Li-Rich Mn-Based Layered Oxide Cathodes for Solid-State Rechargeable Batteries

Solid-state batteries (SSBs) are a promising next-generation secondary battery due to their potential for high energy density and enhanced safety, offering solutions to the problems inherent in conventional lithium-ion batteries (LIBs) with organic liquid electrolytes (i.e., flammability, corrosion susceptibility, and high-voltage instability). In the construction of SSBs, the selection of the cathode materials is critical to achieving a high energy density, particularly when coupled with lithium metal anodes. However, conventional cathodes often affect the energy density of SSBs due to their constrained specific capacities. It is thus crucial for achieving substantial improvements in the energy density of SSBs to develop high specific capacity cathodes. The Li-rich Mn-based layered oxide (LRMO) is a promising cathode material for SSBs for energy densities of above 600 W·h·kg^–1 due to their high discharge specific capacities. Furthermore, LRMO cathodes offer some additional advantages, i.e., reduction of Co and Ni content, leverage of the abundance of Mn to achieve lower materials costs and improved safety. Their application in SSBs also mitigates the dissolution of TM-ions into the electrolyte, thereby enhancing the structural stability and capacity retention during long-term cycling progress. In addition, the resource-efficient composition of LRMO cathodes also align with environmentally friendly and sustainable development goals.

This review represents that the LRMO cathode materials are characterized by a composite crystal structure comprising two key components, i.e., Li₂MnO₃ phase and LiMO₂(M=Mn, Ni, Co) phase. Li₂MnO₃ phase can be considered as a superlattice-structured variant of LiMO₂, formulated as Li[Li_xMn_1-x]O₂. This superlattice-structured introduces unique unhybridized O 2p states, arising from Li—O—Li configurations. These unique oxygen states enable the participation of oxygen in charge compensation processes. Consequently, the high capacity in LRMO cathodes is attributed to the synergistic contributions of both TM cations and oxygen redox reactions.

LRMO cathodes, while exhibiting a distinctive biphasic structure, encounter significantly some challenges in SSBs. Specifically, the application of LRMO cathodes in SSBs is hindered by two primary issues. Firstly, the inherent incompatibility between Li₂MnO₃ phase and SEs interfaces results in sluggish reaction kinetics, severely restricting the activation of oxygen redox activity and consequently reducing the associated capacity contribution. Secondly, a chemical potential mismatch between SEs and LRMO cathodes drives spontaneous reactions at the composite cathode interfaces. These reactions lead to the formation of mixed ionic/electronic conductive CEI. Furthermore, irreversible oxygen escape further oxidizes the SEs interface, generating the passivation layers. These passivation layers increase interfacial impedance and imped ion transport, ultimately hindering practical advancements in SSBs technology.

LRMO cathodes hold a significant promise for SSBs, as evidenced by research progress across various SEs, including sulfides, halides, polymers, and oxides. To fully realize this potential, some strategies addressing the inherent incompatibility between LRMO cathodes and SEs are crucial. These strategies encompass bulk/ interfacial structure design, nanostructured particle engineering, and the construction of stable Li⁺/e^– transport pathways. These approaches can suppress oxygen escape, enhance the high-voltage stability of solid-solid interfaces, and ultimately stabilize oxygen redox while optimizing interfacial dynamics. Consequently, the implementation of these strategies leads to a significant enhancement in the electrochemical performance of LRMO-based SSBs.

LRMO cathodes have attracted considerable attention for SSBs due to their high discharge specific capacity and energy density. Advancements in SSBs utilizing sulfide, halide, polymer, and oxide SEs demonstrate a potential of LRMO cathodes to overcome limitations currently hindering their industrial applications in liquid electrolyte systems. These limitations include gas evolution, TM dissolution, and voltage decay. However, the practical application of LRMO cathodes in SSBs faces some challenges stemming from their inherent properties, such as poor electronic conductivity attributed to their biphasic structure, sluggish interfacial charge transfer kinetics, oxygen escape, high-voltage interfacial instability, and electrochemical-mechanical degradation. Consequently, a comprehensive understanding of failure mechanisms and the development of advanced modification strategies for LRMO cathodes in SSBs are urgently needed. This necessitates several key research directions. Firstly, optimizing large-scale synthesis techniques for single-crystal LRMO cathodes is crucial, coupled with systematic investigation into their degradation mechanisms within SSBs. Such studies should elucidate the complex interplay of mechanical, electrical, and chemical coupling within SSBs. Secondly, the development of zero-strain LRMO cathodes, designed to maintain structural integrity with minimal volume changes during cycling, can effectively mitigate mechanical stress, suppress crack formation (both intergranular and intragranular), and significantly improve long-term cycling stability. Furthermore, machine learning-driven multiscale modeling offers an effective tool for the rational design of bulk/interfacial structures, facilitating superior compatibility and high-voltage stability at the solid-solid interfaces. Finally, the exploration of high-voltage-tolerant SEs specifically tailored for LRMO cathodes, alongside innovations in scalable fabrication processes for ultrathin electrolyte membranes and electrode films, is essential. The synergistic convergence of materials innovation, interfacial engineering and scalable manufacturing offers a transformative potential for realizing the full capabilities of LRMO cathodes. This convergence is crucial for advancing SSBs toward unprecedented levels of energy density, reliability and sustainability. Specifically, these combined efforts will facilitate the production of large-format batteries at the A·h-level, ultimately enabling the large-scale commercialization of SSBs incorporating LRMO cathodes.