High‐Rate LiFePO4 Electrode Material Synthesized by a Novel Route from FePO4 · 4H2O

A LiFePO₄ material with ordered olivine structure is synthesized from amorphous FePO₄ · 4H₂O through a solid–liquid phase reaction using (NH₄)₂SO₃ as the reducing agent, followed by thermal conversion of the intermediate NH₄FePO₄ in the presence of LiCOOCH₃ · 2H₂O. Simultaneous thermogravimetric–differential thermal analysis indicates that the crystallization temperature of LiFePO₄ is about 437 °C. Ellipsoidal particle morphology of the resulting LiFePO₄ powder with a particle size mainly in the range 100–300 nm is observed by using scanning electron microscopy and transmission electron microscopy. As an electrode material for rechargeable lithium batteries, the LiFePO₄ sample delivers a discharge capacity of 167 mA h g^–1 at a constant current of 17 mA g^–1 (0.1 C rate; throughout this study n C rate means that rated capacity of LiFePO₄ (170 mA h g^–1) is charged or discharged completely in 1/n hours), approaching the theoretical value of 170 mA h g^–1. Moreover, the electrode shows excellent high‐rate charge and discharge capability and high electrochemical reversibility. No capacity loss can be observed up to 50 cycles under 5 C and 10 C rate conditions. With a conventional charge mode, that is, 5 C rate charging to 4.2 V and then keeping this voltage until the charge current is decreased to 0.1 C rate, a discharge capacity of ca. 134 mA h g^–1 and cycling efficiency of 99.2–99.6 % can be obtained at 5 C rate.     

Electronic Conductivity and Defect Chemistry of Heterosite FePO4

Electrochemical investigations on polycrystalline orthorhombic FePO₄ (heterosite), the lithium‐poor part of the LiFePO₄/FePO₄ redox couple, gives insight into its charge‐carrier chemistry. The material obtained by chemical delithiation exhibits a predominant electronic conductivity. A residual lithium content of 0.03 wt% was found and has to be considered as lithium interstitials in the FePO₄ ground structure. Compensation by electrons induces n‐type conduction, confirmed by the pO₂ dependence of the electronic conductivity. The pO₂ dependence is primarily ascribed to the formation of an oxidic surface composition leading to bulk depletion of lithium, rather than to filling of oxygen vacancies.     

Isolation of Solid Solution Phases in Size‐Controlled LixFePO4 at Room Temperature

State‐of‐the‐art LiFePO₄ technology has now opened the door for lithium ion batteries to take their place in large‐scale applications such as plug‐in hybrid vehicles. A high level of safety, significant cost reduction, and huge power generation are on the verge of being guaranteed for the most advanced energy storage system. The room‐temperature phase diagram is essential to understand the facile electrode reaction of Li_xFePO₄ (0 < x < 1), but it has not been fully understood. Here, intermediate solid solution phases close to x = 0 and x = 1 have been isolated at room temperature. Size‐dependent modification of the phase diagram, as well as the systematic variation of lattice parameters inside the solid‐solution compositional domain closely related to the electrochemical redox potential, are demonstrated. These experimental results reveal that the excess capacity that has been observed above and below the two‐phase equilibrium potential is largely due to the bulk solid solution, and thus support the size‐dependent miscibility gap model.     

Kinetic Monte Carlo Simulations of Anisotropic Lithium Intercalation into LixFePO4 Electrode Nanocrystals

The kinetic anisotropy of lithium ion adsorption and lithium absorption for Li_xFePO₄ olivine nanocrystals is simulated and reported. The kinetics depend on the orientation of the electrolyte/Li_xFePO₄ interface with respect to the far‐field ionic flux. As a consequence of these kinetics and a Li miscibility gap in Li_xFePO₄, the particle geometry and orientation also have an effect on the morphology of the two‐phase evolution. These processes accompany the charge and discharge behavior in battery microstructures and a direct influence on battery behavior is suggested. A kinetic Monte Carlo (KMC) algorithm based on a cathode particle rigid lattice is used to simulate the kinetics in this system. In these simulations the adsorption kinetics of the electrolyte/electrode interface are treated by coupling the normal flux outside the particle from a continuum numerical simulation of Li‐ion diffusion in the electrolyte to the atomistic KMC model within the particle. The interfacial reaction depends on local concentration and the potential drop at the interface via the Butler–Volmer (B–V) relation. The atomic potentials for the KMC simulation are derived from empirical solubility limits (as determined by OCV measurements). The main results show that the galvanostatic lithium‐uptake/cell‐voltage has three regimes: 1) a decreasing cell potential for Li‐insertion into a Li‐poor phase; 2) a nearly constant potential after the nucleation of a Li‐rich phase Li_(1‐β)FePO₄; 3) a decreasing cell potential after the Li‐poor phase has been evolved into a Li‐rich phase. The behavior in the second regime is sensitive to crystallographic orientation.     

Pulsed Laser Deposition and Electrochemical Characterization of LiFePO4–Ag Composite Thin Films

A simple approach is proposed to enhance the electrical conductivity of olivine‐structured LiFePO₄ thin films by uniformly dispersing small fractions of highly conductive silver (ca. 1.37 wt %) throughout the LiFePO₄ film. In this approach, a highly densified (>85 %) LiFePO₄–Ag target was first fabricated by coating conductive silver nanoparticles onto the surfaces of hydrothermally synthesized LiFePO₄ ultrafine particles by a soft chemical route. Pulsed laser deposition (PLD) was then employed to deposit LiFePO₄–Ag composite thin films on the Si/SiO₂/Ti/Pt substrates. The PLD experimental parameters were optimized to obtain well‐crystallized and olivine‐phase pure LiFePO₄–Ag composite thin films with smooth surfaces and homogeneous thicknesses. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectrometry (Raman), X‐ray photoelectron spectroscopy (XPS), DC conductivity measurements, cyclic voltammetry(CV), as well as galvanostatic measurements were employed to characterize the as‐obtained LiFePO₄–Ag composite films. The results revealed that after silver incorporation, the olivine LiFePO₄ film cathode shows a superior electrochemical performance with a good combination of moderate specific capacity, stable cycling, and most importantly, a remarkable tolerance against high rates and over‐charging and ‐discharging.     

Ultra-Fine Nano-Mg(OH)2 Electrodeposited in Flexible Confined Space and its Enhancement of the Performance of LiFePO4 Lithium-Ion Batteries

To improve the Li-ion diffusion and extreme-environment performance of LiFePO₄ (LFP) lithium-ion batteries, a composite cathode material is fabricated using ultra-fine nano-Mg(OH)₂ (MH). First, a flexible confined space is designed in the local area of the cathode surface, through the transition of charged xanthan gum polymer molecules under electric field force and the self-assembly of the xanthan gum network. Then, the 20 nm nano-Mg(OH)₂ is prepared through cathodic electrodeposition within the local flexible confined space, and subsequent in situ surface modification as it traverses the xanthan gum network under gravity. LFP-MH significantly changes the density and homogeneity of the cathode electrolyte interphase film and improves the electrolyte affinity. The Li||LFP-MH half-cell demonstrates excellent rate capability (110 mAh g⁻¹ at 5 C) and long-term cycle performance (116.6 mAh g⁻¹ at 1 C after 1000 cycles), and maintains over 100 mAh g⁻¹ after 150 cycles at 60 °C, as well as no structural collapse of the cathode material after 400 cycles at 5 V high cut-off voltage. The cell also shows an obvious decrease in inner resistance after 100 cycles (99.53/133.12 Ω). This work provides a significant advancement toward LiFePO₄ lithium-ion batteries with excellent electrochemical performance and tolerance to extreme-environment.     

LD抽运Nd:YVO4/LBO腔内和频橙黄光连续激光器

报道了一种激光二极管抽运Nd:YVO4晶体、腔内Ⅰ类临界相位匹配LBO和频、连续波输出的全固态橙黄色激光器的设计和实验结果。橙黄色激光由Nd:YVO4晶体的1064nm和1342nm谱线腔内和频产生,输出波长为593.5nm。实验采用了双镜谐振腔结构,在1.6W的808nm注入抽运功率下,获得了最高功率为84mW连续波TEM00的橙黄色低噪声激光输出,光-光转换效率为5.3%,光束质量因子M21.2。实验和分析表明,采用激光二极管抽运Nd:YVO4晶体、LBOⅠ类临界相位匹配腔内和频是获得橙黄色激光的实用方法,并可以应用到Nd:YVO4晶体的其它谱线或具有多条谱线的其它激光增益介质,获得更多不同颜色的单谱线激光输出。     

Feasibility of Prelithiation in LiFePO4

Lithium iron phosphate (LiFePO₄) is widely applied as the cathode material for the energy storage Li-ion batteries due to its low cost and high cycling stability. However, the low theoretical specific capacity of LiFePO₄ makes its initial capacity loss more concerning. Therefore, lithium compensation by way of prelithiation and applications of sacrificial Li-rich additives in LiFePO₄ is imminent in elevating the energy density and/or prolonging the lifetime of the LiFePO₄-based Li-ion batteries (LIBs). Prelithiation in LiFePO₄ is herein carried out by electrochemical and chemical methods and its feasibility is proved on the basis of the electrochemical evaluations such as the initial charge capacity and the cycling stability. In addition, the site of the pre-intercalated Li-ions is found via comprehensive physical characterizations and the density functional theory (DFT) calculations. These findings open a new avenue for elevating the energy density and/or prolonging the lifetime of the high-energy-density batteries.     

Thermal stability of luminous YAG: Ce bulk ceramic as a remote phosphor prepared through silica-stabilizing valence of activator in air

A prototype of YAG: Ce (Y3Al5O12) luminous bulk ceramic as a remote phosphor for white LED illumination was fabricated in air through a strategy of silica addition. With increasing the amount of silica in a specific range, the opaque sample turns to be semi-transparent. The precipitation of crystals is verified to be in pure YAG phase by X-ray diffraction (XRD). Beyond the limit of silica amount, the dominant phase of YAG crystal is found to coexist with a small amount of newly-formed Y2Si2O7, Al2O3 and the amorphous phase. The YAG crystals are with a grain size of approximately 2 mm and distribute evenly. The YAG hosts after structural modification via addition of silica result in yellow band emission of 5d→4f transition peaked around 535 nm as excited by a blue LED, owing to the self-reduction of Ce4+ to Ce3+ even in the absence of reductive atmosphere.     

Dichotomy in the Lithiation Pathway of Ellipsoidal and Platelet LiFePO4 Particles Revealed through Nanoscale Operando State‐of‐Charge Imaging

LiFePO₄ is a promising phase‐separating battery electrode and a model system for studying lithiation. The role of particle synthesis and the corresponding particle morphology on the nanoscale insertion and migration of Li is not well understood, and elucidating the intercalation pathway is crucial toward improving battery performance. A synchrotron operando liquid X‐ray imaging platform is developed to track the migration of Li in LiFePO₄ electrodes with single‐particle sensitivity. Lithiation is tracked in two particle types—ellipsoidal and platelet—while the particles cycle in an organic liquid electrolyte, and the results show a clear dichotomy in the intercalation pathway. The ellipsoidal particles intercalate sequentially, concentrating the current in a small number of actively intercalating particles. At the same cycling rate, platelet particles intercalate simultaneously, leading to a significantly more uniform current distribution. Assuming that the particles intercalate through a single‐phase pathway, it is proposed that the two particle types exhibit different surface properties, a result of different synthesis procedures, which affect the surface reactivity of LiFePO₄. Alternatively, if the particles intercalate through nucleation and growth, the larger size of platelet particles may account for the dichotomy. Beyond providing particle engineering insights, the operando microscopy platform enables new opportunities for nanoscale chemical imaging of liquid‐based electrochemical systems.     

Interaction of Carbon Coating on LiFePO4: A Local Visualization Study of the Influence of Impurity Phases

Carbon coating is a proven successful approach for improving the conductivity of LiFePO₄ used in rechargeable Li‐ion batteries. Different impurity phases can be formed during LiFePO₄ synthesis. Here, a direct visualization of the impact of impurity phases in LiFePO₄ on a carbon coating is presented; they are investigated on a model material using various surface‐characterization techniques. By using polished ingot model materials, impurity phases can be clearly observed, identified, and located on the surface of the sample by scanning electron microscopy (SEM), focused‐ion‐beam lithography (FIB), high‐resolution transmission electron microscopy (HR‐TEM), and Raman spectroscopy. During the carbon‐coating process, the phosphorus‐rich phase is found to have an inhibiting effect (or no positive catalytic effect) on carbon formation, while iron‐rich phases (mainly iron phosphides) promote carbon growth by contributing to more carbon deposition and a higher graphitic carbon content. This finding, and the methodological evaluation here, will help us to understand and reveal the influencing factors of impurity phases on the basic carbon‐deposition process to obtain high‐performance LiFePO₄ material for future energy‐storage devices.     

Effect of Indium Substitution on the Thermoelectric Properties of Orthorhombic Cu4SnS4

We report the thermoelectric properties of Cu₄In _x Sn_1?x S₄ (x = 0–0.02), which undergoes a first-order structural phase transition at ~230 K. Substitution of In³⁺ for Sn⁴⁺ suppresses the phase transition temperature (T _t). Indium substitution reduces the electrical resistivity, and degenerate conduction by the orthorhombic phase is observed. The Seebeck coefficient increases over the whole temperature range and a maximum value occurs in the monoclinic phase as a result of indium substitution. Thermal conductivity decreases as x increases, which enhances the dimensionless figure of merit, ZT. We therefore expect optimization of the chemical composition of indium-doped Cu₄SnS₄ to result in an even larger ZT value.     

Low‐Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas‐phase methods and requires high‐temperature condition. As an alternative to the gas‐phase synthetic approach, lower temperature eutectic liquid‐phase synthesis presents a very promising approach with the potential for larger‐scale and controllable growth of high‐quality thin metallic TMD single crystals. Here, the first realization of low‐temperature eutectic liquid‐phase synthesis of type‐II Dirac semimetal PtTe₂ single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe₂ reveals a record‐high conductivity of as high as 3.3 × 10⁶ S m⁻¹ at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type‐II Dirac semimetal PtTe₂ for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe₂ crystal by thinning as‐synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low‐temperature eutectic liquid‐phase synthesis and layer‐by‐layer thinning of high‐quality thin PtTe₂ single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high melting‐point transition metal.     

Dual Phase‐Controlled Synthesis of Uniform Lanthanide‐Doped NaGdF4 Upconversion Nanocrystals Via an OA/Ionic Liquid Two‐Phase System for In Vivo Dual‐Modality Imaging

A novel OA/ionic liquid two‐phase system combining the merits of thermal decomposition method, the IL‐based strategy, and the two‐phase approach is introduced to synthesize high‐quality lanthanide‐doped NaGdF₄ upconversion nanocrystals with different crystal‐phases in OA‐phase and IL‐phase through a one‐step controllable reaction. Oil‐dispersible cubic‐phase NaGdF₄:Yb, Er (Ho, Tm) nanocrystals with ultra‐small size (～5 nm) and monodispersity are obtained in the OA phase of the two‐phase system via an IL‐based reaction. More importantly, water‐soluble hexagonal‐phase NaGdF₄:Yb, Er nanocrystals are obtained in the same system simply by adopting an extremely facile method to complete the dual phase‐transition (crystal‐phase transition and OA‐phase to IL‐phase transition) simultaneously. The synthesized lanthanide‐doped NaGdF₄ upconversion nanocrystals are effective for dual‐mode UCL imaging and CT imaging in vivo.     

Toward Optimal Performance and In‐Depth Understanding of Spinel Li4Ti5O12 Electrodes through Phase Field Modeling

Computational modeling is vital for the fundamental understanding of processes in Li‐ion batteries. However, capturing nanoscopic to mesoscopic phase thermodynamics and kinetics in the solid electrode particles embedded in realistic electrode morphologies is challenging. In particular for electrode materials displaying a first order phase transition, such as LiFePO₄, graphite, and spinel Li₄Ti₅O₁₂, predicting the macroscopic electrochemical behavior requires an accurate physical model. Herein, a thermodynamic phase field model is presented for Li‐ion insertion in spinel Li₄Ti₅O₁₂ which captures the performance limitations presented in literature as a function of all relevant electrode parameters. The phase stability in the model is based on ab initio density functional theory calculations and the Li‐ion diffusion parameters on nanoscopic nuclear magnetic resonance (NMR) measurements of Li‐ion mobility, resulting in a parameter free model. The direct comparison with prepared electrodes shows good agreement over three orders of magnitude in the discharge current. Overpotentials associated with the various charge transport processes, as well as the active particle fraction relevant for local hotspots in batteries, are analyzed. It is demonstrated which process limits the electrode performance under a variety of realistic conditions, providing comprehensive understanding of the nanoscopic to microscopic properties. These results provide concrete directions toward the design of optimally performing Li₄Ti₅O₁₂ electrodes.     

Silicon‐Doped LiFePO4 Single Crystals: Growth,Conductivity Behavior,and Diffusivity

Single crystals of silicon doped LiFePO₄ with a silicon content of 1% are grown successfully by the floating zone technique and characterized by single‐crystal and powder X‐ray diffraction, secondary ion mass spectroscopy, and chemical analysis. Electron paramagnetic resonance demonstrates the presence of only Fe²⁺; no traces of Fe³⁺ are found. Impedance spectroscopy as well as step‐function polarization/depolarization (DC) measurements are carried out using the cells Ti/LiFe(Si)PO₄/Ti and LiAl/LiI/LiFe(Si)PO₄/LiI/LiAl. The electronic and ionic conductivities as well as the Li‐diffusivity of the sample in the major crystallographic directions ([h00], [0k0], and [00l]) are determined. Within experimental error the transport properties along the b‐ and c‐axes are found to be the same but differ significantly from the a‐axis, which exhibits lower values. Compared to undoped LiFePO₄, Si‐doping leads to an increase of the ionic conductivity while the electronic conductivity decreases, which is in agreement with a donor effect. The activation energies of conductivities and diffusivities are interpreted in terms of defect chemistry and relevant Brouwer diagrams are given.     

Composition-induced valence variation of europium and the photoluminescence properties for Eu2+ doped borophosphate luminous glass in air

A simultaneous blue-light and red-light emitting glass of SrO-B2O3-P2O5 doped with Eu2O3 is prepared in air, and then heat-treated without any reductive reagent. A transition combination is found to consist of a band emission peaked around 430 nm and a series of line emission from 593 nm to 611 nm, corresponding to the typical 4f65d→ 4f7 transition of Eu2+ and 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transitions of Eu3+, respectively. Some unidentified crystals such as Sr (PO3)2 and SrB2O4 as hosts for Eu2...     

LiFePO4 Particles Embedded in Fast Bifunctional Conductor rGO&C@Li3V2(PO4)3 Nanosheets as Cathodes for High‐Performance Li‐Ion Hybrid Capacitors

The sluggish kinetics of Faradaic reactions in bulk electrodes is a significant obstacle to achieve high energy and power density in energy storage devices. Herein, a composite of LiFePO₄ particles trapped in fast bifunctional conductor rGO&C@Li₃V₂(PO₄)₃ nanosheets is prepared through an in situ competitive redox reaction. The composite exhibits extraordinary rate capability (71 mAh g^?1 at 15 A g^?1) and remarkable cycling stability (0.03% decay per cycle over 1000 cycles at 10 A g^?1). Improved extrinsic pseudocapacitive contribution is the origin of fast kinetics, which endows this composite with high energy and power density, since the unique 2D nanosheets and embedded ultrafine LiFePO₄ nanoparticles can shorten the ion and electron diffusion length. Even applied to Li‐ion hybrid capacitors, the obtained devices still achieve high power density of 3.36 kW kg^?1 along with high energy density up to 77.8 Wh kg^?1. Density functional theory computations also validate that the remarkable rate performance is facilitated by the desirable ionic and electronic conductivity of the composite.     

Direct Physical Imaging and Chemical Probing of LiFePO4 for Lithium‐Ion Batteries

The control of unexpectedly rapid Li intercalation reactions without structural instability in olivine‐type LiFePO₄ nanocrystals is one of the notable scientific advances and new findings attained in materials physics and chemistry during the past decade. A variety of scientific studies and technological investigations have been carried out with LiFePO₄ to elucidate the origins of many peculiar physical aspects as well as to develop more effective synthetic processing techniques for better electrochemical performances. Among the several features of LiFePO₄ that have attracted much interest, in this article we address four important issues—regarding doping of aliovalent cations, distribution of Fe‐rich secondary metallic phases, nanoparticle formation during crystallization, and antisite Li/Fe partitioning—by means of straightforward atomic‐scale imaging and chemical probing. The direct observations in the present study provide significant insight into alternative efficient approaches to obtain conductive LiFePO₄ nanocrystals with controlled defect structures.     

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe₂-ZnIn₂Se₄ alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn₂Se₄ are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe₂-ZnIn₂Se₄ single crystals containing 15 and 20 mol % of ZnIn₂Se₄, the hopping mechanism of conductivity is dominant at temperatures of T ～ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ～15 and 20 mol % of ZnIn₂Se₄ is a single narrow peak at a wavelength of λ_max = 1190–1160 nm.