Antisite Disorder and Bond Valence Compensation in Li₂FePO₄F Cathode for Li-Ion Batteries

Abstract

T ransition-metal (TM) cation migration has always been considered detrimental for the performance of cathode (positive electrode) materials for metal-ion batteries. In layered oxides based on the rock-salt structure, the TM cations can migrate from their original octahedral sites toward the empty octahedra in the Li layers upon charge and return upon discharge, as observed in the Li-rich Li 2 Ru 1−y Ti y O 3 phases. 1 This process is only partially reversible and leaves a fraction of the TM cations trapped at the tetrahedral interstices of the close-packed oxygen array, resulting in a gradual voltage fade. 1,2 Migration of the TM cations to the Li positions can also induce irreversible phase transformations (e.g., layered Li 0.5 MO 2 to spinel LiM 2 O 4 ) and negatively affect the electrode kinetics due to blocking the Li diffusion pathway. 3−7 Although some cationdisordered materials can demonstrate facile Li diffusion due to the presence of percolation diffusion channels, 8 the perfect ordering of the TM and alkali cations is generally regarded beneficial for the electrochemical performance. 9,10 Compared to the layered oxides much less is known on the TM-alkali metal disorder in the polyanion cathodes. Olivinestructured LiFePO 4 as the commercially deployed material is, perhaps, most scrutinized in this aspect. Owing to the similar ionic radii of the Li + and Fe 2+ cations in the octahedral oxygen environment (0.76 and 0.78 Å, respectively), 11 antisite Li/Fe disorder is possible corresponding to the Kroger−Vink equation:

Solid state chemistry for developing better metal-ion batteries

“…Even more interesting are the polyanionic structures exemplified with (but not restricted to) fluoride-phosphates A 2 MPO 4 F (A = Li, Na, M = Fe, Co), within which the tetrahedral oxygens are not even bound to the transition metal; the reasons why they are termed “dangling” or “semilabile” (Fig. 2b ) 25 , 26 . The lone pairs of these oxygens are equivalent to oxygen non-bonding states and could potentially act as electron sources.…”

“…By extraction of the A + cations these oxygens experience severe underbonding due to the depleted coordination environment (Fig. 2b ) that can be partially compensated by migration of the M cation to the vacant A positions 26 . At the same time, the undercoordinated semilabile oxygens with their sp 3 lone electron pairs carry excessive negative charge acting as traps for the A + cations and increasing their hopping barriers 25 , 55 .…”

“…The relative sensitivity of EDT to lithium compared to that of X-rays scales as ~[Z heavy /3] 1/2 , where Z heavy is the atomic number of the heaviest element in the structure, hence enabling a ~3 times better Li detection in the structures like LiCoO 2 and its derivatives. Thus, Li positions, occupancy factors and antisite disorder can be determined rather precisely 26 . Moreover, EDT is so far the only single-crystal diffraction method to retrieve the crystal structure quantitatively from in situ experiments inside the electron microscope using an electrochemical cell with liquid electrolyte (Fig.…”

Fluorophosphates: Next Generation Cathode Materials for Rechargeable Batteries

“…These oxide layers act as an inert layer between electrode and electrolyte especially in the case where HF is generated during side reactions. In this pursuit, Amaresh [108] Copyright 2016, American Chemical Society. b) Reproduced with permission.…”

Structure solution and refinement of metal-ion battery cathode materials using electron diffraction tomography

“…A formation of antisite defects was studied for the polyanion LIB cathode material Li 2 FePO 4 F (Karakulina et al, 2016) obtained by electrochemical replacement of Na for Li in NaLiFePO 4 F. Structure analysis from EDT data showed massive antisite disorder after merely ten cycles, with up to 40% antisite occupancy (Li at Fe sites or Fe at Li sites) for some positions. This was compared to analogously cycled and characterized LiFePO 4 , where no formation of antisite defects was detected at the same cycling conditions.…”