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38 Focus | Battery technology The structure of the battery also has a major influence on its performance. Most modern lithium-ion cells use a semi- liquid lithium-polymer electrolyte (often in a pouch, cylindrical cell or a prismatic cell) to make ion transport easier and charging quicker. The pouches typically have a capacity of 20 mAh to 2 Ah, while the prismatic cells are packaged in welded aluminium housings and provide capacities of 20- 30 Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Some car makers are using multiple cylindrical cells. The 18650 cell for example is 18 mm in diameter, 65 mm long and has a volume of 66 cm 3  with a capacity of around 3 Ah, while the larger 21700 cell is 21 mm in diameter, 70 mm long with a 97 cm 3  volume and has a capacity of up to 6 mAh, doubling the capacity with a 50% increase in volume. Lithium-polymer cells have a couple of disadvantages though. If the pouch is punctured the electrolyte can leak out and catch fire. Worse, as the cell is charged and discharged, lithium metal builds up on the electrodes. That leads to the formation of crystalline branching structures called dendrites that grow through the cell, getting longer with each charge-discharge cycle. That limits the lifetime of the battery cell to 1000-2000 cycles, or a few years of operation. A greater problem is that the dendrites can eventually connect, causing a short- circuit in the cell. The dendrites heat up, and the lithium electrolyte can then catch fire. As the cells are all stacked together to provide power, the rest of the battery can also catch fire. This has happened with a number of electric vehicles. Dendrites Many laboratories and companies around the world are working to solve the problem of dendrite formation. One way around it is to use magnesium, which does not form dendrites, but the magnesium technology is still at the research stage. Magnesium is much more abundant than lithium, has a higher melting point, forms smooth surfaces when recharging, and has the potential to deliver a more than five-fold increase in energy density if an appropriate cathode can be identified, and researchers have used a type of magnesium salt capable of combining with lithium to stop the dendritic branching. Other teams have used a vanadium pentoxide cathode that can work well with magnesium ions. Other researchers have added a compound of phosphorus and sulphur to the electrolyte that carries electrical charge within batteries. The compound reacts with a lithium metal electrode in a pre-assembled battery to spontaneously coat it with an extremely thin protective layer that prevents dendrite growth, and also prevents corrosion of the electrode, allowing higher energy density.  This is a simple, scalable way to protect the lithium metal and increase storage capacity or energy density, and could boost the distance electric vehicles can travel on a single charge, from about 200 to 600 km.  Yet other teams have developed sensors that can be integrated directly into the battery to allow a lithium-ion cell to charge five times faster. These work as part of a battery’s normal operation and have been tested on standard, commercially available automotive battery cells.  This system uses miniature reference electrodes and an optical fibre temperature sensor that are threaded through a protection layer, while an outer skin of fluorinated ethylene propylene is applied around the fibre to provide chemical protection from the corrosive electrolyte. The result is a sensor with direct contact with all the key parts of the battery that can withstand electrical, chemical and mechanical stresses. This gives a novel instrumentation design for use on standard commercial cylindrical 18650 cells. The data from the sensor is much more precise than external sensing, and this has been used to show that commercially available lithium batteries could be charged at least five times faster than the current recommended maximum rates of charge. Another approach has only a reference electrode that can be used to measure the performance of the anode and the cathode in real time, but this requires careful design of the electrode material to ensure it doesn’t interact with the cell. April/May 2018 | Unmanned Systems Technology Prismatic cells have been combined here into a modular 24 V battery pack (Courtesy of Beckett Energy Systems/Steatite)