High Reversibility of Lattice Oxygen Redox in Na-ion and Li-ion Batteries Quantified by Direct Bulk Probes of both Anionic and Cationic Redox Reactions

Abstract: 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 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.

• The paper demonstrates that NaMg₂/₃Mn₁/₃O₂ exhibits 79% initial lattice oxygen redox reversibility and retains 87% capacity after 100 cycles.
• The paper employs mRIXS to directly probe anionic and cationic redox states, distinguishing oxygen redox from transition metal contributions.
• The paper reveals that Li-rich Li₁.₁₇Ni₀.₂₁Co₀.₀₈Mn₀.₅₄O₂ shows a significant decline in redox reversibility from 76% to 44% over 500 cycles, impacting battery longevity.

High Reversibility of Lattice Oxygen Redox in Na-ion and Li-ion Batteries

This paper provides a comprehensive study on the reversibility and cyclability of lattice oxygen redox processes in sodium-ion (Na-ion) and lithium-ion (Li-ion) batteries. Utilizing mapping of resonant inelastic X-ray scattering (mRIXS), the study effectively quantifies the anionic and cationic redox states in two distinct electrode materials: NaMg$_{2/3}$Mn$_{1/3}$O$_2$ and Li$_{1.17}$Ni$_{0.21}$Co$_{0.08}$Mn$_{0.54}$O$_2$. The employment of soft X-ray spectroscopy, specifically mRIXS, enables a direct, bulk-sensitive probing of both oxygen and transition metal (TM) redox processes over extended electrochemical cycles, addressing previously recognized discrepancies in redox quantification.

Key Findings

The study emphasizes the contrasting behavior of lattice oxygen redox in Na-ion and Li-ion systems. For NaMg$_{2/3}$Mn$_{1/3}$O$_2$, the lattice oxygen redox shows significant stability and reversibility with 79% reversibility during the initial cycle, maintaining 87% of capacity after 100 cycles. The study points out that the Mn-L mRIXS-iPFY spectra evidence minimal transition metal redox during these cycles, correlating the majority of the electrode's capacity to the highly reversible oxygen redox reactions.

Conversely, for the Li-rich compound, Li$_{1.17}$Ni$_{0.21}$Co$_{0.08}$Mn$_{0.54}$O$_2$, the mRIXS-sPFY analysis highlights a 76% initial-cycle reversibility of lattice oxygen redox, which declines significantly to 44% retention after 500 cycles. The data suggests contributions from non-lattice oxygen activities (e.g., oxygen release and surface reactions) interfere with lattice oxygen redox, explaining the capacity fade witnessed in these systems.

Experimental and Theoretical Implications

This investigation provides a robust framework for distinguishing lattice oxygen redox from other overlapping oxygen activities. The mRIXS technique facilitates the realization of these distinctions by offering finer resolution and sensitivity compared to traditional XAS techniques. By disaggregating the redox activities of oxygen from those of transition metals, the study advances our understanding of battery chemistry, potentially guiding future enhancements in cathode design.

The implications stretch beyond mere academic curiosity; these insights can influence the development of high-capacity and stable energy storage systems. The quantification of lattice oxygen redox serves not only to clarify its reversibility but also to spotlight material compositions and configurations optimizing these redox activities. The findings suggest the promise of TM oxide cathode materials with minimal manganese redox during operation, emphasizing the utility of non-redox active dopants to stabilize cycling performance.

Future Directions

While the paper successfully delineates the nature of lattice oxygen redox, it leaves room for further exploration into the underlying mechanisms governing these redox behaviors. Additionally, expanding the mRIXS analysis to a wider array of materials could yield novel insights into other redox-active systems. The high reversibility observed in Na-ion and Li-ion systems presented in this study serves as a benchmark against which future materials may be evaluated, especially considering potential applications in grid storage and portable electronics.

As the field progresses, integrating such spectroscopic techniques with computational models may unravel the complexities that still obscure our understanding of redox dynamics in composite cathode materials. This paper underscores the importance of precise, direct, and reliable measurement techniques as cornerstones for future battery research and development initiatives.