Controlling bulk and surface reactions in high-performance sodium ion battery cathodes

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Sodium-ion batteries (NIBs) provide an alternative to lithium-ion batteries at a significantly lower cost, potentially enabling sustainable large-scale storage of renewable energy. However, NIB cathodes are known to undergo large structural phase transitions and side reactions during repeated cycling, leading to rapid degradation. Interestingly, it has been observed that layered sodium transition metal oxide cathodes with compositions featuring a combination of redox-active and redox-inactive ions can show improved stability and cyclability, although the mechanisms of this stabilisation are not well understood. In order to rationally design optimal NIB cathodes with higher energy densities and longer service lifetimes, control of cathode structure at multiple scales are needed: the long-range crystallographic structure, nanoscale local ordering of metals, electronic structure, and interfacial structure all play decisive roles.This project seeks to design improved sodium ion battery cathodes by using atomic substitution to control the nanoscale, electronic, and interfacial structures. "Unsubstituted" (single metal) and "substituted" (mixed metal) layered NIB cathodes will be prepared, electrochemically tested, and studied using a powerful collection of characterisation techniques sensitive at the nanoscale: solid-state NMR, synchrotron XRD and PDF, in situ SQUID magnetometry and scanning electrochemical microscopy. Based on these results, further optimised cathodes will be proposed and synthesised.