SECONDARY BATTERY

A secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode includes a positive electrode active material. The electrolytic solution includes a multi-nitrile compound. The positive electrode active material includes a lithium-nickel composite oxide and a boron compound. The lithium-nickel composite oxide has a layered rock-salt crystal structure. The positive electrode active material has a crystallite size of a (104) plane that is greater than or equal to 40.0 nm and less than or equal to 74.5 nm. The crystallite size is calculated by X-ray diffractometry and Scherrer equation. The positive electrode active material has an element concentration ratio that is greater than or equal to 0.15 and less than or equal to 0.90. The element concentration ratio is calculated on the basis of a B1s spectrum, a Ni2pspectrum, a Co2pspectrum, a Mn2pspectrum, and an Al2s spectrum of the positive electrode active material detected by X-ray photoelectron spectroscopy.