States of thermochemically or electrochemically synthesized Na_xP_y compounds analyzed by solid state ²³Na and ³¹P nuclear magnetic resonance with theoretical calculation

Phosphorus is a promising material for the electrode in sodium ion batteries (NIBs). In this study, the states of Na_xP_y compounds synthesized by thermochemical reaction and electrochemical sodiation were compared using solid state ²³Na magic angle spinning (MAS) NMR, ²³Na multiple quantum (MQ) MAS NMR, and ³¹P MAS NMR. The NMR signals in thermochemically synthesized Na_xP_y compounds (Na₃P, NaP, Na₃P₇, Na₃P₁₁, NaP₇) are assigned in reference to theoretical chemical shifts, based on first-principles calculations. Furthermore, the NMR signals in electrochemically prepared Na_xP_y compounds after two sodiation/desodiation cycles are ascribed to Na₃P compounds and three amorphous compositions. The amorphous compounds ascribed to Na_1−αP (0 < α < 1), Na_2−βP (0 < β < 1), and Na_3−γP (0 < γ < 1) are formed below 0.58 V in the charge and discharge process. These Na₃P and the amorphous phases overlapped in the range from 0.20 V (sodiation process) to 0.58 V (desodiation process). ²³Na and ³¹P NMR spectra reveal reversible sodiation and desodiation processes in the third cycle.