Mark Patrick, Mouser Electronics
As the roll out of billions of sensors and wireless nodes in the Internet of Things (IoT) progresses, more attention is being paid to the battery technology that powers them. At the same time developers of electric cars are also looking for better battery solutions.
Regularly changing the batteries of a billion devices in the IoT represents a major challenge - in terms of both cost and logistics. Consequently developers are keen to implement rechargeable battery cells that can use energy harvested direct from the environment, such as via solar cells or thermal gradients.
Lithium-based batteries have been taking over from nickel cadmium (NiCd) and nickel metal hydride (NiMH) cells in recent years by providing a higher energy density levels. However, these come with their own challenges, especially when placed in an industrial or electric vehicle context.
Rechargeable lithium batteries are prone to the development of dendrites which can cause short circuits and, in the worst case scenario, lead to fires. The small filaments grow during charging and can eventually connect across the anode and cathode. This has made it more difficult to transport cells from offshore manufacturing plants, due to the risk of the batteries igniting in the hold of cargo planes.
Lithium cells also have a limited number of charging cycles. This is fine for consumer applications, such as mobile phones that are replaced every two to three years, but poses a problem for industrial applications that are expected to last for 10-20 years - and yet the demand for smaller size and longer life continues to increase. So researchers and battery companies are looking at different ways of providing higher energy density in a cell that is safer to use.
Applying Solid State Technology
One way is to take a solid state approach. Current lithium polymer cells have an energy density of around 160Wh/l and use a thick polymer liquid that allows the lithium ions to move but which can leak and allow the growth of dendrites. This also prevents the cells being soldered to a board in a surface mount reflow process, thereby increasing production costs.
As the name suggests, solid state batteries have solid layers and so inhibit the growth of dendrites. However, getting the ions to move easily between the anode and cathode has significant difficulties associated with it. These need to be addressed - as the easier they move, the faster the cell will charge.
TDK for example has launched the first solid state lithium cell that can be surface mounted. The cell uses ceramic layers that allow the ions to move, and can be recharged up to 1000 times. It has a capacity of 100µAh at a nominal voltage of 1.4V allowing currents of a few mA to be drawn and comes in a compact EIA 1812 package. It is aimed predominantly at applications such as real-time clocks, Bluetooth beacons, wearables and IoT nodes using energy harvesting.
Toshiba has likewise developed a solid state battery. This employs a titanium niobium oxide anode material that can be charged in a matter of minutes, and is aiming this at electric vehicles. The SCiB cell has double the storage capacity by volume of the current generation of lithium batteries with graphite-based anodes (where a 20Ah cell has an energy density of 176Wh/l) and will start shipping next year.
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Figure 2: Toshiba SCiB battery
Samsung has also been working on new forms of lithium cell, using a 3D ball of graphene and silicon dioxide to coat both the anode and the cathode. This doubles the energy density and stops the growth of dendrites. The technology has been patented and is set to appear in batteries for IoT and mobile phones soon.
Researchers are, however, also looking at many other materials for battery cells, from aluminium and sodium to magnesium and graphene. The staff at the Clemson Nanomaterials Institute, in South Carolina, for instance, have developed an aluminium battery that could be cheaper and more reliable than lithium-ion technology. Apparao Rao, Professor of Physics at Clemson University and Director of the Clemson Nanomaterials Institute, has used aluminium foil and a graphene anode to construct a battery that can operate over 10,000 cycles and has an energy density of 200Wh/kg.
The battery cell uses thin sheets of graphite called few-layer graphene as the electrode to store electrical charge from aluminium ions present in the electrolyte.
“We knew that aluminium ions could be stored inside few-layer graphene,” said Ramakrishna Podila, Assistant Professor of Physics who also worked on the project. “But the ions need to be packed efficiently to increase the battery capacity. The arrangement of aluminium ions inside graphene is critical for better battery performance.”
In Spain, a Euro 3 million research project has started developing a new type of rechargeable solid state battery also using aluminium. The sulphur-aluminium battery with advanced polymeric gel electrolytes (SALBAGE) project could produce a solid state battery with an energy density over five times that of today’s lithium-ion cells. It is led by Spanish battery developer Albufera Energy Storage, in Madrid, along with several European companies and universities. The new battery is expected to have a high energy density of 1000Wh/kg and low price (at around 60% of today’s lithium-based cells).
Magnesium is an increasingly popular choice for battery cell technology too. Researchers at the University of Houston have found a way to make magnesium batteries that are intrinsically safer and twice the capacity of current lithium ion batteries. This project has already spanned several years and involved scientists from three universities and three leading US scientific laboratories. The key is expanding a titanium disulphide cathode to allow magnesium chloride molecules in whole, rather than having to use energy to break the molecular bonds. “We are combining a nanostructured cathode and a new understanding of the magnesium electrolyte,” said Yan Yao, Associate Professor of Electrical Engineering at the University of Houston, which is heavily involved. This allows batteries with a cathode capacity of 400mAh/g (up from 100mAh/g for earlier magnesium batteries) and twice that of commercial lithium-ion batteries (which generally have a capacity of 200mAh/g). Though admittedly, the voltage of the new battery remains low at about 1V, compared to 3V to 4V for lithium-based batteries for IoT applications.
Sodium is another material with potential for next generation batteries. Switzerland scientists have developed a prototype of a 3V solid state sodium battery with the potential for higher energy densities than lithium cells. "We still had to find a suitable solid ionic conductor that, as well as being non-toxic, was chemically and thermally stable, and that would allow the sodium to move easily between the anode and the cathode," said Hans Hagemann, Professor of Physical at the University of Geneva. The researchers discovered that a boron-based substance, a closo-borane, enables sodium ions to circulate freely. As the closo-borane is an inorganic conductor, it removes the risk of the battery catching fire while recharging. "The difficulty was establishing close contact between the battery's three layers: the anode, consisting of solid metallic sodium; the cathode, a mixed sodium chromium oxide; and the electrolyte, the closo-borane," said Léo Duchêne, a researcher at Empa's Materials for Energy Conversion Laboratory. Sheffield-based start-up Faradion is also working to commercialise its liquid sodium battery technology for electric scooters and cars, while Oxis Energy in Oxford has developed lithium-sulphur cells for the same types of applications.
A lot of engineering effort is going into replacing current liquid lithium-ion polymer batteries, in order to better attend to both IoT and electric vehicle applications. Solid state lithium designs that can be surface mounted and versions that charge quickly in electric vehicles are already coming to market. Research on other materials, such as sodium and sulphur, are in early stages of market acceptance, while entirely new cells with much higher performance levels are on the horizon. All of these aim to provide higher density, longer charging cycles and faster charging, without the risks of fire, in order to provide system designers with the energy they need.