T. N. Ramesh

Scalable Synthesis of Tavorite LiFeSO4F and NaFeSO4F Cathode Materials

Rechargeable lithium-ion cells are considered the most advanced energy storage systems. Commercial cells utilize layered metal-based oxides as the positive electrode, but their high cost and safety could limit use in large-scale applications. In an intensive search for alternative materials, phosphates such as LiFePO4, [2–5] and other polyanion (“XO4”) structures have also been explored, including those based on silicates. In addition to possessing high redox potentials and promising Li transport, polyanion frameworks can exhibit remarkable electrochemical and thermal stability. These make them particularly suitable where safety and longevity are a concern. Fluorinated polyanion moieties have also received much attention owing to their similar properties. The addition of fluoride engenders a charge difference and modification to the dimensionality of the lattice along with an increase in redox potential, thus offering the tantalizing promise of new architectures and electrochemical behavior. 2D layered alkali iron fluorophosphates A2FePO4F (A = Na, Li) have been reported—as well as a family of 3D alkali metal fluorophosphate ion conductors known as tavorites. Named after the mineral LiFePO4OH, [9] this structure does not display the inherent limitations of the 1D ion conductivity of the well-known LiFePO4 olivine lattice. The tavorite framework possesses intersecting channels housing Li that afford multidimensional pathways for ion transport. It is adopted by many minerals; and the redox-active LiMPO4F members (V, Fe, Ti, Mn) exhibit excellent electrochemical properties. The newest member of the tavorite family, LiFeSO4F, is reported to be an exciting intercalation host that is anticipated to vie with the stellar LiFePO4 for prominence on the basis of its superior electrochemical properties. These can be achieved with submicron—not nanosized— particles providing considerable advantages for material processing, and the opportunity for even faster rate behavior with further decrease in ion transport path length. The synthesis of LiFeSO4F cannot be accomplished by typical solid-state routes, however. This is due to its low thermodynamic stability in comparison to the temperatures needed to overcome the kinetics necessary for reaction to occur. It was reported that its poor hydrolytic stability, and that of other related LiMFSO4 fluorosulfates means that crystallization must be effected at low (< 400 8C) temperatures in hydrophobic ionic liquids (ILs) in order to control the reactivity of the hydrated iron precursor. IL media have been demonstrated to be an elegant and versatile means to facilitate reactivity at intermediate temperatures. Nonetheless, such exotic solvents, while offering much promise of tailoring reactivity, are prohibitively expensive at ca.

Tavorite Lithium Iron Fluorophosphate Cathode Materials: Phase Transition and Electrochemistry of LiFePO4F – Li2FePO4F

500/g, and removal of excess LiF reagent is almost impossible owing to its low solubility. Here we demonstrate that single-phase tavorite LiFeSO4F is easily crystallized by reaction in hydrophilic tetraethylene glycol at 220 8C to give a highly electrochemically active material. The use of the low-cost solvent obviates the necessity of recycling precious ionic liquids. Reversible Li insertion is facilitated by the close structural similarity of the lattice with the parent tavorite-type monoclinic C21/c FeSO4F framework, whose structure we have solved. We furthermore show that a new possible Na-ion battery material, NaFeSO4F, can be similarly synthesized as a single phase that is closely related to tavorite. Control of reaction parameters leads to ca. 200 nm homogeneously sized crystallites which exhibit surprising ion mobility properties. Our synthetic approach relies on the formation of macroporous FeSO4·H2O formed by rapidly heating FeSO4·7 H2O in a N2/7 % H2 atmosphere, and reacting this with either NaF or LiF in tetraethylene glycol (TEG) at 220 8C for 60 h (Li) or 48 h (Na). The materials were shown to be single phase as determined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The structure type of MSO4·H2O has a strong topological resemblance to the tavorite structure of the fluorosulfate LiMSO4F, as noted. [17–19] The H OH in MSO4·H2O (or HMSO4OH) is replaced by Li F to generate LiMSO4F. As previously described, the reaction between the alkali metal fluorides and FeSO4·H2O involves two simultaneous processes: 1) loss of water from FeSO4·H2O and 2) insertion of LiF or NaF in the structure. To obtain a pure product, the reaction rate (step 2) must be higher than the dehydration rate (step 1) to avoid the crystallization of FeSO4. In previous studies it was argued that a hydrophobic reaction medium is crucial to slow down the dehydration, so that the reaction with LiF can proceed. However, the opposite approach is more effective: namely, the reaction can be performed in a hydrophilic organic solvent such as TEG (Scheme 1). The reaction proceeds by the dissolution of LiF (or NaF) in the solvent (which is sparingly soluble in TEG at elevated temperature) followed by exchange with the H2O molecule in FeSO4·H2O. The reaction must be performed at temperatures low enough to minimize the dehydration of the iron sulfate precursor to [*] R. Tripathi, T. N. Ramesh, B. L. Ellis, Prof. L. F. Nazar Department of Chemistry, University of Waterloo 200 University Avenue West, Waterloo Ontario N2L 3G1 (Canada) Fax: (+ 1)519-746-0435 E-mail: lfnazar@uwaterloo.ca

DISORDER IN LAYERED HYDROXIDES: DIFFaX SIMULATION OF THE X-RAY POWDER DIFFRACTION PATTERNS OF NICKEL HYDROXIDE

We have synthesized LiFePO 4 F by a simple solid-state route as a pure single phase, which we show is isostructural with that of the minerals tavorite and amblygonite, and we report the first isolation of its fully lithium-inserted crystalline analog, Li 2 FePO 4 F. We show that the latter adopts a triclinic P1 tavorite-type framework that is very closely related to the parent phase. The redox activity between these two compositions is very facile and occurs with an 8% change in volume to result in a reversible and stable capacity of about 145 mAh/g. The electrochemical cycling at both room temperature and 55 °C is very stable.

Neutral Nanosheets that Gel: Exfoliated Layered Double Hydroxides in Toluene

Layered metal hydroxides exhibit non-uniform broadening of lines in their X-ray powder diffraction (XRPD) patterns, which cannot always be explained on the basis of crystallite size effects. In the case of hexagonal solids such as nickel hydroxide, DIFFaX simulations of the XRPD patterns show that: (1) stacking faults and turbostratic disorder at low (<30%) incidence selectively broaden the h0l reflections; (2) turbostratic disorder at high (>40%) incidence causes asymmetric broadening of the hk0 reflections and a complete extinction of the hkl reflections while leaving 00l unchanged; (3) interstratification selectively broadens the non-hk0 reflections; and (4) cation vacancies reduce the relative intensity of the 100 reflection. In contrast, a reduction in the thickness of the crystallites along the stacking direction of the layers selectively broadens the 00l reflections while a reduction in the disc diameter causes the progressive broadening and extinction of the hk0 reflections. Comparison with experimental data shows that several kinds of disorders have to be invoked to account for the observed broadening. DIFFaX simulations enable the quantification of the different kinds of disorder.

Solvothermal synthesis of electroactive lithium iron tavorites and structure of Li2FePO4F

A simple strategy to exfoliate inorganic layered double hydroxide (LDH) solids to their ultimate constituent, intact single layers of nanometer thickness and micrometer size, is presented. The procedure involves intercalation of an ionic surfactant that forms a hydrophobic anchored surfactant bilayer in the galleries of the solid followed by simply stirring the intercalated solid in toluene. The method is rapid but at the same time gentle enough to produce exfoliated nanosheets of regular morphology that are electrically neutral and form stable gels at higher concentrations. In this Letter, we describe the phenomena and use molecular dynamics simulations to show that exfoliation of the LDH in toluene is a consequence of the modification of the cohesive dispersive interactions between surfactant chains anchored on opposing inorganic sheets by the toluene molecules. The toluene molecules function as a molecular glue, holding the surfactant-anchored LDH sheets together, leading to gel formation.

Ammonia induced precipitation of cobalt hydroxide: Observation of turbostratic disorder

Lithium transition metal fluorophosphates with a tavorite structure have been recognized as promising electrode materials for lithium-ion batteries because of their good energy storage capacity combined with electrochemical and thermal stability. We report here a new low-cost and environmentally friendly solvothermal process to prepare LiFePO4F, which exhibits a complex single phase regime followed by a two-phase plateau at 2.75 V on electrochemical lithium insertion. The structure of the pure single phase end member Li2FePO4F was synthesized by lithiation of LiFePO4F, and solved via Rietveld refinement of the combined X-ray and neutron diffraction patterns, showing that Li+ occupies multiple sites in the tavorite lattice. LiFePO4OH was prepared by a new synthetic route and the electrochemical capacity for this material is the highest reported to date. LiFePO4OH was found to intercalate lithium at 2.40 V and the reduced Li2FePO4OH phase was found to be amorphous.

Classification of stacking faults and their stepwise elimination during the disorder → order transformation of nickel hydroxide

Cobalt hydroxide freshly precipitated from aqueous solutions of Co salts using ammonia, is a layered phase having a 9.17 A interlayer spacing. DIFFaX simulations of the PXRD pattern reveal that it is turbostratically disordered.

Effect of lightweight supports on specific discharge capacity of β-nickel hydroxide

Nickel hydroxide samples obtained by strong alkali precipitation are replete with stacking faults. The local structures of the stacking faults resemble the stacking patterns of different polytypic modifications that are theoretically possible among the layered hydroxides. This resemblance becomes a basis for the classification of stacking faults into different types. Each type of stacking fault produces a characteristic non-uniform broadening of peaks in the X-ray powder diffraction pattern of nickel hydroxide. DIFFaX simulations aid the classification and quantification of stacking faults. Hydrothermal treatment of a poorly ordered nickel hydroxide slurry at different temperatures (338-473 K) and different durations (5-48 h) shows that the stacking faults are removed in a stepwise manner. The as-precipitated sample has 17-20% stacking faults of the 3R(2) variety, which evolve into the 2H(2) type at 413 K. The 2H(2) stacking faults persist up to 443 K. The stacking faults are completely removed only at 473 K. At this temperature an ordered beta-Ni(OH)(2) phase is observed.

The Effect of the Moisture Content on the Reversible Discharge Capacity of Nickel Hydroxide

While the performance of βbc (bc: badly crystalline)-nickel hydroxide is relatively unaffected by the use of different lightweight supports, the specific discharge capacity of crystalline β-Ni(OH)2 doubles to approximately 355 mAh g–1 Ni (theoretical, 456 mAh g−1) when pasted to a fibre support compared with 170 mAh g–1 Ni when pasted to a nickel foam. Consequently, the fibre is a superior support as it is unaffected by the quality of the active material and extracts a consistently high performance irrespective of the degree of crystallinity, moisture content, morphology and composition of the active material. Electrochemical impedance measurements indicate that a lower charge-transfer resistance at low states-of-charge is responsible for the superior performance of fibre supported β-Ni(OH)2 electrodes.

X-ray Diffraction Studies on the Thermal Decomposition Mechanism of Nickel Hydroxide

The moisture content of I²-nickel hydroxide samples obtained by strong alkali precipitation from nickel salt solutions varies as a function of the pH at precipitation. A low (7-9) pH at precipitation yields nickel hydroxides with 5-6 moisture content, whereas a high (>13) pH at precipitation yields a moisture content of nearly 12. In all cases, the moisture content is lost at moderate (65-125°C) temperatures but is restored on cooling and equilibration (24 h) in the ambient. The restoration of the original moisture content in its entirely in all samples shows that the nickel hydroxide phases obtained at low and high pH are distinct from each other. DlFFaX simulations show that the phases with different moisture contents differ in the degree of turbostratic disorder and the proportion of I±-phase interstratified by them; the high moisture content phase has a higher proportion of the I±-motifs. This is reflected in the characteristic broadening of lines in the power X-ray diffraction patterns of the two phases. The high moisture content hydroxide delivers a higher (385 mAh g-1 of Ni) (0.85 e- exchange) reversible discharge capacity compared to the low moisture content sample (185 mAh g-1) (0.4 e- exchange) in foam pasted electrodes. These observations highlight (i) the decisive role of moisture content in high performance electrode materials and (ii) the role of the pH during precipitation as the sole determining factor that yields high moisture content.