Unravelling the mechanism of lithium insertion into and extraction from trirutile-type LiNiFeF₆ cathode material for Li-ion batteries

Abstract

LiNiFeF 6 was used as cathode material in lithium-ion cells and studied by in situ X-ray diffraction (XRD), in operando X-ray absorption spectroscopy (XAS) and 7 Li MAS NMR spectroscopy. An optimised electrochemical in situ cell was employed for the structural and electrochemical characterisation of LiNiFeF 6 upon galvanostatic cycling. The results for the first time reveal the lithium insertion process into a quaternary lithium transition metal fluoride with a trirutil-type host structure (space group P4 2 /mnm). The in situ diffraction experiments indicate a preservation of the structure type after repeated lithium insertion and extraction. The lithium insertion reaction can be attributed to a phase separation mechanism between Li-poor Li 1+x1 NiFeF 6 and Li-rich Li 1+x2 NiFeF 6 (x1 ≲ 0.16 ≲ x2), where not only the weight fractions, but also the lattice parameters of the reacting phases change. The insertion of Li ions into [001]-channels of the trirutile structure causes an anisotropic lattice expansion along the tetragonal a-axes. An overall increase in the unit cell volume of~6% and a reduction in the c/a ratio of~4% are detected during discharge. Changes of atomic coordinates and distances suggest the accommodation of intercalated lithium in the empty six-fold coordinated 4c site. This is confirmed by 7 Li MAS NMR spectroscopy showing two Li environments with similar intensities after discharging to 2.0 V. Furthermore, in operando XAS investigations revealed that only Fe 3+ cations participate in the electrochemical process via an Fe 3+ /Fe 2+ redox reaction, while Ni 2+ cations remain electrochemically inactive. CrystEngCommThis journal is

Phase Transformation Behavior and Stability of LiNiO2 Cathode Material for Li‐Ion Batteries Obtained from In Situ Gas Analysis and Operando X‐Ray Diffraction

“…During galvanostatic charging and discharging at C/20 in the voltage range of 3.0-4.3 V, diffraction data were recorded using al aboratory diffractometer optimized for battery research. [41] Patterns were acquired with at ime resolution of 180 s, and they were evaluated by Rietveld refinement analysis. Refinement of both ND and XRD data was performed using the software TOPAS-Academic V5.…”

“…Interestingly,O 2 evolution can be clearly observed at the end of charging and during dischargingafter the plateau at high voltage. In this work, operando XRD measurementsw ere performed on Li/LNO cells using al aboratoryd iffractometer optimized for battery research, [41] allowing highquality data with good time resolution to be collected. Apparently,O 2 evolutioni sc orrelated with structural rearrangements in the CAM.…”

“…In addition, they only provide an approximate overview of phase regions in the phase diagram of Li x NiO 2 .A nu pdated version of structurald ata on LNO was recently reported by Li et al [40] However,t he resolution [in terms of x(Li)] is still not sufficient to draw unequivocal conclusions about the interconnection of structural changes and the observedg as formation. In this work, operando XRD measurementsw ere performed on Li/LNO cells using al aboratoryd iffractometer optimized for battery research, [41] allowing highquality data with good time resolution to be collected. Hence, better identification of the two-phase regions and direct correlation of structuralc hanges with cell potential are ensured.…”

Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni‐Rich NCM and Li‐Rich HE‐NCM Cathode Materials in Li‐Ion Batteries

“…Crystallographic volume changes and multiphase transformations are assumed to be responsible for the capacity fading and to be one of the main factors governing the material fatigue. [135] The high-flux beam (Mo Kα 1,2 ) and the fast 2D detector of the setup allowed performing operando XRD experiments with good time resolution and tracking crystallographic parameters, such as unit cell volume, atomic positions, and crystallographic strain, directly as a function of Li content in the electrode. Hence, some of our studies aimed at analyzing the changes in the NCM crystal structure during Li (de-)intercalation under realistic conditions.…”

Kinetics and Structural Investigation of Layered Li9V3(P2O7)3(PO4)2 as a Cathode Material for Li‐Ion Batteries

“…The detector was mounted perpendicular to the incoming beam with a sample‐to‐detector distance of 320 mm. Details of the diffraction setup can be found in the reference . The experimental geometry was determined with an annealed CeO 2 powder sample and the pyFAI software package was used for the detector calibration and azimuthal integration of the 2D diffraction images .…”