The present invention provides a lithium ion secondary battery, an electric tool, an electric vehicle and an electric power storage system. The lithium ion secondary battery comprises an anode, a cathode and electrolyte, wherein the anode comprises lithium composite oxide. The cathode comprises at least one selected from silicon Si and tin Sn as a forming element. The lithium composite oxide comprises the following components: lithium Li in a composition ratio a, first element in a composition ratio b and second element in a composition ratio c. The first element comprises at least two members selected from manganese Mn, nickel Ni and cobalt Co and comprises manganese at least. The second element comprises at least one selected from aluminum Al, titanium Ti, magnesium Mg and boron B. Furthermore the composition ratios satisfy the following relationships: 1.1b+c.