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42 Focus | Battery technology parasitic hydrogen evolution caused by anode corrosion during discharge. It has been a long-standing barrier to the commercialisation of the technology. Not only does it cause additional consumption of the anode material, it also increases losses in the cell and reduces the performance of the battery over time. Instead, the battery’s structure can be changed by placing ceramic and carbonaceous materials between an aqueous electrolyte and electrodes as an internal layer. That suppresses both the anode corrosion and the build-up of the by-products. This has resulted in a cell with a 0.7- 0.8 V output and 400-800 mA per cell (each cell measuring 10 x 10 cm), and it works for at least 14 days by refilling with water occasionally to provide the oxygen needed for the cell. Aluminium-air cells have been combined for batteries with output voltages of 10-12 V or 4.0-8 V for applications such as underwater systems, and moves to produce a version for electric cars are progressing. Sodium has also been used as the main material in cells, with a 10 Ah (32 Wh) prototype cell being built using traditional lithium-ion manufacturing methods. Silicon One material that has attracted a great deal of attention for a battery is silicon. Self-healing silicon nanoparticle composite materials are being used on the cathode, and represent a new way of holding the composite together rather than using carbon. This is particularly new to self-healing materials research because it is applied to materials that store energy. Previous research has found that cathodes made from nano-sized silicon particles are less likely to break down, but suffer from problems as the silicon nanoparticles start to break away from the polymer binder. Adding self-healing through a reversible chemical bond at the interface between the nanoparticles and the binder helps to prevent that. The resulting cell retains 80% of its initial capacity after a typical 400 cycles, much higher than previous versions. Other materials The search for new battery technologies can lead researchers in strange directions. Some are looking at a combination of graphene and asphalt, for example, which is more commonly used of course for surfacing roads. Porous carbon anodes made from asphalt have exhibited high stability after more than 500 charge-discharge cycles. A density of 20 mA/cm 2 allows batteries using them to fully charge in just five minutes. The researchers mixed asphalt with graphene nano-ribbons and coated the composite with lithium metal through electrochemical deposition, combining it with a sulphurised-carbon cathode to make full batteries for testing. They have a specific energy of 1.322 kW/kg and an energy density of 943 Wh/kg. The coated carbon anode prevents the formation of dendrites, allowing the faster charging and the manufacturing process is simpler and cheaper than other carbon anode technologies as there is no chemical vapour deposition step, no e-beam deposition step and no need to grow nanotubes from graphene. Ultracapacitors Ultracapacitors are a similar technology to batteries, and are of increasing interest. They have a similar structure to batteries but instead of a chemical transfer of electrons they use an electromagnetic approach (hence the capacitor name). This provides a longer lifetime with much faster charging but much less charge retention. Ultracapacitors store energy in an electric field, which allows them to charge and discharge much faster than batteries. They can also survive more than a million charge-discharge cycles, offering much longer lifecycles. They have very little internal resistance (as low as 0.12 m Ω ), allowing them to work at close to 100% efficiency. They are also much lighter than batteries and generally don’t contain harmful chemicals or toxic metals. An ultracapacitor can charge in 2 s but may only hold that charge for two to three minutes. However, it can do this millions of times, compared to a battery that would last perhaps 1000 charge-discharge cycles. In an electric car, for example, an ultracapacitor can provide the power needed for acceleration, while a battery provides the range and recharges the ultracapacitor between surges. This has also led to the technology being adopted in kinetic energy recovery systems to capture energy that would April/May 2018 | Unmanned Systems Technology A test cell using a silicon anode with lithium- ion electrolyte (Courtesy of Nexeon)