Method of diagnosing degradation of electrode active material for lithium secondary battery

A method of diagnosing degradation by beta phase formation of an electrode active material for a secondary battery, comprising:

• preparing a lithium secondary battery which includes a positive electrode including a positive electrode active material of a lithium transition metal oxide containing nickel (Ni) in an amount of 60 mol % or more based on a total number of moles of transition metals except lithium, and a negative electrode facing the positive electrode;
• performing first charging and first discharging of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V, measuring an initial charge/discharge curve of the lithium secondary battery, and then differentiating the initial charge/discharge curve to obtain a first differential curve (dQ/dV) of the lithium secondary battery;
• performing second charging and second discharging of the lithium secondary battery in a voltage range of 2.5 V to 4.2 V, measuring a second charge/discharge curve of the lithium secondary battery, and then differentiating the second charge/discharge curve to obtain a second differential curve (dQ/dV) of the lithium secondary battery, and
• diagnosing that a beta phase of the positive electrode active material has been formed by determining when spaced-apart peaks are observed at 4.0 V or more with a maximum discharge peak value of the second differential curve at 4.0 V or more compared to a maximum discharge peak value of at 4.0 V or more of the first differential curve is spaced apart within a range of 0.01 V to 0.1 V.