# Revealing the Local Structure and Dynamics of the Solid Li Ion Conductor Li3P5O14

## TL;DR

This paper uses NMR and DFT to study the structure and lithium ion movement in Li3P5O14, a promising solid electrolyte for batteries.

## Contribution

The study provides a detailed NMR and DFT-based analysis of the local structure and 3D lithium ion dynamics in Li3P5O14.

## Key findings

- The 31P and 6Li MAS NMR data reveal the structure of the ultraphosphate layer and Li6O1626– chains.
- Li1 and Li5 are the most mobile lithium sites and are adjacent in both intralayer and interlayer directions.
- 6Li NMR relaxation and exchange spectroscopy confirm a 3D lithium ion diffusion pathway through the material.

## Abstract

The development of fast Li ion-conducting materials for
use as
solid electrolytes that provide sufficient electrochemical stability
against electrode materials is paramount for the future of all-solid-state
batteries. Advances on these fast ionic materials are dependent on
building structure-ionic mobility-function relationships. Here, we
exploit a series of multinuclear and multidimensional nuclear magnetic
resonance (NMR) approaches, including 6Li and 31P magic angle spinning (MAS), in conjunction with density functional
theory (DFT) to provide a detailed understanding of the local structure
of the ultraphosphate Li3P5O14, a
promising candidate for an oxide-based Li ion conductor that has been
shown to be a highly conductive, energetically favorable, and electrochemically
stable potential solid electrolyte. We have reported a comprehensive
assignment of the ultraphosphate layer and layered Li6O1626– chains through 31P and 6Li MAS NMR, respectively, in conjunction with DFT. The chemical
shift anisotropy of the eight resonances with the lowest 31P chemical shift is significantly lower than that of the 12 remaining
resonances, suggesting the phosphate bonding nature of these P sites
being one that bridges to three other phosphate groups. We employed
a number of complementary 6,7Li NMR techniques, including
MAS variable-temperature line narrowing spectra, spin-alignment echo
(SAE) NMR, and relaxometry, to quantify the lithium ion dynamics in
Li3P5O14. Detailed analysis of the
diffusion-induced spin-lattice relaxation data allowed for experimental
verification of the three-dimensional Li diffusion previously proposed
computationally. The 6Li NMR relaxation rates suggest sites
Li1 and Li5 (the only five-coordinate Li site) are the most mobile
and are adjacent to one another, both in the a-b plane
(intralayer) and on the c-axis (interlayer). As shown
in the 6Li-6Li exchange spectroscopy NMR spectra,
sites Li1 and Li5 likely exchange with one another both between adjacent
layered Li6O1626– chains and
through the center of the P12O3612– rings forming the three-dimensional pathway. The understanding of
the Li ion mobility pathways in high-performing solid electrolytes
outlines a route for further development of such materials to improve
their performance.