1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g⁻¹, which the value is near to theoretical specific capacity (170 mAh g⁻¹). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g⁻¹) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g⁻¹ when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.     

Properties of LiMn2O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide

LiMn₂O₄ was examined as a cathode material for lithium-ion batteries, working together with a room temperature ionic liquid electrolyte, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MePrPipNTf₂), with the formation of a liquid LiNTf₂-MePrPipNTf₂ system. The Li/LiMn₂O₄ cell was tested by galvanostatic charging/discharging and by impedance spectroscopy. The LiMn₂O₄ cathode showed good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate (VC) as an additive to the ionic liquid. The flash point of the LiNTf₂-MePrPipNTf₂-VC(10%) electrolyte was estimated to be above 300 °C.     

Investigation on capacity fading of LiFePO4 in aqueous electrolyte

The capacity loss rate of LiFePO₄ in aqueous electrolyte was found to be much faster than it in organic electrolyte. The cycling stability in aqueous electrolyte with various dissolved oxygen content and pH value was extensively studied by cyclic voltammetry. It was found that both high OH⁻ and dissolve O₂ content can accelerate the cycling fading of LiFePO₄. It has been proved that the capacity fading of LiFePO₄ is due to not only chemical instability but also electrochemical instability. Mössbauer spectroscopy demonstrated that the Fe(III)-containing species was formed in the active materials arisen from the irreversible side reaction. The carbon-coated LiFePO₄ prepared by chemical vapor decomposition method shows significantly improvement in cycling stability. The carbon coating technology provides an effective approach to enhance cycling performance in aqueous electrolyte as well as proof of proposed fading mechanism.     

Properties of Li-graphite and LiFePO4 electrodes in LiPF6–sulfolane electrolyte

Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF₆ and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO₄ lithium-ion battery. Charging/discharging capacity of the LiFePO₄ electrode was ca. 150 mAh g⁻¹ (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g⁻¹ (C/10 rate). In the case of the full graphite |1 M LiPF₆ + TMS + VC 10 wt% |LiFePO₄ cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g⁻¹. Exchange current density for Li⁺ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was j_o(Li/Li⁺) = 8.15 × 10⁻⁴ A cm⁻². Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO₄ electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF₆ + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 °C. The LiFePO₄(solid) + 1 M LiPF₆ + TMS + 10 wt% VC system shows a flash point of ca. 150 °C. This was much higher in comparison to that characteristic of a classical LiFePO₄ (solid) + 1 M LiPF₆ + 50 wt% EC + 50 wt% DMC system (T_f ≈ 37 °C).     

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.     

Improved electrochemical performance of LiFePO4 by increasing its specific surface area

Cathode material LiFePO₄ with an excellent rate capability has been successfully prepared by a simple solid state reaction method using LiCH₃COO·2H₂O, FeC₂O₄·2H₂O and (NH₄)₂HPO₄ as the starting materials. We have investigated the effects of the sintering temperature and mixing time of the starting materials on the physical properties and electrochemical performance of LiFePO₄. It was found that the rate capability of LiFePO₄ is mainly controlled by its specific surface area and it is an effective way to improve the rate capability of the sample by increasing its specific surface area. In the present study, our prepared LiFePO₄ with a high specific surface area of 24.1 m² g⁻¹ has an excellent rate capability and can deliver 115 mAh g⁻¹ of reversible capacity even at the 5 C rate. Moreover, we have prepared lithium ion batteries based on LiFePO₄ as the cathode material and MCMB as the anode material, which showed an excellent cycling performance.     

Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process

Carbon coated LiFePO₄ particles were first synthesized by sol-gel and freeze-drying method. These particles were then coated with La_0.7Sr_0.3MnO₃ nanolayer by a suspension mixing process. The La_0.7Sr_0.3MnO₃ and carbon co-coated LiFePO₄ particles were calcined at 400 °C for 2 h in a reducing atmosphere (5% of hydrogen in nitrogen). Nanolayer structured La_0.7Sr_0.3MnO₃ together with the amorphous carbon layer forms an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high-resolution transmission electron microscopy. X-ray diffraction results proved that the co-coated composite still retained the structure of the LiFePO₄ substrate. The twin coatings can remarkably improve the electrochemical performance at high charge/discharge rates. This improvement may be attributed to the lower charge transfer resistance and higher electronic conductivity resulted from the twin nanolayer coatings compared with the carbon coated LiFePO₄.     

Rate performance and structural change of Cr-doped LiFePO4/C during cycling

Structural change of Cr-doped LiFePO₄/C during cycling is investigated using conventional and synchrotron-based in situ X-ray diffraction techniques. The carbon-coated and Cr-doped LiFePO₄ particles are synthesized by a mechanochemical process followed by a one-step heat treatment. The LiFe_0.97Cr_0.03PO₄/C has shown excellent rate performance, delivering the discharge capacity up to 120 mAh/g at 10 C rate. The results suggest that the improvement of the rate performance is attributed to the chromium doping, which facilitates the phase transformation between triphylite and heterosite during cycling, and conductivity improvement by carbon coating. Structural analysis using the synchrotron source also indicates that the doped Cr replaces Fe and/or Li sites in LiFePO₄.     

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.     

Synthesis of FePO4·xH2O for fabricating submicrometer structured LiFePO4/C by a co-precipitation method

Amorphous hydrated iron (III) phosphate has been synthesized by a coordinate precipitation method from equimolecular Fe(NO₃)₃ and (NH₄)₂HPO₄ solutions at an elevated temperature. Hydrated iron (III) phosphate samples and the corresponding LiFePO₄/C products were characterized by XRD and SEM. The electrochemical behavior was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The LiFePO₄/C fabricated from as-synthesized FePO₄ delivered discharge capacities of 162.5, 147.3, 133.0, 114.7, 97.2, 91.3 and 88.5 mAh g⁻¹ at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 3C and 4C with satisfactory capacity retention, respectively.     

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.     

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

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

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.     

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

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

Comparative studies on C-coated and uncoated LiFePO4 cycling at various rates and temperatures using synchrotron based in situ X-ray diffraction

The structural changes of LiFePO₄ and C-coated LiFePO₄ during charging at various C-rates and temperatures are investigated using synchrotron based in situ X-ray diffraction technique. The XRD patterns collected during cycling show the structural evidence of the positive effects of carbon coating on LiFePO₄ for the electrochemical performance improvements at different temperatures, especially at low temperatures. At −10 °C, the C-coated LiFePO₄ shows comparable capacities with the sample cycled at room temperature when cycled at C/5 rate with a slight shift of the plateau to a higher voltage during charging. The in situ XRD patterns collected simultaneously show a complete phase transformation from triphylite to heterosite. At −20 °C, the C-coated LiFePO₄ delivers 55.6% of its theoretical capacities at C/5 rate. However, the plateau in the charging curve becomes sloppy and shifts to a higher voltage. The in situ XRD patterns show that the phase transformation from triphylite to heterosite is not completed when charged to 4.5 V due to the larger polarization when charged at −20 °C.     

Emulsion drying synthesis of olivine LiFePO4/C composite and its electrochemical properties as lithium intercalation material

The electroactive LiFePO₄/C nano-composite has been synthesized by an emulsion drying method. During burning-out the oily emulsion precipitates in an air-limited atmosphere at 300 °C, amorphous or low crystalline carbon was generated along with releasing carbon oxide gases, and trivalent iron as a cheap starting material was immediately reduced to the divalent one at this stage as confirmed by X-ray photoelectron spectroscopy, leading to a low crystalline LiFePO₄/C composite. Heat-treatment of the low crystalline LiFePO₄/C in an Ar atmosphere resulted in a well-ordered olivine structure, as refined by Rietveld refinement of the X-ray diffraction pattern. From secondary electron microscopic and scanning transmission electron microscopic observations with the corresponding elemental mapping images of iron and phosphorous, it was found that the LiFePO₄ powders are modified by fine carbon. The in situ formation of the nano-sized carbon during crystallization of LiFePO₄ brought about two advantages: (i) an optimized particle size of LiFePO₄, and (ii) a uniform distribution of fine carbon in the product. These effects of the fine carbon on LiFePO₄/C composite led to high capacity retention upon cycling at 25 and 50 °C and high rate capability, resulting from a great enhancement of electric conductivity as high as 10⁻⁴ S cm⁻¹. That is, the obtained capacity was higher than 90 mAh (g-phosphate)⁻¹ by applying a higher current density of about 1000 mA g⁻¹ (11 C) at 50 °C.     

Synthesis and characterization of LiFePO4/(Ag + C) composite cathodes with nano-carbon webs

LiFePO₄/(Ag + C) composite cathodes with a new type of nano-sized carbon webs were synthesized by two methods of an aqueous co-precipitation and a sol-gel process, respectively. Simultaneous thermogravimetric-differential thermal analysis indicates that the crystallization temperature of LiFePO₄ is about 455-466 °C, which is close to the pyrolysis temperature of polypropylene, 460 °C. The silver and carbon co-modifying does not affect the olivine structure of LiFePO₄ but improves its kinetics in terms of discharge capacity and rate capability. Discharge capacities were improved from 153.4 mA h g^− 1 of LiFePO₄/C to 160.5 mA h g^− 1and 162.1 mA h g^− 1 for LiFePO₄/(Ag + C) cathodes synthesized by the co-precipitation and sol-gel methods, respectively. The possible reasons for the small difference in discharge capacity of two LiFePO₄/(Ag + C) cathodes were discussed. AC impedance measurements show that the Ag + C co-modification decreases the charge transfer resistance of LiFePO₄/(Ag + C) cathodes.     

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.     

Design and synthesis of high-rate micron-sized, spherical LiFePO4/C composites containing clusters of nano/microspheres

A micron-sized LiFePO₄/C composite with a spherical morphology was reduced carbothermally from precursor particles prepared by ball milling-assisted spray-drying. The specific capacity of the electrode at a 10 C (1700 mA/g) rate was 110 mAh/g and a high voltage plateau was achieved. The high-rate performance of the composite electrode was due to its unique spherical structure, comprising clusters of nano- or sub-micron-sized spherical particles. This morphology increases the effective conductive surface area, reduces the charge-transfer reaction resistance and improves the diffusion of lithium ions.     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.