Phosphorous Acid Route Synthesis of Iron Tavorite Phases, LiFePO₄(OH)ₓF₁₋ₓ [0 ≤ X ≤ 1] and Comparative Study of Their Electrochemical Activities

New synthesis routes were employed for the synthesis of three derivatives of iron hydroxo-, fluoro-, and mixed hydroxo-fluoro phosphates LiFePO 4(OH)xF1-x where 0 ? x ? 1 with the tavorite structure type, and their detail electrochemical activities have been presented. The hydrothermal synthesis of the pure hydroxo-derivative, LiFePO4OH, using phosphorous acid as a source of phosphate yielded good quality crystals from which the crystal structure was solved for the first time using SC-XRD (single crystal X-ray diffraction). The fluoro derivative, LiFePO4F, was prepared as a very fine powder at low temperature in a solvent-less flux-based method employing phosphorous acid and mixed alkali metal nitrates. A mixed anionic hydroxo-fluoro iron tavorite phase, LiFePO 4(OH)0.32F0.68, was also synthesized by a hydrothermal route. The electrochemical performance of the three phases was studied with galvanostatic charge-discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). All three phases showed facile Li-insertion through the reduction of Fe3+ to Fe2+ at an average voltage in the range of 2.4-2.75 V, through the variation of the anion from pure OH to pure F. An increase of 0.35 V was observed as a result of F substitution in the OH position. Also, good cyclability and capacity retention were observed for all three phases and a reversible capacity of more than 90% was achieved for LiFePO4F. The results of EIS indicated that lithium ion mobility is highest in the mixed anion

New polyanion-based cathode materials for alkali-ion batteries

A number of new materials have been discovered through exploratory synthesis with the aim to be studied as the positive electrode (cathode) in Li-ion and Na-ion batteries. The focus has been set on the ease of synthesis, cost and availability of active ingredients in the battery, and decent cycle-life performance through a combination of iron and several polyanionic ligands. An emphasis has been placed also on phosphite (HPO32-) as a polyanionic ligand, mainly due to the fact that it has not been studied seriously before as a polyanion for cathode materials. The concept of mixed polyanions, for example, boro-phosphate and phosphate-nitrates were also explored. In each case the material was first made and purified via different synthetic strategies, and the crystal structure, which dominantly controls the performance of the materials, has been extensively studied through Single-Crystal X-ray Diffraction (SCXRD) or synchrotron-based Powder X-ray Diffraction (PXRD). This investigation yielded four new compositions, namely Li3Fe2(HPO3)3Cl, LiFe(HPO3)2, Li0.8Fe(H2O)2B[P2O8]·H2O and AFePO4NO3 (A = NH4/Li, K). Furthermore, for each material the electrochemical performance for insertion of Li+ ion has been studied by means of various electrochemical techniques to reveal the nature of alkali ion insertion. In addition Na-ion intercalation has been studied for boro-phosphate and AFePO4NO3. Additionally a novel synthesis procedure has been reported for tavorite LiFePO4F1-x(OH)x, where 0≤ x ≤1, an important class of cathode materials. The results obtained clearly demonstrate the importance of crystal structure on the cathode performance through structural and compositional effects. Moreover these findings may contribute to the energy storage community by providing insight into the solid-state science of electrode material synthesis and proposing new alternative compositions based on sustainable materials --Abstract, page iv

Accelerating Cathode Material Discovery through Ab Initio Random Structure Searching

The choice of cathode material in Li-ion batteries underpins their overall performance. Discovering new cathode materials is a slow process, and all major commercial cathode materials are still based on those identified in the 1990s. Discovery of materials using high-throughput calculations has attracted great research interest; however, reliance on databases of existing materials begs the question of whether these approaches are applicable for finding truly novel materials. In this work, we demonstrate that ab initio random structure searching (AIRSS), a first-principles structure prediction method that does not rely on any pre-existing data, can locate low energy structures of complex cathode materials efficiently based only on chemical composition. We use AIRSS to explore three Fe-containing polyanion compounds as low-cost cathodes. Using known quaternary LiFePO4 and quinary LiFeSO4F cathodes as examples, we easily reproduce the known polymorphs, in addition to predicting other, hitherto unknown, low energy polymorphs and even finding a new polymorph of LiFeSO4F that is more stable than the known ones. We then explore the phase space for Fe-containing fluoroxalates, predicting a range of redox-active phases that are yet to be experimentally synthesized, demonstrating the suitability of AIRSS as a tool for accelerating the discovery of novel cathode materials

Electronic changes in poly(3,4-ethylenedioxythiophene)-coated LiFeSO4F during electrochemical lithium extraction

The redox activity of tavorite LiFeSO4F coated with poly (3,4-ethylenedioxythiophene), i.e. PEDOT, is investigated by means of several spectroscopic techniques. The electronic changes and iron-ligand redox features of this LiFeSO4F-PEDOT composite are probed upon delithiation through X-ray absorption spectroscopy. The PEDOT coating, which is necessary here to obtain enough electrical conductivity for the electrochemical reactions of LiFeSO4F to occur, is electrochemically stable within the voltage window employed for cell cycling. Although the electronic configuration of PEDOT shows also some changes in correspondence of its reduced and oxidized forms after electrochemical conditioning in Li half-cells, its p-type doping is fully retained between 2.7 and 4.1 V with respect to Li+/Li during the first few cycles. An increased iron-ligand interaction is observed in LixFeSO4F during electrochemical lithium extraction, which appears to be a general trend for polyanionic insertion compounds. This finding is crucial for a deeper understanding of a series of oxidation phenomena in Li-ion battery cathode materials and helps paving the way to the exploration of new energy storage materials with improved electrochemical performances

1. Introduction

Criticism of the methods and conclusions of the Enlightenment was initiated almost as soon as the movement itself had begun. It is for this reason that this chapter follows immediately after the one on the Enlightenment, rather than after the later chapters on nationalism, liberalism, industrialism, evolutionary biology, and the social sciences. These movements made their appearance during the latter part of the eighteenth century, but often served only to broaden and strengthen the earlier criticisms of the Enlightenment and the demands for a more adequate way of thinking than it offered. The movements of thought with which we are concerned in this chapter — evangelism, utilitarianism, romanticism, and idealism — started in the eighteenth century and became characteristic elements of Western culture during the first part of the nineteenth century. After about 1848 other currents of thought, mainly social and scientific, tended to supplant them. [excerpt

Structurally and electronically diverse polyanion-based cathode materials for alkali-ion batteries

”Through this investigation polyanion-based cathode materials including sulfates, phosphates, and phosphites of transition metal have been synthesized for lithium and sodium-ion batteries and their electrochemical performances were evaluated. The emphasis was on soft chemical routes to discover metastable phases which are often missed in high temperature synthesis. Iron-based compounds were the main focus of this investigation, however, concurrently a few vanadium-analogues of iron compounds were investigated to evaluate the effect of multi-electron process on achievable voltage and capacity. This investigation resulted in the discovery of several compounds with unique crystal structures, namely AFe3(SO4)2(OH)6 (A = Na/NH4), NaFe(HPO4)2, Li2Fe(H0.5PO4)2, Li3Fe(PO4)2, Fe3(PO4)2(OH)2, LiV(HPO3)2 and Na3(VO)2(PO4)2F. Single-crystal and synchrotron powder X-ray diffraction techniques have been used to determine the crystal structures. Partial fluoro-substitution in NaFe3(SO4)2(OH)6 improved Li-ion insertion voltage and achievable capacity owing to a synergistic effect of smaller particle size and inductive effect. A new composition, NaFe(HPO4)2, was synthesized through a hydrothermal route. Subsequent partial and full ion-exchange produced two new electroactive compounds, Li2Fe(H0.5PO4)2 and Li3Fe(PO4)2, respectively. A mineral, barbosalite, Fe3(PO4)2(OH)2, has been evaluated as a cathode for Li-ion battery and exhibited capacity enhancement on cycling. LiV(HPO3)2 and Na3(VO)2(PO4)2F exhibited facile electrochemistry with an average voltage of 4.0 and 3.8 V in Li- and Na-ion batteries, respectively. Both of these compounds showed two electron processes with capacities exceeding 125 and 150 mAh.g-1, respectively --Abstract, page iv

Novel High Voltage Electrodes for Li-ion Batteries

An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family