Design of amphiphilic poly(vinylidene fluoride-<Emphasis Type="Italic">co</Emphasis>-hexafluoropropylene)-based gel electrolytes for high-performance lithium-ion batteries

Gel polymer electrolytes (GPE) based on electrospun polymer membranes, poly(vinylidene fluoride-co-hexafluoropropylene), grafted poly(poly(ethylene glycol) methyl ether methacrylate) (PVDF-HFP-g-PPEGMA), and poly(vinylidene difluoride-co-hexafluoropropylene) (PVDF-HFP) are prepared for lithium ion batteries by incorporating with 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI). The uniform porosity and the compatibility of blend electrospun membranes avoiding the pore blocking are beneficial to enhance the electrolyte uptakes. The GPE based on the fibrous PVDF-HFP-g-PPEGMA/PVDF-HFP activated with 1 M LiTFSI (BMITFSI) show a maximum ionic conductivity of 2.3 × 10^?3 S cm^?1 at room temperature and electrochemical stability of up to 5.2 V. The Li/GPE/LiFePO₄ cells with GPE based on PVDF-HFP-g-PPEGMA/PVDF-HFP blend electrospun membrane deliver specific capacities of 163, 141, and 125 mAh g^?1 at 0.1, 0.5, and 1C rates, respectively, and remains well after 50 cycles for each rate. Therefore, the novel GPE have been demonstrated to be suitable for lithium-ion battery applications.     

Triethylbutylammonium bis(trifluoromethanesulphonyl)imide ionic liquid as an effective electrolyte additive for Li-ion batteries

Ionic liquids are promising additives for Li-ion batteries owing to its desirable physicochemical properties. Triethylbutylammonium bis(trifluoromethanesulphonyl)imide ([N₂₂₂₄][Tf₂N]) ionic liquid was synthesized and their physical and electrochemical properties were investigated. Among several quaternary ammonium ionic liquids, [N₂₂₂₄][Tf₂N] exhibited higher conductivity (1.31 mS?cm^?1), better thermal and electrochemical stabilities, and wide electrochemical window, i.e., more than 5.9 V. Standard solution was prepared by dissolving lithium bis(trifluoromethanesulphonyl)imide (LiTf₂N) in ethylene carbonates/dimethyl carbonate (1:1, by weight). The conductivity for the electrolyte containing [N₂₂₂₄][Tf₂N] and the mixed electrolyte without additives at 25 °C are 10.24 and 8.79 mS?cm^?1, respectively. LiFePO₄ half-cell containing 0.6 mol?L^?1 LiTf₂N-based organic electrolyte with [N₂₂₂₄][Tf₂N] showed relatively high initial discharge capacity and coulombic efficiency at first cycle. It is found that the mix [N₂₂₂₄][Tf₂N] electrolyte exhibits relatively high-rate capacity. The capacity retention of half-cell containing [N₂₂₂₄][Tf₂N] is 2 % more than without additive at 0.2 C. However, the rate capacity retention of the half-cell with mix [N₂₂₂₄][Tf₂N] electrolyte is above 10 % more than without additive at 0.5 C. The results showed that [N₂₂₂₄][Tf₂N] was an effective electrolyte additive in LiFePO₄ half-cell.     

Polymer electrolytes based on polycarbonates and their electrochemical and thermal properties

Polycarbonates (4a–d) with various side chain lengths were synthesized by the reaction of 1,4-bis(hydroxyethoxy)benzene derivatives and triphosgene in the presence of pyridine. The polymer electrolytes composed of 4a–d with lithium bis(trifluoromethanesulfonyl)imide (LiN(SO₂CF₃)₂, LiTFSI) were prepared, and their ionic conductivities and thermal and electrochemical properties were investigated. 4d-Based polymer electrolyte showed the highest ionic conductivity values of 1.0?×?10^?4?S/cm at 80 °C and 1.5?×?10^?6?S/cm at 30 °C, respectively, at the [LiTFSI]/[repeating unit] ratio of 1/2. Ionic conductivities of these polycarbonate-based polymer electrolytes showed the tendency of increase with increasing the chain length of oxyethylene moieties as side chains, suggestive of increased steric hindrance by side chains. Unique properties were observed for the 4a(n?=?0)-based polymer electrolyte without an oxyethylene moiety. All of polycarbonate-based polymer electrolytes showed good electrochemical and thermal stabilities as polymer electrolytes for battery application.     

A novel and shortcut method to prepare ionic liquid gel polymer electrolyte membranes for lithium-ion battery

The ionic liquid polymer electrolyte (IL-PE) membrane is prepared by ultraviolet (UV) cross-linking technology with polyurethane acrylate (PUA), methyl methacrylate (MMA), ionic liquid (Py₁₃TFSI), lithium salt (LiTFSI), ethylene glycol dimethacrylate (EGDMA), and benzoyl peroxide (BPO). N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (Py₁₃TFSI) ionic liquid is synthesized by mixing N-methyl-N-propyl pyrrolidinium bromide (Py₁₃Br) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The addition of Py₁₃TFSI to polymer electrolyte membranes leads to network structures by the chain cross-linking. The resultant electrolyte membranes display the room temperature ionic conductivity of 1.37 × 10^?3 S cm^?1 and the lithium ions transference number of 0.22. The electrochemical stability window of IL-PE is about 4.8 V (vs. Li⁺/Li), indicating sufficient electrochemical stability. The interfacial resistances between the IL-PE and the electrodes have the less change after 10 cycles than before 10 cycles. IL-PE has better compatibility with the LiFePO₄ electrode and the Li electrode after 10 cycles. The first discharge performance of Li/IL-PE/LiFePO₄ half-cell shows a capacity of 151.9 mAh g^?1 and coulombic efficiency of 87.9%. The discharge capacity is 131.9 mAh g^?1 with 95.5% coulombic efficiency after 80 cycles. Therefore, the battery using the IL-PE exhibits a good cycle and rate performance.     

Performance of solid-state hybrid supercapacitor with LiFePO4/AC composite cathode and Li4Ti5O12 as anode

We report the studies on quasi-solid battery-supercapacitor (BatCap) systems fabricated using sol–gel-prepared LiFePO₄ and its composites (LACs) with activated charcoal (AC) as hybrid cathode and Li₄Ti₅O₁₂ powder as anode separator by flexible gel polymer electrolyte (GPE) film. The GPE film comprises 1.0 M lithium trifluoromethane sulfonate (LiTf) solution in ethylene carbonate (EC)–propylene carbonate (PC) mixture, immobilized poly(vinylidene fluoride-co-hexafluoro-propylene) (PVdF-HFP), which is of high ionic conductivity (∼3.8 × 10⁻³ S cm⁻¹ at 25 °C) and electrochemical stability window (∼3 V). The effect of the addition of AC in composite electrode LACs has been analyzed using various techniques such as X-ray diffraction, porosity analysis, and electrochemical methods. The interfaces of composite LACs and GPE film not only offer high rate performance but also show high specific energy (>27.8 Wh kg⁻¹) as compared to the symmetric supercapacitors and pristine lithium iron phosphate (LiFePO₄)-based lithium ion batteries. The full BatCap systems have been characterized by cyclic voltammetry and galvanostatic charge–discharge tests. The BatCap systems with composite electrodes (LACs) offer better cyclic performance as compared to that of pristine LiFePO₄-based BatCap or LIB LiFePO₄/Li₄Ti₅O₁₂.

     

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.     

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.     

Thermal and electrochemical properties of poly(butylene sulfite)-based polymer electrolyte

Poly(butylene sulfite) (poly-1) was synthesized by cationic ring-opening polymerization of butylene sulfite (1), which was prepared by the reaction of 1,4-butanediol and thionyl chloride, with trifluoromethanesulfonic acid (TfOH) in bulk. The polymer electrolytes composed of poly-1 with lithium salts such as bis(trifluoromethanesulfonyl)imide (LiN(SO₂CF₃)₂, LiTFSI) and bis(fluorosulfonyl)imide (LiN(SO₂F)₂, LiFSI) were prepared, and their ionic conductivities, thermal, and electrochemical properties were investigated. Ionic conductivities of the polymer electrolytes for the poly-1/LiTFSI system increased with lithium salt concentrations, reached maximum values at the [LiTFSI]/[repeating unit] ratio of 1/10, and then decreased in further more salt concentrations. The highest ionic conductivity values at the [LiTFSI]/[repeating unit] ratio of 1/10 were 2.36?×?10^?4 S/cm at 80 °C and 1.01?×?10^?5 S/cm at 20 °C. On the other hand, ionic conductivities of the polymer electrolytes for the poly-1/LiFSI system increased with an increase in lithium salt concentrations, and ionic conductivity values at the [LiFSI]/[repeating unit] ratio of 1/1 were 1.25?×?10^?3 S/cm at 80 °C and 5.93?×?10^?5 S/cm at 20 °C. Glass transition temperature (T _g) increased with lithium salt concentrations for the poly-1/LiTFSI system, but T _g for the poly-1/LiFSI system was almost constant regardless of lithium salt concentrations. Both polymer electrolytes showed high transference number of lithium ion: 0.57 for the poly-1/LiTFSI system and 0.56 for the poly-1/LiFSI system, respectively. The polymer electrolytes for the poly-1/LiTFSI system were thermally more stable than those for the poly-1/LiFSI system.     

Li3V2(PO4)3 modified LiFePO4/C cathode materials with improved high-rate and low-temperature properties

LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ is prepared with a sol–gel combustion method. The structure and electrochemical behavior of the material are studied using a wide range of techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. It is found that LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ has the better electrochemical performance. The discharge capacity of the as-prepared material can reach up to 153.1, 137.7, 113.6, and 93.3 mAh g^?1 at 1, 2, 5, and 10 C, respectively. The capacitance of the LiFePO₄/C modified by Li₃V₂(PO₄)₃ is higher under lower discharging rate at ?20 °C, and the initial discharge capacity of 0.2 C is 131.4 mAh g^?1. It is also demonstrated that the presence of Li₃V₂(PO₄)₃ in the sample can reduce the charge transfer resistance in the range of ?20 to 25 °C, resulting in the enhanced electrochemical catalytic activity.     

Synthesis of high-performance LiFePO4/C composite with a grape bunch structure through the hydrothermal method

The LiFePO₄/C composite with a grape bunch structure was synthesized through the hydrothermal method at 170 °C for 7 h and followed by being fired at 750 °C for 4 h. Commercial Li₂CO₃, (NH₄)₂Fe(SO₄)₂?·?6H₂O, and (NH₄)₂HPO₄ were used as raw materials. Glucose was used as in situ coating carbon source, and the hydroxylated MWCNTs were used as connecting carbon wires which could be embedded into the carbon coating to form a uniform grape bunch structure. The resultant samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), elementary analysis (EA), Raman spectroscopy, and electrochemical tests. The results show that the grape bunch structure with a low disordered/graphene (D/G) ratio was found to be well dispersed in the LiFePO₄/C composite, and a three-dimensional carbonaceous network was formed which could enhance the electronic conductivity of the LiFePO₄/C composite remarkably. The resultant LiFePO₄/C composite shows a high discharge capacity of 160.3 mAh g^?1 at 0.1 C and 110.9 mAh g^?1 even at 10 C, and the cycling capacity retention rate reaches 99.6 % over 60 cycles. Besides, it also exhibits high conductivity, good reversibility, and excellent stability in EIS and CV tests.     

Enhanced electrochemical performance of LiFePO4/C prepared by sol–gel synthesis with dry ball-milling

LiFePO₄/C was prepared by a modified aqueous sol–gel route developed by incorporating an additional ball-milling step where the dry gel was milled with the additives of synthetic graphite and carbon black. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and elemental analysis. Results showed that the LiFePO₄/C synthesized by suitable ball-milling process had pure, fine and homogenous LiFePO₄ particles. Results of cyclic voltammetry and charge/discharge plateaus demonstrated that the LiFePO₄/C composite synthesized by milling for 2 h had much better electrochemical kinetics. High performances were achieved with its discharge capacities of 157 mA h g^?1 at 0.1?C and 133 mA h g^?1 at 1?C between 2.5 and 4.2 V (1?C?=?170 mA g^?1). And no obvious capacity fading was observed upon cycling. The simple and convenient synthesis route is promising for large-scale production of LiFePO₄/C.     

Influence of surface modification by vanadium oxide and carbon on the electrochemical performance of LiFePO4/C

Surface modification with metal oxides is an efficient method to improve the performance of LiFePO₄. Carbon and V₂O₃ co-coated LiFePO₄ is synthesized by carbothermal reduction method combined with star-balling technique, and vanadium oxide is produced in situ. The structure and pattern of LiFePO₄/C modified with different amounts of vanadium oxide (0–5 mol%) were studied by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy. The electrochemical performance of material electrodes was analyzed by constant current charge–discharge and electrochemical impedance spectra (EIS). Electrochemical test results show that sample B (1.0 mol%) exhibits the best electrochemical performance, whose discharge capacity is up to 160.1, 127.2, and 88.4 mAh?g^?1 at 1, 5, and 10 °C, respectively. It indicates that V₂O₃ modification efficiently improves specific capacity and rate capability. The EIS experiment demonstrates that catalytic activity and reversibility of the cathode electrode are obviously increased by the surface modification of vanadium oxide.     

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.     

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.     

Properties of a gel polymer electrolyte based on lithium salt with poly(vinyl butyral)

Poly(vinyl butyral) (PVB) is of particular interest because of its low cost, extremely wide temperature work range (? 20 to 120 °C), and efficient chemical stability. In this study, a gel polymer electrolyte (GPE) containing Li⁺ ions was fabricated by using dimethylacetylamine (DMA), lithium perchlorate (LiClO₄), and PVB. The experimental results indicated that a highly transparent GPE with a high ionic conductivity (σ) could be obtained by mixing glue (DMA with a PVB content of 10 wt%) with a LiClO₄ content of 6 wt%. It was found that the ionic conductivity (σ) of the GPE depended on the LiClO₄ content, and the GPE with a LiClO₄ content of 6 wt% exhibited a maximum σ of 7.73 mS cm^?1, a viscosity coefficient of 3360 mPa s, and a transmittance greater than 89% (visible region) at room temperature. Furthermore, PVB improved the electrolyte solution leakage, and the LiClO₄ was used as an ion supply source for the high σ of the GPE.     

A carbothermal reduction method for enhancing the electrochemical performance of LiFePO4/C composite cathode materials

LiFePO₄/C composite cathode material has been synthesized by a carbothermal reduction method using β-FeOOH nanorods as raw materials and glucose as both reducing agent and carbon source. The results indicate that the content of carbon and the morphology of raw material have effect on the electrochemical performance of the final LiFePO₄/C material. Sample LFP14 with a carbon content of 2.79 wt.% can deliver discharge capacities of 158.8, 144.3, 111.0, and 92.9 mAh g^?1 at 0.1, 1, 10, and 15 C, respectively. When decreasing the current from 15 C back to 0.1 C, a discharge capacity of 157.5 mAh g^?1 is recovered, which is 99.2 % of its initial capacity. Therefore, as a kind of cathode material for lithium ion batteries, this LiFePO₄/C material synthesized via a carbothermal reduction method is promising in large-scale production, and has potential application in upcoming hybrid electric vehicles or electric vehicles.     

An investigation on the performance of LiFePO4/C derived from various FePO4

LiFePO₄/C composites were synthesized by carbothermal reduction method using commercial FePO₄ and Tween#80-assisted synthesized nano-FePO₄ as starting materials, glucose as reducing agent, and also carbon source. The FePO₄ intermediates were characterized by X-ray diffraction and scanning electron microscopy. A suitable mole ratio of Li to Fe was investigated, and the performances of samples synthesized under different temperatures were studied. It seems that the residual carbon content, which determine the electrochemical polarization of the cathode composites, greatly depend on the synthesis temperature when carbothermal reduction method was used. The electrochemical measurements showed that the discharge capacity first increase and then decrease with the rise of temperature. The optimal sample synthesized at 600 °C for 10 h using homemade FePO₄ as iron source exhibit 142 mAh?g^?1 at 0.2 C and a capacity retention rate of 98.8 % after 50 cycles.     

Electrochemical performance of Mo-doped LiFePO4/C composites prepared by two-step solid-state reaction

To improve the performance of LiFePO₄, LiFe_1?x Mo _x PO₄/C (x?=?0, 0.005, 0.010, 0.015, 0.020, 0.025) cathode materials were synthesized via two-step ball milling solid-state reaction. The prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectra, and galvanostatic charge–discharge test. It is apparent from XRD analysis that Mo doping enlarges the interplanar distance of crystal plane parallel to [010] direction in LiFePO₄. In other words, it widens one-dimensional diffusion channels of Li⁺ along the [010] direction. The results of electrochemical test indicate that the LiFe_0.99Mo_0.01PO₄/C composite exhibits a discharge capacity of 144.8 mAh g^?1 at 1 C rate, a decreased charge transfer resistance of 162.4 Ω and better reversibility of electrode reactions. The present synthesis route is promising and practical for the preparation of LiFePO₄ materials.     

A polymer electrolyte based on poly(vinylidene fluoride-hexafluoropylene)/hydroxypropyl methyl cellulose blending for lithium-ion battery

A polymer electrolyte based on the blending of poly(vinylidene fluoride-hexafluoropylene) (PVDF-HFP) and hydroxypropyl methyl cellulose (HPMC) was prepared for the first time. The structure and performance of the gel polymer electrolyte were characterized and measured by X-ray diffraction, Fourier transform infrared, thermogravimetric analysis, scanning electron microscopy, electrochemical impedance spectroscopy, linear sweep voltammetry, and by a charge/discharge test. The results show that the gel polymer electrolyte has the best performance when PVDF-HFP/HPMC ratio (w/w) is 4:1. At room temperature, the ionic conductivity can reach 0.38?×?10^?3 S cm^?1, the electrochemical stable window is up to 5.0 V (vs. Li/Li⁺), and the half cell of Li/GPE/LiMn₂O₄ shows high-discharge-specific capacity and good cycling performance.     

Synthesis and electrochemical behavior of Li2CoSiO4 cathode with pyrrolidinium-based ionic liquid electrolyte for lithium ion batteries

Li₂CoSiO₄, a silicate olivine cathode for lithium rechargeable batteries, is synthesized for the first time by sol–gel method using polyacrylic acid (PAA) as the chelating agent. Coupled thermal and vibrational analysis of the gel and also the X-ray diffraction pattern confirms the formation of the sample at 800 °C. 1-Butyl-1-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMPyTFSI) solutions of lithium bis(trifuloromethansulfonyl)imide (LiTFSI) having a concentration of 0.2 mol kg^?1 is used as electrolyte. The electrochemical stability window of this electrolyte is found to be >5 V by linear sweep voltammetry technique. The compatibility of Li₂CoSiO₄ with 0.2 mol kg^?1 LiTFSI-BMPyTFSI electrolyte is tested by charge–discharge cycles which show charging and discharging capacities of about 204 and 32 mAh g^?1, respectively, in the first cycle.