Structure and Dynamics in a Layered Na-ion Battery Cathode
Sodium-ion batteries (NIBs) are a significantly cheaper energy storage alternative to lithium-ion batteries and as such will play a vital role in future sustainable grid-based energy storage to meet a rising demand for electricity. To date, the capacity of NIBs is too low for real-world applications and is limited by the cathode, Therefore, new, high-capacity cathodes are required to address the energy storage problem. One of the major limitations to the reversible capacity delivered by layered, transition metal-oxide-based cathodes are capacity losses due to irreversible or kinetically limited phase transformations. To increase the capacity of cathode materials, the mechanism of charge storage and the effects of these phase transformations must be understood in terms of the changes to the crystallographic and electronic structures of the cathode. Among the plethora of layered TM oxides which may be used as NIB cathodes, Mn-based materials have garnered significant interest, owing to the low cost and low toxicity of Mn. However, layered Mn-based NIB cathodes are known to undergo several phase transformations, which leads to significant capacity loss on prolonged cycling, due to the loss and formation of Jahn-Teller-active Mn(III) centres during charge and discharge. One strategy which has proven to be successful at increasing capacity retention is to dope the transition metal layers for species with lower oxidation states, for example Zn(II) and Li(I). In this work, we investigate how the bulk and local structure of a layered, Mg-doped, Mn-based-oxide NIB cathode evolves during charge and discharge. We use operando X-ray diffraction and 23Na NMR to examine the structural changes during charge and discharge. With the aid of hybrid DFT calculations, we have been able to assign the 23Na NMR spectra observed and have rationalised the effect of Na mobility on the observed NMR spectra using variable temperature 23Na NMR.