Method of charging and maintaining lithium ion secondary battery, battery system, vehicle and battery-mounted device

A battery system comprising:

• a lithium ion secondary battery that uses positive active material particles made from a two-phase coexistence type positive active material in its positive electrode plate;
• and
• charge/discharge control means for controlling the charge and discharge of the lithium ion secondary battery,
• the charge/discharge control means comprising:
• overcharge means for charging the lithium ion secondary battery to an overcharge SOC (state of charge) that is not higher than 100% SOC but higher than a target SOC;
• return discharge means for discharging, after operation of the overcharge means, the lithium ion secondary battery to make the SOC of the battery equal to the target SOC;
• and
• maintaining means for maintaining the SOC of the lithium ion secondary battery at the target SOC.

A battery system including:

• a lithium ion secondary battery using positive active material particles in its positive electrode plate, the positive active material particles being made from a two-phase coexistence type positive active material in which a first phase including Li ions inserted therein and a second phase from which the Li ions have been desorbed coexist;
• and
• charge/discharge control means for controlling the charge and discharge of the lithium ion secondary battery,
• wherein the battery system comprises:
• SOC maintenance prediction means for predicting whether the lithium ion secondary battery will be maintained at a target SOC for no less than one hour;
• first phase forming means for making the SOC of the lithium ion secondary battery equal to the target SOC and making the outermost peripheries of the positive active material particles be the first phase if the SOC maintenance prediction means predicts that the battery will be maintained for no less than one hour;
• and
• maintaining means for maintaining the SOC of the lithium ion secondary battery at the target SOC.