Suppressing the Dynamic Oxygen Evolution of Sodium Layered Cathodes through Synergistic Surface Dielectric Polarization and Bulk Site-Selective Co-Doping

Annealing Modulation Defect Chemistry toward High-Performance Sodium-Layered Cathodes

Layered sodium transition-metal oxides generally encounter severe capacity decay and inferior rate performance during cycling, especially at a high state of charge. Herein, defect concentration is rationally modulated to explore the impact on electrochemical behavior in NaNi1/3Fe1/3Mn1/3O2 layered oxides. Bulk vacancies are increased through annealing in an oxygen-rich atmosphere, demonstrated by electron paramagnetic resonance measurement. It is found that the cathode with enriched oxygen vacancies exhibits significantly enhanced reversibility of redox reactions with a higher initial Coulombic efficiency of 90.0%. Furthermore, the reduced volume variations during the initial charge/discharge process are also confirmed by in situ X-ray diffraction. As a result, the oxygen-vacancy-rich cathode shows great cycling stability and superior rate performances. Also, full cells deliver a specific capacity of approximately 145.2 mAh g-1 at 0.5 C, with a high capacity retention of 78.3% after 100 cycles. This work presents a viable strategy for designing Na+ intercalated cathodes with a high-energy density.

Unexpected Elevated Working Voltage by Na+/Vacancy Ordering and Stabilized Sodium-Ion Storage by Transition-Metal Honeycomb Ordering

Na+/vacancy ordering in sodium-ion layered oxide cathodes is widely believed to deteriorate the structural stability and retard the Na+ diffusion kinetics, but its unexplored potential advantages remain elusive. Herein, we prepared a P2-Na0.8Cu0.22Li0.08Mn0.67O2 (NCLMO-12 h) material featuring moderate Na+/vacancy and transition-metal (TM) honeycomb orderings. The appropriate Na+/vacancy ordering significantly enhances the operating voltage and the TM honeycomb ordering effectively strengthens the layered framework. Compared with the disordered material, the well-balanced dual-ordering NCLMO-12 h cathode affords a boosted working voltage from 2.85 to 3.51 V, a remarkable ~20 % enhancement in energy density, and a superior cycling stability (capacity retention of 86.5 % after 500 cycles). The solid-solution reaction with a nearly "zero-strain" character, the charge compensation mechanisms, and the reversible inter-layer Li migration upon sodiation/desodiation are unraveled by systematic in situ/ex situ characterizations. This study breaks the stereotype surrounding Na+/vacancy ordering and provides a new avenue for developing high-energy and long-durability sodium layered oxide cathodes.

Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

P2-type Na0.67Ni0.33Mn0.67O2 has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the Na0.67Ni0.33Mn0.67O2 cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (Na0.67Ni0.23Cu0.1Mn0.67O2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal-oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode maintains the P2 phase within the voltage window of 2.5-4.3 V, whereas the P2-Na0.67Ni0.33Mn0.67O2 cathode transforms entirely into O2-type Na0.67Ni0.33Mn0.67O2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode occurred at 4.6 V, indicating that Cu2+ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na0.67Ni0.23Cu0.1Mn0.67O2 cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g-1) and retaining more than 78 % of its capacity after 500 cycles.

Facilitating Layered Oxide Cathodes Based on Orbital Hybridization for Sodium-Ion Batteries: Marvelous Air Stability, Controllable High Voltage, and Anion Redox Chemistry

Layered oxides have become the research focus of cathode materials for sodium-ion batteries (SIBs) due to the low cost, simple synthesis process, and high specific capacity. However, the poor air stability, unstable phase structure under high voltage, and slow anionic redox kinetics hinder their commercial application. In recent years, the concept of manipulating orbital hybridization has been proposed to simultaneously regulate the microelectronic structure and modify the surface chemistry environment intrinsically. In this review, the hybridization modes between atoms in 3d/4d transition metal (TM) orbitals and O 2p orbitals near the region of the Fermi energy level (EF) are summarized based on orbital hybridization theory and first-principles calculations as well as various sophisticated characterizations. Furthermore, the underlying mechanisms are explored from macro-scale to micro-scale, including enhancing air stability, modulating high working voltage, and stabilizing anionic redox chemistry. Meanwhile, the origin, formation conditions, and different types of orbital hybridization, as well as its application in layered oxide cathodes are presented, which provide insights into the design and preparation of cathode materials. Ultimately, the main challenges in the development of orbital hybridization and its potential for the production application are also discussed, pointing out the route for high-performance practical sodium layered oxide cathodes.

Regulating Oxygen Redox Chemistry through the Synergistic Effect of Transition-Metal Vacancy and Substitution Element for Layered Oxide Cathodes

Reversible oxygen redox (OR) is considered as a paradigmatic avenue to boost the energy densities of layered oxide cathodes. However, its activation is largely coupled with the local coordination environment around oxygen, which is usually accompanied with irreversible oxygen release and unfavorable structure distortion. Herein, it is revealed that the synergistic effect of transition-metal (TM) vacancy and substitution element for modulating the OR activity and reversibility of layered Na0.67 MnO2 through multimodal operando synchrotron characterizations and electrochemical investigations. It is disclosed that TM vacancy can not only suppress the complicated phase transition but also stimulate the OR activity by creating nonbonding O 2p states via the Na─O─vacancy configurations. Notably, the substitution element plays a decisive role for regulating the reversibility of vacancy-boosted OR activity: the presence of strong Al─O bonds stabilizes the Mn-O motifs by sharing O with Al in the rigid Mn─O─Al frameworks, which mitigates TM migration and oxygen release induced by TM vacancy, leading to enhanced OR reversibility; while the introduction of weak Zn─O bonds exacerbates TM migration and irreversible oxygen release. This work clarifies the critical role of both TM vacancy and substitution element for regulating the OR chemistry, providing an effective avenue for designing high-performance cathodes employing anionic redox.

Electronic States Tailoring and Pinning Effect Boost High-Power Sodium-Ion Storage of Oriented Hollow P2-Type Cathode Materials

Fierce phase transformation and limited sodium ion diffusion dynamics are critical obstacles that hinder the practical energy storage applications of P2-type layered sodium transition metal oxides (NaxTMO2). Herein, a synergistic strategy of electronic state tailoring and pillar effect was carefully implemented by substituting divalent Mg2+ into Na0.67Ni0.33Mn0.67O2 material with unique oriented hollow rodlike structures. Mg2+substitution can not only facilitate the anionic oxygen redox reactions and electronic conductivity through increasing the electronic states at Femi energy but also act as pillars within TMO2 layers to alleviate the severe phase transformation to improve structure stability. Moreover, the oriented hollow structure incorporating sufficient buffer spaces and rationally exposed electrochemically active facets effectively alleviates the stresses induced by low volume changes of 8% and provides more open channels for Na+ ion diffusion without crossing multiple grain boundaries. Hence, the Na0.67Mg0.08Ni0.25Mn0.67O2 cathode showed a superior rate capability with high energy density and cycling stability for sodium-ion storage. The underlying mechanisms of these achievements were deciphered through diversified dynamic analysis and the first principle calculations, providing new insights into P2-type NaxTMO2 cathodes for the infinite prospect as an alternative to lithium-ion batteries.

Cooperative Catalysis of Polysulfides in Lithium-Sulfur Batteries through Adsorption Competition by Tuning Cationic Geometric Configuration of Dual-active Sites in Spinel Oxides

Fundamentally understanding the structure-property relationship is critical to design advanced electrocatalysts for lithium-sulfur (Li-S) batteries, which remains a formidable challenge. Herein, by manipulating the regulable cations in spinel oxides, their geometrical-site-dependent catalytic activity for sulfur redox is investigated. Experimental and theoretical analyses validate that the modulation essence of cooperative catalysis of lithium polysulfides (LiPSs) is dominated by LiPSs adsorption competition between Co³⁺ tetrahedral (Td) and Mn³⁺ octahedral (Oh) sites on Mn³⁺ _Oh -O-Co³⁺ _Td backbones. Specifically, high-spin Co³⁺ _Td with stronger Co-S covalency anchors LiPSs persistently, while electron delocalized Mn³⁺ _Oh with adsorptive orbital (d_z ² ) functions better in catalyzing specialized LiPSs conversion. This work inaugurates a universal strategy for sculpting geometrical configuration to achieve charge, spin, and orbital topological regulation in electrocatalysts for Li-S batteries.