Diagnosis of cause of degradation of lithium secondary battery

A method of diagnosing a cause of degradation of a lithium secondary battery, comprising:

• (A) manufacturing a lithium secondary battery including a positive electrode including a layered positive electrode active material, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte;
• (B) obtaining, from first X-ray diffraction data obtained during first charging of the lithium secondary battery, a first graph showing the change of the c-axis d-spacing value of the layered positive electrode active material according to a number of moles of lithium ions deintercalated from the layered positive electrode active material during the first charging;
• (C) obtaining, from second X-ray diffraction data obtained during second charging of the lithium secondary battery, a second graph showing the change of the c-axis d-spacing value of the layered positive electrode active material according to a number of moles of lithium ions deintercalated from the layered positive electrode active material during the second charging;
• and
• (D) classifying the cause of degradation of the secondary battery by comparing the first graph and the second graph.