Heteroatom-Substituted P2-Na2/3Ni1/4Mg1/12Mn2/3O2 Cathode with {010} Exposing Facets Boost Anionic Activity and High-Rate Performance for Na-Ion Batteries

Boosting the Structural Stability of High-Voltage Sodium Layered Cathodes via Site-Selective La/Ti Co-Doping

P2-type sodium layered oxide (Na2/3Ni1/3Mn2/3O2, NNMO) material is considered a potential cathode candidate for sodium-ion batteries (SIBs) due to their compact crystal structure, and high cationic and anionic redox at high voltage. However, their structural instability and capacity fading, due to P2-O2 phase transitions and lattice oxygen evolution, are significant challenges for their long-term cycling performance. To address these problems, co-doping is recognized as an effective technique. Here, a strategy to address some of the issues with the P2-type Na2/3Ni1/3Mn2/3O2 material following La/Ti co-doping at the Na and transition metal (TM) site is demonstrated. The modified electrode, LT-NNMO-0.01, exhibits remarkably improved electrochemical performance with high-voltage plateaus in the charging and discharging processes, indicative of a reversible oxygen redox reaction facilitated by enhanced TM─O bonding strength and suppressed oxygen migration. Systematic investigations including electrochemical, in situ X-ray diffraction (XRD) studies and Density functional theory (DFT) calculations demonstrate the effectiveness of this co-doping strategy in improving structural stability and minimizing volume change, allowing for reversible structural evolution of the material. The present study provides an effective pathway for designing layered metal oxide cathode materials toward advancing the energy density of sodium-ion batteries.

Constructing Low-Strain Cation Storage via High-Entropy Doping to Stabilize Layered Oxide Cathodes for Advanced Sodium-Ion Batteries

P2-type manganese-based layered cathode materials hold great promise for sodium-ion battery applications. However, their practical implementation is hindered by structural instability caused by Jahn-Teller distortion and complex phase transitions. Herein, this work proposes a simple and efficient high-entropy doping strategy by introducing redox-active and lattice-stabilizing cations to elaborate sodium-deficiency P2-type Na0.67Al0.02Fe0.02Ni0.02Cu0.02Zn0.02Mn0.9O2 cathodes with low-strain operation, which can significantly reduce the Mn3+ Jahn-Teller active centers and enhance structural stability. In situ X-ray diffraction results confirm that the high-entropy doping strategy effectively mitigates volume strain of 1.58% during cycling with increased interplanar spacing and fast Na+ transfer kinetics, further maintaining a capacity retention of 88.81% with an average Coulombic efficiency of 99.90% after 600 cycles. This work provides valuable insights into the design of advanced sodium cathode materials for future low-cost energy storage devices.

A universal strategy for bridging Prussian blue analogues and sodium layered oxide cathodes: direct fast conversion, dynamic structural evolution, and sodium storage mechanisms

Prussian blue analogues (PBAs) are widely recognized as one of the most promising cathode materials for sodium-ion batteries (SIBs). However, many unqualified PBAs with unsatisfactory electrochemical performance are difficult to dispose of and pose a risk of environmental contamination. Additionally, the production process of layered oxides, another popular cathode material for SIBs, requires prolonged high-temperature sintering, resulting in significant energy consumption. To address the aforementioned issues, a "two birds with one stone" strategy is proposed. This approach not only demonstrates the feasibility of directly preparing layered oxides with PBAs as precursors through a fast sintering process but also simultaneously addresses the challenge of treating unqualified PBAs while minimizing excessive energy consumption during the preparation of layered oxides. Furthermore, a series of binary, ternary, and quaternary layered oxides were synthesized directly by utilizing PBAs with varying compositions, showcasing the universality of this strategy. This innovative approach breaks the boundaries between different types of sodium cathode materials and builds a distinctive bridge for the direct conversion of PBAs into layered oxides, thereby widening the feasibility of the cathode for SIBs.

In situ-Induced Crystal Facet Engineering to Enhance the Rate Performance of Zn2+ Storage

In recent years, aqueous zinc-ion batteries (ZIBs) have shown considerable promise in the energy storage sector, attributed to their inherent high safety and cost-effectiveness. Zn3V2O7(OH)2·2H2O (ZVO) has emerged as a promising candidate for Zn2+ storage in recent years, owing to its exceptional structural stability that endows it with an excellent cycle life. However, an unsatisfactory rate performance is a limiting factor for its development in ZIBs. Crystal facet engineering can be employed to rationally adjust the number of exposed diffusion channel entrances, thereby effectively enhancing the rate performance of ZIBs. Here, theoretical calculations are employed to analyze the diffusion energy barrier of Zn2+, providing guidance for selecting the optimal crystal facet. The obtained ZVO via electrochemical in situ conversion (e-ZVO) with increased exposed diffusion channel entrances of Zn2+ was proposed to enhance the rate performance. The modulation of ZVO crystal facet orientation and exposed diffusion channel entrances is achieved by manipulating the adsorption-induced surface energy of (CF3SO3)- on crystal facets. As a result, the e-ZVO achieves a high-rate performance of 57 mAh g-1 at 50 A g-1 and a capacity output of 41 mAh g-1 at 40 A g-1 even at -20 °C.

Crystal Modulation of Mn-Based Layered Oxide toward Long-Enduring Anionic Redox with Fast Kinetics for Sodium-Ion Batteries

Mn-based layered oxide cathodes for sodium-ion batteries with anionic redox reactions hold great potential for energy storage applications due to their ultra-high capacity and cost effectiveness. However, achieving high capacity requires overcoming challenges such as oxygen-redox failure, sluggish kinetics, and structural degradation. Herein, we employ an innovative crystal modulation strategy, using Mn-based Na0.72Li0.24Mn0.76O2 as a representative cathode material, which shows that the highly exposed {010} active facets enable an enhanced rate capability (119.6 mAh g-1 at 10 C) with fast kinetics. Meanwhile, the reinforced Mn-O bond inhibits excessive oxidation of lattice oxygen and O-O cohesion loss, stabilizing and maintaining a long-enduring reversible oxygen-redox activity (100 % high capacity retention after 100 cycles at 0.5 C and 84.28 % retention after 300 cycles at 5 C). Time-resolved operando two-dimensional X-ray diffraction reveals the robust structural stability, zero-strain behavior, and suppressed phase transition with ultra-low volume variation during cycling at different rates (0.1 C: 1.75 %, 1 C: 0.31 %, 5 C: 0.04 %). Additionally, the full cell coupled with hard carbon achieves a high energy density of approximately 211 Wh kg-1 with superior performance. This work highlights the significance of crystal modulation and presents a universal approach in developing Mn-based oxide cathodes with stable anionic redox for high-performance sodium-ion batteries.

Recent Advances in High-Entropy Layered Oxide Cathode Materials for Alkali Metal-Ion Batteries

Since the electrochemical de/intercalation behavior is first detected in 1980, layered oxides have become the most promising cathode material for alkali metal-ion batteries (Li+/Na+/K+; AMIBs) owing to their facile synthesis and excellent theoretical capacities. However, the inherent drawbacks of unstable structural evolution and sluggish diffusion kinetics deteriorate their electrochemical performance, limiting further large-scale applications. To solve these issues, the novel and promising strategy of high entropy has been widely applied to layered oxide cathodes for AMIBs in recent years. Through multielement synergy and entropy stabilization effects, high-entropy layered oxides (HELOs) can achieve adjustable activity and enhanced stability. Herein, the basic concepts, design principles, and synthesis methods of HELO cathodes are introduced systematically. Notably, it explores in detail the improvements of the high-entropy strategy on the limitations of layered oxides, highlighting the latest advances in high-entropy layered cathode materials in the field of AMIBs. In addition, it introduces advanced characterization and theoretical calculations for HELOs and proposes potential future research directions and optimization strategies, providing inspiration for researchers to develop advanced HELO cathode materials in the areas of energy storage and conversion.

Bismuth-doping boosting Na+ diffusion kinetics of layered oxide cathode with radially oriented {010} active lattice facet for sodium-ion batteries

O3-type layered oxide cathodes (NaxTMO2) for sodium-ion batteries (SIBs) have attracted significant attention as one of the most promising potential candidates for practical energy storage applications. The poor Na+ diffusion kinetics is, however, one of the major obstacles to advancing large-scale practical application. Herein, we report bismuth-doped O3-NaNi0.5Mn0.5O2 (NMB) microspheres consisting of unique primary nanoplatelets with the radially oriented {010} active lattice facets. The NMB combines the advantages of the oriented and exposed electrochemical active planes for direct paths of Na+ diffusion, and the thick primary nanoplatelets for less surface parasitic reactions with the electrolyte. Consequently, the NMB cathode exhibits a long-term stability with an excellent capacity retention of 72.5% at 1C after 300 cycles and an enhanced rate capability at a 0.1C to 10C rate (1C = 240 mA g-1). Furthermore, the enhancement is elucidated by the small volume change, thin cathode-electrolyte-interphase (CEI) layer, and rapid Na+ diffusion kinetics. In particular, the radial orientation-based Bi-doping strategy is demonstrated to be effective at boosting electrochemical performance in other layered oxides (such as Bi-doped NaNi0.45Mn0.45Ti0.1O2 and NaNi1/3Fe1/3Mn1/3O2). The results provide a promising strategy of utilizing the advantages of the oriented active facets of primary platelets and secondary particles to develop high-rate layered oxide cathodes for SIBs.

Layered Cathode with Ultralow Strain Empowers Rapid-Charging and Slow-Discharging Capability in Sodium Ion Battery

The development of the electric vehicle industry has spurred demand for secondary batteries capable of rapid-charging and slow-discharging. Among them, sodium-ion batteries (SIBs) with layered oxide as the cathode exhibit competitive advantages due to their comprehensive electrochemical performance. However, to meet the requirements of rapid-charging and slow-discharging scenarios, it is necessary to further enhance the rate performance of the cathode material to achieve symmetrical capacity at different rates. Simultaneously, minimizing lattice strain during asymmetric electrochemical processes is also significant in alleviating strain accumulation. In this study, the ordered distribution of transition metal layers and the diffusion pathway of sodium ions are optimized through targeted K-doping of sodium layers, leading to a reduction of the diffusion barrier and endowment of prominent rate performance. At a 20C rate, the capacity of the cathode can reach 94% of that at a 0.1C rate. Additionally, the rivet effect of the sodium layers resulted in a global volume strain of only 0.03% for the modified cathode during charging at a 10C rate and discharging at a 1C rate. In summary, high-performance SIBs, with promising prospects for rapid-charging and slow-discharging capability, are obtained through the regulation of sodium layers, opening up new avenues for commercial applications.

Kinetically controlled synthesis of low-strain disordered micro-nano high voltage spinel cathodes with exposed {111} facets

High-voltage LiNi0.5Mn1.5O4 (LNMO) is one of the most promising cathode candidates for rechargeable lithium-ion batteries (LIBs) but suffers from deteriorated cycling stability due to severe interfacial side reactions and manganese dissolution. Herein, a micro-nano porous spherical LNMO cathode was designed for high-performance LIBs. The disordered structure and the preferred exposure of the {111} facets can be controlled by the release of lattice oxygen in the high-temperature calcination process. The unique configuration of this material could enhance the structural stability and play a crucial role in inhibiting manganese dissolution, promoting the rapid transport of Li+, and reducing the volume strain during the charge/discharge process. The designed cathode exhibits a remarkable discharge capacity of 136.7 mA h g-1 at 0.5C, corresponding to an energy density of up to 636.4 W h kg-1, unprecedented cycling stability (capacity retention of 90.6% after 500 cycles) and superior rate capability (78.9% of initial capacity at 10C). The structurally controllable preparation strategy demonstrated in this work provides new insights into the structural design of cathode materials for LIBs.

A Feasible Dual Modification Strategy of Internal Anion Redox Chemistry and Surface Engineering on P2 Layer-Structured Cathodes in Sodium-Ion Batteries

Boosting the anion redox reaction opens up a possibility of further capacity enhancement on transition-metal-ion redox-only layer-structured cathodes for sodium-ion batteries. To mitigate the deteriorating impact on the internal and surface structure of the cathode caused by the inevitable increase in the operation voltage, probing a solution to promote the bulk-phase crystal structure stability and surface chemistry environment to further facilitate the electrochemical performance enhancement is a key issue. A dual modification strategy of establishing an anion redox hybrid activation trigger agent inside the crystal structure in combination with surface oxide coating is successfully developed. P2-type layer structure cathode materials with Zn/Li (Na-O-Zn@Na-O-Li) anion redox hybrid triggers and a ZnO coating layer possess superior capacity and cycle performance, along with outstanding structural stability, decreased Mn-ion dissolution effect, and less crystal particle cracking during the cycling process. This study represents a facile modification solution to perform structure optimization and property enhancement toward high-performance layered structure cathode materials with anion redox features in sodium-ion batteries.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Enhancing Structure Stability by Mg/Cr Co-Doped for High-Voltage Sodium-Ion Batteries

P2-Na2/3Ni1/3Mn2/3O2 cathode materials have garnered significant attention due to their high cationic and anionic redox capacity under high voltage. However, the challenge of structural instability caused by lattice oxygen evolution and P2-O2 phase transition during deep charging persists. A breakthrough is achieved through a simple one-step synthesis of Cr, Mg co-doped P2-NaNMCM, resulting in a bi-functional improvement effect. P2-NaNMCM-0.01 exhibits an impressive capacity retention rate of 82% after 100 cycles at 1 C. In situ X-ray diffraction analysis shows that the "pillar effect" of Mg mitigates the weakening of the electrostatic shielding and effectively suppresses the phase transition of P2-O2 during the charging and discharging process. This successfully averts serious volume expansion linked to the phase transition, as well as enhances the Na+ migration. Simultaneously, in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy tests demonstrate that the strong oxygen affinity of Cr forms a robust TM─O bond, effectively restraining lattice oxygen evolution during deep charging. This study pioneers a novel approach to designing and optimizing layered oxide cathode materials for sodium-ion batteries, promising high operating voltage and energy density.

The Magnesium Insertion Effects into P2-Type Na2/3 Ni1/3 Mn2/3 O2

A Mg-cell with P2-Na2/3 Ni1/3 Mn2/3 O2 layered oxide cathode provides novel reaction mechanism not observed in Na-cells. The sodium/vacancy ordering and Jahn-Teller effects are suppressed with the insertion of magnesium ion. The Mg-cell exhibits different features when operating between 4.5 and 0.15 V and 3.9 and 0.15 V versus Mg2+ /Mg. To analyze the structural and chemical changes during Mg insertion, the cathode is first charged to obtain the Na1/3 Ni1/3 Mn2/3 O2 compound, which is formally accompanied by an oxidation from Ni2+ to Ni3+ . As structure models Mg1/6 Na1/3 Ni1/3 Mn2/3 O2 and Mg1/12 Na1/2 Ni1/3 Mn2/3 O2 are utilized with a large2 3 a $2\sqrt 3 a$ ×2 3 a $2\sqrt 3 a$ supercell. On discharge, the Mg-cell exhibits a multistep profile which reaches ≈100 mA h g-1 with the valence change from Ni3+ to Ni2+ . Such profile is quite different from its sodium counterpart (230 mA h g-1 ) which exhibits the sodium/vacancy ordering and deleterious presence of Mn3+ . Depending on how the two interlayer spacings are filled by Na and Mg the "staged," "intermediated," and "average" models are analyzed for Mgy Na8 Ni8 Mn16 O48 supercell. This fact suggests differences in the cell performance when Mg is used as counter electrode providing some tips to improve the structure engineering on cathode materials.

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.

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.

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.