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Lithium’s burning need for improvement is producing a wide range of battery research and development initiatives

The lithium-ion cell is a testament to the problems of battery chemistry. Lithium-ion products are prone to burst into flames when they get too hot and, even under good conditions, need careful management during charging, simply to avoid capacity-sapping damage.

First commercialised by Sony in 1991, Li-ion has gradually eaten into the market for high-density rechargeables carved out by nickel-based chemistries. Although they present plenty of problems for battery manufacturers and users, there are strong reasons for using lithium-based batteries.

One key reason for Li-ion’s endurance is the core element’s high electrochemical potential. Although it has only 75% of the electrochemical potential of cadmium or strontium, lithium’s much lower weight makes it possible to achieve very high energy densities.

The problems start when trying to integrate lithium into a safe, high-capacity battery that can handle the peak demands of today’s electronic systems, as well as the slowly growing fleet of electric and hybrid vehicles. So, much of the work is centred on materials that can go around the core element to increase safety and boost capacity.

Because the anode largely controls how much lithium inside the battery can be used to store charge, developing anodes has become a major focus in improving overall battery capacity. Modern Li-ion batteries for mobile phones and tablets mostly use a carbon anode and a cobalt oxide cathode.

During charging, lithium ions captured at the anode form complexes with carbon atoms lying in graphite sheets. The anode stores up to one lithium atom for each hexagonal ring of carbon atoms in the graphite sheet.

During discharge, each lithium atom loses an electron and migrates to the cathode to form part of a lithium cobaltate molecule. Power tools and electric traction batteries use alternatives to cobaltate for safety reasons –manganese oxide or iron phosphate need to reach higher temperatures before they encounter the thermal runaway that characterises most lithium-battery fires.

Although carbon is cheap and effective, it only has an energy density of 370mAh/g. Silicon offers a higher storage efficiency – potentially up to 4210mA/g – and its use is being researched by companies like Nexeon.

One silicon atom can form a complex with more than four lithium atoms, almost reversing the ratio encountered with carbon. But that much improved ratio comes with a major drawback: the lithium-silicon complex balloons and the increase in volume can be as much as four-fold. The expansion and contraction cycles shatter the electrodes, fragments of silicon break off from the cathode and the battery quickly loses capacity.

Nanostructures and mechanical buffer materials may provide a solution by giving the silicon room to expand and by controlling stresses. Calculations performed five years ago by a team from a variety of US and Chinese labs indicated that lithium tends to favour accumulation along one face of the silicon crystal. Wires can balloon outwards and form stress fractures close to where a silicon nanowire is anchored to the electrode. Altering the orientation of the silicon crystal and the overall shape of the nanowires could help prevent the stress fractures.

Fig1: Argonne National Laboratory explains the operation of a Li-ion cell

Researchers at Argonne National Laboratory in the US focused on an electrolyte that can double up as a shock absorber. The team added fluorine to ethylene carbonate to make it behave more like rubber, stretching as the electrode grows. Another option is to make a sponge that allows lithium to be absorbed without forcing the overall structure to grow. One example was a mesoporous sponge – containing openings of the scale of tens of microns – in the silicon structure developed by researchers at the Pacific Northwest National Laboratory and the University of California at San Diego. The structure doubled the gravimetric storage capability compared to graphite, but with only a 30% expansion in size at full charge.

The cathode also provides potential for optimisation. Researchers at the University of St Andrews developed the concept of letting lithium ions capture oxygen from the air as the battery discharges, filling pores in a mesh of mesoporous carbon that forms the core part of the cathode. However, the lithium-air chemistry suffers from problems caused by unwanted products, such as lithium peroxide, blocking the pores and preventing full discharge.

Moving away from oxygen as the main element used in the cathode material provides another opportunity to improve density, as well as safety. Sulphur can perform a similar role to oxygen with, in principle, less danger of bursting into flames.

Using sulphur as the cathode partner and in the electrolyte makes it possible to have a battery that charges to the point where lithium can plate the anode, instead of using chemistries that avoid the element from entering its highly reactive metal state. Not only that, the batteries could be operated at higher voltages. This provides a potential five-fold boost in energy density, compared to today’s lithium-ion chemistries, according to UK lithium-sulphur battery startup Oxis Energy, which is aiming primarily at electric vehicle and home storage for renewables.

Unfortunately, similar to the problem with silicon anodes, the sulphur cathode expands as it acquires lithium ions, almost doubling in volume, again calling for nanostructural techniques that increase manufacturing complexity. Intermediate lithium-sulphur products also tend to react with the liquid organic solvents needed to allow free transfer of the lithium ions between anode and cathode, which reduces the capacity of the battery over time.

Although sulphur is an extremely poor conductor by itself, one option is to add other elements. Rather than the usual liquid electrolyte, some combinations have been used to form a solid version that should improve safety and reliability. Five years ago, a team from Toyota Research and a group of research institutes developed a solid electrolyte based on the combination of lithium, germanium, phosphor and sulphur. An immediately obvious problem was the availability of germanium: there is enough Ge available to satisfy small-scale use in chip production, not for batteries, where the requirements are much higher.

The hunt for an alternative to Ge provided an application for the Materials Project, founded by MIT materials researcher Professor Gerbrand Ceder and Lawrence Berkeley National Laboratory staff scientist Kristin Persson. The project, an attempt to create a database akin to that built by the Human Genome Project, contains material properties that will let computer simulations explore the likely chemical and thermodynamic behaviour of different combinations, avoiding the need to perform trial and error experiments. Simulations performed by Prof Ceder’s group found possible alternatives to Ge in tin and silicon – both of which occupy the same column in the periodic table. A recent experiment by a team from Munich and Stuttgart indicated the silicon variant is likely to perform better than the tin version.

Although continues on numerous fronts, the fact that the same core materials have remained at the heart of rechargeable batteries for 25 years shows how difficult it is to move any new battery design from research to commercialisation. But it seems likely that, with today’s greater understanding of materials properties and the impetus from high-demand markets, such as transport and electricity-grid storage, changes will come faster in the next 25 years.

Author
Chris Edwards

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