Search

Abstract: Carbon-coated lithium iron phosphate (LiFePO4/C) composites were synthesized by conventional mechanical activation (MA) process and also by a modified MA process. Phase-pure particles were obtained of ∼100nm size with a nano-meter thick web of carbon surrounding the particles. The composite prepared by the modified MA process shows good performance as cathode material in lithium cells at room temperature. A high performance was achieved at 0.1C-rate with >96% utilization of the active material. A stable cycle performance even at higher C-rates was achieved with a cathode that has a total carbon content of only 12wt%. The use of the modified MA process to synthesize LiFePO4/C has promise to be an efficient process to decrease the total carbon content of the cathode, resulting in the enhanced energy density. [Copyright &y& Elsevier]

Abstract: The effect of incorporating varying amounts (in the range 2.5–10vol%) of toluene additive in 1M LiCF3SO3 in tetra(ethylene glycol) dimethyl ether (TEGDME) liquid electrolyte on the electrochemical performance of Li/S cell at room temperature was studied. Cyclic voltammetry measurements showed the same active voltage range for the electrolytes with and without toluene, consisting of an oxidation peak at 2.5V and a pair of reduction peaks at 2.35 and 1.9V. Electrolytes with toluene have higher redox currents resulting from increased ion mobility and ionic conductivity. Toluene addition enhanced initial discharge capacity; a maximum of 750mAhg−1 (45% of the theoretical specific capacity of sulfur) was obtained with 5% of toluene, which was 1.8 times that of the cell without toluene. The electrolyte with 5% toluene exhibited a stable cycle performance with the highest discharge capacity and charge–discharge efficiency. AC impedance analyses of Li/S cells with the electrolytes showed that toluene addition resulted in a lower initial interfacial resistance and a fast stabilization of electrode/electrolyte interfaces in the cell. Addition of toluene in low amounts is thus an effective means to enhance the electrochemical performance of 1M LiCF3SO3 in TEGDME electrolyte in Li/S cells at room temperature. [Copyright &y& Elsevier]

Abstract: Carbon-coated LiFePO4 (LiFePO4/C) composite cathode-active materials of submicron size and phase-pure olivine structure were synthesized by a mechanical activation (MA) process. The conductive carbon coating was achieved by addition of (i) carbon powder directly or (ii) sucrose as the carbon precursor, during synthesis. LiFePO4/C containing 6wt% carbon prepared by the two methods showed good electrochemical properties as cathodes in lithium batteries at room temperature with high active material utilization of >94% at 0.1C rate and stable cycle performance. The sucrose precursor method leads to the formation of a thin, porous and more uniform carbon coating around the particles and results in a slightly higher discharge capacity (∼3%) compared to the other sample. [Copyright &y& Elsevier]

Abstract: Poly(vinylidene fluoride-co-hexafluoropropylene) {P(VdF-HFP)} membranes incorporating 0, 6 and 10wt.% of nano-meter sized particles of SiO2 were prepared by electrospinning. These membranes served as host matrix for the preparation of polymer electrolytes (PEs) by activating with the non-volatile and safe room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (BMITFSI). The membranes consisted of layers of fibers with average fiber diameter of 2–5μm and had a porosity of ∼87%. PEs with SiO2 exhibited higher ionic conductivity with a maximum of 4.3×10−3 Scm−1 at 25°C obtained with 6% SiO2. The optimum PE based on the membrane with 6% SiO2 exhibited better compatibility with lithium metal electrode on storage and resulted in enhanced charge–discharge performance in Li/LiFePO4 cells at room temperature, delivering the theoretical specific capacity of 170mAhg−1 at 0.1C-rate. The PEs exhibited a very stable cycle property as well, demonstrating their suitability for lithium battery applications. [Copyright &y& Elsevier]

Abstract: A new class of polymer electrolytes (PEs) based on an electrospun polymer membrane incorporating a room-temperature ionic liquid (RTIL) has been prepared and evaluated for suitability in lithium cells. The electrospun poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) membrane is activated with a 0.5M solution of LiTFSI in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) or a 0.5M solution of LiBF4 in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4). The resulting PEs have an ionic conductivity of 2.3×10−3 Scm−1 at 25°C and anodic stability at >4.5V versus Li+/Li, making them suitable for practical applications in lithium cells. A Li/LiFePO4 cell with a PE based on BMITFSI delivers high discharge capacities when evaluated at 25°C at the 0.1C rate (149mAhg−1) and the 0.5C rate (132mAhg−1). A very stable cycle performance is also exhibited at these low current densities. The properties decrease at the higher, 1C rate, when operated at 25°C. Nevertheless, improved properties are obtained at a moderately elevated temperature of operation, i.e. 40°C. This is attributed to enhanced conductivity of the electrolyte and faster reaction kinetics at higher temperatures. At 40°C, a reversible capacity of 140mAhg−1 is obtained at the 1C rate. [Copyright &y& Elsevier]

Abstract: The effect of incorporating a room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI), in the polymer electrolyte (PE) based on poly(ethylene oxide)-(lithium bis(trifluoromethanesulfonyl) imide) [PEO-LiTFSI] was investigated. BMITFSI content was varied between 20 and 80 parts by weight (pbw) in 100 pbw of PEO-LiTFSI and the influence on ionic conductivity, electrochemical stability and interfacial properties on lithium electrode was studied. A remarkable increase in ionic conductivity was achieved with BMITFSI addition, the effect being most pronounced at lower temperatures. Compared to PEO-LiTFSI, the PEs containing BMITFSI exhibited well-defined redox peaks corresponding to stripping and deposition of lithium. The PEs containing BMITFSI exhibited good electrochemical stability and significantly low interfacial resistance with the lithium electrode. Good discharge performance with 82% active material utilization and stable cycling property was achieved when the PE containing 60 pbw of BMITFSI was evaluated in Li/LiFePO4 cells at 40 °C. [Copyright &y& Elsevier]

Abstract: Electrospinning parameters are optimized for the preparation of fibrous membranes of poly(vinylidene fluoride-co-hexafluoropropylene) {P(VdF-HFP)} that consist of layers of uniform fibres of average diameter 1μm. Electrospinning of a 16wt.% solution of the polymer in acetone/N,N-dimethylacetamide (DMAc) (7/3, w/w) at an applied voltage of 18kV results in obtaining membranes with uniform morphology. Polymer electrolytes (PEs) are prepared by activating the membrane with liquid electrolytes. The fully interconnected porous structure of the host polymer membrane enables high electrolyte uptake and ionic conductivities of 10−3 Scm−1 order at 20°C. The PEs have electrochemical stability at potentials higher than 4.5V versus Li/Li+. A PE based on a membrane with 1M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC), which exhibits a low and stable interfacial resistance on lithium metal, is evaluated for discharge capacity and cycle properties in Li/LiFePO4 cells at room temperature and different current densities. A remarkably good performance with a high initial discharge capacity and low capacity fading on cycling is obtained. [Copyright &y& Elsevier]

Abstract: Pure, nano-sized LiFePO4 and carbon-coated LiFePO4 (LiFePO4/C) positive electrode (cathode) materials are synthesized by a mechanical activation process that consists of high-energy ball milling and firing steps. The influence of the processing parameters such as firing temperature, firing time and ball-milling time on the structure, particle size, morphology and electrochemical performance of the active material is investigated. An increase in firing temperature causes a pronounced growth in particle size, especially above 600°C. A firing time longer than 10h at 600°C results in particle agglomeration; whereas, a ball milling time longer than 15h does not further reduce the particle size. The electrochemical properties also vary considerably depending on these parameters and the highest initial discharge capacity is obtained with a LiFePO4/C sample prepared by ball milling for 15h and firing for 10h at 600°C. Comparison of the cyclic voltammograms of LiFePO4 and LiFePO4/C shows enhanced reaction kinetics and reversibility for the carbon-coated sample. Good cycle performance is exhibited by LiFePO4/C in lithium batteries cycled at room temperature. At the high current density of 2C, an initial discharge capacity of 125mAhg−1 (73.5% of theoretical capacity) is obtained with a low capacity fading of 0.18% per cycle over 55 cycles. [Copyright &y& Elsevier]

Abstract: The phospho-olivines LiMPO4 (where M denotes Fe, Mn, Co, Ni) have attracted a lot of attention in recent years as potential positive electrode materials for lithium batteries. The solid–solution Li(Mn y Fe1− y )PO4 has been particularly promising since it operates at the desirable cell voltage of 3.4–4.1V vs. Li+/Li. In this work, Li(Mn0.4Fe0.6)PO4 synthesized by a modified mechanical activation method was evaluated for electrochemical performance as the cathode material in lithium metal battery at room temperature. Phase-pure particles of size ∼100nm were prepared with a conductive, thin web of carbon surrounding them, by incorporating carbon powder as additive during the synthesis. Cyclic voltammetry showed the active voltage range of the material which consists of two pairs of redox peaks: 3.47V corresponding to Fe3+/Fe2+ reaction and 4.02V corresponding to Mn3+/Mn2+ reaction. An initial discharge capacity of 139mAh/g and a stable cycling property were attained at 0.1 C-rate, attributed to the small particle size and the effective conductive coating achieved for the material by the efficient synthesis process. [Copyright &y& Elsevier]

Abstract: An efficient synthesis based on mechanical activation (MA) was developed for carbon-coated lithium iron phosphate (LiFePO4/C). The conventional MA process was modified by introducing two initial steps of slurry phase blending of the ingredients and solvent removal by rotary evaporation, so as to get an intimate mixing and homogenous dispersion of conductive carbon in the sample. Phase-pure, nanometer-sized particles of the active material covered with a porous, nanometer-sized web of carbon were obtained. LiFePO4/C exhibited remarkably good electrochemical properties when evaluated as cathodes in room temperature lithium cells. An initial discharge capacity of 166 mAh/g (corresponding to 97.6% of theoretical capacity) was achieved at 0.1 C-rate. A very stable cycle performance was also realized; good capacity retention up to 100 cycles was achieved at different current densities. [Copyright &y& Elsevier]

Abstract: The suitability of some single/binary liquid electrolytes and polymer electrolytes with a 1M 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 450mAhg−1 was obtained with a medium content (12μl) 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 5vol.% of toluene in TEGDME had a remarkable effect of increasing the initial discharge capacity from 386 to 736mAhg−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. [Copyright &y& Elsevier]