SECONDARY BATTERY

To provide a secondary battery that is composed of resource-abundant materials, has high energy density, and is less likely to decrease in capacity even with charging and discharging repeated. The above-described problem is solved by a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte. The positive electrode is an olivine-type phosphate, the negative electrode contains silicon or a silicon compound predoped with lithium, and a ratio of a capacity per unit area of the negative electrode to a capacity per unit area of the positive electrode (an AC ratio) is 1.5 or higher.