LiFePO4 Particles Research Articles

Understanding the Effects of Defects on Phase Transformation Kinetics in Olivine LiFePO4 Particles

Effects of anti-site defects on reducing Li diffusion anisotropy in LiFePO4 single crystal have been experimentally [1] and theoretically [2, 3] examined. The strongly anisotropic Li diffusion in olivine phosphates were firstly reported by first-principle calculations by Morgan et al. [4], and later confirmed by experiment [5]. However, the theoretical Li diffusivity (D=1x10-8 cm2/s) in defect-free LiFePO4 single crystals are several orders of magnitude larger than experimentally measured values in the range of 10-11 – 10-14 cm2/s. Subsequent calculations by Malik et al. [2] and Dathar et al. [3] confirmed that anti-site defects not only block the [010] diffusion channels but also decrease the energy barrier for inter-channel Li hopping, effectively reducing Li diffusion anisotropy. In particular, Amin et al. [1] reported the comparable Li diffusivities along the three axes in millimeter-sized LiFePO4 single crystals containing 2.5-3% Li-Fe anti-site defects. Although the reduced Li diffusion anisotropy in LiFePO4 single crystal is understood, the effects of anti-site defects on phase transformation kinetics in olivine LiFePO4 particles have not been clarified. Here we show that both the phase transformation kinetics and the surface reaction kinetics of LiFePO4 particles will be significantly affected by the anti-site defects. Recently, we combine operando hard X-ray spectroscopic imaging and phase-field modeling to show two-dimensional (2D) Li diffusion behaviors in micro-sized LiFePO4 rod containing ~3% Li-Fe anti-site defects. We obtain direct evidence that Li ions can be intercalated through the (100)/(001) surfaces, contradicting a common belief that only the (010) surface of LiFePO4 is electrochemically active. This study not only presents the first experimental confirmation of the previously hypothesized surface-reaction-limited (SRL) [6] phase boundary migration in LiFePO4, but also reveals a new hybrid mode of phase transformation, where the growth of new phase is controlled by surface reaction or Li diffusion in different crystallographic directions. Based on these findings, we propose the existence of three distinct kinetic regimes (SRL, hybrid, and bulk-diffusion-limited (BDL)) of phase transformations of LiFePO4 and potentially many other intercalation compounds undergoing first-order phase transformations (as shown in Figure 1a). In particular, our phase-field simulations show that 2D Li diffusion can significantly enhance Li insertion kinetics in SRL regime. In a LiFePO4 nanoparticle of 200 nm x 50 nm x L nm ([100] x [010] x [001]), our simulation results (Figure 1b) show that the traveling constant velocity of (100) straight phase boundary driven by 2D Li diffusion (D=1x10-11 cm2/s along [100] and [010]) is one magnitude larger than the velocity of the phase boundary driven by 1D Li diffusion with infinite diffusivity along [010]. The latter one is calculated by SCB theory [6]. Anti-site defects are commonly believed to deteriorate high rate performances of LiFePO4, due to its blocking to the 1D Li diffusion along [010]. Our counterintuitive finding, however, show that the enhanced Li diffusion along [100] by Li inter-channel hopping around the defects [2, 3] significantly enhance surface reaction kinetics by activating large fraction of electrochemically surface for Li insertion. The maximum phase boundary velocity driven by 2D Li diffusion is linear to particle size along [100] and inversely linear to thickness along [010]. Therefore, we propose that LiFePO4 nanosheets with large (010) surface and small thickness along [010] is beneficial for high rate capability and high energy density. This is consistent to experimental observations. Our studies show the important effects of anti-site defects on phase transformation kinetics and surface reaction kinetics in LiFePO4. This will provide new insights on potential avenues to improve performances of olivine electrodes via engineering anti-site defect formation and distribution.

Effect of operating parameters on synthesis of lithium iron phosphate (LiFePO4) particles in near- and super-critical water

Hydrothermal synthesis of lithium iron phosphate (LiFePO4) particles in near- and super-critical water was investigated in a batch reactor system. The phase identification and morphology of synthesized LiFePO4 particles were performed by Fourier Transform Infrared (FT-IR), powder X-ray diffraction (XRD) and field emission scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). The effect of operating parameters including temperature (400–450°C), pressure (200, 275 and 350bar) and reaction time (10 and 60min) were examined on particle size, size distribution, purity, crystalline structure, morphology, and electrochemical performance of the synthesized LiFePO4 in the presence of sodium sulphite (Na2SO3) as reducing agent. Highly crystalline LiFePO4 particles were synthesized at near-critical conditions (400°C, 200bar and 60min) with initial discharge capacity of 87mAhg−1 at 0.1 C-rate.

Precursor‐Directed Nucleation and Self‐Assembly Growth: From Hollow Microprisms to Nanoplatelets

AbstractDirectly controlling the morphology and orientation of nanocrystals is a key step for their tailored applications. Achieving such controlled synthesis requires insight into the formation mechanism. Using Li3PO4 microprisms as a solid precursor sealed in capillary tube, we unravel the structural and morphological evolution from the precursor to the resultant LiFePO4 nanoplatelets by means of in situ temperature monitoring, ex situ Raman spectroscopy and TEM/STEM. Under water‐deficient conditions, the precursor dissolves in a sluggish manner, facilitating simultaneous heterogeneous nucleation of LiFePO4 on undissolved Li3PO4 and homogeneous nucleation in solution. Vertically aligned nanoribbons consisting of primary LiFePO4 particles on the Li3PO4 surface undergo self‐assembly growth though oriented attachment. The [100]‐orientation is explained by the fact that H3O+ preferably adsorbed on the LiFePO4 (100) facet and suppressed the crystal growth more along the [100] direction than the others.

Addressing the Interface Issues in All-Solid-State Bulk-Type Lithium Ion Battery via an All-Composite Approach.

All-solid-state bulk-type lithium ion batteries (LIBs) are considered ultimate solutions to the safety issues associated with conventional LIBs using flammable liquid electrolyte. The development of bulk-type all-solid-state LIBs has been hindered by the low loading of active cathode materials, hence low specific surface capacity, and by the high interface resistance, which results in low rate and cyclic performance. In this contribution, we propose and demonstrate a synergistic all-composite approach to fabricating flexible all-solid-state LIBs. PEO-based composite cathode layers (filled with LiFePO4 particles) of ∼300 μm in thickness and composite electrolyte layers (filled with Al-LLZTO particles) are stacked layer-by-layer with lithium foils as negative layer and hot-pressed into a monolithic all-solid-state LIB. The flexible LIB delivers a high specific discharge capacity of 155 mAh/g, which corresponds to an ultrahigh surface capacity of 10.8 mAh/cm2, exhibits excellent capacity retention up to at least 10 cycles and could work properly under harsh operating conditions such as bending or being sectioned into pieces. The all-composite approach is favorable for improving both mesoscopic and microscopic interfaces inside the all-solid-state LIB and may provide a new toolbox for design and fabrication of all-solid-state LIBs.

Utilizing Phosphating Sludge to Synthesize Lithium Iron Phosphate with a Hybrid Coating of Metal Oxides and Carbon

In order to utilize the hazardous phosphating sludge, a novel approach including FeCl3 solution replacement leaching and MeO (metal oxides) coprecipitation coating is proposed to prepare LiFePO4 coated with metal oxides and carbon, where FePO4 is recycled from phosphating sludge and its valuable impurities are utilized as coating materials. The FeCl3 solution replacement leaching is a process for PO43+ enrichment and FePO4 purifying. The metal ions (Zn2+, Ca2+, Mn2+, etc.) of the impurity phosphates can be leached into the solution, meanwhile their corresponding PO43– are reprecipitated as FePO4, whose Fe3+ come from the FeCl3 leaching solution. Taking the obtained FePO4 as the core material, the MeO can be coprecipitated from the leached solution and coated onto the FePO4 particle uniformly under the critical pH and appropriate concentration. This uniform hybrid coating is helpful to avoid the asymmetrical conductivity both for electron and lithium ion on the surface of the LiFePO4 particle, and hence im...

Enhanced electrochemical performance of LiFePO4 cathode with synergistic modification of Na3V2(PO4)3 and carbon network

A high performance 9LiFePO4–Na3V2(PO4)3/C composite is successfully prepared by a sol–gel technique followed by subsequent carbonthermal reduction process. The structure and performance of 9LiFePO4–Na3V2(PO4)3/C composite have been investigated by XRD, SEM, TEM, EDS, CV and EIS. The result shows that the Na3V2(PO4)3 acting as ionic conductor improves the lithium ion transmission rate, while a thin amorphous carbon layer on the surface of nanoparticles and interfacial structure between the two phase enhances the electronic transmission rate of the composite. The synergistic effect of continual conductive carbon network and Na3V2(PO4)3 phase facilitates the diffusion of lithium ion and electron, ensures the stability of the LiFePO4 nanoparticles by limiting the direct contact of LiFePO4 particle with the electrolyte and ultimately improve the rate capability and long-cycling life of the material. The 9LiFePO4–Na3V2(PO4)3/C composite delivers discharge capacities of 151.2, 144.9 and 118.1mAhg−1 at 1.0, 2.0, 5.0 C, respectively, with high capacity retention up to 88.2% after 2000cycles at 2.0 C, exhibiting good electrochemical performance compared with that of the pristine one.

Effect of Lorentz force on the electrochemical performance of lithium-ion batteries

Lorentz force theory demonstrates that electric current density and magnetic force are proportional, indicating that they compensate each other. In a battery operated at high magnetic forces, the electrons in the active material move fast in a specific magnetic field. γ-Fe2O3, a highly magnetic material, is used to prepare LiFePO4 electrodes to study the effect of the Lorentz force on lithium-ion battery performance. The magnetic field created by γ-Fe2O3 induces magnetic forces on the charged LiFePO4 particles, accelerating electron movement. Superconducting quantum interference measurements reveal that saturation magnetization and remanence are prominent when γ-Fe2O3 is added to the LiFePO4 electrodes. The LiFePO4 electrode containing 15wt% γ-Fe2O3 led to superior battery capacity (69.8mAhg−1 at 10C) compared with the pure LiFePO4 electrode (1.8mAhg−1 at 10C). In this study, Lorentz force theory is applied to improve the specific capacity and cycle life at high current rates of a battery containing LiFePO4 cathode materials, suggesting that incorporating γ-Fe2O3 into the cathode is an easy and cheap strategy for increasing the power density and cycle life of lithium-ion batteries.

Synthesis and Electrochemical Properties of Coral-Like LiFePO4 Particles as Supercapacitor Electrodes

Facile synthesis of materials with high specific surface areas is very urgent for their potential applications in the field of energy storage. In the present work, coral-like LiFePO4 particles etched for 60, 90, and 120 min (labeled as S-60, S-90, and S-120, respectively) were successfully synthesized through a facile chemical etching method with rugby-like LiFePO4 (named as LFP) particles as precursors synthesized by the solvothermal method. They were then applied as electrode materials for high-performance electrochemical capacitors. In particular, the specific surface area of S-120 reaches 52 m2 g−1, which is 5.2 times greater than that of unetched counterpart. Therefore, S-120 can achieve a higher capacitance of 359 F g−1 at a scan rate of 2 mV s−1 compared with the unetched counterpart. Furthermore, S-120 presents a good cycling stability of 82.3% capacitive retention even after 2000 cycles, which is much higher than that of unetched LFP particles (64%). As a result, the coral-like LFP particles are promising candidate electrode materials for supercapacitor applications.

Synthesis and Electrochemical Performance of LiFePO4/C Composite by Improved Solid-State Method Using a Complex Carbon Source

A high-performance LiFePO4/C composite was synthesized by improved solid-state method using starch and PEG6000 as a complex carbon source. The raw materials were mixed by wet-mixing method using de-ionized water as medium. For comparison, a LiFePO4/C composite was prepared using PEG6000 as single carbon source. XRD results show that the sample prepared by improved solid-state method has ordered olivine structure and high purity due to the perfect mixing of the raw materials by wet-mixing method. Fine particles with an average particle size about 400 nm are examined by scanning electron microscopy (SEM). TEM observation illustrates that the complex carbon source leads to the homogeneous and complete carbon coating on LiFePO4 particle surface, resulting in its uniform particle size distribution and enhanced electronic conductivity. All the above factors lead to the better room-temperature and low-temperature electrochemical performances of LiFePO4/C composite synthesized by improved solid-state method in both LiFePO4/Li half-cell and 18650 cylindrical full-cell.

Carbon nanostructures modified LiFePO4 cathodes for lithium ion battery applications: optimized porosity and composition

Lithium iron phosphate (LiFePO4) battery cathode was fabricated without using any metallic current collector and polymeric binder. Carbon nanostructures (CNS) were used as microbinders for LiFePO4 particles and at the same time as a 3D current collector. A facile and cost effective method of fabricating composite cathodes of CNS and LiFePO4 was developed. Thick electrodes with high loading of active material (20–25 mg cm−2) were obtained that are almost 2–3 folds higher than commercial electrodes. SEM images confirm that the 3D CNS conductive network encapsulated the LiFePO4 particles homogenously facilitating the charge transfer at the electrode–CNS interface. The composition, scan rate and porosity of the paper-like cathode were sequentially varied and their influence was systematically monitored by means of linear sweep cyclic voltammetry and AC electrochemical impedance spectroscopy. Addition of CNS improved the electrode’s bulk electronic conductivity, mechanical integrity, surface area and double layer capacitance, yet compromised the charge transfer resistance at the electrode–electrolyte interface. Based on a range of the tested binder-free electrodes, this study proposes that electrodes with 20 wt% CNS having 49 ± 2.5% porosity had realized best improvements of two folds and four folds in the electronic conductivity and diffusion coefficient, respectively.

Investigation on high performance LiFePO4 nanoplates with the {010} face prominent for lithium battery cathode materials

LiFePO4 nanoplates were synthesized by using ethylene glycol (EG) as a solvent. The morphologies and sizes of the LiFePO4 particles were strongly dependent on synthetic parameters such as concentrations and mole ratio of reactants. LiFePO4 particles are nanoplates with the {010} face prominent, namely, with a short b-axis and the samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) test analysis. Fourier transform infrared spectroscopy (FTIR) analysis implied that the defect concentrations of the Fe•Li antisite in LiFePO4 nanoplates were very low. The electrochemical behaviors were investigated by cyclic voltammetry measurements in the Li2SO4 aqueous electrolyte. It was shown that all samples could undergo lithium-ion deintercalation and intercalation upon oxidation and reduction at a scan rate range of 5–20mV/s. Only the sample (formed in a FeSO4·7H2O to H3PO4 and LiOH·H2O ratio of 1:1.5:2.7 with appropriate concentration) could undergo lithium-ion deintercalation and intercalation at a large scan rate even at 280mV/s and showed excellent rapid charge and discharge performance. This provided a facile way to prepare high performance LiFePO4 nanoplate cathode material for lithium ion batteries.

Improved Electrochemical Performance of LiFePO4@N-Doped Carbon Nanocomposites Using Polybenzoxazine as Nitrogen and Carbon Sources.

Polybenzoxazine is used as a novel carbon and nitrogen source for coating LiFePO4 to obtain LiFePO4@nitrogen-doped carbon (LFP@NC) nanocomposites. The nitrogen-doped graphene-like carbon that is in situ coated on nanometer-sized LiFePO4 particles can effectively enhance the electrical conductivity and provide fast Li+ transport paths. When used as a cathode material for lithium-ion batteries, the LFP@NC nanocomposite (88.4 wt % of LiFePO4) exhibits a favorable rate performance and stable cycling performance.

Enhancing Surface Diffusion in Battery Electrodes Using Solvent Adsorbates

LiFePO4 is one of the most well-studied positive electrode materials. A well-established doctrine is that Li only diffuses within one-dimensional channels, with negligible diffusivities in the other directions, and almost no movement of Li between channels. However, existing experimental measurements and density functional theory calculations have generally been conducted for bulk LiFePO4, and have largely neglected surface diffusion at the solid/liquid interface. In our work, we directly study the ambipolar Li diffusion along these nominally slow directions perpendicular to the fast conduction channels. Because ambipolar transport of Li is controlled by the slow species, it is difficult to quantify using standard electrical measurements, which generally probe the fast species. To overcome this challenge, we measure the time for a solid solution Li0.5FePO4 electrode to separate into Li-rich and Li-poor phases. Because phase separation necessitates the movement of Li within a particle between the fast conduction channels, this technique directly provides the ambipolar transport of Li. Our results show that phase separation is extremely slow under dry, inert conditions. However, when placed in an organic solvent containing ethylene carbonate but no electrolyte salt, the rate of phase separation is enhanced by more than two order of magnitude. Because the ethylene carbonate only interacts with the surface of the LiFePO4 particles, the enhancement in Li transport demonstrate that surface diffusion is many orders of magnitude faster than bulk diffusion. Additionally, the rate of surface diffusion can be tuned by environmental adsorbates. We confirm our findings through density functional theory calculations, which show a significant reduction in migration enthalpy in the presence of a solvent adsorbate. Our results show that surfaces diffusion enhances the conductivity of Li along the nominally slow directions by several orders of magnitude, and thus reduces the anisotropy of Li diffusion in LiFePO4. Additionally, by demonstrating that the conductivity can be tuned using environmental adsorbates, we offer a novel approach towards enhancing the conductivity of electrode materials. By choosing the correct adsorbates to activate surface diffusion, nominally insulating materials can be conductive at the solid/liquid interface. This introduces a new paradigm in interfacial engineering of electrodes, and could significantly expand the class of insulating materials which are suitable as electrodes in Li-ion batteries.

Evolution of Fe-Antisite Defects in Standard Hydrothermal LiFePO4 Synthesis and Their Accelerated Removal with Ca2+

Based on neutron powder diffraction (NPD), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), Electron Energy Loss Spectroscopy (EELS), X-Ray Photoelectron Spectroscopy (XPS) we show that Fe-antisites defects are surface defects (see Figure 1) and slowly eliminated by Fe-Li cation exchange during standard hydrothermal synthesis1. Recently2 we demonstrated that the calcium ions help eliminate the Fe antisite defects by controlling the nucleation and evolution of the LiFePO4 particles during their hydrothermal synthesis. The final Ca:LFP particles present Fe-antsite defects confined in a thin surface layer compared to standard LFP. Fig. 1. Figure 1: Schematic representation of Fe-antisite defects elimination during standard hydrothermal synthesis of LFP Reference 1. Paolella, A. et al. Cation exchange mediated elimination of the Fe-antisites in the hydrothermal synthesis of LiFePO 4. Nano Energy 16, 256–267 (2015). 2. Paolella, A. et al. Accelerated Removal of Fe-Antisite Defects while Nanosizing Hydrothermal LiFePO 4 with Ca 2+. Nano Lett. 2–7 (2016). doi:10.1021/acs.nanolett.6b00334 Figure 1

Electrochemical Studies of Carbon Nanotube-LiFePO4 Nanocomposite Cathode for High-Capacity Lithium-Ion Batteries

LiFePO4 nanoparticles were incorporated with multi-walled carbon nanotubes (CNTs) via a facile one-step polyol process at a mild temperature. CNTs were uniformly embedded into LiFePO4 particles and formed a conductive network to enhance the electrochemical performance of the CNT-LiFePO4 nanocomposite cathode material. Structural composition and morphology of the composite were investigated in details by X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM, TEM). The electrochemical properties were analyzed in the voltage of 2.0 and 4.0 V by charge/discharge testing and cyclic voltammetry. Cycling behavior of the electrode was examined with current densities up to 30 C. Experimental results showed that these CNT-LiFePO4 nanocomposite cathodes exhibited enhanced electrochemical performance with both high capacity and good reversibility.

Thermal annealing dynamics of carbon-coated LiFePO4 nanoparticles studied by in-situ analysis

The thermal behavior of core-shell carbon-coated lithium iron phosphate (LiFePO4-C) nanoparticles made by flame spray pyrolysis (FSP) during annealing was investigated by in-situ transmission electron microscopy (TEM), in-situ X-ray powder diffraction (XRD) as well as ex-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Crystallization of the initially glassy LiFePO4-C nanoparticles starts at quite low temperatures (T=400°C), forming single crystals inside the confinement of the carbon shell. Upon increasing the temperature to T≥700°C, LiFePO4 starts to diffuse through the carbon shell resulting in cavities inside the mostly intact carbon shell. By increasing the temperature further to T≥800°C, the initial core-shell morphology converts into open carbon shells (flakes and cenospheres) and bulky LiFePO4 particles (diameter in the range 300–400nm), in agreement with ex-situ experiments.

Ultrathin and Uniform Carbon Coating on Active Materials Using Liquid CO2 As a Coating Solvent

Some of cathode and anode active materials (e.g., LiFePO4, Li4Ti5O12, Li2FeSiO4, TiO2, MoO2) exhibit inherently low electronic conductivities and low ionic conductivities; thus, these materials typically suffer from poor electrochemical performance. The reduction of particle size to a nanoscale dimension to enhance lithium ion diffusivity and carbon coating to increase electronic conductivity are the most widely used strategies to improve the performance of the potential active materials. Incorporation of too much carbon content, thick carbon layer coating, and non-uniform carbon coating on individual particles can also deteriorate energy density and battery performance. The complete, uniform, and thin carbon layer coverage ensures that the particles receive electrons from all directions and the electrons penetrate easily through the carbon layer, resulting in low polarization and high-rate performance. Herein, the use of liquid carbon dioxide (l-CO2) for uniform and ultrathin carbon layer coverage on highly porous LiFePO4 and MoO2 particles is discussed. The extremely low surface tension and the low viscosity associated with l-CO2 make it an excellent coating solvent for uniform and ultrathin carbon layer formation on the highly porous active materials. In addition, the carbon content in the range of 1-10 wt% can be easily controlled by adjusting process parameters such as solution concentration or evaporation velocity. The hierarchically porous MoO2 microspheres are synthesized in supercritical methanol without using any surfactants or structure-directing chemicals. The highly porous LiFePO4 particles are synthesized using a co-precipitation method. The MoO2 and LiFePO4 particles exhibits hierarchically porous nano/micro structure, in which primary particles with 30-65 mn size are loosely aggregated and form secondary spheres with 0.5-10 mm size. Owing to the low viscosity and low surface tension of l-CO2, the coating solution is penetrated into the pores of the particles and uniform and ultrathin carbon layer is covered on the primary particles. The thickness of carbon layers were in range of 0.8-2.0 nm by adjusting the carbon precursor concentration in l-CO2. The TEM analysis on the cross-sectional area of the particles shows that the uniform incorporation of the carbon layer on the whole surface of the micron-sized secondary particles. The carbon-coated LiFePO4 exhibits ~108.6 mAh g-1 at a high charge-discharge rate of 30 C. In addition, after 1000 cycles of 10 C, ~ 85% of an initial capacity is maintained. The carbon-coated MoO2 exhibits enhanced high-rate performance and cell stability. Figure 1

Electrochemical Studies of Carbon Nanotube-LiFePO4 Nanocomposite Cathode for High-Capacity Lithium-Ion Batteries

LiFePO4 nanoparticles were incorporated with multi-walled carbon nanotubes (CNTs) via a facile one-step polyol process at low temperature. CNTs were uniformly embedded into LiFePO4 particles and formed a conductive network to enhance the electrochemical performance of the CNT-LiFePO4 nanocomposite cathode. Structural composition and morphology of the composite were investigated in details by X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM, TEM). The electrochemical properties were analyzed in the voltage of 2.0 and 4.0 V by charge/discharge testing and cyclic voltammetry. Primary experimental results showed that these CNT-LiFePO4 nanocomposite cathodes exhibited enhanced electrochemical performance with both high capacity and good reversibility which has been clearly demonstrated in the Fig. 1. Figure 1

Enhanced Rate Capability in Two Phase Reaction in LiFePO4 By Interfacial Modification Between Cathode and Electrolyte

Olivine-type LiFePO4 is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO4 and FePO4. During the phase transition between LiFePO4 and FePO4, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.1 To improve the rate capability of LiFePO4, Park et al performed the surface modification by nitrogen on LiFePO4 particles.2 However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.3 In this study, we prepared LiFePO4thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy. LiFePO4 thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4in propylene carbonate liquid electrolyte were prepared. Annealing of ammonia gas introduces nitrogen into LiFePO4 surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO4 thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO4. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO4 and FePO4, which is the origin of surface modification effect.

Designing and Demonstrating Advanced Core-Shell Structured Carbon Coated Electrode Materials for High Power Li-Ion Battery Application

Li-ion batteries (LIBs) have become an integral part of most electronic gadgets, automobiles and (large-scale) grid-power devices due to its intriguing properties such as high energy and power density, no memory effect and excellent cyclic stability. After conquering mobile devices, currently LIBs are under the scanner to gain wide-scale use in commercial (plug-in) hybrid electric vehicles (EVs). These EVs based on LIBs are projected as alternative transport system vis-à-vis the conventional gasoline-based automobiles. However short driving range, slow recharge rate and high cost are the major obstacles and limit their extended use in EVs. Hence, demonstration of large scale LIBs for EV is far more challenging owing to the complex demand of energy density (to deliver 100-200 km per charge) as well as operational safety at viable cost. State-of-the-art lithium-ion cells use layered transition metal oxides, or phosphates or Mn based spinel as cathode and graphitic carbon as anode. Among these, LiFePO4 (LFP) has received much attention as a promising cathode material for high-power lithium-ion batteries used in EVs owing to its abundance, low manufacturing cost, thermal stability, safety, and high theoretical specific capacity. However, low ionic (~ 10-11 to 10-13 cm2 S-1) and intrinsic electronic conductivity (< 10-9 S cm-2) hinder its electro-chemical performance at high rate. Various efforts including (a) doping with supervalent cation, (b) reducing the particle size, and (c) carbon coating have been attempted to overcome these intrinsic defects. Among these efforts, carbon coating has become a widely used solution as it improves the conductivity, prevents metal ion dissolution, avoids direct contact of electrolyte with active material and restricts the crystal growth of electrode material during carbonization process. Though carbon coating is investigated by various methods, most of these methods are struggled by tedious synthesis conditions, non uniform distribution and lack of ordered carbon, which drastically decrease the performance of electrode materials. More importantly, a suitable carbon coating technique which provides collective characteristics of carbon such as (a) low carbon content, (b) thin layer carbon coating, (c) formation of graphitic carbon (d) homogeneous distribution of carbon coating and (e) core-shell structured of carbon coated electrode is yet to be realized. With this point in mind, a simple, unique and cost effective carbon coating technique to improve the electronic conductivity of electrodes has been discovered. In the present study, we focus on developing carbon coating of lab and large scale (Fig.1A) by dehydration assisted polymerization process, in which usage of dehydrating agent is considered important to form uniform carbon network on LFP. Carbon coating without dehydration agent is also carried out for comparison. The resulting carbon coated electrode material was extensively characterized by various characterization techniques. Structural studies revealed that carbon coating did not alter the crystal structure of LFP and HR-TEM analysis shows the formation of core-shell structure (Fig.1B), i.e., formation of thin layer of carbon (6-8 nm) with less carbon content (3 wt.%) on LiFePO4 particles; ideal for fast lithium ion diffusion during charge/discharge process. The developed carbon coating process led to the formation of sp2 hybridized carbon rich layer by proper catenation of carbon in the presence of dehydration agent, indicating the dominance of graphitic carbon in carbon coated LFP (C-LFP). Characterization studies of carbon coated LFP without dehydrating agent showed that the process was not eminent in yielding required carbon characteristics. The electro-chemical performance of C-LFP was evaluated in half/full cell configuration and benchmark with commercial electrode materials. C-LFP using dehydrating agent exhibit capacity of ~ 85 mA h g-1 at 5C rate with excellent cyclic stability and rate capability, whereas C-LFP without dehydrating agent exhibit a capacity of ~ 43 mA h g-1 at 5 C rate. The specific capacity of C-LFP is 131 mA h g-1 at 1C, whereas the specific capacity of commercially available C-LFP, i.e., UNTPL and TODA is 87 and 143 mA h g-1 respectively (Fig.1C & 1D), demonstrating that developed C-LFP in the present study showing better electro-chemical performance than C-LFP (conventional & UNTPL) and on par with C-LFP (TODA). The excellent electro-chemical performances of C-LFP developed in the present study is attributed to the presence of uniform, thin layer, core-shell structure and high graphitic nature of carbon in carbon coated LFP. Full cell studies of C-LFP with lithium titanate as counter electrode was also carried out and the results are promising. The novel, simple and cost-effective carbon coating process developed in the present study is of potential interest to other low conductive cathode materials and to develop high-power LIBs for EVs application. Figure 1