Boosting reaction homogeneity through reverse gradient fluorination for high-performance Li-rich layered cathodes

Abstract

Co-free lithium-rich cathode materials (LRMNO) demonstrate advantages in capacity and cost, however, a continuous decline in specific capacity during cycling, impedes their commercialization in Li-ion batteries. Ultimately, irreversible anion redox and cumulative bulk strain induce structural instability in the LRMNO cathode. The performance can be enhanced through partial fluorine substitution in oxygen, which adjusts the redox contributions of anions and cations. Nevertheless, the effectiveness of surface F substitution for practical applications remains limited due to the low solubility of F in the cathode material and its modest effect on the bulk lattice. In this study, a new Li_1.2Mn_0.6Ni_0.2O₂ cathode featuring a reverse F concentration gradient (with F increasing linearly from the particle center to the surface) by a secondary sol-gel method was presented, for tailored the effects of fluoridation at varying depths. The gradient-engineered sample demonstrated improved cycle stability and enhanced rate performance, achieving a capacity retention of 95.79 % after 100 cycles at 25 mA g^–1. These significant enhancements result from reducing the capacity contribution from the less reversible oxygen redox, optimizing bulk strain, and suppressing side reactions. This research proposes a universal strategy to effectively tackle challenges related to heterogeneous lithium-ion transport processes and asynchronous multiredox reactions, particularly for foreign elements with low solubility and prone to passivation layers, thereby mitigating performance degradation.