Na-ion Mobility in P2-type NaMgNiMnO (0 ≤ ≤ 0.07) from Electrochemical and Muon Spin Relaxation Studies

Sodium transition metal oxides with a layered structure are one of the most widely studied cathode materials for Na-ion batteries. Since the mobility of Na in such cathode materials is a key factor that governs the performance of material, electrochemical and muon spin rotation and relaxation techniques are here used to reveal the Na-ion mobility in a P2-type NaMgNiMnO ( = 0, 0.02, 0.05 and 0.07) cathode material. Combining electrochemical techniques such as galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique with μSR, we have successfully extracted both self-diffusion and chemical-diffusion under a potential gradient, which are essential to understand the electrode material from an atomic-scale viewpoint. The results indicate that a small amount of Mg substitution has strong effects on the cycling performance and the Na mobility. Amongst the tested cathode systems, it was found that the composition with a Mg content of = 0.02 resulted in the best cycling stability and highest Na mobility based on electrochemical and μSR results. The current study clearly shows that for developing a new generation of sustainable energy-storage devices, it is crucial to study and understand both the structure as well as dynamics of ions in the material on an atomic level.