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

Abstract

LiNiFeF₆ 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 ⁷Li MAS NMR spectroscopy. An optimised electrochemical in situ cell was employed for the structural and electrochemical characterisation of LiNiFeF₆ 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 P4₂/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+x1NiFeF₆ and Li-rich Li_1+x2NiFeF₆ (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 ⁷Li MAS NMR spectroscopy showing two Li environments with similar intensities after discharging to 2.0 V. Furthermore, in operando XAS investigations revealed that only Fe³⁺ cations participate in the electrochemical process via an Fe³⁺/Fe²⁺ redox reaction, while Ni²⁺ cations remain electrochemically inactive.