Dendrites – thin conductive filaments that form inside lithium batteries – reduce the life of these cells and are often responsible for them catching fire. Scientists working at the Pacific Northwest National Laboratory (PNNL) of the US Department of Energy claim to have produced a new electrolyte for lithium batteries that not only completely eliminates dendrites, but also promises to increase battery efficiency and vastly improve current carrying capacity.
Many of the rechargeable batteries used in portable devices today are of the lithium-ion (Li-ion) type, composed of two electrodes in a recharging battery (a positive one made of lithium and a negative one created from graphite) and a chemical electrolyte. Basically, the electrolyte chemically contains the electric charge and also acts as the medium through which the current flows between electrodes when the battery is connected in a circuit.
Unfortunately, as the electrolyte in a Li-ion battery also contains a solution of lithium, the lithium electrode tends to react with this medium, causing dendrites to form.
When these fibers snake their way out from the electrode and into the electrolyte, they tend to break down the controlled path that the electrons generally take by producing conductive paths haphazardly throughout the structure. As a result, there is often a sudden and rapid discharge that allows excess current to flow. At best, this causes the battery to fail prematurely. At worst, this heats up the battery to such an extent that it can set fire to its own packaging or even the device in which it is contained.
In an attempt to counteract this dendrite formation, some researchers have tried such things as coating the anode of Li-ion batteries with carbon nanospheres or tweaking the formula of the electrolyte with additives. Others have even added Kevlar to the mix, but this doesn’t stop the dendrites growing, it merely stops them from expanding too far into the electrolyte.
The new electrolyte developed by the PNNL researchers, however, aims to completely replace the electrolyte with one that does not promote the growth of dendrites at all. And, as a fortunate aside, it also ups the capacity and efficiency of the battery too.
"Our new electrolyte helps lithium batteries be more than 99 percent efficient and enables them to carry more than 10 times more electric current per area than previous technologies," said Doctor Ji-Guang "Jason" Zhang of PNNL. "This new discovery could kick-start the development of powerful and practical next-generation rechargeable batteries such as lithium-sulfur, lithium-air and lithium-metal batteries."
Doctor Zhang and his team based their research on the premise that Li-ion batteries with graphite electrodes may well be approaching their useful energy carrying capacity, and that an increase in efficiency could be wrought with the use of a higher-capacity lithium electrode. Of course, with the fact that an increased capacity lithium electrode would just add to the dendrite problem, the researchers realized that this would be difficult to achieve.
Building on other research that showed electrolytes containing particularly high salt concentrations also exhibited far less dendrite growth into the medium, Doctor Zhang and his co-workers chose to use large amounts of the lithium bis(fluorosulfonyl)imide salt, an organosilicon compound, added to the solvent dimethoxyethanein to produce their experimental electrolyte.
To test its new composition, the team constructed a circular test cell slightly less than 25 mm (0.955 in) in diameter. When this test cell was charged, rather than growing long filament dendrites, the lithium electrode instead developed a thin sheet of lithium nodules across its surface that showed no signs of growing into the electrolyte and short-circuiting the battery.
The team then subjected the test battery to more than 1,000 charge and discharge cycles, and claims that it managed to maintain a rather outstanding 98.4 percent of its initial charge, all of this while supporting a current of around 4 milliamps per square centimeter. Varying the current density also slightly affected the efficiency of the test battery, with 10 milliamps per square centimeter resulting in an efficiency of approximately 97 percent, while just 0.2 milliamps per square centimeter managed to produce an exceptionally high 99.1 percent efficiency.
According to the researchers, this is an outstanding set of figures, particularly as the large majority of Li-ion batteries with lithium electrodes run at a current density of 1 milliamp per square centimeter or less and are prone to failure after less than 300 charge/discharge cycles.
Given the high efficiency of the new electrolyte, the researchers also believe that this raises the possibility of a radical new design in battery technology – an "anode-free" cell. In other words, the electrolyte itself could act as an electrode. The actual construction would need to be refined, but with an electrolyte that runs at up to 99-plus percent efficiency, there may be an opportunity to manufacture a battery that only has a negative current accumulator, without a reactive material coating on the anode.
"Not needing an anode could lower the cost and size of rechargeable batteries and would also significantly improve the safety of these batteries," said Doctor Zhang.
Of course the electrolyte compound and its battery need further testing and refinement before potentially being made available commercially, and Doctor Zhang and his co-workers are gauging ways to achieve this along with numerous other additives that may further improve their electrolyte. This is a necessary step in achieving somewhere around 99.9 percent efficiency, which is a prerequisite to commercial production and release.
Testing and incorporating possible new cathode metals that work well in an amalgam with the new electrolyte are also being considered by the researchers.
The results of this research have recently been published in the journal Nature Communications.
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