Rongji Chen

The hydrothermal synthesis of new oxide materials

Abstract The development of advanced secondary lithium batteries depends on cathode structures that can reversibly intercalate lithium ions. The initial prototypical system Li/TiS 2 showed the feasibility of high energy density systems with extensive reversibility. However, with the advent of the safe lithium-carbon anodes by Sony there is a need for higher voltage cathodes. LiCoO 2 presently fulfills this need in small cells. However, for large systems where cost is an issue new oxides are needed. One approach to the formation of such oxide is low temperature hydrothermal synthesis. Mild hydrothermal reactions lead to the formation of new metastable transition metal oxide structures, not accessible by conventional high temperature methods, which have relatively open crystal structures. The nature of the cations present in solution (the “templating ion”) has a dramatic effect on the crystal structure of the phase formed, as also does the pH of the reaction medium and the particular transition metal. Thus, by appropriate choice of reaction medium new structures containing large tunnels or channels, similar to those found in aluminosilicate zeolites, can be formed that will offer unique properties for the Materials Scientist. In particular, it is expected to be possible to realize enhanced diffusion in such materials. Here, the hydrothermal synthesis of tungsten, molybdenum and vanadium oxides is considered. The role of the cation in the synthesis is described, as well as the key interfacial reactions that are taking place. Cases are described where the cation controls the structure formed and is retained in the structure, to instances where the cation critically controls the reactions occurring but is not retained in the lattice.

Cathodic Behavior of Alkali Manganese Oxides from Permanganate

Extensive research is currently underway to find promising candidates for cathode materials in lithium secondary batteries, and a manganese oxide that behaved like the layered LiCoCO{sub 2} would be a prime candidate for this application because of its high free-energy of reaction with lithium and relatively low cost. The reaction of potassium, sodium, and lithium permanganate in water at 170 C leads directly to potassium, sodium, and lithium manganese dioxides, A{sub y}MnO {center_dot} nH{sub 2}O, with a R{bar 3}m rhombohedral structure. These crystalline layered structures after dehydration readily and reversibly react with lithium through an intercalation mechanism. The capacity for lithium is a function of the alkali ion present, and the larger potassium ion maintains the capacity best. For lithium there is a tendency to convert to the spinel structure which leads to loss of capacity.

The hydrothermal synthesis of sodium manganese oxide and a lithium vanadium oxide

Abstract We report here the direct synthesis of the hexagonal form of sodium manganese dioxide using the mild hydrothermal decomposition of sodium permanganate in water at 170 °C. Sodium manganate, Na 0.35 MnO 2 · 0.7H 2 O, has an R3m rhombohedral structure like Li x (H 2 O)TiS 2 with a 7 A repeat distance indicative of a monolayer of water between the manganese dioxide layers. The water may be readily removed by gently heating to 150 °C in air. This manganese oxide and a new vanadium oxide, also formed hydrothermally, react readily and reversibly with lithium ions and are under evaluation for use as the cathode in high energy density lithium batteries.

The intercalation and hydrothermal chemistry of solid electrodes

Abstract Intercalation chemistry plays a key role in the electrochemical reduction and oxidation by lithium of many solid electrodes, including transition metal compounds and graphite. For 25 years, from TiS 2 and graphite to Li x CoO 2 and LiMn 2 O 4 , the electrode reactions of such intercalation compounds have been extensively studied. Much emphasis has been placed on two simple classes of structures, layer and spinel, both of which are formed by Li x TiS 2 and Li x MnO 2 . More recently, much effort has been directed at synthesizing new structures that might show enhanced electrochemical activity. Soft chemistry approaches have been harnessed for this purpose. Mild hydrothermal reactions are one such approach. Several new vanadium oxides have been formed, as well as layered forms of manganese oxide. A number of these new compounds reversibly react with lithium and therefore may be used as the cathode in lithium batteries.

The hydrothermal synthesis of the new manganese and vanadium oxides, NiMnO3H, MAV3O7 and MA0.75V4O10·0.67H2O (MA=CH3NH3)

The hydrothermal reaction of manganese or vanadium oxides in the presence of organic cations leads to the formation of new layered structures. When nickel salts are hydrothermally reacted with tetramethylammonium permanganate a new orthorhombic form of nickel manganese oxide, NiMnO 3 H x (where x≤1) is formed. It readily chemically intercalates lithium; it has space group Cmc2 1 , a=2.861(1) A, b=14.650(1) A, and c=5.270(1) A. The magnetic properties of this compound are quite different from the ilmenite NiMnO 3 phase, showing paramagnetism above 390 K and more complex behavior below that temperature. The hydrothermal reaction of vanadium pentoxide with methylamine leads to a series of new layered vanadium oxides, which differ in structure from the corresponding ones prepared in the presence of the tetramethylammonium ion because of the existence of hydrogen bonding. Methylamine is the first organic to form a double sheet vanadium oxide, (CH 3 NH 3 ) 0.75 V 4 O 10 ·0.67H 2 O, with the δ-Ag x V 2 O 5 structure. (CH 3 NH 3 ) 0.75 V 4 O 10 ·0.67H 2 O is monoclinic, space group C2/m with a=11.673(1) A, b=3.668(1) A, c=11.095(1) A and β=99.865(5)°. (CH 3 NH 3 )V 3 O 7 shows significant buckling of the layers compared with N(CH 3 ) 4 V 3 O 7 , and has a monoclinic unit cell, space group P2 1 /c with a=11.834(8) A, b=6.663(4) A, c=15.193(9) A and β=138.104(1)°.

Low Temperature Synthesis of Lamellar Transition Metal Oxides Containing Surfactant Ions

Recently there has been much interest in reacting vanadium oxides hydrothermally with cationic surfactants to form novel layered compounds. A series of new transition metal oxides, however, has also been formed at or near room temperature in open containers. Synthesis, characterization, and proposed mechanisms of formation are the focus of this work. Low temperature reactions of vanadium pentoxide and ammonium (DTA) transition metal oxides with long chain amine surfactants, such as dodecyltrimethylammonium bromide yielded interesting new products many of which are layered phases. DTA{sub 4}H{sub 2}V{sub 10}O{sub 28}{center_dot}8H{sub 2}O, a layered highly crystalline phase, is the first such phase for which a single crystal X-ray structure has been determined. The unit cell for this material was found to be triclinic with space group P {bar 1} and dimensions a = 9.895(1){angstrom}, b = 11.596(1){angstrom}, c = 21.924(1){angstrom}, {alpha} = 95.153(2){degree}, {beta} = 93.778(1){degree}, and {gamma} = 101.360(1){degree}. Additionally, the authors synthesized a dichromate phase and a manganese chloride layered phase, with interlayer spacings of 26.8{angstrom}, and 28.7{angstrom} respectively. The structure, composition, and synthesis of the vanadium compound are described, as well as the synthesis and preliminary characterization of the new chromium and manganese materials.