Contents
Why This Matters
Batteries are essential for powering many of our everyday technologies. Increased demand in areas such as transport and storage of electrical grids will require batteries with a longer lifespan and more capacity. Scientific advances in batteries can meet the demand for more energy storage while ensuring that these next-generation batteries are safe, cost-effective and sustainable. However, challenges remain.
The Technology
A battery is an energy storage device consisting of a chemical solution called an electrolyte and a separator that acts as a barrier between two terminals – an anode and a cathode. In use, the electrolyte allows the flow of charged particles, such as lithium ions, from the anode to the cathode. This produces an electric current that flows out of the battery to a device through an external circuit. Charging the battery reverses this process. Different applications, such as electric vehicles or electric grid storage, require different battery characteristics – such as size, weight, portability or duration of use – each of which comes with trade-offs.
Figure 1. Example of how rechargeable lithium-ion batteries work during use
Most current battery research focuses on lithium-based systems, which can store a lot of energy in a small volume and undergo many charge cycles. According to the American Chemical Society, lithium-ion batteries will account for 70 percent of the rechargeable battery market by 2025. Lithium supply must increase to meet this demand, prompting efforts to develop advanced battery technologies that use more earth-rich materials and reduce dependence on foreign manufactured materials.
Researchers are investigating how to replace critical elements in various components of lithium-ion batteries to improve performance and safety while using more sustainable, widely available and cost-effective materials. For example, the standard material used for the anode of lithium-ion batteries is graphite – the same flaky carbon material used in pencils. However, silicon is a cheap and more readily available material that is safer and can potentially store 10 times as much lithium by weight.
Alternative cathode materials are also being tested for lithium-ion batteries. For example, different metal oxides are usually used in the cathode to interact with the lithium and give the battery different properties. Alternatively, lithium-sulfur batteries contain a sulfur-based cathode that reacts with lithium ions to form lithium sulfide, which can allow cells to store 5 times as much energy as a conventional lithium-ion battery. Sulfur is an abundant element that can be mined in the United States. This makes it a more sustainable alternative to other commonly used metals in lithium-ion battery cathodes, such as cobalt, which is costly and may come from foreign mines with controversial labor or mining practices.
Another advance replaces the typically liquid electrolyte – which can be flammable and can catch fire when overheated – with safer, more stable materials. For example, using a solid electrolyte such as a ceramic or glassy material can prevent the build-up of lithium salt crystals that can short circuit the battery and cause a fire. These solid-state batteries have the potential to store twice as much energy as conventional lithium-ion batteries, increasing how long the battery can operate before needing to be recharged.
Lithium-ion batteries are usually limited to short-term use. Rechargeable metal-air batteries and flow batteries can allow longer storage durations, which can provide advantages in storing intermittent energy produced from renewable sources for use on demand. Metal-air batteries use a metal anode along with a porous cathode to allow oxygen flow from the surrounding air. Because a terminal is porous, these batteries are lighter than normal batteries. Scientists have examined a number of metals – such as aluminium, lithium, sodium, tin and zinc – for potential use. Each comes with different advantages and disadvantages. For example, the aluminum-air battery is light, recyclable, made from common materials and cheap, but is difficult to charge due to a tendency to corrode.
Unlike standard rechargeable batteries, flow batteries store liquid electrolytes in external tanks. Because there is no size limit for external tanks, the storage capacity of the flow battery can be scaled up as needed. This makes them ideal for storing large amounts of energy for the grid, but less useful in portable applications such as electric vehicles.
Figure 2. Example of how flow batteries work for a grid application
One of the most advanced flow batteries uses vanadium ions in the electrolyte. Vanadium is expensive and scarce; Vanadium’s ions, however, are stable and can be cycled through the battery over and over again without experiencing unwanted side effects, theoretically providing unlimited storage. However, vanadium batteries cannot store much energy in a small volume, and therefore require large external tanks to hold enough power to be useful. Researchers are investigating a variety of chemistries for flow batteries—including zinc-bromine, which uses inexpensive, readily available materials.
Advances in lithium-ion batteries are in various stages of research, but none are currently in commercial use for electric vehicles or grid storage. Batteries with sulfur-based cathodes, silicon-based anodes, and solid electrolytes are all in the pilot phase for transportation applications, with the latter two being piloted for use in electric vehicles. Batteries with silicon-based anodes are only commercially available in small electronics.
Non-rechargeable metal-air batteries can be found in devices such as hearing aids; However, no rechargeable metal-air battery chemistry has reached large-scale commercialization. Much of the research on flow battery chemistry has been conducted on a small scale in laboratories; however, flow batteries are used commercially for several grid and storage applications – including in the US, Japan and Australia.
Opportunities
Challenges
Policy Context and Questions
For more information, contact Karen L. Howard at (202) 512-6888 or HowardK@gao.gov.
What is the most promising battery technology?
Sodium-ion batteries promise well. They are energy dense, non-flammable and work well in colder temperatures, and sodium is cheap and abundant. In addition, sodium-based batteries will be more environmentally friendly and even more affordable than lithium-ion batteries are now.
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What controls the voltage of a battery?
The voltage of a battery is a fundamental characteristic of a battery, which is determined by the chemical reactions in the battery, the concentration of the battery components and the polarization of the battery. The voltage calculated from equilibrium conditions is usually known as the nominal battery voltage.
How does a battery create voltage?
Batteries produce electricity A chemical reaction between the metals and the electrolyte releases more electrons in one metal than it does in the other. The metal that releases more electrons develops a positive charge, and the other metal develops a negative charge.
How is tension created? Electric generators move magnets near coils of wire to create the voltages on the electrical grid. DC generation creates voltages using the energy from light in photovoltaic cells, or the energy from chemical reactions, usually inside batteries, and even temperature differences using thermocouples.
How does a battery create potential energy? As the electrons move from the cathode to the anode, they increase the chemical potential energy, thus charging the battery; when they move the other direction, they convert this chemical potential energy into electricity in the circuit and discharge the battery.
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