Unblocking Oxygen Charge Compensation for Stabilized High-Voltage Structure in P2-Type Sodium-Ion Cathode

Gradient and De-Clustered Anionic Redox Enabled Undetectable O2 Formation in 4.5 V Sodium Manganese Oxide Cathodes

Anionic redox chemistry presents a promising approach to enhancing the energy density of oxide cathode materials. However, anionic redox reactions invariably lead to O2 formation, either as free gaseous O2 or trapped molecular O2, which destabilizes the material's structure. Here, this critical challenge is addressed by constructing a crystal structure with both gradient redox activity and de-clustered redox-active oxygen. This design strategy is directly validated by operando differential electrochemical mass spectrometry and ex situ 50 K electron paramagnetic resonance, revealing no release of O2 or trapped O2 in the 4.5 V P2-type sodium manganese-based layered oxide. Notably, the material exhibits a highly reversible capacity of 247 mA h g-1 at 20 mA g-1 and exceptional capacity retention of 91.4% after 300 cycles at 300 mA g-1. In situ X-ray diffraction further suggests that the absence of O2 formation suppresses the typical P2-O2 phase transition, resulting in a minimal lattice volume change of only 0.5%. Ex situ neutron diffraction studies and theoretical calculations further elucidate that the locally ordered lattice is well-preserved, attributable to reduced cationic migrations during cycling.

K-Doping Suppresses Oxygen Redox in P2-Na0.67Ni0.11Cu0.22Mn0.67O2 Cathode Materials for Sodium-Ion Batteries

In P2-type layered oxide cathodes, Na site-regulation strategies are proposed to modulate the Na+ distribution and structural stability. However, their impact on the oxygen redox reactions remains poorly understood. Herein, the incorporation of K+ in the Na layer of Na0.67Ni0.11Cu0.22Mn0.67O2 is successfully applied. The effects of partial substitution of Na+ with K+ on electrochemical properties, structural stability, and oxygen redox reactions have been extensively studied. Improved Na+ diffusion kinetics of the cathode is observed from galvanostatic intermittent titration technique (GITT) and rate performance. The valence states and local structural environment of the transition metals (TMs) are elucidated via operando synchrotron X-ray absorption spectroscopy (XAS). It is revealed that the TMO2 slabs tend to be strengthened by K-doping, which efficiently facilitates reversible local structural change. Operando X-ray diffraction (XRD) further confirms more reversible phase changes during the charge/discharge for the cathode after K-doping. Density functional theory (DFT) calculations suggest that oxygen redox reaction in Na0.62K0.03Ni0.11Cu0.22Mn0.67O2 cathode has been remarkably suppressed as the nonbonding O 2p states shift down in the energy. This is further corroborated experimentally by resonant inelastic X-ray scattering (RIXS) spectroscopy, ultimately proving the role of K+ incorporated in the Na layer.

Achieving Stable Cycling Performance in a P2-Type Layered Oxide Cathode through a Synergic Li/Zn Doping for Sodium-Ion Batteries

It has been suggested that sodium layered transition metal oxides could potentially serve as excellent cathodes for sodium-ion batteries (SIBs) because of their appropriate operating potentials and high capacities. However, the growing reliance on energy requirements has necessitated a higher energy density of SIBs. It has been demonstrated that activating oxygen-related activities for SIBs is a viable method to improve energy density. Herein, we suggest applying the synergy of Li and Zn codoping to activate the anionic redox reactions (ARRs) and improve their reversibility in Na0.75Ni0.3Mn0.7O2. The dual ion doping alleviates the phase transition and inhibits the Na+/vacancy arrangements, consequently improving the rate capacity and cyclic stability. The Na0.75Li0.15Ni0.1Zn0.05Mn0.7O2 delivers a discharge capacity of 134 mA h g-1 and no significant capacity loss at 100 mA g-1 between 2 and 4.5 V after 200 cycles. More importantly, the density functional theory (DFT) calculation proves that the codoping strategy triggers more ARRs compared to single-element doping, thereby providing enhanced capacity. The codoping induced oxygen redox strategies will create a new path for rational design of cathodes to enhance the energy density for SIBs.

Cation Configuration and Structural Degradation of Layered Transition Metal Oxides in Sodium-Ion Batteries

Given the pressing depletion of lithium resources, sodium-ion batteries (SIBs) stand out as a cost-effective alternative for energy storage solutions in the near future. Layered transition metal oxides (LTMOs) emerge as the leading cathode materials for SIBs due to their superior specific capacities and abundant raw materials. Nonetheless, achieving long-term stability in LTMOs for SIBs remains a challenge due to the inevitable structural degradation during charge-discharge cycles. The complexity and diversity of cation configurations/superstructures within the transition metal layers (TMO2) further complicate the understanding for newcomers. Therefore, it is critical to summarize and discuss the factors leading to structural degradation and the available strategies for enhancing LTMOs' stability. In this review, the cationic configurations of TMO2 layers are introduced from a crystallographic perspective. It then identifies and examines four key factors responsible for structural decay, alongside the impacts of various modification strategies. Finally, more effective and practical research approaches for investigating LTMOs have been proposed. The work aims to enhance the comprehension of the structural deterioration of LTMOs and facilitate a substantial improvement in their cycle life and energy density.

Trace Y Doping Regulated Bulk/Interfacial Reactions of P2-Layered Oxides for Ultrahigh-Rate Sodium-Ion Batteries

P2-phase layered cathodes play a pivotal role in sodium-ion batteries due to their efficient Na+ intercalation chemistry. However, limited by crystal disintegration and interfacial instability, bulk and interfacial failure plague their electrochemical performance. To address these challenges, a structural enhancement combined with surface modification is achieved through trace Y doping. Based on a synergistic combination of experimental results and density functional theory (DFT) calculations, the introduction of partial Y ions at the Na site (2d) acts as a stabilizing pillar, mitigating the electrostatic repulsions between adjacent TMO2 slabs and thereby relieving internal structural stress. Furthermore, the presence of Y effectively optimizes the Ni 3d-O 2p hybridization, resulting in enhanced electronic conductivity and a notable rapid charging ability, with a capacity of 77.3 mA h g-1 at 40 C. Concurrently, the introduction of Y also induces the formation of perovskite nano-islands, which serve to minimize side reactions and modulate interfacial diffusion. As a result, the refined P2-Na0.65 Y0.025[Ni0.33Mn0.67]O2 cathode material exhibits an exceptionally low volume variation (≈1.99%), an impressive capacity retention of 83.3% even at -40 °C after1500 cycles at 1 C.

Revealing the Nature of Binary-Phase on Structural Stability of Sodium Layered Oxide Cathodes

The emergence of layered sodium transition metal oxides featuring a multiphase structure presents a promising approach for cathode materials in sodium-ion batteries, showcasing notably improved energy storage capacity. However, the advancement of cathodes with multiphase structures faces obstacles due to the limited understanding of the integrated structural effects. Herein, the integrated structural effects by an in-depth structure-chemistry analysis in the developed layered cathode system NaxCu0.1Co0.1Ni0.25Mn0.4Ti0.15O2 with purposely designed P2/O3 phase integration, are comprehended. The results affirm that integrated phase ratio plays a pivotal role in electrochemical/structural stability, particularly at high voltage and with the incorporation of anionic redox. In contrast to previous reports advocating solely for the enhanced electrochemical performance in biphasic structures, it is demonstrated that an inappropriate composite structure is more destructive than a single-phase design. The in situ X-ray diffraction results, coupled with density functional theory computations further confirm that the biphasic structure with P2:O3 = 4:6 shows suppressed irreversible phase transition at high desodiated states and thus exhibits optimized electrochemical performance. These fundamental discoveries provide clues to the design of high-performance layered oxide cathodes for next-generation SIBs.

Rational Regulation of High-Voltage Stability in Potassium Layered Oxide Cathodes

Layered oxide cathode materials may undergo irreversible oxygen loss and severe phase transitions during high voltage cycling and may be susceptible to transition metal dissolution, adversely affecting their electrochemical performance. Here, to address these challenges, we propose synergistic doping of nonmetallic elements and in situ electrochemical diffusion as potential solution strategies. Among them, the distribution of the nonmetallic element fluorine within the material can be regulated by doping boron, thereby suppressing manganese dissolution through surface enrichment of fluorine. Furthermore, in situ electrochemical diffusion of fluorine from the surface into the bulk of the materials after charging reduces the energy barrier of potassium ion diffusion while effectively inhibiting irreversible oxygen loss under high voltage. The modified K0.5Mn0.83Mg0.1Ti0.05B0.02F0.1O1.9 layered oxide cathode exhibits a high capacity of 147 mAh g-1 at 50 mA g-1 and a long cycle life of 2200 cycles at 500 mA g-1. This work demonstrates the efficacy of synergistic doping and in situ electrochemical diffusion of nonmetallic elements and provides valuable insights for optimizing rechargeable battery materials.

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.

Constructing P2/O3 biphasic structure of Fe/Mn-based layered oxide cathode for high-performance sodium-ion batteries

Fe/Mn-based layered oxide cathode is regarded as a competitive candidate for sodium-ion batteries (SIBs) because of its high theoretical capacity, earth abundance and low cost. However, its poor cycling stability still remains a major bottleneck. Herein, P2/O3 biphasic Na0.67Fe0.425Mn0.425Cu0.15O2 layered oxide is successfully synthesized via a sol-gel method. It is observed that Cu substitution can facilitate the conversion of P2 to O3 phase, and the P2/O3 composite structure can be obtained with an appropriate amount of Cu. Meanwhile, in-situ XRD reveals that constructing P2/O3 composite structure can realize the highly reversible phase transition process of P2/O3-P2/P3-OP4/OP2 and decrease the lattice mismatch during Na+ insertion/extraction. Consequently, the biphasic P2/O3-Na0.67Fe0.425Mn0.425Cu0.15O2 electrode exhibits 87.1 % capacity retention after 100 cycles at 1C, while the single phase P2-Na0.67Fe0.5Mn0.5O2 electrode has only 36.4 %. Therefore, the constructing biphasic structure is proved to be an effective strategy for designing high-performance Fe/Mn-based layered oxide cathodes.

Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries

Though considerable Mg-doped layered cathodes have been exploited, some new differences relative to previous reports can be concluded by doping a heavy dose of Mg via two rational strategies. Unlike the common unit cell of the P6₃/mmc group by X-ray diffraction, neutron diffraction reveals a large supercell of the P6₃ group and enhanced ordering for Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ with Mg occupying both the Na and Mn sites. Compared with only one obvious voltage plateau of Na_0.5[Ni_0.25Mn_0.75]O₂ (NNM), Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ (NMNMM) shows more severe voltage plateaus but with excellent electrochemical performance. Na_0.5[Mg_0.25Mn_0.75]O₂ (NMM) with Mg only occupying the Ni site displays a highly reversible whole-voltage-range oxygen redox chemistry and smooth voltage curves without any voltage hysteresis. Cationic Ni²⁺/Ni⁴⁺ couples are responsible for the charge compensations of NNM and NMNMM, while only the oxygen anionic reaction accounts for the capacity of NMM between 2.5 and 4.3 V. Interestingly, the Mn³⁺/Mn⁴⁺ pair contributes all capacity for all cathodes between 1.5 and 2.5 V. All cathodes undergo a double-phase mechanism: an irreversible P2-O2 phase transition for NNM, an enhanced reversible P2-O2 phase transition for NMNMM, and a highly reversible P2-OP4 phase transition for NMM. In addition, the designed cathodes display excellent rate capability and long-term cycling stability but with a large difference in the various voltage ranges of 2.5-4.3 and 1.5-2.5 V, respectively. This work provides a good understanding of ion doping and some new insights into exploiting high-performance materials.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Rational design of intergrowth P2/O3 biphasic layered structure with reversible anionic redox chemistry and structural evolution for Na-ions batteries

Layered oxides have attracted unprecedented attention for their outstanding performance in sodium-ion battery cathodes. Among them, the two typical candidates P2 and O3 type materials generally demonstrate large diversities in specific capacity and cycling endurance with their advantages. Thus, composite materials that contain both P2 and O3 have been widely designed and constructed. Nevertheless, the anionic/cationic ions' behavior and structural evolution in such complex structures remain unclear. In this study, a deep analysis of an advanced Na_0.732Ni_0.273Mg_0.096Mn_0.63O₂ material that contains 78.39 wt% P2 phase and 21.61 wt% O3 phase is performed based on two typical cathodes P2 Na_0.67Ni_0.33Mn_0.67O2 and O3 NaNi_0.5Mn_0.5O₂ that have the same elemental constitution but different crystal structures. Structural analysis and density functional theory (DFT) calculations suggest that the composite is preferred to form a symbiotic structure at the atomic level, and the complex lattice texture of the biphase structure can block unfavorable ion and oxygen migration in the electrode process. Consequently, the biphase structure has significantly improved the electrochemical performance and kept preferable anionic oxygen redox reversibility. Furthermore, the hetero-epitaxy-like structure of the intergrowth of P2 and O3 structures share multi-phase boundaries, where the inconsistency in electrochemical behavior between P2 and O3 phases leads to an interlocking effect to prevent severe structural collapse and relieves the lattice strain from Na⁺ de/intercalation. Hence, the symbiotic P2/O3 composite materials exhibited a preferable capacity and cyclability (∼130 mAh g^-1 at 0.1 C, 73.1% capacity retention after 200 cycles at 1 C), as well as reversible structural evolution. These findings confirmed the advantages of using the bi/multi-phase cathode for high-energy Na-ion batteries.

Completely suppressed high-voltage phase transition of P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2via Li/Ni co-doping for sodium storage

A novel P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2 cathode is prepared via Li/Ni co-doping, and delivers attractive cycling and rate performances due to the high Na+ diffusion coefficient and the complete suppression of the high-voltage P2–Z phase transition.