Gel electrolyte for lithium-ion batteries

The electrochemical performance of gel electrolytes based on crosslinked poly[ethyleneoxide-co-2-(2-methoxyethyoxy)ethyl glycidyl ether-co-allyl glycidyl ether] was investigated using graphite/Li_1.1[Ni_1/3Mn_1/3Co_1/3]_0.9O₂ lithium-ion cells. It was found that the conductivity of the crosslinked gel electrolytes was as high as 5.9 mS/cm at room temperature, which is very similar to that of the conventional organic carbonate liquid electrolytes. Moreover, the capacity retention of lithium-ion cells comprising gel electrolytes was also similar to that of cells with conventional electrolytes. Despite of the high conductivity of the gel electrolytes, the rate capability of lithium-ion cells comprising gel electrolytes is inferior to that of the conventional cells. The difference was believed to be caused by the poor wettability of gel electrolytes on the electrode surfaces.     

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).     

Enhanced rate performance of nano–micro structured LiFePO4/C by improved process for high-power Li-ion batteries

A spherical carbon-coated nano–micro structured LiFePO₄ composite is synthesized for use as a cathode material in high-power lithium-ion batteries. The composites are synthesized through carbothermal reduction with two sessions of ball milling (before and after pre-sintering of precursor) followed by spray-drying with the dispersant of polyethylene glycol added. The structure, particle size, and surface morphology of the cathode active material and the properties of the coated carbon are investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Results indicate that the LiFePO₄/C composite has a spherical micro-porous morphology composed of a large number of carbon-coated nano-spheres linked together with an ordered olivine structure. The carbon on the surface of LiFePO₄ effectively reduces inter-particle agglomeration of the LiFePO₄ particles. A galvanostatic charge–discharge test indicates that the LiFePO₄/C composites exhibit initial discharge capacities of 155 mAh g⁻¹ and 88 mAh g⁻¹ at 0.2 C and 20 C rates with the end of discharge voltage of 2.5 V, respectively. This behavior is ascribed to the unique spherical structure, which shortens lithium ions diffusion length and improves the electric contact between LiFePO₄ particles.     

Electrochemical characteristics of LiMxFe1−xPO4 cathode with LiBOB based electrolytes

The conductivity of LiBOB based electrolyte, which we formed in our lab, has been compared with commercialized LiPF₆ based electrolyte firstly. The charge and discharge capacity of the LiFePO₄/Li half-cell with two kind of electrolyte has compared both at room temperature and elevated temperature. LiBOB cell presented better charge/discharge stability at elevated temperature than the counterpart. ICP method was adopted to analyze the reaction of electrolytes with cathode material at high temperature. Cyclic voltammogram was conducted to analyze the charge/discharge process of the cell in both LiBOB based electrolyte and LiPF₆ based electrolyte at different temperature.     

High flash point electrolyte for use in lithium-ion batteries

The high flash point solvent adiponitrile (ADN) was investigated as co-solvent with ethylene carbonate (EC) for use as lithium-ion battery electrolyte. The flash point of this solvent mixture was more than 110 °C higher than that of conventional electrolyte solutions involving volatile linear carbonate components, such as diethyl carbonate (DEC) or dimethyl carbonate (DMC). The electrolyte based on EC:ADN (1:1 wt) with lithium tetrafluoroborate (LiBF₄) displayed a conductivity of 2.6 mS cm⁻¹ and no aluminum corrosion. In addition, it showed higher anodic stability on a Pt electrode than the standard electrolyte 1 M lithium hexafluorophosphate (LiPF₆) in EC:DEC (3:7 wt). Graphite/Li half cells using this electrolyte showed excellent rate capability up to 5C and good cycling stability (more than 98% capacity retention after 50 cycles at 1C). Additionally, the electrolyte was investigated in NCM/Li half cells. The cells were able to reach a capacity of 104 mAh g⁻¹ at 5C and capacity retention of more than 97% after 50 cycles. These results show that an electrolyte with a considerably increased flash point with respect to common electrolyte systems comprising linear carbonates, could be realized without any negative effects on the electrochemical performance in Li-half cells.     

Electrochemical behavior of carbon-coated SnS2 for use as the anode in lithium-ion batteries

Carbon-coated SnS₂ nanoparticles were prepared by a simple solvothermal route at low temperature. A carbon coating with a thickness of about 5 nm was deposited on nano-sized SnS₂ particles to serve as the anode in lithium-ion batteries. Both the nanostructure and the morphology of the SnS₂ powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The coated samples were used as active anode materials for lithium-ion batteries, and their electrochemical properties were examined by constant current charge-discharge cycling, cyclic voltammetry and electrochemical impedance spectroscopy. The reversible capacity of the carbon-coated SnS₂ after 50 cycles was 668 mAh/g, which was much higher than that of the uncoated SnS₂ (293 mAh/g). The carbon-coated SnS₂ also had a better rate capability than the uncoated SnS₂ in the range of 0.008-1 C. The capacity retention of the carbon-coated SnS₂ was improved due to its good conductivity and the effective buffer matrix that alleviated volume expansion during the charge-discharge process.     

Microwave-assisted one-pot synthesis of SnC2O4/graphene composite anode material for lithium-ion batteries

Currently, SnC₂O₄ is considered as one of the most promising anode materials for high-energy lithium-ion batteries (LIBs) because its charge capacity is higher than that of metal oxides. Herein, a facile microwave-assisted solvothermal method was employed to obtain SnC₂O₄/GO composites within only 30?min, which is time-efficient. The amount of SnC₂O₄ was increased to 95.3?wt% to improve the capacity of the composite. Pure SnC₂O₄ with a high specific surface area of 19.6?m² g^?1 without any other tin compound was used for fabrication. The SnC₂O₄/GO composite exhibited excellent electrochemical performance, with reversible discharge/charge capacity of 657/659?mA?h?g^?1 after 100 cycles at 0.2?A?g^?1. Furthermore, at high current densities of 1.0 and 2.0?A?g^?1, the SnC₂O₄/GO composite anode exhibited high reversible discharge/charge capacities of 553/552 and 418/414?mA?h?g^?1, respectively, after 200 cycles at room temperature. These improvements were likely obtained because SnC₂O₄ was well composited with graphene, which not only offered rapid electron transfer but also released the tension produced by the volumetric effect during repeated lithiation/delithiation. Cyclic voltammetry (CV) was also performed to further study the electrochemical reactions of SnC₂O₄/GO. The facile microwave-assisted solvothermal method used herein is considered as a highly efficient method to fabricate metal oxalate/graphene composites for use as anode materials in LIBs.     

Electrochemical properties of anatase TiO2 nanotubes as an anode material for lithium-ion batteries

Titanium oxides with a one-dimensional nanostructure are of great significance in electrochemical lithium insertion due to their high specific surface area and pore volume. In this paper, anatase TiO₂ nanotubes with diameters of about 10 nm and lengths of 200–400 nm were synthesized by a hydrothermal process. The phase structure and morphology were analyzed by X-ray diffraction, Raman scattering, and transmission electron microscopy. The electrochemical properties were investigated by constant current discharge–charge and cyclic voltammetry. There is a potential plateau at 1.73 and 1.88 V in the process of Li insertion and extraction, and the initial Li insertion/extraction capacity is 290 and 238 mAh g⁻¹ at 36 mA g⁻¹, respectively. The Li insertion capacity at the potential plateau of 1.73 V in the first cycle is about 150 mAh g⁻¹. In the 20th cycle, the reversible capacity still remains at about 200 mAh g⁻¹, and the coulombic efficiency is approximately 98%, exhibiting excellent cycling stability. The discharging capacity is about 168 mAh g⁻¹ in the 30th cycle at 210 mA g⁻¹, demonstrating a good high-rate performance. Anatase TiO₂ nanotubes might be a promising negative material for lithium-ion batteries.     

Additives-containing functional electrolytes for suppressing electrolyte decomposition in lithium-ion batteries

Several olefinic compounds such as vinyl acetate, divinyl adipate and allyl methyl carbonate were studied as additives for propylene carbonate (PC)-based electrolytes in lithium-ion battery, which kind of electrolytes always exfoliate graphitic carbon and decompose drastically to liberate organic gas. Three kinds of graphitic carbons commonly used in lithium-ion batteries, namely, natural graphite, MCMB 6-28 and MCF were chosen to test the decomposition-suppressing ability of additives. The effects of the type of graphitic anodes and the structure of additives on the electrolyte decomposition have been investigated in the terms solid electrolyte interface (SEI) formation, which was characterized by cyclic voltammetry (CV), ac impedance, SEM, XPS analyses, and auger electron spectroscopy (AES). The electrochemical performance of the additives-containing electrolytes in combination with LiCoO₂ cathode and graphitic carbon anode was also tested in coin cells.     

Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol-formaldehyde polymer as carbon precursor

LiFePO₄/C composites were synthesized by two methods using home-made amorphous nano-FePO₄ as the iron precursor and soluble starch, sucrose, citric acid, and resorcinol-formaldehyde (RF) polymer as four carbon precursors, respectively. The crystalline structures, morphologies, compositions, electrochemical performances of the prepared powders were investigated with XRD, TEM, Raman, and cyclic voltammogram method. The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a deficient carbon coating on the surface of LiFePO₄ particle, but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO₄ particle, and the corresponding thicknesses of the uniform carbon films are 2.5 nm and 4.5 nm, respectively. When RF polymer was used as the carbon precursor, the material showed the highest initial discharge capacity (138.4 mAh g^− 1 at 0.2 C at room temperature) and the best rate performance among the four materials.     

Graphene as a conductive additive to enhance the high-rate capabilities of electrospun Li4Ti5O12 for lithium-ion batteries

Spinel Li₄Ti₅O₁₂ (LTO) is a promising candidate anode material for Li-ion batteries due to its well-known zero-strain merits. To improve the electronic properties of spinel LTO, which are intrinsically poor, we processed the material into a nanosized architecture to shorten the distance for Li-ion and electron transport using the versatile electrospinning method. Graphene was chosen as an effective carbon coating to improve the surface conductivity of the nanocomposites. The as-prepared graphene-embedded LTO anode material showed improved discharging/charging and cycling properties, particularly at high rates, such as 22 C, which makes the nanocomposite an attractive anode material for applications in electric vehicles.     

Effects of Cr doping on the electrochemical properties of Li2FeSiO4 cathode material for lithium-ion batteries

Cr-doped Li₂FeSiO₄ was prepared by a sol-gel method. The effects of Cr doping on the characteristics of Li₂FeSiO₄ were carefully investigated. Compared with the XRD pattern of the Li₂FeSiO₄ sample, the XRD patterns of the Cr-doped samples have no extra reflections. This indicates that Cr enters the structure of Li₂FeSiO₄ rather than forming impurities. As indicated by the charge-discharge measurements, the highest capacity is obtained by 3% Cr doping. The particle size of the Li₂Fe_0.97Cr_0.03SiO₄ sample was smaller than that of the Li₂FeSiO₄ sample, and the BET surface area of the Li₂Fe_0.97Cr_0.03SiO₄ sample was more than twice as high as that of the Li₂FeSiO₄ sample. Compared with the Li₂FeSiO₄ sample, the Li₂Fe_0.97Cr_0.03SiO₄ sample shows faster activation, higher reversible capacity, and better rate capability, which can be attributed to the smaller particle size and larger surface area as well as the crystal defects induced by Cr doping.     

One-step electrodeposition synthesis and electrochemical properties of Cu6Sn5 alloy anodes for lithium-ion batteries

Cu₆Sn₅ alloys were successfully electrodeposited on rough Cu foils and smooth Cu sheets using a facile one-step electrodepositing method, and their structural and electrochemical properties were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charging/discharging testing and electrochemical impedance spectroscopy (EIS). The influence of surface morphology of the current collectors on the cycleability and the interfacial performance of the Cu₆Sn₅ alloy electrode are both discussed. The results demonstrate that the Cu₆Sn₅ alloy electrode on the rough Cu foil presented better electrochemical performance than that on the smooth Cu sheet because its rough surface could buffer the volume changes to some extent. The first discharging (lithiation) and charging (delithiation) capacities were measured at 462 and 405 mAh g⁻¹ respectively with high initial coulomb efficiency of 88%, with charging capacity in the 50th cycle remaining 76% of that in the first cycle. The phase transformation during initial lithiation was detected by electrochemical impedance spectroscopy (EIS) and its trend versus electrode potential is also discussed.     

Preparation of carbon-coated TiO2 nanostructures for lithium-ion batteries

Carbon-coated TiO₂ one-dimensional nanostructures are synthesized by hydrothermal reaction followed by post-calcination at various temperatures. Post-calcination induces crystallization of TiO₂ and the complete crystallization of anatase phase is observed at 600 °C of the calcination temperature. Carbon-coated TiO₂ nanostructures show relatively poor crystallinity as compared with the pristine counterparts, but their lithiation capacity and high rate capability are improved throughout all calcination temperatures. The coated carbon suppresses severe agglomeration of TiO₂ nanotubes which allows easy access of Li-ions and electrons to the whole surface of primary nanotubes, leading to the better lithiation performance. Higher calcination temperatures cause excessive growth of nanotube walls, leading to the collapse of tubular morphology and deterioration of lithiation performance. At 700 °C of the calcination temperature, the enhanced electronic conductivity from the graphitization of the coated carbon seems to be the main reason for the improved capacity of TiO₂ nanowires.     

Efficient construction of a CoCO3/graphene composite anode material for lithium-ion batteries by stirring solvothermal reaction

Transition-metal carbonates have recently been investigated as anode materials for lithium-ion batteries because of their relatively high capacity compared with that of the corresponding transition-metal oxides. In this work, a facile stirring solvothermal reaction is used to prepare a CoCO₃/graphene composite without the use of an additional organic chelating agent. The as-prepared CoCO₃/graphene composite exhibits a smaller cubic particle size of 1–2 µm and a larger specific surface area than the composite obtained by a traditional solvothermal reaction. The composite prepared with stirring delivers a highly reversible capacity of 602 mAh g^?1 after 100 cycles. Even at a high current density of 2.0 A g^?1, the composite maintains charge–discharge capacities of 605/598 mAh g^?1. The composites contained the same amount of graphene, indicating that the improved electrochemical properties are attained independently of the amount of the graphene. In addition, the results of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)experiments also reveal that the CoCO₃/graphene composite electrode materials synthesised via a stirring solvothermal reaction exhibit substantially enhanced kinetics. The stirring solvo/hydrothermal reaction develops in this work is considered a promising candidate for efficiently preparing carbonate/graphene composites with better electrochemical properties for practical applications, without the use of an extra chelating agent.     

CuO/graphene composite as anode materials for lithium-ion batteries

CuO/graphene composite is synthesized from CuO and graphene oxide sheets following reduced by hydrazine vapor. As the electrode material for lithium-ion batteries, CuO nanoparticles with sizes of about 30 nm homogeneously locate on graphene sheets, and act as spacers to effectively prevent the agglomeration of graphene sheets, keeping their high active surface. In turn, the graphene sheets with good electrical conductivity server as a conducting network for fast electron transfer between the active materials and charge collector, as well as buffered spaces to accommodate the volume expansion/contraction during discharge/charge process. The synergetic effect is beneficial for the electrochemical performances of CuO/graphene composite, such as improved initial coulombic efficiency (68.7%) and reversible capacity of 583.5 mAh g⁻¹ with 75.5% retention of the reversible capacity after 50 cycles.     

An investigation of polypyrrole-LiFePO4 composite cathode materials for lithium-ion batteries

A series of polypyrrole-LiFePO₄ (PPy-LiFePO₄) composites were synthesised by polymerising pyrrole monomers on the surface of LiFePO₄ particles. AC impedance measurements show that the coating of polypyrrole significantly decreases the charge-transfer resistance of LiFePO₄ electrodes. The electrochemical reactivity of polypyrrole and PPy-LiFePO₄ composites for lithium insertion and extraction was examined by charge/discharge testing. The PPy-LiFePO₄ composite electrodes demonstrated an increased reversible capacity and better cyclability, compared to the bare LiFePO₄ electrode.     

Redox shuttles for safer lithium-ion batteries

Overcharge protection is not only critical for preventing the thermal runaway of lithium-ion batteries during operation, but also important for automatic capacity balancing during battery manufacturing and repair. A redox shuttle is an electrolyte additive that can be used as intrinsic overcharge protection mechanism to enhance the safety characteristics of lithium-ion batteries. The advances on stable redox shuttles are briefly reviewed. Fundamental studies for designing stable redox shuttles are also discussed.     

Electrochemical reactivity of Co-Li2S nanocomposite for lithium-ion batteries

The fabrication of Co-Li₂S nanocomposite thin film is reported by pulsed laser deposition (PLD) for the first time. Li₂S-Co nanocomposite thin film is used as storing Li electrodes that have led to promising electrochemical activity and good electrochemical performance. The releasing Li process from the as-deposited Li₂S-Co nanocomposite thin films is confirmed by the ex situ high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements and may come from the decomposition of Li₂S with and without the interaction of metal Co into CoS₂ and S. The electrochemical reaction mechanism of Co-Li₂S nanocomposite film electrode involving both the formation and decomposition of Li₂S and the lithium extraction/insertion of CoS₂ after the initial charging process is proposed. Our results demonstrate the advantages of using Co-Li₂S nanocomposite in storage lithium materials.     

V2O3/rGO composite as a potential anode material for lithium ion batteries

V₂O₃ is a promising anode material and has attracted the interests of researchers because of its high theoretical capacity of 1070?mAh?g^?1, low discharge potential, inexpensiveness, abundant sources, and environmental friendliness. However, the development and application of V₂O₃ have been hindered by the low conductivity and drastic volume change of V₂O₃ composites. In this work, V₂O₃/reduced graphene oxide (rGO) nanocomposites are successfully prepared through a facile solvothermal method and annealing process. In this synthesis protocol, V₂O₃ nanoparticles (NPs) are encapsulated by rGO. This unique structure enables rGO to inhibit volume changes and improve the ion and electronic conductivity of V₂O₃. In addition, V₂O₃ NPs, which exhibit sizes of 5–40?nm, are uniformly dispersed on rGO sheets without aggregation. The Li⁺ storage behavior of V₂O₃/rGO is systematically investigated in the potential range 0.01–3.0?V. The V₂O₃/rGO nanocomposite can achieve a high reversible specific capacity of 823.4?mAh?g^?1 under the current density of 0.1?A?g^?1, and 407.3 mAh g^?1 under the high current density of 4.0?A?g^?1. The results of this study provide insight into the fabrication of rGO-based functional materials with extensive applications.