Hirokazu Kitaura

Improvement of High-Rate Performance of All-Solid-State Lithium Secondary Batteries Using LiCoO2 Coated with Li2O – SiO2 Glasses

Development of rate capability is one of the most important issues to be solved in all-solid-state lithium secondary batteries. The electrochemical performance of these batteries has been highly improved by coating LiCoO 2 particles with Li 2 O-SiO 2 thin film. The interfacial resistance between LiCoO 2 and the glass-ceramic electrolyte was decreased by those coatings. The rate capabilities of the cells using the coated LiCoO 2 particles were superior to that of the cell using noncoated LiCoO 2 . The Li 2 Si 3 coating was more effective than the SiO 2 coating in enhancing rate performance, suggesting that lithium-ion conductivity of the coating materials is important for high-rate performance.

Fabrication of electrode–electrolyte interfaces in all-solid-state rechargeable lithium batteries by using a supercooled liquid state of the glassy electrolytes

The softening behavior of a 80Li2S·20P2S5 (mol%) glass electrolyte was investigated and a favorable electrode–electrolyte interface was fabricated by sticking the supercooled liquid state of the 80Li2S·20P2S5 electrolyte on active material particles. A dense pellet of the glass electrolyte without an obvious grain boundary or any voids was prepared by softening the 80Li2S·20P2S5 glass by means of a hot press. The electrical conductivity of the pellet was 8.8 × 10−4 S cm−1 at room temperature. Sticking the solid electrolyte on the Li4Ti5O12 active material particles increased the contact area at the electrode–electrolyte interface and the utilization of the active material was increased in the all-solid-state cells. However, LiCoO2 reacted with the solid electrolyte during the hot press and the electrochemical performance of the cells using hot-pressed LiCoO2 with the glass electrolyte degraded. LiNbO3 coating suppressed the reaction of LiCoO2 with the solid electrolyte. The all-solid-state full-cell Li4Ti5O12/80Li2S·20P2S5 glass/LiNbO3-coated LiCoO2 prepared by hot press showed a larger reversible capacity of 120 mAh g−1 at 0.064 mA cm−2 compared with the full-cell prepared by cold press. The softening of the 80Li2S·20P2S5 glass electrolyte is an effective way for increasing the contact area between the active materials and solid electrolyte.

Synthesis of nanosized nickel sulfide in high-boiling solvent for all-solid-state lithium secondary batteries

Nickel sulfide nanoparticles were synthesized by thermal decomposition of nickel acetylacetonate in a mixed solution of 1-dodecanethiol and a high-boiling solvent. The crystal phase and morphology of the obtained samples by changing the reaction time, reaction temperature, and the kind of the solvents were characterized by X-ray diffraction measurements and transmission electron microscopy. By using oleylamine as a coordinating solvent, Ni9S8 nanorods with the size of about 100 nm were obtained at 280 °C for 5 hours. On the other hand, NiS nanoparticles with the size of about 50 nm were obtained by using 1-octadecene as a noncoordinating solvent. Nickel sulfide crystals with different phase and morphology were obtained by selecting the solvent. The all-solid-state cells using NiS nanoparticles with the size of about 50 nm as active materials were fabricated. The 80Li2S·20P2S5 (mol%) glass–ceramic was used as a solid electrolyte. The cell Li–In/NiS exhibited the initial discharge capacity of 780 mA h g−1 at 0.13 mA cm−2 and the good cycle performance during 20 cycles.

Preparation of α-Fe2O3 Electrode Materials via Solution Process and Their Electrochemical Properties in All-Solid-State Lithium Batteries

α-Fe 2 O 3 particles of various sizes were prepared by a solution process and were applied to the electrodes for all-solid-state lithium rechargeable batteries. The prepared α-Fe 2 O 3 was monodispersed particles with sizes controlled from about 250 nm to 3.7 μm. Their electrochemical properties were examined for the all-solid-state cells with highly conductive Li 2 S-P 2 S 5 solid electrolytes and the influences of the particle size of α-Fe 2 O 3 on the electrochemical performance of the cells were investigated. The first discharge capacities increased with decreasing the particle size of α-Fe 2 O 3 and the cell using the smallest α-Fe 2 O 3 particles of 250 nm showed the highest capacity of 1050 mAh g -1 . The charge-discharge reaction mechanism in the all-solid-state cells was investigated by means of X-ray diffraction and was revealed to be similar to that in conventional cells with a liquid electrolyte. The lithium intercalation to α-Fe 2 Ο 3 occurred in the initial stage of the first discharge only in the cases using α-Fe 2 O 3 particles with small crystallite size, large specific surface area and large pore volume. By controlling the cutoff voltage, the all-solid-state cell retained the constant capacity of about 130 mAh g -1 for 15 cycles at the current density of 0.064 mA cm -2 . The rate capability at 25°C and temperature dependence of the first discharge capacity in the all-solid-state cells were also investigated. The cell at 60°C showed the larger first-discharge capacity of 940 mAh g -1 at the current density of 1.27 mA cm -2 .

All-solid-state lithium secondary batteries with metal-sulfide-coated LiCoO2 prepared by thermal decomposition of dithiocarbamato complexes

LiCoO2 particles were coated with cobalt and nickel sulfides by thermal decomposition of their respective diethyldithiocarbamato complexes. All-solid-state lithium secondary batteries were fabricated using the coated LiCoO2 positive electrode and a Li2S–P2S5 solid electrolyte. The coatings reduced the interfacial resistance between LiCoO2 and the Li2S–P2S5 solid electrolyte of the all-solid-state batteries after the first charge, resulting in an improved cell performance. The all-solid-state cell with NiS-coated LiCoO2 was charged and discharged at a high rate of 10 C. The coatings reduced deterioration of the interface between LiCoO2 and the Li2S–P2S5 solid electrolyte, indicating that they function as an effective buffer layer at the interface.

Electrochemical Analysis of Li4Ti5O12 Electrode in All-Solid-State Lithium Secondary Batteries

All-solid-state cells using the solid electrolyte were assembled to investigate their rate performances and impedance behaviors. Furthermore, cells using dissolved in ethylene carbonate and diethyl carbonate were fabricated for comparison to the all-solid-state cells. In the all-solid-state cells, resistance attributable to the lithium-ion transfer from the solid electrolyte to spinel phase was observed during the lithium insertion into spinel phase during the discharge process. Results clarified that the cells using the solid electrolyte showed higher resistance and larger activation energy for this lithium-ion transfer than the cells using the liquid electrolyte. Resistance was not observed in the charge process. The all-solid-state cell showed the average charge voltage of (vs Li) and reversible capacity of at the charge current density of and the discharge current density of .

All-Solid-State Lithium Secondary Batteries Using LiMn2O4 Electrode and Li2S – P2S5 Solid Electrolyte

All-solid-state In/LiMn 2 O 4 cells with the 80Li 2 S·20P 2 S 5 (mol %) solid electrolyte were assembled. Cycle and rate performances were evaluated. The cell showed the first discharge capacity of 55 mAh g -1 at a current density of 0.064 mA cm -2 and retained the reversible capacity of 47 mAh g -1 after 100 cycles. The capacity decreased with increase in the current density, and the overpotential at the beginning of discharge was observed. The discharge capacity was about 23 mAh g -1 at a current density of 2.6 mA cm -2 To identify the cause of the overpotential, impedance measurements were carried out, and resistance components were analyzed by changing the state of charge and by evaluating an all-solid-state Li 1-x Mn 2 O 4 symmetric cell. The interfacial resistance between the LiMn 2 O 4 electrode and the solid electrolyte was identified after the charge process. The transmission electron microscopy observation and energy-dispersive X-ray analysis, showed that Mn slightly diffused from LiMn 2 O 4 to the solid electrolyte at the interface.

Electrochemical performance of all-solid-state lithium secondary batteries using Li 4 Ti 5 O 12 electrode and Li 2 S–P 2 S 5 solid electrolytes

All-solid-state Li–In/Li 4 Ti 5 O 12 cells using Li 2 S–P 2 S 5 solid electrolytes were assembled to investigate their electrochemical properties in the wide voltage range of 0–3 V (versus Li). The Li/Li 4 Ti 5 O 12 cells using 1 M LiPF 6 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 70Li 2 S·27P 2 S 5 ·3P 2 O 5 (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/cm 2 . The all-solid-state cell was cycled at 1.3 mA/cm 2 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/cm 2 .