The Normal Spinel, Li

It is worthwhile to point out that lithium extraction from inverse spinels V[LiM]04, such as V[LiNi]04 and V[LiCo]04 takes place at high voltage, typically between 4 and 5V [153]. Lithium is extracted from the octahedral 16d sites of these spinels with a concomitant oxidation of the divalent nickel or cobalt ions. From a structural point of view, this can be readily understood because lithium must be dislodged from the 16d octahedral sites, which are of low-energy, into neighboring energetically unfavorable 8b tetrahedra, which share all four faces with 16d sites that are occupied by nickel or cobalt and by lithium. Lithium extraction reactions 

Note that this reaction is similar to that used by Hunter in his preparation of L-Mn02 in acid medium at room temperature [121]  

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. 

Lithium-titanium-oxide spinels provide a relatively low voltage of 1.5V vs. lithium. They are, therefore, of interest as possible negative electrode materials for lithium-ion cells [161-163] they can be coupled, for example, to Li[Mn2104 (4V vs. Li) to yield a 2.5V lithium-ion cell, or to LixMn02 (3V vs. Li) to yield a 1.5V lithium-ion cell. Although these cells have a voltage lower than that of commercial 

LixC/Li, xCo02 cells (3.5V), they are attractive from a safety point of view because the titanium spinel operates at a potential significantly far away from that of metallic lithium. 

---

The normal spinel Li[Ti2]04 is a metallic oxide with a superconducting transition temperature of 13.7 K. The nominal formula is Li+[Ti3+Ti4+]04, in which the Li+ ions occupy the tetrahedral sites while the octahedral sites contain titanium with an average charge Ti3 5+, although as the material is metallic at room temperature the electrons are delocalized in a partly filled 3d band. 

Thomas MGSR, David WIF, Goodenough JB (1985) Synthesis and structural characterization of the normal spinel Li[Ni204]. Mat Res Bull 20 1137-1146... 

The spectra of the doped materials (Cr, Ni, Zn +, Li+, Co +, AP+) are similar to those seen for the nominally stoichiometric materials, and sets of resonances between 500 and 700 ppm are seen on cation doping in addition to that of the normal spinel environment (at ca. 500 ppm). Again, these resonances are assigned to lithium ions near manganese-(IV) cations. The lower intensity of the additional resonances seen on Cr + substitution, in comparison to Zn + or Ni + substitution, is consistent with the oxidation of fewer manganese ions near the depart ions. For the Li- and Zn-doped spinels, resonances at ca. 2300 ppm were also observed, which are assigned to lithium ions in the octahedral sites of the spinel structure. In the case of Zn doping, it is clear that the preference of Zn + for the tetrahedral site of the spinel structure forces the lithium onto the octahedral site. 

Other half plus the minority cations (Te) occupy the B sites of the normal spinel structure. The structure is shown in Fig. 32, which shows that the C 9-like network of comer-connected tetrahedra (of atoms on B sites) is now composed of strictly alternating Li and Te atoms. The truncated tetrahedral interstices thus formed are centred by the remaining Li atoms (on the A sites). 

The striking features of the structure (when compared with that of a normal spinel, Fig. 29) are (a) that the Li(2)04 tetrahedra are very tilted, so that the anion array is grossly distorted from the approximate cubic eutaxy observed in the normal spinel structure and (b) that, by contrast, the cation array is almost exactly that of the normal structure, and very regular indeed. [There is a small tetragonal distortion of the unit cell -symmetry P4i22-which has c/( /2 a) = 0.9697, compared with unity for the equivalent ratio of the cubic cell of the normal spinel structure.]... 

The nature of a spinel is described by a parameter X, the fraction of B atoms in tetrahedral holes some authors refer to the degree of inversion y (= 2X). For a normal spinel X = 0, and for an inverse spinel X =. Intermediate values are found (e.g. in a random spinel), and X is not necessarily constant for a given spinel but can in some cases be altered by appropriate heat treatment. For NiMn204 X varies from 0 37 (quenched) to 0-47 (slow-cooled). Values of X have been determined by X-ray and neutron diffraction, by measurements of saturation magnetization, and also by i.r. measurements. In favourable cases l.r. bands due to tetrahedral AO4 groups can be identified showing, for example, that in Li(CrGe)04 Li occupies tetrahedral positions. ... 

James Hunter of Eveready Battery Co. was the first to patent spinel cathode material. The application of material to Li-ion system has been developed by J. M. Tarascon [59] and extensively studied by M. Thackeray [60]. Generally, lithium spinel oxides suitable for the cathode are limited to those with a normal spinel in which the lithium ions occupy the tetrahedral (8a) sites and the transition-metal ions reside at the octahedral (16d) sites. Currently, spinel is the center of much interest as the cathode material for large format lithium-ion cell for hybrid electric vehicle applications where high power, safety, and low cost are the strongly required features. 

---