SECONDARY BATTERY

A secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode includes a lithium-nickel composite oxide. The negative electrode includes a lithium-titanium composite oxide. The electrolytic solution includes a carboxylic acid ester. The carboxylic acid ester includes at least one of ethyl acetate, propyl acetate, ethyl propionate, or propyl propionate. A first oxygen spectrum and a second oxygen spectrum are detectable by a surface analysis of the positive electrode by X-ray photoelectron spectroscopy. The first oxygen spectrum has a peak within a range of binding energy that is greater than or equal to 528 eV and less than or equal to 531 eV. The second oxygen spectrum has a peak within a range of binding energy that is greater than 531 eV and less than or equal to 535 eV. A ratio of an intensity of the first oxygen spectrum to an intensity of the second oxygen spectrum is greater than or equal to 0.30 and less than or equal to 0.80. A third oxygen spectrum and a fourth oxygen spectrum are detectable by a surface analysis of the negative electrode by the X-ray photoelectron spectroscopy. The third oxygen spectrum has a peak within a range of binding energy that is greater than or equal to 528 eV and less than or equal to 531 eV. The fourth oxygen spectrum has a peak within a range of binding energy that is greater than 531 eV and less than or equal to 535 eV. A ratio of an intensity of the third oxygen spectrum to an intensity of the fourth oxygen spectrum is greater than or equal to 0.82 and less than or equal to 1.35.