Morphological Evolution of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Materials for Lithium-Ion Batteries: The Critical Effects of Surface Orientations and Particle Size

Abstract

An evolution panorama of morphology and surface orientation of high-voltage spinel LiNi(0.5)Mn(1.5)O4 cathode materials synthesized by the combination of the microwave-assisted hydrothermal technique and a postcalcination process is presented. Nanoparticles, octahedral and truncated octahedral particles with different preferential growth of surface orientations are obtained. The structures of different materials are studied by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption near edge spectroscopy (XANES), and transmission electron microscopy (TEM). The influence of various morphologies (including surface orientations and particle size) on kinetic parameters, such as electronic conductivity and Li(+) diffusion coefficients, are investigated as well. Moreover, electrochemical measurements indicate that the morphological differences result in divergent rate capabilities and cycling performances. They reveal that appropriate surface-tailoring can satisfy simultaneously the compatibility of power capability and long cycle life. The morphology design for optimizing Li(+) transport and interfacial stability is very important for high-voltage spinel material. Overall, the crystal chemistry, kinetics and electrochemical performance of the present study on various morphologies of LiNi(0.5)Mn(1.5)O4 spinel materials have implications for understanding the complex impacts of electrode interface and electrolyte and rational design of rechargeable electrode materials for lithium-ion batteries. The outstanding performance of our truncated octahedral LiNi(0.5)Mn(1.5)O4 materials makes them promising as cathode materials to develop long-life, high energy and high power lithium-ion batteries.

Influence of Synthesis, Dopants and Cycling Conditions on the Cycling Stability of Doped LiNi0.5Mn1.5O4Spinels

“…In Figure 3 the activation energies of the different materials are shown in dependence of the lattice parameters (s. Table II). As it was observed previously 28,38 an increase of the lattice parameter (from 8.1808 to 8.1903 Å) leads to a decrease of the activation energy (from 0.5 to 0.07 eV). However in the previous studies this was observed on undoped LNMO spinels, which were annealed at different temperatures.…”

“…42 This process leads to the reduction of Mn 4+ to the larger Mn 3+42 and thus the expansion of the lattice parameter. 28 Additionally, a greater formation of the rock-salt impurity phase is observed.…”

“…Hirayama et al and Liu et al found, that the {110} facets are vulnerable to manganese dissolution. 28,33,34 Since the cycling of 2 Li eq. involves Mn 4+ /Mn 3+ as the active redox couple below 3.5 V vs. Li + /Li, there is a higher risk of manganese disproportionation and dissolution during cycling.…”

“…In Figure S6 the influence of different C-rates on the length of the 2.7 V plateau for both materials is visible. Liu et al 28 found that spinel particles with {100} facets show a higher lithium diffusion coefficient than particles with mainly {111} facets. As LNMRTO AP displays {100} facets in contrast to LNMRTO HT , the assumption of a kinetic effect is supported.…”

Tortuosity of Battery Electrodes: Validation of Impedance-Derived Values and Critical Comparison with 3D Tomography

“…This was certainly expected, since the electronic conductivity of graphite electrodes is on the order of > S/cm, 9 so that R El. /R Ion quite clearly is <10 −2 , However, for cathode electrodes based on oxide active materials with typically rather low electronic conductivities, e.g., LNMO, 10 and particularly for cathodes with a low amount of conductive carbon additive, the electronic resistance might not be negligible anymore and the measured value of (R Ion · κ) could change when switching from low to high conductivity electrolytes. Such a variance of the apparent value of (R Ion · κ) obtained by impedance analysis when changing the electrolyte conductivity would be a clear indication that the electronic resistance of the electrode cannot be neglected, and that the simple transmission line model would not yield the correct R Ion -value (i.e., R app Ion = R Ion ).…”