A large proportion of the function of batteries arises from the electrodes, and these are in turn mediated by the atomic-scale perturbations or changes in the crystal structure during an electrochemical process (e.g. battery use). Sodium insertion and extraction into/from electrodes during charge/discharge can lead to dramatic changes in the crystal structure and these are typically detrimental to performance. By understanding how and when large structural changes occur, electrochemical limits and solid state chemistry can be invoked to provide solutions. Therefore, to understand battery function and improve performance it is critical to probe the crystal structure evolution in situ or operando - while an electrochemical process is occurring inside a battery.

This presentation will showcase some of our recent work on tracking the structural evolution of electrodes during battery operation with a particular emphasis on the sodium location and distribution. Examples will include studies on the Na₃V₂O_2x(PO₄)₂F_3-2x system, Fe[Fe(CN)₆]1-x.yH₂O framework materials, layered P2 and O3 polymorphs of Na_2/3Fe_2/3Mn_1/3O₂ at varying current rates and layered P2 Na_2/3Mn_0.8Ti_0.1Fe_0.1O₂ and Na_2/3Mn_0.8Mg_0.2O₂materials. The relationship between electrochemical parameters such as current rate, number of cycles and battery history will be tied with phase transitions and sodium site occupancy evolution. Such detailed relationships will help to build an atomic scale picture of electrode functionality.

The message in the presentation will be the ability to literally “see” how sodium behaves in such electrodes and thus build a sodium-centred picture of battery function. For room temperature sodium-ion batteries based on intercalation chemistry to become a reality the insertion/extraction process needs to be understood and manipulated.