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, ³¹P 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 ³¹P 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 ³¹P-¹⁴N proximities are probed by a transfer of population in double resonance (TRAPDOR) experiment, whereas ³¹P-³¹P proximities are observed using one-dimensional (1D) ³¹P double-quantum (DQ)-filtered and two-dimensional (2D) ³¹P 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 ³¹P NMR signals of LiPON thin films. With the help of this assignment, the simulation of the quantitative 1D ³¹P NMR spectrum indicates that PO₄^3- orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [O₃PNPO₃]^5- dimeric anions. PO₃N^4- isolated tetrahedra and [O₃POPO₃]^4- anions are also present but in smaller amounts.