Structure Design for High-Performance Li-Rich Mn-Based Layered Oxides, O2- or O3-Type Cathodes, What's Next?

Li-rich layered oxides have received extensive attention as promising high-energy-density cathodes for next-generation Li-ion batteries. Different from traditional cathodes such as LiCoO2, LiFePO4, and Li2MnO4, Li-rich oxides generally can harvest superior discharge capacities exceeding 250 mAh g(-1), which originated from the contribution of oxygen redox chemistry. However, lattice oxygen release and irreversible TM transition would induce severe structure distortion and capacity degradation as well as voltage attenuation within Li-rich cathodes during electrochemical processes, which greatly limits their industrial applications in next-generation Li-ion batteries. To address these issues, structure design has been proposed and demonstrated as an efficient strategy to improve both the structural and electrochemical stability of Li-rich oxide cathodes. In particular, burgeoning O2-type Li-rich cathodes designed by adjusting the stacking sequence of oxygen atoms exhibited unique electrochemical properties that are superior to those of the traditional O3 counterparts. Nevertheless, it raises a crucial question regarding the selection of prevailing design prototypes: the O2- or O3-type of Li-rich oxide cathode greatly determines the future development direction of next-generation Li-ion batteries. In this Account, we mainly summarize our recent progress and understanding of the design of the O2- and O3-types from the perspectives of oxygen redox behaviors and structural evolution, aiming to provide insightful guidance for the rational design of high-performance Li-rich cathode materials. This Account begins with presenting representative structure designs based on a layered O3-type platform, including regulations of Li content within both transition metal (TM) and alkali metal (AM) layers and designs of TM proportions and superstructure units. Moreover, unique configuration designs have been combed and discussed in which Li-O-square and Li-O-Na configurations greatly facilitate the invertibility of oxygen redox reactions. In parallel, when altering the oxygen stacking sequence from ABCABC to ABCB, unique characteristics such as inhibited voltage decay and enhanced cycling stability as well as reversible TM ion migration can be achieved within the O2-type structures, where the synthesis routes and underlying mechanism of reversible TM migration in the O2-type cathodes have been summarized in detail. Additionally, our latest progress on structural designs of Li+ coordination environment regulation and biphasic layered structure were also presented, which could support the elevation of structure stability and cyclability of Li-rich cathodes upon long cycles, paving new structural design directions in addition to prevailing O3- and O2-type counterparts. At last, the challenges faced by the O3- and O2-type cathode materials and consequent solutions have been proposed. We hope this Account can provide fundamental insights and a route map for the proper design of high-energy-density Li-rich cathodes to achieve stable oxygen redox reactions.