@article{31891,
  author = {Kehua Dai and Jinpeng Wu and Zengqing Zhuo and Qinghao Li and Shawn Sallis and Jing Mao and Guo Ai and Chihang Sun and Zaiyuan Li and William E Gent and William C Chueh and Yi- de Chuang and Rong Zeng and Zhi-xun Shen and Feng Pan and Shishen Yan and Louis F.J Piper and Zahid Hussain and Gao Liu and Wanli Yang},
  title = {High Reversibility of Lattice Oxygen Redox Quantified by Direct Bulk Probes of Both Anionic and Cationic Redox Reactions},
  abstract = {<p>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 Na<sub>2/3</sub>Mg<sub>1/3</sub>Mn<sub>2/3</sub>O<sub>2</sub>. 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 Li<sub>1.17</sub>Ni<sub>0.21</sub>Co<sub>0.08</sub>Mn<sub>0.54</sub>O<sub>2</sub>, 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 Na<sub>2/3</sub>Mg<sub>1/3</sub>Mn<sub>2/3</sub>O<sub>2</sub> and&nbsp;<span>Li</span><sub>1.17</sub><span>Ni</span><sub>0.21</sub><span>Co</span><sub>0.08</sub><span>Mn</span><sub>0.54</sub><span>O</span><sub>2</sub> systems suggests the importance of distinguishing lattice oxygen redox from other oxygen activities for clarifying its intrinsic properties. 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 Na<sub>2/3</sub>Mg<sub>1/3</sub>Mn<sub>2/3</sub>O<sub>2</sub> (79%) and&nbsp;<span>Li</span><sub>1.17</sub><span>Ni</span><sub>0.21</sub><span>Co</span><sub>0.08</sub><span>Mn</span><sub>0.54</sub><span>O</span><sub>2</sub> (76%) is revealed during the initial cycle. While&nbsp;<span>Na</span><sub>2/3</sub><span>Mg</span><sub>1/3</sub><span>Mn</span><sub>2/3</sub><span>O</span><sub>2</sub> 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. Reversibility of lattice oxygen redox holds the key to its practical employments, especially in 3d transition-metal compounds. Here, direct, independent, and quantitative bulk probes of both cationic and lattice anionic redox chemistry are achieved through mapping of resonant inelastic X-ray scattering. We found highly reversible lattice oxygen redox in&nbsp;<span>Na</span><sub>2/3</sub><span>Mg</span><sub>1/3</sub><span>Mn</span><sub>2/3</sub><span>O</span><sub>2</sub> (79%) and&nbsp;<span>Li</span><sub>1.17</sub><span>Ni</span><sub>0.21</sub><span>Co</span><sub>0.08</sub><span>Mn</span><sub>0.54</sub><span>O</span><sub>2</sub> (76%) during the initial cycle, with 87% and 44% capacity retention after 100 and 500 cycles, respectively. Therefore, aside from other oxygen activities, lattice oxygen redox could be highly reversible in batteries.</p>},
  year = {2019},
  journal = {Joule},
  volume = {3},
  pages = {518 - 541},
  month = {02/2019},
  issn = {25424351},
  doi = {10.1016/j.joule.2018.11.014},
  language = {eng},
}