The separation of cobalt and nickel from sulfatic leach liquors of spent lithium-ion batteries is described in this paper. In addition to the base metals (e.g., cobalt and nickel), components such as manganese and lithium are also present in such leach liquors. The co-precipitation of these contaminants can be prevented during leach liquor processing by selective precipitation. For the recovery of a cobalt-nickel mixed material, oxalic acid serves as a suitable reagent. For the optimization of the precipitation retention time and yield, the dependence of the oxalic acid addition must be taken into account. In addition to efficiency, attention must also be given to the purity of the product. After this procedure, further processing of the products by calcination into oxides leads to better marketability. A series of experiments confirms the suitability of oxalic acid for precipitation of cobalt and nickel as a mixed oxalate from sulfatic liquors and also suggests a possible route for further processing of the products with increased marketability. The impurities in the resulting oxides are below 3%, whereby a sufficiently high purity of the mixed oxide can be achieved.