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37 The size of the metal ions is a key consideration, as these have to fit into the lattice of the electrodes in a process called intercalation. The more ions that can fit into an anode or cathode, the higher the battery’s energy density. That is why lithium, with the smallest metal ions, is popular for systems with a need for high energy density. Its ions can fit efficiently into an electrode and deliver a battery capable of up to 400 Wh/kg, or 1 MJ/kg, but there are many other battery technologies with much higher energy densities. That compares to the energy density of 3 MJ/kg for explosives such as TNT, so batteries are essentially an explosive whose energy is released slowly rather than in a few microseconds. Lithium can also be used for the electrodes, but so can many other materials that can accommodate more ions. These range from the most common – carbon – to graphene, silicon, aluminium and zinc. Cobalt is also widely used as an electrode material. The battery industry currently consumes 42% of global cobalt production, and the metal provides 0.22 kg/kWh in the nickel cobalt aluminium- based batteries used in some electric vehicles now in mass production. That compares to 0.36 kg/kWh for the nickel manganese cobalt batteries used by other electric vehicle makers. However, most of the world’s cobalt is sourced from the Democratic Republic of Congo, where child labour is known to have been used for mining it; supplies are also vulnerable to political instability. Many battery makers are therefore looking for alternative supplies of cobalt or designing battery cells without it, such as lithium iron phosphate or lithium manganese oxide chemistries, although these have a lower energy density. Other metals with a higher number of available electrons (known as valency) can be used as transport ions in the electrolyte to deliver more electrons from one electrode to the other, allowing faster charging. Iron phosphate, magnesium, sodium and sulphur are growing in popularity, as is aluminium. The faster the ions can travel through the electrolyte during charging without overheating the cell, the quicker it will charge. Battery technology | Focus Unmanned Systems Technology | April/May 2018 The operation of a lithium-ion battery cell using a silicon anode (Courtesy of Nexeon) Different materials produce different amounts of power in a cell (Courtesy of Faradion)