Sodium Rich Layered Oxide Cathodes for High Capacity Sodium-Ion Batteries

Recently there has been much interest in developing and understanding the chemistries of lithium-rich materials with stoichiometric ratios of Li:TM > 1 1-2 where TM is a transition metal. These materials can deliver improved capacity by redox involving the anions in the structure (i.e. O 2- → O (2-n)- + ne - ) 3-5 to compensate Li + removal. However, to meet the needs of a globally electrified society, electrochemical storage systems not reliant on lithium are required. Sodium-ion technologies are a promising alternative, but Na-ion batteries typically exhibit lower energy densities. Na-rich cathode technologies offer the opportunity to overcome this. However, the greater ionic radius of sodium means that for it to occupy a TM site, as Li + does in Li-rich species, significant strain is induced. As such, most Na-rich species reported to date contain rare 2 nd and 3 rd -row transition metals Ru 6,7 and Ir 8 , whose more diffuse orbitals can accommodate the larger Na + ion. We have developed a synthetic approach which allows us to synthesise Na-rich materials using only earth abundant elements. These materials show great promise as the first industrially relevant Na-rich materials with the ability to undergo reversible (de)sodiation. Here our efforts to understand the charge compensation mechanism of these materials using various techniques including synchrotron spectroscopic (RIXS and XAS), and scattering (XRD and PDF) experiments are presented. From these studies we have resolved the charge compensation mechanism in these materials and demonstrated how it contrasts with that of Ru and Ir containing materials. This is critically valuable information for the rational design of energy materials as it reveals how we can control material behaviour through compositional tuning. References A. House, et al . Nat. Energy , 2021 , 6, 781–789 Sharpe, et al ., J. Am. Chem. Soc., 2020 , 142, 21799–21809 A. House , et al ., Nat. Energy, 2023 , 8, 351–360 A. House, et al ., Energy Environ. Sci., 2022 , 15, 376–383 A. House, et al ., Nature, 2020 , 577, 502–508 Tamaru et al. Electrochemistry Communications . 2013 , 33, 23–26 Mortemardde Boisse et al. Nat. Commun. 2016 7:11397 Perez et al. Chem.Mater . 2016 , 28, 8278−8288 Figure 1