RESUMEN
Sodium-ion batteries (SIBs) have attracted widespread attention for large-scale energy storage, but one major drawback, i.e., the limited capacity of cathode materials, impedes their practical applications. Oxygen redox reactions in layered oxide cathodes are proven to contribute additionally high specific capacity, while such cathodes often suffer from irreversible structural transitions, causing serious capacity fading and voltage decay upon cycling, and the formation process of the oxidized oxygen species remains elusive. Herein, a series of Al-doped P2-type Na0.6 Ni0.3 Mn0.7 O2 cathode materials for SIBs are reported and the corresponding charge compensation mechanisms are investigated qualitatively and quantitatively. The combined analyses reveal that Al doping boosts the reversible oxygen redox reactions through the reductive coupling reactions between orphaned O 2p states in NaOAl local configurations and Ni4+ ions, as directly evidenced by X-ray absorption fine structure results. Additionally, Al doping also induces an increased interlayer spacing and inhibits the unfavorable P2 to O2 phase transition upon desodiation/sodiation, which is common in P2-type Mn-based cathode materials, leading to the great improvement in capacity retention and rate capability. This work provides deeper insights into the development of structurally stable and high-capacity layered cathode materials for SIBs with anion-cation synergetic contributions.
RESUMEN
Sodium layered transition-metal oxides have attracted great attention for advanced Na-ion batteries (NIBs) because of their rich structural diversity and superior specific capacity provided by not only cation redox reactions but also possible oxygen-related anionic redox reactions. However, they usually undergo severe electrochemical performance fading, especially the voltage retention during the cationic and anionic redox processes. Herein, we design and synthesize a couple of novel sodium lithium magnesium aluminum manganese oxides (Na0.75Li0.2Mg0.05Al0.05Mn0.7O2) with the same Na+ coordination environment but different oxide layer stacking sequences, namely, P2-NLMAMO and P3-NLMAMO. We systematically investigate and compare the voltage decay phenomenon and the cationic/anionic redox processes under different electrochemical cycling windows combined with ex situ hard and soft X-ray absorption spectroscopy techniques. The results clearly indicate that the P2-NLMAMO electrode with a lower extent of Mn redox is prone to deliver a superior capacity retention and rate performance, more importantly, a higher average voltage in contrast to the P3-type counterpart. In addition, negligible change is detected for the average discharge voltage upon extended cycling when increasing the discharge cutoff voltage to 2.5 V for both P2-NLMAMO and P3-NLMAMO. This unique feature work provides an effective strategy for developing high-capacity P-type layered cathodes based on both cationic and anionic redox chemistry under controlled crystal structure arrangement, which could lead to a deeper understanding of the correlation between crystal structure and electrochemical performance for NIBs.
RESUMEN
Harnessing anionic redox reactions is of prime importance for boosting the capacity of sodium-ion batteries (NIBs). However, quantifying the cyclability of anionic redox reactions is still challenging. Herein, we conduct a qualitative and quantitative investigation of the cationic and anionic redox reactions of a prototype Na-rich layered oxide, namely, Na3RuO4, by a combination of bulk-sensitive X-ray absorption spectroscopy and full-range mapping of resonant inelastic X-ray scattering. We unequivocally reveal that both Ru cations and oxygen anions are involved in the charge compensation process of Na3RuO4. Ru redox is highly reversible over extended electrochemical cycles, while the cyclability of lattice oxygen redox gradually decreases with the retention of only 36% after 30 cycles, which is mainly responsible for the capacity fading of Na3RuO4. Our findings provide deeper insights into the complex oxygen redox mechanism, which plays a decisive role for designing high-energy Na-rich electrode materials for NIBs.
RESUMEN
The ever-increasing demand for large-scale energy storage has driven the prosperous investigation of sodium-ion batteries (NIBs). As a promising cathode candidate for NIBs, P2-type Na2/3Ni1/3Mn2/3O2 (NaNMO), a prototype sodium-layered oxide, has attracted extensive attention because of its high operating voltage and high capacity density. Although its electrochemical properties have been extensively investigated, the fundamental charge compensation mechanism, that is, the cationic and anionic redox reactions, is still elusive. In this report, we have systematically investigated the transition metal and oxygen redox reactions of NaNMO nanoflakes using bulk-sensitive soft X-ray absorption spectroscopy and full-range mapping of resonant inelastic X-ray scattering from an atomic-level view. We show that the bulk Mn3+/Mn4+ redox couple emerges from the first discharge process with the increment of inactive Mn3+ upon cycling, which may have a negative effect on the cyclability. In contrast, the bulk Ni redox mainly stems from the Ni2+/Ni3+ redox couple, in contrast to the conventional wisdom of the Ni2+/Ni4+ redox couple. The quantitative analysis provides unambiguous evidence for the continuous reduction of the average valence state of Mn and Ni over extended cycles, leading to the voltage fading. In addition, we reveal that the oxygen anions also participate in the charge compensation process mainly through irreversible oxygen release rather than reversible lattice oxygen redox. Such understanding is vital for the precise design and optimization of NaNMO electrodes for rechargeable NIBs with outstanding performance.
