E-Mobility Engineering 016 l Aurora Powertrains eSled dossier l In Conversation: Thomas de Lange l Automated manufacturing focus l Torque sensing insight l Battery Show Europe 2022 report l Sodium batteries insight l User interfaces focus

Issues with lithium-ion cells are promoting a resurgence in sodium cell technology, reports Nick Flaherty , but various challenges irst need to be overcome Charging back R esearch into sodium-ion batteries (NIBs) began in the 1970s and ’80s but was overtaken by lithium-ion battery (LIB) research in the 1990s. However, they have recently re-emerged as a potentially sustainable and less expensive alternative to LIBs as the demand for lithium drives up its cost, and there is growing concern over the safety of LIB cells, which can be prone to short-circuits and thermal runaway. NIB cells can be constructed from abundant materials in the same way as LIBs. However, the combination of materials has so far struggled to reach the energy levels of LIBs, but research over the past decade has brought the performance of sodium cells up to that of lithium iron phosphate (LFP) cells. “Sodium can be extracted from seawater, the anode materials can be obtained from renewable sources, and the cathodes generally contain iron and manganese rather than high levels of cobalt and nickel,” says Emma Kendrick at the School of Metallurgy and Materials, University of Birmingham, in the UK. The manufacturing processes are expected to be the same as those for LIBs, so no investment in new manufacturing lines will be needed. The cost difference between LIBs and NIBs can therefore be considered to be due to materials and performance only. However, Kendrick points to the challenges of increasing the lifespan of the cells. This is governed by various chemical and cell engineering optimisations relating not only to the materials but also to the design of electrodes and cells and their operation. For example, dendrites and plating can occur very readily in sodium-ion cells, and this is controlled not only by the electrolyte’s composition and the ability to form low- resistance, stable interfaces, but also through electrode design. This has been demonstrated by the use of sodium cells from Faradion in e-bikes and heavy trucks in India, and Chinese e-bike developer NIU is planning a version of its two-wheeler in 2023 using sodium batteries. Major battery maker CATL is also developing the technology. There are three main advantages of sodium cells over lithium- ion. They are more powerful in terms of charge and discharge performance, so offer advantages for applications with high power requirements and faster charging, such as onboard batteries and small vehicles. However, they are not yet seen as a good enough fit for electric cars. Another advantage is their low- temperature performance. Lithium cells struggle to operate below 0 ºC, and need other batteries to drive heaters to warm them up so they can work. This is not necessary for NIB cells. Then there is the safety aspect, both in operation and in transport. While NIB cells can form dendrites in the same way as LIB cells, they do not cause short-circuits, and any heating is quickly quenched by their water-based electrolytes. The cells can also be transported with zero charge, something that is not possible with LIB cells, making shipping them a lot simpler and safer. Cathodes Most cathodes use layers of oxide materials and are air-sensitive, which leads to a higher manufacturing cost for NIBs and is holding back their development. This poor air stability needs to be understood and resolved, say researchers in China who have reviewed the latest understanding of and solutions to air sensitivity. The mechanism of air sensitivity is complex and involves multiple chemical and physical reactions. Components in air, such as oxygen, water and CO 2 , participate in the reaction with NIU in China is planning an electric bike using sodium-ion battery cells (Courtesy of NIU) 56 Autumn 2022 | E-Mobility Engineering