Resolving the Amorphous Structure of Lithium Phosphorus Oxynitride (Lipon)

Abstract

Lithium phosphorus oxynitride, also known as Lipon, solid-state electrolytes are at the center of the search for solid-state Li metal batteries. Key to the performance of Lipon is a combination of high Li content, amorphous character, and the incorporation of N into the structure. Despite the material's importance, our work presents the first study to fully resolve the structure of Lipon using a combination of ab initio molecular dynamics, density functional theory, neutron scattering, and infrared spectroscopy. The modeled and experimental results have exceptional agreement in both neutron pair distribution function and infrared spectroscopy. Building on this synergy, the structural models show that N forms both bridges between two phosphate units and nonbridging apical N. We further show that as the Li content is increased the ratio of bridging to apical N shifts from being predominantly bridging at Li contents around 2.5:1 Li:P to only apical N at higher Li contents of 3.38:1 Li:P. This crossover from bridging to apical N appears to directly correlate with and explain both the increase in ionic conductivity with the incorporation of N and the ionic conductivity trends found in the literature.

Elucidating Interfacial Stability between Lithium Metal Anode and LiPON via In Situ Electron Microscopy

“…It needs to be emphasized that no observable changes in the morphology of LiPON or its interface have been observed after an external bias of 5 V is applied to the interface inside the microscope, indicating that interphase layer here is selflimiting and is stable. 30 Peaks "a" and "c" in Figure 4A primarily arise from the coordination of Li with O, while peak "b" can be attributed to the coordination with N, based on previously reported EELS ne structure of Li oxides and nitrides. 31,32 Similar ne structure changes also occurred to P L-edges ( Figure 4B), where four distinctive peaks are clearly visible in the pristine spectra, the one from the interphase appear as a delayed edge.…”

“…~100nm) using radio frequency magnetron sputtering in a reactive N 2 atmosphere following previous reports. 18,19,23,27 This con guration assures the sample to be electron beam transparent with a suitable thickness for EELS measurements. The anode was fabricated by coating a tungsten tip with Li metal.…”

“…31,32 Similar ne structure changes also occurred to P L-edges ( Figure 4B), where four distinctive peaks are clearly visible in the pristine spectra, the one from the interphase appear as a delayed edge. The nearest neighbors of the P atoms in LiPON include O and N, 30 the P-L ne structures, such as peaks "a" and "b" may largely corresponds to its bonding with O and N. In contrast, the interphase P-L contain less feature with a relatively earlier starting edge "e", similar to P 3-EELS characters (EELS atlas, 1987), which could be a result from valence change from a phosphate to phosphide. As the beam current and the acquisition time used in this experiment are low, the signal-to-noise ratio of EELS at the higher energy loss, e.g.…”

“…This difference in the TEM contrast can be linked to the formation of compounds with decreased density, among other factors. The density of LiPON was experimentally determined to be 2.3 g/cc while Li metal has a density of 0.53 g/cc 27. The density is 1.27 g/cc for Li 3 N, 1.44 g/cc for Li3 P, and 2.01 g/cc for Li 2 O, all of which are lower than that of pristine LiPON.…”

A Lithium Oxythioborosilicate Solid Electrolyte Glass with Superionic Conductivity

“…While resolving the structure of amorphous materials is a longstanding challenge, the combination of NMR and Raman spectroscopies is a very useful local probe. Techniques such as ab initio molecular dynamics coupled with neutron pair distribution function (PDF) analysis have also proven to be powerful tools for elucidation of the local structure in LiPON . These methods, along with detailed NMR studies of Li diffusion will be employed in subsequent studies to gain a better understanding of ion conduction in these oxysulfide glasses.…”