Stabilizing Cobalt-free Li-rich Layered Oxide Cathodes through Oxygen Lattice Regulation by Two-phase Ru Doping

The limited theoretical capacity of traditional cathode materials solely based on transition metal (TM) redox has been a primary factor for the ceiling placed on the energy density of LIBs. In this regard, Li-rich layered oxides (LLOs) have received a great deal of attention, due to their high reversible capacity (&gt; 250 mAh g^-1) and high energy density (900 Wh kg^-1), courtesy of the TM redox and the additional high-voltage O^2- redox.^[1] Developing Co-free cathodes is inevitably a holy grail to overcome the limitations faced by conventional Co-containing LIB cathode materials (e.g., LiCoO₂, NMC) due to the high-cost, toxicity, and limited geological distribution of cobalt. Therefore, studies on Co-free LLOs are necessary for creating long-term benefits for the cathode industry. Like typical LLOs, Co-free LLOs suffer from dramatic voltage decay on cycling, which hinders their applications in commercial LIBs. It is well accepted that this voltage decay is primarily attributed to the irreversible anionic redox and structural degradation in LLOs. The doping strategy has been widely used to address the voltage decay of LLOs.^[2] While the dopant is typically introduced to the layered Rm component, the lithium-rich component (C2/c) in the LLOs, which is the source of anionic redox, has always been ignored.<br/><br/>In this work, a small quantity (3%) of the electrochemically active 4<i>d</i> metal, Ruthenium (Ru), is successfully doped into TM octahedral sites in both hexagonal and monoclinic phases in the typical Co-free Li-rich LNMO cathode.^[3] The effects of Ru doping on the structure and redox activities of the LNMO are investigated using various morphological, local structure, and crystallographic techniques. Neutron-based pair distribution function (PDF) analysis, neutron powder diffraction (NPD), and <i>in operando</i> synchrotron-based X-ray powder diffraction (XRPD) is employed to characterize the short- and long-range cation arrangement, the chemical environment of lattice oxygen, and the structural evolution on cycling. Post-mortem synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and X-ray absorption characterizations (XAS) are utilized to investigate the cationic/anionic redox process. The correlation between voltage decay with structural changes is reported in detail. Owing to enhanced structural stability, Ru-doped LNMO exhibits an extraordinarily low voltage decay (&lt; 0.45 mV per cycle) and meanwhile retains a high reversible capacity (215 mAh g^-1 at 1 C) contributed by both cationic and O^2- redox. This work unravels the role of heavier ions in modifying the structural evolution and suppressing the voltage decay and sheds light on improving the performance of cathodes for practical applications in LIBs.<br/><br/><b>Reference</b><br/><br/>[1] a) S. Hu, A. S. Pillai, G. Liang, W. K. Pang, H. Wang, Q. Li, Z. Guo, <i>Electrochem. Energy Rev. </i><b>2019</b>, <i>2</i>, 277-311; b) M. Han, J. Jiao, Z. Liu, X. Shen, Q. Zhang, H.-J. Lin, C.-T. Chen, Q. Kong, W. K. Pang, Z. Guo, R. Yu, L. Gu, Z. Hu, Z. Wang, L. Chen, <i>Adv. Energy Mater. </i><b>2020</b>, <i>10</i>, 1903634.<br/>[2] Y. Fan, W. Zhang, Y. Zhao, Z. Guo, Q. Cai, <i>Energy Stor. Mater. </i><b>2021</b>, <i>40</i>, 51-71.<br/>[3] Y. Fan, E. Olsson, G. Liang, Z. Wnag, A. D'Angelo, B. Johannessen, L. Thomsen, B. Cowie, J. Li, F. Zhang, Y. Zhao, W. K. Pang, Q. Cai, Z. Guo, <i>Angew. Chem. Int. Ed. </i>Under review<i>.</i>