Is Cobalt in Li‐Rich Layered Oxides for Li‐Ion Batteries Necessary?

Cobalt is considered an essential element for layered cathode active materials supporting enhanced lithium-ion conductivity and structural stability. Herein, we investigated the influence of Co concentration on the physicochemical properties and electrochemical performance of lithium-rich layered oxides (LRLOs) with different Co content (Li1.2Ni0.2-x/2Mn0.6-x/2CoxO2, x=0, 0.04, and 0.08). Though the presence of Co grants structural stability to LRLOs, superior long-term cycling stability is achieved with the Co-free LRLO retaining 88.1 % of the initial specific capacity (vs. 75.9 % of Li1.2Ni0.16Mn0.56Co0.08O2) after 300 galvanostatic cycles at 250 mA g−1 (1 C). The chemical stability on the surface of LRLOs containing Co declines faster, indicating a higher bulk structural stability not being the primary determinant of the LRLOs’ cycling performance. Ex-situ investigations indicate that the superior cycling stability of Co-free LRLO is obtained by reducing the Mn-related redox at discharge, which contributes to the large degree of polarization and low energy efficiency. Finally, the full-cell configured with the optimized LRLO as cathode and graphite anode delivers an energy density of 464 Wh kg−1 at C/10, and 74.4 % and 94.3 % of retention in discharge specific capacity and average voltage at the 1000th cycle, demonstrating the applicability of Co-free LRLO for sustainable LIBs.