Tailoring Anionic Redox Activity in a P2-Type Sodium Layered Oxide Cathode via Cu Substitution

A High-Entropy Intergrowth Layered-Oxide Cathode with Enhanced Stability for Sodium-Ion Batteries

Layered transition metal oxides are widely considered as ideal cathode materials for SIBs. However, the existing P2 and O3 structures possess specific issues, which limit their practical applications. To address these issues, this work designed a novel intergrowth layered oxide cathode with P2 and O3 phases by implementing Cu and Ti into the structure with the formation of high-entropy cathode materials with superior performance for SIBs. The electrochemical test results show that the optimized high-entropy cathode with the P2/O3 intergrowth structure possesses a high initial discharge capacity of 157.85 mAh g-1 at 0.1 C, an excellent rate performance of 84.41 mAh g-1 at 10 C, and long-term stability with capacity retention of 83.25 % after 500 cycles at 5 C. Furthermore, the analysis results of ex situ XRD and in situ XRD indicate that the adverse phase transition of P2-O2 under high voltage is effectively suppressed. This work indicates that the integration of high-entropy strategy with the two-phase intergrowth structure can effectively stabilize the layered structure, suppress the slipping of transition metal layers, and improve electrochemical performance, which provides a new approach for designing high-performance and practical layered transition metal oxide cathode materials for advanced SIBs.

Achieving Long-Enduring High-Voltage Oxygen Redox in P2-Structured Layered Oxide Cathodes by Eliminating Nonlattice Oxygen Redox

Triggering reversible lattice oxygen redox (LOR) in oxide cathodes is a paradigmatic approach to overcome the capacity ceiling determined by orthodox transition-metal (TM) redox. However, the LOR reactions in P2-structured Na-layered oxides are commonly accompanied by irreversible nonlattice oxygen redox (non-LOR) and large local structural rearrangements, bringing about capacity/voltage fading and constantly evolving charge/discharge voltage curves. Herein, a novel Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 (◻ = TM vacancies) cathode with both NaOMg and NaO◻ local configurations is deliberately designed. Intriguingly, the activating of oxygen redox at middle-voltage region (2.5-4.1 V) via NaO◻ configuration helps in maintaining the high-voltage plateau from LOR (≈4.38 V) and stable charge/discharge voltage curves even after 100 cycles. Hard X-ray absorption spectroscopy (hXAS), solid-state NMR, and electron paramagnetic resonance studies demonstrate that both the involvement of non-LOR at high-voltage and the structural distortions originating from Jahn-Teller distorted Mn3+ O6 at low-voltage are effectively restrained in Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 . Resultantly, the P2 phase is well retained in a wide electrochemical window of 1.5-4.5 V (vs Na+ /Na), resulting in an extraordinary capacity retention of 95.2% after 100 cycles. This work defines an effective approach to upgrade the lifespan of Na-ion battery with reversible high-voltage capacity provided by LOR.

Mitigating the Formation of Tetrahedral Zn in Layered Oxides Enables Reversible Lattice Oxygen Redox Triggering by the Na-O-Zn Configuration

Na-ion layered oxides with Na-O-A' local configurations (A' represents nonredox active cations such as Li⁺, Na⁺, Mg²⁺, Zn²⁺) are attractive cathode choices for energy-dense Na-ion batteries owing to the accumulation of cationic and anionic redox activities. However, the migration of A' would degrade the stability of the Na-O-A' configuration, bringing about drastic capacity decay and local structural distortions upon cycling. Herein, we uncover the close interplay between irreversible Zn migration and the inactivation of lattice oxygen redox (LOR) for layered oxides based on Na-O-Zn configuration by ²³Na solid-state NMR and Zn K-edge EXAFS techniques. We further design a Na_2/3Zn_0.18Ti_0.10Mn_0.72O₂ cathode in which irreversible Zn migration is effectively prevented, and the LOR reversibility is significantly enhanced. Theoretical insights demonstrate that the migrated Zn²⁺ is more inclined to occupy the tetrahedral site rather than the prismatic site and can be effectively minimized by incorporation of Ti⁴⁺ into the transition-metal layer. Our findings substantiate that the Na-O-Zn configuration can be utilized as an appropriate structure to achieve stable LOR by the cautious manipulating of intralayer cation arrangements.

Measuring T1 relaxation in paramagnetic solids with solid-state NMR: a case study on the milling induced phase transition in Li6CoO4

Solid-state NMR has been a vital tool for the study of structural evolution of cathodes in lithium-ion and sodium-ion batteries. However, the differentiation of relaxation parameters for certain sites is difficult owing to limited spectral resolution associated with strong anisotropic hyperfine interaction. Here we propose a novel IR-pjMATPASS method that can measure T₁ relaxation with site-specific resolution for paramagnetic solids. We apply this method to the characterization of ball-milling induced order-disorder phase transition in Li₆CoO₄ as a case study. The quasi-quantitate ⁷Li NMR enables the synthetic optimization of high energy ball-milling conditions to harvest a disordered cubic phase through site-specific ⁷Li T₁ measurements. The example study shown here provides a quantitative strategy for NMR studies of paramagnetic solids.