Synthesis and characterization of Carbon Nano Fiber/LiFePO4 composites for Li-ion batteries

Carbon Nano Fibers (CNFs) coated with LiFePO₄ particles have been prepared by a non-aqueous sol–gel technique. The functionalization of the CNFs by HNO₃ acid treatment has been confirmed by Raman and XPS analyses. The samples pure LiFePO₄ and LiFePO₄–CNF have been characterized by XRD, SEM, RAMAN, XPS and electrochemical analysis. The LiFePO₄–CNF sample shows better electrochemical performance compared to as-prepared LiFePO₄. LiFePO₄–CNF (10 wt.%) delivers a higher specific capacity (∼140 mAh g⁻¹) than LiFePO₄ with carbon black (25 wt.%) added after synthesis (∼120 mAh g⁻¹) at 0.1C.     

Optimized solid-state synthesis of LiFePO4 cathode materials using ball-milling

Olivine-type LiFePO₄ cathode materials were synthesized by a solid-state reaction method and ball-milling. The ball-milling time, heating time and heating temperature are optimized. A heating temperature higher than 700 °C resulted in the appearance of impurity phase Fe₂P and growth of large particle, which was shown by high resolution X-ray diffraction and field emission scanning electron microscopy. The impurity phase Fe₂P exhibited a considerable capacity loss at the 1st cycle and a gradual increase in discharge capacity upon cycling. Moreover, it exhibited an excellent high-rate capacity of 104 mAh g⁻¹ at 3 C in spite of the large particle size. The optimum synthesis conditions for LiFePO₄ were ball-milling for 24 h and heat-treatment at 600 °C for 3 h. LiFePO₄/Li cells showed an enhanced cycling performance and a high discharge capacity of 160 mAh g⁻¹ at 0.1 C.     

Synthesis of carbon-coated LiFePO4 nanoparticles with high rate performance in lithium secondary batteries

A novel preparation technique was developed for synthesizing carbon-coated LiFePO₄ nanoparticles through a combination of spray pyrolysis (SP) with wet ball milling (WBM) followed by heat treatment. Using this technique, the preparation of carbon-coated LiFePO₄ nanoparticles was investigated for a wide range of process parameters such as ball-milling time and ball-to-powder ratio. The effect of process parameters on the physical and electrochemical properties of the LiFePO₄/C composite was then discussed through the results of X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), the Brunauer-Emmet-Teller (BET) method and the use of an electrochemical cell of Li|1 M LiClO₄ in EC:DEC = 1:1|LiFePO₄. The carbon-coated LiFePO₄ nanoparticles were prepared at 500 °C by SP and then milled at a rotating speed of 800 rpm, a ball-to-powder ratio of 40/0.5 and a ball-milling time of 3 h in an Ar atmosphere followed by heat treatment at 600 °C for 4 h in a N₂ + 3% H₂ atmosphere. SEM observation revealed that the particle size of LiFePO₄ was significantly affected by the process parameters. Furthermore, TEM observation revealed that the LiFePO₄ nanoparticles with a geometric mean diameter of 146 nm were coated with a thin carbon layer of several nanometers by the present method. Electrochemical measurement demonstrated that cells containing carbon-coated LiFePO₄ nanoparticles could deliver markedly improved battery performance in terms of discharge capacity, cycling stability and rate capability. The cells exhibited first discharge capacities of 165 mAh g⁻¹ at 0.1 C, 130 mAh g⁻¹ at 5 C, 105 mAh g⁻¹ at 20 C and 75 mAh g⁻¹ at 60 C with no capacity fading after 100 cycles.     

High reversible capacities of graphite and SiO/graphite with solvent-free solid polymer electrolyte for lithium-ion batteries

The combination of graphite or silicon monoxide (SiO)/graphite = 1/1 mixture with a solvent-free solid polymer electrolyte (SPE) was fabricated using a new preparation process, involving precoating the electrode with vapor-grown carbon fiber (VGCF) and binders (polyvinyl difluoride: PVdF or polyimide: PI), followed by the overcoating of the SPE. The reversible capacity of [graphite | SPE | Li] and [SiO/graphite | SPE | Li] cells were >360 and >1000 mAh g⁻¹ with 78% and 77% for the 1st Coulombic efficiency, respectively. The reversible capacities were 75% at the 250th cycle for [graphite | SPE | Li] and 72% at the 100th cycle for [SiO/graphite | SPE | Li]. The electrode used was compatible with that of the conventional liquid electrolyte system, and the SPE film could be formed on the electrode by the continuous overcoating process, which will lead to a low-cost electrodes and low-cost battery production. The solid-state lithium-ion polymer battery (SSLiPB) developed in this study, which consisted of [LiFePO₄ | SPE | graphite], showed the reversible capacity of 128 mAh g⁻¹ (based on the LiFePO₄ capacity) with favorable cycle performance.     

Electrochemical properties of cross-linked polymer electrolyte by electron beam irradiation and application to lithium ion batteries

A polymer electrolyte was successfully fabricated for a room temperature operation lithium battery by cross-linking the mixture of oligomeric poly (ethylene glycol) dimethylether (PEGDME) and poly (ethylene glycol) diacrylate (PEGDA) with Li(CF₃SO₂)₂N using electron beam irradiation. The maximum ionic conductivity achieved for the cross-linked solid polymer electrolyte (c-SPE) at room temperature was 2.1 × 10⁻⁴ S cm⁻¹ and the lithium transport number of the electrolyte was around 0.2. The c-SPE showed no reaction heat with lithium metal up to 300 °C. The interface resistance of Li/c-SPE/Li at room temperature was about 45 Ω cm², which is considerable lower than that of 210 Ω cm² for Li/PEO₁₀Li(CF₃SO₂)₂N/Li. The electrochemical window of the polymer electrolyte was above 4 V (versus Li⁺/Li). The initial discharge capacity for the Li/SPE/LiFePO₄-C cell was approximately 90 mAh g⁻¹ for LiFePO₄-C at 1/10 °C rate at room temperature and showed a good cyclability and a high coulombic efficiency of 99.2%.     

Cathode performance of olivine-type LiFePO4 synthesized by chemical lithiation

Chemical lithiation with LiI in acetonitrile was performed for amorphous FePO₄ synthesized from an equimolar aqueous suspension of iron powder and an aqueous solution of P₂O₅. An orthorhombic LiFePO₄ olivine structure was obtained by annealing a chemically lithiated sample at 550 °C for 5 h in Ar atmosphere. The average particle size remained at approximately 250 nm even after annealing. The lithium content in the sample was quantitatively confirmed by Li atomic absorption analysis and ⁵⁷Fe Mössbauer spectroscopy. While an amorphous FePO₄/carbon composite cathode has a monotonously decreasing charge–discharge profile with a reversible capacity of more than 140 mAh g⁻¹, the crystallized LiFePO₄/carbon composite shows a 3.4 V plateau corresponding to a two-phase reaction. This means that the lithium in the chemically lithiated sample is electrochemically active. Both amorphous FePO₄ and the chemically lithiated and annealed crystalline LiFePO₄ cathode materials showed good cyclability (more than 140 mAh g⁻¹ at the 40th cycle) and good discharge rate capability (more than 100 mAh g⁻¹ at 5.0 mA cm⁻²). In addition, the fast-charge performance was found to be comparable to that with LiCoO₂.     

Enhancement of electrochemical performance of lithium dry polymer battery with LiFePO4/carbon composite cathode

LiFePO₄/carbon composite electrode was prepared and applied to the dry polymer electrolyte. Enhanced low-temperature performance of LiFePO₄ was achieved by modifying the interface between LiFePO₄ and polymer electrolyte. The molecular weight of the polymer and the salt concentration as the Li/O ratio were optimized at 3 × 10⁵ and 1/10, respectively. Impedance analysis revealed that a small resistive component occurred in the frequency range of the charge transfer process. The reversible capacity of the laminate cell was 140 mAh g⁻¹ (C/20) and 110 mAh g⁻¹ (C/2) at 40 °C, which is comparable to the performance in the liquid electrolyte system.     

Simple and fast synthesis of LiFePO4-C composite for lithium rechargeable batteries by ball-milling and microwave heating

A very simple and rapid method for synthesizing LiFePO₄-C composite has been developed by vibrant type ball-milling for 30 min and microwave heating for 2–4 min. X-ray diffraction and Mössbauer spectroscopy verify that well-crystallized LiFePO₄ without Fe³⁺ impurities is obtained. From laser particle size analysis and transmission electron microscopy, it is confirmed that a LiFePO₄-C composite with fine and uniform particle size (mean particle size ≤0.640 μm, D75 in volume distribution ≤0.592 μm) and with extremely uniform carbon distribution is prepared by vibrant type ball-milling and microwave heating. The LiFePO₄-C delivers a high initial discharge capacity of 161 mAh g⁻¹ at C/10 and shows very stable cycling behaviour.     

The effect of carbon coating thickness on the capacity of LiFePO4/C composite cathodes

Two types of carbon source and precursor mixing pellets were employed simultaneously to prepare the LiFePO₄/C composite materials: Type I using the LiFePO₄ precursor with 20 wt.% polystyrene (PS) as a primary carbon source, and Type II using the LiFePO₄ precursor with 50 wt.% malonic acid as a secondary carbon vapor source. During final sintering, a Type I pellet was placed down-stream and Type II precursor pellet(s) was(were) placed upstream next to a Type I precursor pellet in a quartz-tube furnace. The carbon-coated product of the sintered Type I precursor pellet was obtained by using both PS and malonic acid as carbon sources. When two Type II pellets were used as a carbon vapor source (defined as Product-2), a more uniform film between 4 and 8 nm was formed, as shown in the TEM images. In the absence of a secondary carbon source (defined as Product-0), the discharge capacity of Product-0 was 137 mAh g⁻¹ with 100 cycles at a 0.2C-rate, but Product-2 demonstrated a high capacity of 151 mAh g⁻¹ with 400 cycles. Our results indicate that electrochemical properties of LiFePO₄ are correlated to the amount of carbon and its coating thickness and uniformity.     

Improved high-rate charge/discharge performances of LiFePO4/C via V-doping

V-doped LiFePO₄/C cathode materials were prepared through a carbothermal reduction route. The microstructure was characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrochemical Li⁺ intercalation performances of V-doped LiFePO₄/C were compared with those of undoped one through galvanostatic intermittent titration technique, cyclic voltamperometry, and electrochemical impedance spectrum. V-doped LiFePO₄/C showed a high discharge capacity of ∼70 mAh g⁻¹ at the rate of 20 C (3400 mA g⁻¹) at room temperature. The significantly improved high-rate charge/discharge capacity is attributed to the increase of Li⁺ ion “effective” diffusion capability.     

Synthesis of LiFePO4/C composite with high-rate performance by starch sol assisted rheological phase method

LiFePO₄/C composite was synthesized at 600 °C in an Ar atmosphere by a soluble starch sol assisted rheological phase method using home-made amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/C composite has good crystallinity, ultrafine sphere-like particles of 100-200 nm size and in situ carbon. The synthesized LiFePO₄ could inherit the morphology of FePO₄ precursor. The electrochemical performance of the LiFePO₄ by galvanostatic cycling studies demonstrates excellent high-rate cycle stability. The Li/LiFePO₄ cell displays a high initial discharge capacity of more than 157 mAh g⁻¹ at 0.2C and a little discharge capacity decreases from the first to the 80th cycle (>98.3%). Remarkably, even at a high current density of 30C, the cell still presents good cycle retention.     

Optimized performances of core-shell structured LiFePO4/C nanocomposite

A nanosized LiFePO₄/C composite with a complete and thin carbon-shell is synthesized via a ball-milling route followed by solid-state reaction using poly(vinvl alcohol) as carbon source. The LiFePO₄/C nanocomposite delivers discharge capacities of 159, 141, 124 and 112 mAh g⁻¹ at 1 C, 5 C, 15 C and 20 C, respectively. Even at a charge-discharge rate of 30 C, there is still a high discharge capacity of 107 mAh g⁻¹ and almost no capacity fading after 1000 cycles. Based on the analysis of cyclic voltammograms, the apparent diffusion coefficients of Li ions in the composite are in the region of 2.42 × 10⁻¹¹ cm² s⁻¹ and 2.80 × 10⁻¹¹ cm² s⁻¹. Electrochemical impedance spectroscopy and galvanostatic intermittent titration technique are also used to calculate the diffusion coefficients of Li ions in the LiFePO₄/C electrode, they are in the range of 10⁻¹¹-10⁻¹⁴ cm² s⁻¹. In addition, at −20 °C, it can still deliver a discharge capacity of 122 mAh g⁻¹, 90 mAh g⁻¹ and 80 mAh g⁻¹ at the charge-discharge rates of 0.1 C, 0.5 C and 1 C, respectively.     

Structural and electrochemical properties of Cl-doped LiFePO4/C

Cl-doped LiFePO₄/C cathode materials were synthesized through a carbothermal reduction route, and the microstructure and electrochemical performances were systematically studied. Cl-doped LiFePO₄/C cathode materials presented a high discharge capacity of ∼90 mAh g⁻¹ at the rate of 20 C (3400 mA g⁻¹) at room temperature. Electrochemical impedance spectroscopy and cyclic voltamperometry indicated the optimized electrochemical reaction and Li⁺ diffusion in the bulk of LiFePO₄ due to Cl-doping. The improved Li⁺ diffusion capability is attributed to the microstructure modification of LiFePO₄ via Cl-doping.     

Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries

Porous nanostructured LiFePO₄ powder with a narrow particle size distribution (100–300 nm) for high rate lithium-ion battery cathode application was obtained using an ethanol based sol–gel route employing lauric acid as a surfactant. The synthesized LiFePO₄ powders comprised of agglomerates of crystallites <65 nm in diameter exhibiting a specific surface area ranging from 8 m² g⁻¹ to 36 m² g⁻¹ depending on the absence or presence of the surfactant. The LiFePO₄ obtained using lauric acid resulted in a specific capacity of 123 mAh g⁻¹ and 157 mAh g⁻¹ at discharge rates of 10C and 1C with less than 0.08% fade per cycle, respectively. Structural and microstructural characterization were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray (EDX) analysis while electronic conductivity and specific surface area were determined using four-point probe and N₂ adsorption techniques.     

Li/LiFePO4 batteries with gel polymer electrolytes incorporating a guanidinium-based ionic liquid cycled at room temperature and 50 °C

Gel polymer electrolytes composed of PVdF-HFP microporous membrane incorporating a guanidinium-based ionic liquid with 0.8 mol kg⁻¹ lithium bis(trifluoromethanesulfonylimide) are characterized as the electrolytes in Li/LiFePO₄ batteries. The ionic conductivity of these gel polymer electrolytes is 3.16 × 10⁻⁴ and 8.32 × 10⁻⁴ S cm⁻¹ at 25 and 50 °C, respectively. The electrolytes show good interfacial stability towards lithium metal and high oxidation stability, and the decomposition potential reaches 5.3 and 4.6 V (vs. Li/Li⁺) at 25 and 50 °C, respectively. Li/LiFePO₄ cells using the PVdF-HFP/1g13TFSI-LiTFSI electrolytes show good discharge capacity and cycle stability, and no significant loss in discharge capacity of the battery is observed over 100 cycles. The cells deliver the capacity of 142 and 150 mAh g⁻¹ at the 100th cycling at 25 and 50 °C, respectively.     

Cathode performance of LiMnPO4/C nanocomposites prepared by a combination of spray pyrolysis and wet ball-milling followed by heat treatment

LiMnPO₄/C nanocomposites could be prepared by a combination of spray pyrolysis and wet ball-milling followed by heat treatment in the range of spray pyrolysis temperature from 200 to 500 °C. The ordered LiMnPO₄ olivine structure without any impurity phase could be identified by X-ray diffraction analysis for all samples. It could be also confirmed from scanning electron microscopy and transmission electron microscopy observations that the final samples were the LiMnPO₄/C nanocomposites with approximately 100 nm in primary particles size. The LiMnPO₄/C nanocomposite samples were used as cathode active materials for lithium batteries, and the electrochemical tests were carried out for the cell Li|1 M LiPF₆ in EC:DMC = 1:1|LiMnPO₄/C at various charge/discharge rates in three charge modes. As a result, the final sample which was synthesized at 300 °C by spray pyrolysis showed the best electrochemical performance due to the largest specific surface area, the smallest primary particle size and a well distribution of carbon. At galvanostatic charge/discharge rates of 0.05 C, the cell delivered first discharge capacities of 123 and 165 mAh g⁻¹ in correspondence to charge cutoff voltages of 4.4 and 5.0 V, respectively. Furthermore, in a constant current-constant voltage charge mode at 4.4 V, the cells also exhibited initial discharge capacities of 147 mAh g⁻¹ at 0.05 C, 145 mAh g⁻¹ at 0.1 C, 123 mAh g⁻¹ at 1 C and 65 mAh g⁻¹ at 10 C. Moreover, the cells showed fair good cycleability over 100 cycles.     

Effects of carbon coating and iron phosphides on the electrochemical properties of LiFePO4/C

Carbon coated LiFePO₄ (LiFePO₄/C) with different contents of high electron conductive iron phosphide phase was synthesized by an aqueous sol–gel method in a reductive sintering atmosphere. Different synthesis parameters were used for adjusting the microstructure and phase compositions of the products. The effects of the carbon coating and iron phosphides on the electrochemical properties of the LiFePO₄/C electrodes were studied by means of testing the discharge capacities at rates of 0.1–5C (1C = 170 mAh g⁻¹) and analyzing the CV curves. The results show that carbon coating in a content of 1.5 wt.% derived from the carbon source of ethylene glycol greatly decreases the particle size of LiFePO₄ in one order in the specific surface area, and significantly improves the rate capability of LiFePO₄. The effect of the content of FeP on the capacity of the carbon coated LiFePO₄ was different at different discharge rates. Increasing the content of FeP from 1.2 to 3.7 wt.% slightly decreases the capacity of LiFePO₄/C at low discharge rate (0.1C and 1C), but obviously increases the capacity of LiFePO₄/C when the discharge rate is increased to 5C. For the carbon free sample, even it also has 1.8 wt.% FeP, it still possesses poor capacity due to the large particle size of LiFePO₄ and the lack of conductivity. And too much iron phosphides lowers the discharge capacity of the electrode since they are inert for the deinsertion/insertion of lithium ion.     

Study on γ-butyrolactone for LiBOB-based electrolytes

To solve the problems of LiBOB-based electrolytes, small salt solubility and low conductivity, a sort of cyclic carboxylate, γ-butyrolactone (GBL) was applied in the lithium-ion battery electrolyte as the main solvent of lithium bis(oxalate)borate (LiBOB). LiBOB–GBL electrolyte exhibits good electrochemical stability, which is suitable to be the candidate of the lithium-ion battery electrolyte. Using GBL as the solvent of the LiBOB salt can increase the solubility and conductivity dramatically. At room temperature, LiFePO₄/LiBOB–GBL/Li half cell shows satisfying cycle performance with no capacity fading in the first 50 cycles and promising capacity performance with stable discharge capacity of about 125 mAh g⁻¹. EA is mixed with GBL to get lower viscosity solvent. In LiFePO₄/Li half cell with 0.5 C discharge rate, 0.2 M LiBOB–GBL/EA (1:1, wt) electrolyte exhibits best at room temperature and 0.7 M LiBOB–GBL/EA (1:1, wt) electrolyte exhibits best at elevated temperature.     

Electrochemical properties of lithium sulfur cells using PEO polymer electrolytes prepared under three different mixing conditions

Lithium sulfur cells were prepared by composing with sulfur cathode (PEO)₆LiBF₄ polymer electrolyte and lithium anode. (PEO)₆LiBF₄ polymer electrolyte was prepared under three different mixing conditions: stirred polymer electrolyte (SPE), ball-milled polymer electrolyte (BPE) and ball-milled polymer electrolyte with 10 wt%Al₂O₃ (BCPE). The effects of ball milling and additive were investigated by discharge test according to depth of discharge. The initial discharge capacity of lithium sulfur cell using BCPE was 1670 mAh g⁻¹-sulfur, which was better than those of SPE and BPE, and approximately equal to the theoretical capacity. The cycle performance of Li/(PEO)₆LiBF₄/S cell was remarkably improved by the addition of Al₂O_3.     

Electrochemical properties of LiFePO4 prepared via hydrothermal route

LiFePO₄ as a cathode material for rechargeable lithium batteries was prepared by hydrothermal process at 170 °C under inert atmosphere. The starting materials were LiOH, FeSO₄, and (NH₄)₂HPO₄. The particle size of the obtained LiFePO₄ was 0.5 μm. The electrochemical properties of LiFePO₄ were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate (1:1 in volume) containing 1.0 mol dm⁻³ LiClO₄. The hydrothermally synthesized LiFePO₄ exhibited a discharge capacity of 130 mA h g⁻¹, which was smaller than theoretical capacity (170 mA h g⁻¹). The annealing of LiFePO₄ at 400 °C in argon atmosphere was effective in increasing the discharge capacity. The discharge capacity of the annealed LiFePO₄ was 150 mA h g⁻¹.