May 17, 2010
Scientists Close In On Lithium Battery Safety Flaws
Scientists at Cambridge have developed a simple, accurate way of "seeing" chemistry in action inside a lithium-ion battery.
By helping them understand how these batteries behave under different conditions the new method "“ which involves Nuclear Magnetic Resonance (NMR) spectroscopy "“ could help researchers solve the fire safety problems that have dogged the development of these batteries.
Lithium battery technology will also be crucial for the development of the next generation of electric cars such as the Nissan Leaf, due to be built in the UK from 2013.
But lithium batteries have one serious disadvantage: over several charge and discharge cycles, particularly if the batteries are charged quickly, minute fibers of lithium, known as dendrites, can form on the carbon anodes. These lithium fibers can cause short circuits, causing the battery to rapidly overheat and catch fire.
Writing in the journal Nature Materials, Professor Clare of the University of Cambridge says: "These dead lithium fibers have been a significant impediment to the commercialization of new generations of higher capacity batteries that use lithium metal as the anode instead of the carbons used today."
Scientists have use theoretical models and optical and scanning electron microscopes to study dendrite formation, but finding a way of quantifying the amount of dendrites formed has proved elusive until now.
The paper describes using a new method based on NMR spectroscopy to see chemistry in action within a tiny, 1cm long, battery enclosed in the same kind of aluminum bags used to keep coffee fresh.
According to Professor Grey: "Fire safety is a major problem that must be solved before we can get to the next generation of lithium-ion batteries and before we can safely use these batteries in a wider range of transportation applications. Now that we can monitor dendrite formation inside intact batteries, we can identify when they are formed and under what conditions.
Our new method should allow researchers to identify which conditions lead to dendrite formation and to rapidly screen potential fixes to prevent the problem."
Rangeet Bhattacharyya et al, 'In situ NMR observation of the formation of metallic lithium microstructures in lithium batteries' was published online in Nature Materials on 16 May 2010.
Lithium battery technology is based on three components, a cathode, an anode and an electrolyte that can allow for rapid transport between the two electrodes. On charging, lithium ions are extracted from the cathode, along with electrons. The lithium ions move through the electrolyte, while the electrons are driven through the external circuit, to reach the anode where the anode material, typically a graphitic carbon reacts with the lithium ions and is reduced by the electrons. Highly oxidized (cathode) and highly reduced (anode) materials result. On discharge "“ ie, when the cell is used to generate energy "“ this process occurs in reverse.
NMR spectroscopy, as in Magnetic Resonance Imaging used in medical imaging, relies on the ability to detect certain types of atoms, for example, lithium-7, whose nuclei possess "spin". These atoms, when excited by radio frequency waves in the presence of a magnetic field give rise to characteristic signals that allow their local environments to be identified. For example, lithium (Li) in a metal gives rise to a signal that is quite distinct from that of a Li ion in an electrolyte. The scientists at Cambridge and Stony Brook University in the US showed it is possible to identify the signal from a Li dendrite and monitor the growth of these species as the lithium batteries are cycled.
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