Film Preparation And First-Principles Studies Of Cathode Materials LiCoO₂ For Li-ion Batteries

Experimental and theoretical studies were performed on cathode material LiCoO₂ for Li-ion batteries.1. LiCoO₂ thin film samples have been prepared by pulsed laser deposition under different deposition conditions. Crystal structures of these samples were characterized by XRD method. Surface morphologies of the prepared films were analyzed by SEM, FESEM, and AFM techniques. Results revealed that LiCoO₂ thin films preferred grown along c-axis on the smooth Si(100), Si(111) and stainless steel substrates; the size of crystal grain were mainly affected by the repetition rate of the laser pulse and the temperature of the substrate; under the condition of high repetition rate(35Hz), LiCoO₂ thin films were grown with random growth mode; at the normal range, oxygen pressure had little effect on the films; at the very low oxygen pressure, however, the LiCoO₂ crystal lattice will appear the phenomena of loss of oxygen, at the same time the surface of LiCoO₂ films were coarser compared to the samples prepared at the higher oxygen pressure. In addition, the LiCoO₂/MgO double-layer samples are also prepared on the Si (100). The XRD patterns and FESEM images show that the MgO(111) can be epitaxially grown on LiCoO₂ (003).2. The geometrical and electronic structures of LiCoO₂(104) surface had been studied using the first-principles projector augmented-wave(PAW) method within the generalized gradient approximation (GGA), based on the density functional theory (DFT). Results shown that the geometrical structure of LiCoO₂(104) surface was obviously distorted compared to bulk under the influence of magnetism, in particular the bond of Co1-O3 was lengthen from 1.936? to 2.062?. In addition, there was distinct difference between surface and bulk electronic structures; the O valence state alter into non-saturation, that is to say the O valence was less than -2. These changes occurred at LiCoO₂ (104) surface is harmful to the long-term structural stability of LiCoO₂ grain dipped in electrolyte.