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

through while preserving battery life, but electrolytes in current use tend to dissolve it, reducing the lifetime. Developing a new electrolyte that works with the different materials in the cathode and anode is a challenge, but the PNNL team has found an electrolyte that produces a cell with an energy density comparable to LFP at 150 Wh/litre and a lifetime of at least 300 cycles. The research cell uses a sodiated lithium NMC cathode loaded with sodium ions and a hard carbon anode with the new electrolyte. The key is the SEl layer that forms on both the anode and the cathode, and this layer depends on the composition of the electrode and the interaction with the electrolyte. “That’s one reason why a high-voltage sodium battery does not have a long cycle life,” says Jiguang (Jason) Zhang, the team’s lead researcher. “We have shown in principle that sodium-ion has the potential to be a long-lasting and environmentally friendly battery technology. We selected a salt and solvent to control the amount and quality of the SEI layer on the anode, which reduced the solubility to protect the anode longer,” he says. Rather than using the traditional NaPF 6 as the salt in the electrolyte, the team chose a salt sodium, fluorine, sulphur iodide material. “This salt can decompose to form an inorganic SEI layer,” Zhang says. “We selected the solvent with a smaller dielectric constant so that it is difficult to dissolve the SEI layer.” The team has developed a coin cell to test the performance of the materials. It held 90% of its capacity after 300 cycles at 4.2 V, which is higher than most high-voltage NIBs previously reported. “Normally the gold standard for cycle life is the discharge to 80% capacity,” Zhang says. “In this work it was measured to 90%, so it has the potential to last even longer.” The electrolyte is non-flammable, and the cell can operate at high voltages. “We also measured the production of gas vapour at the cathode,” says Phung Le, a PNNL battery chemist and one of the lead authors of the study. “We found very minimal gas production, which provides new insights into developing a stable electrolyte for sodium-ion batteries that might operate at elevated temperatures.” There is more work to do though to improve the cell’s energy density with the new electrode materials. “We need to look further at the cathode material, and we have an effort to improve the specific capacity of the carbon anode by more than 50% using different structures,” he says. “Once we find suitable materials we will scale up to a pouch cell, but it is still early days as we continue to improve both the anode and cathode.” Shuttle challenges The poor durability of sodium cells stems from a specific atomic reshuffling in the battery’s operation – the P2-O2 phase transition – as ions travelling through the battery cause disorder in the crystal structures and eventually break them. the cathode’s layered material, and depends on the crystal structure of the material. There are different strategies to improve the air stability, including the introduction of a surface coating, washing cathode materials, and element substitution in either transition- metal sites or sodium sites. But the researchers found there is a lack of instructive information for rationally designing practical materials. Electrolyte boost Researchers at Pacific Northwest National Laboratory (PNNL) in the US have been working on refining sodium cells to boost their lifetime. There are two trends in sodium batteries: have a long cycle life but lower voltage and lower energy density for stationary storage applications, or a higher energy density and higher voltage for e-mobility applications, but that has an impact on the lifetime. The PNNL work is focused on a new electrolyte and solvent to boost NIB lifetime by optimising the protective film, or solid-electrolyte interface (SEI) layer, that forms on the electrodes. The film is critical, as it allows sodium ions to pass 9esearcOers at 7acific 5ortOwest 5ational 3aIoratory Oave developed a sodium ion coin cell tOat Oolds of its capacity after cycles at = *ourtesy of 7553 Autumn 2022 | E-Mobility Engineering 57 Deep insight | Sodium batteries