Takahisa Ohsaki

Rechargeable Lithium‐Ion Cells Using Graphitized Mesophase‐Pitch‐Based Carbon Fiber Anodes

The electrochemistry of lithium intercalation into a graphitized mesophase-pitch-based carbon fiber with a radial-like texture used as the anode material in rechargeable lithium-ion cells was characterized. The radial-like texture in the cross section of the carbon fiber contributed to the rapid diffusion of lithium ions, resulting in the high rate capability. The anode performance of the graphitized carbon fiber was superior to that of the graphite. Experimental flat-plate C/LiCoO{sub 2} lithium-ion cells using the graphitized carbon fiber anode exhibited a high mid-discharge voltage of 3.7 V, a high rate capability, and a long cycle life of more than 400 cycles at 2 mA/cm{sup 2} mA/cm{sup 2} during charge-discharge cycling between 4.2 and 2.7 V. The long cycle life obtained for the cell was due to no significant change in resistance associated with the passivating films on the graphitized carbon fiber with extended cycles. It was also demonstrated that A size C/LiCoO{sub 2} cells using the graphitized carbon fiber anode have excellent rate performance at discharge currents between 0.25 and 3 A, a large discharge capacity of 0.95 Ah, and a high energy density of 310 Wh/dm{sup 3} and 120 Wh/kg.

Lithium insertion and extraction for high-capacity disordered carbons with large hysteresis

Disordered carbons heat-treated from 550 to 1000 °C containing hydrogen atoms showed high specific capacities with large hysteresis in the potential when used as anodes in lithium-ion cells. The lithium storage mechanism in the disordered carbons has been investigated by the charge-discharge test, X-ray diffraction (XRD) and solid-state 7Li NMR measurements. Variation of the layer spacing of the disordered carbon heat-treated at 900 °C with insertion and extraction indicated that lithium was inserted into the unorganized carbon site (U-site) near 0 V vs LiLi+ after insertion into the layer structure site (L-site) and removed from the U-site near 1 V after the extraction from the L-site. 7Li NMR spectra of the lithiated disordered carbons heat-treated at 550 °C showed two bands with a relatively small shift (< 10 ppm) from 0 ppm vs LiCl, indicating that stored lithium had an ionic character. The results of 7Li NMR analysis revealed the existence of the ionic lithium stored in the reversible storage sites and lithium trapped in the irreversible storage site. The high capacity with large hysteresis was attributed to the ionic lithium stored on the condensed aromatic ring in the U-site.

Laminated Thin Li-Ion Batteries Using a Liquid Electrolyte

Thin Li-ion batteries with a laminated film bag as a casing were developed by using a liquid electrolyte and a graphitized boron-doped mesophase-pitch-based carbon fiber (B-MCF) anode. The thin Li-ion batteries exhibited excellent discharge performance, long cycle life, and very low swelling under high-temperature storage. A 1.5 M solution of LiBF4 in an ethylene carbonate (EC)/γ-butyrolactone (GBL) (1:3 by volume) mixed solvent is advantageous for use as the electrolyte in the laminated film bag because of its high flame point of 129°C, high boiling point of 216°C, low vapor pressure, and high conductivity of 2.1 mS/cm at -20°C The B-MCF anode in the LiBF 4 -EC/GBL electrolyte exhibited a high reversible capacity of 345 mAh/g. a high coulombic efficiency of 94% at the first cycle, and high rate capability. It was demonstrated that the thin Li-ion battery with a thickness of 3.6 mm has a high energy density of 172 Wh/kg. high rate capability between 0.2 and 3C rate discharge, a high capacity ratio of 50% at 1C rate discharge and -20°C, and a long cycle life of more than 500 cycles at 1C rate charge-discharge cycling. The B-MCF anode led to the high rate discharge performance and the long cycle life of the thin Li-ion batteries using the LiBF 4 -EC/GBL electrolyte. The very low swelling and small evolution of gas under the high-temperature storage at 85°C were attributable to the stability of LiBF 4 -EC/GBL electrolyte against the fully charged LiCoO 2 cathode material.

Large Hysteresis during Lithium Insertion into and Extraction from High‐Capacity Disordered Carbons

Perylene-based disordered carbon (PBDC) heat-treated at 550 °C for anodes in Li-ion cells showed large hysteresis and a high reversible capacity of 800 mAh/g. The hysteresis was analyzed by polarization and impedance measurements. The overpotential during lithium extraction increased markedly in the range of open-circuit potential, 0.5-1 V vs Li/Li + . The impedance spectra of PBDC during lithium insertion were significantly different from those during extraction. The charge-transfer resistance for lithiated PBDC during extraction above 0.5 V was much larger than that for the PBDC during insertion. The chemical diffusion coefficient of lithium, D Li , of PBDC during lithium insertion decreased almost linearly from 5 x 10 -10 to 3 x 10 -12 cm 2 s -1 with increasing lithium storage capacity. The values of D Li during lithium extraction above 0.5 V were much smaller than those during insertion. The large hysteresis was due to the large charge-transfer resistance and the slow diffusion of lithium during lithium extraction from the fully lithiated PBDC. The large charge-transfer resistance during lithium extraction has been interpreted as the rectification of lithiated PBDC, which is similar to that of n-type semiconductors under anodic polarization

Electrochemical intercalation of lithium into graphitized carbons

Abstract The change of the carbon structure with electrochemical intercalation of lithium has been investigated by X-ray diffraction (XRD) method. Graphitized carbons showed the first and the second stage structures clearly during the intercalation process. However, the layer spacing corresponding to the 1st stage structure of graphitized carbon was smaller than that of graphite. This is because the first stage structure of graphitized carbon is the mixed structure of lithiated graphite crystallites and lithiated turbostratic disordered layers. The lithium is mainly intercalated into turbostratic disordered layers above 0.1 V versus Li Li + , and intercalated into graphite crystallites rather than turbostratic disordered layers below 0.1 V versus Li Li + .

The Impedance of Lithium Electrodes in LiPF6 ‐ Based Electrolytes

The impedance spectrum of a lithium electrode in an LiPF 6 -based electrolyte exhibits one or two semicircles. The variation of the two semicircles with immersion time indicates the presence of two kinds of passivating surface films. The change in resistance (Δ R1) associated with the first film during the first 24 h immersion time increased with increasing LiPF 6 concentration. The large value of Δ R1 in a degraded LiPF 6 electrolyte indicated that the decomposition of PF 6 - would lead to the formation of a thick passivating film

7Li NMR and ESR analysis of lithium storage in a high-capacity perylene-based disordered carbon

Abstract The lithium storage mechanism of perylene-based disordered carbon (PBDC) heat-treated at 550 °C, which is a promising material for use as the anode in lithium-ion cells, was studied by solid-state 7 Li NMR and ESR analysis. PBDC is one of carbonaceous materials containing condensed aromatic rings, and showed a high reversible specific capacity of about 800 mAh/g with large hysteresis in the charge/discharge profile. 7 Li NMR spectra for the lithiated PBDCs exhibited two bands at the insertion of above 900 mAh/g. Band A at 7 ppm and band B at 0.3 ppm versus LiCl were assigned to lithium in reversible and irreversible storage sites, respectively. The results of 7 Li NMR analysis supported the presence of ionic lithium located on aromatic rings. ESR spectra for PBDC lithiated by over 300 mAh/g showed sharp and broad signals. The intensity of the broad signal varied significantly with lithium insertion. The variations of 7 Li NMR and ESR spectra with lithium insertion were interpreted by the presence of two kinds of insertion sites: the layer structure site (L-site) and the unorganized carbon site (U-site) located between the L-sites.

High-capacity lithium-ion cells using graphitized mesophase-pitch-based carbon fiber anodes

Abstract We have developed high-capacity lithium-ion cells using graphitized mesophase-pitch-based carbon fiber (MCF) as an anode material. The graphitized MCF is a highly graphitized carbon fiber with a radial-like texture in the cross section. This structure contributes to the rapid diffusion of lithium ions inside the carbon fiber. The diffusion coefficient of lithium ions in the graphitized MCF was one order of magnitude larger than those for graphite, resulting in an excellent high-rate performance of the carbon electrode. The graphitized MCF anode showed larger capacity, a higher rate capability, and better reversibility than the graphite anode. The 863448 size (8.6 mm × 34 mm × 48 mm) prismatic cell with the graphitized MCF anode exhibited a large capacity of > 1000 mAh. At 3 A discharge, the prismatic cell had 95% of its capacity at 0.5 A discharge with a mid-discharge voltage of 3.35 V. The cell maintained > 85% of its initial capacity after 500 cycles and showed high capacity at −20 °C. It has thus been demonstrated that the prismatic cell using the graphitized MCF anode has excellent performance, and is an attractive choice for the power sources of cellular phones and other appliances.

Reduction of voltage delay in the Li/SOCl2 system — A study of polymers with chlorine substituent groups as delay reducing additives

Abstract Several kinds of vinyl polymers with chlorine substituent groups have been evaluated as voltage-delay reducing additives. The reduction of the voltage delay was affected by the properties of substituent groups, and polyvinyl chloride (PVC) and vinyl chloride-vinylidene chloride copolymer exhibited excellent performance. Observations using energy dispersive X-ray analysis, scanning electron microscopy, and impedance analysis indicate that the growth of the LiCl passivating film is slowed by the polymer film formed on the lithium anode in the PVC-added electrolyte.