Editorial Feature

Materials Used In Electric Car Batteries

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There are a range of materials being used in batteries for electric vehicles. Lithium-ion batteries are utilized in the majority of all-electric and plug-in hybrid electric vehicles; nickel-metal-hydride are common for hybrid cars; and newer materials are being introduced, such as lithium polymer and lithium iron phosphate, with more on the horizon to challenge those in commonly used.

Each battery material has it's own strengths and weaknesses. Nickel-metal-hydride has a moderate cost but a lower power density than other batteries, while a lithium iron phosphate battery uses lithium-ion chemistry but with an iron phosphate cathode so that, when compared to other lithium-ion battery types, it offers superior heat and chemical stability without the risk of fire but with lower energy density.

Although lithium polymer can be shaped in any way, it is considered more expensive and has a lower energy density than standard lithium-ion.

Lithium-Ion

Lithium-ion is the material of choice for Tesla Motors electric cars. Working with Panasonic, they have halved the cost of the battery and increased its storage capacity by 60%. DuPont also sells lithium-ion batteries branded Energain for electric cars that it claims are 15% to 30% more powerful than other electric car batteries of the same type.

In the lithium-ion batteries, the negative electrode is made of graphite, a form of carbon, the positive electrode is made of a metal oxide, such as lithium cobalt oxide, and the electrolyte is a lithium salt dissolved in an organic solvent. The movement of lithium ions between the electrodes creates the energy of the battery. Lithium-ion batteries are known to be lightweight but they decay easily with age.

UK firm Nexeon makes its lithium-ion batteries with silicon as opposed to carbon anodes with the advantages of higher energy density per unit mass relative to other batteries and an increased capacity up to 40% and increased longevity. The company also claims lower manufacturing costs relative to other materials and  lighter design possibilities suitable for hybrid vehicles.

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Lithium-Sulfur

In the pipeline also is a lithium-sulfur battery with a rechargeable sulfur cathode to, in theory, power an electric car three times more than current lithium-ion batteries for the same weight and at a lower cost. The theory is being tested by researchers led by Professor of Chemistry Linda Nazar, Ph.D. at the University of Waterloo. In 2009 her group demonstrated the feasibility of a lithium-sulfur battery using nanomaterials. Since then, the group has focused on stabilizing sulfur, given that while it is abundant, light and cheap, it also dissolves easily from incoming electrons.

Experiments have now shown that an oxygenated surface of an ultra-thin manganese dioxide (MnO2) nanosheet chemically recycles the sulfides in a process that involves a surface-bound intermediate called polythiosulfate so much so that, rather than dissolving quickly in a battery, recharges it through more than 2000 cycles.

The group categorizes the reaction as one similar to one found in early sulfur chemistry experiments done in Germany around 1845.

Lithium-Oxygen

Also able to recharge over 2,000 cycles is a lithium-oxygen battery developed by researchers from the University of Cambridge. Professor Clare Grey, leader of the research, believes an applied version of their prototype could be a decade away. Their version consists of a carbon electrode made from graphene (comprising one-atom-thick sheets of carbon atoms), and additives that alter the chemical reactions at work in the battery, making it more stable and efficient. Specifically, it uses lithium hydroxide instead of lithium peroxide, and adds water and lithium iodide as a mediator so that the battery undergoes fewer chemical reactions, making it more stable.

Researchers reduced the 'voltage gap' between charge and discharge to 0.2 volts compared to previous less-efficient versions of a lithium-air battery. The efficiency, traceable to a highly porous graphene electrode, greatly increases the capacity of the demonstrator that can only be cycled in pure oxygen. The group is working on protecting the battery’s metal electrode so that it doesn't form lithium metal fibers, or dendrites, that can short-circuit the battery.

Sources and Further Reading

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