"High Reversibility of Lattice Oxygen Redox Quantified by Direct Bulk Probes of Both Anionic and Cationic Redox Reactions"

Kehua Dai: Jinpeng Wu, Zengqing Zhuo, Qinghao Li, Shawn Sallis, Jing Mao, Guo Ai, Chihang Sun, Zaiyuan Li, William E. Gent, William C. Chueh, Yi-de Chuang, Rong Zeng, Zhi-xun Shen, Feng Pan, Shishen Yan, Louis F.J. Piper, Zahid Hussain, Gao Liu, Wanli Yang; Joule, 02/20/19.

Additional Authors: Jinpeng Wu, Zengqing Zhuo, Qinghao Li, Shawn Sallis, Jing Mao, Guo Ai, Chihang Sun, Zaiyuan Li, William E. Gent, William C. Chueh, Yi-de Chuang, Rong Zeng, Zhi-xun Shen, Feng Pan, Shishen Yan, Louis F.J. Piper, Zahid Hussain, Gao Liu, Wanli Yang

Abstract:

Context and Scale

Battery cathodes based on 3d-transition-metal oxides need viable improvements in their energy density. Recent proposals of O redox have enabled conceptual possibilities, although the assessment of its reversibility remains elusive. This work reports independent and direct quantifications of the evolving O-2p and Mn-3d redox states through O-K and Mn-L mapping of resonant inelastic X-ray scattering (mRIXS). The high reversibility of the lattice O redox in Na2/3Mg1/3Mn2/3O2 (79%) and Li1.17Ni0.21Co0.08Mn0.54O2 (76%) is revealed during the initial cycle. While Na2/3Mg1/3Mn2/3O2 displays decent O-redox capacity retention (87% after 100 cycles), the Li-rich system shows significant decay (44% after 500 cycles). We demonstrate direct quantifications of the reversibility of lattice O redox through photon-in-photon-out bulk-sensitive mRIXS. The quantification results directly show that the reversibility of lattice O redox could be very high in both Li-ion and Na-ion batteries.

Summary

The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na2/3Mg1/3Mn2/3O2. The bulk Mn redox emerges from initial discharge and is quantified by inverse partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K super-partial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen redox reversibility during the initial cycle, with 87% capacity sustained after 100 cycles. In Li1.17Ni0.21Co0.08Mn0.54O2, lattice oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice oxygen redox in both Li-ion and Na-ion systems. The contrast between Na2/3Mg1/3Mn2/3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing lattice oxygen redox from other oxygen activities for clarifying its intrinsic properties.