Yoshinari Makimura

Solid-State Chemistry and Electrochemistry of LiCo1 ∕ 3Ni1 ∕ 3Mn1 ∕ 3O2 for Advanced Lithium-Ion Batteries III. Rechargeable Capacity and Cycleability

Reaction mechanism of LiCo 1/3 Ni 1/3 Mn 1/3 O 2 was examined by ex situ X-ray diffraction (XRD) and electrochemical methods. According to XRD results, the change in hexagonal lattice parameters was quite similar to that of Li 1-x Co 1/2 Ni 1/2 O 2 , i.e., the unit cell volume was almost constant at 0 ≤ x < 2/3 in Li 1-x Co 1/3 Ni 1/3 Mn 1/3 O 2 , indicating homogeneous phase reaction over an entire range. Reversible-potential measurements on Li 1-x Co 1/3 Ni 1/3 Mn 1/3 O 2 (0 ≤ x < 1) against a lithium electrode were also carried out and the solid-state redox reactions were described by applying the concept of electrochemical density of states (EDS). Gibbs free-energy change for the reaction of Li + □CO 1/3 N 1/3 Mn 1/3 O 2 → LiCo 1/3 Ni 1/3 Mn 1/3 O 2 was obtained to be AG = -398 kJ mol -1 , which was slightly higher than that for LiCo 1/2 Ni 1/2 O 2 (-393 kJ mol -1 ). Rechargeable capacity (Q) as a function of charge-end voltage (E ce ) was examined for Li/LiCo 1/3 Ni 1/3 Mn 1/3 O 2 cells and the relation between Q in mAh g -1 and E ce in V was given to be Q = 151 + 121 (E ce - 4.2). From these results, rechargeable capacity and cycleability as a function of charge-end voltage are discussed in relation to the reaction mechanism of LiCo 1/3 Ni 1/3 Mn 1/3 O 2 .

Systematic research on insertion materials based on superlattice models in a phase triangle of LiCoO2-LiNiO2-LiMnO2. I. First-principles calculation on electronic and crystal structures, phase stability and new LiNi1/2Mn1/2O2 material

The first-principles calculations on the lithium insertion materials have been done to give the rationale for materials research on advanced lithium-ion batteries. To apply the computational methods, the model crystals are constructed by assuming in-plane two-dimensional superlattices based on α-NaFeO 2 -type structure for three sides of a phase triangle of LiCoO 2 -LiNiO 2 -LiMnO 2 . Model crystals consisting of a [2 x 2] superlattice gives two transition metal elements in the ratio of 3 to 1 and a [√3 x √3]R30° superlattice model gives 2 to 1. For the ratio of 1 to I, two model crystals are assumed, i.e., so-called straight and zigzag models. The first-principles calculations for these model crystals indicate that (i) LiCo 1-x Ni x O 2 is a solid solution between LiCoO 2 and LiNiO 2 , (ii) LiCo 1-x Mn x O 2 is not expected to be a solid solution between LiCoO 2 and LiMnO 2 (unstable, phase separation is expected), and (iii) LiNi 1-x Mn x O 2 is not a solid solution between LiNiO 2 and LiMnO 2 (LiNi 1/2 Mn 1/2 O 2 is expected to be a stable phase). Among these compounds calculated, LiNi 1/2 Mn 1/2 O 2 having a layered structure may be a new compound consisting of Ni 2+ and Mn 4+ in their formal charges, which is neither a solid solution between LiNiO 2 and LiMnO 2 nor between NiO and Li 2 MnO 3 . From these theoretical results, materials design toward the implementation of novel lithium insertion materials for advanced lithium-ion batteries is discussed.

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.