NOVELTY - Manufacturing positive electrode active material (301) for energy storage device, involves: forming mixture by mixing raw materials to be positive electrode active material; performing first baking on mixture; grinding mixture; adding graphene oxide to ground mixture; performing second baking to form reaction product, and to reduce graphene oxide; and coating surface of reaction product with graphene.    USE - For manufacturing positive electrode active material selected from lithium iron phosphate, lithium nickel phosphate, lithium cobalt phosphate, or lithium manganese phosphate; useful for energy storage device (claimed) such as secondary batteries e.g. lithium-ion secondary battery, and electric double layer capacitors; used for power supply for hybrid vehicles.    ADVANTAGE - The positive electrode material is suitable for forming energy storage device having high capacity per weight or volume. A surface of main material i.e. positive electrode material is coated with two-dimensional carbon formed of graphene (where two-dimensional carbon is formed by stacking 1-10 sheets of graphene); which is highly conductive material having structure expanding two-dimensionally and having ignorable thickness, so that amount of carbon coating can be reduced and energy storage device having capacity close to theoretical capacity can be obtained even when conduction auxiliary agent is not used or amount of conduction auxiliary agent is extremely small. Thus, amount of carbon coating in positive electrode and volume of conduction auxiliary agent can be reduced; and consequently volume and weight of positive electrode can be reduced. Graphene has high conductivity, such as greater than or equal to 106 S/cm and is higher than that of silver. In addition, graphene doped with alkali metal or alkaline earth metal has high conductivity. When two-dimensional carbon is used for energy storage device in which charge and discharge are performed by transfer of alkali metal ions or alkaline earth metal ions, such as lithium-ion secondary battery, two-dimensional carbon may be doped with alkali metal or alkaline earth metal without particularly performing doping treatment and conductivity is improved. The main material included in positive electrode active material has small particle diameter, such as less than or equal to 50 nm. Thus surface area of positive electrode active material can be increased and charge and discharge characteristics are improved. Thus, energy storage device manufactured using positive electrode active material can have favorable charge and discharge characteristics and high capacity per volume and weight.    DETAILED DESCRIPTION - Manufacturing positive electrode active material (301) for energy storage device, involves: forming mixture by mixing raw materials to be positive electrode active material; performing first baking on mixture; grinding mixture; adding graphene oxide to ground mixture; performing second baking to form reaction product, and to reduce graphene oxide; and coating surface of reaction product with graphene. The mixture is ground before first baking. The thickness of graphene is 0.34-3.4 nm. The first and second baking are performed in atmosphere of rare gas, nitrogen, or hydrogen, or under reduced pressure. The method further involves grinding reaction product after performing second baking. The particle diameter of reaction product is less than or equal to 50 nm. The positive electrode active material optionally comprises conduction auxiliary agent ( less than or equal to 1 wt.%). An INDEPENDENT CLAIM is included for energy storage device comprising: positive electrode (321) provided with active material; negative electrode (322); and electrolyte (311) provided between positive electrode and negative electrode, where active material of positive electrode comprises mixture of binder, solvent, and main material of positive electrode active material whose surface is coated with graphene.    DESCRIPTION OF DRAWING(S) - The figure shows lithium-ion secondary battery comprising positive electrode active material.    Positive electrode current collector (300)    Positive electrode active material (301)    Negative electrode current collector (305)    Negative electrode active material (306)    Separator (310)    Electrolyte solution (311)    Housing (320)    Positive electrode (321)    Negative electrode (322)