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.     

Rechargeable lithium/sulfur battery with suitable mixed liquid electrolytes

The suitability of some single/binary liquid electrolytes and polymer electrolytes with a 1 M solution of LiCF₃SO₃ was evaluated for discharge capacity and cycle performance of Li/S cells at room temperature. The liquid electrolyte content in the cell was found to have a profound influence on the first discharge capacity and cycle property. The optimum, stable cycle performance at about 450 mAh g⁻¹ was obtained with a medium content (12 μl) of electrolyte. Comparison of cycle performance of cells with tetra(ethylene glycol)dimethyl ether (TEGDME), TEGDME/1,3-dioxolane (DIOX) (1:1, v/v) and 1,2-dimethoxyethane (DME)/di(ethylene glycol)dimethyl ether (DEGDME) (1:1, v/v) showed better results with the mixed electrolytes based on TEGDME. The addition of 5 vol.% of toluene in TEGDME had a remarkable effect of increasing the initial discharge capacity from 386 to 736 mAh g⁻¹ (by >90%) and stabilizing the cycle properties, attributed to the reduced lithium metal interfacial resistance obtained for the system. Polymer electrolyte based on microporous poly(vinylidene fluoride) (PVdF) membrane and TEGDME/DIOX was evaluated for ionic conductivity at room temperature, lithium metal interfacial resistance and cycle performance in room-temperature Li/S cells. A comparison of the liquid electrolyte and polymer electrolyte showed a better performance of the former.     

Performance evaluation of printed LiCoO2 cathodes with PVDF-HFP gel electrolyte for lithium ion microbatteries

In order to improve the discharge capacity in lithium ion microbatteries, a thick-film cathode was fabricated by a screen printing using LiCoO₂ pastes. The printed cathode showed a different discharge curves when the cell was tested using various (liquid, gel and solid-state) electrolytes. When a cell test was performed with organic liquid electrolyte, the maximum discharge capacity was 200 μAh cm⁻², which corresponded to approximately 133 mAh g⁻¹ when the loading weight of LiCoO₂ was calculated. An all-solid-state microbattery could be assembled using sputtered LiPON electrolyte, an evaporated Li anode, and printed LiCoO₂ cathode films without delamination or electrical problems. However, the highest discharge capacity showed a very small value (7 μAh cm⁻²). This problem could be improved using a poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) gel electrolyte, which enhanced the contact area and adhesion force between cathode and electrolyte. The discharge value of this cell was measured as approximately 164 μAh cm⁻² (≈110 mAh g⁻¹). As the PVDF-HFP electrolyte had a relatively soft contact property with higher ionic conductance, the cell performance was improved. In addition, the cell can be fabricated in a leakage-free process, which can resolve many safety problems. According to these results, there is a significant possibility that a film prepared using the aforementioned paste with screen printing and PVDF-HFP gel electrolyte is feasible for a microbattery.     

Rechargeable Li/LiFePO4 cells using N-methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI electrolyte incorporating polymer additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (∼0-∼30 °C).     

Enhanced electrochemical properties of LiFePO4/C synthesized with two kinds of carbon sources,PEG-4000 (organic) and Super p (inorganic)

Olivine structured LiFePO₄/C cathode was synthesized via a freeze-drying route and followed by microwave heating with two kinds of carbon sources: PEG-4000 (organic) and Super p (inorganic). XRD patterns indicate that the as-prepared sample has an olivine structure and carbon modification does not affect the structure of the sample. Image of SEM shows a uniform and optimized particles size, which greatly improves the electrochemical properties. TEM result reveals the amorphous carbon around the surface of the particles. At a low rate of 0.1 C, the LiFePO₄/C sample presents a high discharge capacity of 157.8 mAh g⁻¹ which is near the theoretical capacity (170 mAh g⁻¹), and it still attains to 149.1 mAh g⁻¹ after 200 cycles. It also exhibits an excellent rate capacity with high discharge capacities of 143.2 mAh g⁻¹, 137.5 mAh g⁻¹, 123.7 mAh g⁻¹ and 101.6 mAh g⁻¹ at 0.5 C, 1.0 C, 2.0 C and 5.0 C, respectively. EIS results indicate that the charge transfer resistance of LiFePO₄ decreases greatly after carbon coating.     

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.     

Small capacity decay of lithium iron phosphate (LiFePO4) synthesized continuously in supercritical water: Comparison with solid-state method

Nanosize lithium iron phosphate (LiFePO₄) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25 MPa and 400 °C under various flow rates. The properties of LiFePO₄ synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200-800 nm, higher BET surface area ranging 6.3-15.9 m² g⁻¹ and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3-15 μm; 2.4 m² g⁻¹). LiFePO₄ synthesized in supercritical water exhibit higher discharge capacity of 70-80 mAh g⁻¹ at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO₄ (60 mAh g⁻¹), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125 mAh g⁻¹ is observed during the 30 cycles in carbon-coated LiFePO₄ synthesized using supercritical water while rapid capacity decay from 158 to 140 mAh g⁻¹ is observed in the carbon-coated LiFePO₄ synthesized using the solid-state method.     

Polyterthiophene/CNT composite as a cathode material for lithium batteries employing an ionic liquid electrolyte

A polyterthiophene (PTTh)/multi-walled carbon nanotube (CNT) composite was synthesised by in situ chemical polymerisation and used as an active cathode material in lithium cells assembled with an ionic liquid (IL) or conventional liquid electrolyte, LiBF₄/EC-DMC-DEC. The IL electrolyte consisted of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) containing LiBF₄ and a small amount of vinylene carbonate (VC). The lithium cells were characterised by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The specific capacity of the cells with IL and conventional liquid electrolytes after the 1st cycle was 50 and 47 mAh g⁻¹ (based on PTTh weight), respectively at the C/5 rate. The capacity retention after the 100th cycle was 78% and 53%, respectively. The lithium cell assembled with a PTTh/CNT composite cathode and a non-flammable IL electrolyte exhibited a mean discharge voltage of 3.8 V vs Li⁺/Li and is a promising candidate for high-voltage power sources with enhanced safety.     

One-step microwave synthesis and characterization of carbon-modified nanocrystalline LiFePO4

The precursors of LiFePO₄ were prepared by a sol-gel method using lithium acetate dihydrate, ferrous sulfate, phosphoric acid, citric acid and polyethylene glycol as raw materials, and then the carbon-modified nanocrystalline LiFePO₄ (LiFePO₄/C) cathode material was synthesized by a one-step microwave method with the domestic microwave oven. The effect of microwave time and carbon content on the performance of the resulting LiFePO₄/C material was investigated. Structural characterization by X-ray diffraction and scanning electron microscopy proved that the olivine phase LiFePO₄ was synthesized and the grain size of the samples was several hundred nanometers. Under the optimal conditions of microwave time and carbon content, the charge-discharge performance indicated that the nanosized LiFePO₄/C had a high electrochemical capacity at 0.2 C (152 mAh g⁻¹) and improved capacity retention; the exchange current density was 1.6977 mA cm⁻². Furthermore, the rate capability was improved effectively after LiFePO₄ was modified with carbon, with 59 mAh g⁻¹ being obtained at 20 C.     

In situ construction of carbon nano-interconnects between the LiFePO4 grains using ultra low-cost asphalt

LiFePO₄/C composite cathode materials with carbon nano-interconnect structures were synthesized by one-step solid state reaction using low-cost asphalt as both carbon source and reducing agent. Based on the thermogravimetry, differential scanning calorimetry, transmission electron microscopy and high-resolution transmission electron microscopy, a growth model was proposed to illustrate the formation of the carbon nano-interconnect between the LiFePO₄ grains. The LiFePO₄/C composite shows enhanced discharge capacity (150 mAh g⁻¹) with excellent capacity retention compared with the bare LiFePO₄ (41 mAh g⁻¹) due to the electronically conductive nanoscale networking provided by the asphalt-based carbon. The results prove that the asphalt is a perfect carbon source and reduction agent for cost-effective production of high performance LiFePO₄/C composite.     

Cycling-induced stress in lithium ion negative electrodes: LiAl/LiFePO4 and Li4Ti5O12/LiFePO4 cells

In this work, we examined the electrochemical behaviour of lithium ion batteries containing lithium iron phosphate as the positive electrode and systems based on Li-Al or Li-Ti-O as the negative electrode. These two systems differ in their potential versus the redox couple Li⁺/Li and in their morphological changes upon lithium insertion/deinsertion. Under relatively slow charge/discharge regimes, the lithium-aluminium alloys were found to deliver energies as high as 438 Wh kg⁻¹ but could withstand only a few cycles before crumbling, which precludes their use as negative electrodes. Negative electrodes consisting solely of aluminium performed even worse. However, an electrode made from a material with zero-strain associated to lithium introduction/removal such as a lithium titanate spinel exhibited good performance that was slightly dependent on the current rate used. The Li₄Ti₅O₁₂/LiFePO₄ cell provided capacities as high as 150 mAh g⁻¹ under C-rate in the 100th cycle.     

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.     

Effect of synthesis condition on the structural and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by the metal acetates decomposition method

In order to get homogeneous layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ as a lithium insertion positive electrode material, we applied the metal acetates decomposition method. The oxide compounds were calcined at various temperatures, which results in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_1/3Mn_1/3Co_1/3]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry and SEM. XRD experiment revealed that the layered Li[Ni_1/3Mn_1/3Co_1/3]O₂ material can be best synthesized at temperature of 800 °C. In that synthesized temperature, the sample showed high discharge capacity of 190 mAh g⁻¹ as well as stable cycling performance at a current density of 0.2 mA cm⁻² in the voltage range 2.3-4.6 V. The reversible capacity after 100 cycles is more than 190 mAh g⁻¹ at room temperature.     

Lithium-polymer battery based on an ionic liquid-polymer electrolyte composite for room temperature applications

A lithium-polymer battery based on an ionic liquid-polymer electrolyte (IL-PE) composite membrane operating at room temperature is described. Utilizing a polypyrrole coated LiV₃O₈ cathode material, the cell delivers >200 mAh g⁻¹ with respect to the mass of the cathode material. Discharge capacity is slightly higher than those observed for this cathode material in standard aprotic electrolytes; it is thought that this is the result of a lower solubility of the LiV₃O₈ material in the IL-PE composite membrane.     

Synthesis and electrochemical performance of novel loose structured submicro/micro-sized NiSnx alloy composites for lithium batteries

An effective method of carbothermal reduction was employed to prepare spherical microcrystal NiSn_x alloy powders from oxides of Sn and Ni used as anode materials for Li-ion battery. According to XRD, SEM and TEM analysis, the synthesized spherical NiSn_x powders show a loose submicro/micro-sized structure and a multi-phase composition. The prepared NiSn_x alloy composite electrode exhibits a stable discharge capacity of electrode is ca. 380 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and can be retained at 350 mAh g⁻¹ after 25 cycles. Moreover, NiSn_x alloys exhibit excellent high rate performance, i.e. stable discharge capacities of 300-310 mAh g⁻¹ and the coulombic efficiencies of 97.5-99.5% have been obtained at the current density of 500 mA g⁻¹. The loose submicro-sized particle structural characteristic and the Ni addition in Sn matrix should be responsible for the improvement of cycling stability of NiSn_x electrode. The carbothermal reduction method is simple, low-cost and mass-productive, which should be viable to other alloy composite materials system of rechargeable lithium ion batteries.     

Improving electrochemical properties of room temperature ionic liquid (RTIL) based electrolyte for Li-ion batteries

Room temperature ionic liquids (RTILs) with high safety characteristic usually have high viscosity and melting point, which is adverse for the application of RTIL-based electrolytes in Li-ion batteries. In this investigation, a promising RTIL, i.e. PP13TFSI consisting of N-methyl-N-propylpiperidinium (PP13) cation and bis(trifluoromethanesulfonyl)imide (TFSI) anion is synthesized. The effect of the content of Li salt in the electrolytes containing PP13TFSI and LiTFSI on the ionic conductivity and cell performance is investigated. The electrolyte of 0.3 mol kg⁻¹ LiTFSI/PP13TFSI is recommended for its higher lithium transference number and discharge capacity in the LiCoO₂/Li cell than other electrolytes. In addition, it is found that, by introducing 20% diethyl carbonate (DEC) as a co-solvent into pure RTIL electrolyte, the rate capability and low-temperature performance of the LiCoO₂/Li cells are improved obviously, without sacrificing its safety characteristics. It suggests that a component with low viscosity and melting point, i.e. DEC, is necessary to effectively overcome the shortcomings of RTIL for the application in Li-ion batteries.     

An optimized Ni doped LiFePO4/C nanocomposite with excellent rate performance

Both Ni doping and carbon coating are adopted to synthesize a nano-sized LiFePO₄ cathode material through a simple solid-state reaction. It is found that the Ni²⁺ has been successfully doped into LiFePO₄ without affecting the phospho-olivine structure from the XRD result. The images of SEM and TEM show that the size of particles is distributed in the range of 20-60 nm, and all the particles are coated with carbon completely. The results of XPS show the valence state of Fe and Ni in the LiFePO₄. The electronic conductivity of the material is as high as 2.1 × 10⁻¹ S cm⁻¹, which should be ascribed to the coefficient of the conductive carbon network and Ni doping. As a cathode material for lithium-ion batteries, the Ni doped LiFePO₄/C nanocomposite delivers a discharge capacity of 170 mAh g⁻¹ at 0.2 C, approaching the theoretical value. Moreover, the material shows excellent high-rate charge and discharge capability and long-term cyclability. At the high rates of 10 and 15 C, this material exhibits high capacities of 150 and 130 mAh g⁻¹, retaining 95% after 5500 cycles and 93% after 7200 cycles, respectively. Therefore, the as-prepared material is capable of such large-scale applications as electric vehicles and plug-in hybrid electric vehicles.     

TiO2 nanotube arrays annealed in CO exhibiting high performance for lithium ion intercalation

Anatase titania nanotube arrays were fabricated by means of anodization of Ti foil and annealed at 400 °C in respective CO and N₂ gases for 3 h. Electrochemical impendence spectroscopy study showed that CO annealed arrays possessed a noticeably lower charge-transfer resistance as compared with arrays annealed in N₂ gas under otherwise the same conditions. TiO₂ nanotube arrays annealed in CO possessed much improved lithium ion intercalation capacity and rate capability than N₂ annealed samples. At a high charge/discharge current density of 320 mA g⁻¹, the initial discharge capacity in CO annealed arrays was found to be as high as 223 mAh g⁻¹, 30% higher than N₂ annealed arrays, ∼164 mAh g⁻¹. After 50 charge/discharge cycles, the discharge capacity in CO annealed arrays remained at ∼179 mAh g⁻¹. The improved intercalation capacity and rate capability could be attributed to the presence of surface defects like Ti-C species and Ti³⁺ groups with oxygen vacancies, which not only improved the charge-transfer conductivity of the arrays but also possibly promoted phase transition.     

Ionic liquid-based gel polymer electrolyte for LiMn0.4Fe0.6PO4 cathode prepared by electrospinning technique

Two polymer electrolytes (PEs), one consisting of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (PE_EC/DMC) and the other consisting of LiTFSI in room temperature ionic liquid (RTIL), 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI) (PE_IL), were prepared by using electrospun P(VdF-HFP) membranes. The PEs showed typical impedance spectroscopic responses with high conductivity and good anodic stability. The PEs were applied with carbon coated LiMn_0.4Fe_0.6PO₄ cathode material prepared by sol-gel method. The charge-discharge kinetics of LiMn_0.4Fe_0.6PO₄ cathode cells were studied by electrochemical impedance spectroscopy. The excellent performance with high capacity and good cycle stability was observed for both the cells. The cell comprising of PE_IL showed a better performance than the other cell. The cells having PE_EC/DMC and PE_IL delivered discharge capacities of 150 and 141 mAh g⁻¹, and 168 and 162 mAh g⁻¹, respectively, after cycle 1 and 50. The differences in the performance of the PEs originate from the differences in viscosity, ionic conductivity and also from the different levels of interactions of a RTIL and EC/DMC with the polymer. The evaluation of lithium ion diffusion coefficients shows its fast diffusion in both the cases, the trend of which changed with the increase in the number of cycles.     

A novel micro-spherical CoSn2/Sn alloy composite as high capacity anode materials for Li-ion rechargeable batteries

Micro-scaled spherical CoSn₂/Sn alloy powders synthesized from oxides of Sn and Co via carbothermal reduction at 800 °C were examined for use as anode materials in Li-ion battery. The phase composition and particle morphology of the CoSn₂/Sn alloy composite powders were investigated by XRD, SEM and TEM. The prepared CoSn₂/Sn alloy composite electrode exhibits a low initial irreversible capacity of ca. 140 mAh g⁻¹, a high specific capacity of ca. 600 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and a good rate capability. The stable discharge capacities of 500-515 mAh g⁻¹ and the columbic efficiencies of 95.8-98.1% were obtained at current density of 500 mA g⁻¹. The relatively large particle size of CoSn₂/Sn alloy composite powder is apparently favorable for the lowering of initial capacity loss of electrode, while the loose particle structural characteristic and the Co addition in Sn matrix should be responsible for the improvement of cycling stability of CoSn₂/Sn electrode.