Cathode active material for secondary battery and secondary battery comprising same

A lithium secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte solution,

• wherein the positive electrode comprises a positive electrode active material,
• wherein the negative electrode comprises a negative electrode active material, wherein the negative electrode active material comprises a metallic lithium thin film
• wherein the electrolyte solution comprises an organic solvent and a lithium salt, and
• wherein the positive electrode active material comprises:
• a core including a lithium composite metal oxide;
• and
• a film-shaped coating layer disposed to surround the core,
• wherein the coating layer comprises at least one lithium ion conductive compound having a band gap of 5.5 eV to 10 eV,
• wherein the lithium ion conductive compound is selected from the group consisting of LiBH4, LiBH4—LiI, and Li2NH, and
• a thickness (d) of the coating layer satisfies X<d≤100X, where X is a maximum distance at which electrons are capable of passing through the coating layer by a dielectric breakdown electric field when charging to an open circuit voltage of the lithium composite metal oxide versus lithium, wherein X is determined by Equation 1, wherein d is less than 500 nm:
• X=V/Fb [Equation 1]
• wherein, in Equation 1, V is the open circuit voltage of the lithium composite metal oxide versus lithium, and Fb is a dielectric breakdown electric field value calculated by Equation 2, and
• F
• b
• =
• 24.442
• ⁢
• ⁢
• exp
• ⁡
• (
• 0.315
• ⁢
• ⁢
• E
• g
• ω
• ⁢
• max
• )
• [
• Equation
• ⁢
• ⁢
• 2
• ]
• wherein, in Equation 2, Eg is a band gap, and ωmax is a maximum phonon frequency.