Cathode Electrolyte Interface Engineering by Gradient Fluorination for High-Performance Lithium Rich Cathode

Despite their ultrahigh specific capacity, lithium-rich layered oxide cathodes are still plagued by challenges such as poor cycle stability and notorious voltage decay, which are primarily attributed to surface issues such as the release of lattice oxygen and interfacial side reactions. In this study, a facial strategy of gradient fluorination is adopted to construct a thin but robust LiF-rich cathode electrolyte interface (CEI), highly enhancing the stability of the interface of lithium-rich oxides. Experimental results and theoretical calculations both demonstrate that the stable CEI not only promotes oxygen participation in redox reactions and simultaneously inhibits oxygen release and structural transition, but also facilitates the transport kinetics of lithium ions. As a result, the gradient fluorinated lithium-rich cathode delivers highly enhanced rate performance (133 mAh g(-1) at 5 C), superior cycling stability with a capacity retention of 81.9% after 100 cycles at 1 C (250 mAh g(-1)), and alleviated voltage fade (only 1.75 mV per cycle). Moreover, a unique formation mechanism for LiF-rich surfaces is proposed according to theoretical calculations. This work not only provides a fresh understanding of the CEI formation mechanism, but also show a promising avenue for designing LiF-rich CEIs applicable to other layered oxide cathodes.