Rechargeable Li/LiFePO4 cells using N-methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI electrolyte incorporating polymer additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (∼0-∼30 °C).     

Improved electrochemical performance of LiFePO4 by increasing its specific surface area

Cathode material LiFePO₄ with an excellent rate capability has been successfully prepared by a simple solid state reaction method using LiCH₃COO·2H₂O, FeC₂O₄·2H₂O and (NH₄)₂HPO₄ as the starting materials. We have investigated the effects of the sintering temperature and mixing time of the starting materials on the physical properties and electrochemical performance of LiFePO₄. It was found that the rate capability of LiFePO₄ is mainly controlled by its specific surface area and it is an effective way to improve the rate capability of the sample by increasing its specific surface area. In the present study, our prepared LiFePO₄ with a high specific surface area of 24.1 m² g⁻¹ has an excellent rate capability and can deliver 115 mAh g⁻¹ of reversible capacity even at the 5 C rate. Moreover, we have prepared lithium ion batteries based on LiFePO₄ as the cathode material and MCMB as the anode material, which showed an excellent cycling performance.     

Synthesis of LiFePO4/C cathode material from ferric oxide and organic lithium salts

LiFePO₄/C cathode material has been simply synthesized via a modified in situ solid-state reaction route using the raw materials of Fe₂O₃, NH₄H₂PO₄, Li₂C₂O₄ and lithium polyacrylate (PAALi). The sintering temperature of LiFePO₄/C precursor is studied by thermo-gravimetric (TG)/differential thermal analysis (DTA). The physical properties of LiFePO₄/C are then investigated through analysis using by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and the electrochemical properties are investigated by electrochemical impedance spectra (EIS), cyclic voltammogram (CV) and constant current charge/discharge test. The LiFePO₄/C composite with the particle size of ∼200 nm shows better discharge capacity (156.4 mAh g⁻¹) than bare LiFePO₄ (52.3 mAh g⁻¹) at 0.2 C due to the improved electronic conductivity which is demonstrated by EIS. The as-prepared LiFePO₄/C through this method also shows excellent high-rate characteristic and cycle performance. The initial discharge capacity of the sample is 120.5 mAh g⁻¹ and the capacity retention rate is 100.6% after 50 cycles at 5 C rate. The results prove that the using of organic lithium salts can obtain a high performance LiFePO₄/C composite.     

Synthesis of FePO4·2H2O nanoplates and their usage for fabricating superior high-rate performance LiFePO4

Monoclinic phase FePO₄·2H₂O nanoplates are synthesized very easily in a waterbath and are lithiated to LiFePO₄/C nanoparticles by a simple rheological phase method. The thickness of the nanoplates can be tuned simply by changing the concentrations of the reactants. The LiFePO₄/C nanoparticles lithiated from the thin FePO₄·2H₂O nanoplates, with the sizes about 50 nm and the carbon coating layer at the surface 1–2 nm, show excellent high-rate performance and long-term cyclability as the cathode for lithium ion batteries, delivering discharge capacities of more than 150, 120, 110, 100, and 75 mAh g⁻¹ at rates of 5 C, 10 C, 15 C, 20 C and 30 C, respectively.     

Structural and electrochemical studies of PPy/PEG-LiFePO4 cathode material for Li-ion batteries

A simple chemical oxidative polymerization of pyrrole (Py) directly onto the surface of LiFePO₄ particles was applied to the synthesis of polypyrrole-LiFePO₄ (PPy-LiFePO₄) powder. The LiFePO₄ sample without carbon coating was synthesized by a solvothermal method. The polyethylene glycol (PEG) was used as additive during Py polymerization for increasing the PPy-LiFePO₄ conductivity. Properties of resulting LiFePO₄, PPy-LiFePO₄ and PPy/PEG-LiFePO₄ samples were characterized by XRD, SEM, TGA and galvanostatic charge-discharge measurements. These methods confirmed the presence of polypyrrole on LiFePO₄ particles and its homogeneous distribution in the resulting powder material. The PPy/PEG-LiFePO₄ composites show higher discharge capacity than pure LiFePO₄, as PPy/PEG network improves the electron conductivity. It presents specific discharge capacity of 153 mAh/g at C/5 rate.     

Synthesis and characterization of high-density LiFePO4/C composites as cathode materials for lithium-ion batteries

To achieve a high-energy-density lithium electrode, high-density LiFePO₄/C composite cathode material for a lithium-ion battery was synthesized using self-produced high-density FePO₄ as a precursor, glucose as a C source, and Li₂CO₃ as a Li source, in a pipe furnace under an atmosphere of 5% H₂-95% N₂. The structure of the synthesized material was analyzed and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical properties of the synthesized LiFePO₄/carbon composite were investigated by cyclic voltammetry (CV) and the charge/discharge process. The tap-density of the synthesized LiFePO₄/carbon composite powder with a carbon content of 7% reached 1.80 g m⁻³. The charge/discharge tests show that the cathode material has initial charge/discharge capacities of 190.5 and 167.0 mAh g⁻¹, respectively, with a volume capacity of 300.6 mAh cm⁻³, at a 0.1C rate. At a rate of 5C, the LiFePO₄/carbon composite shows a high discharge capacity of 98.3 mAh g⁻¹ and a volume capacity of 176.94 mAh cm⁻³.     

Effect of CeO2-coating on the electrochemical performances of LiFePO4/C cathode material

The effect of CeO₂ coating on LiFePO₄/C cathode material has been investigated. The crystalline structure and morphology of the synthesized powders have been characterized by XRD, SEM, TEM and their electrochemical performances both at room temperature and low temperature are evaluated by CV, EIS and galvanostatic charge/discharge tests. It is found that, nano-CeO₂ particles distribute on the surface of LiFePO₄ without destroying the crystal structure of the bulk material. The CeO₂-coated LiFePO₄/C cathode material shows improved lithium insertion/extraction capacity and electrode kinetics, especially at high rates and low temperature. At −20 °C, the CeO₂-coated material delivers discharge capacity of 99.7 mAh/g at 0.1C rate and the capacity retention of 98.6% is obtained after 30 cycles at various charge/discharge rates. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

Reaction mechanism and electrochemical performance of LiFePO4/C cathode materials synthesized by carbothermal method

Spray drying and carbothermal method was employed to investigate reaction mechanism and electrochemical performance of LiFePO₄/C cathode by using different carbon sources. Micro-structural variations of LiFePO₄/C precursors using different carbon sources were studied by Thermo-gravimetric (TG)/Differential Thermal Analysis (DTA). The LiFePO₄/C samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) absorption spectroscopy. The results indicated that the crystallization temperature of LiFePO₄ was 453 °C, while the transform temperature was 539 °C from Li₃Fe₂(PO₄)₃ to LiFePO₄. At 840 °C, LiFePO₄/C sample with an excess of impurity phase Fe₂P gave much poorer electrochemical performance. The severe decomposition of LiFePO₄/C happened at 938 °C and generated impurity phases Li₄P₂O₇ and Fe₂P. The clear discharge platform of Fe₂P emerged at around 2.2 V.     

Enhanced electrochemical performances of LiNi0.5Mn1.5O4 spinel via ethylene glycol-assisted synthesis

A simple and effective method, ethylene glycol-assisted co-precipitation method, has been employed to synthesize LiNi_0.5Mn_1.5O₄ spinel. As a chelating agent, ethylene glycol can realize the homogenous distributions of metal ions at the atomic scale and prevent the growth of LiNi_0.5Mn_1.5O₄ particles. XRD reveals that the prepared material is a pure-phase cubic spinel structure (Fd3m) without any impurities. SEM images show that it has an agglomerate structure with the primary particle size of less than 100 nm. Electrochemical tests demonstrate that the as-prepared LiNi_0.5Mn_1.5O₄ possesses high capacity and excellent rate capability. At 0.1 C rate, it shows a discharge capacity of 137 mAh g⁻¹ which is about 93.4% of the theoretical capacity (146.7 mAh g⁻¹). At the high rate of 5 C, it can still deliver a discharge capacity of 117 mAh g⁻¹ with excellent capacity retention rate of more than 95% after 50 cycles. These results show that the as-prepared LiNi_0.5Mn_1.5O₄ is a promising cathode material for high power Li-ion batteries.     

Preparation of synthetic rutile and metal-doped LiFePO4 from ilmenite

The synthetic rutile and metal-doped LiFePO₄ are prepared from the high-titanium residue and iron-rich lixivium, which are obtained from the ilmenite by a mechanical activation and leaching process. ICP results show that the rutile contains 92.01% TiO₂, 1.59% Fe₂O₃, 0.034% MnO₂ and 0.60% (MgO + CaO), which meet the requirement of the titanium dioxide chlorination process. The results also reveal that small amounts of Al³⁺, Ca²⁺ and Ti⁴⁺ precipitate in the FePO₄·xH₂O precursor. XRD and Rietveld-refine results show that the metal-doped LiFePO₄ is single olivine-type phase and well crystallized, and Ti⁴⁺ occupy M1 site, Ca²⁺ occupy M2 site and Al³⁺ occupy both sites, which indicates the formation of cation-deficient solid solution. The sample exhibits a capacity of 123 mAh g⁻¹ at 5C rate, and retains 94.3% of the capacity after 100 cycles.     

Synthesis and electrochemical performance of LiNi0.5Mn1.5O4 spinel compound

Spinel compound LiNi_0.5Mn_1.5O₄ was synthesized by a chemical wet method. Mn(NO₃)₂, Ni(NO₃)₂·6H₂O, NH₄HCO₃ and LiOH·H₂O were used as the starting materials. At first, Mn(NO₃)₂ and Ni(NO₃)₂·6H₂O reacted with NH₄HCO₃ to produce a precursor, then the precursor reacted with LiOH·H₂O to synthesize product LiNi_0.5Mn_1.5O₄. The product showed a single spinel phase under appropriate calcination conditions, and exhibited a high voltage plateau at about 4.6-4.8 V in the charge/discharge process. The LiNi_0.5Mn_1.5O₄ had a discharge specific capacity of 118 mAh/g at about 4.6 V and 126 mAh/g in total in the first cycle at a discharge current density of 2 mA/cm². After 50 cycles, the total discharge capacity was above 118 mAh/g.     

Synthesis of spindle-shaped nanoporous C–LiFePO4 composite and its application as a cathodes material in lithium ion batteries

In this work, LiFePO₄/C composites were prepared in hydrothermal system by using iron gluconate as iron source, and two feeding sequences during the preparation were comparatively studied. 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 results showed that the feeding sequences and iron gluconate seriously affected the microstructures and electrochemical properties of the resulting LiFePO₄ cathodes in lithium ion batteries. The spindle-shaped LiFePO₄ with hierarchical microporous structure self-assembled by nanoparticles has been successfully synthesized by synthesis route B. In addition, the cell performance of the synthesized LiFePO₄ by synthesis route B was better than that of LiFePO₄ by synthesis route A. Specially at high rates, the superior rate performance of the spindle-shaped LiFePO₄/C microstructure (LFP/C-B) was revealed. And special reversible capacities of ∼118 and ∼95 mAh g⁻¹ were obtained at rates of 2 C and 5 C, comparing to ∼96 and ∼68 mAh g⁻¹ for LFP/C-A.     

Chemical lithiation route to size-controllable LiFePO4/C nanocomposite

Chemical lithiation of amorphous FePO₄ with LiI in acetonitrile is performed to form amorphous LiFePO₄. The amorphous FePO₄·2H₂O precursor is synthesized by co-precipitation method from equimolar aqueous solutions of FeSO₄·7H₂O and NH₄H₂PO₄, using H₂O₂ (hydrogen peroxide) as the oxidizing agent. The nanocrystalline LiFePO₄/C is obtained by annealing the amorphous LiFePO₄ and in situ carbon coating with sucrose in a reducing atmosphere. The particle size of FePO₄·2H₂O precursor decreases with increasing reaction temperature. The final LiFePO₄/C products completely maintain the shape and size of the precursor even after annealing at 700 °C for 2 h. The excellent electrochemical properties of these nanocrystalline LiFePO₄/C composites suggest that to decrease the particle size of LiFePO₄ is very effective in enhancing the rate capability and cycle performance. The specific discharge capacities of LiFePO₄/C obtained from the FePO₄·2H₂O precursor synthesized at 75 °C are 151.8 and 133.5 mAh g^?1 at 0.1 and 1 C rates, with a low capacity fading of about 0.075 % per cycle over 50 cycles at 0.5 C rate.     

Synthesis of LiFePO4/carbon composite from nano-FePO4 by a novel stearic acid assisted rheological phase method

LiFePO₄/carbon composite was synthesized at 600 °C for 4 h in an Ar atmosphere by a stearic acid assisted rheological phase method using amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/carbon composite has good crystallinity, ultrafine and well-dispersed particles of 60–150 nm size and in situ carbon coated on the surface of LiFePO₄ crystallites. The synthesized LiFePO₄/carbon composite shows a high discharge capacity of 160 mAh g⁻¹ and 155 mAh g⁻¹ at rates of 0.5 C and 1 C, respectively. Even at a high current density of 30 C, the material still presents a discharge capacity of 93 mAh g⁻¹ and exhibits an excellent cycling performance.     

High power performance of nano-LiFePO4/C cathode material synthesized via lauric acid-assisted solid-state reaction

A nano-LiFePO₄/C composite has been directly synthesized from micrometer-sized Li₂CO₃, NH₄H₂PO₄, and FeC₂O₄·2H₂O by the lauric acid-assisted solid-state reaction method. The SEM and TEM observations demonstrate that the synthesized nano-LiFePO₄/C composite has well-dispersed particles with a size of about 100–200 nm and an in situ carbon layer with thickness of about 2 nm. The prepared nano-LiFePO₄/C composite has superior rate capability, delivering a discharge capacity of 141.2 mAh g⁻¹ at 5 °C, 130.9 mAh g⁻¹ at 10 C, 121.7 mAh g⁻¹ at 20 °C, and 112.4 mAh g⁻¹ at 30 °C. At −20 °C, this cathode material still exhibits good rate capability with a discharge capacity of 91.9 mAh g⁻¹ at 1 °C. The nano-LiFePO₄/C composite also shows excellent cycling ability with good capacity retention, up to 100 cycles at a high current density of 30 °C. Furthermore, the effect of lauric acid in the preparation of nano-LiFePO₄/C composite was investigated by comparing it with that of citric acid. The SEM images reveal that the morphology of the LiFePO₄/C composite transformed from the porous structure to fine particles as the molar ratio of lauric acid/citric acid increased.     

Synthesis of LiFePO4/C by solid-liquid reaction milling method

LiFePO₄/C was synthesized by the method of solid-liquid reaction milling, using FeCl₃·6H₂O, Li₂CO₃ and (NH₄)₂HPO₄ and glucose, which was used as reductant (carbon source). The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), TG-DTA analysis, infrared absorption carbon-sulfur analysis and electrochemical performance test. The sample synthesized at 680 °C for 8 h showed, at initial discharge, a capacity of 155.8, 153.2, 148.5, 132.7 mAh g^− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate respectively. The sample also showed an excellent capacity retention as there was no significant capacity fade after 10 cycles.     

Preparation and electrochemical properties of a LiFePO4/C composite cathode material by a polymer-pyrolysis–reduction method

A LiFePO₄/C composite was successfully prepared by a polymer-pyrolysis–reduction method, using FePO₄·2H₂O and lithium polyacrylate (PAALi) as raw materials. The structure of the LiFePO₄/C composites was investigated by X-ray diffraction (XRD). The micromorphology of the precursor and LiFePO₄/C powders was observed using scanning electron microscopy (SEM), and the in situ coating of carbon on the particles was observed by transmission electron microscopy (TEM). Furthermore, the electrochemical properties were evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge cycling tests. The results showed that the sample synthesized at 700 °C had the best electrochemical performance, exhibiting initial discharge capacities of 157, 139 and 109 mAh g⁻¹ at rates of 0.1, 1 and 5 C, respectively. Moreover, the sample presented excellent capacity retention as there was no significant capacity fade after 50 cycles.     

Preparation and characterization of nano-particle LiFePO4 and LiFePO4/C by spray-drying and post-annealing method

Pure, nano-sized LiFePO₄ and LiFePO₄/C cathode materials are synthesized by spray-drying and post-annealing method. The influence of the sintering temperature and carbon coating on the structure, particle size, morphology and electrochemical performance of LiFePO₄ cathode material is investigated. The optimum processing conditions are found to be thermal treatment for 10 h at 600 °C. Compared with LiFePO₄, LiFePO₄/C particles are smaller in size due to the inhibition of crystal growth to a great extent by the presence of carbon in the reaction mixture. And that the LiFePO₄/C composite coated with 3.81 wt.% carbon exhibits the best electrode properties with discharge capacities of 139.4, 137.2, 133.5 and 127.3 mAh g⁻¹ at C/5, 1C, 5C and 10C rates, respectively. In addition, it shows excellent cycle stability at different current densities. Even after 50 cycles at the high current density of 10C, a discharge capacity of 117.7 mAh g⁻¹ is obtained (92.4% of its initial value) with only a low capacity fading of 0.15% per cycle.     

Emulsion drying synthesis of olivine LiFePO4/C composite and its electrochemical properties as lithium intercalation material

The electroactive LiFePO₄/C nano-composite has been synthesized by an emulsion drying method. During burning-out the oily emulsion precipitates in an air-limited atmosphere at 300 °C, amorphous or low crystalline carbon was generated along with releasing carbon oxide gases, and trivalent iron as a cheap starting material was immediately reduced to the divalent one at this stage as confirmed by X-ray photoelectron spectroscopy, leading to a low crystalline LiFePO₄/C composite. Heat-treatment of the low crystalline LiFePO₄/C in an Ar atmosphere resulted in a well-ordered olivine structure, as refined by Rietveld refinement of the X-ray diffraction pattern. From secondary electron microscopic and scanning transmission electron microscopic observations with the corresponding elemental mapping images of iron and phosphorous, it was found that the LiFePO₄ powders are modified by fine carbon. The in situ formation of the nano-sized carbon during crystallization of LiFePO₄ brought about two advantages: (i) an optimized particle size of LiFePO₄, and (ii) a uniform distribution of fine carbon in the product. These effects of the fine carbon on LiFePO₄/C composite led to high capacity retention upon cycling at 25 and 50 °C and high rate capability, resulting from a great enhancement of electric conductivity as high as 10⁻⁴ S cm⁻¹. That is, the obtained capacity was higher than 90 mAh (g-phosphate)⁻¹ by applying a higher current density of about 1000 mA g⁻¹ (11 C) at 50 °C.     

LiFePO4 cathode power with high energy density synthesized by water quenching treatment

A water quenching (WQ) method was developed to synthesize LiFePO₄ and C-LiFePO₄. Our results indicate that this synthesis method ensures improved electrochemical activity and small crystal grain size. The synthetic conditions were optimized using orthogonal experiments. The LiFePO₄ sample prepared at the optimized condition showed a maximum discharge capacity of 149.8 mAh g⁻¹ at a C/10 rate. C-LiFePO₄ with a low carbon content of 0.93% and a high discharge specific capacity of 163.8 mAh g⁻¹ has also been obtained using this method. Water quenching treatment shows outstanding improvement of the electrochemical performance of LiFePO₄.