INTERNATIONAL JOURNAL OF ELECTRIC AND HYBRID VEHICULES
I.J.E.H.V.
contact : b.maisseu@ehcar.net
I.J.E.H.V.
Conferences
Congress - Meeting
Library
News
News Records

2018-01-05

How to make Graphene Batteries




The molecular structure of a graphene sheet

Want to learn how to make Graphene Batteries? Our Graphene Battery Userís Guide, which has been created for scientists and non-scientists alike, details how graphene batteries work, their benefits, and provides immediate, actionable steps that you can take to begin developing your own graphene battery. Donít miss out on the next phase of nano evolution. Our Graphene Battery Userís Guide includes four well established graphene electrode designs of experiments (DOE) from cutting edge academic research including one for a Graphene-Lithium-Sulphur battery, the current front runner technology, and are included for reference
Graphene-based batteries are being actively researched for many commercial applications. The improved performance and life cycle advantages when developing graphene-based batteries over traditional metal-ion batteries are well worth the resource investment. Elon Muskís Tesla Motors provides a famous example of innovative companies actively pursuing graphene battery research and commercialization. We believe that the real graphene battery breakthroughs will be from graphene-lithium-ion hybrid chemistries incorporated into the cathodes of lithium-sulfur cells. This type of technology is still years away from commercialization and intensive research is ongoing. The more innovative graphene battery technologies will require significant R&D expenditures and will take many years to commercialize.

Traditional Battery Technology


A cross sectional view of a traditional battery

Batteries, whether they are rechargeable are not, commonly employ lithium. Zinc and alkaline based batteries are available, but they generally have a shorter lifespan due to their high charge density. Unlike lithium-based batteries, they canít operate at higher voltages. A primary (non-rechargeable) battery consists of two electrodes which allows the current to flow in one direction only, via an intermediary electrolyte. Secondary (rechargeable) batteries still consist of two electrodes however the lithium ions can flow in both directions depending on if charging or discharging. The anode is generally a lithium-based (metal oxide) compound and the cathode a porous carbon. Both the cathode and anode have a rigid structure with defined holes, which allows for the absorption of lithium ions into the holes when the current is applied. When there is no current being applied, the ions desorb into the electrolyte solution. Absorption of the lithium ions can occur on both the anode and the cathode. When a battery is in use, the ions move to the cathode. When charging, the current is reversed and the ion absorb into the anode. This process allows for many cycles to be produced, leading to an enhanced lifespan. The cathode of choice is traditionally graphite and the anode can vary, but common types are LiCoO2, LiMn2O4, LiNiMnCoO2 (NMC), LiFePO4, LiNiCoAlO2 and Li4Ti5O12.

Graphene Battery Technology


Graphene Batteries can reduce the environmental impact of battery use

Graphene battery technology has a similar structure to traditional batteries in that they have two electrodes and an electrolyte solution to facilitate ion transfer. The main difference between solid-state batteries and graphene-based batteries is in the composition of one or both electrodes. The change primarily lies in the cathode, but carbon allotropes can be utilized in the anode as well. The cathode in a traditional battery is purely composed of solid-state materials, where as in a graphene battery the cathode is a composite-a hybrid material consisting of a solid-state metallic material and graphene. The amount of graphene in the composite can vary, depending upon the intended application. The amount of graphene incorporated into the electrode generally depends upon the performance requirements and is based upon the existing efficiencies and/or weaknesses of the solid-state precursor material.

Graphene Battery Breakthrough
The real graphene battery breakthrough are the graphene-lithium-ion hybrid chemistries incorporated into the cathodes of lithium-sulfur cells as detailed in this guide. There are no pure graphene electrodes in a graphene battery, many graphene-based electrodes are fabricated and work in a similar way to traditional batteries. Their performance is enhanced via the addition of graphene to the electrode formulation. Generally, inorganic-based electrodes will have limitations which are typically surface area, density, capacity, cycle times, conductivity or capacitance to name a few. As graphene is a versatile molecule with many unique and desirable properties, it can be adopted in a variety of ways as there is no Ďone size fits allí solution for using graphene. Graphene is used to enhance many of the benefits already present with traditional materials, but it also helps to breakthrough previous battery limitations, leading to increased battery performance or life. Graphene works in electrodes in two general ways, either as a support or a composite/hybrid. As a support material, graphene helps to keep metal ions in a regular order, which generally helps with electrode efficiency. As a composite material in an electrode, it plays a different role as they are generally more involved in the facilitation of the charge itself, where its high conductivity and well-ordered structure are critical to providing an improvement against itís non-graphene predecessors. Below is a detailed account of how graphene specifically enables certain battery applications.

Lithium-Ion Batteries
Graphene-based batteries are quickly becoming more favorable than their graphite predecessors. Graphene batteries are an emerging technology which allows for increased electrode density, faster cycle times, as well as possessing the ability to hold the charge longer thus improving the batteryís lifespan. Graphite batteries are well-established and come in many forms. Similar to graphite, there are now various types of functional graphene derivative electrodes and researchers are discovering multiple benefits when compared to pure graphite electrodes.

Graphene-Metal Oxide Hybrids
As mentioned, graphite has been historically used as the primary cathode material, where the lithium ions migrate into the structured holes. However, graphene doesnít have this capability. Instead, due to the large surface area of graphene, the lithium ions can be stored via surface adsorption and induced bonding. Induced bonding generally occurs when a graphene derivative is present and the lithium ions bind to the functionalized surface. In addition to a large surface area, graphene electrodes also possess high conductivity. Many of the metal oxides traditionally used in batteries have impairments such as low conductivity, low volumetric energy density and the loss of contact points. By hybridizing the metal oxide matrix with graphene, many of these problems are removed and the conductivity becomes greater because the interaction between the interstitial ions and the hybrid matrix is vastly improved. To produce Graphene-Metal Oxide nanoparticle hybrids, the graphene acts as a template during the synthesis which produces an evenly distributed matrix due to the regular repeating structure of graphene. This process also limits nanoparticle aggregation which promotes the large nanoparticle surface area during the lithium charge and discharge cycles. As such, the specific capacity and cycling performance is improved against pure MO-based electrodes. Graphene-MO hybrid electrodes can exhibit up to 1100 mAh g-1 for the first 10 cycles. The specific energy density is maintained at 1000 mAh g-1, even after 130 cycles.

Graphene-Carbon Nanotube/Fullerene Hybrids
While not as widely reported, an electrode consisting of both graphene and either carbon nanotubes and/or fullerenes have been implemented. Electrodes that consist of 6-15 monolayers of graphene, which are layered on top of each other, show a specific capacity of 540 mAh g-1, which is a vast improvement on their graphitic counterparts due to their higher surface area. By dispersing the graphene sheets with either carbon nanotubes or fullerenes, the inter-graphene spacing is increased. This extra spacing creates extra cavities for the lithium ions to occupy and can increase the specific capacity by up to 40% when compared with stacked graphene electrodes.

Graphene Lithium Sulphur Batteries


A Graphene-Lithium-Sulphur Battery

Lithium sulphur batteries have the potential to replace lithium-ion batteries in commercial applications due to their low cost, low toxicity and the potential for possessing an energy density of 2567 W h kg-1, which is five times than that of lithium-based batteries currently available. As such, they have attracted a lot of interest. However, lithium sulphur batteries have several significant drawbacks that have stopped them from reaching the commercial marketplace. The first is the deposition of inorganic salts at the cathode, due to the cell possessing highly soluble reactants. The deposition of the salts causes a loss in active material, low coulombic efficiency, low utilization of the sulphur cathode and a degradation of the cycle life. The second main concern is that sulphur suffers from an inherent low conductivity.
Using graphene as a support for the sulphur ions eradicates some of the associated problems in sulphur batteries due to many properties previously mentioned; itís high electrical conductivity, good mechanical strength, high chemical/thermal stability and a large surface area. The high surface area provides a good dispersion of sulphur, which regulates the mobilization of sulphur ions and prevents a build-up of sulphur ions on the cathode. The production of graphene supported sulphur particles can also be produced through a one-pot synthesis.
The performance of lithium-sulphur batteries depends heavily upon the concentration of sulphur (and effectively the graphene to sulphur ratio) in the electrode. From electrodes produced to date, the higher sulphur containing electrodes provide the best performance. Electrodes containing 63 wt% sulphur produce a capacity of 731 mAh g-1, with the potential of obtaining up to 1160 mAh g-1. After 50 cycles, the capacity is maintained at 700 mA h g-1. This is at least twice the capacity of other sulphur-graphene electrodes produced and they are also stable under high rate cycling. Some degradation of the cell does occur as the sulphur binding has not yet reached 100% efficiency. Nonetheless, graphene supported sulphur electrodes show a vast improvement when compared to non-graphene-based sulphur electrodes.

Graphene Supercapacitors


A Graphene Supercapacitor Design

Supercapacitors are a valuable commodity in the field of electronics and can store up more than a hundred times more energy than standard capacitors. Supercapacitors are also able to work in low temperature conditions and are regularly used in lieu of electrochemical batteries.
One of the main properties of a supercapacitance material is the ability to form double-electric layers. This is essential in electric double-layer capacitors (EDLC) supercapacitors. Supercapacitors work by accumulating charges at the electrode-electrolyte interface through polarization, so that the energy is stored. Activated carbon has been the traditional electrode choice, but it suffers from the inability to operate at high voltages. Graphene and its derivatives, are useful due to their high specific surface area, high conductivity, open-pore structure, production potential and low cost; all of which are desirable properties for a supercapacitor.

Graphene-Metal Oxide Composite Electrodes


Graphene Molybdenum Oxide Electrodes

Graphene and metal oxide composite electrodes have provided a new area in which the capacitance is increased compared to standard capacitor electrodes. Common metal oxides include ZnO2, SnO2, Co3O4 and MnO2. MnO2 graphene composites are the most promising due to the variable oxidation states that manganese ions can adopt. The redox reaction between the III and IV states involves the intercalation of metal ions in the electrolyte solution (e.g. Li+, Na+), which helps to facilitate energy storage. In addition to the efficient ion intercalation, the graphene sheets yield a conductive network with a large surface area, which helps to further promote ion-electrode interactions. Graphene-MO composites show a high specific capacitance of 310 F g-1, which is three times higher than an electrode made of either pure graphene or metal oxide. 95.6% of the specific capacitance can also be retained after 2000 cycles when using graphene-MO composites.
As mentioned, supercapacitors are currently being favored over electrochemical batteries for some applications. However, one issue with using supercapacitors is the lower energy density. Graphene has been incorporated into the negative electrodes of supercapacitors to increase the energy density, while keeping the power density high. Supercapacitors have been developed using graphene as the negative electrode and a nanowire-graphene composite as the positive electrode. This capacitor is set up asymmetrically and has found to give a high-energy density of 30.4 Wh kg-1 at an operating voltage of 2 V. The graphene supercapacitors have surpassed other capacitors in terms of both energy and power density and is a promising prospect in the field of supercapacitors.

Graphene-Polymer Composite Electrodes


A Graphene-Polymer Composite Electrode

While graphene-polymer composites donít possess high conductivity when compared to other graphene-based composites, they do possess a high charge/discharge rate, flexibility, and high doping-undoping capability. Graphene-polymer composites operate by n and p doping redox reactions where electrons are lost/gained to convert and store energy. Graphene oxide and a nitrogen containing polymer are best used to create a graphene-polymer electrode composite. Polymerization of the functional groups facilitates strong pi-pi interactions between the two components of the composite which leads to a large surface area and a semi-flexible structure that can mechanically deform during the cycle charge-discharge processes. These graphene-polymer composites can exhibit up to 531 F g-1 and retain up to 74% of its capacitance after 2000 cycles.

Graphene-CNT Electrodes


A representation of aligned carbon nanotubes in between graphene sheets

Like lithium-ion batteries, carbon nanotubes are used to increase the inter-graphene spacing between the various monolayers. Stacked graphene electrodes can suffer from a decreased surface area compared to monolayered graphene. By introducing nanotubes into the electrode, both the nanopores and the conductivity of the electrode are increased. Nanotubes can be employed into the graphene matrix in either two or three-dimensions, with an average specific capacitance of 120 F g-1 and 386 F g-1, respectively. In the three-dimensional graphene-CNT, the capacitance can even increase by up to 20% after 2000 cycles, showing that these electrodes have an excellent electrochemical stability.

Graphene Fuel Cells


A Fuel Cell Inside View

Fuel cells are different from batteries and capacitors, as they produce electricity rather than store it. Many fuel cells contain a platinum-based catalyst, which are very expensive to produce. To minimize the cost of fuel cells, carbon allotropes are used as supports for the platinum catalysts. Graphene is one such catalyst support. Graphene oxide provides a good dispersion, large surface area and high conductivity (when reduced), so the integrity and efficiency of the device is enhanced by integrating graphene into the composite materials used in fuel cells.
Graphene is used in some fuel cells to help facilitate the oxidation of methanol and has been found to be much more effective than other carbon allotropes, such as carbon nanotubes and carbon black. Grapheneís two-dimensional sheets provide a greater active surface area for electron/ion transport, as both sides of the sheet are exposed to the solution within the fuel cell. The uniformity of the graphene surface also prevents aggregation and promotes an even distribution of the platinum particles across the supports. The surface defects in the graphene structure also increases the interaction between the graphene support and the platinum particles. Platinum-graphene supported fuel cells can exhibit a current density up to 0.12 mA cm-2, which is at least three times higher than other carbon-based supports.
Graphene can be doped with nitrogen after undergoing a nitrogen plasma treatment. Doped graphene has nitrogen-based functional groups on its surface which allow for a better dispersion and decoration by the platinum nanoparticles. Doped graphene produces a greater conductivity and electrocatalytic activity compared to undoped graphene. The oxidation current of doped graphene is twice that of undoped graphene.