Spinel, formation structure

Electrodes that are prepared from acid-leached LT-LiCo, xNix02 compounds (0< x<0.2) show significantly enhanced electrochemical behavior over the parent LT-LiCo1 xNix02 structure. The improved performance has been attributed to the formation of compounds with a composition and cation arrangement close to the ideal Li[B2]04 spinel structure (B = Co, Ni) [62]. These spinel-type structures have cubic symmetry, which is maintained on lithiation the unit cells expand and contract by only 0.2 percent during lithium insertion and extraction. 

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

Let us now turn to diffusion in the general case, without worrying about the exact mechanism or the rates of diffusion of the various species. As an example to illustrate how we would analyze a diffusion-limited solid state reaction, we use the general equation describing formation of a compound with spinel (cubic) structure and stoichiometry ... 

CCP close-packed structures,or pillars are placed between the layers to provide the stabilization. We reported on compounds KMn02 i o,i84 (VO)j-Mn02, which are examples of such pillared structures. The former is stable to spinel formation at low current densities, and the latter shows excellent stability but poor rate capability. The groups of Dahn and Doeff among others have pursued non-ccp structures by looking at tunnel structures such as... 

Experiments have shown that Aoxide spinel formation is on the order of 10 4cm at ca. 1000°C [C.A. Duckwitz, H. Schmalzried (1971)]. Using Eqns. (10.45) and (10.46) with the accepted cation diffusivities (on the order of 10 10 cm2/s), one can estimate from j% that each A particle crosses the boundary about ten times per second each way. In other words, quenching cannot preserve the atomistic structure of a moving interface which developed during the motion by kinetic processes. This also means that heat conduction is slower than a structural change on the atomic scale, unless one quenches extremely small systems. 

Modification of Y-AI2O3 by Mg and leads to the diffusion of these cations into the y-AI2O3 lattice and results in non-stochiometric spinel formation MgAl204 and LiAlsOg, as was confirmed by X-ray data [4], The amount of incorporated cation seems to determine the surface basicity the benzoic acid (b.a.) adsorption per surface unit of the catalyst increased with the Mg " amount added ( Table 1). This way the surface basicity can be regulated without changing the structure of the catalyst surface or bulk, since Y-AI2O3 is known to have the same spinel structure with unit cell of cubic type and cell parameter a = 7.907 A. 

In high radiation fields, the spinel crystal structure has been shown to change. The structure, while still cubic, becomes disordered with a reduction in lattice parameter. The disordered rock-salt structure has a smaller unit cell reflecting the more random occupation of the octahedral sites by both trivalent and divalent ions. Increased radiation damage results in the formation of completely amorphous spinels. Radial distribution functions (g(r)) of these amorphous phases have Al-0 and Mg-O radial distances that are different from equivalent crystalline phases. The Al-0 distance in the amorphous form is reduced from Al-O of 0.194nm in the crystalline phase to 0.18nm in the amorphous phase, while the Mg-O distance is increased (0.19nm in the crystal to 0.21 nm in the amorphous phase). Differences between the Al-O distances of crystalline and amorphous phases are a characteristic of both calcium and rare earth aluminates. 

DRS spectra indicated the change of promoter structure, with spinel formation at temperatures exceeding 700°C, i.e. incorporation of Ni(Co) in alumina support causing the destruction of previously formed active phases of the catalyst. 

Evidence of spinel structure within some of the alumina and titania catalysts was established by x-ray powder techniques. Samples were removed from the reaction bed of some of the runs where high yields of sodium sulfate had been obtained using either alumina or titanium dioxide as catalyst. The presence of compounds having spinel type structure was established by x-ray powder diffraction. The amount of spinel was variable and of small quantity because neither the proportions of necessary ions nor the reaction conditions were ideal for spinel formation. 

When or-Fe203 was used as the positive electrode in high-temperature lithium cells, the introduction of a small amount of lithium into the corundum-type structure caused the hexagonal-close-packed oxygen array to shear irreversibly to cubic-close packing which generated a defect /-Li FCjOj (spinel-type) structure. Further lithiation resulted in the formation of LiFe,Ojj thereafter, the reaction followed the same sequence as that shown in reactions (4), (5) and (6) [100]. The stability of the spinel structures at elevated temperatures, as well as the ability of the cubic close-packed oxygen array to accommodate lithium at the expense of... 

The chemical structures of the above three reaction products were identified by X-ray dif-fractometry. Using a DTA-EGD coupled technique, the calcining temperature range of spinel formation can be confirmed. 

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