LiPF6 In Ethylene Carbonate Research Articles

A High Precision Coulometry Study of the SEI Growth in Li/Graphite Cells

The charge and discharge endpoint capacities as well as the coulombic efficiency of Li/graphite coin cells have been examined using the high precision charger at Dalhousie University. Cells were charged and discharged at different C-rates and temperatures to observe trends in the formation of the solid electrolyte interphase (SEI) on the graphite electrode. The experiments show that time and temperature, not cycle count, are the dominant contributors to the growth of the SEI. The charge consumed by the SEI and hence the SEI thickness, increase approximately with time1/2 consistent with a process where the temperature-dependent SEI growth rate is inversely proportional to the SEI thickness. The charge consumed by the SEI is proportional to the electrode surface area and this increased consumption on high surface area electrodes continues during cycling, at least with the 1 M LiPF6 ethylene carbonate:diethyl carbonate electrolyte used.

Electrochemical reaction of lithium with CoCl 2 in nonaqueous electrolyte

The electrochemical properties of cobalt chloride was investigated in 1 M LiPF 6-ethylene carbonate (EC)/diethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1:1:1 by volume) electrolyte within 3.0 V to 0 V. We for the first time found that CoCl 2 material can electrochemically reversibly react with lithium via a conversion reaction, and delivers a reversible capacity of ca. 400 mA h g −1 at the 1 C rate at room temperature. This finding proves that conversion reactions in lithium-ion batteries can be expanded to transition-metal chlorides.

Influence of surface modification of LiCoO2 by organic compounds on electrochemical and thermal properties of Li/LiCoO2 rechargeable cells

LiCoO2 is the most famous positive electrode (cathode) for lithium ion cells. When LiCoO2 is charged at high charge voltages far from 4.2V, cycleability of LiCoO2 becomes worse. Causes for this deterioration are instability of pure LiCoO2 crystalline structure and an oxidation of electrolyte solutions LiCoO2 at higher charge voltages. This electrolyte oxidation accompanies with the partial reduction of LiCoO2. We think more important factor is the oxidation of electrolyte solutions. In this work, influence of 10 organic compounds on electrochemical and thermal properties of LiCoO2 cells was examined as electrolyte additives. As a base electrolyte solution, 1M (M: molL−1) LiPF6-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio=3:7) was used. These compounds are o-terphenyl (o-TP), Ph-X (CH3)n (n=1 or 2, X=N, O or S) compounds, adamantyl toluene compounds, furans and thiophenes. These additives had the oxidation potentials (Eox) between 3.4 and 4.7V vs. Li/Li+. These Eox values were lower than that (6.30V vs. Li/Li+) of the base electrolyte. These additives are oxidized on LiCoO2 during charge of the LiCoO2 cells. Oxidation products suppress the excess oxidation of electrolyte solutions on LiCoO2. As a typical example of these organic compounds, o-TP (Eox: 4.52V) was used to check the fundamental properties of these organic additives. Charge–discharge cycling tests were carried out for the Li/LiCoO2 cells with and without o-TP. Constant current charge at 4.5V is mainly used as a charge method. Cells with 0.1wt.% o-TP exhibited slightly better cycling performance and lower polarization than those without additives. Lower polarization arises from a decrease in a resistance of interface between electrolyte solutions and LiCoO2 by surface film formation resulted from oxidation of o-TP. Oxidation products were found by mass spectroscopy analysis to be mixture of several polycondensation compounds made from two to four terphenly monomers. Thermal stability of LiCoO2 with electrolyte solutions did not improve by addition of o-TP. Slightly better charge–discharge cycling properties were obtained by using organic modifiers. However, when industrial applications were considered, drastic improvements have not been obtained yet. One of reasons may be too large influence of the deterioration of stability of pure LiCoO2 structure at high voltage charging for industrial use. We hope to realize the tremendous improvements of high energy, long cycle life and safe lithium cells by the combination of both LiCoO2 with more stable structure such as LiCoO2 treated with MgO and new organic additives with molecular structure more carefully designed.

Electrochemical performance of all-solid-state lithium secondary batteries using Li4Ti5O12 electrode and Li2S–P2S5 solid electrolytes

All-solid-state Li–In/Li4Ti5O12 cells using Li2S–P2S5 solid electrolytes were assembled to investigate their electrochemical properties in the wide voltage range of 0–3 V (versus Li). The Li/Li4Ti5O12 cells using 1 M LiPF6 in ethylene carbonate and diethyl carbonate were fabricated for comparison with the all-solid-state cells. The capacity of the all-solid-state cell using the 70Li2S·27P2S5·3P2O5 (mol%) solid electrolyte decreased with an increase in the current density as well as the cell using the liquid electrolyte. However, the all-solid-state cell was charged and discharged even at a high current density of 10 mA/cm2. The all-solid-state cell was cycled at 1.3 mA/cm2 and retained 90% of the first reversible capacity of about 120 mAh/g after 500 cycles. The all-solid-state cell cycling at 100 °C showed the small overpotential and reversible capacity of about 120 mAh/g at 13 mA/cm2.

Characterization of poly(vinylidenefluoride-co-hexafluoroprolylene) membranes containing nanoscopic AlO(OH)n filler with Li/LiFePO4 cell

This paper describes the nanoscopic AlO(OH)n filled in porous poly(vinylidenefluoride-co-hexafluoroprolylene) membranes by phase inversion technique. The membranes were gelled with 0.5M LiPF6 in ethylene carbonate and diethyl carbonate mixture for characterization studies. The inclusion of AlO(OH)n nanoparticles substantially enhances the ionic conductivity and mechanical and thermal stabilities, which were observed through ac impedance, tensile strength, and differential scanning calorimetry. For example, the ionic conductivity has been increased from 2.1×10−3 to 3.9×10−3 S cm−1 after the inclusion of nanoparticles. Similarly, the elongation break value is improved from 277% to 464% for the incorporation of nanoparticles. A morphological feature of the membrane was analyzed by scanning electron microscopy. Further, physicochemical properties, such as liquid uptake, porosity measurements, activation energy, and percentage of crystallinity, have also been presented. Finally, Li/polymer membrane/LiFePO4 cell was fabricated, and cycling performance of the cell was evaluated at C/10 rate. The cell delivers the initial discharge capacity 149 mAh/g at ambient temperature conditions.

Carbonate-modified siloxanes as solvents of electrolyte solutions for rechargeable lithium cells

Influence of mixing carbonate-modified siloxanes into LiPF 6-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge–discharge cycling properties of lithium was examined. As the solute, 1 M (M: mol L −1) LiPF 6 was used. As siloxanes, 4-(2-trimethylsilyloxydimethylsilylethyl)-1,3-dioxolan-2-one and 4-(2-bis(trimethylsilyloxy)methylsilylethyl)-1,3-dioxolan-2-one were investigated. These siloxanes are derivatives of butylene cyclic carbonate or vinyl ethylene carbonate. Charge–discharge cycling efficiencies of lithium metal anodes improved and an impedance of anode/electrolyte interface decreased by mixing siloxanes, compared with those in 1 M LiPF 6-EC/MEC alone. Slightly better cycling behavior of natural graphite anode was obtained by adding siloxanes. Si–C/LiCoO 2 cells exhibited better anode utilization and good cycling performance by using 1 M LiPF 6-EC/MEC + siloxane electrolytes. Thermal behavior of electrolyte solutions toward graphite–lithium anodes was evaluated with a differential scanning calorimeter. By adding siloxanes, temperature starting the large heat-output of graphite–lithium anodes with 1 M LiPF 6-EC/MEC electrolyte solutions shifted to higher temperature about 100 °C. However, amount of heat-output did not decrease by adding siloxanes.

Improved thermal stability of graphite electrodes in lithium-ion batteries using 4-isopropyl phenyl diphenyl phosphate as an additive

To enhance the thermal stability of graphite electrodes for lithium-ion batteries, 4-isopropyl phenyl diphenyl phosphate (IPPP) was investigated as an additive in the electrolyte of 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 in weight). The electrochemical performance of Li/IPPP-electrolyte/C half cells was evaluated. The thermal behavior of LixC6 and LixC6-IPPP-electrolytes were examined using a C80 micro-calorimeter. Electrolytes with 5 and 10% IPPP improve the thermal stability of the graphite electrode in the tests. The electrochemical performance of Li/IPPP-electrolyte/C cells is not degraded by the addition of this amount of IPPP to the electrolyte.

Effect of Fluoroethylene Carbonate Additive on the Performance of Lithium Ion Battery

Fluoroethylene carbonate (FEC) with a volume ratio of 2% was added to the electrolyte containing 1 mol· L-1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) (1∶1∶1 by volume). The effects of FEC on lithium ion battery performance and on the mesocarbon microbead (MCMB) electrode/electrolyte interphase were studied by cyclic voltammetry (CV), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electrochemical impedance spectroscopy (EIS). The results indicated that the application of a 2% (volume ratio) of FEC suppressed electrolyte decomposition and caused the formation of an excellent solid electrolyte interphase (SEI) film on the MCMB electrode. The battery resistance decreased while the specific capacity and cyclic stability of the battery increased.

Li-Ion Electrolytes Containing Ester Co-Solvents for Wide Operating Temperature Range

As part of our continuing efforts to develop advanced electrolytes to improve the performance of lithium-ion cells, especially at low temperatures, we have identified a number of electrolyte co-solvents that can be incorporated into multi-component electrolyte formulations for enhanced performance, especially at very low temperatures (down to -70oC). In the current work, we investigated a number of ester co-solvents, namely methyl propionate (MP), ethyl propionate (EP), methyl butyrate (MB), ethyl butyrate (EB), propyl butyrate (PB), and butyl butyrate (BB), in multi-component electrolytes of the following composition: 1.0 M LiPF6 in ethylene carbonate (EC) + ethyl methyl carbonate (EMC) + X (20:60:20 v/v %) [where X = ester co-solvent]. These electrolytes have been optimized to provide good low temperature performance (down to -60oC) while still offering reasonable high temperature resilience to produce the desired wide operating temperature systems (-60 to +60oC). This has primarily been achieved by fixing the EC-content at 20% and the ester co-solvent at 20%, in contrast to the previously developed systems.

Electrochemical properties of LiFePO4/C synthesized by mechanical activation using sucrose as carbon source

Olivine lithium iron phosphate (LiFePO4) is attracting much attention as a safe, low cost and high capacity cathode material for lithium batteries, especially for applications in electric and hybrid-electric vehicles. In the present work, carbon-coated LiFePO4 (LiFePO4/C) materials are synthesized from lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and sucrose (varying content) as starting materials, by the process of mechanical activation followed by thermal treatment. A uniform, in situ coating of carbon surrounding the crystals of LiFePO4 is achieved by the pyrolysis of sucrose during the thermal treatment. The structural properties of LiFePO4/C are characterized by X-ray diffraction, and the morphological characteristics are studied by electron microscopy methods. Phase-pure particles of approximately 100 nm size and a homogeneous particle size distribution are obtained with 6 wt% carbon content. The electrochemical properties are evaluated at room temperature by cyclic voltammetry and charge–discharge performance in coin cells fabricated with lithium metal anode, LiFePO4/C cathode, and 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) electrolyte. LiFePO4/C containing 6 wt% of carbon exhibits high discharge capacity of 165 mAh/g (corresponding to 97% of theoretical capacity) at 0.1 C-rate and 145 mAh/g at 1 C-rate. Good rate capability and stable cycle performance are realized for lithium cells employing LiFePO4/C prepared by adding the cheap and easily available sucrose as carbon source during the synthesis of LiFePO4.

Low Temperature Electrolyte for Li-Ion Batteries

Electrolyte of LiPF6-ethylene carbonate (EC)-methyl acetate (MA) was prepared. 1 M LiPF6-EC-MA electrolyte presented the conductivity in excess of 1 mS cm-1 at -50 oC. The cyclic voltammetry showed that the electrolyte was better than conventional electrolyte of LiPF6-ethylene carbonate (EC)-diethyl carbonate (DEC) for the formation of SEI film. 1 M LiPF6-EC-MA electrolyte showed much better cycleability than conventional electrolyte of 1 M LiPF6-EC-DE·C for LiCoO2/Li cell. It is a promising alternative electrolyte for Li-ion batteries.

Poly-ether modified siloxanes as electrolyte additives for rechargeable lithium cells

Influence of four poly-ether modified siloxanes as electrolyte additives on charge–discharge cycling properties of lithium was examined. As siloxanes, diethylene glycol methyl-(3-dimethyl(trimethylsiloxy)silyl propyl)ether (sample A), diethylene glycol methyl-(3-dinethyl(trimethylsiloxy)silyl propyl)-2-methylpropyl ether (sample B), diethylene glycol methyl-(3-bis(trimethylsiloxy)silyl propyl)ether (sample C) and diethylene glycol-(3-methyl-bis(trimethylsiloxy)silyl-2-methylpropyl)ether (sample D) were investigated. As a base electrolyte solution, 1 M (M, mol L −1) LiPF 6-ethylene carbonate (EC)/methylethyl carbonate (MEC) (mixing volume ratio = 3:7) was used. As the anodes, lithium metal, natural graphite carbon and silicon–SiO 2–carbon (Si–C) composite electrodes were used. Lithium cycling efficiencies of these three anodes improved and an impedance of anode/electrolyte interface decreased by adding poly-ether modified siloxanes. Graphite/LiCoO 2 and Si–C/LiCoO 2 cells exhibited better anode utilization and good cycling performance by using 1 M LiPF 6–EC/MEC + siloxane electrolytes. It was also found that thermal stability of the electrolyte solutions improved by adding siloxanes. Thermal decomposition temperature of 1 M LiPF 6–EC/MEC shifted to higher temperature by adding siloxanes. Amount of heat-output of graphite–lithium anodes with M LiPF 6–EC/MEC electrolyte solutions decreased and the temperature starting the heat-output shifted to higher temperature by adding siloxanes. Among siloxanes examined here, samples B and D exhibited much better performance.

New Insight into the Solid Electrolyte Interphase with Use of a Focused Ion Beam

The formation and evolution of the solid electrolyte interphase (SEI) film on the surface of natural graphite spheres in the electrolyte of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:1) were investigated with use of focused ion beam (FIB) technology. Secondary electron FIB images clearly show the surface and cross-section morphology of the SEI film. The composition variation along the surface and cross section of the SEI film was also explored by the elemental line scan analysis (ELSA). The initial SEI film with an apparent thickness range of approximately 450 to approximately 980 nm is rough in morphology and nonuniform in composition, and contains small splits. After certain electrochemical cycles, the thickened SEI film displays microscale holes and obvious cracks on the surface, and the content of organic compounds increases. In addition, the concept of "internal SEI film" is first proposed based on the characterization of the cross section of the natural graphite spheres with the aid of FIB. Finally, the capacity fading mechanisms of the natural graphite spheres corresponding to different electrochemical stages are discussed.

Thermalgravimetry–mass spectrometry studies on the thermal stability of graphite anodes with electrolyte in lithium-ion battery

Thermal reactions of a lithiated graphite anode in 1 M LiPF 6-ethylene carbonate (EC)/dimethyl carbonate (DMC) (50:50 vol.%) in the temperature range 40–320 °C were investigated by TG–MS analysis. Studies by TG–MS during thermal reactions detected a small exothermic peak around 140 °C due to CO 2 ( m/ z = 44) evolution, which suggests partial destruction of the SEI formed on the graphite and/or decomposition of the electrolyte through the SEI. In addition, the main exothermic reaction above 280 °C, which is associated with simultaneous evolution of C 2H 4O ( m/ z = 44), is caused by direct reaction of the lithiated graphite with solvent.

Fabrication and characterization of a LiCoO 2 battery–supercapacitor combination for a high-pulse power system

The performance of portable electronic equipment can often be improved by including an electrochemical capacitor alongside the battery. The capacitor extends battery life by reducing its peak output power. A CR2032 coin-type battery cell was used for the LiCoO 2 cathode and Li anode. A mixture of 1 M LiPF 6-ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 vol, Merk) was used as the electrolyte. The electrochemical characteristics of a supercapacitor are able to provide a much higher pulse current capability than the battery system. By combining a supercapacitor and a battery, the pulse performance of a battery can be significantly improved according to various pulse times.

Preparation and Characterization of Tin/Carbon Composites for Lithium-Ion Cells

AbstractA number of Sn/C composites were prepared for evaluation as anode materials for Li-ion cells. In one case, samples were prepared by incorporation of Sn species into organic precursors that were then pyrolyzed under an Ar/H2 cover gas to prepare the Sn/C composites. They were also prepared by decoration of various types of carbon with nanoparticles of Sn by electroless deposition using hydrazine. The carbons examined included a disordered carbon prepared in house from poly(methacrylonitrile), a mesocarbon microbead (MCMB) carbon, and a platelet graphite. The Sn/C composites were examined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) and were also analyzed for Sn content. They were then tested as anodes in three-electrode cells against Li metal using 1M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) solution. The best overall electrochemical performance was obtained with a Sn/C composite made by electroless deposition of 10% Sn onto platelet graphite.

In-situ spectroelectrochemical analysis of the passivating surface film formed on a carbon film electrode as a function of the water content in 1 M LiPF 6 -EC/DEC solution

The in-situ spectroelectrochemical technique has been applied to investigate the role of water in the formation of a passivating surface film on a plasma enhanced chemical vapor deposited (PECVD) carbon film electrode in 1 M LiPF6-ethylene carbonate (Li-EC) and diethyl carbonate (DEC) solution, combined with cyclic voltammetry. In-situ Fourier transform infra-red (FTIR) spectra of the surface film showed that all the peak intensities of the Li2CO3, ROCO2Li, and LiPF6 constituents significantly increase with increasing water content under application of the negative potentials with respect to open circuit potential (OCP). It is suggested that the reduction of Li-EC to ROCO2Li runs via a one-electron transfer pathway with the help of the unrestricted supply of the electron transfer path as a result of diffusion of water through the surface film; then Li2CO3 formation proceeds concurrently by the chemical reaction of ROCO2Li with water. Moreover, the compact sedimentation of ROCO2Li in the presence of water in the electrolyte is subjected to severe interference of the salt reduction product, LixPFy, than in the absence of water in the electrolyte. These FTIR results coincide well with those of cyclic voltammetry. From the combined results of in-situ FTIR spectroscopy and cyclic voltammetry, it is indicated that, unlike other salt and solvent reaction products, ROCO2Li, Li2CO3 and LixPFy simultaneously increase to constitute the outer layer of the surface film with equal amounts.