Lithium-ion secondary batteries using occlusion and release of lithium ions have advantages including an energy density higher than those of nickel-cadmium (Ni—Cd) batteries and nickel-hydrogen (Ni—MH) batteries having the same capacity and a high operating voltage and are, therefore, being widely used in information processing devices and communication device, such as personal computers and portable telephones, which need to be reduced in size and weight.
In recent years, use of a lithium-ion secondary battery as a power source for an electric vehicle, a hybrid vehicle or the like has been studied. Further, use of a lithium-ion secondary battery as a battery for storing electric power generated by a renewable power source such as a solar battery or wind power generation that is being introduced for realization of low-carbon society in view of the global warming problem is also being studied.
Putting a lithium-ion secondary battery into widespread use for power storage or as a large power source for an electric vehicle or the like requires not only reducing the manufacturing cost but also reducing the cost required for maintenance or the like. In a scheme to do so, it is important to extend the product life.
Although it is thought that the product life of lithium ion secondary batteries can be extended by re-evaluating the materials that comprise them and the structure of the batteries, there is a method that can reduce the shortening of their product life cycles that is caused by inappropriate usage of the battery and so forth. For example, Patent Literature 1 and Patent Literature 2 propose techniques that reduce the shortening of the life cycles of lithium ion secondary batteries by controlling charging and discharging of these batteries.
Patent Literature 1 describes controlling charging and discharging of a lithium-ion secondary battery so that the amount of lithium ions moving between a positive-electrode active material and a negative-material active material during charging and discharging is equal to or smaller than 95% of the amount of lithium ions that move in the reverse direction.
Patent Literature 2 describes controlling charging and discharging of a lithium-ion secondary battery so that the discharge end voltage at the time of discharging is 3.2 to 3.1 V and the upper limit voltage at the time of charging is 4.0 to 4.5 V.
Lithium-ion secondary batteries using lithium cobaltate, lithium manganate or lithium nickelate as a positive-electrode material (positive-electrode active material) are known. Lithium-ion secondary batteries using a graphite-based material or a coke-based material as a negative-electrode material (negative-electrode active material) are also known.
The applicant of the present invention has found that when a manganese-based lithium-ion secondary battery using lithium manganate as a positive-electrode material in such lithium-ion secondary batteries is stored in a particular state of charge (SOC), the battery performance degrades rapidly (the battery capacity is reduced).
“SOC” represents the ratio of an amount of electricity with which a lithium-ion secondary battery is charged to the capacity of the lithium-ion secondary battery. The particular SOC at which the battery performance degrades rapidly is a value of, for example, SOC=approximately 40%, lower than the maximum SOC, which is a charge limit point, and higher than the minimum SOC, which is a discharge limit point. “Storage” referred to in the specification of the present invention means leaving a lithium-ion secondary battery in a state at the SOC value.
Degradation of the battery performance of a lithium-ion secondary battery at such a particular SOC is not a serious problem when the battery is used in a mode of use in which the battery is ordinarily stored in a fully charged state, that is, when the battery is used as an uninterruptible power supply (UPS) or the like.
However, in a mode of use in which a lithium-ion secondary battery is stored at an arbitrary SOC between the maximum SOC and the minimum SOC, for example, in use for storing electric power generated by the above-described renewable power source, there is a possibility of the battery being stored at the above-described particular SOC. In such a case, the battery performance of the lithium-ion secondary battery degrades rapidly.