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40 Solid-state batteries A very different approach is to use a solid-state battery structure. This uses a solid electrolyte that prevents dendrites from growing. However, the challenge is to get the high-speed ion transport through a solid material to provide fast charging. The latest solid-state cells can provide a range of 320 km for an electric vehicle after only six minutes of charging. This latest solid-state battery, called SCiB, uses titanium niobium oxide as the anode material; it has twice the storage capacity by volume of the graphite-based anodes generally used in lithium-ion batteries. The current 20 Ah SCiB cell has an energy density of 176 Wh/litre. This technology required a new way of synthesising crystals of titanium niobium oxide and storing lithium ions more efficiently in the crystal structure than previous versions, and is expected to start production in 2019. A 50 Ah prototype of the new battery technology has also been developed. Like the 20 Ah version, this maintains more than 90% of its initial capacity after going through 5000 charge-discharge cycles, and ultra-rapid recharging can be carried out in temperatures as low as  -10 C, in just 10 minutes. One US electric car maker says it has filed a patent for a 3D solid-state battery structure that can charge in one minute. Each layer is charged simultaneously, and the design could hold 2.5 times the energy of modern lithium-ion cells. The patent has not been published, however, and there are some potential drawbacks anyway. Charging a solid- state battery pack with a 500-mile range in just one minute would take several megawatts of energy in a very short time. Transferring such large amounts of energy would also have safety implications. The company says though that production of the technology could start in 2023. Another solid-state lithium-ion battery technology, from a Japanese supplier, is set for commercialisation over the next two years. At the moment it is comparable in performance to liquid cells but is more durable and has better temperature performance. The supplier has shipped samples of the battery to potential customers in the aerospace and automobile industries, and plans to commercialise the technology in small cells by 2020. One of the challenges with solid- state cells is that they are still more expensive than lithium-polymer cells as they are only made in small quantities. Manufacturers plan to start with small batteries for specialist UAVs for example, as the higher cost can be more acceptable. Artificially produced garnet can also be used as the electrolyte in solid-state cells as a result of its high ionic conductivity, approaching 1mS/cm, and cells built with the material have a higher output voltage than cells with other chemistries or solid-state cells, at 6 V. One problem though is the high impedance between the garnet electrolyte and electrode materials, but this has been addressed with an ultra- thin layer of aluminium oxide, which decreases the impedance by a factor of 300. This virtually eliminates the barrier to electricity flow within the battery, allowing for efficient charging and discharging. The layer is created using atomic layer deposition, and reduces the impedance from 1710   Ω cm 2  to 1 Ω cm 2  at room temperature, effectively negating the lithium metal/garnet interfacial impedance. This has led to a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode. Lithium sulphur In contrast to solid-state cells, lithium- sulphur batteries are already in mass production for applications such as electric mopeds. Sulphur is a natural cathode partner for metallic lithium, but operates quite differently from lithium-ion systems. The sulphur cell uses a metal lithium anode, a sulphur-based cathode and a non- flammable safe electrolyte protecting the lithium metal. The structure of the cathode and the composition of the electrolyte are areas for patented technologies. For a sulphur cell, the anode actually dissolves and is plated with lithium while charging, preventing the formation of dendrites. This provides a theoretical specific energy of over 2700 Wh/kg, which is nearly five times higher than that of lithium-ion cells.  Cells rated at 400 Wh/kg have already been developed using a ceramic lithium April/May 2018 | Unmanned Systems Technology A roll of anode material for silicon lithium-ion cells (Courtesy of Nexeon)