Long-term cyclability of nanostructured LiFePO4

Amorphous LiFePO₄ was obtained by lithiation of FePO₄ synthesized by spontaneous precipitation from equimolar aqueous solutions of Fe(NH₄)₂(SO₄)₂·6H₂O and NH₄H₂PO₄, using hydrogen peroxide as oxidizing agent. Nano-crystalline LiFePO₄ was obtained by heating amorphous nano-sized LiFePO₄ for different periods of time. The materials were characterized by TG, DTA, X-ray powder diffraction, scanning electron microscopy (SEM) and BET. All materials showed very good electrochemical performance in terms of energy and power density. Upon cycling, a capacity fading affected the materials, thus reducing the electrochemical performance. Nevertheless, the fading decreased upon cycling and after the 200th cycle the cell was able to cycle for more than 500 cycles without further fading.     

Investigation on diffusion behavior of Li in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient () of lithium-ion in LiFePO₄ cathode material. The values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10⁻¹⁶ to 8.9 × 10⁻¹⁴ cm² s⁻¹ and from 1.2 × 10⁻¹⁶ to 8.5 × 10⁻¹⁴ cm² s⁻¹ in the voltage range from 3.2 to 4 V (vs. Li⁺/Li). The transfer coefficients of cathode (0.32-0.42) and anodic (0.26-0.3), and the standard rate constant (1.58 × 10⁻⁹ to 1.30 × 10⁻⁸ cm s⁻¹) were measured from the Tafel plots of LiFePO₄ in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of . The revised values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10⁻¹⁵ to 2.21 × 10⁻¹³ cm² s⁻¹ and from 5.81 × 10⁻¹⁶ to 3.22 × 10⁻¹³ cm² s⁻¹ in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

A kinetic study of electrochemical lithium insertion into oriented V2O5 thin films prepared by rf sputtering

The kinetics of the electrochemical lithium insertion reaction in crystalline V₂O₅ thin films in liquid electrolyte has been investigated using ac impedance spectroscopy. The experimental data are obtained for sputtered films characterized by a common morphology corresponding to an arrangement of V₂O₅ platelets perpendicular to the substrate (h 0 0 or 1 1 0 preferred orientation). The results are discussed as a function of the Li content for 0 < x < 1 in Li_xV₂O₅, the film thickness in the range of 0.6-3.6 μm and temperature 15-55 °C. The moderate evolution of the chemical diffusion coefficient D vs. the lithium content is related with the specific structural response of these pure thin film materials which exhibit a single phase behavior. A comparison of the kinetic parameters for different thickness values allows to indicate the same Li diffusion rate whatever the film thickness and the diffusion pathway does not correspond to the thickness but to the length of the edge (≈1 μm) of V₂O₅ platelets. For the first time, an experimental evaluation of the activation energy for Li diffusion in crystalline V₂O₅ is obtained. A value of 0.98 eV is found for a diffusion phenomenon along the b direction. This work demonstrates the excellent capacity-rate performance as well as the efficient and homogeneous behavior of these oriented films can be explained by their specific microstructure.     

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.     

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.     

An effective and simple way to synthesize LiFePO4/C composite

The cathode material is synthesized from FeC₂O₄·2H₂O and LiH₂PO₄ by a solid-state reaction using citric acid as a carbon source. The electric conductivity of the synthesized LiFePO₄ has been raised by eight orders of magnitude from 10⁻⁹ S cm⁻¹. The LiFePO₄/C composite shows a greatly enhanced rate performance and the cyclic stability at room temperature. It delivers an initial discharge capacity of 128 mAh g⁻¹ at 4C, which is retained as high as 92% after 1000 cycles. In addition, the tested low temperature character is attractive. At −20 °C, the composite exhibits a discharge capacity of 110 mAh g⁻¹ at 0.1C. The homogenous morphology, the porous surface, the small particles inside and the conductive carbon observed contribute much to obtain the favorable electrochemical performance.     

Electrochemical performance of nano-SiO2 modified LiCoO2 thin films fabricated by electrostatic spray deposition (ESD)

With a mixture of a SiO₂ sol and a solution of lithium and cobalt acetates as the precursor, nano-SiO₂ modified LiCoO₂ films were fabricated by the electrostatic spray deposition (ESD) technique. The SiO₂ content of these films was 0, 5, 10, 15 and 20 wt%, respectively. Their structure and electrochemical properties were characterized by means of X-ray diffraction, scanning electron microscopy, galvanostatic cell cycling, AC impedance spectroscopy and cyclic voltammetry. Li₂CoSiO₄ was found formed in the SiO₂-containing films. The film with 15 wt% SiO₂ shows the best cycling stability with the capacity of 130 mAh/g in the voltage range between 2.7 and 4.3 V at the current density of 0.1 mA/cm². Due to its resulted small cell impedance, it has excellent rate capability. A LiCoO₂ (shell)/SiO₂ (core) structure model is proposed to explain the improved properties of these films.     

Li-ion diffusion kinetics in LiFePO4 thin film prepared by radio frequency magnetron sputtering

LiFePO₄ thin films were prepared by radio frequency (RF) magnetron sputtering and were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscope (AFM). Li-ion chemical diffusion coefficients, , were measured by potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The effects of Ag content, film thickness, and film orientation on the electrochemical performance and Li-ion chemical diffusion coefficients of the LiFePO₄ thin films were investigated. values were measured using the liquid electrolyte and the solid electrolyte, and the obtained values were discussed. The values by PITT and EIS were in the range of 10⁻¹⁴ to 10⁻¹² and 10⁻¹⁵ to 10⁻¹² cm² s⁻¹, respectively and that by CV was in the order of 10⁻¹⁴ cm² s⁻¹.     

Factor affecting rate performance of undoped LiFePO4

Undoped lithium iron phosphate (LiFePO₄) was prepared and characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The material has a single crystal globular structure with grain-sizes ca. 100-150 nm. It was used to prepare composite electrodes containing different amounts of carbon (10, 15 and 20 wt.%, respectively) used as cathodes in non-aqueous lithium cells. By increasing the carbon content, an increase in the overall electrochemical performance was observed. Impedance spectroscopy was used to investigate the ohmic and kinetic contributions to the cell overvoltage. It was found that increasing the carbon content leads to a reduction of the cell impedance as a consequence of the reduction of the charge transfer resistance. The poor performance exhibited at very high discharge rates is a direct consequence of the high value of the charge transfer resistance. A further decrease of the charge transfer resistance in high carbon content cathodes (20 wt.% carbon) was obtained by improving the powder mixing procedure. The cell performance of well mixed, high carbon content electrodes was better than our previously obtained results in terms of higher capacity retention both for different discharge rates and repeated cycling. For currents larger than a 3 C rate, a severe capacity fade affected the electrodes. It was concluded that the electronic contact at the LiFePO₄/carbon interface plays a decisive role in material utilization at different discharge rates which affects the capacity fade upon cycling.     

Antagonistic effects of copper on the electrochemical performance of LiFePO4

In the last few years, several strategies towards boosting the electrochemical performance of LiFePO₄ cathodes have been envisaged. Copper addition to the phosphate seems to be a simple, inexpensive method for this purpose. However, it has a serious drawback: at voltages slightly higher than that required for lithium extraction from LiFePO₄, the copper is oxidized to either Cu(I) or Cu(II) with partial decomposition of the electrolyte. XRD patterns are consistent with the disappearance of copper from pristine composites upon charging at up to 4.0 V. Moreover, a copper deposit is formed on the lithium surface in the discharged state that creates a barrier hindering the release of Li ion from the electrode. Therefore, copper electroactivity strongly influences the capacity and cycling life of the cell.     

Enhanced rate performance of nano–micro structured LiFePO4/C by improved process for high-power Li-ion batteries

A spherical carbon-coated nano–micro structured LiFePO₄ composite is synthesized for use as a cathode material in high-power lithium-ion batteries. The composites are synthesized through carbothermal reduction with two sessions of ball milling (before and after pre-sintering of precursor) followed by spray-drying with the dispersant of polyethylene glycol added. The structure, particle size, and surface morphology of the cathode active material and the properties of the coated carbon are investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Results indicate that the LiFePO₄/C composite has a spherical micro-porous morphology composed of a large number of carbon-coated nano-spheres linked together with an ordered olivine structure. The carbon on the surface of LiFePO₄ effectively reduces inter-particle agglomeration of the LiFePO₄ particles. A galvanostatic charge–discharge test indicates that the LiFePO₄/C composites exhibit initial discharge capacities of 155 mAh g⁻¹ and 88 mAh g⁻¹ at 0.2 C and 20 C rates with the end of discharge voltage of 2.5 V, respectively. This behavior is ascribed to the unique spherical structure, which shortens lithium ions diffusion length and improves the electric contact between LiFePO₄ particles.     

Enhanced performance of LiFePO4 through hydrothermal synthesis coupled with carbon coating and cupric ion doping

A hydrothermal reaction has been adopted to synthesize pure LiFePO₄ first, which was then modified with carbon coating and cupric ion (Cu²⁺) doping simultaneously through a post-heat treatment. X-ray diffraction patterns, transmission electron microscopy and scanning electron microscopy images along with energy dispersive spectroscopy mappings have verified the homogeneous existence of coated carbon and doped Cu²⁺ in LiFePO₄ particles with phospho-olivine structure and an average size of 400 nm. The electrochemical performances of the material have been studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. The carbon-coated and Cu²⁺-doped LiFePO₄ sample (LFCu5/C) exhibited an enhanced electronic conductivity of 2.05 × 10⁻³ S cm⁻¹, a specific discharge capacity of 158 mAh g⁻¹ at 50 mA g⁻¹, a capacity retention of 96.4% after 50 cycles, a decreased charge transfer resistance of 79.4 Ω and superior electrode reaction reversibility. The present synthesis route is promising in making the hydrothermal method more practical for preparation of the LiFePO₄ material and enhancement of electrochemical properties.     

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.     

Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol-formaldehyde polymer as carbon precursor

LiFePO₄/C composites were synthesized by two methods using home-made amorphous nano-FePO₄ as the iron precursor and soluble starch, sucrose, citric acid, and resorcinol-formaldehyde (RF) polymer as four carbon precursors, respectively. The crystalline structures, morphologies, compositions, electrochemical performances of the prepared powders were investigated with XRD, TEM, Raman, and cyclic voltammogram method. The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a deficient carbon coating on the surface of LiFePO₄ particle, but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO₄ particle, and the corresponding thicknesses of the uniform carbon films are 2.5 nm and 4.5 nm, respectively. When RF polymer was used as the carbon precursor, the material showed the highest initial discharge capacity (138.4 mAh g^− 1 at 0.2 C at room temperature) and the best rate performance among the four materials.     

Electrochemical properties of carbon-coated LiFePO4 cathode using graphite, carbon black, and acetylene black

Electrochemical properties of LiFePO₄ were investigated by incorporating conductive carbon from three different carbon sources (graphite, carbon black, acetylene black). SEM observations revealed that the carbon-coated LiFePO₄ were smaller than the bare LiFePO₄ particles. The carbon-coated LiFePO₄ showed much better performance in terms of the discharge capacity and cycling stability than the bare LiFePO₄. Among carbon-coated LiFePO₄, the particles coated with graphite exhibited better electrochemical properties than others coated with carbon black or acetylene black.     

Effects of neodymium aliovalent substitution on the structure and electrochemical performance of LiFePO4

LiFe_1−xNd_xPO₄/C (x = 0-0.08) cathode material was synthesized using a solid-state reaction. The synthesis conditions were optimized by thermal analysis of the precursor and magnetic properties of LiFePO₄/C. The structure and electrochemical performances of the material were studied using XRD, FE-SEM, EDS, electrochemical impedance spectroscopy and galvanostatic charge-discharge. The results show that a small amount of aliovalent Nd³⁺ ion-dopant substitution on Fe²⁺ ions can effectively reduce the particle size of LiFePO₄/C. Cell parameters of LiFe_1−xNd_xPO₄ (x = 0.04-0.08) were calculated, and the results showed that LiFe_1−xNd_xPO₄/C had the same olivine structure as LiFePO₄. LiFe_0.4Nd_0.6PO₄/C delivers the discharge capacity of 165.2 mAh g⁻¹ at rate of 0.2 C and the capacity retention rate is 92.8% after 100 cycles. Charge-transfer resistance decreases with the addition of glucose and Nd³⁺ ions. Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) was synthesized and PZS nanorods were used as a carbon source to coat LiFePO₄. All of the results show that aliovalent doping substitution of Fe in LiFePO₄ is well tolerated.     

Effect of carbon coating on low temperature electrochemical performance of LiFePO4/C by using polystyrene sphere as carbon source

Carbon perfectly coated LiFePO₄ cathode materials are synthesized by carbon-thermal reduction method using polystyrene (PS) spheres as carbon source. The PS spheres with diameters of 150–300 nm used for the pyrolysis reaction not only inhibit the particle growth but also lead to uniform distribution of carbon coating on the surface of LiFePO₄ particles. Rate capability and cycling stability of LiFePO₄/C with the carbon contents ranging from 1.4 wt% to 3.7 wt% are investigated at −20 °C. The LiFePO₄/C with 3.0 wt% C exhibits excellent electrochemical capability at low temperature, which delivers 147 mAh g⁻¹ at 0.1 C. After 100 cycles at a charge–discharge rate of 1 C, there is still 100% of initial capacity retained for the LiFePO₄/C electrode at −20 °C. According to the transmission electron microscope analysis and cyclic voltammetry measurement, this can be attributed to the good carbon coating morphology and optimal carbon coating thickness.     

Enhanced electrochemical performance of LiFePO4/C via Mo-doping at Fe site

A series of Mo-doped LiFe_1−3xMo_xPO₄/C (x = 0.000, 0.025, 0.050, 0.100, 0.150) cathode materials are synthesized by sol–gel method. XRD, ICP and Rietveld refinement results reveal that Mo doped in the crystal lattice and probably occupied Fe site. The structure benefits the transportation of Li⁺ and the diffusion of Li⁺ in the doped materials are enhanced remarkably than that of the undoped one, which leads to excellent electrochemical performance. The doped sample with x = 0.025 exhibits the best electrochemical performance, with the initial discharge capacity of 162.3 mAh g⁻¹ at 0.1 C rate.     

Electrochemical lithiation and delithiation of LiMnPO4: Effect of cation substitution

The electrochemical lithiation-delithiation reaction was examined for LiMnPO₄ in which different cations were substituted for part of Mn. The X-ray diffraction analysis indicated that LiMnPO₄ is tolerant, to some extent, to substitution of Mg²⁺, Ca²⁺ and Zr⁴⁺. The substitution of Mg²⁺ and/or Zr⁴⁺ led to an increased reversible capacity and a reduced polarization, whereas Ca²⁺ substitution had a detrimental effect on the electrochemical properties. The potential transient analysis showed that LiMn_0.88Mg_0.1Zr_0.02PO₄ has higher lithium diffusivities than pure LiMnPO₄, indicating facilitated diffusion kinetics in the substituted material. Upon the first charge-discharge cycle, LiMn_0.88Mg_0.1Zr_0.02PO₄ suffered less irreversible capacity loss when compared with LiMnPO₄, and smaller amounts of electrolyte salt-based species were detected on the electrode surface of LiMn_0.88Mg_0.1Zr_0.02PO₄.