Structural Evolution of Li[sub 3+x]Fe(MoO[sub 4])[sub 3] upon Lithium Insertion in the Compositional Range 0≤x≤1

Abstract

Li 3 Fe͑MoO 4 ͒ 3 reversibly inserts 1 lithium ion per formula down to 2 V vs Li. A preliminary room temperature phase diagram for Li 3+x Fe͑MoO 4 ͒ 3 ͑0 Ͻ x Ͻ 1͒ is constructed combining electrochemical and in situ X-ray diffraction results. Single-phase regions are detected at x = 0, 0.75, and 1. The crystalline structure of the final compound Li 3+1 Fe͑MoO 4 ͒ 3 is derived from that of Li 3 Fe͑MoO 4 ͒ 3 by completely filling the tunnel formed by square pyramidal sites along the a-axis of the structure. The close structural relationship between the host and inserted compounds ensures a good capacity retention of this material over prolonged electrochemical cycling in lithium cells.Since the demonstration of reversible electrochemical lithium insertion-extraction in LiFePO 4 in 1997, 1 compounds built up from polyatomic anions have attracted much attention as an alternative to LiMO 2 ͑M = transition metal͒ layered materials as positive electrode for rechargeable lithium batteries. Materials where the redox activity is due to the Fe +2 /Fe +3 couple seem to be the most attractive in terms of electrochemical performance, low cost, and environmental friendliness. Besides iron phosphates, 2-9 the electrochemistry of iron molybdates Fe 2 ͑MoO 4 ͒ 3 , 10,11 FeMoO 4 , 12 and more recently Li 3 Fe͑MoO 4 ͒ 3 have also been investigated. 13 The compound Li 3 Fe͑MoO 4 ͒ 3 presents a NASICON-related structure 14,15 built up from interconnected octahedra, tetrahedra, and trigonal prisms, displaying good lithium conductivity that appears to be related to the motion of lithium within the prismatic channels along the a axis ͑ 300°C = 6.6 ϫ 0 −7 Scm −1 , 600°C = 1.4 ϫ 10 −3 Scm −1 ͒. 16 In a previous paper 13 we have shown that the high mobility of lithium ions is also evidenced during the insertion of lithium, up to 1 ion per formula unit, that is associated with the reduction of Fe +3 to Fe +2 . Lithium intercalation within Li 3+x Fe͑MoO 4 ͒ 3 in the range 0 ഛ x ഛ 1 proceeds with slight structural modifications, excellent reversibility, and fast kinetics, characteristics that make this compound attractive as electrode material in electrochemical devices. To complete the study of this compound, it is important to fully understand the evolution of the electrode upon lithium insertion. With the aim of analyzing the structural changes induced by lithium insertion into Li 3 Fe͑MoO 4 ͒ 3 , and to identify the different phases eventually formed along the intercalation reaction, in this work we have followed the lithium insertion process by in situ synchrotron X-ray diffraction ͑SXRD͒. As it will be shown, although the framework of Li 3 Fe͑MoO 4 ͒ 3 is retained all along the insertion reaction, at least two other single phases are formed: Li 3+0.8 Fe͑MoO 4 ͒ 3 and Li 3+1 Fe͑MoO 4 ͒ 3 . ExperimentalThe starting material Li 3 Fe͑MoO 4 ͒ 3 was prepared by solid-state reaction from Fe 2 O 3 , LiNO 3 , and Mo 7 O 24 ͑NH 4 ͒ 6 ·4H 2 O. Stoichiometric mixtures of these reactants were uniaxially pressed into 12 mm diameter pellets and fired i...

Recent advances in the research of polyanion-type cathode materials for Li-ion batteries

“…219 Lithium intercalation into Li 3 Fe(MoO 4 ) 3 was found to occur at 2.4 V with the formation of Li 4 Fe(MO 4 ) 3 . 228,229 Recently, Mikhailova et al 230 investigated both the Li extraction and insertion behavior of orthorhombic NASICON-Li 3 V(MoO 4 ) 3 . Lithium extraction from Li 3 V(MoO 4 ) 3 occurs at 3.7 V with the formation of Li 2 V (MoO 4 ) 3 and preserving the space-group symmetry.…”

Lithium Insertion into Li2MoO4: Reversible Formation of (Li3Mo)O4 with a Disordered Rock-Salt Structure

“…Detailed structural studies of deep Li-insertion into ternary molybdenum oxides with 3d transition metals often registered an unknown phase with a cubic rock-salt type structure with a lattice parameter of about 4.12 Å. ,− For example, such a compound was found during Li-insertion into MnMoO 4 (ref ) and ZnMoO 4 (ref ). A formation of a defect Li x Zn 1‑ x O rock-salt structure was proposed by Leyzerovich et al A cubic NaCl-type compound was also detected by in situ synchrotron measurements during Li-intercalation into isostructural Li 3 V­(MoO 4 ) 3 (ref ) or Li 3+ x Fe­(MoO 4 ) 3 with x > 1 (ref ) materials. A compound with a similar cubic lattice parameter was also observed after chemical lithiation of Fe 2 (MoO 4 ) 3 , where a formation of iron oxide with a small amount of Li was proposed …”

“…The NASICON-like structure of these molybdates is built on a three-dimensional framework of ( M ,Li)­O 6 -octahedra sharing corners with MoO 4 -tetrahedra. The electrochemical behavior of such quaternary molybdenum oxides with 3d transition metals as cathode materials was studied in numerous works. − The oxidation and reduction processes during lithium extraction and insertion are associated with a change in the oxidation state of the 3d transition metal, either alone or with molybdenum simultaneously . In oxide compounds, molybdenum typically features oxidation states between 3+ and 6+ and adopts different oxygen surroundings such as tetrahedra, tetragonal pyramids and octahedra.…”

“…Li 3-x MoO4 , carbon black and PVdF with a mass of about 20 mg were charged with a C/10 rate to 2.6 V and 3.5 V and immediately disassembled. In order to remove LiPF 6 from the surface the samples were washed with DMC.…”

NASICON Open Framework Structured Transition Metal Oxides for Lithium Batteries

“…The position of the redox couple Fe 3+ /Fe 2+ is located at 3.3 -3.4 eV below the Fermi energy of lithium and this material delivered a reversible capacity of 110 mAh/g relative to a Li-metal anode. In an independent analysis, Li 3 Fe(MoO 4 ) 3 was shown to reversibly insert lithium down to 2 V akin to Fe 2 (MoO 4 ) 3 (Dompablo et al, 2006). They conducted a comprehensive phase diagram study for Li 3+x Fe(MoO 4 ) 3 which revealed preservation of the structural framework for low lithium contents (0<x<1) ensuring good cyclability of the material in lithium cells, however, with a slight change of the cell volume (0.85%) (Vega et al, 2005).…”