Probing Local Structure, Electronic Structure and Dynamics in Layered P2-Na _x MO ₂ battery Materials by ²³ Na MAS NMR

In this presentation 23 Na MAS NMR was used to probe the local structure, electronic structure and dynamics in several layered Na x MO 2 oxides for application in Na-ion batteries. Among them, P2-Na x CoO 2 appears to be a model material with a very specific phase diagram and unusual physical properties. 23 Na MAS NMR was used to characterize the changes in the electronic structure during Na deintercalation. 23 Na MAS was also used to characterize the local structure and dynamics versus temperature in several P2 or P3 Na x CoO 2 materials and P2-Na 1/2 VO 2 . In the latter, an increase of two orders of magnitude in its electronic conductivity has been observed at approximately 322 K and a structural transition has been found to occur simultaneously. At room temperature, high resolution powder diffraction and pair distribution function analysis reveal the triangular lattice formed by vanadium ions to be distorted by the formation of pseudo-trimers vanadium clusters. At 350 K, the mobility of the electronic charge carried by vanadium ions increases as evidenced by the increase in the electronic conductivity. The arrangement of sodium ordering in P2-Na 1/2 VO 2 , which maximizes sodium-sodium distances to lower electrostatic repulsions between alkali ions, is found to be unchanged across this transition, but the ionic mobility as observed by NMR, clearly change. Finally, the P2- Na 2+x Ni 2-x/2 TeO 6 (0 ≤ x ≤ 0.5) system provides a framework for investigating the effect of dual Na + substitution into the sodium layer and the transition-metal layer of the structure, and its effects on the electrochemical properties of the materials. The sodium substitution disrupts ordering within the transition-metal layer thereby disrupting Na + ordering in the adjacent sodium layers. Beyond a critical sodium concentration, the layer stacking shifts, and all voltage plateaus of the P2-Na 2 Ni 2 TeO 6 material are no longer observed. These results highlight how sodium ordering between the MO 2 layers and the electronic transport within the MO 2 layers are intimately correlated in Na x MO 2 type sodium layered oxides.