A method for processing spent nickel-based cathode material useful in lithium-based batteries, the method comprising: (i) producing an initial mixture containing the spent nickel-based cathode material and a molten salt system comprising cations and anions, wherein the cations comprise lithium cations; (ii) heating the initial mixture to a temperature of 700° C. to 900° C. for at least 1 hour to produce a relithiated cathode material; and (iii) washing the relithiated cathode material to remove any residual salt. In a further method, the cations comprise nickel and lithium cations and the anions comprise chloride or bromide anion in combination with at least one of nitrate, sulfate, and carbonate anions in further combination with hydroxide anion, wherein the method results in upcycling of the nickel-based cathode material to produce a version of said relithiated cathode material having a greater nickel content.