Understanding and designing high power and high energy density cathode materials for lithium ion batteries by experiments and first principles computations

Abstract

A new layered compound LiNi2/3Sb1/30 2 was synthesized and tested electrochemically to understand the effect of the transition metals on the structural stability of the layered compound upon Li de-intercalation. The electrochemical results show that the structure of LiNi2/3Sb1 /302 degrades upon cycling. XRD pattern refinement and TEM diffraction on the cycled LiNi2/3 Sb1/3O2 indicate that the structure degradation is associated with the migration of Ni into the Li layer. First principles calculation also shows a very low barrier for the migration of a divalent Ni from the transition metal layer to the tetrahedral sites of the lithium layer in the partially delithiated Li2/3Ni2/3Sbl/3O2. The divalent Ni becomes highly mobile because of the strong electrostatic repulsion from the surrounding three Ni3+ and three Sb5+. The effect of the alkali ions on the structural stability in the layered AMO 2 (A = alkali ion; M = transition metals) compounds is discussed by comparing layered LiMnO2 and NaMnO2. The structure of layered LiMnO 2 transforms rapidly into a spinel-like structure upon delithation due to the formation of a Li/Mn dumbbell configuration. However, such kind of dumbbell does not form in NaxMnO2, indicating its better structural stability upon deintercalation. The electrochemical results of NaMnO 2 show a much better capacity retention than that of LiMnO2, confirming that NaMnO2 is more stable than LiMnO2 upon deintercalation. The XRD results of the cycled NaMnO2 also show no significant structural change. The pronounced voltage steps and plateaus of NaMnO2 upon cycling were also investigated. First principles calculations show that the Li diffusivity in LiNi0.5Mn5O4 is in the order of 10-9 cm 2/s, implying that LiNi0.5Mn1.5O4 can be a high rate material even with a large particle size. The electrochemical tests of the micron-sized LiNiO0.5Mn1.5O4 show higher rate capability than nano LiNi0.5Mn1.5O4 by Shaju, et al, indicating that the ionic and electronic transport may not be the rate limiting factors. It was also found that cell configurations, such as separators, mechanical pressure of the cell and the carbon content in the electrode, could dramatically affect the rate capability of the cell. When the cell is highly optimized in configuration, more than half of the theoretical capacity is obtained at a discharge rate of 167C (corresponding to 22 seconds) with a particle size in the range of 3-5 [mu]m, which agrees with the high Li diffusivity by my calculation.