Brian L. Ellis

Synthesis of nanocrystals and morphology control of hydrothermally prepared LiFePO4

Lithium transition metal phosphate olivines such as LiFePO4 have been recognized as very promising electrodes for lithium-ion batteries because of their energy storage capacity combined with electrochemical and thermal stability. A key issue in these materials is to determine the synthetic conditions for optimum control of particle size and morphology, and ideally to find those that result in nanocrystalline products. Here, we report a full study that examines the synthesis of the material via hydrothermal methods to give single phase nanocrystalline materials for LiFePO4 and LiMnPO4, and their solid solutions with Mg2+. A reaction mechanism is proposed. Variation of the synthesis parameters showed that increasing reactant concentration strongly favours the formation of nanocrystalline products, but as less defect-free materials are formed at temperatures above 180 °C, and ideally above 200 °C, control of nucleation and growth can (and should) also be effected using polymeric or surfactant additives. The nature of the precursor and carbon-containing additives in the autoclave also have profound effects on morphology and the electrochemical properties.

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.

On the Stability of LiFePO4 Olivine Cathodes under Various Conditions (Electrolyte Solutions, Temperatures)

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

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

LiFePO 4 is one of the most important cathode materials for Li-ion batteries studied over the past few years. Impressive work has revealed important structural aspects and the correlations between structure and composition and electrochemical properties. Fewer efforts have been devoted to the surface chemical aspects of this material. We report herein on a study of the stability aspects of LiFePO 4 at two temperatures, 30 and 60°C. Three types of solutions were used based on EC-DMC 1:1 solvent mixtures those involving no acidic contamination (using LiClO 4 as the electrolyte), those contaminated by HF(using LiPF 6 as the Li salt), and LiPF 6 solutions deliberately contaminated with H 2 O. Iron dissolution from LiFePO 4 in these electrolytes, as well as the electrochemical response as a function of solution composition and aging, were studied at the two temperatures. The effect of additives that neutralize acidic species in solution was also studied. In general, LiFePO 4 develops a unique surface chemistry. Highly stable behavior of LiFePO 4 cathodes, without any substantial iron dissolution at elevated temperatures, was observed and measured when the solution contains no acidic or protic contaminants.

Nanostructured materials for lithium-ion batteries: Surface conductivity vs. bulk ion/electron transport

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.

Lithium metal fluorosulfate polymorphs as positive electrodes for Li-ion batteries: synthetic strategies and effect of cation ordering

Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.

Direct synthesis of nanocrystalline Li0.90FePO4: observation of phase segregation of anti-site defects on delithiation

Transition-metal fluorosulfates are currently being extensively explored for their use as cathodes in Li-ion batteries. Several new polymorphs of LiMSO4F (M = Fe, Mn, Zn) crystallizing in the tavorite, triplite and sillimanite structures have captured much recent interest, but synthetic access is limited and the underlying phase stability and ion transport in these materials are poorly understood. Here we report that solvothermal routes to LiMSO4F (M = Fe, Mn, Zn) offer significant advantage over both exotic ionothermal methods and solid state synthesis by enabling greater control of the chemistry. We show new limits for the onset of triplite crystallization, and report new phases in the Li[Fe,Zn]SO4F system that enable a fuller understanding of the complex chemistry and thermodynamics underlying these fascinating materials. The transformation of LiFeSO4F from the tavorite to the triplite polymorph is triggered in the absence of any substituents, proving that tavorite is an intermediate in the reaction pathway. As a result of structural changes between tavorite and triplite, their Li+ transport paths are quite different. Combined X-ray/neutron diffraction studies of the triplites suggest that distinct inter-site zig-zag paths must be involved, owing to complete cation disorder that impacts the electrochemical behavior.

Solvothermal synthesis of electroactive lithium iron tavorites and structure of Li2FePO4F

Solid solutions of LixFePO4 are of tremendous interest because of a proposed increase in ion transport properties, but the formation of these solutions at high temperatures is difficult if not impossible and direct synthesis is difficult and rarely reported. Here we report modified polyol syntheses which produce nanocrystalline Li1−yFePO4 directly, where the maximum Li substoichiometry on the M1 site sustained at synthesis temperatures of 320 °C is about 10%. High target lithium vacancy concentrations promote the increase in anti-site disorder of Li+ and Fe2+, as this process is driven by vacancy stabilization. Combined neutron and X-ray diffraction on partial delithiated substoichiometric olivines reveals segregated defect-free (where Li is extracted) and defect-ridden (where Li remains) regions. This proves (1) that the anti-site defects obstruct Li+ diffusion explaining the detrimental electrochemistry and (2) that the anti-site defects form clusters. Finally, preferential anisotropic strain broadening in the bc-plane indicates the existence of a coherent interface between the Li-poor and Li-rich phases. Along with the size broadening upon delithiation this proves that in nano-sized LixFePO4 the two phases coexist within a single particle, which is not expected based on thermodynamics arguments due to the energy penalty associated with the coherent interface. Thereby, these results give important and unique insight and understanding in the properties of nano sized LiFePO4.

Positive Electrode Materials for Li-Ion and Li-Batteries†

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.