Journal of Power Sources, Vol.413, 418-424, 2019
States of thermochemically or electrochemically synthesized NaxPy compounds analyzed by solid state Na-23 and P-31 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 NaxPy compounds synthesized by thermochemical reaction and electrochemical sodiation were compared using solid state Na-23 magic angle spinning (MAS) NMR, Na-23 multiple quantum (MQ) MAS NMR, and P-31 MAS NMR. The NMR signals in thermochemically synthesized NaxPy 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 NxPy compounds after two sodiation/desodiation cycles are ascribed to Na3P compounds and three amorphous compositions. The amorphous compounds ascribed to Na1-alpha P (0 < alpha < 1), Na2-beta P (0 < beta < 1), and Na3-gamma P (0 < gamma < 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). Na-23 and P-31 NMR spectra reveal reversible sodiation and desodiation processes in the third cycle.