Showing papers by "Yuji Miyahara published in 2023"

Correlation between Local Structure and Reaction Mechanism in Disordered Rock-Salt-Type Lithium–Manganese Oxides

Abstract: First-principles (FP) calculations of disordered rock-salt-type lithium–manganese oxides were performed by assuming a quasi-random structure model with the composition of Li16Mn16O32. The effects of randomness in the crystal structure and the changes in local atomic configurations during the Li extraction process on electronic structures, phase stability, and electrochemical properties were investigated by the FP calculations. The calculations indicate that the randomness of local atomic configurations, which originates from the disordered structure, makes it possible for Mn and O sites to take on a variety of electronic states. Analyses of densities of states, magnetic moments of Mn atoms, and Bader charges indicate the possibility that not only Mn but also oxygen atoms contribute to the redox reaction. The migration of Li and Mn is found to appear from octahedral to tetrahedral sites at a high state of charge. The FP calculations suggest that the change in local atomic configurations and electronic structures causes the transition in voltage profiles from sloping to 3 V plateau shape as the cycle progresses, which is characteristic of spinel-related positive electrode materials.

Scalable Synthesis of Porous Silicon by Acid Etching of Atomized Al-Si Alloy Powder for Lithium-Ion Batteries.

Abstract: Si anodes have attracted considerable attention for their potential application in next-generation lithium-ion batteries because of their high specific capacity (Li15Si4, 3579 mAh g-1) and elemental abundance. However, Si anodes have not yet been practically applied in lithium-ion batteries because the volume change associated with lithiation and delithiation degrades their capacity during cycling. Instead of considering the active material, we focused on the structural design and developed a scalable process for producing Si anodes with excellent cycle characteristics while precisely controlling the morphology. Al-Si alloy powders were prepared by gas atomization, and porous Si with a skeletal structure was prepared by leaching Al using HCl. Porous Si (p-Si12, p-Si19) prepared from Al88Si12 and Al81Si19 comprised resinous eutectic Si, and porous Si (p-Si25) prepared from Al75Si25 comprised lumpy primary Si and resinous eutectic Si. The porosity of the Si anodes varied from 63% to 76%, depending on the Si composition. The p-Si19 anode displayed the finest pore distribution (20-200 nm), excellent rate characteristics, a reversible discharge capacity of 1607 mAh g-1 after 200 cycles at a rate of 0.1 C with a Coulombic efficiency of over 97%, and high stability. The performances of the p-Si25 and p-Si19 electrodes began to decrease after 250 and 850 cycles, respectively, with a constant-charge capacity of 1000 mAh g-1 and at a rate of 0.2 C. In contrast, the p-Si12 anode maintained its discharge capacity at 1000 mAh g-1 for up to 1000 cycles without degradation. Therefore, the developed manufacturing process is expected to produce porous Si as an active material in lithium-ion batteries for high capacity and long life at an industrial scale.