Identification of phosphorus sites in amorphous LiPON thin film by observing internuclear proximities.

Amorphous lithium phosphorus oxynitrides (LiPON), prepared by reactive magnetron sputtering, have become the electrolytes of choice for all-solid-state thin film microbatteries since its discovery in early 1990s. Nevertheless, there is still a lack of understanding of their atomic-level structure and its influence on ionic conductivity. Solid-state NMR spectroscopy represents a promising technique to determine the atomic-level structure of LiPON glasses but is challenging owing to its low sensitivity in the case of thin film materials. Recently, P solid-state NMR spectra of LiPON thin films were acquired under magic-angle spinning (MAS) conditions and assigned with the help of density functional theory (DFT) calculations of NMR parameters. However, the identification of the different P local environments in these materials is still a challenge owing to their amorphous structure and the lack of resolution of the P MAS NMR spectra. We show herein how the NMR observation of internuclear proximities helps to establish the nature of P sites in LiPON thin films. The P-N proximities are probed by a transfer of population in double resonance (TRAPDOR) experiment, whereas P-P proximities are observed using one-dimensional (1D) P double-quantum (DQ)-filtered and two-dimensional (2D) P homonuclear correlation spectra as well as dipolar dephasing experiments using DQ-DRENAR (DQ-based dipolar-recoupling effects nuclear alignment reduction) technique. The obtained NMR data further support the recently proposed assignment of P NMR signals of LiPON thin films. With the help of this assignment, the simulation of the quantitative 1D P NMR spectrum indicates that PO orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [OPNPO] dimeric anions. PON isolated tetrahedra and [OPOPO] anions are also present but in smaller amounts.