L. C. De Jonghe

Electrochemical Insertion of Sodium into Carbon

Electrochemical insertion of sodium ions into carbon using solid polymer electrolytes or organic liquid electrolytes is described. Cells with the configuration Na/P(EO)sNaCF3SOJCP(EO) = polyethylene oxide) or Na/liquid electrolyte/C were galvanostatically discharged, charged, and cycled. The extent of insertion into C (Le., x in Na§ was found to be a strong function of the type and particle size of the carbon used, and the reversibility of the process was highly dependent upon the type of electrolyte used. The possibility of designing a sodium ion rocking chair cell is discussed, and a first-generation example, using a petroleum coke anode, polymer electrolyte, and sodium cobalt bronze cathode is described. Rocking chair batteries, in which both the anode and cathode are intercalation materials, have recently been commercialized. Because the anodes are commonly inexpensive carbons such as petroleum coke or graphite, reductive intercalation of lithium into these materials is now the subject of intense scrutiny] Similar sodium insertion reactions into carbons have been observed, 2 but have not yet been exploited for use in batteries. We now describe a preliminary study of these insertion reactions and discuss the possibility of developing a sodium ion cell analogous to the wellknown lithium ion systems. Experimental Conoco petroleum coke, Shawinigan black, and JohnsonMatthey microcrystalline graphite were either ground in an attritor mill or used as received after heat-treatment. Polymer electrolytes of composition P(EO)sNaCF3SO3 (PEO = polyethylene oxide) and composite cathodes containing the carbon of interest, PEO, and NaCF3SO3 were made as described previously. 3 Electrodes for use in cells with liquid electrolytes consisted of carbon and ethylene propylene diene monomer (EPDM) binder (2% by weight) and were vacuum dried prior to use. Battery-grade solvents from Mitsubishi Petrochemical Company were stored in an inert atmosphere glove box (02 < 1 ppm) and used as supplied. Sodium was purified as described previously?

Room temperature synthesis of crystalline metal oxides

Crystalline titanium dioxide powders have been synthesized as either rutile or anatase from aqueous solutions at low temperatures (T≤100°C) and atmospheric pressure. First, a sol is prepared by the hydrolysis of a titanium alkoxide in an acidic solution. The sol is subsequently heated at different rates to produce the different crystalline phases of titanium dioxide. Powder characterization was carried out using X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. In general, the precipitate size was observed to be between 50 and 100 nm. Possible mechanisms involved in determining the crystal variants are discussed.

TRANSIENT STRESSES IN BIMODAL COMPACTS DURING SINTERING

Abstract A method is described and used to evaluate the transient stresses in a sintering compact of ZnO containing a hard, dense dispersion of SiC. A hard second phase can severely limit densification rates by generating a mean hydrostatic stress, σh, which opposes the compressive sintering stress of the matrix. σh rapidly increases with increasing volume fraction, ƒ, of the second phase. The interface stress, σi., at the ZnO SiC boundary increases with decreasing ƒ, σi can attain large values, especially in the intermediate stage of sintering. The effect of these stresses on microstructural development is considered.

Orthorhombic Na{sub x}MnO{sub 2} as a Cathode Material for Secondary Sodium and Lithium Polymer Batteries

The use of orthorhombic Na[sub x]MnO[sub 2] as a cathode material for alkali metal polymer electrolyte batteries is described for the first time. This sodium manganese bronze has a tunnel structure and can reversibly intercalate up to 0.55 to 0.6 alkali metal ions (Li[sup +] or Na[sup +]) per manganese at moderate current densities, corresponding to capacities of 160 to 180 mAh/g. Li/PEO/Na[sub 0.2]Li[sub x]MnO[sub 2] cells [PEO = poly(ethylene oxide)] to date have been cycled over ninety times at 0.1 mA/cm[sup 2], with excellent capacity retention. Na/PEO/Na[sub x]MnO[sub 2] and Na[sub x]MnO[sub 2]/PEO/Na[sub x]MnO[sub 2] cells have been cycled over sixty times to date at the same rate, showing moderate capacity fading.

Electrochemical properties of organic disulfide thiolate redox couples

The redox behavior, kinetic reversibility, chemical reversibility, and stability, and the specific adsorption or chemisorption at electrode surfaces of a diverse group of organodisulfide cathode materials have been studied by potential-sweep and potential-step methods. The number of electrons involved in the redox reaction and the diffusion coefficients of the organodisulfide species in electrolyte solutions were determined with a rotating disk electrode in conjunction with chronocoulometry/chronoamperometry. Observations indicate that the overall, stoichiometric reaction of those redox couples is RSSR + {r reversible} where R represents an organic moiety. These reactions are chemically reversible, yet kinetically hindered, especially at ambient temperatures. The microscopic reversibility of the redox couples promises the possibility of constructing secondary energy conversion systems based on these materials. The slow electrode kinetics, however, indicates that the introduction of electrocatalysts to assist the electrode reaction may be effective in improving battery performance. The negligible adsorption of these materials at platinum electrodes, in addition, implies that the electrode kinetics can be formulated by simple electrodic equations without consideration of surface coverage.

ROLE OF THE GRAIN-BOUNDARY PHASE ON THE ELEVATED-TEMPERATURE STRENGTH, TOUGHNESS, FATIGUE AND CREEP RESISTANCE OF SILICON CARBIDE SINTERED WITH Al, B AND C

The high-temperature mechanical properties, specifically strength, fracture toughness, cyclic fatigue-crack growth and creep behavior, of an in situ toughened silicon carbide, with Al, B and C sintering additives (ABC-SiC), have been examined at temperatures from ambient to 1500°C with the objective of characterizing the role of the grain-boundary film/phase. It was found that the high strength, cyclic fatigue resistance and particularly the fracture toughness displayed by ABC-SiC at ambient temperatures was not severely compromised at elevated temperatures; indeed, the fatigue-crack growth properties up to 1300°C were essentially identical to those at 25°C, whereas resistance to creep deformation was superior to published results on silicon nitride ceramics. Mechanistically, the damage and shielding mechanisms governing cyclic fatigue-crack advance were essentially unchanged between ∼25°C and 1300°C, involving a mutual competition between intergranular cracking ahead of the crack tip and interlocking grain bridging in the crack wake. Moreover, creep deformation was not apparent below ∼1400°C, and involved grain-boundary sliding accommodated by diffusion along the interfaces between the grain-boundary film and SiC grains, with little evidence of cavitation. Such unusually good high-temperature properties in ABC-SiC are attributed to crystallization of the grain-boundary amorphous phase, which can occur either in situ, due to the prolonged thermal exposure associated with high-temperature fatigue and creep tests, or by prior heat treatment. Moreover, the presence of the crystallized grain-boundary phase did not degrade subsequent ambient-temperature mechanical properties; in fact, the strength, toughness and fatigue properties at 25°C were increased slightly.

Sintering stress of homogeneous and heterogeneous powder compacts

Abstract The thermodynamic meaning of the sintering stress is considered for some simple 2-dimensional porous bodies. The sintering stress acts as an effective mean grainboundary stress and can thus be meaningfully combined with applied stresses. The sintering stress can, in general, not be described in rigorous thermodynamic terms, especially for heterogeneous compacts. Simplifications do allow, however, to assess the effects of some microstructural heterogeneities, including the presence of large pores or of dispersed second phases, on the sintering stress.

A High-Rate Manganese Oxide for Rechargeable Lithium Battery Applications

Li x MnO 2 made by ion exchange of glycine-nitrate combustion synthesis-processed (GNP) orthorhombic Na 0.44 MnO 2 (GNP-Li x MnO 2 ) has been cycled in lithium/liquid electrolyte cell configurations at room temperature and lithium/polymer cell configurations at 85°C over one hundred times without showing capacity fading or phase conversion to spinel. At 2.5 mA/cm 2 in liquid cells (5C rate) or I mA/cm 2 (1.5C rate) in polymer cells, 80-95% of the expected capacity is delivered. The remarkable stability is attributable to the unusual double tunnel structure, which cannot easily undergo rearrangement to spinel. The enhanced rate capability of GNP-Li x MnO 2 compared to conventionally prepared materials is attributable to the shorter particle length, which allows faster diffusion of lithium ions along the tunnels.

Thin-film ceramic electrolytes deposited on porous and non-porous substrates by sol-gel techniques

Abstract Successful deposition of ultra-thin ceramic membranes by the sol-gel process could lead to greatly improved electrolyte devices. The parameters of the sol-gel technique which affect film cracking and continuity have been investigated, and the effect of substrate conditions and substrate porosity have been determined. Crack-free yttria-stabilized zirconia films having thickness of approximately 0.2 to 0.5 μm have been deposited on continuous quatz substrate. Preliminary results have also indicated that continuous sub-micron electrolyte films can be deposited on porous substrates.

The Use of Polydisulfides and Copolymeric Disulfides in the Li/PEO/SRPE Battery System

Solid redox polymerization electrodes (SRPEs) have recently been used successfully as cathodes in lithium solid polymer electrolyte batteries. SRPEs contain organopolydisulfides, (SRS) n , as the electroactive material; upon cell discharge these materials are reductively depolymerized via scission of the disulfide linkages to di- or trithiolate salts. The thiolate salts are reoxidized to the polymeric disulfides when the cell is recharged. Organopolydisulfides are easily synthesized via a one-step process, are inexpensive, and exhibit high performance levels in batteries