A lithium transition metal composite oxide, comprising a twin crystal structure. The twin crystal structure includes a first crystalline region and a second crystalline region. A grain boundary exists between the first crystalline region and the second crystalline region. The first crystalline region includes a first region located within 20 nm from the grain boundary. The second crystalline region includes a second region located within 20 nm from the grain boundary. An angle between a transition metal layer in the first region and a transition metal layer in the second region is 65° to 80°. By adjusting the angle between the transition metal layer in the first region and the transition metal layer in the second region to fall within 65° to 80°, it can improve stability of the twin crystal structure, and in turn, improve the cycle performance of the electrochemical device at a high voltage.