E-Mobility Engineering 019 | In conversation: Stephen Lambert l WAE EVR l Battery case materials focus l Quality control insight l Clipper Automotive Clipper Cab digest l Optimising battery chemistries insight l Powertrain testing focus

Deep insight | Optimising battery materials This HOS-PFM polymer was used to coat aluminium and silicon electrodes and tested in a lithium-ion battery set- up. The coating significantly prevents silicon and aluminium-based electrodes from degrading during battery cycling while delivering high battery capacity over 300 cycles with a capacity of 3.0mAh/cm 2 . The HOS-PFM coating could allow the use of electrodes containing as much as 80% silicon, and increase the energy density of lithium-ion batteries by at least 30% as a result. The team next plans to work with companies to scale up HOS-PFM for mass manufacturing. Flame-retardant materials Other materials are aiming to make battery cells more resistant to catching fire, even with a short-circuit. For example, the Electronics and Telecommunications Research Institute (ETRI) in South Korea has developed a fluorosulphate-based flame retardant additive with significantly improved electrochemical stability, and cell performance compared to triphenyl phosphate (TPP). TPP is a conventional phosphorous flame retardant; existing phosphate additives have flame-retardant properties but there has to be a lot of them in the electrolyte. That increases the interfacial resistance (the chemical reaction between the electrolyte and the electrode) which reduces the performance of the battery and its lifetime. The ETRI teamanalysedmore than 10 types of commercially available phosphorous flame retardants through electrochemical experiments, synthesised a fluorosulphate-based flame-retardant additive for the first time, and improved the shortcomings of conventional phosphorous flame retardants. The team used the fluorosulphate material in a cell with a nickel, manganese, and cobalt cathode (with a nickel content of more than 90 %) and a lithiummetal anode that is more susceptible to short-circuits and growth of dendrites. The results showed the flame- retardant properties were improved by 2.3 times, and the performance of the battery by 160%, compared to an electrolyte with a conventional flame- retardant additive. The new additive is also easy to commercialise, as it can be produced without changing the existing lithium- ion battery production process. “We wanted to overcome the limitations of exiting flame-retardant additives and realise highly safe lithium- ion batteries through this research,” says Jimin Oh, a senior researcher at the Intelligent Sensor Lab at ETRI. “In the future, we will make efforts to contribute to the commercialisation of fluorosulphate-based flame-retardant additive for EV cells.” The next stage is to apply a flame- retardant additive to anodes using lithiummetal oxide anodes and lithium metal cathodes, as well as cells with silicon cathodes. Conclusion New additivematerials and coatings are boosting the performance and lifetime of different types of battery cells. The ceramic and polymer layers in solid-state batteries are allowing lithiummetal to be used safely to boost the energy density. More effective flame-retardant materials mean the same safety levels can be achieved with less material, allowingmore electrolyte to be used to boost the performance of the cells. 63 Lithium sulphur Lithium sulphur (LiS) batteries can provide a lighter option for short-haul aircraft and light goods vehicles in particular, with energy densities of 400-600 Wh/kg, but finding the right combination of materials is still a challenge. The Listar research project, led by University College London and involving the universities of Birmingham, Cambridge, Coventry, Imperial College London, Nottingham, Oxford, Southampton and Surrey, is therefore looking at the best-performingmaterials for LiS cells. “This work is important, because understanding what is happening inside a LiS battery is harder than with lithium-ion,” says Dr Daniel Auger at Cranfield University, which is developing simulations to model the behaviour of the battery in particular vehicle types. “There is just the one stage of electrochemical processes in lithium-ion, but four in LiS,” he says. “The charge is also very ‘flat’, meaning there are regions of the battery where it is very difficult to ‘see’ the charge. We have to look for different kinds of indicators of what’s happening.” The project is developing a LiS pouch cell using the most promising anode, cathode and electrolyte components previously tested individually in coin cells. It will investigate the cathode-electrolyte interfaces of quasi-solid-state batteries with Oxlid in Oxford, in the UK. It is also developing a solid-state composite cathode for an all-solid-state LiS battery. An optical microscope image showing the formation of lithiummetal dendrites during lithium sulphur cell cycling, captured as part of the Listar project (Courtesy of University of Coventry) May/June 2023 | E-Mobility Engineering