LabNotes - June 2011
Materials for Energy Storage
Large-scale energy storage technology options now available are expensive, require high temperatures, or have large footprints. Some of the technology is also potentially dangerous. NETL is conducting research on alternative options to reduce costs and make large-scale energy storage safer and more practical. This includes research on appropriate anodes, cathodes, and electrolytes for magnesium (Mg)-, sodium (Na)-, and lithium (Li)-based batteries and novel transition metal oxide- and nitride-based super-capacitor electrode materials.
Improved means of energy storage will allow smarter and more efficient use of energy, allowing regional and national “smart grid” systems to better manage peak loads and minute-to-minute fluctuations in electricity demand (Figure 1). It will also make renewable energy sources, such as solar and wind power, more practical since these energy sources vary so much over time (Figure 2). The goal is to develop energy storage systems with high-energy density to meet the ever-demand for the electric grid to provide reliable, distributed power. To accomplish this, we are simultaneously investigating various potential ways to make high-energy density energy storage less expensive. Research is underway to make magnesium batteries practical, create new cathode materials that should make sodium batteries work better, create new anode materials to improve the performance of lithium batteries, and to use transition metal non-oxide and oxide superstructures in electric double layer capacitors to provide back-up pulse power for long-term energy storage devices, such as batteries and fuel cells. Innovative fabrication methods can also lead to dramatic energy storage system improvements (Figure 3).
All of this research is being performed through teamwork with local universities: the University of Pittsburgh, the Pennsylvania State University, West Virginia University, and the University of Maryland.
Figure 1. Storage allows constant base load generation to meet variable demand and makes electricity distribution more reliable and more efficient.
Figure 2. Storage allows shifting of power from intermittent sources, such as solar and wind power.
Figure 3. Storage costs can be reduced with innovative fabrication methods. Nanotechnology and composite microstructures will reduce the cost of producing batteries and enhance performance and durability.
High-energy Density Magnesium Batteries for Smart Electrical Grids
Magnesium-based batteries are an attractive alternative to other batteries, such as lithium (Li) batteries, because magnesium (Mg) is cheap, safe to use, and its compounds are usually non-toxic. Magnesium is much more abundant in the Earth’s crust, making it less expensive than Li by a factor of 24. Magnesium is safer because it is stable when exposed to the atmosphere. Magnesium is also lightweight and provides a theoretical energy density of 2205 Ah/kg, making it an attractive high-energy density battery system. Furthermore, it provides two electrons per atom and has similar electrochemical characteristics to Li (12 gram (g) per Faraday (F), compared to 7 g/F for Li or 23 g/F for Na). NETL scientists believe that metallic Mg or its alloys should be feasible candidates as positive electrodes for power systems in which cost is critical.
Proper design and architecture could lead to Mg-based batteries with energy densities of 400 - 1100 watt hour per kilogram (Wh/kg) for an open circuit voltage in the range of 0.8 - 2.1 volts, which would make it an attractive candidate for electrical grid energy storage and for stationary back-up energy. To make magnesium-based batteries practical, NETL researchers are developing novel alloys of Mg doped with different elements, such as Ca, Zn, Y, etc. These alloys are being produced by melting and casting as well as powder metallurgy approaches. Concurrently, innovative approaches are being explored to increase gravimetric and volumetric capacity by curtailing undesired volume expansion. A new displacement reaction hypothesis, based on the reaction of nano-structured transition metal compounds with Mg, has resulted in a thermodynamically favorable reversible displacement reaction of transition metals and Mg-alloys.
Recent accomplishments include a new intermetallic anode compound created by melting/casting and synthesis of a new MgMn1-xFexSiO4/C composite, and other transition metal oxide spinel cathode systems. Mg-based electrolytes and other ionic electrolytes have also been developed and are being tested. Figure 4 shows the results of a recent charge/discharge study on MgMnSiO4 in an Mg-based electrolyte that demonstrates the feasibility of a Mg battery.
Figure 4. Charge/discharge curves for MgMnSiO4 in magnesium-based (MgBu2(AlCl2Et)2/THF) electrolyte.
Novel Cathodes and Anodes
Sodium is another element that is less expensive than lithium. NETL has developed low cost Na-based cathodes such as NaFePO4 and Na3Fe2(PO4)3 that potentially can be used in large-scale energy storage systems. Figure 5 shows recent charge/discharge curves for the NaFePO4 and Na3Fe2(PO4)3 systems, which indicate a steady increase in their capacity and indicates that a low cost sodium battery is possible (for more details, click on the hyperlinked paper).
Figure 5. Charge/discharge curves for NaFePO4 and Na3 Fe2(PO4)3 cathodes vs. Li in LiPF6.
NETL researchers are also exploring ways to improve lithium-ion batteries. The low specific capacity of commercially used graphite anodes limits the development of high-energy density Li-ion batteries. Although silicon (Si) possesses a theoretical specific capacity of 4,200 mAh g-1, the high energy density of Si cannot be realized until three major challenges are overcome: (1) poor cyclability due to the large Si volume change, (2) inconsistent power density, and (3) large first cycle irreversible capacity and low columbic efficiency during subsequent charge/discharge cycles. NETL researchers have synthesized Si-C (carbon fiber, carbon nanotube, carbon mattes, graphene) composite anodes. The nanometer-sized Si particles (in amorphous, crystalline, and nanocrystalline forms) are being homogenously deposited on various carbon structural morphologies of carbon nanotubes, carbon mattes, carbon fibers, and graphene layers, and then the Si anchored graphene will be self-assembled onto a Si-graphene stack (similar to graphite) composite (Figure 6). In this unique approach, the particles size, volume, mass fraction, and loading of the Si will be controlled, so that the matrix will maintain the graphitic layered structure even when the size of the Si nanoparticles increases by 300 percent (in volume) at a fully lithiated state, and recover to the original state after delithiation, resulting in a reasonably high capacity and, more importantly, the desired long cycle life.
Figure 6. Cross-sectional schematic drawing of a graphene-Si composite anode prepared by electrodeposition and reassembly
Double-layer Supercapacitor Materials Development
There is considerable interest in supercapacitor or ultracapacitor materials for providing back-up pulse power for long-term energy storage devices, such as batteries and fuel cells. Most supercapacitor systems to date rely on carbon-based structures utilizing electrochemical double layer capacitance (EDLC) phenomenon based on charge transfer occurring from adsorbed species. In contrast, pseudo-capacitor technology relies on charge transfer reactions involving Faradaic transitions. A combination of Faradaic and EDLC response would generate supercapacitors that exhibit high capacitance for pulse power as well as sustained energy. In the pseudocapacitor arena, noble metal oxides typically have a high capacitance (about 720 F/g). However, cost and economics limit the use of noble metal oxides and carbon-based graphene or carbon nanotube related structures. Therefore, there is a need to explore alternative systems that have good electronic conductivity, adequate surface area, and the ability to undergo Faradaic electrochemical redox mechanism of charge storage. Transition metal oxides and non-oxides are well known for their ability to undergo Faradaic electrochemical redox mechanism of charge storage while also exhibiting excellent electronic conductivities. Vanadium nitride (VN) is one such transition metal non-oxide that has electronic conductivity comparable to carbon.
NETL researchers are attempting to generate high surface area transition metal non-oxide and oxide superstructures with high capacitance, scan rate response, and cyclability for sustained short- and long-term pulse power. Our goal is to improve EDLC using activated carbon or graphene, possibly in combination with transition metal non-oxide materials.
We are focusing on high surface area carbons, to achieve desirable power and energy densities along with VN and other transition metal oxides and non-oxides to generate composite structures for power grid storage applications (Figure 7). Experiments are being conducted to increase the lifetime, rated voltage, range of operating temperatures, and combined power density/energy density. These investigations are targeting increased capacitance on the order of 1000 F/g. We have shown that synthesized vanadium nitride (VN) nanoparticles exhibit these excellent capacitance values due to the formation of a thin amorphous oxide/oxynitride layer on the surface of the nitride (Figure 8).
Figure 7. Schematic diagrams of an electrochemical double layer type capacitor showing the charged (left) and discharged (right) states.
Figure 8. Schematic showing the presence of a thin oxide layer over a VN nanoparticle generated by a two-step ammonolysis approach, shown to exhibit very high capacitances of up to 1,300 F/g.
In transition metal oxide electrode materials, we have produced single-crystalline metal oxide nanoarrays that would be highly suitable for energy conversion and storage devices. Vertically-aligned Ni(OH)2 (001) and NiO (111) nanoplatelets have been synthesized via a simple wet-chemical method, and the electrochemical properties of NiO (111) arrays as electrodes for supercapacitors have been determined. Figure 9 shows the typical nanosheets of Ni(OH)2 and its arrayed architecture.
Figure 9. Top view (left) and side view (right) of Ni(OH)2. The nickel hexagonal structure is shown (bottom). The nanoplatelets are stacked compactly as multilayer nanosheets. This is normal for Ni(OH)2 because it is a typical layered double hydroxide.