Formation of solid solution and its effect on lithium insertion schemes for advanced lithium-ion batteries:: X-ray absorption spectroscopy and X-ray diffraction of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2

The X-ray absorption spectra (XAS) of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2 were examined together with X-ray diffraction (XRD). Co and Ni K-edge XANES spectra of LiCo1/2Ni1/2O2 are quite similar to that of LiCoO2 or LiNiO2, suggesting that electronic states of Co and Ni in LiCo1/2Ni1/2O2 are Co3+ and Ni3+. Analytical results of Co and Ni K-edge EXAFS oscillations on the first coordination shell of nickel and cobalt ions in LiCo1/2Ni1/2O2 indicate that the local environment around the targeted species is the same as that in LiCoO2 or LiNiO2. Since there is no doubt about the crystal and electronic structures of LiCoO2 and LiNiO2, the results indicate that LiCo1/2Ni1/2O2 consists of low-spin states of Co3+ and Ni3+ distributed at equivalent positions in triangular lattice of sites forming homogeneous transition metal oxide layers. Thus, XAS complements XRD in describing solid solution LiCo1/2Ni1/2O2 of LiCoO2 and LiNiO2. The electrochemical behaviors of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2 are also restated and the effects of the formation of solid solution on the change in lattice dimension during topotactic electrochemical reactions are discussed.