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

Synthesis, Electrochemistry and Solid-Solution Behaviour of Energy Storage Materials Based on Natural Minerals

Polyanionic compounds have been heavily investigated as possible electrode materials in lithium- and sodium-ion batteries. Chief among these is lithium iron phosphate (LiFePO4) which adopts the olivine structure and has a potential of 3.5 V vs. Li/Li+. Many aspects of ion transport, solid-solution behaviour and their relation to particle size in olivine systems are not entirely understood. Morphology, unit cell parameters, purity and electrochemical performance of prepared LiFePO4 powders were greatly affected by the synthetic conditions. Partially delithiated olivines were heated and studied by Mössbauer spectroscopy and solid-solution behaviour by electron delocalization was observed. The onset of this phenomenon was around 470-500 K in bulk material but in nanocrystalline powders, the onset of a solid solution was observed around 420 K. The isostructural manganese member of this family (LiMnPO4) was also prepared hydrothermally. Owing to the thermal instability of MnPO4, partially delithiated LiMnPO4 did not display any solid-solution behaviour. Phosphates based on the tavorite (LiFePO4OH) structure include LiVPO4F and LiFePO4(OH)1-xFx which may be prepared hydrothermally or by solid state routes. LiVPO4F is a high capacity (2 electrons/transition metal) electrode material and the structures of the fully reduced Li2VPO4F and fully oxidized VPO4F were ascertained. Owing to structural nuances, the potential of the iron tavorites are much lower than that of the olivines. The structure of Li2FePO4F was determined by a combined X-ray and neutron diffraction analysis. The electrochemical properties of very few phosphates based on sodium are known. A novel fluorophosphate, Na2FePO4F, was prepared by both solid state and hydrothermal methods. This material exhibited two two-phase plateau regions on cycling in a half cell versus sodium but displayed solid-solution behaviour when cycled versus lithium, where the average potential was 3.3 V. On successive cycling versus Li a decrease in the sodium content of the active material was observed, which implied an ion-exchange reaction occurred between the material and the lithium electrolyte. Studies of polyanionic materials as positive electrode materials in alkali metal-ion batteries show that some of these materials, namely those which contain iron, hold the most promise in replacing battery technologies currently available

Recent Advances in Fluorophosphate and Orthosilicate Cathode Materials for Lithium Ion Batteries

Corresponding authors.SHI Zhi-Cong, Email: [email protected]; Tel: +86-411-39893938. YANG Yong, Email: [email protected]; Tel: +86-592-2185753.[中文文摘]综述了用于锂离子电池的氟磷酸盐和正硅酸盐正极材料的研究现状,重点对各种材料的结构及合成方法与性能的关系,特别是对如何改善材料的电化学性能进行了总结和探讨.展望了这两类锂离子电池正极材料的发展趋势.[英文文摘]We review recent research on fluorophosphate and orthosilicate cathode materials for lithium ion batteries.Emphasis is placed on the relationship between structures,methods of preparation and properties of the cathode materials.We especially focus on factors leading to an improvement in their electrochemical performance.Trends of research into fluorophosphate and orthosilicate cathode materials are also discussed.高等学校博士学科点专项科研基金(20090041120020); 中央高校基本科研业务费专项资金(DUT10JN06)资助项

Sol-gel synthesis and electrochemical properties of fluorophosphates Na(2)Fe(1-x)Mn(x)PO(4)F/C (x=0, 0.1, 0.3, 0.7, 1) composite as cathode materials for lithium ion battery

Fluorophosphates Na(2)Fe(1-x)Mn(x)PO(4)F/C (x = 0, 0.1, 0.3, 0.7, 1) composite were successfully synthesized via a sol-gel method. The structure, morphology and electrochemical performance of the as prepared materials were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge/discharge measurements. XRD results show that, consistent with Na(2)FePO(4)F, Na(2)Fe(0.9)Mn(0.1)PO(4)F (x = 0.1) crystallize in a two-dimensional (2D) layered structure with space group Pbcn. However, increasing the content of Mn to x >= 0.3 results in a structure transition of Na(2)Fe(1-x)Mn(x)PO(4)F from the 2D layered structure of Na(2)FePO(4)F to the three-dimensional (3D) tunnel structure of Na(2)MnPO(4)F. SEM and TEM analysis indicates nanostructured primary particles (about tens of nanometres in diameter) are obtained for all samples due to uniform carbon distribution and low calcining temperature used. Na(2)FePO(4)F is able to deliver a reversible capacity of up to 182 mA h g(-1) (about 1.46 electrons exchanged per unit formula) with good cycling stability. Compared with Na(2)FePO(4)F, partial replacement of Fe by Mn in Na(2)Fe(1-x)Mn(x)PO(4)F increases the discharge voltage plateau. Similar to Na(2)FePO(4)F, iron-manganese mixed solid solution Na(2)Fe(1-x)Mn(x)PO(4)F (x 0.1, 0.3, 0.7) also show good cycling performance. Furthermore, Na(2)MnPO(4)F with high electrochemical activity was successfully prepared for the first time, which is able to deliver a discharge capacity of 98 mA h g(-1). The good electrochemical performance of Na(2)Fe(1-x)Mn(x)PO(4)F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.National Basic Research Program of China (973 program)[2011CB935903, 2007CB209702]; National Natural Science Foundation of China[20873115, 21021002, 90606015

LiY(SO4_4)2_2: A Superionic Material Synthesized by Superionic State Hidden in no-Superionic Literature

A potential superionic material LiY(SO$_4$)$_2$ has been excavated from the published literatures because its synthesis method and experiment data implied it exists the superionic state. We use \textit{ab initio} calculation to analyzing the differences between solid state and superionic state. We found the diffusion of Li$^+$ from the lattice site to the interstitial site will change the nearest neighbor numbers of O atom from 4 to 8. In order to reduce energy, the reorientation of SO$_4^{2-}$ must exist accompany with the diffusion of Li$^+$ so the nearest neighbor number of O will keep about 5 in the superionic state. Our work not only presents an example for discovering materials from literatures based on prior knowledge but also reveals the micromechanism of cation-anion coupled dynamics for superionic state.Comment: 18pages 5figure

KFe(C2O4)F : a fluoro-oxalate cathode material for Li/Na-ion batteries

Funding: The authors want to thank EPSRC (EP/R030472/1) and the Faraday Institution (FIRG018) for their financial support. In addition, AGM wishes to thank the Faraday Institution for financial support and training (Grant number FITG033). The authors also thank EPSRC Light Element Analysis Facility Grant EP/T019298/1 and the EPSRC Strategic Equipment Resource Grant EP/R023751/1.The iron-based polyanionic fluoro-oxalate material, KFe(C2O4)F (KFCF), has been synthesized by hydrothermal methods. This compound shows promising reversible lithium and sodium insertion properties as a cathode material. The material delivered a first-cycle discharge capacity of 120 mAh g-1 at ∼3.3 V (Li+/Li) and 97.4 mAh g-1 at ∼3.0 V (Na+/Na) in LIB and NIB, respectively. Stable cycling performance was observed in both cases. The involvement of reversible Fe2+/Fe3+ redox was confirmed by ex-situ Mössbauer spectroscopy supported by first-principles calculations. This study reveals promising performance from a mixed oxalate-fluoride based polyanionic material thereby opening up further possibilities for materials discovery in the design of new electrode materials.Publisher PDFPeer reviewe

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

In the past decades, research efforts on polyanion-type cathode materials by the scientific community intensified significantly. This paper reviews the latest advances in the exploration and development of polyanion-type compounds as high performance cathode materials for Li-ion batteries. It focuses on the synthesis, structure and physicochemical (especially electrochemical) properties of several classes of polyanion compounds. The relationship between composition-structure-performance of the novel electrode materials is also summarized and analyzed. The main approaches, achievements and challenges in this field are briefly commented and discussed.National Natural Science Foundation of China[20873115, 21021002, 90606015]; National Basic Research Program of China (973 program)[2007CB209702, 2011CB935903

Stress Analysis of Electrode Particles in Lithium-Ion Batteries

This chapter reviews several theoretical models that are used to compute the stress fields inside the electrode particles of lithium-ion batteries during discharging/charging process and provides a guideline for researchers to choose the appropriate models. Due to the limitation of the existing models, a general electrochemo-mechanical framework is presented to model the concentration and stress fields of the electrode during the phase transformation. The interaction between stresses fields and phase transformation is addressed, which is a novel discovery in the research of lithium-ion batteries. The electrodes with different sizes and geometries are compared. The structural and electrochemical advantages of hollow core-shell structure particles are highlighted. The present work could help to accurate predict stress profile in electrode particles with different sizes, geometries, and charging operations and contributes to finding the optimal electrode. Therefore, this chapter is helpful for the material and structure design of electrodes of lithium-ion batteries