Structure and electron density analysis of electrochemically and chemically delithiated LiCoO2 single crystals

Single crystals of Li0.68CoO2, Li0.48CoO2, and Li0.35CoO2 were successfully synthesized for the first time by means of electrochemical and chemical delithiation processes using LiCoO2 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 Li0.68CoO2; a=2.8090(15) Å, c=14.3890(17) Å, and c/a=5.122 for Li0.48CoO2; and a=2.8070(12) Å, c=14.4359(14) Å, and c/a=5.143 for Li0.35CoO2. The crystal structures were refined to the conventional values R=1.99% and wR=1.88% for Li0.68CoO2; R=2.40% and wR=2.58% for Li0.48CoO2; and R=2.63% and wR=2.56% for Li0.35CoO2. The oxygen–oxygen contact distance in the CoO6 octahedron was determined to be shortened by the delithiation from 2.6180(9) Å in LiCoO2 to 2.5385(15) Å in Li0.35CoO2. The electron density distributions of these LixCoO2 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.