Calcium-pillared layered Na(Mn,Fe,Ni)O2 cathode materials for sodium ion batteries

Sodium-ion batteries (NIBs) are a more sustainable, cheaper and safer energy alternative to lithium-ion batteries (LIBs). The best performing NIBs contain sodium layered oxide cathode materials. O3 phase materials have Na ions in elongated octahedral environments, which can accommodate large quantities of Na giving high capacities but often poor cycling stability. Fe based layered oxides are desirable due to their availability and inexpensive starting materials. O3 phase materials combining Mn, Fe and Ni in the TM layer are now widely commercialised and show a combination of acceptable capacity and capacity retention, combined with satisfactory working voltage (~3.1V vs. Na+/Na). However, the performance of Fe based layered oxides often suffers from Fe migration to tetrahedral sites in the Na layer and poorly reversible phase transitions caused by TM layer gliding in the highly charged state. We have substituted Ca in the Na layer as a means of mitigating these drawbacks. Divalent calcium is a similar size to sodium and may act as a pillar, thus increasing the interaction between TM layers and Na layers and reducing the extent of layer gliding. The Ca-pillared materials demonstrate better cycling stability and rate performance and also show much less voltage fade. By using a combination of 44Ca isotopic enrichment of as-prepared materials and varying the transition metal ratios (cycled materials) we aim to unravel the origins of this performance enhancement using Polaris.