Structural analysis of LiNi1/3Mn1/3Co1/3O2, Li0.96 Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials synthesized by Pechini method

Abstract

Layered tri-transition metal oxides, specially LiNi1/3Co1/3Mn1/3O2 (NMC 333), have become a promising alternative to LiCoO2 electrode material in the rechargeable Lithium-Ion Battery (LIB). The electrochemical performances of NMC 333 mainly depend on its crystallographic structural properties including lattice parameters, the unit-cell, c/a ratio, volume, crystallite size (D), dislocation density(δ), and lattice strain. This study aims to synthesize LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials and study their structural properties. The Pechini method was used for powder synthesis in this study. The synthesized materials were characterized using X-ray diffraction (XRD). X-ray characterization confirmed the formation of only the single-phase layered hexagonal lattice (α-NaFeO2-type) structure without any impurity phase for all these prepared materials. Interestingly, while confirming the formation of layered structures, a better splitting of the (006)/(102) and (108)/(110) peaks appeared for Li0.96K0.04Ni1/3Mn1/3Co1/3O2 than that of LiNi1/3Mn1/3Co1/3O2 and Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 in the diffractograms. The lattice parameters, i.e. a, c, c/a, the unit-cell volume, the crystallite size (D), and dislocation density(δ) are 2.8641(Å)̇, 14.2143(Å)̇, 4.9629, 100.979(Å3)̇, 77.45 nm,1.666×1014 m−2, for LiNi1/3Mn1/3Co1/3O2. While they are 2.8675(Å)̇, 14.2317(Å), 4.9630, 101.347(Å3)̇, 85.06 nm, 1.382×1014 m−2 for Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and 2.869 (Å)̇, 14.2421(Å)̇, 4.9641, 101.528(Å3)̇, 128.38 nm, 0.606×1014 m−2 for Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It is also observed that the lattice parameters, the unit-cell volume, c/a, and the crystallite size are increased with the substitution of Li+ by Na+ and K+. It may be due to the radii of Na+ and K+ are bigger than that of Li+ and that will pave the way for increasing the interlayer space of the substituted materials with the substitution of bigger ions. The c/a ratio constitutes a direct indication of the cation mixing. Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 exhibit higher c/a values than LiNi1/3Mn1/3Co1/3O2, supporting the observation that the substituting bigger ions such as Na+ and K+ into LiNi1/3Mn1/3Co1/3O2 suppresses the cation mixing and forms a well-defined layered structure. The micro-strain calculated for the LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 are 1.38×10−3, 2.17×10−3 and 1.46×10−3, respectively. This implies a slight difference in the crystallinity of the materials, as the micro-strain was slightly affected by substituting Na+ and K+. Crystallite size (D) was 77.45 nm, 85.06 nm, and 128.38 nm for LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It exhibits an increment of crystallite size, indicating a lowering of the dislocation density with the substitution of bigger ions. Altogether, this study reveals that substituting Li+ with bigger ions of Na+ and K+ is improving the structural stability of NMC 333.