Carbon-coated LiFePO4 Cathode Research Articles

Direct regeneration of fluorine-doped carbon-coated LiFePO4 cathode material from spent lithium-ion battery

Direct regeneration of fluorine-doped carbon-coated LiFePO4 cathode material from spent lithium-ion battery

Three-Dimensional Carbon-Coated LiFePO4 Cathode with Improved Li-Ion Battery Performance

LiFePO4 (LFPO)has great potential as the cathode material for lithium-ion batteries; it has a high theoretical capacity (170 m·A·h·g−1), high safety, low toxicity and good economic benefits. However, low conductivity and a low diffusion rate inhibit its future development. To overcome these weaknesses, three-dimensional carbon-coated LiFePO4 that incorporates a high capacity, superior conductivity and low volume expansion enables faster electron transport channels. The use of Cetyltrimethyl Ammonium Bromid (CTAB) modification only requires a simple water bath and sintering, without the need to add a carbon source in the LFPO synthesis process. In this way, the electrode shows excellent reversible capacity, as high as 159.8 m·A·h·g−1 at 2 C, superior rate capability with 97.3 m·A·h·g−1 at 5 C and good cycling ability, preserving ~84.2% capacity after 500 cycles. By increasing the ion transport rate and enhancing the structural stability of LFPO nanoparticles, the LFPO-positive electrode achieves excellent initial capacity and cycle life through cost-effective and easy-to-implement carbon coating. This simple three-dimensional carbon-coated LiFePO4 provides a new and simple idea for obtaining comprehensive and high-performance electrode materials in the field of lithium cathode materials.

An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Summary High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level.

Graphene encapsulated iron nitrides confined in 3D carbon nanosheet frameworks for high-rate lithium ion batteries

Metal nitrides with high theoretical capacities and exceptional electrical conductivities hold great potentials as high-rate anode materials for lithium ion batteries (LIBs), but suffer from severe pulverization and air instability of electrodes. Herein, we developed a general strategy for synthesizing few-layer graphene encapsulated transition metal nitride (e.g., Fe2N) nanoparticles confined in three dimensional (3D) ultrathin carbon nanosheet frameworks (denoted as Fe2N@CNFs) via the iron nitrate accelerated polyvinylpyrrolidone (PVP) blowing and subsequent in-situ nitridation process, as high-rate anode for LIBs. Benefiting from the well-confined Fe2N nanoparticles by graphene shells for alleviating structural pulverization and air sensitivity, and 3D carbon nanosheet porous frameworks for fast electrolyte ion diffusion and fast electron transport, the resulting Fe2N@CNFs anodes exhibited high reversible capacity of 587 mA h g−1 at 0.1 A g−1, and remarkable rate capacity of 215 mA h g−1 at high current density of 10 A g−1, outperforming most reported metal nitride electrodes. When coupled with carbon-coated LiFePO4 cathode, the resulting full LIBs still delivered an initial discharge capacity of 243 mA h g−1 at 0.1 A g−1. Therefore, this work will pave a new way for rational construction of a series of high-performance metal nitride electrodes for energy-related devices.

Solid Polymer Electrolytes for Lithium Metal Battery via Thermally Induced Cationic Ring-Opening Polymerization (CROP) with an Insight into the Reaction Mechanism

We report the synthesis of solid polymer electrolytes (SPEs) using a thermally induced and a lithium salt catalyzed cationic ring-opening polymerization (CROP) technique. A synergistic approach using two salts such as lithium tetrafluoroborate-LiBF4 and lithium bis(trifluoromethane sulfonyl)imide-LiTFSI has assured a complete monomer to polymer conversion and fast reaction kinetics during the CROP process. The initiation mechanism of lithium salt-induced CROP is elucidated using molecular dynamic simulation, quantum chemical calculation, real-time FT-Raman spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetry–mass spectrometry analysis techniques. The cross-linked 3D network of ethylene oxide based SPE is prepared without the use of any solvents or external catalysts. In particular, a mixture of poly(ethylene glycol) diglycidyl ether, LiBF4, and LiTFSI in appropriate proportions after a baking process produced a freestanding, flexible, and nontacky film. The synthesized SPEs exhibit low glass transition temperature (< −50 °C), high ionic conductivity (>0.1 mS cm–1), and excellent oxidation stability (>5.5 V vs Li/Li+). The SPE is polymerized directly onto a carbon-coated LiFePO4 cathode film and successfully cycled in a lithium metal battery configuration at 40 and 60 °C. As evidence, the SPE is galvanostatically cycled against a high-voltage LiNi1/3Mn1/3Co1/3O2 cathode, and the preliminary results indicated exciting characteristics in terms of specific capacity and Coulombic efficiency.

In-Situ X-Ray Study of Carbon Coated LiFePO4 for Li-Ion Battery in Different State of Charge

This paper is focused on the structural characterization of commercially exploited carbon coated LiFePO4 cathode material in different state of charge. The material was measured inside of modified ECC-Opto-Std cell and X-ray characterization was performed using Rigaku Miniflex 600 HR diffractometer. The study compare two approaches used for characterization of electrodes in different state of charge: a) Fast galvanostatic mode with potential limitation typically used in the ex-situ characterizations and b) Operando measurement at slow galvanostatic charging (0.1 C). Lattice parameters and volume changes were evaluated by Rietveld refinement and results indicate that carbon coating of particles does not have any influence on the volume changes of materials. We found the full compliance of lattice parameters changes and therefore also volume changes with those reported for pure LFP material in other reported studies.

An organic-skinned secondary coating for carbon-coated LiFePO4 cathode of high electrochemical performances

Carbon coating technologies for olivine-structured LiFePO4 have been successfully commercialized in lithium ion industries. However, up to today, there are still many challenges overcoming the low intrinsic ionic conductivity and surface instability limitations. Herein, we propose a lithium ion conductive organic-skinned secondary coating technique by applying sodium maleate onto the surface of a commercial carbon-coated LiFePO4 cathode. Very distinctive features of the organic skin secondary coating are highly uniform and continuous. More importantly, the abundant carbonyl groups on the LFP surface facilitates facial Li ion transport. The nanostructure tuned organic layer is able to effectively suppress the undesired side reactions at the electrode/electrolyte interface. With 6.0 wt% of the sodium maleate coating, rate capability and long-term cycling behavior of the LiFePO4 cathode at different temperatures are significantly improved. Notably, the well-known impedance rise of LFP cathode with increasing electrochemical cycles is effectively suppressed. An insight into the mechanisms of the secondary sodium maleate coating on the carbon-coated LiFePO4 cathode is discussed.

Synthesis of single-crystalline LiFePO4 with rhombus-like morphology

In this investigation, carbon-coated LiFePO4 cathode materials were synthesized with a facile hydrothermal method. The structure and electrochemical properties of the materials were investigated by X-ray diffraction (XRD), Roman, transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS), and electrochemical impedance spectroscopy (EIS). By adjusting the mixing concentration of starting materials, a single-crystalline LiFePO4 with an anisotropic rhombus morphology (Space Group: Pmnb No. 62) were successfully synthesized. In addition, the carbon coated on the surface of LiFePO4 material prepared has a lower ID/IG (0.80), which indicates an optimized carbon structure with an increased amount of sp2-type carbon. Electrochemical performance test shows that the carbon-coated LiFePO4 cathode materials have an initial discharge capacity of 146 mAh g−1 at 0.2C.

Synthesis and characterization of LiFePO4/C cathode materials by sol-gel method.

The carbon coated LiFePO4 cathode materials (LiFePO4/C) were successfully synthesized by sol-gel method with glucose, citric acid and PEG-4000 as dispersant and carbon source, respectively. The microstructure and grain size of LiFePO4/C composite were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy. The results showed that the carbon source and calcination temperature had important effect on the graphitization degree of carbon; the carbon decomposed by citric acid had higher graphitization degree; with calcination temperature rising, the graphitization degree of carbon increased and the particles size increased. The graphitization degree and grain size were very important for improving the electrochemical performance of LiFePO4 cathode materials, according to the experimental results, the sample LFP-700 (LFP-C) which was synthesized with citric acid as dispersant at 700 degree C had lower polarization and larger discharge capacity.

One Pot Synthesis of Zr4+doped Carbon Coated LiFePO4 Cathode Material for Rechargeable Li-ion Battery

One Pot Synthesis of Zr4+doped Carbon Coated LiFePO4 Cathode Material for Rechargeable Li-ion Battery

Effects of surface fluorination on the electrochemical properties and thermal stability of LiFePO4 cathode for lithium-ion batteries

The electrochemical properties and thermal stability of carbon coated LiFePO4 cathode were improved by the surface fluorination using NF3 gas at a pressure less than 6.67kPa at 25°C. Especially, in case of F1-LiFePO4, the discharge capacity was about 10% higher than that of untreated LiFePO4. Also it indicated the lowest resistance (Rct=35.55Ω) and largest exchange current density (i0=0.72mA) from the EIS results. However, in case of F2-LiFePO4, the electrochemical properties were negatively affected by the formation of some resistive fluoride films. In terms of thermal stability, the decomposition temperature (222.0°C) of F1-LiFePO4 was 14°C higher than that of untreated LiFePO4. Therefore, the surface fluorination led to the improvement of electrochemical properties and thermal stability of LiFePO4 as a promising cathode in high power lithium-ion cells for HEVs.

LiFePO4@C cathode materials synthesized from FePO4@PAn composites

Carbon coated LiFePO4 cathode materials were successfully synthesized by heating the precursors of FePO4@PAn (polyaniline) and equimolar LiOH·H2O under Ar flow. We demonstrated a method that could precisely control the carbon shell in a single reaction step. The precursors of FePO4@PAn were prepared by in situ polymerization of aniline and precipitation of FePO4 in one pot in the presence of hydrogen peroxide as oxidizer. In the obtained core–shell cathode composite, the LiFePO4 core phase was crystalline while the carbon shell had the thickness of several nanometers. The optimized cathode material exhibited satisfactory rate and cycle performance, which virtues are quite suitable for power lithium ion batteries.

Particle size effects of carbon sources on electrochemical properties of LiFePO4/C composites

Carbon-coated LiFePO4 cathode materials were prepared by a solid-state method incorporating different sizes of polystyrene (PS) spheres as carbon sources. In scanning electron microscope images, small PS spheres appear more effective at preventing aggregation of LiFePO4 particles. From transmission electron microscopy images, it was found that the LiFePO4 particles were completely uniformly coated with 5-nm carbon layer when the carbon source was 0.22 μm PS spheres. When the size of PS sphere was increased to 2.75 μm, a network of carbon was formed and wrapped around the LiFePO4 to create a conductive web. Raman spectroscopy and four-point probe conductivity measurement showed that using larger sizes of PS spheres as carbon sources leads to greater conductivity of LiFePO4/C. The LiFePO4 precursor sintered with 0.22 μm PS spheres delivered an initial discharge capacity of 145 mAh g−1 at a 0.2 C rate, but it only sustained 289 cycles at 80% capacity. When the diameter of PS spheres was increased to 2.75 μm, the discharge capacity of LiFePO4/C decreased, but the cycle life reached 755 cycles, the highest number in this work probably due to the network formation of carbon wrapping around LiFePO4 particles.

Thermal Characterization of LiFePO4 Cathode in Lithium Ion Cells

The thermal behavior of carbon-coated LiFePO4 cathode material was investigated using three different calorimetric techniques. Heat released during charge and discharge of cylindrical 18650 size cells was measured in an isothermal calorimeter. Accelerating rate calorimetry was used to investigate the cause of thermal runaway of the cylindrical 18650 size cells. On the other hand, differential scanning calorimeter was used to study the contribution of the delithiated carbon-coated LiFePO4 and the lithiated graphite to the overall thermal runaway through coin cells of 2032 size. The carbon-coated LiFePO4 showed very good thermal stability.

Electrochemical and Thermal Studies of Carbon-Coated LiFePO4 Cathode

The carbon-coated Li-ion cathode material was studied for its electrochemical and thermal performance. This electrode exhibited a reversible capacity corresponding to more than 90% of the theoretical capacity when cycled between 2.5 and 4.0 V. The material also showed good capacity retention at high powers, implying that the carbon coating improves the electronic conductivity and hence the cycling of this material. The diffusion coefficient of this material was calculated from its electrochemical impedance spectroscopy. The heat generation during charge and discharge was studied using an isothermal microcalorimeter. Thermal studies were also investigated by using a differential scanning calorimeter and an accelerating rate calorimeter, which showed that is safer than the commonly used lithium metal oxide cathodes with layered structures.