Efficient Method of Designing Stable Layered Cathode Material for Sodium Ion Batteries Using Aluminum Doping

A Comprehensive Review on Strategies for Enhancing the Performance of Polyanionic-Based Sodium-Ion Battery Cathodes

The significant consumption of fossil fuels and the increasing pollution have spurred the development of energy-storage devices like batteries. Due to their high cost and limited resources, widely used lithium-ion batteries have become unsuitable for large-scale energy production. Sodium is considered to be one of the most promising substitutes for lithium due to its wide availability and similar physiochemical properties. Designing a suitable cathode material for sodium-ion batteries is essential, as the overall electrochemical performance and the cost of battery depend on the cathode material. Among different types of cathode materials, polyanionic material has emerged as a great option due to its higher redox potential, stable crystal structure, and open three-dimensional framework. However, the poor electronic and ionic conductivity limits their applicability. This review briefly discusses the strategies to deal with the challenges of transition-metal oxides and Prussian blue analogue, recent developments in polyanionic compounds, and strategies to improve electrochemical performance of polyanionic material by nanostructuring, surface coating, morphology control, and heteroatom doping, which is expected to accelerate the future design of sodium-ion battery cathodes.

Optimizing electrochemical performance of Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure via preparing concentration-gradient particles for sodium-ion batteries

P2-type layered oxides for rechargeable sodium-ion batteries have drawn a lot of attention because of their excellent electrochemical performance. However, these types of cathodes usually suffer from poor cyclic stability. To overcome this disadvantage, in this work, novel ball-shaped concentration-gradient oxide Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure modified by Mn-rich surface is successfully prepared using co-precipitation method. The concentration of Mn increased from the inner core to the surface, endowing the material with an excellent cyclic stability. The cathode exhibits enhanced electrochemical properties than that of the sample synthesized by solid-state method and concentration-constant material. It shows 143.2 mAh/g initial discharge capacity and retains 131 mAh/g between 2 V and 4.5 V after 100 rounds. The significant improvement in the electrochemical properties of the sample benefits from the unique concentration-gradient structure, and the Mn-rich surface that effectively stabilizes the basic P2 structure. The relatively higher Ni content in the core leads to a slight improvement in the discharge capacity of the sample. This strategy may provide new insights for preparing layered cathodes for sodium-ion batteries with high electrochemical performance.

Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g^-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g^-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF₆ electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg^-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na⁺ kinetics facilitating Na⁺ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na⁺ kinetics, boosting the overall Na storage.

Unprecedented Cyclability and Moisture Durability of NaCrO2 Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr2O3 Coating

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO₂ remains one of the most attractive materials due to its reasonable level of capacity, nearly flat reversible voltages, and high thermal stability. However, the cyclic stability of NaCrO₂ needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr₂O₃-coated and Al-doped NaCrO₂, which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr₂O₃ shell and a Na(Cr_1-2xAl_2x)O₂ core, rather than xAl₂O₃/NaCrO₂ or Na_1/1+2x(Cr_1/1+2xAl_2x/1+2x)O₂, through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr₂O₃-coated NaCrO₂ without Al dopants or Al-doped NaCrO₂ without shells because of their synergistic contributions. As a result, Na(Cr_0.98Al_0.02)O₂ with a thin Cr₂O₃ layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO₂. In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr₂O₃-coated Na(Cr_1-2xAl_2x)O₂.

Anionic Redox Activities Boosted by Aluminum Doping in Layered Sodium-Ion Battery Electrode

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 Na_0.6 Ni_0.3 Mn_0.7 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.

A dual-modification strategy for P2-type layered oxide via bulk Mg/Ti co-substitution and MgO surface coating for sodium ion batteries

P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 → O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na⁺. Moreover, the as-prepared MgO-coated Na_0.67Ni_0.17Co_0.17Mn_0.66Mg_0.1O₂ exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g^-1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g^-1 within 2-4.5 V, which has been obviously enhanced than that of Na_0.67Ni_0.17Co_0.17Mn_0.66O₂. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.

Dual Doping with Cation and Anion for Enhancing the Structural Stability of Sodium-Ion Layered Cathode

First-principles-based calculations were implemented to explore the ideal combination of cations and anions as dual dopants for enhancing the structural stability of the sodium-ion layered cathode for application in sodium...

Gradient Hydrogen Migration Modulated with Self-Adapting S Vacancy in Copper-Doped ZnIn2S4 Nanosheet for Photocatalytic Hydrogen Evolution

It is a challenge to regulate charge flow synergistically at the atomic level to modulate gradient hydrogen migration (H migration) for boosting photocatalytic hydrogen evolution. Herein, a self-adapting S vacancy (Vs) induced with atomic Cu introduction into ZnIn₂S₄ nanosheets was fabricated elaborately, which can tune charge separation and construct a gradient channel for H migration. Detailed experimental results and theoretical simulations uncover the behavior mechanism of Vs generation with Cu introduction after substituting a Zn atom tendentiously. Cu-S bond shrinkage and Zn-S bond distortion are presented around Vs areas. Besides, Vs induced by Cu introduction lowers the internal electric field to restrain electron transmission between layers, which are enriched on the Vs area because of the lower surface electrostatic potential. Atomic Cu and Vs show a synergistic effect for regulating regional charge separation due to the Cu dopant being a hole trap and Vs being an electron trap. The channels for H migration with gradient ΔG_H⁰ are constructed by different S atom sites, which are modulated by Vs. Gradient H migration driven by a photothermal effect occurs on an identical surface without striding across a heterogeneous interface, which is a valid pathway with lower resistance for boosting H₂ release. Ultimately, 5 mol % Cu confined in ZnIn₂S₄ nanosheets achieves an optimum photocatalytic hydrogen evolution activity of 9.8647 mmol g^-1 h^-1, which is 14.8 times higher than 0.6640 mmol g^-1 h^-1 for ZnIn₂S₄, and apparent quantum efficiency reaches 37.11% at 420 nm. This work demonstrates the behavior mechanism of atomic substitution and provides cognition for hydrogen evolution mechanism deeply.

Pinning Effect Enhanced Structural Stability toward a Zero-Strain Layered Cathode for Sodium-Ion Batteries

Layered oxides as the cathode materials of sodium-ion batteries are receiving extensive attention due to their high capacity and flexible composition. However, the layered cathode tends to be thermodynamically and electrochemically unstable during (de)sodiation. Herein, we propose the pinning effect and controllable pinning point in sodium storage layered cathodes to enhance the structural stability and achieve optimal electrochemical performance. 0 %, 2.5 % and 7.3 % transition-metal occupancies in Na-site as pinning points are obtained in Na_0.67 Mn_0.5 Co_0.5-x Fe_x O₂ . 2.5 % Na-site pinned by Fe³⁺ is beneficial to restrain the potential slab sliding and enhance the structural stability, resulting in an ultra-low volume variation of 0.6 % and maintaining the smooth two-dimensional channel for Na-ion transfer. The Na_0.67 Mn_0.5 Co_0.4 Fe_0.1 O₂ cathode with the optimal Fe³⁺ pinning delivers outstanding cycle performance of over 1000 cycles and superior rate capability up to 10 C.

Tailoring P2/P3 Biphases of Layered Nax MnO2 by Co Substitution for High-Performance Sodium-Ion Battery

P-type layered oxide is a promising cathode candidate for sodium-ion batteries (SIBs), but faces the challenge of simultaneously realizing high rate capability and long cycle life. Herein, Co-substituted Na_x MnO₂ nanosheets with tunable P2/P3 biphase structures are synthesized by a novel dealloying-annealing strategy. The optimized P2/P3-Na_0.67 Mn_0.64 Co_0.30 Al_0.06 O₂ cathode delivers an excellent rate capability of 83 mA h g^-1 at a high current density of 1700 mA g^-1 (10 C), and an outstanding cycling stability over 500 cycles at 1000 mA g^-1 . This excellent performance is attributed to the unique P2/P3 biphases with stable crystal structures and fast Na⁺ diffusion between open prismatic Na sites. Moreover, operando X-ray diffraction is applied to explore the structural evolution of Na_0.67 Mn_0.64 Co_0.30 Al_0.06 O₂ during the Na⁺ extraction/insertion processes, and the P2-P2' phase transition is effectively suppressed. Operando Raman technique is utilized to explore the structural superiority of P2/P3 biphase cathode compared with pure P2 or P3 phase. This work highlights precisely tailoring the phase composition as an effective strategy to design advanced cathode materials for SIBs.

A comprehensive study of the multiple effects of Y/Al substitution on O3-type NaNi0.33Mn0.33Fe0.33O2 with improved cycling stability and rate capability for Na-ion battery applications

O3-NaNi0.33Mn0.33Fe0.33O2 layered oxide has attracted increasing attention as one of the most promising materials for Na-ion battery applications due to air stability and environmental friendliness, but the complex phase transitions and inferior cycling stability are extremely challenging to overcome. Cation substitution has been widely used to stabilize crystal structures and improve electrochemical performance for SIBs. Based on past experimental results, it was discovered that the transition metal-oxygen bond energy of the introduced dopant is an important factor for optimizing electrochemical performance. In this study, we validated our hypothesis that yttrium (Y)-which possesses high bond energy for oxygen-is most likely to be an ideal doping element by conducting a comparative study of substituting Mn in O3-NaNi0.33Mn0.33Fe0.33O2 layered oxide with aluminum (Al) and Y through elemental doping. As hypothesized, the electrochemical properties of NaNi0.33Mn0.33Fe0.33O2 have increased markedly by introducing a small amount of Y and Al, and the Y-doped materials showed superior rate performance and cycling stability due to enhanced Na+ diffusion reaction kinetics and layered structure stability. Furthermore, the substitution of Y for Mn can improve thermal stability and alleviate phase transformations. The improvement mechanism of Y substitution can be attributed to a larger d-spacing and stronger metal-oxygen bond. These results suggest that structural modulation is an effective strategy to reinforce electrochemical properties of layered oxides and provides some guidance about designing promising electrode materials.

Atomic layer deposition of Al2O3 on P2-Na0.5Mn0.5Co0.5O2 as interfacial layer for high power sodium-ion batteries

Surface modification is one of the impressive and widely used technique to improve the electrochemical performance of sodium-ion batteries by modifying the electrode-electrolyte interface. Herein, we used the atomic layer deposition (ALD) to modify the surface of P2-Na_0.5Mn_0.5Co_0.5O₂ by sub-monolayer Al₂O₃ coating on the prefabricated electrodes. Phase purity is confirmed using various structural and morphological studies. The pristine electrode delivered an initial discharge capacity of 154 mAh g^-1 at 0.5C, and inferior rate performance of 23 mAh g^-1 at 40C rate. On the other hand, the interfacial modified cathode with 5 cycles of ALD coating delivers a high capacity of 174 and 45 mAh g^-1 at 0.5C and 40C rate, respectively. The Co^2+/3+ redox couple is utilized for the faradaic process with high reversibility along with suppressed P2-O2 phase transition. The presence of the Al₂O₃ layer acts as an artificial cathode electrolyte interface by suppressing the electrolyte oxidation at higher cutoff potentials. This is clearly validated by the reduced charge transfer resistance of surface modified electrodes after cycling at various current rates. Even at an elevated temperature condition (50 °C), interfacial layer significantly improves the safety of the cell and ensures the stability of the cathode.

Deciphering the Structure-Property Relationship of Na-Mn-Co-Mg-O as a Novel High-Capacity Layered-Tunnel Hybrid Cathode and Its Application in Sodium-Ion Capacitors

Developing novel cathode materials with a high energy density and long cycling stability is necessary for Na-ion batteries and Na-ion hybrid capacitors (NICs). Despite their high energy density, structural flexibility, and ease of synthesis, P-type Na layered oxides cannot be utilized in energy-storage applications owing to their severe capacity fading. In this regard, we report a novel composite layered-tunnel Na_0.5Mn_0.5Co_0.48Mg_0.02O₂ cathode whose binary structure was confirmed via scanning electron microscopy and high-resolution transmission electron microscopy. Combination of the two-dimensional (2D) layered oxides with the three-dimensional tunnel structure, as well as the presence of Mg²⁺ ions, resulted in a high capacity of 145 mAh g^-1 at a current density of 85 mA g^-1, along with a high stability and rate capability. An NIC was fabricated with composite layered-tunnel structure as a battery-type electrode and commercial activated carbon as a counter electrode. The NIC exhibited a maximum energy density of 35 Wh kg^-1 and good stability retaining 72% of its initial energy density after 3000 cycles. This integrated approach provides a new method for designing high-energy and high-power cathodes for NICs and NIBs.

Ni-Doped Layered Manganese Oxide as a Stable Cathode for Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are one of the promising alternatives to lithium-ion batteries (LIBs). Layered potassium manganese oxides are more attractive as cathodes for PIBs due to their high capacity, low cost, and simple synthesis method but suffer from the Jahn-Teller effect of Mn³⁺ in material synthesis. Here, a layered P3-type K_0.67Mn_0.83Ni_0.17O₂ material with a suppressed Jahn-Teller effect was successfully synthesized. K_0.67Mn_0.83Ni_0.17O₂ delivers a specific capacity of 122 mAh g^-1 at 20 mA g^-1 in the first discharge, superior rate performance, and good cycling stability (75% capacity retention cycled at a high rate of 500 mA g^-1 after 200 cycles). Besides, the K ion diffusion coefficient of the K_0.67Mn_0.83Ni_0.17O₂ electrode can reach 10^-11 cm² s^-1, which are larger than the Ni-free electrode. The X-ray diffraction and electron diffraction analyses demonstrate that appropriate nickel could suppress the Jahn-Teller effect and reduce the structural deterioration, resulting in more migration pathways for K ions, thus enhancing the rate capability and cycling performance. These results provide a strategy to develop high-performance cathode materials for PIBs and deepen the understanding of structural deterioration in layered manganese-based oxides.

Probing the Structural Transition Kinetics and Charge Compensation of the P2-Na0.78Al0.05Ni0.33Mn0.60O2 Cathode for Sodium Ion Batteries

Although the layered P2-type Na_0.67Ni_0.33Mn_0.67O₂ materials show high discharge voltage and specific capacity, they suffer from severe structural instabilities and surface reaction upon Na exchange for sodium-ion batteries (SIBs). Therefore, it is quite necessary to reveal the underlying structural evolution mechanism and diffusion kinetics to improve the structural/electrochemical stability for application in SIBs. Here, we synthesize a P2-type Na_0.78Al_0.05Ni_0.33Mn_0.60O₂ material by a small dose of Al replacing the Mn, aiming at enhancing the structural stability without sacrificing the average discharge voltage and theoretical capacity. The etching X-ray photoelectron spectroscopy and energy-dispersive X-ray mapping/line scan results indicate that the Al doping induces dual effects of the Al₂O₃ surface coating and the bulk lattice doping, which efficiently suppress the accumulation of structural irreversible changes from P2 to O2, the volume changes, and surface reactions at high voltage. Obvious improvements are further found on the diffusion kinetics of Na ions as well as the decrease of overall voltage polarization. Interestingly, the dual effects of Al doping lead to the significant increase of capacity retention after 50 cycles and improvement of rate capability compared with the undoped counterpart between 2.0 and 4.5 V. Hence, this work sheds new light on stabilizing the P2-Na-Ni-Mn-O materials, which provides a rewarding avenue to develop better SIBs.

Manganese-Based Na-Rich Materials Boost Anionic Redox in High-Performance Layered Cathodes for Sodium-Ion Batteries

To improve the energy and power density of Na-ion batteries, an increasing number of researchers have focused their attention on activation of the anionic redox process. Although several materials have been proposed, few studies have focused on the Na-rich materials compared with Li-rich materials. A key aspect is sufficient utilization of anionic species. Herein, a comprehensive study of Mn-based Na_1.2 Mn_0.4 Ir_0.4 O₂ (NMI) O3-type Na-rich materials is presented, which involves both cationic and anionic contributions during the redox process. The single-cation redox step relies on the Mn³⁺ /Mn⁴⁺ , whereas Ir atoms build a strong covalent bond with O and effectively suppress the O₂ release. In situ Raman, ex situ X-ray photoelectron spectroscopy, and soft-X-ray absorption spectroscopy are employed to unequivocally confirm the reversibility of O₂ ^2- species formation and suggest a high degree of anionic reaction in this NMI Na-rich material. In operando X-ray diffraction study discloses the asymmetric structure evolution between the initial and subsequent cycles, which also explains the effect of the charge compensation mechanism on the electrochemical performance. The research provides a novel insight on Na-rich materials and a new perspective in materials design towards future applications.

Manganese based layered oxides with modulated electronic and thermodynamic properties for sodium ion batteries

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn–Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na_0.833[Li_0.25Mn_0.75]O₂ can not only suppress the Jahn–Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

Mn-based layered oxides are promising cathode materials for next generation sodium ion batteries. To address two existing issues facing the system, here the authors show that a simple zinc doping can suppress both Jahn–Teller distortion and phase separation, enabling enhanced cycling performance.