Hikari Takahara

All-Solid-State Lithium Secondary Battery Using Oxysulfide Glass Addition and Coating of Carbon to Positive Electrode

Carbon addition and carbon coating to LiCoO 2 has been attempted to improve the rate performance of all-solid-state battery using oxysulfide glass. The discharge capacity decreased with the current density (0.064 and 0.32 mAcm -2 ) for the given acetylene black content (0, 0.25, 2.5, and 5 wt %); the mere addition of acetylene black contributed to lower the rate capability. On the other hand, for the carbon-coated LiCoO 2 by Spark-Plasma-Sintering method, the higher discharge capacity of above 100 mAhg -1 was exhibited even at the higher current density (0.32 mAcm -2 ). Carbon coating to active material, rather than the merely mixing or addition, is effective for improving rate performance in all-solid-state batteries.

Application of Lithium Metal Electrodes to All-Solid-State Lithium Secondary Batteries Using Li3 PO 4 ­ Li2 S ­ SiS2 Glass

The Li 3 PO 4 -Li 2 S-SiS 2 glass electrolyte exhibited instability against a Li metal electrode in the charge-discharge cycle using a LiCoO 2 positive electrode. The interface products between the Li electrode and the glass electrolyte were investigated by Si and S-K edge near-edge X-ray absorption fine structure analyses. It was suggested that Li 2 S and Si coordinated to three sulfur atoms formed after charge-discharge cycles. This side reaction could be suppressed by modifying the surface of Li metal by N 2 gas, leading to improvement of the charge-discharge property compared to unmodified Li electrode. The operating voltage attained to about 4 V in the modified Li/Li 3 PO 4 -Li 2 S-SiS 2 glass electrolyte/LiCoO 2 cell, which was comparable to Li-ion battery using a liquid electrolyte.

Study of the Capacity Fading Mechanism for Fe-Substituted LiCoO2 Positive Electrode

To find the origin of a large initial irreversible capacity and capacity fading with cycling for Fe-substituted LiCoO 2 (LiCo 1-y Fe y O 2 ), the LiCo 0.8 Fe 0.2 O 2 positive electrode was selected for study by ex situ X-ray diffraction, Co and Fe K-edge X-ray absorption, and 57 Fe Mossbauer spectroscopies. A disordering of Fe ions from 3b(0, 0, 1/2) to 6c(0, 0, 3/8) sites was detected for initial charged samples through X-ray Rietveld analysis and Co and Fe K-edge X-ray absorption near-edge structures and extended X-ray fine structures spectra. The valence state of Fe ions in the 6c site was determined to be a 3+/4+ mixed valence state from the isomer shift values obtained by 57 Fe Mossbauer spectra and mean M-O distance values. 50% of the iron ions become disordered after the initial charge process and more than 20% remain in 6c sites after the first and tenth discharge processes. The existence of Fe 3+δ (0 < δ < 1) ions on the interstitial 6c site can block fast Li conduction in the Li layer of the layered rock-salt structure (R3m). This leads to a lack of reversibility in Fe-substituted LiCoO 2 positive electrode materials.

Hydrogen analysis in diamond-like carbon by glow discharge optical emission spectroscopy

Glow discharge-optical emission spectroscopy (GD-OES) was evaluated for hydrogen analysis in diamond-like carbon (DLC) films. DLC film samples were prepared using the plasma-based ion-implantation and deposition method (PBIID). Their hydrogen contents were determined using elastic recoil detection analysis (ERDA). The hydrogen intensity obtained by GD-OES increased gradually concomitantly with increasing hydrogen contents at the lower hydrogen content region. However, the intensity increased drastically at the higher hydrogen content region of more than about 30 at%. When the hydrogen contents increased, the correlation between GD-OES hydrogen intensity and ERDA hydrogen contents was deviated from the linear relation obtained for hydrogen-implanted silicon samples as reference materials. The sputtering rate of the DLC sample was found to vary at the high H content region. A linear relationship was obtained between the hydrogen contents and GD-OES intensities corrected with the sputtering rate of samples.