The invention discloses a lithium ion battery with a positive electrode and a negative electrode which are arranged at two sides and a preparation method of the lithium ion battery. The lithium ion battery comprises a shell, a naked battery cell and two shell covers, wherein an upper end and a lower end of the shell are of opening structures, the naked battery cell is accommodated in the shell, tabs are respectively arranged at an upper end and a lower end of the naked battery cell, the two shell covers are respectively arranged at openings at the upper end and the lower end of the shell, pole posts are arranged on the two shell covers, tab connection holes corresponding to the tabs are formed in the pole posts in a penetrating way, the tabs extend to the tab connection holes in the pole posts, the tabs and the pole posts are connected by a stirring and friction welding process, and the shell covers and the shell are also connected by the stirring and friction welding process. By the lithium ion battery, the problem existing in connection of the battery tabs and the pole posts and the battery shell and the shell covers due to the adoption of ultrasonic welding and laser technologies is solved, the efficiency and the yield of battery production are improved, and the equipment investment cost during the production process of the battery is reduced.