A carbon–LiFePO<Subscript>4</Subscript> nanocomposite as high-performance cathode material for lithium-ion batteries

A cathode material of an electrically conducting carbon–LiFePO₄ nanocomposite is synthesized by wet ball milling and spray drying of precursor powders prior to a solid-state reaction. The structural characterization shows that the composite is composed of LiFePO₄ crystals and 4.8 wt.% amorphous carbon. Galvanostatic charge/discharge measurements indicate that the composite exhibits a superior high energy and high cycling stability. This composite delivers a discharge capacity of 159.1 mAh g⁻¹ at 0.1 C, 150.8 mAh g⁻¹ at 1 C, and 140.1 mAh g⁻¹ at 2 C rate. The capacity retention of 99% is achieved after 200 cycles at 2 C. The 18,650 cylindrical batteries are assembled using the composite as cathode materials and demonstrate the capacity of 1,400 mAh and the capacity retention of 97% after 100 cycles at 1 C. These results reveal that the as-prepared LiFePO₄–carbon composite is one of the promising cathode materials for high-performance, advanced lithium-ion batteries directed to the hybrid electric vehicle and pure electric vehicle markets.     

LiFePO<Subscript>4</Subscript>/C with high capacity synthesized by carbothermal reduction method

The olivine-type LiFePO₄/C cathode materials were prepared via carbothermal reduction method using cheap Fe₂O₃ as raw material and different contents of glucose as the reducing agent and carbon source. Their structural and morphological properties were investigated by X-ray diffraction, scanning electron microscope, transmission electron microscope, and particle size distribution analysis. The results demonstrated that when the content of the carbon precursor of glucose was 16 wt.%, the synthesized powder had good crystalline and exhibited homogeneous and narrow particle size distribution. Even and thin coating carbon film was formed on the surface of LiFePO₄ particles during the pyrolysis of glucose, resulting in the enhancement of the electronic conductivity. Electrochemical tests showed that the discharge capacity first increased and then decreased with the increase of glucose content. The optimal sample synthesized using 16 wt.% glucose as carbon source exhibited the highest discharge capacity of 142 mAh g⁻¹ at 0.1C rate with the capacity retention rate of 90.4% and 118 mAh g⁻¹ at 0.5C rate.     

Improved of cathode performance of LiFePO<Subscript>4</Subscript>/C composite using different carboxylic acids as carbon sources for lithium-ion batteries

Among the several materials under development for use as a cathodes in lithium-ion batteries olivine-type LiFePO₄ is one of the most promising cathode material. However, its poor conductivity and low lithium-ion diffusion limits its practical application. In this study, we report seven different carboxylic acids used to synthesize LiFePO₄/C composite, and influences of carbon sources on electrochemical performance were intensively studied. The structure and electrochemical properties of the LiFePO₄/C were characterized by X-ray diffraction, scanning electron microscopy, electrical conductivity, and galvanostatic charge–discharge measurements. Among the materials studied, the sample E with tartaric acid as carbon source exhibited the best cell performance with a maximum discharge capacity of 160 mAh g⁻¹ at a 0.1 C-rate. The improved electrochemical properties were attributed to the reduced particle size and enhanced electrical contacts by carbon.     

Synthesis of the LiFePO<Subscript>4</Subscript> by a solid-state reaction using organic acids as a reducing agent

Olivine LiFePO₄ using organic acid as a reducing agent has been synthesized utilizing a solid-state method. Samples were characterized by an X-ray diffraction and a scanning electron microscope. The single-phase LiFePO₄ and small grain size of the crystallite were obtained without the use of a carbon-coating process. In such LiFePO₄ powder, the initial specific capacity was 142 mAhg⁻¹ at a current rate of 0.1 C. After the 50th cycle test, the reversible specific capacity was 132 mAhg⁻¹ at a 2 C rate, showing a retention ratio to the initial capacity as 98.4%.     

Enhanced electrochemical properties of LiMnPO<Subscript>4</Subscript>/C via Li-site substitution with Mg

Li_0.94Mg_0.03MnPO₄/C composite cathode materials for lithium ion battery with different carbon contents are synthesized by sol–gel method followed by heat treatment in the air. Environmental scanning electron microscopy measurements show that both firing temperature and carbon content affect the morphology of the end products. X-ray powder diffraction analysis indicates that the samples are olivine-structured. The galvanostatic charge–discharge results show that the optimal firing temperature registers 400 °C and that the electrochemical performances of Li_0.94Mg_0.03MnPO₄/C are improved by elevating its carbon amount. The sample with an initial conductive carbon content of 20 wt.% gives the best performances; when tested at the rate of 0.02C, 0.1C, and 1.0C between 2.8 and 4.4 V, its initial discharge capacity reaches 145.8, 103.0, and 72.8 mAhg⁻¹, respectively, and maintains at 100.1, 77.6, and 65.4 mAhg⁻¹, respectively, after 100 cycles.     

Performance of solid-state hybrid supercapacitor with LiFePO4/AC composite cathode and Li4Ti5O12 as anode

We report the studies on quasi-solid battery-supercapacitor (BatCap) systems fabricated using sol–gel-prepared LiFePO₄ and its composites (LACs) with activated charcoal (AC) as hybrid cathode and Li₄Ti₅O₁₂ powder as anode separator by flexible gel polymer electrolyte (GPE) film. The GPE film comprises 1.0 M lithium trifluoromethane sulfonate (LiTf) solution in ethylene carbonate (EC)–propylene carbonate (PC) mixture, immobilized poly(vinylidene fluoride-co-hexafluoro-propylene) (PVdF-HFP), which is of high ionic conductivity (∼3.8 × 10⁻³ S cm⁻¹ at 25 °C) and electrochemical stability window (∼3 V). The effect of the addition of AC in composite electrode LACs has been analyzed using various techniques such as X-ray diffraction, porosity analysis, and electrochemical methods. The interfaces of composite LACs and GPE film not only offer high rate performance but also show high specific energy (>27.8 Wh kg⁻¹) as compared to the symmetric supercapacitors and pristine lithium iron phosphate (LiFePO₄)-based lithium ion batteries. The full BatCap systems have been characterized by cyclic voltammetry and galvanostatic charge–discharge tests. The BatCap systems with composite electrodes (LACs) offer better cyclic performance as compared to that of pristine LiFePO₄-based BatCap or LIB LiFePO₄/Li₄Ti₅O₁₂.

     

Synthesis of spherical LiFePO4/C via Ni doping

Spherical LiFePO₄/C powders were synthesized by the conventional solid-state reaction method via Ni doping. Low-cost asphalt was used as both the reduction agent and the carbon source. An Ni-doped spherical LiFePO₄/C composite exhibited better electrochemical performances compared to an un-doped one. It presented an initial discharge capacity of 161 mAhg⁻¹ at 0.1 C rate (the theoretical capacity of LiFePO₄ with 5 wt% carbon is about 161 mAhg⁻¹). After 50 cycles at 0.5 C rate, its capacity remained 137 mAhg⁻¹ (100% of the initial capacity) compared to 115 mAhg⁻¹ (92% of the initial capacity) for an un-doped one. The electrochemical impedance spectroscopy analysis and cyclic voltammograms results revealed that Ni doping could decrease the resistance of LiFePO₄/C composite electrode drastically and improve its reversibility.     

A facile hydrothermal method to prepare LiFePO4/C submicron rod with core–shell structure

Submicron rod LiFePO₄/C has been synthesized via a facile hydrothermal process. The morphology, crystal structure, and charge–discharge performance of the prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and galvanostatic charge–discharge testing. The SEM and TEM illustrate that submicron rods with a width of about 140 nm and a length of up to 400 nm have been obtained. The TEM test also indicates a “core–shell” structure with a 1.5–2 nm carbon shell on the LiFePO₄ core. Even though the separate carbon-coated procedure is not used in this method, the electrochemical behavior results are satisfied. It displays that LiFePO₄/C has highly crystalline and a desirable core–shell structure with uniform carbon film. Galvanostatic battery testing shows that LiFePO₄/C delivers 104 mAh g^?1 at 5 C rates. The highest specific capacity of 166 mAh g^?1 is achieved at 0.1 C rate, and 99.8 % of the initial specific capacitance remained after 30 cycles.     

Electrochemical and electrical properties of Nb- and/or C-containing LiFePO4 composites

A study of LiFePO₄-based electrodes prepared through various synthesis conditions is presented. From X-Ray diffraction, high resolution transmission electron microscopy, electrochemical Li⁺ extraction/insertion and electrical conductivity data we conclude that the use of starting precursors such as Li₂CO₃, FeC₂O₄·2H₂O and/or Nb(OC₆H₅)₅ produces LiFePO₄-based composites containing significant amounts of carbon. We never succeeded in doping LiFePO₄ with Nb to yield Li_1−xNb_xFePO₄ but produced, instead, crystalline β-NbOPO₄ and/or an amorphous (Nb, Fe, C, O, P) “cobweb” around LiFePO₄ particles which is responsible for superior electrochemical activity. AC-conductivity measurements conclude to a total electrical conductivity of ∼10^− 9 S cm^− 1 at 25 °C with an activation energy of ca. 0.65 eV for pure LiFePO₄ and LiFePO₄/β-NbOPO₄ composites. C-containing LiFePO₄ samples, including those that were tentatively but unsuccessfully doped with Nb, are much more conductive (up to 1.6 · 10^− 1 S cm^− 1) with an activation energy ΔE∼0.08 eV.     

Structural and electrochemical properties of Nd-doped LiFePO4/C prepared without using inert gas

To further improve the electrochemical performance of LiFePO₄/C, Nd doping has been adopted for cathode material of the lithium ion batteries. The Nd-doped LiFePO₄/C cathode was synthesized by a novel solid-state reaction method at 750 °C without using inert gas. The Li_0.99Nd_0.01FePO₄/C composite has been systematically characterized by X-ray diffraction, EDS, SEM, TEM, charge/discharge test, electrochemical impedance spectroscopy and cyclic stability. The results indicate that the prepared sample has olivine structure and the Nd³⁺ and carbon modification do not affect the structure of the sample but improve its kinetics in terms of discharge capacity and rate capability. The Li_0.99Nd_0.01FePO₄/C powder exhibited a specific initial discharge capacity of about 161 mAh g^− 1 at 0.1 C rate, as compared to 143 mAh g^− 1 of LiFePO₄/C. At a high rate of 2 C, the discharge capacity of Li_0.99Nd_0.01FePO₄/C still attained to 115 mAh g^− 1 at the end of 20 cycles. EIS results indicate that the charge transfer resistance of LiFePO₄/C decreases greatly after Nd doping.     

An improved solid-state reaction route to Mg2+-doped LiFePO4/C cathode material for Li-ion battery

An improved solid-state reaction route has been employed to synthesize Mg²⁺-doped LiFePO₄/C nanocomposite cathode by calcining the precursor obtained via evaporating the mixture of ascorbic acid, LiCH₃COO·2H₂O, Mg(CH₃COO)₂·4H₂O, and amorphous FePO₄ nanoparticles in anhydrous ethanol under continuous stirring. Ascorbic acid used here acted as both reducing agent and carbon source. The amorphous FePO₄ was pre-prepared via a simple and fast oxidic precipitation method. Electrochemical tests showed that the final product exhibited good rate and cycling performance, with discharge capacities of 145.2 mAh g^?1 at 0.2 C, 129.8 mAh g^?1 at 1 C, 107.6mAh g^?1 at 5 C, and 81.4 mAh g^?1 at 20 C, respectively. The Mg²⁺-doped LiFePO₄/C showed enhanced charge–discharge performance compared with undoped LiFePO₄/C, especially at high rates. The enhanced electrochemical performance of the composite could be attributed to a combination result of the fine particle size with narrow particle size distribution, homogeneous carbon coating on the surface of the particles, and magnesium ion doping.     

Electrochemical behavior of carbonic precursor with Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>nanostructured material in hybrid battery system

The nanostructured Na₃V₂(PO₄)₃ (NVP) cathode material has been synthesized using the sol-gel route for different molar fractions of citric acid as a carbon source during the synthesis. The nanostructured NVP as cores with carbonic shell structures are fabricated with two different citric acid molar ratios. The thermal treatment has been optimized to convert the amorphous carbon shell into graphitic carbon to realize the better electrical conductivity and thus effective electron transfer during the electrochemical charge transfer process. The X-ray diffraction measurements confirmed the rhombohedral crystallographic phase (space group R-3c) with average crystallite size ~28 ± 5 nm. The coin cells are assembled in a hybrid rechargeable electrochemical cell configuration with lithium as a counter electrode and LiPF₆-EC:DEC:DMC (1:1:1 ratio) as the electrolyte. The electrochemical charge/discharge measurements are carried out at C/10 and C/20 rates and the measured specific capacities are 80 and 120 mAhg^?1 for samples with lower and higher citric acid molar ratios, respectively. The studies suggest that NVP can be used as an effective cathode material in hybrid electrochemical cells, and a higher concentration of citric acid may result in the effective carbonic shell for optimal electron transfer and thus enhanced electrochemical performance.     

In-situ synchrotron X-ray absorption studies of LiMn0.25Fe0.75PO4 as a cathode material for lithium ion batteries

Recently, lithium bi-metal phosphates (LiM′M″PO₄) have been synthesized for use as cathode materials in order to increase cell voltages and electrical performances. In this work, we have substituted Mn²⁺ at the 4c site of LiFePO₄ to prepare the lithium bi-metal phosphate LiMn_0.25Fe_0.75PO₄ and have found that it greatly enhances the cell voltage. At a 0.05 C discharge rate, the cell capacity was about 153 mAhg^− 1 and the average working voltage rose to 3.53 V due to the Mn substitution. However, the capacity and working voltage both decrease as the discharge rate increases. By in-situ metal K-edge absorption analysis, it reveals that the substituted metal Mn²⁺ does not work completely at a higher discharge rate, due to poor electrical conductivity and a serious Jahn–Teller effect.     

Effect of milling time on the performance of bowl-like LiFePO4/C prepared by wet milling-assisted spray drying

LiFePO₄/C was prepared by wet milling-assisted spray drying. The effects of ball-milling time on the characteristics of LiFePO₄/C were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, cyclic voltammograms, electrochemical impedance spectra, and galvanostatic charge–discharge testing. Bowl-like material was obtained, surrounded by a network of carbon, which display larger specific surface area. The specific surface area of particle first increased and then decreased, as the increasing of ball-milling time; when ball-milling time reach 2.5 h, it showed the largest specific surface area of 29.350 m² g^?1, primary particles with size of ～50 nm, delivered a discharge capacity of 162 mAh g^?1 at 0.5 C and 123 mAh g^?1 at 10 C, and with no capacity loss.     

Low temperature synthesis of layered LiNiO2 cathode material in air atmosphere by ion exchange reaction

The layered LiNiO₂ cathode material for lithium ion battery was synthesized by ion-exchange reaction at low temperature in air atmosphere. The influence of synthesis conditions on the electrochemical performance of the resulting LiNiO₂ was investigated. The LiNiO₂ samples were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and infrared (IR) analysis. The results indicate that low temperature fabricated LiNiO₂ powders keep a single layered hexagonal structure and homogenous spheric shape like the raw material NiOOH. Charge and discharge tests show that the resultant LiNiO₂ exhibits good electrochemical properties. The first charge and discharge capacities of the sample are 183.4 mA h g^− 1 and 169.5 mA h g^− 1 at 0.5 mA cm^− 2, respectively. Galvanic charge/discharge and cyclic voltammetry tests reflect that LiNiO₂ electrode exhibits good cycle reversibility.     

Propylene oxide-assisted fast sol–gel synthesis of mesoporous and nano-structured LiFePO4/C cathode materials

LiFePO₄/C nanocomposites are synthesized by a propylene oxide-assisted fast sol–gel method using FeCl₃, LiNO₃, NH₄H₂PO₄, and sucrose as the starting materials. It was found that after adding propylene oxide into the solution containing the starting materials, a monolithic jelly-like FePO₄ gel containing lithium and carbon source is generated in a few minutes without controlling the pH value of the solution and a time-consuming heating process. Propylene oxide plays a key role in the fast generation of the precursor gel. The final products of LiFePO₄/C are obtained by sintering the dry precursor gel. The structures, micro-morphologies, and electrochemical properties of the LiFePO₄/C composites are investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption analysis, electrochemical impedance spectrum, and charge–discharge cycling tests. The results indicate that the LiFePO₄/C composite prepared by sintering the precursor gel at 680 °C for 5 h is about 30 nm in size with a meso-porous structure (the main pore size distribution is around 3.4 nm). It delivers 166.7 and 105.8 mAh g^?1 at 0.2 and 30 C, respectively. The discharge specific capacity is 97.8 mAh g^?1 even at 40 C. The cycling performance of the prepared LiFePO₄/C composite is stable. The excellent electrochemical performance of the LiFePO₄/C composite is attributed to the nano-sized and mesoporous structure of LiFePO₄/C and the in-situ surface coating of the carbon. It was also found that propylene oxide is crucial for the generation of mesoporous and nano-structured LiFePO₄/C.     

Synthesis of carbon microtubules core structure LiFePO4 via a template-assisted method

The carbon microtubules core structure LiFePO₄ is synthesized using a cotton fiber template-assisted method. The crystalline structure and morphology of the product is characterized by X-ray diffraction and field emission scanning electron microscopy. The charge–discharge kinetics of the LiFePO₄ electrode is investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The result shows that the well-crystallized carbon microtubules core structure LiFePO₄ is successfully synthesized. The as-synthesized material exhibits a high initial discharge capacity of 167 mAh g^?1 at 0.2 C rate. The material also shows good high-rate discharge performance and cycling stability, about 127 mAh g^?1 and 94.7 % capacity retention after 100 cycles even at 5.0 C rate.     

A PEG-assisted rheological phase reaction synthesis of 5LiFePO<Subscript>4</Subscript>⋅Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as cathode material for lithium ion cells

5LiFePO₄⋅Li₃V₂(PO₄)₃/C composite cathode material is synthesized by a polyethylene glycol (PEG)-assisted rheological phase method. As a surfactant and dispersing agent, PEG can effectively inhabit the aggregation of colloidal particles during the formation of the gel. Meanwhile, PEG will coat on the particles to play the role of carbon source during the sintering. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy, and electrochemical methods. XRD results indicate that the 5LiFePO₄⋅Li₃V₂(PO₄)₃/C composites are well crystallized and contain olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. The composite synthesized at 650 °C exhibits the initial discharge capacities of 134.8 and 129.9 mAh g⁻¹ and the capacity retentions of 96.2 and 97.1 % after 50 cycles at 1C and 2C rates, respectively.     

Influence of coated MnO2 content on the electrochemical performance of Li1.2Ni0.2Mn0.6O2 cathodes

Layered cathode material Li_1.2Ni_0.2Mn_0.6O₂ has been synthesized using a coprecipitation method and coated by MnO₂ with varying amounts (1, 3, 5, and 9 wt%). The physical properties and electrochemical performances of the materials are characterized by XRD, SEM, charge/discharge tests, cycle life, and rate capability tests. XRD patterns show that the pristine and coated Li_1.2Ni_0.2Mn_0.6O₂ powders exhibit layered structure. The discharge capacities and coulombic efficiencies of Li_1.2Ni_0.2Mn_0.6O₂ in the first cycle have been improved and increase with the increasing content of coated MnO₂. The 9 wt% MnO₂-coated Li_1.2Ni_0.2Mn_0.6O₂ delivers 287 mAhg^?1 for the first discharge capacity and 86.7 % for the first coulombic efficiency compared with 228 mAhg^?1 and 65.9 % for pristine Li_1.2Ni_0.2Mn_0.6O₂. However, the 5 wt% MnO₂-coated Li_1.2Ni_0.2Mn_0.6O₂ shows the best capacity retention (99.9 % for 50 cycles) and rate capability (88.6 mAhg^?1 at 10 C), while the pristine Li_1.2Ni_0.2Mn_0.6O₂ only shows 91.5 % for 50 cycles and 25.3 mAhg^?1 at 10 C. The charge/discharge curves and differential capacity vs. voltage (dQ/dV) curves show that the coated MnO₂ reacts with Li⁺ during the charge and discharge process, which is responsible for higher discharge capacity after coating. Electrochemical impedance spectroscopy results show that the R _ct of Li_1.2Ni_0.2Mn_0.6O₂ electrode decreases after coating, which is responsible for superior rate capability.     

Enhanced electrochemical performance of LiFePO4/C prepared by sol–gel synthesis with dry ball-milling

LiFePO₄/C was prepared by a modified aqueous sol–gel route developed by incorporating an additional ball-milling step where the dry gel was milled with the additives of synthetic graphite and carbon black. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and elemental analysis. Results showed that the LiFePO₄/C synthesized by suitable ball-milling process had pure, fine and homogenous LiFePO₄ particles. Results of cyclic voltammetry and charge/discharge plateaus demonstrated that the LiFePO₄/C composite synthesized by milling for 2 h had much better electrochemical kinetics. High performances were achieved with its discharge capacities of 157 mA h g^?1 at 0.1?C and 133 mA h g^?1 at 1?C between 2.5 and 4.2 V (1?C?=?170 mA g^?1). And no obvious capacity fading was observed upon cycling. The simple and convenient synthesis route is promising for large-scale production of LiFePO₄/C.