Field of the Invention

The present invention relates to synthesis of graphene by microfluidization method and use thereof in potential anode, cathode and electrolyte active materials and additives in electrochemical energy storage devices.

Background of the Invention

Graphite has been widely used in the field of electrochemical energy storage in the recent years because of its layered structure and relatively low price. Graphene, which is a single layer of graphite, is considered as a promising candidate for use in electrochemical energy storage applications.

Graphene can be synthesized by various methods such as exfoliation and cleavage, CVD, thermal decomposition, ultrasonication, ultracentrifugation and microfluidization. Among the said methods, microfluidization is high shear-rate treat of water or alcohol suspensions of starting material. With the help of extremely high shear rate, both size reduction and expansion of layer can be seen.

The microfluidization method is performed by high shear-rate treat of water or alcohol suspensions of starting material. As it is used in obtaining graphene, the method is capable of reduction of both the plate size and the particle size simultaneously by means of the high shear rate which is a characteristic feature of the method. The microfluidization method and the features of the products obtained by this method are schematically shown in Figure 1. A prior art patent discloses the potential of use of graphenes, which are obtained by starting from expandable graphite starting material using microfluidization method such that the plate size is reduced and the distance between the plates is increased, in Li-ion battery applications.

The United States patent document numbered US20110195308A1, known in the state of the art, discloses size reduction and grounding steps performed by the microfluidization device produced by Microfluidics Corp and the process of mixing the Li-ion battery materials.

Chinese patent document numbered CN106159199, known in the state of the art, discloses a method of preparing three-dimensional graphene electrode and use thereof in energy storage applications. The three-dimensional graphene electrode is applied to a high-density lithium- sulfur battery, a high-density lithium-silicon battery, a high-density super capacitor, a high-density Faraday capacitor and a high-density battery capacitor, and can be applied to a high-working-voltage and high-energy- storage-density battery capacitor.

The United States patent document numbered US2012045688, known in the state of the art, discloses an electrochemical energy storage device and lithium super batteries comprising a positive electrode, a negative electrode, a porous separator disposed between the two electrodes, and a lithium-containing electrolyte in physical contact with the two electrodes. In the said invention, nanosized graphene has been obtained from graphite particles by exfoliation method. The lithium super-battery of the said invention exhibits a gravimetric energy 5 times higher than conventional supercapacitors and a power density 10 times higher than conventional lithium-ion batteries.

Expandable graphite used in the art is one of the precursors used for synthesizing graphene. It is a common and cost-effective material. However, the synthesis of graphene is hard and costly as it is time-consuming and requires relatively high temperatures. Conventional methods such as ball milling are more likely to create micron or nanosized few layer graphenes instead of single layer nanosized graphene and the said methods are relatively expensive.

Summary of the Invention

The objective of the present invention is to provide a single layer and nanosized graphene by microfluidization method.

Another objective of the present invention is to use the obtained graphene in anode, cathode and electrolyte active materials and additives in electrochemical energy storage devices.

A further objective of the present invention is to be able to produce high energy capacity electrode and/or electrolyte materials in the field of electrochemical energy storage at low cost.

Another objective of the present invention is to enable graphene to exhibit high performance at high charge-discharge rates since it is synthesized by microfluidization method.

Detailed Description of the Invention

“Graphene synthesis by microfluidization method” developed to fulfill the objectives of the present invention is illustrated in the accompanying figures, in which:

Figure 1 shows the SEM images of expandable graphite powders after 4 cycle microfluidization.

Figure 2 is the graphic of the first three charge-discharge curves of 4 cycle microfluidized expandable graphite (Cycle rate: 500 mA/g). Figure 3 is the graphic of the charge-discharge capacities of 4 cycle microfluidized expandable graphite (Cycle rate: 500 mA/g) for the first 10 cycles. Figure 4 is the graphic of the Coulombic Efficiencies of 4 cycle microfluidized expandable graphite (Cycle rate: 500 mA/g).

The present invention is a graphene synthesis method by microfluidization method for being used in high energy capacity electrode and/or electrolyte materials in the field of electrochemical energy storage, and comprises the following steps:

- purchasing expandable graphite of 30 pm thickness and 300 pm diameter,

- sieving by a sieve with 63 pm mesh size,

- weighing 1 g of expandable graphite,

- dispersing it in 400 mL 2-propanol to prepare a suspension,

- conducting ultrasonication for 1 hour at 20 kHz frequency and 40% amplitude,

- pouring the suspension into the microfluidizer, and

- performing 4 cycles.

Expandable graphite of 30 pm thickness and 300 pm diameter was purchased and sieved through a sieve with 63 pm mesh size. 1 g of expandable graphite was weighed and dispersed in 400 mL 2-propanol to prepare a suspension. Then Ultrasonication was conducted for 1 hour at 20 kHz frequency and 40% amplitude. The resulting suspension was poured to the Microfluidizer (Microfluidics Corp.) for performing the 4 cycles. 290 MPa pressure was applied during the cycles.

Microstructural changes of the expandable graphite obtained as a result of 4 cycles of microfluidization have been observed by Scanning Electron Microscope (Zeiss SUPRA 50VP). Before the analyses, the samples were sputtered with Au- Pd sputter coating technique to ensure adequate electronic conductivity. Figure 1 shows the Scanning Electron Microscope (Zeiss SUPRA 50VP) images.

Electrode slurries were prepared by dissolving 0.1 gram of Polyvinylidene Fluoride (PVdF) as a binder in 4.0 mL N-Methyl Pyrollidone (NMP) solvent. Then, expandable graphite processed by the microfluidizer was added as the active material and mixed in a planetary grinder for 1 hour at 200 RPM rotation speed. The active material binder ratio was selected to be 90:10. The prepared slurries were tape-casted on copper (Cu) foil and dried in a vacuum oven at a temperature of 80°C. Afterwards, electrodes were punched with a diameter of 16 mm, and pouch-type half-cells were assembled. Lithium (Li) foil was used as a counter electrode in the half cells. 1M LiPFe in EC:DMC (1:1) (1 M Lithium Hexafluoro Phosphate dissolved in Ethylene Carbon-Diethyl Carbonate mixture at a ratio of 1:1) was used as the electrolyte.

The charge-discharge curves were obtained between 0.02 - 2.00 V from BasyTec multichannel battery test system at 500 mA/g cycle rates. The obtained potential values were provided with respect to Li/Li-i- reference.

As can be seen in Figure 2, after 4 cycle microfluidization process, the particle sizes of the expandable graphites are from a few nanometer up to 5pm. In this sense, it can be stated that both size reduction and thinning mechanism were seen after 4 cycle microfluidization.

According to the cycle curves formed with the cells prepared by the powders obtained after 4 cycle microfluidization, shown in Figure 3; at relatively higher cycle rates such as 500 mA/g, the specific discharge capacity of 127.1 mAh/g was obtained. At the end of 3 cycles, specific discharge capacity of 125.0 mAh/g was maintained. The plateau below 0.1V and 0.02V corresponds to the staging mechanism which occurs due to Lithium intercalation within carbonaceous materials.

The capacity value for the first 10 cycles at a cycle rate of 500 mA/g given in Figure 4 shows that the specific discharge rate of 117.9 mAh/g is maintained. At the end of 10 cycles, the total capacity decrease is limited to 7%.

The Coulombic efficiencies of 4 cycle microfluidized expandable graphite are given in Figure 5 for the first 10 cycles. It was observed that at a cycle rate of 500 mA/g, Coulombic efficiencies gradually increased. After 10 cycles, Coulombic Efficiency reached to 94.7%.

The method of the invention, wherein high shear-rate techniques such as microfluidization are used, enables to obtain graphene and to reduce size of the obtained graphene as well as increasing the distance between the layers simultaneously by using expandable graphite.

Since the method of the invention is semi-continuous, it is feasible to adapt for mass-production. Thus the costs will be reduced.

In addition, graphene synthesized by microfluidization method using an original raw material such as expandable graphite for electrochemical energy storage shows high performance at high charge/discharge rates.