Electrochemically active material for a positive electrode of a secondary lithium battery

Electrochemically active material (I) comprises more than 97 wt.% of a rhombohedral crystal phase comprising a mixed oxyfluoride (II) of lithium, nickel and manganese. Electrochemically active material comprises more than 97 wt.% of a rhombohedral crystal phase comprising a mixed oxyfluoride of formula Li(Ni ( 1 - x - y - z - t )Co xMn yLi zM t)O 2 - eF e (II) x : greater than or equal to 0 and less than 0.7; y : greater than 0.05 and less than or equal to 0.5; z : greater than or equal to 0 and less than or equal to 0.2; t : greater than 0 and less than or equal to 0.3; e : greater than 0.01 and less than or equal to 0.5; (1-x-y-z-t) : less than or equal to 0.20. M : Mg, Al, Bi, Ti, Si, Zr, Fe, Zn and/or Cu. Independent claims are also included for: (1) positive electrode for a rechargeable lithium battery, comprising a conductive support covered with an active layer containing (I); (2) rechargeable lithium battery comprising a positive electrode containing (I) and a negative electrode whose active material is metallic lithium, a lithium alloy or a material into whose structure lithium can be inserted.