Future demand for lithium-ion batteries is predicted to increase continuously due to their use in portable electronic devices and larger-scale energy storage, such as hybrid electric vehicles and static energy storage systems. Sodium is ubiquitous and significantly cheaper than lithium.  Recent advances in sodium-ion battery technology has achieved promising results, with energy, power and cycle life comparable to Li-ion technology.¹

Among cathode candidates, layered oxides offer many advantages due to their simple structures, high capacities, and ease of synthesis. However, there are major challenges to overcome, such as: low operating voltage, long term stability of host structure, high polarisation and the stability of the sodium in the P2-phase.² In general, ternary and quaternary transitional metal (T_M­) systems offer the most advantages, due to the ability to tune the material with different T_M at different percentages. SHARP Corp. have been developing a variety of layered oxides for sodium-ion battery applications. One of the most promising candidates to date is the quaternary T_M system, NaNi_xSn_yMn_zTi_yO₂ giving an experimental capacity of 175 mAh/g and an average voltage of 3.2 V. However, the cycle stability of this material requires improvement. In order to understand this loss in capacity over cycle life, in-operando x-ray diffraction was carried out in coin cell configuration vs hard carbon, containing either one or two beryllium windows depending on the x-ray source used. Initially, this project looked at the material in-situusing low energy laboratory x-ray diffraction, but the energy of the x-ray source is not sufficient to identify the different phase transitions throughout sodiation/desodiation. By means of using a synchrotron x-ray source and following real-time electrochemical performance, this experiment has allowed the identification of key phase transitions throughout initial sodiation at a rate of C/20 then continued cycling at a rate of C/10. What is particularly interesting is the corresponding phase transition that occurs at high voltage, with a defined plateau from 3.9 to 4.3 V. The corresponding redox reaction is unstable thus small loses of capacity is observed through cycling. By identifying this phase, and understanding what the structural changes to the cathode material are, there is the possibility to tune this material and therefore stabilising this high voltage phase-transition.