to 1.0 mg graphene/cm³. Thus, graphene is prevented from forming a graphitic structure during the process and thereby, the anode capacity is increased.

In the invention, various chemical and/or thermal reduction processes are not applied for the anode electrode obtained before or after the coating of graphene on the copper foil, i.e. said coating process does not include an additional reduction step. Therefore, the method of the invention is very advantageous in terms of coating time and coating cost.

An adjustment using hot press in the coating thickness obtained during the casting and spray coating methods is made by setting the EPD parameters in the invention. In other words, said adjustment may be made using the graphene concentration, the charging salt concentration, the voltage applied and the inter-electrode distance parameters. Thus, the coating thickness and the amount of material coated on a unit surface may be adjusted by setting the EPD parameters. The amount of anode active material at which an optimum anode capacity is obtained with the present invention is 0.4 to 2.0 mg graphene/cm² copper. Above 2.0 mg graphene/cm², the electrical conductivity decreases as the electrode thickness increases, and below 0.4 mg graphene/cm², the total energy storage capacity decreases as Li storage capacity per unit copper mass decreases. Thus, the range selected is 0.4 to 2.0 graphene/cm² copper.

On the other hand, the anode capacity is increased to provide a mechanical strength using the method of the invention without any polymer binder. Binders cannot store Li as they are not electroactive. No use of them at the anode increases the rate of electroactive material per unit anode mass, thus increasing the anode capacity. The mechanical strength may be achieved in the present invention without using a binder.

On the other hand, the disadvantage of battery swelling in the state of the art is eliminated by using graphene in the invention. The sponge-like porous structure of the graphene may tolerate volume expansion during the charge/discharge process.

The electrode coating unit of the invention comprises an anode electrode, a cathode electrode, a solution comprising a predetermined amount of charged graphene particles, and an anode electrode which has mobility due to an electrical field formed as a result of a potential difference applied to both the anode electrode and the cathode electrode, and has the capacity thereof increased by coating the positively-charged graphene particles on a copper foil due to the negative charge of the cathode electrode and by reducing the same.

The preferred embodiment of the electrode produced by the electrode coating unit is a Li-ion battery.

Description of the Figures

Figure 1 is a symbolic view of a storage unit cell and the components thereof.

Description of the References in the Figures

In order to better understand the invention, the numbers in the figures are given below: 1. Coating unit

1.1 Anode electrode

1.2 Cathode electrode

1.3 Solution

1.4 Graphene particle

D. Distance V. Voltage source 100. Method

Detailed Description of the Invention:

The coating unit (1) of the invention comprises an anode electrode (1.1), a cathode electrode (1.2), and a solution (1.3) comprising charged graphene particles (1.4) which are homogenously distributed in an aprotic solvent between said anode electrode (1.1) and the cathode electrode (1.2).

In the invention, a voltage source (V) is used, which applies a potential at a predetermined voltage value to the anode electrode (1.1) and the cathode electrode (1.2). The potential applied is 70 V to 150 V.

The anode electrode (1.1) is of metal, and preferably stainless steel sheet, and the anode electrode (1.1) is positively charged.

In the cathode electrode (1.2), the graphene particles (1.4) are coated on the copper foil using the voltage applied, and the cathode electrode (1.2) is negatively charged.

The invention consists of graphene particles (1.4) coated on an electrical conductor, preferably copper foil, in the cathode electrode (1.2) to be used in the energy storage unit, preferably in Li- ion batteries. A cathodic EPD method is used to produce said cathode electrode (1.2). Electrophoretic deposition (EPD) is a method based on the movement of the charged molecules or particles under the electric field strength formed by the voltage difference. In the cathodic EPD method, the graphene particles (1.4), on the one hand, are allowed to be deposited on the copper foil in the cathode electrode (1.2) due to the negative charge of the cathode electrode (1.2), while the deposited graphene particles (1.4), on the other hand, are reduced. A method (100) of producing an anode electrode comprising graphene particles (1.4) for use in energy storage units, preferably in Lithium-ion batteries, comprises the following method steps: preparing a suspension by dissolving the graphene particles (1.4) with a predetermined concentration in solvents such as isopropyl alcohol (IPA) and/or ethanol and/or methanol and/or N-methylpyrrolidone (NMP) (101) allowing the positively-charged particles to be deposited on the negatively-charged cathode electrode (1.2) and to be coated on the cathode electrode (1.2), as a result of a positive charging of the graphene particles (1.4) by adding Mg(N0₃)₂ into the mixture in the predetermined ratios (102) coating a copper foil on the cathode electrode (1.2) in an EPD cell, and with cathode electrode (1.2) positioning the anode electrode (1.1) with respect to the cathode electrode (1.2) so as to be at a predetermined distance (D) (103) coating the charged graphene particles (1.4) on the cathode electrode (1.2) by directing towards the cathode electrode (1.2) under an electrical field caused by the application of a potential difference of a predetermined value both to the anode electrode (1.1) and cathode electrode (1.2) using a voltage source (V) (104).

In the cathodic EPD method, the graphene particles (1.4), on the one hand, are allowed to be deposited on the copper foil in the cathode electrode (1.2) due to the (-) charge of the cathode, while the deposited graphene particles (1.4), on the other hand, are reduced. Said reduction is achieved without additional reduction steps such as various chemical and/or thermal reduction processes before or after the coating process.

In step 101, said concentration is 0.05 to 5 mg graphene/cm³, preferably 0.2 to 1.0 mg graphene/cm³.

The solvents used in step 101 are aprotic solvents and do not undergo electrolysis which causes gas evolution at high potentials. Thus, a homogenous and mechanically durable coating process is performed. No polymer binder is added to the mixture in step 101. Thus, the disadvantages caused by the polymer binder are eliminated.

In the suspension prepared in step 102, the graphene particles (1.4) are negatively charged due to the decomposition of the oxygen functional groups thereon. That is, the remaining carboxylate groups are negatively charged as the naturally occurring carboxyl groups on the surface of the graphenoxide have their hydrogen decomposed in the solvent. The graphene particles ( 1.4) become positively-charged graphene particles (1.4) due to the positive charge of magnesium as a result of Mg(N0₃)₂ addition to the mixture. In step 102, the ratio of said Mg(N0₃)₂ is in the range of 2.0 to 8.0 mg/solvent. Electrodes produced by the current method apart from this range do not function as an anode electrode in Li-ion batteries.

In step 103, the distance between the cathode electrode (1.2) and the anode electrode (1.1) is 0.5 to 2.0 cm.

In step 104, said potential difference is 70 to 150 V. In case that a lower potential difference is applied, a low amount of coating is obtained, and the mechanical strength is weaker, but when a higher potential difference is applied, graphitization occurs as a result of the tight stacking of planar graphene plates, thereby the capacity of the anode electrode (1.1) is reduced.

The graphene particles (1.4) may be coated on the copper bilaterally in the method (100).

Electrophoretic deposition (or electrophoresis) method (EPD) is a method which allows the charged particles in a suspension to hold onto the surface by directing them to the conductive electrodes under the effect of an electric field between the conductive electrodes charged with a certain potential. The positively (+) charged particles move towards the negative (cathode) electrode, while the negatively (-) charged particles move towards the positive (anode) electrode. Material deposition at the electrode occurs by the particle coagulation. First, stable suspensions consisting of the charged particles of the material to be coated in an appropriate solvent should be prepared in order to perform the EPD technique. The parameters such as the amount of material in the suspension, the type and charge of the solvent, the distance between the electrodes, the potential applied, and the duration of application influence the characteristics of the coating obtained by the EPD. The duration of the application is 2 to 100 minutes, preferably 10 minutes.

In Table-1, the values of the zeta potential of the graphene particles obtained by adding Mg(N0₃)₂ of varying concentrations to the 0.5 mg/cm³ of graphene/IPA mixture with a pH of 5.8 are provided. The zeta potential is an indicative of the electrical charge on the surfaces of the colloidal particles in the solution. The value of zeta potential of the graphene particles is measured to be - 19.3 mV in 0.5 mg/cm³ of pure graphene/IPA mixture with a pH of 5.8. An increase of this value above 27 mV after addition of Mg(N0₃)₂ to the mixture indicates that the graphene particles (1.4) are charged with a (+) charge and that the particles will move towards the cathode electrode (1.2) (-) under the electrical field. As may be seen from the table, the concentration of Mg(N0₃)₂ is kept in the range of 0.2 to 8 mg/cm³ as it has been proven that the zeta potential and thus, EPD efficacy are reduced when the value of 8 mg/cm³ is exceeded. That is, the optimal range for the concentration of Mg(N0₃)₂ is 0.2 to 8 mg/cm³.

Table 1. values of the zeta potential of the graphene particles (1.4) obtained by adding Mg(N0₃)₂ of varying concentrations to the 0.5 mg/cm³ of graphene/IPA mixture with a pH of

5.8

On the other hand, the application of the graphene -based anode produced by the present invention in Li-ion battery is shown in Graph- 1. Graph 1(a) represents the first 5 charge/discharge cycle of the anode electrode produced by coating the graphene particles (1.4) on LiFeP0₄ as the cathode electrode, 1.0 M LiPF₆ dissolved in ethylenecarbonate/dimethylcarbonate (1:1 v/v) as the electrolyte, a polypropylene membrane as the separator and copper foil as the anode electrode using the casting method in a Li-ion battery. Graph (b) represents the first 5 charge/discharge cycle of CR2032-type Li-ion battery at a density of 0.5 A/g current, in which an anode electrode (1.1) produced by coating of the graphene particles (1.4) using the cathodic EPD (100) method is used. The graphene particles (1.4) produced in the same way was used in both coating methods (a and b). The initial discharge capacity of the anode electrode produced by the casting method was determined as 385 mAh/g graphene. This value has a graphene capacity of 960 mAh/g for the anode electrode (1.1) produced by coating the same using the cathodic EPD method and may be used in Li-ion batteries.

Therefore, the anode capacity of the graphene particles (1.4) was increased 2.5 times by the cathodic EPD method.

Graph 1

Industrial Applicability of the Invention:

The invention has been developed for the use of an anode electrode with increased capacity in the energy storage units and is applicable to the industry.

The invention is not limited to the above examples, and a technical person can easily suggest different other uses of the invention. These should be considered within the scope of the invention.

Claims

1. An electrode coating unit (1) comprising an anode electrode (1.1), and a cathode electrode (1.2), characterized in that it comprises a solution (1.3) comprising a predetermined amount of charged graphene particles, and an electrode which has mobility due to an electrical field formed as a result of a potential difference applied to both the anode electrode (1.1) and the cathode electrode (1.2), and has the capacity thereof increased by coating the positively-charged graphene particles (1.4) on a copper foil due to the negative charge of the cathode electrode (1.2) and by reducing the same.

2. An electrode coating unit (1) as claimed in claim 1, characterized in that an anode electrode with a graphene capacity of 960 mAh/g may be generated, which will be used in Li-ion batteries.

3. An electrode coating unit (1) as claimed in claim 2, characterized in that it is a Li-ion battery.

4. A method of producing an anode electrode (100), comprising graphene particles for use in energy storage units, preferably in Lithium-ion batteries, characterized in that it comprises the following steps: preparing a suspension by dissolving the graphene particles (1.4) with a predetermined concentration in solvents such as isopropyl alcohol (IPA) and/or ethanol and/or methanol and/or N-methylpyrrolidone (NMP) (101) allowing the positively-charged particles to be deposited on the negatively- charged cathode electrode (1.2) and to be coated on the cathode electrode (1.2), as a result of a positive charging of the graphene particles (1.4) by adding Mg(N03)2 into the mixture in the predetermined ratios (102) coating a copper foil on the cathode electrode (1.2) in an EPD cell, and with cathode electrode (1.2) positioning the anode electrode (1.1) with respect to the cathode electrode (1.2) so as to be at a predetermined distance (D) (103) coating the charged graphene particles (1.4) on the cathode electrode (1.2) by directing towards the cathode electrode (1.2) under an electrical field caused by the application of a potential difference of a predetermined value both to the anode electrode (1.1) and cathode electrode (1.2) using a voltage source (V) (104).

5. A method (100) as claimed in claim 4, characterized in that the concentration of said graphene particles (1.4) in step 101 is 0.05-5 mg graphene/cm³, preferably 0.2 to 1.0 mg graphene/cm³.

6. A method (100) as claimed in claim 5, characterized in that said Mg(N0₃)₂ ratio in step 102 is 2.0 to 8.0 mg/solvent.

7. A method (100) as claimed in claim 6, characterized in that the distance (D) between the cathode electrode (1.2) and the anode electrode (1.1) in step 103 is 0.5 to 2.0 cm.

8. A method (100) as claimed in claim 7, characterized in that said potential difference in step 104 is 70 to 150 V.

9. A method (100) as claimed in claim 8, characterized in that the graphene particles (1.4) are able to be coated on the copper bilaterally.