Minato Egashira

Thermal stability of graphite anode with electrolyte in lithium-ion cells

Abstract Thermal stability of electrochemically lithiated graphite with 1 M LiPF6/EC+DMC and PVdF-binder has been investigated. DSC measurements using an airtight sample case reveal a mild heat generation started from 130 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. The heat evolved in the small peak at 140 °C decreased by storage of the lithiated graphite with PVdF and the electrolyte at 50 °C for 3 days before the DSC measurements. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 °C. The peak at 140 °C seems to be caused by the reaction (the Solid Electrolyte Interphase (SEI) formation) of the electrolyte and lithiated graphite, whose surface is covered by poly(vinylidene flouride) (PVdF)-binder without formation of SEI at a lower temperature. The mild heat generation from 140 to 280 °C is the reaction of the lithiated graphite and the electrolyte through SEI (SEI formation), because there was no such mild heat generation when non-lithiated graphite was used. The peak at 280 °C is probably a direct reaction of lithiated graphite and electrolyte by a breakdown of SEI.

Thermal studies of fluorinated ester as a novel candidate for electrolyte solvent of lithium metal anode rechargeable cells

Abstract The thermal stability of fluorinated ester electrolytes with and without lithium metal and the positive electrode material at the charged state were investigated, in terms of application for electrolytes in lithium metal anode cells. The fluorinated ester electrolytes are solutions dissolving LiPF 6 in carboxylic acid esters whose original carboxylic acids are partially fluorinated. The corresponding non-fluorinated ester electrolytes were also studied for comparison. According to differential scanning calorimetry (DSC) measurement, fluorinated ester electrolytes exhibited significant thermal stability when coexisting with lithium metal or Li 0.5 CoO 2 . LiPF 6 /methyl difluoroacetate showed the best stabilization effect, which shifted the exothermic peak of the electrolyte with lithium metal or Li 0.5 CoO 2 to about 300°C. In addition, LiPF 6 /methyl difluoroacetate exhibited a good lithium anode cycling efficiency. We believe that LiPF 6 /methyl difluoroacetate is a very promising electrolyte for use in realizing lithium metal anode secondary cells.

Cathode properties of pyrophosphates for rechargeable lithium batteries

Abstract Two rechargeable pyrophosphates, LiVP 2 O 7 and TiP 2 O 7 , were synthesized by a solid state reaction, and the cathode properties were investigated. The structures were characterized by powder X-ray Rietveld refinement. LiVP 2 O 7 was isostructural with P 2 1 monoclinic LiFeP 2 O 7 and TiP 2 O 7 was isostructural with P a3 cubic ZrV 2 O 7 . Lithium cells using LiVP 2 O 7 cathodes showed that approximately 0.5 Li per unit could be reversibly deintercalated from LiVP 2 O 7 at 4.1 V vs. Li/Li + . On the other hand, 0.6 Li per unit could be reversibly intercalated into TiP 2 O 7 at 2.6 V vs. Li/Li + .

Lithium Dicyanotriazolate as a Lithium Salt for Poly(ethylene oxide) Based Polymer Electrolytes

The synthesis and properties of a new type of polymer electrolyte, formed by blending poly(ethylene oxide), PEO, with lithium 4,5-dicyano-1,2,3-triazolate salt, are reported and discussed. The results show that this electrolyte has a wider temperature range of high conductivity and a higher lithium transference number than those of other, common PEO-based electrolyte membranes.

Properties of carbon anodes and thermal stability in LiPF6/methyl difluoroacetate electrolyte

The thermal stability of 1 M LiPF 6 /CHF 2 COOCH 3 (methyl difluoroacetate, MFA) with lithiated carbon anodes and the electrochemical characteristics of carbon anodes in this electrolyte have been investigated in terms of the use of this electrolyte in lithium-ion batteries. Differential scanning calorimeter measurement of LiPF 6 /MFA with lithiated carbon anodes indicated that the main exothermic peak is around 400°C. The peak temperature was 110°C higher than the peak of LiPF 6 lethylene carbonate-dimethyl carbonate (EC-DMC) (1:1 in vol) with lithiated carbon anodes. The chemical compositions of the solid electrolyte interphase (SEI) on the lithium metal anode in LiPF 6 /MFA electrolyte were characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. These spectroscopic measurements revealed that CHF 2 COOLi existed as a major component of SEI. It was expected that a reaction product of CHF 2 COOLi and lithiated carbon increased the thermal stability. Compared with the case in I M LiPF 6 /EC-DMC [1:1 in vol], the discharge capacity of carbon anodes was slightly smaller in 1 M LiPF 6 /MFA, while the cycling performance was similar in both electrolytes. From the result of the impedance measurement, the reason for the small capacity is the large resistance of SEI. Moreover, good cycle performance was obtained for the lithium ion cell used in 1 M LiPF 6 /MFA, while the cells discharge capacity was slightly lower than that of the lithium-ion cell in I M LiPF 6 /EC-DMC (1:1 in vol). It is noted that I M LiPF 6 /MFA electrolyte is a good candidate to improve the thermal stability of the lithium ion and lithium metal anode battery.

Potential and Thermodynamics of Graphite Anodes in Li‐Ion Cells

Lithium graphite intercalation compound (Li‐GIC) has potential plateaus depending on its staging in Li/Li‐GIC cells. It is believed that the entropy of each stage is different depending on the total number of states created by the inserted and electrons in Li‐GIC. On this basis, the entropy change with Li insertion has been calculated and the potential, using the reaction Gibbs function, has been estimated. The entropy term in the usual potential equation diverges for Li vacant and Li full GICs. The equation was improved and had finite values for these GICs depending on the mean size of the crystallites in the GIC. By the introduction of the intercalation pressure to our model, we confirmed that Li‐GIC is unstable and decomposes into Li‐poor and Li‐rich phases. The potential was calculated using the entropy term for Li full GIC. The result agreed well with the experimental value. The entropy term contribution was very large for the potential difference of each plateau.

Structural changes of fullerene by heat-treatment up to graphitization temperature

Abstract Structural changes of the C60 and C70 mixture during heat-treatment up to 2400 °C were studied by observing the carbonized disk of the fullerene with Raman spectroscopy, X-ray diffraction, FE-SEM, TEM, and AFM/STM. The fullerene lost its five-membered ring and its fcc crystal structure by heat-treatment at 800 °C, as revealed by the Raman spectra and X-ray diffraction, forming hexagonal planes which were randomly arranged by 1300 °C. Further heat-treatment allowed some stacking of layers which grow to dominate, reducing the randomly oriented planes. The graphitized temperature up to 2400 °C provided a very sharp peak at 26 °, suggesting formation of stable turbostratic layers. The TEM characterized turbostratic stacking of 3 to 4 layers. A series of observation under AFM/STM and TEM indicate the crystal of the fullerene, amorphous grain of the hexagonal planes, and the hollow sphere are all in the same range of size around 10–20 nm. Such microdomains induced micro-roughness as observed by FE-SEM on the surface of the carbon disk. Superstructure of hexagonal plane was observed on the surface. A kind of solid state carbonization of the fullerene is suggested to maintain the dimensions of its crystal into the spherical microdomain, even if its marked structural changes take place within the unit.

The effect of the coexistence of anion species in imidazolium cation-based molten salt systems

Abstract Wider application of imidazolium cation-based room temperature molten salts in batteries and electrochemical capacitors will require an improvement of their cathodic and anodic stability. In this study, the authors found that a certain combination of anions can change the anodic stability and other properties of molten salt systems. A 1:1 (in moles) mixture of 1-ethyl-3-methyl imidazolium (EMI) tetrafluoroborate and EMI bis(tetrafluoromethanesulfonyl)imide (TFSI) exhibited an anodic stability similar to that of EMITFSI, the most anodically stable single-anion molten salt among the molten salts used in this study. This anion also altered other characteristics of the salt, such as ionic conductivity and thermal stability, compared to those seen by a simple combination of each of these single-anion salts. In contrast, coexistence of the TFSI anion and trifluoromethanesulfonyl (triflate) anion provided no significant change in any characteristics of the molten salt described above. The 1 H-NMR study of these molten salts revealed that the coexistence of tetrafluoroborate anions and TFSI anions lowered the degree of hydrogen bonding from that in each individual molten salt, while the coexistence of the TFSI anion and triflate anion provided no changes in hydrogen bonding. The coexistence of anions, such as tetrafluoroborate and TFSI, is expected to change the manner of cation–anion association, which inhibits hydrogen bonding. Based on these results, the authors suggest the possibility of controlling the electrochemical stability of a molten salt by varying the manner of its cation–anion association.

Single pentagon in a hexagonal carbon lattice revealed by scanning tunneling microscopy

The electronic structure of a single pentagon in a hexagonal carbon lattice has been revealed on an atomic scale by scanning tunneling microscopy. The pentagon is located at the apex of the conical protuberance of the graphitic particle. The enhanced charge density localized at each carbon atom in the pentagon is identified, and the ringlike pattern of the (∛×∛)R30° superstructure of graphite is clearly observed around the pentagon.

Effects of the electrolyte composition on the electric double-layer capacitance at carbon electrodes

The electric double-layer capacitances at carbon electrodes with different surface morphologies have been determined in organic solution and polymeric gel electrolytes by an ac impedance method. The capacitance depended not only on the surface structure but also on the electrolyte composition. Activated carbon fiber with pore structure showed higher double-layer capacitance in the polymeric gel than in the solution. The capacitance at glassy carbon with smooth surface was higher in the solution electrolyte than in the gel. The polymeric component in the electrolyte gave different effects on the double-layer capacitance at the carbon electrode depending on the surface morphology.