Magnetism as a Tool in the Solid-State Chemistry of Cathode Materials

7 Magnetism as a Tool in the Solid-State Chemistry of Cathode Materials 

Let us start with this material with which we are familiar since the Sect. 13.5.2.2 of this chapter. We have already noted that the lattice of this compound is fragile, with a problem that is recurrent in Ni-based cathode materials Ni has a tendency to occupy Li lattice sites [65]. The problem with Ni is that it has a tendency to mix up with Li, so that instead of having all the Li ions on their (h)-lattice sites, and Ni on their (a)-sites, part of Ni ions are found on the lattice sites of the lithium, forming Ni(3b) defects, also named NIli- The reason is that the ionic radii of Li and Ni are almost the same 0.76 A for Li , 0.69 A for Ni . This is damageable the electrochemical properties, because the NiLi defect is a diffusing center for the Li ions, which affects their mobility. As a consequence, it is very difficult to prepare stoichiometric LiNi02 and the actual chemical composition of the final product after synthesis is usually Lii Nii+j02. The parameter z is thus very important. The most accurate determination of z is obtained by the analysis of magnetic properties. 

1 unpaired electron, so 5 = 1/2, and the effective magnetic moment associated to it is /teff=5(5+1)] /b = 1-73 / b- The experimental value /eff=l-81 is then evidence that all the nickel ions are not in the Ni configuration only a fraction x of them is in the Ni state, the other fraction 1 - x is in the Ni state due to some deviation from stoichiometry. According to the chemical formula Lii Nii+z02, the charge neutrality equation is  

Taking into account that j eff(Ni )= 1-73, y eff(Ni +) = 2.83 Eq. (13.73) gives the value z = 0.027. Note the slope of Curie constant is proportional to so the difference between 1.81j b with respect to 1.73 means a 

Let us now investigate the magnetic properties at low temperature. At temperature Tb = 8 K a peak of susceptibility is observed (see Fig. 13.25). In that case it is important to use two experimental procedures referred field cooled (FC) and zero-field cooled (ZFC). In the FC experiment, the magnetic field is applied at high 

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