Innovative fabrication methods can also lead to significant energy storage system improvements.

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.

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. (Earlier post.) 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 is divalent (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, NETL says.

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 MgMn_1-xFe_xSiO₄/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.

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.

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 homogeneously 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.

In this 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% (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.

Double-layer supercapacitor materials development. 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, NETL says.

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. The goal is to improve EDLC using activated carbon or graphene, possibly in combination with transition metal non-oxide materials.

NETL is 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. 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.

In transition metal oxide electrode materials, NETL has 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.