The low Coulombic efficiency during cycling hinders the application of Cobalt-free lithium-rich materials in lithium-ion batteries. Here we demonstrated that the dissolution of iron, rather than traditionally acknowledged manganese, is mainly responsible for the low Coulombic efficiency of the iron-substituted cobalt-free lithium-rich material. Besides, we presented an approach to inhibit the dissolution of transition metal ions by using concentrated electrolytes. We found that the cathode electrolyte interphase (CEI) layer formed in the concentrated electrolyte is a uniform and robust LiF-rich CEI, which is a sharp contrast with the uneven and fragile organic-rich CEI formed in the dilute electrolyte. The LiF-rich CEI not only effectively inhibits the dissolution of TMs but also stabilizes the cathode structure. The Coulombic efficiency, cycling stability, rate performance, and safety of the Fe-substituted cobalt-free lithium-rich cathode material in the concentrated electrolyte have been improved tremendously.