Structure and electron density analysis of electrochemically and chemically delithiated LiCoO 2 single crystals

Abstract

Single crystals of Li 0.68CoO 2, Li 0.48CoO 2, and Li 0.35CoO 2 were successfully synthesized for the first time by means of electrochemical and chemical delithiation processes using LiCoO 2 single crystals as a parent compound. A single-crystal X-ray diffraction study confirmed the trigonal R3¯m space group and the hexagonal lattice parameters a=2.8107(5) Å, c=14.2235(6) Å, and c/ a=5.060 for Li 0.68CoO 2; a=2.8090(15) Å, c=14.3890(17) Å, and c/ a=5.122 for Li 0.48CoO 2; and a=2.8070(12) Å, c=14.4359(14) Å, and c/ a=5.143 for Li 0.35CoO 2. The crystal structures were refined to the conventional values R=1.99% and w R=1.88% for Li 0.68CoO 2; R=2.40% and w R=2.58% for Li 0.48CoO 2; and R=2.63% and w R=2.56% for Li 0.35CoO 2. The oxygen–oxygen contact distance in the CoO 6 octahedron was determined to be shortened by the delithiation from 2.6180(9) Å in LiCoO 2 to 2.5385(15) Å in Li 0.35CoO 2. The electron density distributions of these Li x CoO 2 crystals were analyzed by the maximum entropy method (MEM) using the present single-crystal X-ray diffraction data at 300 K. From the results of the single-crystal MEM, strong covalent bonding was clearly visible between the Co and O atoms, while no bonding was found around the Li atoms in these compounds. The gradual decrease in the electron density at the Li site upon delithiation could be precisely analyzed.