Correlation between Cation Disordering and the Anionic Redox Chemistry in Li-Rich Layered Oxides
Authors: Yun Soyeong , Muhammad Shoaib , Yoon Won-Sub 
Published: May 2020

To satisfy our increasing dependence on the high energy storage electronics, Li-rich layered oxides (Li[LixM1-x]O2, M = Ni, Co, Mn, Ru, Ir, etc) have attracted significant attention as cathode materials for lithium-ion batteries in recent years. This class of cathode materials exhibit promising capacities, up to 300 mAh/g, due to accumulative transition metal and oxygen redox reaction1–5. Additional capacity in this class of Li-rich cathode material is attributed to the anionic redox which is irreversible or partially reversible oxygen evolution6,7 and reversible O2-/n- redox1,3,4,8. Luo el at.1 demonstrated that the localized electron holes on oxygen coordinated by Mn4+ and Li+ ions is promoted by the relatively more ionic interactions of O-(Mn/Li) than O-(Ni/Co) interactions at the result of the oxygen oxidation. In our previous study2, at least partially reversible oxygen evolution was observed in 0.4Li2MnO3-0.6LiMn0.5Ni0.5O2 by using Reitveld refinement of high resolution powder diffraction patterns. In the case of 4d (Ru) and 5d (Ir) transition metal-based Li-rich cathode materials, reversible anionic redox was confirmed by X-ray photoelectron spectroscopy (XPS)3,4. Even though the participation of oxygen in redox activity in Li2MnO3 results in partial oxygen evolution with irreversible structural changes9, oxygen redox reaction is stabilized by the reversible peroxo-dimers (O2
n-) formation in the Li2MO3-based (M = Ru, Ir) Li-rich cathode materials due to the high covalent metal-oxygen environment in Ru- and Ir-based lithium-rich cathode materials. However, Seo et al.10 identified the structural and chemical origin of the anionic redox activity which preferentially takes place along the Li-O-Li configurations (non-bonding O 2p states) in Li-rich layered oxides and cation-disordered oxides by using ab initio calculations based on DFT. The electrons in Li-O-Li states are higher energy than those in the other O 2p states owing to their unhybridized O 2p and Li 2s orbitals without taking into account the covalency between the transition metal and oxygen. Through the significant research efforts, anionic redox mechanism seems to be well established. In the case of Na2RuO3, extra anionic capacity is enabled in honeycomb-ordered Na2RuO3 due to the ordered intermediate Na1RuO3 phase which can accommodate the frontier orbital reorganization11, meanwhile, transition metal migration into the empty Li sites is closely associated with the anionic redox as a result of the structural disordering in Li1.17Ni0.21Co0.08Mn0.54O2
5 and Li2IrxSn1-xO3 systems12. It is still under debating whether such manifold anionic chemistry will be solved in terms of transition metals (3d, 4d, and 5d) or alkali metals (Li and Na) or bulk and local structure. In addition, the different reaction mechanism is confirmed between multivalent iridium redox (Ir4+/5.5+)12 and cumulative iridium (Ir4+/5+) and oxygen (O2-/n-) redox13 even in the same C2/m space group of Li2IrO3 materials.

Herein, redox mechanism of Li2IrO3 was investigated as a model compound for Li-rich cathode material to understand the key structural differences that lead to anionic redox activity through the synchrotron-based analysis. The local environment, not the composition itself in Li-rich materials, is one of the major factors affecting the redox reaction. We demonstrate that the in-plane disordering in the transition metal layer play a critical role in cation migration and anionic redox, and that cation migration also can trigger the anionic redox reactions. These fundamental insights about Li-rich material family are critical for further development of anionic redox-based cathode materials. In-plane ordering in pristine state with the same composition of Li2IrO3 has a major impact on cation migration and reversible anion redox. Based on the experimental data, detailed findings and discussion will be presented in the 237th ECS meeting.

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