This article presents a study of cathode material LiNi0.8Co0.1Mn0.1O2 using continuous in situ x-ray diffractometry during cycling of a rechargeable cell in the range of 2.7 V to 4.8 V. Changes in the crystal structure LiNi0.8Co0.1Mn0.1O2 during charge–discharge were studied. Capacitance drop due to degradation, and dependence parameters of the material unit cell on the voltage are shown. The causes of irreversible structural degradation of the cathode material during intercalation and deintercalation of lithium ions were investigated. Results of our work can help to find a possible solution to decrease the degradation factors by understanding the degradation mechanism.