[en] Compositions Li(Ni_0.33Mn_0.33Co_0.33)_1 – xFe_xO₂ (0 ≤ х ≤ 1) of the LiNiO₂–LiMnO₂–LiCoO₂–LiFeO₂ concentration tetrahedron have been studied. Samples were synthesized by gel combustion with starch, which afforded, for the first time, the impurity-free stable solid solution LiNi_0.2Mn_0.2Co_0.2Fe_0.4O₂ with an α-NaFeO₂ layered structure used as a cathode matrix of lithium-ion batteries. Electrochemical testing of homogeneous LiNi_0.2Mn_0.2Co_0.2Fe_0.4O₂ and the LiNi_0.3Mn_0.3Co_0.3Fe_0.1O₂ sample with the lowest iron content has been carried out. The results expand the understanding of the possibility of saturation with iron of Li(Ni,Mn,Co)O₂ with an α-NaFeO₂ structure and indicate the feasibility of further studying the homogeneity volume and properties of the Li(Ni,Mn,Co,Fe)O₂ solid solution.