Atsushi Ogata

Electrochemically Reversible Sodium Intercalation of Layered NaNi0.5Mn0.5O2 and NaCrO2

Electrochemical activities of NaNi0.5Mn0.5O2 and NaCrO2, having the analogous layered structure to LiCoO2, were investigated in 1 mol dm-3 NaClO4 propylene carbonate at room temperature. Almost all sodium ions were extracted from the NaNi0.5Mn0.5O2 and NaCrO2 electrodes by galvanostatic oxidation to 4.5 V accompanied with several phase transitions. Layered NaNi0.5Mn0.5O2 electrode showed a highly reversible capacity of 185 mAh g-1 as positive electrode in Na cell in the potential region between 2.5 and 4.5 V versus Na. A NaCrO2 electrode was hardly electroactive after oxidation up to 4.5 V. When galvanostatic cycling was carried in the limited potential domain between 2 and 3.5 V, both electrodes showed discharge capacities of 100 - 120 mAh g-1 with satisfactory capacity retention. Layered LiCrO2 (R-3m) and NaCrO2 (R-3m) possess the quite similar crystal structures and the same transition metal, nevertheless they were inactive and active in Li and Na cells, respectively.

Electrochemical Insertion of Li and Na Ions into Nanocrystalline Fe3O4 and α‐Fe2O3 for Rechargeable Batteries

Fe 3 O 4 powders with different particle sizes on average (400, 100, and 10 nm) were prepared and characterized by X-ray diffraction, transmission electron microscopy, Mossbauer spectroscopy, and electrochemical methods. To examine the electrochemical activity of Fe 3 O 4 in relation to the particle size effect, galvanostatic cycling tests in aprotic electrolytes containing lithium or sodium ions were conducted. The electrochemical activity was significantly enhanced as the mean particle size decreased. The nanocrystallized Fe 3 O 4 (10 nm) prepared by precipitation method delivered 190 mA h g ―1 of the rechargeable capacity in the voltage range of 2-3 V in a lithium-ion containing electrolyte, whereas the 400 and 100 nm Fe 3 O 4 powders showed 10 and 80 mA h g ―1 of the rechargeable capacity, respectively. An ex situ X-ray diffraction study for the electrochemically cycled samples suggested the partly reversible Fe ion migration from the tetrahedral sites to the octahedral sites with a retained spinel framework structure. The nanocrystallized Fe 3 O 4 as well as α-Fe 2 O 3 were highly electrochemically active in the sodium salt electrolyte. The rechargeable capacity of 160 or 170 mA h g ―1 with excellent capacity retention was obtained for nanocrystalline Fe 3 O 4 or α-Fe 2 O 3 , respectively.

Structural, Electrochemical, and Thermal Aspects of Li [ ( Ni0.5Mn0.5 ) 1 − x Co x ] O2 ( 0 ≤ x ≤ 0.2 ) for High-Voltage Application of Lithium-Ion Secondary Batteries

We investigated structural, electrochemical, and thermal properties of layered Li[(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 (0 ≤ x ≤ 0.2) oxides synthesized via coprecipitation. The prepared materials had a well-ordered 03 type α-NaFeO 2 layer structure. The occupation of divalent Ni in the Li layer decreased monotonously with increasing Co amount in Li[(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 . Because of the improved structural integrity and electrical conductivity, the Co substitution for Ni and Mn gave rise to the increment on the initial discharge capacity. However, the replacement brought about severe capacity fading during extensive cycling in a Li-ion cell. To elucidate the possible reasons for the capacity fading, electrochemically and chemically delithiated Li 1-δ [(Ni 0.5 Mn 0.5 ) 1-x Co x ]O 2 powders were examined through the storage at 60°C for 300 h in the electrolyte. With increasing Co content, the amount of dissolved Ni, Co, and Mn greatly increased. Furthermore, the original 03 (R3m) layer structure was completely transformed to O1 (P3ml) phase for the Li 0.1 [(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 , being accompanied by a severe particle degradation. However, Li 0.1 [Ni 0.5 Mn 0.5 ]O 2 maintained its original structure with uniform surface morphology, which would be mainly attributed to the presence of divalent Ni in the Li layer. A high-temperature X-ray diffraction study with a combination of thermal gravimetric analysis also confirmed that the 03 phase was stable to 200-250°C without significant weight loss in that region for the Li 0.1 [Ni 0.5 Mn 0.5 ]O 2 . Whereas the Li 0.1 [(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 having O1 layer structure showed a gradual weight loss at the temperature, which would result from the oxygen loss from the oxide. The Li[Ni 0.5 Mn 0.5 ]O 2 has a large amount of Ni 2+ in the Li layer, which provided significant structural, electrochemical, and thermal stabilities at a highly delithiated state, compared to the Li[(Ni 0.5 Mn 0.5 ) 0.8 Co 0.2 ]O 2 .