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2018-01-06

Design Of Experiments- Synthesizing A Graphene-Based Battery Electrode




Above- Plasma Exfoliated Graphene Nanoplatelets

The conversion of graphene and metal-based inorganic compounds into usable graphene composites for electrodes can be done by many methods. As this new field is advancing, new methods are frequently being invented and subsequently published. To date they include ex-situ hybridization, in-situ crystallization, chemical reduction, electroless deposition, sol-gel methods, hydrothermal methods, electrochemical deposition, thermal evaporation and in-situ self-assembly, to name a few of the most common classes utilized.
As with any method, there are always multiple ways to synthesize the material itself. To cover each one would be impractical, so here we look at some specific ways that you can implement graphene into composite materials for use as electrodes in your graphene battery R&D project. The values used are from published experiments and are used for ratio illustrative purposes only. The amounts and scale can be varied to better suit specific experiments.
Below is a DOE for Graphene-Lithium-Sulphur batteries, a current leading technology.
Synthesis of the Thermally Exfoliated Reduced Graphene. Thermally exfoliated reduced graphene was obtained.
Preparation of Graphene-Sulphur Hybrids- The G/S hybrids were prepared by hydrothermal reduction assembly of GO with a sulfur-dissolving CS2 and alcohol solution. In brief, 50 mL of the GO aqueous dispersion and 15 mL of alcohol were mixed, and then 3 mL of CS2 containing 100, 150, and 200 mg of dissolved sulfur (tuning the sulfur content in the samples) was added to the GO dispersion. The mixture was stirred for 90 min and then sealed in an 80 mL Teflon-lined stainless steel autoclave for a hydrothermal reaction at 180 C for 10 h. The black cylinder of the G/S hydrogel was washed by ethanol and distilled water, and the wet hydrogel was then freeze-dried to obtain the G/S hybrids.
Preparation of Graphene-Sulphur Hybrids (Powder)- G/S hybrids (powder) were prepared by mixing 90 mg of intercalation-exfoliated graphene and thermally exfoliated reduced graphene with 150 mg of sulfur under the same hydrothermal conditions as the G/S hybrids.
Preparation of Graphene-Sulphurmix The G/Smix was prepared by mixing- 50 mL of the GO aqueous dispersion, 15 mL of alcohol, and 150 mg of sulfur under the same conditions but without CS2.
Electrochemical Measurements. The G/S hybrid was cut, compressed, and shaped into a circular pellet with a diameter of 12 mm and directly used as a cathode. The mass loading of a G/S electrode was about 2 mg cm2. The G S59 or G S60 hybrid (powder) cathode was prepared by mixing 90 wt % G S59 or G S60 hybrid (powder) with 10 wt % polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone as a binder to form a slurry, which was then coated on an aluminum foil and dried under vacuum at 70 C for 12 h. The foil was pressed between twin rollers, shaped into a circular pellet with a diameter of 12 mm, and used as a cathode. The electrolyte was 1.0 M lithium bis-trifluoromethanesulfonylimide in 1,3-dioxolane and 1,2-di- methoxyethane (1:1 by volume) with 0.5 wt % LiNO3 additive. A 2025 type stainless steel coin cell was used to assemble a test cell. A lithium metal foil was used as the anode, and a G/S slice as the cathode. A LAND galvanostatic charge discharge instrument was used to perform the measurements. The coin-type cell was assembled in an Ar-filled glovebox (MBraun Unilab). The current density set for cell tests was referred to the mass of sulfur in the cathode and varied from 0.3 to 4.5 A g-1. The charge discharge voltage range was 1.5 2.8 V. The CV was measured using a VSP-300 multichannel potentiostat/galvanostat (Bio-Logic, France) workstation in the voltage range 1.5 2.8 V (vs Li/Li) at a scan rate of 0.1 mV s-1. The G S63 hybrid electrode was discharged to the end of the second plateau and disassembled, dried in the glovebox, and followed by transferring to the vacuum chamber of XPS for structure characterization.1
Below are 3 more types of Graphene Electrode DOEs
To create a pure graphene-based electrode, disperse graphene oxide powder (100 mg) in distilled water (30 mL) and sonicate for 30 minutes. Heat the resulting suspension on a hot plate until it reaches 100 C and add hydrazine hydrate (3 mL). Keep the suspension at 98 C for 24 h to reduce the graphene oxide to rGO. The reduced graphene oxide can be collected by filtration to leave a black powder. Wash the filtrated powder several times with distilled water, so that the excess hydrazine is removed. Re-disperse the graphene powder into water by sonication. Centrifuge the solution (4000 rpm, 3 minutes) to remove the larger particles. Collect the graphene by vacuum filtration and dry in a vacuum. If you have purchased rGO, then this step can be skipped. To create the electrode, disperse graphene in ethanol until a concentration of 0.2 mgmL-1 is achieved. Filter the suspension by vacuum filtration and collect on the microporous filter paper. Cut the filtered graphene into 1 x 2 cm2 (1 mg weight), so that it is ready for use. Attach it to a cell, with an electrolyte buffer, to test the graphene electrode.
This second method details the preparation of a cobalt-graphene hybrid electrode, for use as an electrode in lithium-ion batteries. To prepare the electrode, add graphene oxide (0.1 g) to cobalt acetate (350 mg) and deionised water (400 mL). To the solution add NH4OH (3800 L) and hydrazine (250 L) and stir for 4 h at 100 C. Filter the solution once the reaction has finished. Re-crystallize the solution by heating the product for 6 h at 200 C.
This final method is for creating a tin-graphene nanoribbon composite electrode, for use in lithium-ion batteries. To create the electrode, add graphene nanoribbon (GNR) (75 mg), SnCl2.H2O (1.33g, 5.89 mmol), 2-pyrrolidinone (65 mL) and a magnetic stirrer bar to a dried round-bottom flask. Sonicate the solution for 20 minutes, followed by refluxing for 1 h. Cool the vessel to room temperature and sonicate overnight in an open-air environment. Quench the mixture with acetone and water three times and filter over a PTFE membrane (0.45 m). Dry in a vacuum (60 C) for 24 h and anneal in a quartz furnace (500 C, Ar atmosphere) for 2h. The theoretical yield is 380 mg.

Graphene Products Available

Cheap Tubes Inc offers various graphene-based products that can be implemented into electrode formulations. Graphene Nanoplatelets (GNPs) offer some of the best properties for battery applications. The GNPs can be used to replace other carbon-based materials. GNPs have excellent electrical and thermal conductivity, mechanical stability and can provide a composite with enhanced conductivities, mechanical strength, and lower gas permeation. Our research grade graphene nanoplatelets are produced by plasma exfoliation. Our plasma exfoliation process produces high-grade GNPs which have less defects and a higher internal conductivity. The GNPs consist of several graphene layers and generally have a thickness between 3-10 nm and are friable with high shear methods such as a 3 roll mill or homogenizer. The GNPs are available with varying functional groups, including nitrogen-based, oxygen-based, fluorine and amine groups. Non-functionalised GNPs (argon processed) are also available. The production methods are scalable, so GNPs can be implemented in large volume and larger scale applications.
Graphene Oxide (and reduced graphene oxide) is available as a powder, a dispersion, or a spin coated film. The elemental composition of Cheaptubes graphene oxide consists of 35-42% carbon, 45-55% oxygen and 3-5% hydrogen. Dispersions can be provided in various solvents and in a range of concentrations. We also offer graphene oxide (and reduced graphene oxide) films and coatings. A graphene oxide film comprised of our single layer graphene oxide product spin coated on glass provides a 5-20 nm final thickness. They also possess a conductivity in the range of 104-105 Sm-1, with a sheet resistance of 101-103 Osq-1. In contrast, a single (flexible) graphene sheet on a flexible organic substrate provides the same thickness and area, but it provides a conductivity in the range of 103-104 Sm-1 and a sheet resistance of 102-104 O sq-1.
CVD Graphene Films are also available. Graphene films that have been grown on a range of substrates including silicon, copper, PET and quartz, are available. They can also be transferred to a variety of customer supplied substrates if they are compatible with the solvents used to float the graphene off the substrate so it can be redeposited onto your substrate.
As an alternative to graphene, conductive carbon nanotube composite additives can be used. This product has been specifically designed to improve the efficiency of lithium-ion electrodes. As a blend of both nanotubes and carbon black, it improves the tap density (up to 10%) and capacitance retention of the electrode without decline after multiple charge/discharge cycles. The additive contain multi-walled carbon nanotubes blended with a proprietary carbon black. This additive can be used in both the anode and cathode within a battery cell. Generally 2-3 wt% is used in a cathode and 1-2 wt% in an anode.
Graphene-based batteries are quickly becoming comparable, in terms of efficiency, to traditional solid-state batteries. They are advancing all the time and it wont be long before they surpass their solid-state predecessors. The extra benefits associated with graphene being present in the electrodes can be useful, even if the efficiency isnt as high. For batteries that possess a similar efficiency, graphene batteries are an ideal choice, which is why scientist are trying to further advance this class of batteries. They have started to gain traction in the commercial marketplace and it wont be long before they become the norm and phase-out solid-state batteries. To quote recent forecasts the world graphene battery market is expected to reach $115 million by 2022, growing at a CAGR of 38.4% during the forecast period. The automotive industry is estimated to dominate the market throughout the analysis period. Geographically, Europe is expected to be the leading market in 2016, with a revenue contribution of around 38%.
With increasing energy demands globally, improving energy storage devices while reducing negative environmental impacts related to consumer based battery usage is a noble goal and one that we emphatically support. We hope that this guide has helped you to understand the current graphene battery research trends and inspired you to begin graphene battery development.