Electrochemical study on lithium iron phosphate/hard carbon lithium-ion batteries

The electrochemical performances of lithium iron phosphate (LiFePO₄), hard carbon (HC) materials, and a full cell composed of these two materials were studied. Both positive and negative electrode materials and the full cell were characterized by scanning electron microscopy, transmission electron microscopy, charge–discharge tests, and alternating current (a.c.) impedance techniques. Experimental results show that the LiFePO₄/HC full cell exhibits a gradually decreased cell voltage, and it is capable of delivering a reversible discharge capacity of 122.1 mAh g⁻¹ at 0.2-C rate. At the higher rate of 10 C, the efficiency of the full cell remains almost unchanged from that of 0.2 C. Furthermore, the LiFePO₄/HC battery demonstrated a long life of 2,450 cycles with 40% of capacity change at a 10-C high rate. The internal resistance of the full cell is rather low as it is revealed from a.c. impedance measurements. These properties make the LiFePO₄/HC battery an attractive option for high rate and long cycle life power applications.     

Effects of molecular weight,heating rate,synthetic temperature and sintering duration on electrochemical properties of a LiFePO<Subscript>4</Subscript>/C cathode material pyrolyzed from lithium polyacrylate

A LiFePO₄/C composite was obtained by a polymer pyrolysis reduction method, using lithium polyacrylate (LiPAA) as carbon source and fractional lithium source, and FePO₄·2H₂O as iron and phosphorus source. The structure of the LiFePO₄/C composites was investigated by X-ray diffraction (XRD). The micromorphology of the precursors and LiFePO₄/C powders was observed using scanning electron microscopy (SEM). Laser particle analyzer and BET were also used to characterize the materials. It was found that the micromorphology, particle size distribution and specific surface area of LiFePO₄/C composites were greatly influenced by the molecular weight of LiPAA. The electrochemical properties of the LiFePO₄/C composites were evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge cycling tests. The results showed that the molecular weight of LiPAA, heating rate, synthetic temperature and sintering duration directly affected the electrochemical properties of LiFePO₄/C composites. The sample with the optimized electrochemical properties were obtained in the following conditions, i.e., LiPAA with the molecular weight of 20,000, heating rate of 10 °C min⁻¹, synthetic temperature of 700 °C and sintering duration of 15 h.     

Effect of different conductive additives on charge/discharge properties of LiCoPO<Subscript>4</Subscript>/Li batteries

Single-phase LiCoPO₄ nanoparticles were synthesized by solid-state reaction method and subsequent high-energy ball milling. The electrochemical properties of LiCoPO₄/Li batteries were analyzed by ac impedance experiments, cyclic voltammetry (CV), and charge/discharge tests. The structural and morphological performance of LiCoPO₄ nanoparticles was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The XRD result demonstrated that LiCoPO₄ nanoparticles had an orthorhombic olivine-type structure with a space group of Pmnb. Different conductive additives including acetylene black and carbon black (SP270) were used to fabricate electrodes. The morphologies of the electrodes and different conductive additives were observed by field emission-scanning electron microscopy (FE-SEM). LiCoPO₄/Li battery with acetylene black showed the best electrochemical properties, and exhibited a discharge plateau at around 4.7 V with an initial discharge capacity of 110 mAh g⁻¹ at a discharge current density of 0.05 mA cm⁻² at 25 °C.     

LiFePO<Subscript>4</Subscript>/CA cathode nanocomposite with 3D conductive network structure for Li-ion battery

A novel LiFePO₄/Carbon aerogel (LFP/CA) nanocomposite with 3D conductive network structure was synthesized by using carbon aerogels as both template and conductive framework, and subsequently wet impregnating LiFePO₄ precursor inside. The LFP/CA nanocomposite was characterized by X-ray diffraction (XRD), TG, SEM, TEM, nitrogen sorption, electrochemical impedance spectra and charge/discharge test. It was found that the LFP/CA featured a 3D conductive network structure with LiFePO₄ nanoparticles ca. 10–30 nm coated on the inside wall of the pore of CA. The LFP/CA electrodes delivered discharge capacity for LiFePO₄ of 157.4, 147.2, 139.7, 116.3 and 91.8 mA h g⁻¹ at 1 °C, 5 °C, 10 °C, 20 °C and 40 °C, respectively. In addition, the LFP/CA electrode exhibited good cycling performance, which lost less than 1% of discharge capacity over 100 cycles at a rate of 10 °C. The good high rate performances of LiFePO₄ were attributed to the unique 3D conductive network structure of the nanocomposite.     

Facile low-temperature polyol process for LiFePO4 nanoplate and carbon nanotube composite

Crystalline LiFePO₄ nanoplates were incorporated with 5 wt.% multi-walled carbon nanotubes (CNTs) via a facile low temperature polyol process, in one single step without any post heat treatment. The CNTs were embedded into the LiFePO₄ particles to form a network to enhance the electrochemical performance of LiFePO₄ electrode for lithium-ion battery applications. The structural and morphological characters of the LiFePO₄–CNT composites were investigated by X-ray diffraction, Fourier Transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were analyzed by cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge tests. Primary results showed that well crystallized olivine-type structure without any impurity phases was developed, and the LiFePO₄–CNT composites exhibited good electrochemical performance, with a reversible specific capacity of 155 mAh g⁻¹ at the current rate of 10 mA g⁻¹, and a capacity retention ratio close to 100% after 100 cycles.     

Physical and electrochemical characterizations of LiFePO4-incorporated Ag nanoparticles

Olivine-type LiFePO₄ composite materials for cathode material of the lithium-ion batteries were synthesized by using a sol-gel method and were coated by a chemical deposition of silver particles. As-obtained LiFePO₄/C-Ag (2.1 wt.%) composites were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), conductivity measurements, cyclic voltammetry, as well as galvanostatic measurements. The results revealed that the discharge capacity of the LiFePO₄/C-Ag electrode is 136.6 mAh/g, which is 7.6% higher than that of uncoated LiFePO₄/C electrode (126.9 mAh/g). The LiFePO₄/C coated by silver nanoparticles enhances the electrode conductivity and specific capacity at high discharge rates. The improved capacity at high discharge rates may be attributed to increased electrode conductivity and the synergistic effect on electron and Li⁺ transport after silver incorporation.     

Synthesis,characterization and electrochemical performances of LiFePO4/graphene cathode material for high power lithium-ion batteries

LiFePO₄/graphene (LiFePO₄/G) cathode with exciting electrochemical performance was successfully synthesized by liquid phase method. LiFePO₄ nanoparticles wrapped with multi-layered grapheme can be fabricated in a short time. This method did not need external heating source. Heat generated by chemical reaction conduct the process and removed the solvent simultaneously. The LiFePO₄/G were analyzed by X-ray diffraction (XRD) analysis, scanning electron microscope (SEM), transmission electron microscopy (TEM), magnetic properties analysis and electrochemical performance tests. The LiFePO₄/G delivered a capacity of 160 mAh g⁻¹ at 0.1C and could tolerate various dis-charge currents with a capacity retention rate of 99.8%, 99.2%, 99.0%, 98.6%, 97.3% and 95.0% after stepwise under 5C, 10C, 15C, 20C, 25C and 30C, respectively.     

Biosynthesis and electrochemical characteristics of LiFePO4/C by microwave processing

The yeast cells are adopted as a template and cementation agent to prepare LiFePO₄/C with high surface area by co-precipitation and microwave processing. The electrochemical properties of the resultant products are investigated. The synthesized LiFePO₄/C is characterized by means of X-ray diffraction, transmission electron microscopy (TEM), Brunauer–Emmett–Teller method, and battery test instrument. The LiFePO₄/C particles with average size of 35-100 nm coated by porous carbon are observed by TEM. The LiFePO₄/C, with the specific surface area of 98.3 m²/g, exhibits initial discharge specific capacity of 147 mAh/g and good cycle ability. The yeast cells as a template are used to synthesize the precursor LiFePO₄/cells compounds. In microwave heating process, the use of yeast cells as reducing matter and cementation agent results in the enhancement of the electrochemical properties.     

Effect of Dy3+‐doping on Electrochemical Properties of LiFePO4/C Cathode for Lithium‐Ion Batteries

Dy doping and carbon coating are adopted to synthesize a LiFePO₄ cathode material in a simple solution environment. The samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Their electrochemical properties were investigated by cyclic voltammetry (CV) and galvanostatic charge‐discharge tests. An initial discharge capacity of 153 mAh/g was achieved for the LiDy_0.02Fe_0.98PO₄/C composite cathode with a rate of 0.1 C. In addition the electronic conductivity of Dy doped LiFePO₄/C was enhanced to 1.9 × 10^?2 Scm^?1. The results suggest that the improvement of the electrochemical properties are attributed to the dysprosium doping and carbon coating which facilitates the phase transformation between triphylite and heterosite during cycling. XRD data indicate that doping did not destroy the lattice structure of LiFePO₄. To evaluate the effect of Dy substitution, cyclic voltammetry was used at room temperature. prepared. From Cv measurement a more symmetric curve with smaller interval between the cathodic and anodic peak current was obtained by Dy substitution. This denoted a decreasing of polarization with Dy substitution, which illustrated an enhancement of electrochemical performances.     

Preparation and characterization of high-rate and long-cycle LiFePO4/C nanocomposite as cathode material for lithium-ion battery

Olivine LiFePO₄/C nanocomposite cathode materials with small-sized particles and a unique electrochemical performance were successfully prepared by a simple solid-state reaction using oxalic acid and citric acid as the chelating reagent and carbon source. The structure and electrochemical properties of the samples were investigated. The results show that LiFePO₄/C nanocomposite with oxalic acid (oxalic acid: Fe²⁺= 0.75:1) and a small quantity of citric acid are single phase and deliver initial discharge capacity of 122.1 mAh/g at 1 C with little capacity loss up to 500 cycles at room temperature. The rate capability and cyclability are also outstanding at elevated temperature. When charged/discharged at 60 °C, this materials present excellent initial discharge capacity of 148.8 mAh/g at 1 C, 128.6 mAh/g at 5 C, and 115.0 mAh/g at 10 C, respectively. The extraordinarily high performance of LiFePO₄/C cathode materials can be exploited suitably for practical lithium-ion batteries.     

Surfactant assisted solvothermal synthesis of LiFePO_4 nanorods for lithium-ion batteries

Well-shaped and uniformly dispersed LiFePO_4 nanorods with a length of 400–500 nm and a diameter of about 100 nm, are obtained with participation of a proper amount of anion surfactant sodium dodecyl sulfonate(SDS) without any further heating as a post-treatment. The surfactant acts as a self-assembling supermolecular template, which stimulated the crystallization of LiFePO_4 and directed the nanoparticles growing into nanorods between bilayers of surfactant(BOS). LiFePO_4 nanorods with the reducing crystal size along the b axis shorten the diffusion distance of Li~+ extraction/insertion, and thus improve the electrochemical properties of LiFePO_4 nanorods. Such prepared LiFePO_4 nanorods exhibited excellent specific capacity and high rate capability with discharge capacity of 151 mAh/g, 122 mAh/g and 95 mAh/g at 0.1C, 1 C and 5 C, respectively. Such excellent performance of LiFePO_4 nanorods is supposed to be ascribed to the fast Li~+ diffusion velocity from reduced crystal size along the b axis and the well electrochemical conductivity. The structure, morphology and electrochemical performance of the samples were characterized by XRD, FE-SEM, HRTEM, charge/discharge tests, and EIS(electrochemical impedance spectra).     

Synthesis, characterization, and electrochemical performance of LiFePO4/C cathode materials for lithium ion batteries using various carbon sources: best results by using polystyrene nano-spheres

Olivine LiFePO₄/C cathode materials for lithium ion batteries were synthesized using monodisperse polystyrene (PS) nano-spheres and other carbon sources. The structure, morphology, and electrochemical performance of LiFePO₄/C were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge–discharge tests, electrochemical impedance spectroscopy (EIS) measurements, and Raman spectroscopy measurements. The results demonstrated that LiFePO₄/C materials have an ordered olivine-type structure with small particle sizes. Electrochemical analyses showed that the LiFePO₄/C cathode material synthesized from 7 wt.% PS nano-spheres delivers an initial discharge capacity of 167 mAh g^-1 (very close to the theoretical capacity of 170 mAh g^-1) at 0.1 C rate cycled between 2.5 and 4.1 V with excellent capacity retention after 50 cycles. According to Raman spectroscopy and EIS analysis, this composite had a lower I _D/I _G, sp ³/sp ² peak ratio, charge transfer resistance, and a higher exchange current density, indicating an improved electrochemical performance, due to the increased proportion of graphite-like carbon formed during pyrolysis of PS nano-spheres, containing functionalized aromatic groups.     

Mechanism studies and electrochemical performance optimization on xLiFePO4·(1 ? x)Li3V2(PO4)3 hybrid materials

Hybrid materials xLiFePO₄·(1 − x)Li₃V₂(PO₄)₃ were synthesized by sol–gel method, with phenolic resin as carbon source and chelating agent, methylglycol as surfactant. The crystal structure, morphology and electrochemical performance of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), galvanostatic charge–discharge test and particle size analysis. The results show that LiFePO₄ and Li₃V₂(PO₄)₃ co-exist in hybrid materials, but react in single phase. Compared with individual LiFePO₄ and Li₃V₂(PO₄)₃ samples, hybrid materials have smaller particle size and more uniform grain distribution. This structure can facilitate Li ions extraction and insertion, which greatly improves the electrochemical properties. The sample 0.7LiFePO₄·0.3Li₃V₂(PO₄)₃ retains the advantages of LiFePO₄ and Li₃V₂(PO₄)₃, obtaining an initial discharge capacity of 166 mA h/g at 0.1 C rate and 109 mA h/g at 20 C rate, with a capacity retention rate of 73.3% and an excellent cycle stability.     

The improvement of the high-rate charge/discharge performances of LiFePO4 cathode material by Sn doping

Nanocrystalline LiFePO₄ and LiFe_0.97Sn_0.03PO₄ cathode materials were synthesized by an inorganic-based sol–gel route. The physicochemical properties of samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and elemental mapping. The doping effect of Sn on the electrochemical performance of LiFePO₄ cathode material was extensively investigated. The results showed that the doping of tin was beneficial to refine the particle size, increase the electrical conductivity, and facilitate the lithium-ion diffusion, which contributed to the improvement of the electrochemical properties of LiFePO₄, especially the high-rate charge/discharge performance. At the low discharge rate of 0.5 C, the LiFe_0.97Sn_0.03PO₄ sample delivered a specific capacity of 158 mAh g⁻¹, as compared with 147 mAh g⁻¹ of the pristine LiFePO₄. At higher C-rate, the doping sample exhibited more excellent discharge performance. LiFe_0.97Sn_0.03PO₄ delivered specific capacity of 146 and 128 mAh g⁻¹ at 5 C and 10 C, respectively, in comparison with 119 and 107 mAh g⁻¹ for LiFePO₄. Moreover, the doping of Sn did not influence the cycle capability, even at 10 C.     

Electrochemical properties of LiFePO4/C synthesized using polypyrrole as carbon source

LiFePO₄/C composites were synthesized by pyrolysis of LiFePO₄/polypyrrole (PPy), which was obtained by an in situ chemical polymerization involving pyrrole monomer and hydrothermal synthesis LiFePO₄. All samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge techniques. The results showed the LiFePO₄/C sintered at 800 °C containing 2.8 wt.% carbon exhibited a higher discharge capacity of 49.6 mAh·g⁻¹ at 0.1 C, and bare LiFePO₄ only delivered 11.6 mAh·g⁻¹ in 2 M LiNO₃ aqueous electrolyte. The possible reason for the improvement of electrochemical performance was discussed and could be attributed to the formation of aromatic compounds during the carbonization of PPy.     

Synthesis and characterization of core–shell F-doped LiFePO<Subscript>4</Subscript>/C composite for lithium-ion batteries

Core–shell LiFePO₄/C composite was synthesized via a sol–gel method and doped by fluorine to improve its electrochemical performance. Structural characterization shows that F⁻ ions were successfully introduced into the LiFePO₄ matrix. Transmission electron microscopy verifies that F-doped LiFePO₄/C composite was composed of nanosized particles with a ~3 nm thick carbon shell coating on the surface. As a cathode material for lithium-ion batteries, the F-doped LiFePO₄/C nanocomposite delivers a discharge capacity of 162 mAh/g at 0.1 C rate. Moreover, the material also shows good high-rate capability, with discharge capacities reaching 113 and 78 mAh/g at 10 and 40 C current rates, respectively. When cycled at 20 C, the cell retains 86% of its initial discharge capacity after 400 cycles, demonstrating excellent high-rate cycling performance.     

Preparation and characterization of electrospun LiFePO4/carbon complex improving rate performance at high C-rate

LiFePO₄/carbon complexes were prepared by electrospinning to improve rate performance at high C-rate and their electrochemical properties were investigated to be used as a cathode active material for lithium ion battery. The LiFePO₄/carbon complexes were prepared by the electrospinning method. The prepared samples were characterized by SEM, EDS, XRD, TGA, electrometer, and electrochemical analysis. The LiFePO₄/carbon complexes prepared have a continuous structure with carbon-coated LiFePO₄ and the LiFePO₄ in LiFePO₄/carbon complex has improved thermal stability from carbon coating. The conductivity of LiFePO₄/carbon complex heat-treated at 800 °C is measured as 2.23 × 10^?2 S cm^?1, which is about 10⁶–10⁷ times more than that of raw LiFePO₄. The capacity ratio of coin cell manufactured from raw LiFePO₄ is 40%, whereas the capacity ratio of coin cell manufactured from LiFePO₄/carbon complex heat-treated at 800 °C is 61% (10 C/0.1 C). The improved rate performance of LiFePO₄/carbon complex heat-treated at 800 °C is due to the carbon coating and good electrical connection.     

A novel network composite cathode of LiFePO4/multiwalled carbon nanotubes with high rate capability for lithium ion batteries

A novel network composite cathode was prepared by mixing LiFePO₄ particles with multiwalled carbon nanotubes for high rate capability. LiFePO₄ particles were connected by multiwalled carbon nanotubes to form a three-dimensional network wiring. The web structure can improve electron transport and electrochemical activity effectively. The initial discharge capacity was improved to be 155 mA h/g at C/10 rate (0.05 mA/cm²) and 146 mA h/g at 1C rate. The comparative investigation on MWCNTs and acetylene black as a conducting additive in LiFePO₄ proved that MWCNTs addition was an effective way to increase rate capability and cycle efficiency.     

Electrodeposition and electrocatalytic properties of platinum nanoparticles on multi-walled carbon nanotubes: effect of the deposition conditions

Platinum (Pt) nanoparticles were electrochemically deposited on multi-walled carbon nanotubes (MWCNTs) through a three-step process, including an electrochemical treatment of MWCNT, electro-oxidation of PtCl₄ ²⁻ to Pt(IV) complex, and an electro-conversion of Pt(0) on MWCNT. The effect of formation conditions for Pt(IV) complexes on the Pt nanoparticals transformed was investigated. The structure and elemental composition of the resulting Pt/MWCNT electrode were characterized by transmission electron micrograph (TEM) and energy dispersive X-ray spectroscopy (EDX). The electrocatalytic properties of the resulting Pt/MWCNT electrode for methanol oxidation have been investigated. The high electrocatalytic activity and good stability of Pt/MWCNT electrode may be attributed to the high dispersion of platinum nanoparticles and the particular properties of the MWCNT supports.     

Synthesis of LiFePO4 material with improved cycling performance under harsh conditions

LiFePO₄ material was synthesized at 670 °C in an Ar atmosphere using a sol–gel method. This material showed a well developed XRD pattern (orthorhombic structure, Pnma) without any peaks at 2θ = 41°, indicating the absence of FeP or metallic Fe₂P impurities. The Li/LiFePO₄ cell showed a high initial discharge capacity of more than 150 mA h/g and no capacity decrease until the 70th cycle (>99.9%). This cell also exhibits excellent cycle performance at high current densities of over 30C, without any surface treatment or carbon coating onto the LiFePO₄ particles.