LITHIUM-ION BATTERIES [Chemical Engineering Progress]
(Chemical Engineering Progress Via Acquire Media NewsEdge) Batteries are being asked to meet daunting performance requirements that could soon be pushing up against the limits of existing battery technologies. Portable electronics, electric vehicles, and grid-scale storage require high energy density and power, low cost, and safety.
The ever-increasing function coupled with the shrinking size of portable electronics, the integration of renewable energy into the power grid, and the trend toward all-electric vehicles are weighing heavily on battery technology.
Manufacturers of batteries for mobile phones, tablets, and laptops are continually being called upon to pack more and more power into smaller and smaller form. Moore's Law has enabled the computing power that was once housed in a building to fit into a pocket-size device - putting pressure on the batteries that power these devices to follow suit. The problem is that the charge carriers in a battery (lithium ions) are much larger than electrons, which are the charge carriers in electronics. Thus, battery size is restricted by the relatively bulky ion.
Electric vehicles fall into three categories - hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs) - each requiring slightly different battery performance. The automotive industry requires batteries that pack enough energy to allow for long-range driving coupled with a high power density for acceleration, and these batteries must be lightweight, low cost, ultrasafe, and able to cycle thousands of times.
The lithium-ion battery is not the only energy-storage device being considered for backing up the power grid. Others include supercapacitors, pumped hydro, flywheels, and compressed-air energy storage (CAES), as well as alternative battery technologies such as flow batteries, sodium sulfur (NaS) batteries, and advanced lead-acid batteries. Funded by the U.S. Dept, of Energy (DOE), the Pacific Northwest Smart Grid Demonstration Project is evaluating Li-ion batteries and other power-grid storage technologies. As part of that effort, a 5-MW lithium-ion battery storage unit was unveiled in June of this year at Portland General Electric's Salem Smart Power Center in South Salem, OR - the largest grid-scale battery unit being tested.
Batteries store energy in the form of chemicals and convert this chemical energy into electrical energy on demand. The basic unit of a battery is the electrochemical cell, which consists of an anode and a cathode separated by an electrolyte. These cells are typically combined in series or parallel to form modules, which are then connected (also in series or parallel) to form packs. Packs also contain the necessary electronics (e.g., sensors, voltage converter, and regulator circuit) to facilitate safe and reliable operation.
The Li-ion battery
The components of the cell determine whether a battery is rechargeable (secondary) or nonrechargeable (primary), as well as how much energy it can store (energy density), how fast it can charge and discharge (power density), its cycle life, safety, and cost. Among rechargeable batteries, the lithium-ion battery has become the battery of choice.
Rechargeable batteries came on the scene in 1859 with the invention of the lead-acid battery, followed by the nickel-cadmium battery in 1899. Both battery chemistries are still in use today. Two new rechargeable battery chemistries, which offer much higher energy density than their predecessors, were developed over the past two decades: the nickel metal hydride battery (1990), and the Li-ion battery (1991).
The lithium-ion battery offers many advantages over other rechargeable battery technologies. Li-ion batteries boast high energy densities (typically twice that of NiCd batteries) and high cell voltages (3.6 V). They require little maintenance, do not experience memory effects, and exhibit relatively low self-discharge.
Lithium-ion batteries come in a variety of chemistries, but their principle of operation is the same (Figure 1). The anode undergoes oxidation, releasing electrons, while the cathode is reduced by those electrons. Lithium ions move between the two electrodes through the electrolyte, and the electrons travel to an external electric circuit to charge or power a device. (The electrolyte is electronically insulating and therefore blocks electrons from passing through it.)
Battery chemistry and design are dictated by the performance requirements of the application of interest. While consumers would like every battery for every application to have the best of all attributes - energy density, power density, safety, lifetime, and cost - tradeoffs exist that make this impossible. Instead, some batteries are designed for high power, some for high energy capacity, and so on.
Energy density, which describes the amount of electricity the battery can deliver, is largely governed by the choice of cathode material. On the other hand, power density, which quantifies the rate at which a battery delivers electric current, depends on the materials used for the electrolyte and both electrodes. Because there are many ways for a battery to fail, most of the components within the battery will influence the lifetime. Safety - a key concern for designers of lithium-ion batteries - relates to the thermal stability of the electrodes: At high temperatures, the electrode materials degrade into compounds that can react with a flammable organic electrolyte.
Engineering a better Li-ion battery
If batteries were able to meet all of the demands of all of their existing and future applications, they would not be the topic of a CEP energy supplement. So, while lithium-ion batteries represent a step up from older chemistries and they themselves have made significant improvements over the past two decades, there is still much more to be done.
Energy density, power, and lifetime. Efforts are underway to develop new materials and designs to boost the performance of Li-ion batteries. One area that continues to receive significant attention is the cathode material, as it largely determines the energy density of the battery. Several alternative cathode materials are being explored.
The anode material must be compatible with any new cathode material and be able to accept the increased number of lithium ions that a new cathode material would provide. Thus, researchers are also looking at new anode materials. In theory, silicon anodes provide a tenfold increase in capacity over graphite in Li-ion batteries. The problem, however, has been the large volume changes that silicon undergoes during charging and discharging, which causes cracks to form in the anode.
Safety. Safety features are incorporated within the lithium-ion battery pack to minimize risks associated with this type of battery. However, the need for extreme safety in electric vehicles has energized research efforts to find a more stable electrolyte. (At high operating temperatures, degradation products in the battery can react with the flammable electrolyte, which typically consists of lithium salt dissolved in an organic solvent.) The safety issues associated with Li-ion batteries are evidenced by several incidents aboard Boeing 787 planes involving thermal runaway of the lithium-ion batteries.
Cost. Lithium-ion batteries are more expensive than other rechargeable batteries - a disadvantage that particularly impacts their use in electric vehicles and grid-scale storage. Several of the materials used to make the electrodes (e.g., lithium, cobalt, and nickel) are expensive and require costly methods to extract and process them into usable forms. One potential strategy for reducing the cost of Li-ion batteries would be to develop new extraction and processing methods that are less costly.
Focusing on Li-ion batteries
Batteries present many opportunities for chemical engineers to bring their skills to bear. From the mining and processing of raw materials (e.g., lithium, cobalt, and carbon), to developing better electrode materials through nanotechnology, to component assembly, chemical engineers have the unique skills and training necessary to design next-generation batteries to meet the demands of future applications. This special section provides an overview of lithium-ion batteries, introducing the basic concepts involved in batteries and identifying areas where further development is necessary.
In the first article, Robert Spotnitz of Battery Design LLC sets the stage with some basic information about batteries. He discusses the main components of batteries, emphasizing the role of each and introducing some of the issues that must be considered in the design of these components. Spotnitz works through the thermodynamics and kinetics that characterize electrochemical cells, and he uses fundamental equations to explain limits on performance and to compare the performance of existing battery systems to what could be achieved with good engineering.
In the second article, minerals consultant Gerry Clarke covers the major raw materials that go into Li-ion batteries. He identifies the major resources found around the world, and discusses the challenges related to the mining and production of these raw materials. The article focuses mainly on lithium - the essential ingredient of Li-ion batteries - and addresses recent speculation that the Earth's lithium resources will not be sufficient to meet the rising demand for this metal. The processes for extracting lithium and converting it into lithium carbonate, the primary lithium chemical used to produce lithium-based battery components, are also discussed.
Lithium-ion batteries can be made in a wide variety of cell designs using different combinations of materials. This is the topic of the third article, in which Avani Patel, R&D Director for Dow Energy Materials, discusses the materials used to create Li-ion battery electrodes, ion conductors, and separators. Patel points out that many material combinations are available to cell designers, and that optimizing the pairing of key materials can significantly impact cell performance for a target application.
In the final article, Andrew Jansen of Argonne National Laboratory rounds out the supplement by exploring battery applications, including transportation, portable electronics, and massive electricity storage. Jansen ties together some of the topics introduced in the preceding articles, such as the selection of materials for the electrodes and the performance metrics.
In discussing battery applications, Jansen concludes that the lithium-ion battery will not be able to meet the demands of future-generation applications. While advances continue to provide incremental improvements to the performance and cost of lithium-ion batteries, it is time to look beyond this technology to what is next. Jansen notes several areas of interest, including multivalent intercalated ions that carry more charge than lithium; chemical reactions of the working ion to store more energy; and non-_ aqueous redox flow systems.
MICHELLE BRYNER is a senior editor at Chemical Engineering Progress (Email: email@example.com), where she covers topics ranging from traditional chemical engineering to metamaterials and nanotechnology, to supercomputer advancements, to biotechnology. She has written for Popular Science and Psychology Today, as well as the online publications TechNewsDaily.com, lnnovationNewsDaily.com, and MensHealth.com. Before becoming a writer, she worked as a chemical engineer at W.L. Gore, where she developed new processes and products (including some of the PacLite gear). She received a BChE from the Univ. of Delaware and an MS in science journalism from New York Univ., and is a member of AlChE.
(c) 2013 American Institute of Chemical Engineers
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