Local Structure of Glassy Lithium Phosphorus Oxynitride Thin Films: A Combined Experimental and Ab Initio Approach

Conducting LixPOy Interface Generated From Insulating Residual Lithium Compounds on LiNi0.8Mn0.1Co0.1O2 Surface Improves Cycle Life and Assists in Fast Cycling

LiNi0.8Mn0.1Co0.1O2 (NMC811) is the most promising cathode material for future Li-ion batteries (LIBs). However, the bulk and surface structural instabilities retard its commercial success. Surface chemical instability toward exposure to moisture (H2O and CO2) leads to the formation of residual lithium compounds (RLCs: Li2CO3, LiOH) on the surface. The alkaline RLCs form a resistive layer on the surface of NMC811 by undergoing parasitic side reactions with electrolytes. Herein, an "Adverse-to-Beneficial" approach is proposed to eliminate RLCs by chemically transforming them into a LixPOy (Li3PO4 and LiPO3) interface. The interface protects the NMC811 surface from moisture attack and unwanted side reactions with electrolytes. It enhances the cycle life by retaining 70% of the initial capacity after 300 cycles at a 0.5C rate and 60% after 500 cycles, even at a 5C rate in a voltage window of 3.0-4.3 V versus Li+/Li. The coexistence of two Li-conducting phases lowers the voltage polarization of the kinetically sluggish H1 → M phase transition to unlock fast cycling, reduces cationic disorder, improves coulombic efficiency, enhances ion diffusion kinetics, and minimizes particle crack formation after long-term cycling. Hence, the LixPOy interface yields multifaceted benefits in the storage, processing, and electrochemistry of NMC811.

Tracking dendrites and solid electrolyte interphase formation with dynamic nuclear polarization-NMR spectroscopy

Polymer-ceramic composite electrolytes enable safe implementation of Li metal batteries with potentially transformative energy density. Nevertheless, the formation of Li-dendrites and its complex interplay with the Li-metal solid electrolyte interphase (SEI) remain a substantial obstacle which is poorly understood. Here we tackle this issue by a combination of solid-state NMR spectroscopy and Overhauser dynamic nuclear polarization (DNP) which boosts NMR interfacial sensitivity through polarization transfer from the metal conduction electrons. We achieve detailed molecular-level insight into dendrites formation and propagation within the composites and determine the composition and properties of their SEI. We find that the dendrite's quantity and growth path depend on the ceramic content and correlated with battery's lifetime. We show that the enhancement of Li resonances in the SEI occurs through Li/Li+ charge transfer in Overhauser DNP, allowing us to correlate DNP enhancements and Li transport and directly determine the SEI lithium permeability. These findings have promising implications for SEI design and dendrites management which are essential for the realization of Li metal batteries.

Highly Condensed and Super-Incompressible Be2PN3

Although beryllium and its compounds show outstanding properties, owing to its toxic potential and extreme reaction conditions the chemistry of Be under high-pressure conditions has only been investigated sparsely. Herein, we report on the highly condensed wurtzite-type Be2PN3, which was synthesized from Be3N2 and P3N5 in a high-pressure high-temperature approach at 9 GPa and 1500 °C. It is the missing member in the row of formula type M2PN3 (M = Mg, Zn). The structure was elucidated by powder X-ray diffraction (PXRD), revealing that Be2PN3 is a double nitride, rather than a nitridophosphate. The structural model was further corroborated by 9Be and 31P solid-state nuclear magnetic resonance (NMR) spectroscopy. We present 9Be NMR data for tetrahedral nitride coordination for the first time. Infrared and energy-dispersive X-ray spectroscopy (FTIR and EDX), as well as temperature dependent PXRD complement the analytical characterization. Density functional theory (DFT) calculations reveal super-incompressible behavior and the remarkable hardness of this low-density material. The formation of Be2PN3 through a high-pressure high-temperature approach expands the synthetic access to Be-containing compounds and may open access to various multinary beryllium nitrides.

A free-standing lithium phosphorus oxynitride thin film electrolyte promotes uniformly dense lithium metal deposition with no external pressure

Lithium phosphorus oxynitride (LiPON) is an amorphous solid electrolyte that has been extensively studied over the last three decades. Despite the promise of pairing it with various electrode materials, LiPON's rigidity and air sensitivity set limitations to understanding its intrinsic properties. Here we report a methodology to synthesize LiPON in a free-standing form that manifests remarkable flexibility and a Young's modulus of ∼33 GPa. We use solid-state nuclear magnetic resonance and differential scanning calorimetry to quantitatively reveal the chemistry of the Li/LiPON interface and the presence of a well-defined LiPON glass-transition temperature of 207 °C. Combining interfacial stress and a gold seeding layer, our free-standing LiPON shows a uniformly dense deposition of lithium metal without the aid of external pressure. This free-standing LiPON film offers opportunities to study fundamental properties of LiPON for interface engineering for solid-state batteries.

Finding Order in Disorder: The Highly Disordered Lithium Oxonitridophosphate Double Salt Li8+x P3 O10-x N1+x (x=1.4(5))

The crystalline lithium oxonitridophosphate Li8+x P3 O10-x N1+x , was obtained in an ampoule synthesis from P3 N5 and Li2 O. The compound crystallizes in the triclinic space group P1 - ${\mathrel{\mathop{{\rm { 1}}}\limits^{{\rm -}}}}$ with a=5.125(2), b=9.888(5), c=10.217(5) Å, α=70.30(2), β=76.65(2), γ=77.89(2)°. Li8+x P3 O10-x N1+x is a double salt, the structure of which contains distinctive complex anion species, namely non-condensed P(O,N)4 tetrahedra, and P(O,N)7 double tetrahedra connected by one N atom. Additionally, there is mixed occupation of O/N positions, which enables further anionic species by variation of O/N occupancies. To characterize these motifs in detail, complementary analytical methods were applied. The double tetrahedron exhibits significant disorder in single-crystal X-ray diffraction. Furthermore, the title compound is a Li+ ion conductor with a total ionic conductivity of 1.2×10-7  S cm-1 at 25 °C, and a corresponding total activation energy of 0.47(2) eV.

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 PO43- orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [O3PNPO3]5- dimeric anions. PO3N4- isolated tetrahedra and [O3POPO3]4- anions are also present but in smaller amounts.

Structure Determination of the Crystalline LiPON Model Structure Li5+x P2 O6-x N1+x with x≈0.9

Non-crystalline lithium oxonitridophosphate (LiPON) is used as solid electrolyte in all-solid-state batteries. Crystalline lithium oxonitridophosphates are important model structures to retrieve analytical information that can be used to understand amorphous phases better. The new crystalline lithium oxonitridophosphate Li_5+x P₂ O_6-x N_1+x was synthesized as an off-white powder by ampoule synthesis at 750-800 °C under Ar atmosphere. It crystallizes in the monoclinic space group P2₁ /c with a=15.13087(11) Å, b=9.70682(9) Å, c=8.88681(7) Å, and β=106.8653(8)°. Two P(O,N)₄ tetrahedra connected by an N atom form the structural motif [P₂ O_6-x N_1+x ]^(5+x)- . The structure was elucidated from X-ray diffraction data and the model corroborated by NMR and infrared spectroscopy, and elemental analyses. Measurements of ionic conductivity show a total ionic conductivity of 6.8×10^-7  S cm^-1 at 75 °C with an activation energy of 0.52±0.01 eV.

Solid State NMR Spectroscopy a Valuable Technique for Structural Insights of Advanced Thin Film Materials: A Review

Solid-state NMR has proven to be a versatile technique for studying the chemical structure, 3D structure and dynamics of all sorts of chemical compounds. In nanotechnology and particularly in thin films, the study of chemical modification, molecular packing, end chain motion, distance determination and solvent-matrix interactions is essential for controlling the final product properties and applications. Despite its atomic-level research capabilities and recent technical advancements, solid-state NMR is still lacking behind other spectroscopic techniques in the field of thin films due to the underestimation of NMR capabilities, availability, great variety of nuclei and pulse sequences, lack of sensitivity for quadrupole nuclei and time-consuming experiments. This article will comprehensively and critically review the work done by solid-state NMR on different types of thin films and the most advanced NMR strategies, which are beyond conventional, and the hardware design used to overcome the technical issues in thin-film research.