by Dr. Cynthia Lundgren
Scientists studying new ways to squeeze more energy from batteries are making great strides in developing new methods and materials to potentially increase the energy density of batteries by 30 percent.
The electrochemistry group within the U.S. Army Research Laboratory (ARL) typically works on cutting-edge energy and power solutions for the Soldier, including batteries, fuel cells, fuel reformation, and capacitors. The battery group recently developed new materials that could change how much batteries weigh, how long they last, and how much power they provide.
We’ve done this through fundamental research. Army scientists have studied how batteries work for years, looking at how each component interacts with the others. As a result, we have designedmaterials that allow for stable operation at high voltages and increased energy density, a measure of the amount of energy per unit of weight or volume that can be stored in a battery.
We looked at how to control the interface between the electrode and the electrolyte. That’s what determines how fast electrons can move back and forth; it dictates how fast a battery can charge and discharge, as well as how much power it can have.
At high voltages, batteries are extremely energetic, instable systems. At very high voltages, the electrode eats up the electrolyte. As a result, there has never been a single-cell battery that operated at 5 volts or higher.
yet higher voltages can be a good thing. The higher the voltage, the more energy and power. Voltage is like water pressurein a pipe: The higher the pressure, the more powerful the stream.
A MUCH-NEEDED BREAKTHROUGH
The basics of battery technology haven’t changed much over the centuries. Although we don’t know exactly what early civilizations did with batteries, archeologists have discovered battery remnants in the ancient world. In 1800, scientists harnessed the power of the chemical battery.
Since the introduction of lithium-ion batteries in the 1970s, progress has been slow.
The three components in the electrochemical reactions of any battery are the anode, cathode, and electrolyte. An anode is an electrode through which electric current flows into a polarized electrical device, and a cathode is an
OPENING NEW DOORS
electrode through which electric current flows out. An electrolyte is any substance containing free ions that makes the substance electrically conductive.
This is where our research team made a breakthrough: Our scientists designed an electrolyte additive that we half-jokingly call “magic pixie dust.” When we add it to the electrolyte, it becomes a sort of sacrificial agent, preferentially reacting with the electrode and forming a stable interface that allows the battery to operate at 5 volts.
Army researchers Dr. Kang Xu and Dr. Arthur Cresce designed the substance two years ago, and ARL has filed patent applications for their work. “Westarted synthesizing the basic building blocks of the suspect interface components," Xu explained. "One by one, we pieced together the story and found out exactly what protects the electrolytes from decomposing. This fundamental knowledge enabled us to develop this new 5-volt electrolyte.”
The laboratory held a battery industry day in February 2011, to show the Army’s patent portfolio to the battery industry. More than 40 companies attended the event. Since then, nine have signed material transfer agreements, and the Army is providing additive samples to these companies. Ultimately, the goal is to license this technology for commercial use.
“This is what you would call a quantum leap,” Cresce said. “We’ve gone from circling around a certain type of 4-volt energy for quite a while. All of a sudden, a whole new class of batteries and voltages are open to us. The door is open that was closed before.”
Doors have opened beyond the ARL as a result of the group’s discovery. Other scientists have developed high-voltage cathode materials, but they had no way to show the benefits of their materials because no electrolyte was stable at the higher voltage. now the scientific community is talking about 5-volt cathodes using the additive developed by ARL.
There is much more work to be done. One of our current projects deals with other lithium battery chemistries. Lithium air is a primary battery that researchers are examining to see if they can make it rechargeable. Unfortunately, lithium-air batteries have encountered difficulties with stability, in part because of the electrolyte. Our work focuses on a solid electrolyte-a ceramic that conducts lithium ions-that could improve that stability.
Lithium-ion batteries are also quite expensive on a large scale, so we’re looking at other means to improve efficiency,lower costs, and increase safety. Our program on dual intercalation materials uses a carbon anode and a cathode.
Lithium-ion batteries rely on cations intercalating (tunneling) through both the anode and cathode when they charge and discharge, respectively, but cathode materials that intercalate lithium and are stable to mechanical changes when the lithium enters and exits are difficult to find and expensive.
An alternative is to use graphite, a form of carbon, as both anode and cathode, stably intercalating cations and anionssimultaneously. Carbon is not stable at high voltages with a plain electrolyte, but our additive could make this dual-intercalation battery possible.
Before the additive, the average coulombic charge/discharge efficiency-the amount of electricity being used to charge compared with the amount that can be discharged-was quite low in a standard electrolyte, about 60 percent; much of that loss is due to reaction with the electrolyte. With the additive, we have been able to achieve a first charge/discharge of more than 99 percent. The ability to create high-voltage batteries using just
REDUCING SOLDIERS’ LOAD
carbon electrodes has the potential to be a very big deal.
Starting this year, we are also working on molten lithium sulfur for grid storage. Microgrids are becoming increasingly important to the Army to manage power and energy in a smart way by reducing logistical burdens, increasing generator efficiency, and allowing for greater use of renewables. But using renewable energy requires storage; hence the need for new storage solutions.
What can a Soldier expect to have in the field that will be affected by our research today? All of the electronics-sight, night vision, guidance system, lasers, almost anything that is "smart"-has a battery. The average Soldier carries 16 pounds of batteries for a 72-hour mission. Depending on the Soldier’s role in a platoon, it could be up to 32 pounds of batteries. The more electronics Soldiers carry, the more batteries they’re going to have to carry. Our research has the potential to substantially reduce the battery weight, allowing for Soldiers to carry more ammunition or water.
Everything we do at the lab is done with the consideration of empowering, unburdening, and protecting Soldiers. Our main goal is to support the Soldier, whose needs are more stringent than what is needed commercially. For instance, Soldiers need batteries that operate in a wide temperature range, from -40 degrees to +70 degrees Celsius. Commercially, battery users generally are looking at a range of -20 to +40 degrees.
A lot of battery failures in the field are temperature-related. Working on the fundamentals and looking at the interface allows us to understand what limits operations at low temperatures. Throughthat understanding, we have been able to develop these new additives and materials.
We have been able to make only incremental improvements over the years, however. Typically, improvements in energy density have averaged about 1 percent a year, with a few step changes, such as the emergence of lithium-ion batteries.
Ultimately, we believe batteries will start looking more like fuel cells, such as the metal air batteries, or semi-fuel cells.
What limits us? Right now the Lithium 145 battery, which the Army uses, is rated at 145 watt-hours per kilogram. Our goal, which is achievable, is to increase that to 300 watt-hours per kilogram.
In battery chemistry, we are limited by the periodic table, with lithium on oneend and fluorine on the other. we are thermodynamically constrained by the amount of power and energy that we can develop in a battery.
To counter that, we are looking at new strategic areas by designing systems to allow for ubiquitous energy-energy gained in any environment using indigenous or other available sources such as wastewater.
Some of our new programs are looking at how we could make fuel out of water. One of our long-term goals, for example, is to determine whether we can split water and make hydrogen that could be used as fuel in a fuel cell or small engine. Nature splits water, taking water and carbon dioxide and making energy. We are trying to short-circuit this process and take the components that nature uses- proteins that are found in spinach, for instance-and actually split water.
We are researching fuel cells, solid-state capacitors for pulse power. In addition, we're looking at fuel reformation: How do we get hydrogen from a logistics fuel like JP8? Chemicals have the highest energy density. So it is beneficial to store energy in a chemical such as hydrogen or JP8, but it needs to be in a form that can be used. Fuel cells can use only hydrogen. To use JP8 as a fuel, we have to convert it.
We have a new initiative looking at computational methods of multi-scale modeling that we hope will accelerate the process of designing new materials.
This initiative, funded by ARL, looks at multi-scale modeling of electronic materials, part of which is for electrochemical devices. The scientists will be working with our computational team to look at a range of possibilities, from the atomic scale to the systems scale.
Right now, we can look at an atom, or we can look at a system, but it’s hard to draw the lines between them. One of the things we want is to speed up our ability to make changes.
In the future, our hope is not just to makebetter materials, but rather to design new types of power and energy devices that we cannot even fathom today.
We have only a small group of researchers working on these technologies-a small group, but very high-powered. Although the laboratory gives us a variety of effective tools, our best resource is our people.
The ability to do research that can make a difference for Soldiers has allowed us to attract and retain top talent.
A 30 percent increase in battery energy density gives us an idea of what lies ahead.
STABLe OPeRATIOn AT HIgH VOLTAgeS AnD InCReASeD eneRgy DenSITy.
http:// www.arl .army.mil/www/default. cfm?page=556.
DR. CYNTHIA LUNDGREN is a chemist and Chief of the Electrochemistry Branch of the Power and Energy Division in theSensors and Electron Devices Directorate of the U.S. Army Research Laboratory. Her group investigates energy storage and conversion of material components and their interactions in both large and small systems. The technologies include fuel cells, fuel reforming, primary, secondary, and reserve batteries, and capacitors. New areasof research include nanofabrication as well as taking advantage of biological systems in the development of novel power sources. Lundgren holds a B.S. in chemistry from Rutgers University, an M.S. in physical chemistry from Seton Hall University, and a Ph.D. in electrochemistry from the University of North Carolina at Chapel Hill.