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With the increasing desire for all of our favourite gadgets to be portable, the need for extended battery lives is becoming more and more important. Researchers at the United States National Renewable Energy Laboratory (NREL) have discovered a way of using nanotubes to boost the power and durability of lithium-ion batteries.
Rising Demand for Performance
With global communication and transportation relying on lithium-ion batteries, the demand for increased performance of them is rapidly accelerating. In fact, according to analysts Frost and Sullivan, the electric vehicle market is expected to grow from $1 billion in 2009 to $14 billion by 2016.
In conjunction with the Energy Department, automotive battery developers and car manufacturers, NREL's Energy Storage group is looking to enhance the performance and durability of lithium-ion batteries for a cleaner and more secure future of transportation.
The nanotube approach represents an exciting opportunity — improving the performance of rechargeable lithium-ion batteries while making them last longer. Increasing the life and performance of rechargeable batteries will drive down overall electric vehicle costs and make us less reliant on foreign sources of energy.
Ahmad Pesaran - Energy Storage Group Manager
NREL scientists identified and produced crystalline nanotubes and nanorods that are able to alleviate common problems with lithium-ion batteries, such as, overheating, high weight, poor electrical conductivity and rapid loss of charge.
Electrodes based on carbon nanotubes are far superior to the electrodes found in current battery technology. Binder-free and with a higher performance, the technology has rapidly gathered interest from industry and NanoResearch, Inc., are getting a license for volume production of the batteries.
The nanotubes and nanorods are able to aid battery charging whilst lessening the swelling and shrinkage of electrodes which typically leads to a shorter lifetime for batteries.
Under the Battery for Advanced Transportation Technologies (BATT) program, which focuses on the reduction of cost and improvement of performance of batteries in electric vehicles, the Energy Department's Vehicle Technology Office supported NREL's research.
Think of a lithium-ion battery as a bird's nest. The NREL approach uses nanorods to improve what is going on inside, while ensuring that the nest remains durable and resilient. We are changing the architecture, changing the chemistry somewhat, without changing the battery itself.
Chunmei Ban - NREL Scientist
Currently, the price of single-wall carbon nanotubes (SWCNTs) is high. As the use of these devices in electrodes increases, engineers and scientists expect the cost to reduce to a point where they are economically viable.
How do Lithium-Ion Batteries Work?
The structure of a lithium-ion battery consists of a negative electrode made from carbon (cathode), a positive electrode made from a metal oxide (anode) and in between the electrodes, a lithium salt which is dissolved in an organic liquid (electrolyte). The anode and cathode are connected via an external electrical component which allows for electron flow between them.
The electrolyte closes the circuit inside the battery, and so is important for the recharging of the battery, as it allows the ions to transfer back and forth between the positive and negative poles. If this didn't occur, then the battery would cease to conduct electricity.
Use of metal oxides can lead to permanent damage of the battery as, during operation, they experience large volumetric alterations caused by the injection and extraction of the lithium ions from the electrodes. Eventually the electrodes come into contact with one another and the battery will break.
There are some metal oxides that work better than graphite when coupled with another metal oxide as the two electrodes however they still contribute to the expansion in volume inside the battery leading to internal breakdown.
Iron oxide was selected by the NREL research team due to its low cost, abundance and the fact that it is safe to use. These iron oxide nanoparticles had to be a specific size to work effectively and also needed to be held in a strong matrix to ensure that the battery could cope with large volumetric changes whilst optimising its electrical conductivity.
Scientists at NREL said that use of the carbon nanotubes can lower the overall cost as there is no need to incorporate expensive metals, such as Cobalt, into lithium-ion batteries' cathodes. This is because the nanotubes form a conductive rope-like wrap which, when there is shrinkage, allow electrons to continue on a conductive path to the iron oxide uninhibited.
Constructing Better Anodes and Cathodes
The power density of the battery can be tripled compared to graphite when coupling SWCNT with the iron oxide solution. This results in a reduction of the overall weight of the battery which is typically high due to the presence of the graphite. It was essential that the iron oxide particles be uniformly distributed within the encircling nanotubes.
Hyrothermal synthesis and vaccum filtration were used by NREL scientists to construct lithium-ion anodes which don't need to have the binders typically required in conventional batteries. Initially, precursors of iron oxide nanorods were produced for fabrication of the electrodes. It was discovered that annealing of SWCNTs with iron hydroxide nanorods at 450°C, could produce iron oxide.The SWCNTs ensured superior physical and electrical contact between the two materials whilst also helping to form the iron oxide particles.
Lithium nickel manganese cobalt oxide (NMC) was embedded into the nanotubes to produce the cathodic electrodes, this induced high conductivity in them. After 500 charge cycles, it is shown that these nanocomposites can retain 92% of their original charge.
Wet-Chemistry Synthesis Drives the Formation of the Nanomaterials
When the potential difference in a lithium-ion battery is at its maximum, it is fully charged and ready to be used. When this potential difference drops to zero, the battery will need to be recharged to work again.
Chunmei Ban has extensive experience in wet-chemistry synthesis and she utilized this when attempting to form the nanomaterials as rods. This shape made sense to NREL researchers as nanowires would work well with the curvature of nanotubes and thus create a strong electrode.
The nanomaterials attach to the iron oxide particles very strongly and they are porous allowing for effective diffusion.
Nano-Changes Making a Big Difference
NREL's use of nanomaterials to fabricate electrodes can lead to a superior overall performance of lithium-ion batteries.
The founder of NanoResearch, Inc., David Addie Noye, visited NREL with commercializing this innovation in mind. He has now decided to license this technology. His belief is that the fundamental problem with lithium-ion batteries may now be solved thanks to this development.
The newly improved batteries provided by NREL's unique approach to their fabrication may have significant improvements in portable consumer electronics (laptops, tablets, cell phones etc.). It can also have an effect on the energy storage industry with the increase in renewable energy storage by the national grid.
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