16d sites

Some cations with an octahedral-site preference (such as Ni2+, Co3+, and Cr3+) are expected to occupy the 16d sites of the spinel with Mn, whereas cations with a strong tetrahedral-site preference (such as Zn2+) are expected to occupy the 8a sites and to dislodge corresponding lithium ions into the 16d sites. In cases where Mn is substituted by transition metal ions (such as Co, Ni, and Cr) that can partake in the electrochemical reaction, voltage plateaus between 4 and 5V have been observed [135, 136],... 

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... 

Both CU7 38Mn4Sni2S32 (Z = l) and CU7 07Ni4Sni2S32 (Z = l) crystallize in the cubic Fd3m space group with a= 10.4145 and 10.305(1) A respectively. The occupancies of M and Sn (which occupy the 16d sites) refine to 0.25 and 0.75 and hence were fixed in the final refinement. However in both the structures a small amount of Cu deficiency was observed in the 8 a sites. The interatomic distances in both the compounds are in accordance with the earher known compounds [17]. The ionic radii of Mn (VI, HS) is 0.83 and is larger than the size of the Ni (Vl) which is 0.69 [22]. Accordingly the Mn/Sn-S bond distance is greater than the Ni/Sn-S bond distance. 

In spinel LiMn O, the light element, lithium, occupies the tetrahedral 8a site and the heavy metal element, manganese, occupies the octahedral 16d site (32e site) forms the cubic close packing. The spinel of such a construction is called a... 

Fig. 2.2 Structures of common cathode materials (a) The layered structure of LiCo02 with c-axis oriented vertically. The octahedrally coordinated Li ions in 3a sites are represented as spheres and CoOg (Co in 3b sites) as octahedra (b) the cubic structure of LiMu204 spinel, with tetrahedrally coordinated Li ions (in 8a sites) represented as spheres, and MnOg (Mn in 16d sites) as octahedra and (c) the olivine structure of LiFeP04, looking down the h-axis. Octahedrally coordinated Li ions are represented as spheres, and FeOg and PO4 as octahedra and tetrahedia, respectively...

Spinels. The A[M2]04 spinels have a cubic-close-packed array of oxygen atoms with M-site cations in half of the octahedra, the 16d sites of space group Fd3m of Fig. 3.15. The [M2]04 array represents a strongly bonded framework with A atoms in the interstitial tetrahedral sites 8a. The empty 16c octahedral sites share faces with the 8a sites to form, with the 8a sites, a 3D-interconnected interstitial space. On LF-ion insertion into an empty 16c site, the coulombic repulsive force between the LF-ion and the near-neighbor A-site cations pushes all the A-site cations in a cascade into the 16c sites [18],... 

In the case of magnetite, Fe[Fe2]04, insertion of one Li per formula unit produces an ordered rock-salt array with Fe remaining on aU the 16d sites and Li and Fe randomly distributed over the 16c sites. Further insertion of Li extrudes Fe reversibly... 

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. 

For Ln = Sm to Lu, LnaTiaOy crystallizes in the pyrochlore/cubic structure in bulk. In this structure, the Ln cations occupy the 16d site and are coordinated with 8 oxygen ions, while Ti occupy the 16c site and are located at the center of the distorted octahedra (using Wyckoff notation). The pyrochlore/cubic structure is a superstructure of the ideal fluorite with 1/8 anion defect. The lattice parameter evolves from = 10.231(1) A [59,60] to 10.024 A [61] for Sm to Lu, respectively. 

Figure 15. Two quadrants of the A[B2]04 spinel structure, with A (Striate balls) at 8a sites, B (Black alls) at 16d sites, and O (White balls) at 32e sites. For LiMn204, Li will occupy 8a sites and Mn at 16d sites. [93]...

The relationship between the crystal parameter a and g (Al), the number of 16d sites occupied by Al. (Adapted from Komaba, S. et al., Solid State Ionics, 149,2002.)... 

Spinel Li4Ti50i2 is a white crystal and is stable in air. Its crystal structure, which is similar to the spinel LiMn204 shown in Chapter 4, is shown in Figure 8.2. It can also be written as Li[Lii/3Ti5/3]04. It belongs to space group Fd3m with crystal parameter a = 0.836 nm. anions situated at 32e sites form an face-centered cubic (FCC) lattice. Some Li atoms are situated at tetrahedral 8a sites, and the other Li atoms and Ti atoms at octahedral 16d sites. When... 

Li can also be doped into Li4Ti50i2 to increase capacity and improve cycling performance. It is known that the presence of doping elements at tetrahedral 8a sites will increase irreversible capacity and that doping at octahedral 16d sites will lower the reversible capacity. Furthermore, when Li is doped at interstitial 48f sites, phase transitions can be avoided. 

VlCo VCo ) 4.2 V and V (Ni /Ni ) 4.8 V. Orthorhombic LiVtMn]04 was transformed to the spinel V[LiMn]04 under pressure of 55 kbar at 850°C [61] electrochemical delithiation gave V°(Mn /Mn ) 3.8 V aU vv. Li. These values represent a remarkable stabilization of the redox energies compared to their values with Li in the 8a sites. However, these spinels are not of interest as battery cathodes as the Li -ion mobility is reduced by its coexistence with M atoms on the octahedral 16d sites. 

---