And living with them isn’t always easy, either. Two experiences drove the latter point home to me recently.
As we were working on this issue with its special section on lithium-ion batteries, both of my laptop’s batteries — the main battery that powers it when it is running, as well as the internal battery that allows the machine to retain its settings when it is off — failed. It took several days of troubleshooting before we realized that a new computer was the best, probably the only, way to solve the problem. The irony of the battery failures while I was editing articles on batteries did not go un-commented-on.
A few weeks ago, while I was giving my smartphone a boost at a public charging station, I overheard someone else who was doing the same thing remark to her companion, “Why can’t they make batteries that don’t need to be recharged so often?” I was tempted to say “They can. But you wouldn’t want to carry around an eight-pound phone.”
The design of batteries, as well as the devices they power, involves tradeoffs among numerous factors, including size and weight. Maybe eight pounds isn’t exactly right, but as the first number that popped into my head, it would have allowed me to make my point about tradeoffs without expounding on concepts like electrodes and electrolytes and their impacts on energy density, power density, charge/discharge cycling, safety, and so on.
Different applications have different requirements. For instance, the automotive industry needs batteries that pack enough energy to allow for long-range driving coupled with high power density for quick acceleration, and these batteries must be lightweight, low cost, ultrasafe, and able to cycle thousands of times. Manufacturers of portable electronic devices, on the other hand, prefer high energy density and are content with far fewer cycles, since many users upgrade their devices before the batteries have undergone even a thousand cycles. In consumer applications, maximizing volumetric energy density (the highest energy in the smallest package) is typically the major concern, whereas for military applications that involve a soldier carrying a battery pack, maximizing gravimetric energy density (the highest energy in the lightest package) is the most important metric.
Battery chemistries and designs, therefore, are tailored to the intended application, and material selection is key to this tailoring. A lithium-ion battery is a system where all components and interactions among components affect the performance of the battery and the device it powers. Thus, the materials used in the construction of the battery’s cathode, anode, electrolyte, and other components must be carefully chosen to ensure that the characteristics of the operating battery are consistent with the user’s expectations. Appropriate pairing of materials is key to optimizing battery performance for a particular application.
Many of the challenges faced by battery designers and manufacturers — as they seek to design higher-energy, higher-power, lighter, safer, and cheaper batteries — are being addressed through research and development of new materials. CEP covers such activity each month in the Update section. This issue, for example, reports on a microbial battery developed by engineers at Stanford Univ. in which exoelectrogenic microbes produce electricity through the digestion of plant and animal waste in sewage.
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