LiFePO<Subscript>4</Subscript> composites decorated with nitrogen-doped carbon as superior cathode materials for lithium-ion batteries

The development of methods to synthesize electrode materials can improve the performance of lithium ion storage. In this study, a facile and low-cost approach is employed to synthesize LiFePO₄ (LFP/NC) hybrid materials decorated with nitrogen-doped carbon nanomaterials (NC). Melamine was used as nitrogen and carbon source with an NC to LFP ratio of 3.19%. As electrode materials for lithium ion batteries (LIBs), the LFP/NC composites exhibit an optimum performance with a high rate capacity of 144.6 mAh·g^?1 at 1 C after 500 cycles without apparent loss. The outstanding cycling stability may be attributed to the synergetic effects of well-crystallized particles and NC layers.     

Nitrogen-doped carbon nanofiber decorated LiFePO<Subscript>4</Subscript> composites with superior performance for lithium-ion batteries

Nitrogen-doped carbon nanofiber (NCNF) decorated LiFePO₄ (LFP) composites are synthesized via an in situ hydrothermal growth method. Electrochemical performance results show that the embedded NCNF can improve electron and ion transfer, thereby resulting in excellent cycling performance. The as-prepared LFP and NCNF composites exhibit excellent electrochemical properties with discharge capacities of 188.9 mAh g^?1 (at 0.2 C) maintained at 167.9 mAh g^?1 even after 200 charge/discharge cycles. The electrode also presents a good rate capability of 10 C and a reversible specific capacity as high as 95.7 mAh g^?1. LFP composites are a potential alternative high-performing anode material for lithium ion batteries.     

Effect of carbon coating on the electrochemical performance of LiFePO<Subscript>4</Subscript>/C as cathode materials for aqueous electrolyte lithium-ion battery

Organic electrolyte is widely used for lithium-ion rechargeable batteries but might cause flammable fumes or fire due to improper use such as overcharge or short circuit. That weakness encourages the development of tools and materials which are cheap and environmental friendly for rechargeable lithium-ion batteries with aqueous electrolyte. Lithium iron phosphate (LiFePO₄) with olivine structure is a potential candidate to be used as the cathode in aqueous electrolyte lithium-ion battery. However, LiFePO₄ has a low electronic conductivity compared to other cathodes. Conductive coating of LiFePO₄ was applied to improve the conductivity using sucrose as carbon source by heating to 600 °C for 3 h on an Argon atmosphere. The carbon-coated LiFePO₄ (LiFePO₄/C) was successfully prepared with three variations of the weight percentage of carbon. From the cyclic voltammetry, the addition of carbon coatings could improve the stability of cell battery in aqueous electrolyte. The result of galvanostatic charge/discharge shows that 9 % carbon exhibits the best result with the first specific discharge capacity of 13.3 mAh g^?1 and capacity fading by 2.2 % after 100 cycles. Although carbon coating enhances the conductivity of LiFePO₄, excessive addition of carbon could degrade the capacity of LiFePO₄.     

Highly enhanced low-temperature performances of LiFePO<Subscript>4</Subscript>/C cathode materials prepared by polyol route for lithium-ion batteries

Although LiFePO₄/C has been successfully put into practical use in lithium-ion batteries equipped on new energy vehicles, its unsatisfactory low temperature results in poor low performance of lithium-ion batteries, leading to a much smaller continue voyage course at extreme environments with low temperature for electric vehicles. In this paper, the electrochemical performance of the LiFePO₄/C prepared by polyol route was investigated at a temperature range from 25 to ?20 °C. Compared to commercial ones, as-prepared LiFePO₄/C shows a much better low-temperature performance with a reversible capacity of 30 mA h g^?1 even at 5 C under ?20 °C and a capacity retention of 91.1 % after 100 cycles at 0.1 C under 0 °C. Moreover, high-resolution transmission electron microscopy (HRTEM) revealed that this outstanding performance at low temperatures could be assigned to uniform carbon coating and the nano-sized particles with a highly crystalline structure.     

Synthesis and electrochemical performance of LiFePO<Subscript>4</Subscript>/C cathode materials from Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> for high-power lithium-ion batteries

Carbon-coated olivine-structured LiFePO₄/C composites are synthesized via an efficient and low-cost carbothermal reduction method using Fe₂O₃ as iron source at a relative low temperature (600 °C). The effects of two kinds of carbon sources, inorganic (acetylene black) and organic (sucrose), on the structures, morphologies, and lithium storage properties of LiFePO₄/C are evaluated in details. The particle size and distribution of the carbon-coated LiFePO₄ from sucrose (LiFePO₄/SUC) are more uniform than that obtained from acetylene black (LiFePO₄/AB). Moreover, the LiFePO₄/SUC nanocomposite shows superior electrochemical properties such as high discharge capacity of 156 mAh g^?1 at 0.1 C, excellent cyclic stability, and rate capability (78 mAh g^?1 at 20 C), as compared to LiFePO₄/AB. Cyclic voltammetric test discloses that the Li-ion diffusion, the reversibility of lithium extraction/insertion, and electrical conductivity are significantly improved in LiFePO₄/SUC composite. It is believed that olivine-structured LiFePO₄ decorated with carbon from organic carbon source (sucrose) using Fe₂O₃ is a promising cathode for high-power lithium-ion batteries.     

A novel carbon source coated on C-LiFePO<Subscript>4</Subscript> as a cathode material for lithium-ion batteries

The poor electronic conductivity and low lithium-ion diffusion are the two major obstacles to the largely commercial application of LiFePO₄ cathode material in power batteries. In order to improve the defects of LiFePO₄, a novel carbon source polyacrylonitrile (PAN), which would form the hierarchical porous structure after carbonization, is fabricated and used. This work comes up with a simple and facile carbothermal reduction method to prepare porous-carbon-coated LiFePO₄ (C-LiFePO₄-PC) composite and to study the effect of carbon-coated temperature on ameliorating the electrochemical performance. The obtained C-LiFePO₄-PC composite shows a high initial discharge capacity of 164.1 mA h g^?1 at 0.1 C and good cycling stability as well as excellent rate capacity (49.0 mA h g^?1 at 50 C). The most possible factors that improve the electrochemical performance could be related to the enhancement of electronic conductivity and the existence of porous carbon layers. In a word, the C-LiFePO₄-PC material would become an excellent candidate for application in the fields of lithium-ion batteries.     

Effects of yttrium ion doping on electrochemical performance of LiFePO<Subscript>4</Subscript>/C cathodes for lithium-ion battery

The olivine-type LiFe_1-x Y _x PO₄/C (x?=?0, 0.01, 0.02, 0.03, 0.04, 0.05) products were prepared through liquid-phase precipitation reaction combined with the high-temperature solid-state method. The structure, morphology, and electrochemical performance of the samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS), galvanostatic charge-discharge, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). We found that the small amount of Y³⁺ ion-doped can keep the microstructure of LiFePO₄, modify the particle morphology, decrease charge transfer resistance, and enhance exchange current density, thus enhance the electrochemical performance of the LiFePO₄/C. However, the large doping content of Y³⁺ ion cannot be completely doped into LiFePO₄ lattice, but existing partly in the form of YPO₄. The electrochemical performance of LiFePO₄/C was restricted owing to YPO₄. Among all the doped samples, LiFe_0.98Y_0.02PO₄/C showed the best electrochemical performance. The LiFe_0.98Y_0.02PO₄/C sample exhibited the initial discharge capacity of 166.7, 155.8, 148.2, 139.8, and 121.1 mAh g^?1 at a rate of 0.2, 0.5, 1, 2, and 5 C, respectively. And, the discharge capacity of the material was 119.6 mAh g^?1 after 100 cycles at 5 C rates.     

Synthesis and electrochemical performance of carbon-coated LiCoBO<Subscript>3</Subscript> as cathode materials for lithium-ion batteries

Carbon-coated LiCoBO₃ (LiCoBO₃/C) is prepared by sol-gel method and polyethylene glycol 6000 (PEG-6000) is chosen as carbon source. The LiCoBO₃/C sample exhibits an initial discharge capacity of 76.7 mAh g^?1 at 0.1 C, and it can deliver a discharge capacity of 65.9 mAh g^?1 after 50 cycles, while the LiCoBO₃ sample only presents a first discharge capacity of 34.3 and 16.8 mAh g^?1 at the 50th cycle, LiCoBO₃/C sample shows better cycling performance than that of LiCoBO₃. The improved electrochemical properties could be mainly ascribed to the conductive carbon network and the reduced particle size of the LiCoBO₃ powders. Electrochemical impedance spectroscopy (EIS) results confirm that carbon coating decreases the charge transfer resistance and improve the electrochemical reaction kinetics.     

Effect of carbon coating on electrochemical performance of LiFePO<Subscript>4</Subscript> cathode material for Li-ion battery

Pristine LiFePO₄ (LFP) and carbon-coated LiFePO₄ (LFP/C) are synthesized by sol-gel process using citric acid as a carbon precursor. LFP/C is prepared with three different stoichiometric ratios of metal ions and citric acid, namely 1:0.5, 1:1, and 1:2. Prepared LFP and LFP/C powder samples are characterized by X-ray diffractometer, field emission scanning electron microscope, transmission electron microscope, and Raman spectrophotometer. Electrochemical performances of pristine and carbon-coated LFP are investigated by charge-discharge and cyclic voltammetry technique. The results show that LFP/C (1:1) with an optimum thickness of 4.2 nm and higher graphitic carbon coating has the highest discharge capacity of 148.2 mA h g^?1 at 0.1 C rate and 113.1 mA h g^?1 at a high rate of 5 C among all four samples prepared. The sample LFP/C (1:1) shows 96% capacity retention after 300 cycles at 1 C rate. The decrease in discharge capacity (141.4and 105.9 mA h g^?1 at 0.1 and 5 C, respectively) is observed for the sample LFP/C (1:2). Whereas, pristine LFP shows the lowest discharge capacity of 111.1 mA h g^?1 at 0.1 C and capacity was decreased very fast and work only up to 147 cycles. Moreover, cyclic voltammetry has also revealed the lowest polarization of 0.19 V for LFP/C (1:1) and the highest 0.4 V for pristine LFP.     

Synthesis and electrochemical performance of carbon-coated LiMnBO<Subscript>3</Subscript> as cathode materials for lithium-ion batteries

Carbon-coated LiMnBO₃/C is synthesized by a sol-gel method using polyethylene glycol 6000 (PEG-6000) as carbon source. The influences of different sintering temperatures on the crystal structure, morphology, and electrochemical performance of LiMnBO₃/C composites are investigated. XRD results indicate that the samples consist of the monoclinic phase LiMnBO₃ (m-LiMnBO₃) and the hexagonal phase LiMnBO₃ (h-LiMnBO₃), and the amount of m-LiMnBO₃ is reduced and the h-LiMnBO₃ is increased with the increasing sintering temperature. The particle size of the samples is about 500 nm, and the surface of the particles is coated with a thick amorphous carbon layer. The LiMnBO₃/C synthesized at 750 °C exhibits the initial discharge capacities of 213.4, 170.8, and 109.7 mAh g^?1 at 0.025, 0.05, and 0.5 C rates, respectively, and shows better cycling performance than that of bare LiMnBO₃. The enhanced electrochemical performance might be largely attributed to the uniformly coated carbon layers from decomposition of the PEG-6000.     

Influence of carbon additive on the properties of spherical Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> and LiFePO<Subscript>4</Subscript> materials for lithium-ion batteries

Preparing spherical particles with carbon additive is considered as one effective way to improve both high rate performance and tap density of Li₄Ti₅O₁₂ and LiFePO₄ materials. Spherical Li₄Ti₅O₁₂/C and LiFePO₄/C composites are prepared by spray-drying–solid-state reaction method and controlled crystallization–carbothermal reduction method, respectively. The X-ray diffraction characterization, scanning electron microscope, Brunauer–Emmett–Teller, alternating current impedance analyzing, tap density testing, and electrochemical property measurements are investigated. After hybridizing carbon with a proper quantity, the crystal grain size of active materials is remarkably decreased and the electrochemical properties are obviously improved. The Li₄Ti₅O₁₂/C and LiFePO₄/C composites prepared in this work are spherical. The tap density and the specific surface area are as high as 1.71 g cm⁻³ and 8.26 m² g⁻¹ for spherical Li₄Ti₅O₁₂/C, which are 1.35 g cm⁻³ and 18.86 m² g⁻¹ for spherical LiFePO₄/C powders. Between 1.0 and 3.0 V versus Li, the reversible specific capacity of the Li₄Ti₅O₁₂/C is more than 150 mAh g⁻¹ at 1.0-C rate. Between 2.5 and 4.2 V versus Li, the reversible capacity of the LiFePO₄/C is close to 140 mAh g⁻¹ at 1.0-C rate.     

Surfactant-assisted synthesis of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> particles as high-rate and long-life cathode materials for lithium-ion batteries

Herein, we reported the synthesis of uniform LiMn₂O₄ submicroparticles by surfactant-assisted preparation of spherical MnCO₃ precursor followed by solid-state reaction. Polyethylene glycol (Mw = 1000) was used as surfactant to control the morphology and size of the MnCO₃ precursor as well as the MnO₂ intermediate and LiMn₂O₄ product. The influence of particle size, homogeneity, and crystallinity on the electrochemical performance of LiMn₂O₄ was intensively investigated. The test results indicate that the LiMn₂O₄ sample using polyethylene glycol with weight as 10% of reactants shows the best rate capability and long-term cyclability. Due to the homogeneous particles with the average size of ca. 250 nm and high crystallinity, the discharge capacities are as high as 125, 118, 114, and 100 mAh g^?1 at 1, 10, 20, and 50 C rates, respectively, along with high capacity retention of 74% after 1000 cycles at 20 C.     

A piperidinium-based ester-functionalized ionic liquid as electrolytes in Li/LiFePO<Subscript>4</Subscript> batteries

A new functionalized ionic liquid (IL) based on cyclic quaternary ammonium cations with ester group and bis(trifluoromethanesulfonyl)imide ([TFSI]^?) anion, namely, N-methyl-N-methoxycarbonylpiperidinium bis(trifluoromethanesulfonyl)imide ([MMOCPip][TFSI]), was synthesized and characterized. Physical and electrochemical properties, including Li-ion transference number, ionic conductivity, and electrochemical stability, were investigated. The electrochemical window of [MMOCPip][TFSI] was 6 V, which was wide enough to be used as a common electrolyte material. The Li-ion transference number of this IL electrolyte containing 0.1 M LiTFSI was 0.56. The half-cell tests indicated that the [MMOCPip][TFSI] obviously improved the cyclability of a Li/LiFePO₄ cell. For the Li/LiFePO₄ half-cells, after 20 cycles at room temperature at 0.1 C, the discharge capacity was 109.7 mAh g^?1 with 98.7% capacity retention in the [MMOCPip][TFSI]/0.1 M LiTFSI electrolyte. The good electrochemical performance demonstrated that the [MMOCPip][TFSI] could be used as electrolyte for lithium-ion batteries.     

Advanced LiTi<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C anode by incorporation of carbon nanotubes for aqueous lithium-ion batteries

Inferior rate capability is a big challenge for LiTi₂(PO₄)₃ anode for aqueous lithium-ion batteries. Herein, to address such issue, we synthesized a high-performance LiTi₂(PO₄)₃/carbon/carbon nanotube (LTP/C/CNT) composite by virtue of high-quality carbon coating and incorporation of good conductive network. The as-prepared LTP/C/CNT composite exhibits excellent rate performance with discharge capacity of 80.1 and 59.1 mAh g^?1 at 10 C and 20 C (based on the mass of anode, 1 C = 150 mA g^?1), much larger than that of the LTP/C composite (53.4 mAh g^?1 at 10 C, and 31.7 mAh g^?1 at 20 C). LTP/C/CNT also demonstrates outstanding cycling stability with capacity retention of 83.3 % after 1000 cycles at 5 C, superior to LTP/C without incorporation of CNTs (60.1 %). As verified, the excellent electrochemical performance of the LTP/C/CNT composite is attributed to the enhanced electrical conductivity, rapid charge transfer, and Li-ion diffusion because of the incorporation of CNTs.     

Cycle-life and degradation mechanism of LiFePO<Subscript>4</Subscript>-based lithium-ion batteries at room and elevated temperatures

Cycle-life tests of commercial 22650-type olivine-type lithium iron phosphate (LiFePO₄)/graphite lithium-ion batteries were performed at room and elevated temperatures. A number of non-destructive electrochemical techniques, i.e., capacity recovery using a small current density, electrochemical impedance spectroscopy, and differential voltage and differential capacity analyses, were performed to deduce the degradation mechanism of these batteries. To further characterize their internal materials, we disassembled the batteries, and material analyses were performed. All results indicated that loss in active lithium was the main reason for battery aging, and the cells showed diverse recession of active materials at different temperatures. In addition, high discharge rate and growing impedance lead to a capacity fall down at 25 °C at approximately 300–500 cycles.     

Modified carbothermal synthesis and electrochemical performance of LiFePO4/C composite as cathode materials for lithium-ion batteries

LiFePO₄/C active materials were synthesized via a modified carbothermal method, with a low raw material cost and comparatively simple synthesis process. Rheological phase technology was introduced to synthesize the precursor, which effectively decreased the calcination temperature and time. The LiFePO₄/C composite synthesized at 700 °C for 12 h exhibited an optimal performance, with a specific capacity about 130 mAh g^?1 at 0.2C, and 70 mAh g^?1 at 20C, respectively. It also showed an excellent capacity retention ratio of 96 % after 30 times charge–discharge cycles at 20C. EIS was applied to further analyze the effect of the synthesis process parameters. The as-synthesized LiFePO₄/C composite exhibited better high-rate performance as compared to the commercial LiFePO₄ product, which implied that the as-synthesized LiFePO₄/C composite was a promising candidate used in the batteries for applications in EVs and HEVs.     

ZnO-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode material for lithium-ion batteries synthesized by a combustion method

ZnO-coated LiMn₂O₄ cathode materials were prepared by a combustion method using glucose as fuel. The phase structures, size of particles, morphology, and electrochemical performance of pristine and ZnO-coated LiMn₂O₄ powders are studied in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge test, and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that surface-modified ZnO have no obvious effect on the bulk structure of the LiMn₂O₄. TEM and XPS proved ZnO formation on the surface of the LiMn₂O₄ particles. Galvanostatic charge/discharge test and rate performance showed that the ZnO coating could improve the capacity and cycling performance of LiMn₂O₄. The 2 wt% ZnO-coated LiMn₂O₄ sample exhibited an initial discharge capacity of 112.8 mAh g^?1 with a capacity retention of 84.1 % after 500 cycles at 0.5 C. Besides, a good rate capability at different current densities from 0.5 to 5.0 C can be acquired. CV and EIS measurements showed that the ZnO coating effectively reduced the impacts of polarization and charge transfer resistance upon cycling.     

Relationship between local structure and electrochemical performance of LiFePO<Subscript>4</Subscript> in Li-ion batteries

This work is devoted to the study of fundamental properties of LiFePO₄ (LFP) olivine in view of the optimization of this material for its use as a positive electrode material in Li-ion batteries. The investigation of the electronic and magnetic properties appears to be successful for the detection of a small amount of impurities. By the combination of X-ray diffraction, optical spectroscopy, and magnetometry, we characterize the local structure and the morphology of LFP particles. The impact of the ferromagnetic clusters (γ-Fe₂O₃ or Fe₂P) on the electrochemical response is examined. The electrochemical performance of the optimized LFP powders investigated at 60 °C is excellent in terms of capacity retention (153 mAh/g at 2 C) as well as cycling life. No iron dissolution was observed after 200 charge–discharge cycles at 60 °C for cells containing Li foil, Li₄Ti₅O₁₂, or graphite as negative electrodes. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007.     

Cu-supported carbon networks containing SnO<Subscript>2</Subscript> as three-dimensional anodes for lithium-ion batteries

Cu-supported SnO₂@C composite coatings constructed by interconnected carbon-based porous branches were fabricated by annealing Cu foils with films formed by knife coating DMF solution containing SnCl₂, polyacrylonitrile (PAN), and poly(methyl methacrylate) (PMMA) on their surface in vacuum. The carbon-based porous branches consist of amorphous carbon matrices, SnO₂ nanoparticles with a size of 30–100 nm mainly encapsulated inside, and many micropores with a size of 1–5 nm. The three-dimensional (3D) porous network structures of the SnO₂@C composite were achieved by volatilization of PMMA and pyrolysis of SnCl₂. The SnO₂@C composite coatings demonstrate good cyclic performance with a high reversible capacity of 642 mA h g^?1 after 100 cycles at a current density of 50 mA g^?1 without apparent capacity fading during cycling and excellent rate performance with a capacity of 276 mA h g^?1 at a high current density up to 10 A g^?1.     

Synthesis,structure, and electrochemistry of Ag-modified LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials for lithium-ion batteries

The composite of silver-modified lithium manganese oxide were prepared using thermal decomposition method of different mole ratio. Structural characterization was carried out by X-ray diffraction (XRD). XRD analysis revealed different patterns as the content of the dopant in the spinel increases. Phase analysis shows that Ag particles were dispersed on the LiMn₂O₄ surface instead of entering the spinel structure. On the other hand, the electrochemical behavior of cathode powder was examined by using two-electrode test cells consisting of a cathode, metallic lithium as anode, and a solid polymer electrolyte of 0.87PEO-0.13LiCF₃SO₃-0.10DBP. According to the electrochemical tests results, the influence of the Ag additive content on the electrochemical properties of Ag/LiMn₂O₄ composites is clearly shown.