Revealing Defects in Crystalline Lithium-Ion Battery Electrodes by Solid-State NMR: Applications to LiVPO₄F

Abstract

Identifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO 4 F and establish the relative sub-nanometer-scale proximities between them. Well-crystallized LiVPO 4 F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solidstate 7 Li nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic 7 Li environments that can account for up to 20% of the total lithium content. Multi-dimensional and site-selective solid-state 7 Li NMR experiments using finite-pulse radio-frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO 4 F crystal structure, revealing the existence of dipole-dipole-coupled defect pairs. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO 4 F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing lithium defect environments in crystalline solids, including paramagnetic materials with short 7

Understanding Local Defects in Li-Ion Battery Electrodes through Combined DFT/NMR Studies: Application to LiVPO4F

“…Kosova et al 11 suggested more recently that these extra signals would correspond to the three Li sites of the anti-Nasicon Li3V2(PO4)3, which could be present as impurity, even though their positions do not match exactly those reported in other studies 3,12 . In fact, recently 13 , using 2D 7 Li NMR experiments, we could show that the additional signals correspond to Li sites close to structural and/or chemical defects in LiVPO4F, and not to Li in impurity phases as suggested previously 11 . We thus considered several structural and chemical hypotheses for modeling the defects in LiVPO4F and compared the resulting calculated Fermi contact shifts with the experimental ones.…”

“…LiVPO4F exhibits a Tavorite-type structure described in (Figure 2c) 10,13 . In the following, we will first interpret the main signal located at 116 ppm for the Li site in LiVPO4F based on calculated 3D spin density maps.…”

Advances in the Cathode Materials for Lithium Rechargeable Batteries

“…[47] As in olivine-type materials,the presence of defects blocks the 1D channels and the rate is determined by the diffusion of the Li + around the defects. [102] It was shown that the elimination of anti-site defects leads to as ignificant improvement of electrochemical performance of LiVPO 4 F. [103] Tw oL i + sites exist in the LiFeSO 4 Fu nit cell occupying positions in the tunnels that run along the [100] and [010] directions.Itwas found that the lowest activation energy for Li + diffusion is 0.36-0.46 eV,s uggesting ah igh Li + mobility. [104] Them ost favorable migration paths are formed from zigzag jumps of Li + allowing the long-range diffusion along the [100],[ 010],a nd [111] directions.T hese interconnected paths create a3 Dd iffusion network leading to the good electrochemical performance of the LiFeSO 4 Fmaterial.…”

Kathodenmaterialien für wiederaufladbare Lithiumbatterien

“…Dieses Ergebnis belegt die eindimensionale Natur der Li + ‐Mobilität in Tavoritstrukturen, wie sie bereits bei den Olivinen beobachtet wurde. Wie bei Olivin‐Materialien blockiert das Vorhandensein von Defekten die 1D‐Kanäle und die Transportgeschwindigkeit wird von der Diffusion des Li + an den Defekten bestimmt . Es wurde gezeigt, dass die Beseitigung von Defekten zu einer signifikanten Verbesserung der elektrochemischen Leistung von LiVPO 4 F führt .…”