On the difference in cycling behaviors of lithium-ion battery cell between the ethylene carbonate- and propylene carbonate-based electrolytes

Density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations have been performed to gain insight into the difference in cycling behaviors between the ethylene carbonate (EC)-based and the propylene carbonate (PC)-based electrolytes in lithium-ion battery cells. DFT calculations of the lithium solvation, Li⁺(S)_i (S = EC or PC; i = 1–4) with and without the presence of the counter anion showed that the desolvation energy to remove one solvent molecule from the first solvation shell of the lithium ion was significantly reduced by as much as 70 kcal mol⁻¹ (293.08 kJ mol⁻¹) in the presence of the counter anion, suggesting the lithium ion is more likely to be desolvated at high salt concentrations. The thermodynamic stability of the ternary graphite intercalation compounds, Li⁺(S)_iC₇₂, in which Li⁺(S)_i was inserted into a graphite cell, was also examined by DFT calculations. The results suggested that Li⁺(EC)_iC₇₂ was more stable than Li⁺(PC)_iC₇₂ for a given i. Furthermore, some of Li⁺(PC)_iC₇₂ were found to be energetically unfavorable, while all of Li⁺(EC)_i=1–4C₇₂ were stable, relative to their corresponding Li⁺(S)_i in the bulk electrolyte. In addition, the interlayer distances of Li⁺(PC)_iC₇₂ were more than 0.1 nm longer than those of Li⁺(EC)_iC₇₂. MD simulations were also carried out to examine the solvation structures at a high salt concentration of LiPF₆: 2.45 mol kg⁻¹. The results showed that the solvation structure was significantly interrupted by the counter anions, having a smaller solvation number than that at a lower salt concentration (0.83 mol kg⁻¹). We propose that at high salt concentrations, the lithium desolvation may be facilitated due to the increased contact ion pairs so as to form a stable ternary GIC with less solvent molecules without destruction of graphite particles, followed by solid–electrolyte-interface film formation reactions. The results from both DFT calculations and MD simulations are consistent with the recent experimental observations.     

Electrochemical impedance spectroscopic study of the intercalation of lithium and sodium ions into polyparaphenylene in carbonate-based electrolytes

Electrochemical impedance spectroscopy is used to investigate the electrochemical intercalation of lithium and sodium ions into polyparaphenylene under galvanostatic conditions in carbonate-based electrolyte. The evolution of the charge transfer resistance was studied at various selected potentials both during the reduction and the oxidation processes in order to control the stoichiometry of the intercalated compounds. The reversibility of the intercalation process, the effect of the cycling and the stability of the intercalated materials in the electrolyte as a function of the time were examined. A significant decrease of the charge transfer resistance occurs during the intercalation. That is related to an increase of the conductive state especially for the richest compounds Na_0.46(C₆H₄) and Li_0.50(C₆H₄).     

Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

The effect of desiccants on the cycling efficiency of the lithium electrode in propylene carbonate-based electrolytes

Cycling efficiencies of the Li electrode ,in propylene carbonate (PC) 1 M in either LiClO₄ or LiAsF₆ were assessed as a function of electrolyte purification procedure. The use of neutral alumina and galvanostatic pre-electrolysis resulted in the highest efficiency values to date. While cyclic voltammograms at Pt or vitreous C were insensitive to electrolyte impurities, voltammograms on Ni about the Li potential were very informative. Thus, the repeated deposition and subsequent removal of thin (2 mC/cm²) Li plates revealed enhanced nucleation but diminished rate of growth of the nuclei as cycling progresses. A model of Li encapsulation is proposed to account for the eventual failure of the Li electrode.     

Reprint of “Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver”

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Synthesis and properties of polystyrene/graphite nanocomposites

In this paper, graphite/polystyrene nanocomposite is synthesized by in situ polymerization of styrene in a tetrahydrofuran (THF) solution system of potassium (K)-THF-graphite intercalation compound (GIC). K-THF-GIC has proved to initiate polymerization of styrene by the anionic mechanism. Due to the interfacial interaction between the graphite nanolayers and the polymer, the composites exhibit higher glass transition temperature and higher thermal stability when compared to polystyrene. The percolation threshold in the conductivity of the composites is lesser than 8.2 wt% and the dielectric constant can reach as high as 136.     

Effects of copper trifluoromethanesulphonate as an additive to propylene carbonate-based electrolyte for lithium-ion batteries

Addition of copper trifluoromethanesulphonate (CuTF) to propylene carbonate (PC)-based electrolyte effectively suppresses the cointercalation and decomposition of PC in the mesocarbon microbeads (MCMB) electrodes during the first lithiation process. During the first charging cycle, copper ions are reduced at a higher potential (2 V versus Li/Li⁺) than the potential of PC cointercalation and decomposition (0.6-0.8 V versus Li/Li⁺), and predominately form a porous copper layer over the MCMB surface, thereby obstructing PC to cointercalate. An increase in reversible capacity can be achieved by increasing the amount of CuTF. However, above a critical value, the copper layer inhibits the intercalation of lithium ions and lowers the capacity. The AC impedance data reveal that the passivation film and the charge-transfer resistance are both increased when the deposited copper is in excess. An optimum result may be obtained when the addition is approximately 5 wt.%. CuTF is a possibility for PC-based electrolyte additive in lithium-ion batteries.     

Behaviour of highly crystalline graphitic materials in lithium-ion cells with propylene carbonate containing electrolytes: An in situ Raman and SEM study

The microcrystalline flaked graphites SFG6 and SFG44 were evaluated with regard to their compatibility with propylene carbonate (PC) by in situ Raman microscopy and postmortem scanning electron microscopy (SEM) study. PC is employed as electrolyte component in lithium-ion batteries. However, when used with certain types of graphitic materials, exfoliation occurs. To compare the effects of exfoliation, the first lithium insertion properties of these graphitic materials were measured with in situ Raman microscopy. Lithium half-cells containing either 1 M LiClO₄ 1:1 (w/w) ethylene carbonate (EC):dimethyl carbonate (DMC) or 1:1 (w/w) EC:PC were investigated. The commencement of the exfoliation process was detected in SFG44 EC:PC by the appearance of a shoulder band at 1597 cm⁻¹ on the G-band (1584 cm⁻¹) below 0.9 V versus Li/Li⁺. The band (assigned as the exfoliation or E-band) at higher wavenumbers (1597 cm⁻¹) corresponded to solvated lithium ions intercalated into graphite. The in situ Raman spectra of SFG6 in EC:DMC or EC:PC and SFG44 in EC:DMC did not show the E-band and instead displayed regular lithium intercalation spectra.In situ Raman microscopy and SEM were further employed to study the exfoliation process observed for SFG44 in 1:1 (w/w) EC:PC, when the potential was held under steady-state conditions at 0.8, 0.6 and 0.3 V, respectively. A blue-shift in the E-band from 1597 to 1607 cm⁻¹ was observed as the potential was lowered. SEM images showed dissimilar degrees of exfoliation at these three potentials.     

Characterisation of the SEI formed on natural graphite in PC-based electrolytes

The origin of the different Li⁺ intercalation behaviour of raw and jet-milled natural graphite has been investigated. Jet-milled graphite is found to cycle reversibly in equal solvent mixture of propylene carbonate (PC) and ethylene carbonate (EC), whereas raw graphite does not. Using both Al Kα and synchrotron radiation (SR) Photoelectron Spectroscopy, new insight is obtained into the formation of the solid electrolyte interphase (SEI) on the two different graphite materials during electrochemical cycling in 1 M LiPF₆ in either PC:EC (1:1) or in PC with 5% vinylene carbonate (VC) as additive. Solvent reduction products are found at the surface of both raw and jet-milled graphite cycled in PC:EC (1:1), but differed in composition. The addition of VC reduces primarily the quantities of salt reaction products (LiF and Li_xPF_y compounds) and produces a mainly organic SEI layer. Electron diffraction from the edges for raw and jet-milled graphite particles shows a more disordered surface structure in the jet-milled particles than in the raw graphite. The more disordered surface structure can serve as a physical barrier hindering PC co-intercalation and facilitating the formation of a stable SEI layer.     

Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced in order to obtain disordered carbons containing very small content of fluorine and different physical properties according to the reduction treatment (chemical, thermal or electrochemical). After a physical characterization (X-ray diffraction, electron spin resonance and FT-IR spectroscopies), the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO₄-EC/PC, 50:50%, v/v). Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment. Semi-ionic carbon fluoride treated in F₂ atmosphere for 2 h at 150 °C and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g⁻¹ in the second cycle); in this case, the voltage profiles show a large flat portion at potentials lower than 0.3 V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample. Nevertheless, a part of the lithium ions are removed at potentials higher than 0.5 V versus Li⁺/Li limiting the useful capacity.     

Electrochemical intercalation of potassium into graphite in KF melt

Electrochemical intercalation of potassium into graphite in molten potassium fluoride at 1163 K was investigated by means of cyclic voltammetry, galvanostatic electrolysis and open-circuit potential measurements. It was found that potassium intercalated into graphite solely between graphite layers. In addition, the intercalation compound formed in graphite bulk in molten KF was quite unstable and decomposed very fast. X-ray diffraction measurements indicate that a very dilute potassium-graphite intercalation compound was formed in graphite matrix in the fluoride melt. Analysis with scanning electron microscope and transmission electron microscope shows that graphite was exfoliated to sheets and tubes due to lattice expansion caused by intercalation of potassium in molten KF.     

Electrochemical lithium storage of sodium titanate nanotubes and nanorods

Layered hydrated sodium titanate nanotubes are synthesized via a hydrothermal reaction in alkaline solution. The as-prepared nanotubes are calcined at different temperatures (300-600 °C) in air. The microstructure of obtained samples is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is observed that the calcined products maintain their parent tubular morphologies below 500 °C. After calcinations at 600 °C, the hollow tubular morphology could completely be converted to the short solid nanorod morphology. In the meanwhile, the monoclinic sodium hexatitanate as a main phase is formed in nanorods, coexisted with sodium trititanate as a residual phase. The electrochemical lithium storage of obtained samples is studied by galvanostatic method and cyclic voltammetry. It is demonstrated that the nanotubes calcined at 500 °C have relatively large reversible capacity, good reversibility and excellent high rate discharge capability. The lithium intercalation process is shown to have pseudocapacitive feature caused by their layered structure and open lithium insertion tunnels, which is in favor of the high rate charge/discharge capability of sodium titanate nanotubes.     

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.     

Doping effects on structure and electrode performance of K-birnessite-type manganese dioxides for rechargeable lithium battery

The potassium birnessites doped with Al, Ni, and Co were prepared by calcination and aqueous treatment, which showed that single phase products were obtained with Ni and Al up to 5 at.% and Co up to 25 at.% addition to strating KMnO₄. The discharge-recharge capacities and capacity retentions in an aprotic Li cell were not improved by the Ni and Al dopings, but those of the cobalt doped birnessite were improved. The initial discharge capacities of the undoped and cobalt doped birnessites were 170 and 200 mAh g⁻¹ with capacity retentions of 56 and 80% during the initial 20 cycles, respectively. The reasons for the improvement of the battery performance by Co doping were considered as follows: (i) a change in the stacking structure, (ii) a decrease in the charge transfer resistance, and (iii) improved structural stability of the oxide. Their micro structures were evaluated by X-ray diffraction, photoelectron and Raman spectroscopies, and electron microscopy. Also, potassium birnessite synthesized by adding about 3 times excess potassium indicated that the stacking structure was similar to the 30 at.% cobalt doping sample, furthermore, the better capacity retention was achieved as cathode in a Li cell.     

Low temperature performance of graphite electrode in Li-ion cells

We studied low temperature performance of Li/graphite cell. Results show that capacity of the graphite electrode falls significantly in the temperature range of 0 to −20 °C. When lithiation and delithiation are both carried out at −20 °C, graphite only retains 12% of the room temperature capacity. However, delithiation capacity of graphite increases to 92% of the room temperature value if the lithiation is carried out at room temperature. We believe that the poor low temperature performance of the cell is due to slow kinetics of lithium ion diffusion in graphite rather than low ionic conductivity of electrolyte and solid electrolyte interface (SEI) on the graphite surface. During lithiation and delithiation processes, lithium ion has the similar apparent chemical diffusion coefficient of 10⁻⁹-10⁻¹⁰ cm²/s at 20 °C, depending on the state of lithiation of graphite. We observed a dramatic decrease in lithium ion diffusivity in the temperature range of 0 to −20 °C, and that at low temperatures of <−20 °C, lithium ion has higher diffusivity in the delithiated graphite than in the lithiated one. We also observed that temperature dependence of cycling behavior of the Li/graphite cell follows the change of lithium ion diffusivity.     

Electrochemical characteristics of silicon coated graphite prepared by gas suspension spray method for anode material of lithium secondary batteries

Silicon-coated graphite particles were tested as anodes for lithium ion rechargeable batteries. The synthetic graphite particles were first coated with silicon precursor containing solution by gas suspension spray method and then calcined at heat treatment temperature at 500 °C under hydrogen atmosphere. The silicon-coated graphite showed high specific capacity and good cycle performance due to the formation of amorphous silicon-carbon black composite layer on the surface of the graphite particles. It has stable structure under repeated volume expansion and contraction. The silicon-coated graphite still has high irreversible capacity due to the solid electrolyte interface (SEI) formation during the 1st cycle. However, the capacity loss could be lessened to a certain level by controlling the composition of the solvent mixture in the electrolyte.     

On the determination of kinetic characteristics of lithium intercalation into carbon

The diffusional processes proceeding in the intercalation electrode on applying a potential step are theoretically treated. The theory of chronovoltammetry is shown to require the necessary account of the contribution of the surface solid-state film to the overall diffusion resistance of the electrode. For the case of small deviations from equilibrium, analytical solutions of the diffusion problem have been obtained. Ignoring the retarding contribution of the surface film is shown to bring about an error into the diffusion coefficient of lithium in its alloys and intercalates. The theory has been experimentally verified with Li_xC₆ electrodes of various compositions made of pyrocarbon films on a nickel backing as examples. The diffusion coefficients are 10⁻¹¹-10⁻⁹ cm² s⁻¹, depending on composition.     

Electrochemical properties of nanosize Sn-coated graphite anodes in lithium-ion cells

A series of Sn-coated graphite composite materials for lithium-ion batteries were prepared by microencapsulating nanosize Sn particles in graphite. The nanosize Sn particles are homogeneously dispersed in the graphite matrix via electroless chemical reduction. The tin-graphite composite showed a great improvement in lithium storage capacity. Since Sn is an active element to lithium, Sn can react with lithium to form Li_4.4Sn alloys, a reaction accompanied by a dramatic volume increase, whereas the ductile graphite matrix provides a perfect buffer layer to absorb this volume expansion. Therefore, the integrity of the composite electrode is preserved during lithium insertion and extraction. Cyclic voltammetry was employed to identify the reaction process involved in lithium insertion and extraction in the graphite structure, as well as lithium alloying with tin. The tin-graphite composites provide a new type of anode material for lithium-ion batteries with an increased capacity.     

Electrochemical behavior of lithium intercalated in a molybdenum disulfide-crown ether nanocomposite

A new nanocomposite, obtained from the intercalation of the cyclic ether 12-Crown-4 into MoS₂, Li_0.32MoS₂(12-Crown-4)_0.19, is described. The laminar product has an interlaminar distance of 14.4 Å. The electrical conductivity of the nanocomposite varies from 2.5 × 10⁻² to 4.3 × 10⁻² S cm⁻¹ in the range 25-77 °C, being about four times higher than the analogous poly(ethylene oxide) (PEO) derivative at room temperature. The electrochemical step-wise galvanostatic intercalation or de-intercalation of lithium, leading to Li_xMoS₂(12-Crown-4)_0.19 with x in the range 0.07-1.0, indicates a Li/Li⁺ pair average potential of 2.8 V. The electrochemical lithium diffusion coefficients in the crown ether intercalates, determined by galvanostatic pulse relaxation between 15 and 37 °C at different lithium intercalation degrees, are higher than those of the PEO derivatives under similar conditions, being however the diffusion mechanism rather more complex. The variation of both, the lithium diffusion activation enthalpy and the quasi-equilibrium potentials, with the lithium content shows there are two different limit behaviors, at low and high lithium intercalation degree, respectively. These features are discussed by considering the high stability of the Li-crown ether complex and the different chemical environments found by lithium along the intercalation process.     

Electrochemical effect of graphite fluoride modification on Li-rich cathode material in lithium ion battery

Recently, Li-rich layered structure has been used in the cathode of lithium ion batteries because of its high specific capacity. However, this structure still has some problems including large irreversible capacity loss, significant deterioration of cycling performance and poor rate property. Therefore, in our study, graphite fluoride is used to modify the surface of Li_1.14Ni_0.133Co_0.133Mn_0.544O₂ through a facile solvent evaporating method. Due to conversion reaction of the graphite fluoride, the huge discharge capacity compensation during the first discharging can improve the coulombic efficiency significantly. As reaction products, the layer of LiF@carbon reduces the interfacial reactions and increases the reversible capacity. After modification by graphite fluoride, the discharge capacities are improved by 22% from 266 to 325?mAh?g^?1 at 0.1?C, and 13% at 2?C. After 100 cycles, the discharge capability at 1?C is increased by 13% from 180 to 203?mAh?g^?1.