Wan M. Im

Bio: Wan M. Im is an academic researcher from Seoul National University. The author has contributed to research in topics: Crystal structure & Lattice (order). The author has an hindex of 1, co-authored 1 publications receiving 20 citations.

Factors Controlling the Stability of O3− and P2‐Type Layered MnO2 Structures and Spinel Transition Tendency in Li Secondary Batteries

Abstract: Cathode properties of two layered manganese dioxides (AxMnO21d?yH2O, where A is the pillaring alkali cations) having different crystal structures were compared in 3 V Li secondary batteries. The materials were prepared from the mixture of KNO 3, LiOH, and MnO at 800 and 10508C, respectively. The 8008C-prepared MnO2 has a trigonal R3m space group with an O3-type oxide-packing pattern, whereas the 10508C material has an orthorhombic Cmcm symmetry with a P2-type oxide-packing pattern. The gallery space where the pillaring cations and water molecules reside is wider in the case of the 800 8C material. Due to the higher mobility of pillaring cations in the 800 8C material and similarity in the oxide-packing pattern (O3-type) to the spinel phases, the pillaring cations are easily leached out during cell cycling, which ultimately leads to a lattice collapse and structural transition t o the spinel-related phases. By contrast, as the 1050 8C material has rather immobile pillaring cations and its oxide-packing pattern (P2type) is far different from that of the spinel phases, this cathode shows better cycling performance, with its structural integri ty being well maintained.

Optimization of Insertion Compounds Such as LiMn2 O 4 for Li-Ion Batteries

Abstract: The spinel LiMn 2 O 4 , whose electrochemical activity with Li was discovered in the early 1980s, was put forth in the early 1990s as a possible alternative to LiCoO 2 as a positive electrode material for Li-ion batteries. Ten years later, the Li-ion LiMn 2 O 4 /C cells are on the verge of entering the portable electronics and electric/hybrid vehicle market. This paper retraces the key steps of this decade that were necessary to master the intimate physical/electrochemical relationship of LiMn 2 O 4 , and that led to the development of rechargeable Li-ion LiMn 2 O 4 /C technology. During the long development period, the early supremacy of LiMn 2 O 4 as the only alternative to LiCoO 2 diminished with the development of positive electrode materials that present abundance and cost advantages. Despite the uncertainty of the future of the spinel, successfully translating a fundamental success into a commercial one, we stress that the long learning experience will benefit the scientific battery community aiming at rapidly optimizing the electrochemical performance of alternative materials, such as LiFePO 4 .

Cathode compositions for lithium ion batteries

Abstract: A cathode composition for a lithium-ion battery having the formula Li[M 1 (1-x) Mn x ]O 2 where 0

Birnessite polytype systematics and identification by powder X-ray diffraction

Abstract: The polytypes of birnessite with a periodic stacking along the c* axis of one-, two-, and three-layers are derived in terms of an anion close-packing formalism. Birnessite layers may be stacked so as to build two types of interlayers: P-type in which basal O atoms from adjacent layers coincide in projection along the c * axis, thus forming interlayer prisms; and, O-type in which these O atoms form interlayer octahedra. The polytypes can be categorized into three groups that depend on the type of interlayers: polytypes consisting of homogeneous interlayers of O- or P-type, and polytypes in which both interlayer types alternate. Ideal birnessite layers can be described by a hexagonal unit cell ( a h = b h ≈ 2.85 A and γ = 120°) or by an orthogonal C-centered cell ( a = √3 b , b h = 2.85 A, and γ = 90°); and, hexagonal birnessite polytypes (1 H , 2 H 1 , 2 H 2 , 3 R 1 , 3 R 2 , 3 H 1 , and 3 H 2 ) have orthogonal analogs (1 O , 2 O 1 , 2 O 2 , 1 M 1 , 1 M 2 , 3 O 1 , and 3 O 2 ). X-ray diffraction (XRD) patterns from different polytypes having the same layer symmetry and the same number of layers per unit cell exhibit hkl reflections at identical 2𝛉 positions. XRD patterns corresponding to such polytypes differ only by their hkl intensity distributions, thus leading to possible ambiguities in polytype identification. In addition, the characteristics of the birnessite XRD patterns depend not only on the layer stacking but also on the presence of vacant layer sites, and on the type, location, and local environment of interlayer cations. Several structure models are described for birnessite consisting of orthogonal vacancy-free or of hexagonal vacancy-bearing layers. These models differ by their stacking modes and by their interlayer structures, which contain mono-, di-, or trivalent cations. Calculated XRD patterns for these models show that the hkl intensity distributions are determined by the polytype, with limited influence of the interlayer structure. Actual structures of phyllomanganates can thus be approximated by idealized models for polytype identification purpose. General rules for this identification are formulated. Finally, the occurrence of the different polytypes among natural and synthetic birnessite described in the literature is considered with special attention given to poorly understood structural and crystal-chemical features.