Rechargeable　magnesium　(Mg)　batteries　represent　a　promising　energy　storage　system　by　offering　low　cost　and　dendrite-less　propensity.　However,　the　limited　selection　of　cathode　materials,　and　often　with　low　voltage　and　capacity,　constrain　Mg　batteries.　Herein,　by　exploiting　the　ion-docking　effect　between　two　halogen　species-iodine　cations　(I+)　and　chlorine　anions　(Cl-)-we　activate　the　cathodic　activity　of　halogens　and　develop　a　magnesium-iodine/chlorine　(Mg-I/Cl)　battery　prototype　with　high　energy　and　power　density.　The　ion-docking　effect　enables　I+　and　Cl-　to　mutually　balance　and　disperse　their　charges,　and　weakens　the　coordination　strength　between　Cl-　and　Mg2+　while　enhancing　the　stability　of　I+,　thus　facilitating　the　multi-electron　(2　+　1/3)　redox　reactions　of　halogens.　We　also　find　the　solvation　state　of　iodine　species　determines　the　reaction　process　of　the　I0/I3-/I-　redox　couples.　The　here-developed　magnesium-iodine/chlorine　battery　features　an　impressively　high　discharge　plateau　of　up　to　3.0　V　with　a　high　capacity　exceeding　400　mAh　g-1,　and　demonstrates　a　stable　lifespan　for　500　cycles,　with　the　ability　of　ultra-fast　charging　at　20C　and　low-temperature　cycling　under　-30　degrees　C.　These　findings　may　provide　new　insights　for　developing　high-energy-density　Mg　battery　systems.