Electrochemical performance of LiFePO4 thin films with different morphology and crystallinity

LiFePO₄ thin films have been prepared by pulsed laser deposition method on titanium substrates. The influence of the deposition parameters, e.g. substrate temperature, ambient argon pressure, and post-annealing on the crystallinity and morphology of as-deposited thin films are investigated. Well-crystallized pure olivine-phase is obtained under optimized deposition condition (20–30 Pa, 500 °C). It shows a high electrochemical activity (83% theoretical capacity) at low current density (0.33 μA cm⁻², 1/20 C) and elevated testing temperature (45 °C). Moderate post-annealing treatment can enhance the utilization of the films further. The deposition of the film at a too high temperature or post-annealing for too long time could introduce Fe³⁺ impurities, i.e., Li₃Fe₂(PO₄)₃ and Fe₄(P₂O₇)₃, which can be easily detected by extending the electrochemical test voltage down to 2.5 V.     

Optimization of electrochemical properties of LiFePO4/C prepared by an aqueous solution method using sucrose

LiFePO₄ can be used as a positive electrode material for lithium-ion batteries by making composite with electrical conductive carbonaceous materials. In this study, LiFePO₄/C (carbon) composite was prepared by a soft chemistry route, in which sucrose was used as a carbon source of a low price. We tried to optimize a Li/(LiFePO₄/C) cell performance through changing synthetic conditions and discussed the factors affecting the electrochemical performances of the cell, such as the amount of the carbon source, synthetic temperature, gas flow rate of pyrolysis and the formation of secondary phases. It was found that the connection of the residual carbon and Fe₂P to LiFePO₄ particles and the amount of these two phases were important factors. In our experimental conditions, LiFePO₄/C including 9.72 wt.% of residual carbon, prepared at 800 °C for 12 h showed the highest reversible capacity and the best C rate performance among the synthesized materials; 130 mAh g⁻¹ at 10C rate and 50 °C.     

Effect of milling method and time on the properties and electrochemical performance of LiFePO4/C composites prepared by ball milling and thermal treatment

Effects of ball milling way and time on the phase formation, particulate morphology, carbon content, and consequent electrode performance of LiFePO₄/C composite, prepared by high-energy ball milling of Li₂CO₃, NH₄H₂PO₄, FeC₂O₄ raw materials with citric acid as organic carbon source followed by thermal treatment, were investigated. Three ball milling ways and five different milling durations varied from 0 to 8 h were compared. LiFePO₄/C composites could be obtained from all synthesis processes. TEM examinations demonstrated LiFePO₄/C from ball milling in acetone resulted in sphere shape grains with a size of ∼60 nm, similar size was observed for LiFePO₄/C from dry ball milling but in a more irregular shape. The ball milling in benzene resulted in a much larger size of ∼250 nm. The LiFePO₄/C composites prepared from dry ball milling and ball milling in acetone showed much better electrochemical performance than that from ball milling in benzene. SEM examinations and BET measurements demonstrated that the high-energy ball milling effectively reduced the grain size. A ball milling for 4 h resulted in the best electrochemical performance, likely due to the proper amount of carbon and proper carbon structure were created.     

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.     

Influence of carbon sources on LiFePO4/C composites synthesized by the high-temperature high-energy ball milling method

Carbon-coated LiFePO₄ composites were synthesized by a new method of high-temperature high-energy ball milling (HTHEBM). Fe₂O₃ and LiH₂PO₄ were used as raw materials. Glucose, sucrose, citric acid and active carbon were used as reducing agents and carbon sources, respectively. In this method, high-energy ball milling and carbon coating worked together and, therefore, fine and homogeneous LiFePO₄/C particles with excellent properties were obtained in a relatively short synthesis time of 9 h. Moreover, the synthesis process could be completely finished at a relatively lower temperature of 600 °C for high-energy ball milling transforming mechanical energy into thermal energy. The results of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical performance tests indicated that carbon source had an important influence on the properties of LiFePO₄/C composites synthesized by the HTHEBM method. It was proved that the LiFePO₄ composites coated with glucose had the best properties with 1 μm geometric mean diameter and 150.3 mA h g⁻¹ initial discharge capacity at a current rate of 0.1 C. After the 20th cycle test, the reversible capacity was 148 mA h g⁻¹ at 0.1 C, showing a retention ratio to the initial capacity of 98.5%.     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

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.     

Synthesis of spinel Li4Ti5O12 anode material by a modified rheological phase reaction

Using lithium acetate dihydrate and tetra-n-butyl titanate as the raw materials, spinel Li₄Ti₅O₁₂ was successfully synthesized by a modified rheological phase method. Thermogravimetric analysis and differential scanning calorimetry (TG–DSC) of the thermal decomposition process of the precursor and X-ray diffraction (XRD) data indicate the crystallization of lithium titanates has occurred at 580 °C, and main phase Li₄Ti₅O₁₂ has obtained at 600 °C. Laser granulometer and scanning electron microscope (SEM) have been employed to estimate the particle size distribution, morphology and microstructure of the products. It reveals the prepared Li₄Ti₅O₁₂ powder had a small particle size (about 140 nm) and narrow size distribution (d_0.1 = 0.07, d_0.5 = 0.139, d_0.9 = 2.813 μm). Galvanostatic charge and discharge tests were carried out to characterize the electrochemical performances of Li₄Ti₅O₁₂. The result indicates that the Li₄Ti₅O₁₂ electrode material obtained from the precursor that had been experienced heat treatment at 110 °C exhibited discharge capacities of 161.6, 156.5 and 112.3 mAh g⁻¹ after 50 cycles at current rates 1, 2.5 and 10 C, respectively, demonstrating excellent high rate performance, due to the pure and well crystallized Li₄Ti₅O₁₂ with ultrafine particles and narrow size distribution.     

Structure optimization and the structural factors for the discharge rate performance of LiFePO4/C cathode materials

Carbon coating and iron phosphides of high electron conductivity were introduced into the LiFePO₄ materials which were derived via a sol-gel method in order to improve the high discharge rate performance. The start constituents were FeC₂O₄·2H₂O, LiOH·H₂O, NH₄H₂PO₄ and ethylene glycol. Effects of the calcination temperature and the ethylene glycol on the structure and the electrochemical performance of the LiFePO₄ materials were investigated. Structure analyses showed that the addition of ethylene glycol caused an obvious decrease in the particle size of LiFePO₄. Calcination temperature and ethylene glycol jointly affected the formation of iron phosphides. Combining the electrochemical testing, it was found that, at low discharge rate, small particle size and high content of LiFePO₄ were much important for the capacity rather than the iron phosphides, and relative high content of Fe₂P (e.g. 8 wt.%) even worsened the capacity. However, with the increase of the discharge rate, the high electron conductive iron phosphides turned to play important role on the capacity. Fe₂P effectively increased both the reaction and diffusion kinetics and hence enhanced the utilization efficiency of the LiFePO₄ at high discharge rate. Combining relative small particle size, even 2 wt.% of iron phosphides could improve the high rate performance of LiFePO₄/C significantly.     

Enhanced electrochemical performance of nano-sized LiFePO4/C synthesized by an ultrasonic-assisted co-precipitation method

A simple and effective method, the ultrasonic-assisted co-precipitation method, was employed to synthesize nano-sized LiFePO₄/C. A glucose solution was used as the carbon source to produce in situ carbon to improve the conductivity of LiFePO₄. Ultrasonic irradiation was adopted to control the size and homogenize the LiFePO₄/C particles. The sample was characterized by X-ray powder diffraction, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). FE-SEM and TEM show that the as-prepared sample has a reduced particle size with a uniform size distribution, which is around 50 nm. A uniform amorphous carbon layer with a thickness of about 4-6 nm on the particle surface was observed, as shown in the HRTEM image. The electrochemical performance was demonstrated by the charge-discharge test and electrochemical impedance spectra measurements. The results indicate that the nano-sized LiFePO₄/C presents enhanced discharge capacities (159, 147 and 135 mAh g⁻¹ at 0.1, 0.5 and 2 C-rate, respectively) and stable cycling performance. This study offers a simple method to design and synthesis nano-sized cathode materials for lithium-ion batteries.     

Carbon coated Li3V2(PO4)3 cathode material prepared by a PVA assisted sol-gel method

Carbon coated Li₃V₂(PO₄)₃ cathode material was prepared by a poly(vinyl alcohol) (PVA) assisted sol-gel method. PVA was used both as the gelating agent and the carbon source. XRD analysis showed that the material was well crystallized. The particle size of the material was ranged between 200 and 500 nm. HRTEM revealed that the material was covered by a uniform surface carbon layer with a thickness of 80 Å. The existence of surface carbon layer was further confirmed by Raman scattering. The electrochemical properties of the material were investigated by charge-discharge cycling, CV and EIS techniques. The material showed good cycling performance, which had a reversible discharge capacity of 100 mAh g⁻¹ when cycled at 1 C rate. The apparent Li⁺ diffusion coefficients of the material ranged between 9.5 × 10⁻¹⁰ and 0.9 × 10⁻¹⁰ cm² s⁻¹, which were larger than those of olivine LiFePO₄. The large lithium diffusion coefficient of Li₃V₂(PO₄)₃ has been attributed to its special NASICON-type structure.     

Electrochemical properties of carbon-mixed LiFePO4 cathode material synthesized by the ceramic granulation method

LiFePO₄/carbon composite cathode material was prepared using polyvinyl alcohol (PVA) as carbon source by pelleting and subsequent pyrolysis in N₂. The samples were characterized by XRD, SEM and TGA. Their electrochemical performance was investigated in terms of charge–discharge cycling behavior. It consists of a single LiFePO₄ phase and amorphous carbon. The special micro-morphology via the process is favorable for electrochemical properties. The discharge capacity of the LiFePO₄/C composite was 145 mAh/g, closer to the theoretical specific capacity of 170 mAh/g at 0.1 C low current density. At 3 C modest current density, the specific capacity was about 80 mAh/g, which can satisfy for transportation applications if having a more planar discharge flat.     

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.     

Improving the rate performance of LiFePO4 by Fe-site doping

LiFePO₄ doped by bivalent cation in Fe-sites show improved rate performance and cyclic stability. Under 10 C rate at room temperature, the capacities of LiFe_0.9M_0.1PO₄ (M = Ni, Co, Mg) maintain at 81.7, 90.4 and 88.7 mAh/g, respectively, in comparison with 53.7 mAh/g for undoped LiFePO₄ and 54.8 mAh/g for carbon-coated LiFePO₄ (LiFePO₄/C). The capacity retention is 95% after 100 cycles for doped samples while this value is only 70% for LiFePO₄ and LiFePO₄/C. Such a significant improvement in electrochemical performance should be partially related to the enhanced electronic conductivities (from 2.2 × 10⁻⁹ to <2.5 × 10⁻⁷ S cm⁻¹) and probably the mobility of Li⁺ ions in the doped samples.     

Synthesis and electrochemical properties of monoclinic Li3V2(PO4)3/C composite cathode material prepared from a sucrose-containing precursor

Monoclinic structure Li₃V₂(PO₄)₃/C composite powders are synthesized via a novel homogeneous mixing route followed by a one-step heat treatment. The composites were characterized by X-ray diffraction (XRD) and galvanostatic charge/discharge, CV measurements. The influence of the heat treatment on the electrochemical properties of Li₃V₂(PO₄)₃/C composites was investigated. To examine the effect of residual carbon content on the properties of the composites, six samples with 1.2, 2.3, 3.4, 4.4, 5.8, and 7.0 wt% carbon were prepared. The sample with 4.4 wt% carbon exhibited good cycling performance and rate capability in the range of 3.0–4.8 V.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

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.     

Enhanced electrochemical performance of carbon nanospheres-LiFePO4 composite by PEG based sol-gel synthesis

The carbon nanospheres-LiFePO₄ (CNSs-LiFePO₄) composite has been synthesized by PEG (polyethylene glycol, mean molecular weight of 30,000) based sol-gel route. Highly conductive CNSs (30-40 nm) were adopted to improve the electronic conductivity of LiFePO₄. PEG was used to promote the dispersion of CNSs with the surface functionalization of CNSs, which could facilitate the coating of CNSs on the surface of the LiFePO₄ particles. The sample was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman scattering. Electrochemical performance of the CNSs-LiFePO₄ composite was characterized by the charge-discharge test and electrochemical impedance spectra measurement. The results indicated that LiFePO₄ particles were well coated with the conductive CNSs to overcome the intrinsic low electronic conductivity problem of LiFePO₄. The CNSs-LiFePO₄ composite delivered an enhanced rate capability (146, 128 and 113 mAh g⁻¹ at 0.1 C, 1 C and 5 C rate). The PEG based sol-gel route enables LiFePO₄ networked with CNSs, which offered a higher electrochemical performance.     

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.