A lithium ion battery includes a cathode, an anode, and an electrolyte sandwiched between the cathode and the anode. The cathode includes a cathode active material. The anode is spaced from the cathode. The cathode active material includes a sulfur grafted poly(pyridinopyridine). The sulfur grafted poly(pyridinopyridine) includes a poly(pyridinopyridine) matrix and sulfur dispersed in the poly(pyridinopyridine) matrix. The sulfur includes a number of poly-sulfur groups or a number of elemental sulfur particles dispersed in the poly(pyridinopyridine) matrix. The electrolyte is a gel electrolyte.